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By James Ashenhurst

• Infrared Spectroscopy: A Quick Primer On Interpreting Spectra

Last updated: October 31st, 2022 |

How To Interpret IR Spectra In 1 Minute Or Less: The 2 Most Important Things To Look For [Tongue and Sword]

Last post , we briefly introduced the concept of bond vibrations, and we saw that we can think of covalent bonds as a bit like balls and springs:  the springs vibrate, and each one “sings” at a characteristic frequency, which depends on the strength of the bond and on the masses of the atoms.  These vibrations have frequencies that are in the mid-infrared (IR) region of the electromagnetic spectrum.

We can observe and measure this “singing” of bonds by applying IR radiation to a sample and measuring the frequencies at which the radiation is absorbed. The result is a technique known as Infrared Spectroscopy , which is a useful and quick tool for identifying the bonds present in a given molecule.

We saw that the IR spectrum of water was pretty simple – but moving on to a relatively complex molecule like glucose (below) we were suddenly confronted with a forest of peaks!

Your first impression of looking at that IR might be: agh!  how am I supposed to make sense of that??

To which I want to say:  don’t panic! 

Table of Contents

• Let’s Correct Some Common Misconceptions About IR
• Starting With “Hunt And Peck” Is Not The Way To Go
• IR Spectroscopy: The Big Picture
• The Two Main Things To Look For In An IR Spectrum: “Tongues” and “Swords”.
• Alcohols and Carboxylic Acids: More Detail
• Specific Examples of IR Spectra of Carbonyl Functional Groups
• Less Crucial, But Still Useful: Two More Very Diagnostic Areas.
• Glucose, Revisited: The 1 Minute Analysis

1. Let’s Correct Some Common Misconceptions About IR

In this post, I want to show that a typical analysis of an IR spectrum is much simpler than you might think. In fact, once you learn what to look for, it can often be done in a minute or less.  Why?

• IR is not generally used to determine the whole structure of an unknown molecule. For example, there isn’t a person alive who could look at the IR spectrum above and deduce the structure of glucose from it. IR is a tool with a very specific use. [Back in 1945 when IR was one of the few spectral techniques available, it was necessary to spend a lot more time trying to squeeze every last bit of information out of the spectrum. Today, with access to NMR and other techniques, we can do more cherry-picking]
• We don’t need to analyze every single peak  ! (as we’ll see later, that’s what NMR is for : – )  ).   Instead, IR is great for  identifying certain specific functional groups , like alcohols and carbonyls. In this way it’s complimentary to other techniques (like NMR) which don’t yield this information as quickly.

With this in mind, we can simplify the analysis of an IR spectrum by cutting out everything except the lowest-lying fruit. 

See that forest of peaks from 500-1400 cm -1 ? We’re basically going to ignore them all!

80% of the most useful information for our purposes can be obtained by looking at  two specific areas of the spectrum : 3200-3400 cm -1 and 1650-1800 cm -1 . We’ll also see that there are at least two more regions of an IR spectrum worth glancing at, and thus conclude a “first-order” analysis of the IR spectrum of an unknown. [We might write a subsequent post which gets nittier and grittier about the finer points of analyzing an IR spectrum]

Bottom line: The purpose of this post is to show you how to  prioritize your time  in an analysis of an IR spectrum.

[BTW: all spectra are from the NIST database . Thank you, American taxpayers!]

2. Starting With “Hunt And Peck” Is Not The Way To Go

Confronted with an IR spectrum of an unknown (and a sense of rising panic), what does a typical new student do?

They often reach for the first tool they are given, which is a table of common ranges for IR peaks given to them by their instructor.

The next step in their analysis is to go through the spectrum from one side to the next, trying to match every single peak to one of the numbers in the table. I know this because this is exactly what I did when I first learned IR.  I call it “hunting and pecking”.

The only people who “hunt and peck” as their first step are people who have no plan  (i.e. “newbies”).

So by reading the next few paragraphs you can save yourself a lot of time and confusion.

[Hunt and peck has its place, but only AFTER  you’ve looked for “tongues” and “swords”, below. Hunting and pecking is great to make sure you didn’t miss anything big – but as a first step, it’s bloody awful!]

3. The Big Picture

In IR spectroscopy we measure where molecules absorb photons of IR radiation. The peaks represent areas of the spectrum where specific bond vibrations occur. [for more background, see the previous post, especially on the “ball and spring” model] . Just like springs of varying weights vibrate at characteristic frequencies depending on mass and tension, so do bonds.

Here’s an overview of the IR window from 4000 cm  -1  to 500 cm  -1  with various regions of interest highlighted.

An even more compressed overview looks like this: ( source )

Within these ranges, there are  two high-priority areas to focus on , and two lesser-priority areas we’ll discuss further below.

4. The Two Main Things To Look For In An IR Spectrum: “Tongues” and “Swords”.

When confronted with a new IR spectrum, prioritize your time by asking two important questions:

• Is there a broad, rounded peak in the region around 3400-3200 cm -1 ? That’s where hydroxyl groups ( OH ) appear.
• Is there a sharp, strong peak in the region around 1850-1630 cm -1 ? That’s where carbonyl groups ( C=O ) show up.

First, let’s look at some examples of hydroxyl group peaks in the 3400 cm -1 to  3200 cm -1  region,  which Jon describes vividly as “tongues”. The peaks below all belong to alcohols. Hydrogen bonding between hydroxyl groups leads to some variations in O-H bond strength, which results in a range of vibrational energies. The variation results in the broad peaks observed.

Hydroxyl groups that are a part of carboxylic acids have an even broader appearance that we’ll describe in a bit.

[Sometimes it helps to know what not to look for. On the far right hand side is included one example of a very weak peak on a baseline that you can safely ignore.]

The main point is that  a hydroxyl group isn’t generally something you need to go looking for in the baseline noise.

Although hydroxyl groups are the most common type of broad peak in this region, N-H peaks can show up in this area as well (more on them in the Note 1 ). They tend to have a sharper appearance and may appear as one or two peaks depending on the number of N-H bonds.

Next,  let’s look at some examples of   C=O peaks, in the region around 1630-1800 cm -1. . These peaks are almost always the strongest peaks in the entire spectrum and are relatively narrow, giving them a somewhat “sword-like” appearance.

That sums up our 80/20 analysis: look for tongues and swords.

If you learn nothing else from this post, learn to recognize these two types of peaks!

Two other regions of the IR spectrum can quickly yield useful information if you train yourself to look for them.

3. The line at 3000 cm -1 is a useful “border” between alk ene  C–H (above 3000 cm -1 )   and alk ane C–H (below 3000 cm -1  ) This can quickly help you determine if double bonds are present.

4. A peak in the region around 2200 cm -1 – 2050 cm -1  is a subtle indicator of the presence of a triple bond [C≡N or C≡C] . Nothing else shows up in this region.

A Common Sense Reminder

First, some obvious advice:

• if you’re given the molecular formula, that will determine what functional groups you should look for. It makes no sense to look for OH groups if you have no oxygens in your molecular formula, or likewise the presence of an amine if the formula lacks nitrogen.
• Less obviously,  calculate the degrees of unsaturation   if you are given the molecular formula, because it will provide important clues. Don’t look for C=O in a structure like C 4 H 10 O which doesn’t have any degrees of unsaturation.

5. Alcohols and Carboxylic Acids: More Detail

Let’s look at a specific example so we can see everything in perspective. The spectrum below is of 1-hexanol.

Note the hydroxyl group peak around 3300 cm -1  , typical of an alcohol   (That sharp peak around 3600 cm -1  is a common companion to hydroxyl peaks: it represents non-hydrogen bonded O-H). 

To gain some familiarity with variation,  here’s some more examples of entire IR spectra of various alcohols.

• Cyclohexanol 

Carboxylic Acids

Hydroxyl groups in carboxylic acids are considerably broader than in alcohols. Jon calls it a “hairy beard”, which is a perfect description. Their appearance is also highly variable. The OH absorption in carboxylic acids can be so broad that it extends below 3000 cm -1 , pretty much “taking over”  the left hand part of the spectrum.

Here’s an example: butanoic acid.

Here’s some more examples of full spectra so you can see the variation.

• Benzoic acid ,
• Pentanoic acid ,
• Acetic acid

The difference in appearance between the OH of an alcohol and that of a carboxylic acid is usually diagnostic. In the rare case where you aren’t sure whether the broad peak is due to the OH of an alcohol or a carboxylic acid, one suggestion is to check the region around 1700 cm for the C=O stretch. If it’s absent, you are likely looking at an alcohol.

[ Note 1 for more detail on the 3200-3500 cm -1 region : Amines, Amides, and Terminal Alkynes]

6. Specific Examples of IR Spectra of Carbonyl Functional Groups

The second important peak region is the carbonyl C=O stretch area at about 1630-1830 cm. Carbonyl stretches are sharp and strong.

Once you see a few of them they’re impossible to miss. Nothing else shows up in this region.

To put it in perspective, here’s the IR spectrum of hexanal. That peak a little after 1700 cm -1 is the C=O stretch.  When it’s present, the C=O stretch is almost always the strongest peak in the IR spectrum and impossible to miss.

The position of the C=O stretch varies slightly by carbonyl functional group. Some ranges (in cm -1 ) are shown below:

• Aldehydes (1740-1690): benzaldehyde , propanal , pentanal
• Ketones (1750-1680): 2-pentanone , acetophenone
• Esters (1750-1735): ethyl acetate , methyl benzoate
• Carboxylic acids (1780-1710): benzoic acid , butanoic acid
• Amide (1690-1630): acetamide , benzamide ,  N,N -dimethyl formamide (DMF)
• Anhydrides (2 peaks; 1830-1800 and 1775-1740): acetic anhydride , benzoic anhydride

Conjugation will affect the position of the C=O stretch somewhat, moving it to lower wavenumber.

A decent rule of thumb is that you will never, ever see a C=O stretch below 1630. If you see a strong peak at 1500, for example, it is  not C=O. It is something else.

7. Less Crucial, But Still Useful: Two More Very Diagnostic Areas.

• The C-H Stretch Boundary at 3000 cm -1

3000 cm -1 serves as a useful dividing line. Above this line is observed higher frequency C-H stretches we attribute to sp 2 hybridized C-H bonds. Two examples below: 1-hexene (note the peak that stands a little higher) and benzene.

For a molecule with only sp 3 -hybrized C-H bonds, the lines will appear below 3000 cm -1 as in hexane, below.

2. The Distinctive Triple Bond Region around 2200 cm -1

Molecules with triple bonds appear relatively infrequently in the grand scheme of things, but when they do, they do have a distinctive trace in the IR.

The region between 2000 cm -1 and 2400 cm -1   is a bit of a “ghost town” in IR spectra; there’s very little that appears in this region. If you do see peaks in this region, a likely candidate is a triple bonded carbon such as an alkyne or nitrile .

Note how weak the alkyne peaks are.  This is one exception to the rule that one should ignore weak peaks. Still, caution is required: if you’re given the molecular formula, confirm that an alkyne is possible by calculating the degrees of unsaturation and ensuring that it is at least 2 or more.

Terminal alkynes (such as 1-hexyne) also have a strong C-H stretch around 3400 cm -1  that is more strongly diagnostic.

8. Glucose, Revisited: The 1 Minute Analysis

OK. We’ve gone over 4 regions that are useful for a quick analysis of an IR spectrum.

• (important!) O-H around 3200-3400 cm -1
• (important!) C=O around 1700 cm -1
• C-H dividing line at 3000 cm -1
• (rare) Triple bond region around 2050-2250 cm -1

Now let’s go back and look at the IR of glucose. What do we see?

Here are the two big things to note:

• OH present around 3300 cm -1  . (in fact, this was included as one of the “swords” in section #3,  above)
• No C=O stretch present. No strong peak around 1700 cm -1   . (The peak at 1450 cm -1   isn’t a C=O stretch).

Also, if we take a bit of extra time we can see:

• No alkene C-H (no peaks above 3000 cm -1  )
• Nothing in triple bonded region (rare, but still an easy thing to learn to check)

Now: If you were given this spectrum as an “unknown” along with its molecular formula, C 6 H 12 O 6 , what conclusions could you draw about its structure?

• The molecule has at least one OH group (and possibly more)
• The molecule doesn’t have any C=O groups
• The molecule *likely* doesn’t have any alkenes. If any alkenes are present, they don’t bear any C-H bonds, because we’d see their C-H stretch above 3000 cm -1 .

A molecule with one degree of hydrogen deficiency (C 6 H 12 O 6 ) but no C=O, and likely no C=C ?

A good guess would be that the molecule contains a ring . (We know this is the case, of course, but it’s nice to see the IR confirming what we already know).

This is what a 1-minute analysis of the IR of glucose can tell us. Not the whole structure, mind you, but certainly some important bits and pieces.

That’s enough for today. In the next post we’ll do some more 1-minute analyses and give more concrete examples of how to use the information in an IR spectrum to draw conclusions about molecular structure.

Related Articles

• IR Spectroscopy: 4 Practice Problems
• Bond Vibrations, Infrared Spectroscopy, and the “Ball and Spring” Model
• Introduction To UV-Vis Spectroscopy
• UV-Vis Spectroscopy: Practice Questions
• UV-Vis Spectroscopy: Absorbance of Carbonyls
• Degrees of Unsaturation (or IHD, Index of Hydrogen Deficiency)

More on the 3200 region: Amines, Amides, and Terminal Alkyne C-H

While we’re in the 3200 region…. Amines and Amides

Amines and amides also have N-H stretches which show up in this region. [update: a comment from Paul Wenthold mentions some helpful advice about amides – they are rare – look for confirming evidence from the mass spectrum or other sources before assigning an amide based on a stretch in this region, as this region can also contain carbonyl “overtone” peaks]

Notice how the primary amine and primary amide have two “fangs”, while the secondary amine and secondary amide have a single peak.

The amine stretches tend to be sharper than the amide stretches; also the amides can be distinguished by a strong C=O stretch (see below).

Primary amines (click for spectra)

• Benzylamine
• Cyclohexylamine

Secondary amines:

• N-methylbenzylamine
• N,N-dibenzylamine
• N-methylaniline

Primary amides

• Propionamide

Secondary amides

• N-methyl benzamide

Terminal alkyne C-H

Terminal alkynes have a characteristic C-H stretch around 3300 cm -1 . Here it is for ethynylbenzene, below.

• Ethynylbenzene

00 General Chemistry Review

• Lewis Structures
• Ionic and Covalent Bonding
• Chemical Kinetics
• Chemical Equilibria
• Valence Electrons of the First Row Elements
• How Concepts Build Up In Org 1 ("The Pyramid")

01 Bonding, Structure, and Resonance

• How Do We Know Methane (CH4) Is Tetrahedral?
• Hybrid Orbitals and Hybridization
• How To Determine Hybridization: A Shortcut
• Orbital Hybridization And Bond Strengths
• Sigma bonds come in six varieties: Pi bonds come in one
• A Key Skill: How to Calculate Formal Charge
• Partial Charges Give Clues About Electron Flow
• The Four Intermolecular Forces and How They Affect Boiling Points
• 3 Trends That Affect Boiling Points
• How To Use Electronegativity To Determine Electron Density (and why NOT to trust formal charge)
• Introduction to Resonance
• How To Use Curved Arrows To Interchange Resonance Forms
• Evaluating Resonance Forms (1) - The Rule of Least Charges
• How To Find The Best Resonance Structure By Applying Electronegativity
• Evaluating Resonance Structures With Negative Charges
• Evaluating Resonance Structures With Positive Charge
• Exploring Resonance: Pi-Donation
• Exploring Resonance: Pi-acceptors
• In Summary: Evaluating Resonance Structures
• Drawing Resonance Structures: 3 Common Mistakes To Avoid
• How to apply electronegativity and resonance to understand reactivity
• Bond Hybridization Practice
• Structure and Bonding Practice Quizzes
• Resonance Structures Practice

02 Acid Base Reactions

• Introduction to Acid-Base Reactions
• Acid Base Reactions In Organic Chemistry
• The Stronger The Acid, The Weaker The Conjugate Base
• Walkthrough of Acid-Base Reactions (3) - Acidity Trends
• Five Key Factors That Influence Acidity
• Acid-Base Reactions: Introducing Ka and pKa
• How to Use a pKa Table
• The pKa Table Is Your Friend
• A Handy Rule of Thumb for Acid-Base Reactions
• Acid Base Reactions Are Fast
• pKa Values Span 60 Orders Of Magnitude
• How Protonation and Deprotonation Affect Reactivity
• Acid Base Practice Problems

03 Alkanes and Nomenclature

• Meet the (Most Important) Functional Groups
• Condensed Formulas: Deciphering What the Brackets Mean
• Hidden Hydrogens, Hidden Lone Pairs, Hidden Counterions
• Don't Be Futyl, Learn The Butyls
• Primary, Secondary, Tertiary, Quaternary In Organic Chemistry
• Branching, and Its Affect On Melting and Boiling Points
• The Many, Many Ways of Drawing Butane
• Wedge And Dash Convention For Tetrahedral Carbon
• Common Mistakes in Organic Chemistry: Pentavalent Carbon
• Table of Functional Group Priorities for Nomenclature
• Summary Sheet - Alkane Nomenclature
• Organic Chemistry IUPAC Nomenclature Demystified With A Simple Puzzle Piece Approach
• Boiling Point Quizzes
• Organic Chemistry Nomenclature Quizzes

04 Conformations and Cycloalkanes

• Staggered vs Eclipsed Conformations of Ethane
• Conformational Isomers of Propane
• Newman Projection of Butane (and Gauche Conformation)
• Introduction to Cycloalkanes (1)
• Geometric Isomers In Small Rings: Cis And Trans Cycloalkanes
• Calculation of Ring Strain In Cycloalkanes
• Cycloalkanes - Ring Strain In Cyclopropane And Cyclobutane
• Cyclohexane Conformations
• Cyclohexane Chair Conformation: An Aerial Tour
• How To Draw The Cyclohexane Chair Conformation
• The Cyclohexane Chair Flip
• The Cyclohexane Chair Flip - Energy Diagram
• Substituted Cyclohexanes - Axial vs Equatorial
• Ranking The Bulkiness Of Substituents On Cyclohexanes: "A-Values"
• The Ups and Downs of Cyclohexanes
• Cyclohexane Chair Conformation Stability: Which One Is Lower Energy?
• Fused Rings - Cis-Decalin and Trans-Decalin
• Naming Bicyclic Compounds - Fused, Bridged, and Spiro
• Bredt's Rule (And Summary of Cycloalkanes)
• Newman Projection Practice
• Cycloalkanes Practice Problems

05 A Primer On Organic Reactions

• The Most Important Question To Ask When Learning a New Reaction
• The 4 Major Classes of Reactions in Org 1
• Learning New Reactions: How Do The Electrons Move?
• How (and why) electrons flow
• The Third Most Important Question to Ask When Learning A New Reaction
• 7 Factors that stabilize negative charge in organic chemistry
• 7 Factors That Stabilize Positive Charge in Organic Chemistry
• Common Mistakes: Formal Charges Can Mislead
• Nucleophiles and Electrophiles
• Curved Arrows (for reactions)
• Curved Arrows (2): Initial Tails and Final Heads
• Nucleophilicity vs. Basicity
• The Three Classes of Nucleophiles
• What Makes A Good Nucleophile?
• What makes a good leaving group?
• 3 Factors That Stabilize Carbocations
• Equilibrium and Energy Relationships
• What's a Transition State?
• Hammond's Postulate
• Grossman's Rule
• Draw The Ugly Version First
• Learning Organic Chemistry Reactions: A Checklist (PDF)
• Introduction to Addition Reactions
• Introduction to Elimination Reactions
• Introduction to Free Radical Substitution Reactions
• Introduction to Oxidative Cleavage Reactions

06 Free Radical Reactions

• Bond Dissociation Energies = Homolytic Cleavage
• Free Radical Reactions
• 3 Factors That Stabilize Free Radicals
• What Factors Destabilize Free Radicals?
• Bond Strengths And Radical Stability
• Free Radical Initiation: Why Is "Light" Or "Heat" Required?
• Initiation, Propagation, Termination
• Monochlorination Products Of Propane, Pentane, And Other Alkanes
• Selectivity In Free Radical Reactions
• Selectivity in Free Radical Reactions: Bromination vs. Chlorination
• Halogenation At Tiffany's
• Allylic Bromination
• Bonus Topic: Allylic Rearrangements
• In Summary: Free Radicals
• Synthesis (2) - Reactions of Alkanes
• Free Radicals Practice Quizzes

07 Stereochemistry and Chirality

• Types of Isomers: Constitutional Isomers, Stereoisomers, Enantiomers, and Diastereomers
• How To Draw The Enantiomer Of A Chiral Molecule
• How To Draw A Bond Rotation
• Introduction to Assigning (R) and (S): The Cahn-Ingold-Prelog Rules
• Assigning Cahn-Ingold-Prelog (CIP) Priorities (2) - The Method of Dots
• Enantiomers vs Diastereomers vs The Same? Two Methods For Solving Problems
• Assigning R/S To Newman Projections (And Converting Newman To Line Diagrams)
• How To Determine R and S Configurations On A Fischer Projection
• The Meso Trap
• Optical Rotation, Optical Activity, and Specific Rotation
• Optical Purity and Enantiomeric Excess
• What's a Racemic Mixture?
• Chiral Allenes And Chiral Axes
• On Cats, Part 4: Enantiocats
• On Cats, Part 6: Stereocenters
• Stereochemistry Practice Problems and Quizzes

08 Substitution Reactions

• Introduction to Nucleophilic Substitution Reactions
• Walkthrough of Substitution Reactions (1) - Introduction
• Two Types of Nucleophilic Substitution Reactions
• The SN2 Mechanism
• Why the SN2 Reaction Is Powerful
• The SN1 Mechanism
• The Conjugate Acid Is A Better Leaving Group
• Comparing the SN1 and SN2 Reactions
• Polar Protic? Polar Aprotic? Nonpolar? All About Solvents
• Steric Hindrance is Like a Fat Goalie
• Common Blind Spot: Intramolecular Reactions
• The Conjugate Base is Always a Stronger Nucleophile
• Substitution Practice - SN1
• Substitution Practice - SN2

09 Elimination Reactions

• Elimination Reactions (1): Introduction And The Key Pattern
• Elimination Reactions (2): The Zaitsev Rule
• Elimination Reactions Are Favored By Heat
• Two Elimination Reaction Patterns
• The E1 Reaction
• The E2 Mechanism
• E1 vs E2: Comparing the E1 and E2 Reactions
• Antiperiplanar Relationships: The E2 Reaction and Cyclohexane Rings
• Bulky Bases in Elimination Reactions
• Comparing the E1 vs SN1 Reactions
• Elimination (E1) Reactions With Rearrangements
• E1cB - Elimination (Unimolecular) Conjugate Base
• Elimination (E1) Practice Problems And Solutions
• Elimination (E2) Practice Problems and Solutions

10 Rearrangements

• Introduction to Rearrangement Reactions
• Rearrangement Reactions (1) - Hydride Shifts
• Carbocation Rearrangement Reactions (2) - Alkyl Shifts
• Pinacol Rearrangement
• The SN1, E1, and Alkene Addition Reactions All Pass Through A Carbocation Intermediate

11 SN1/SN2/E1/E2 Decision

• Identifying Where Substitution and Elimination Reactions Happen
• Deciding SN1/SN2/E1/E2 (1) - The Substrate
• Deciding SN1/SN2/E1/E2 (2) - The Nucleophile/Base
• SN1 vs E1 and SN2 vs E2 : The Temperature
• Deciding SN1/SN2/E1/E2 - The Solvent
• Wrapup: The Quick N' Dirty Guide To SN1/SN2/E1/E2
• Alkyl Halide Reaction Map And Summary
• SN1 SN2 E1 E2 Practice Problems

12 Alkene Reactions

• E and Z Notation For Alkenes (+ Cis/Trans)
• Alkene Stability
• Addition Reactions: Elimination's Opposite
• Selective vs. Specific
• Regioselectivity In Alkene Addition Reactions
• Stereoselectivity In Alkene Addition Reactions: Syn vs Anti Addition
• Hydrohalogenation of Alkenes and Markovnikov's Rule
• Hydration of Alkenes With Aqueous Acid
• Rearrangements in Alkene Addition Reactions
• Addition Pattern #1: The "Carbocation Pathway"
• Halogenation of Alkenes and Halohydrin Formation
• Oxymercuration Demercuration of Alkenes
• Alkene Addition Pattern #2: The "Three-Membered Ring" Pathway
• Hydroboration Oxidation of Alkenes
• m-CPBA (meta-chloroperoxybenzoic acid)
• OsO4 (Osmium Tetroxide) for Dihydroxylation of Alkenes
• Palladium on Carbon (Pd/C) for Catalytic Hydrogenation
• Cyclopropanation of Alkenes
• Alkene Addition Pattern #3: The "Concerted" Pathway
• A Fourth Alkene Addition Pattern - Free Radical Addition
• Alkene Reactions: Ozonolysis
• Summary: Three Key Families Of Alkene Reaction Mechanisms
• Synthesis (4) - Alkene Reaction Map, Including Alkyl Halide Reactions
• Alkene Reactions Practice Problems

13 Alkyne Reactions

• Acetylides from Alkynes, And Substitution Reactions of Acetylides
• Partial Reduction of Alkynes With Lindlar's Catalyst or Na/NH3 To Obtain Cis or Trans Alkenes
• Hydroboration and Oxymercuration of Alkynes
• Alkyne Reaction Patterns - Hydrohalogenation - Carbocation Pathway
• Alkyne Halogenation: Bromination, Chlorination, and Iodination of Alkynes
• Alkyne Reactions - The "Concerted" Pathway
• Alkenes To Alkynes Via Halogenation And Elimination Reactions
• Alkynes Are A Blank Canvas
• Synthesis (5) - Reactions of Alkynes
• Alkyne Reactions Practice Problems With Answers

14 Alcohols, Epoxides and Ethers

• Alcohols - Nomenclature and Properties
• Alcohols Can Act As Acids Or Bases (And Why It Matters)
• Alcohols - Acidity and Basicity
• The Williamson Ether Synthesis
• Ethers From Alkenes, Tertiary Alkyl Halides and Alkoxymercuration
• Alcohols To Ethers via Acid Catalysis
• Cleavage Of Ethers With Acid
• Epoxides - The Outlier Of The Ether Family
• Opening of Epoxides With Acid
• Epoxide Ring Opening With Base
• Making Alkyl Halides From Alcohols
• Tosylates And Mesylates
• PBr3 and SOCl2
• Elimination Reactions of Alcohols
• Elimination of Alcohols To Alkenes With POCl3
• Alcohol Oxidation: "Strong" and "Weak" Oxidants
• Demystifying The Mechanisms of Alcohol Oxidations
• Protecting Groups For Alcohols
• Thiols And Thioethers
• Calculating the oxidation state of a carbon
• Oxidation and Reduction in Organic Chemistry
• Oxidation Ladders
• SOCl2 Mechanism For Alcohols To Alkyl Halides: SN2 versus SNi
• Alcohol Reactions Roadmap (PDF)
• Alcohol Reaction Practice Problems
• Epoxide Reaction Quizzes
• Oxidation and Reduction Practice Quizzes

15 Organometallics

• What's An Organometallic?
• Formation of Grignard and Organolithium Reagents
• Organometallics Are Strong Bases
• Reactions of Grignard Reagents
• Protecting Groups In Grignard Reactions
• Synthesis Problems Involving Grignard Reagents
• Grignard Reactions And Synthesis (2)
• Organocuprates (Gilman Reagents): How They're Made
• Gilman Reagents (Organocuprates): What They're Used For
• The Heck, Suzuki, and Olefin Metathesis Reactions (And Why They Don't Belong In Most Introductory Organic Chemistry Courses)
• Reaction Map: Reactions of Organometallics
• Grignard Practice Problems

16 Spectroscopy

• Conjugation And Color (+ How Bleach Works)
• Bond Vibrations, Infrared Spectroscopy, and the "Ball and Spring" Model
• 1H NMR: How Many Signals?
• Homotopic, Enantiotopic, Diastereotopic
• Diastereotopic Protons in 1H NMR Spectroscopy: Examples
• C13 NMR - How Many Signals
• Liquid Gold: Pheromones In Doe Urine
• Natural Product Isolation (1) - Extraction
• Natural Product Isolation (2) - Purification Techniques, An Overview
• Structure Determination Case Study: Deer Tarsal Gland Pheromone

17 Dienes and MO Theory

• What To Expect In Organic Chemistry 2
• Are these molecules conjugated?
• Conjugation And Resonance In Organic Chemistry
• Bonding And Antibonding Pi Orbitals
• Molecular Orbitals of The Allyl Cation, Allyl Radical, and Allyl Anion
• Pi Molecular Orbitals of Butadiene
• Reactions of Dienes: 1,2 and 1,4 Addition
• Thermodynamic and Kinetic Products
• More On 1,2 and 1,4 Additions To Dienes
• s-cis and s-trans
• The Diels-Alder Reaction
• Cyclic Dienes and Dienophiles in the Diels-Alder Reaction
• Stereochemistry of the Diels-Alder Reaction
• Exo vs Endo Products In The Diels Alder: How To Tell Them Apart
• HOMO and LUMO In the Diels Alder Reaction
• Why Are Endo vs Exo Products Favored in the Diels-Alder Reaction?
• Diels-Alder Reaction: Kinetic and Thermodynamic Control
• The Retro Diels-Alder Reaction
• The Intramolecular Diels Alder Reaction
• Regiochemistry In The Diels-Alder Reaction
• The Cope and Claisen Rearrangements
• Electrocyclic Reactions
• Electrocyclic Ring Opening And Closure (2) - Six (or Eight) Pi Electrons
• Diels Alder Practice Problems
• Molecular Orbital Theory Practice

18 Aromaticity

• Introduction To Aromaticity
• Rules For Aromaticity
• Huckel's Rule: What Does 4n+2 Mean?
• Aromatic, Non-Aromatic, or Antiaromatic? Some Practice Problems
• Antiaromatic Compounds and Antiaromaticity
• The Pi Molecular Orbitals of Benzene
• The Pi Molecular Orbitals of Cyclobutadiene
• Frost Circles
• Aromaticity Practice Quizzes

19 Reactions of Aromatic Molecules

• Electrophilic Aromatic Substitution: Introduction
• Activating and Deactivating Groups In Electrophilic Aromatic Substitution
• Electrophilic Aromatic Substitution - The Mechanism
• Ortho-, Para- and Meta- Directors in Electrophilic Aromatic Substitution
• Understanding Ortho, Para, and Meta Directors
• Why are halogens ortho- para- directors?
• Disubstituted Benzenes: The Strongest Electron-Donor "Wins"
• Electrophilic Aromatic Substitutions (1) - Halogenation of Benzene
• Electrophilic Aromatic Substitutions (2) - Nitration and Sulfonation
• EAS Reactions (3) - Friedel-Crafts Acylation and Friedel-Crafts Alkylation
• Intramolecular Friedel-Crafts Reactions
• Nucleophilic Aromatic Substitution (NAS)
• Nucleophilic Aromatic Substitution (2) - The Benzyne Mechanism
• Reactions on the "Benzylic" Carbon: Bromination And Oxidation
• The Wolff-Kishner, Clemmensen, And Other Carbonyl Reductions
• More Reactions on the Aromatic Sidechain: Reduction of Nitro Groups and the Baeyer Villiger
• Aromatic Synthesis (1) - "Order Of Operations"
• Synthesis of Benzene Derivatives (2) - Polarity Reversal
• Aromatic Synthesis (3) - Sulfonyl Blocking Groups
• Birch Reduction
• Synthesis (7): Reaction Map of Benzene and Related Aromatic Compounds
• Aromatic Reactions and Synthesis Practice
• Electrophilic Aromatic Substitution Practice Problems

20 Aldehydes and Ketones

• What's The Alpha Carbon In Carbonyl Compounds?
• Nucleophilic Addition To Carbonyls
• Aldehydes and Ketones: 14 Reactions With The Same Mechanism
• Sodium Borohydride (NaBH4) Reduction of Aldehydes and Ketones
• Grignard Reagents For Addition To Aldehydes and Ketones
• Wittig Reaction
• Hydrates, Hemiacetals, and Acetals
• Imines - Properties, Formation, Reactions, and Mechanisms
• All About Enamines
• Breaking Down Carbonyl Reaction Mechanisms: Reactions of Anionic Nucleophiles (Part 2)
• Aldehydes Ketones Reaction Practice

21 Carboxylic Acid Derivatives

• Nucleophilic Acyl Substitution (With Negatively Charged Nucleophiles)
• Addition-Elimination Mechanisms With Neutral Nucleophiles (Including Acid Catalysis)
• Basic Hydrolysis of Esters - Saponification
• Transesterification
• Proton Transfer
• Fischer Esterification - Carboxylic Acid to Ester Under Acidic Conditions
• Lithium Aluminum Hydride (LiAlH4) For Reduction of Carboxylic Acid Derivatives
• LiAlH[Ot-Bu]3 For The Reduction of Acid Halides To Aldehydes
• Di-isobutyl Aluminum Hydride (DIBAL) For The Partial Reduction of Esters and Nitriles
• Amide Hydrolysis
• Thionyl Chloride (SOCl2)
• Diazomethane (CH2N2)
• Carbonyl Chemistry: Learn Six Mechanisms For the Price Of One
• Making Music With Mechanisms (PADPED)
• Carboxylic Acid Derivatives Practice Questions

22 Enols and Enolates

• Keto-Enol Tautomerism
• Enolates - Formation, Stability, and Simple Reactions
• Kinetic Versus Thermodynamic Enolates
• Aldol Addition and Condensation Reactions
• Reactions of Enols - Acid-Catalyzed Aldol, Halogenation, and Mannich Reactions
• Claisen Condensation and Dieckmann Condensation
• Decarboxylation
• The Malonic Ester and Acetoacetic Ester Synthesis
• The Michael Addition Reaction and Conjugate Addition
• The Robinson Annulation
• Haloform Reaction
• The Hell–Volhard–Zelinsky Reaction
• Enols and Enolates Practice Quizzes
• The Amide Functional Group: Properties, Synthesis, and Nomenclature
• Basicity of Amines And pKaH
• 5 Key Basicity Trends of Amines
• The Mesomeric Effect And Aromatic Amines
• Nucleophilicity of Amines
• Alkylation of Amines (Sucks!)
• Reductive Amination
• The Gabriel Synthesis
• Some Reactions of Azides
• The Hofmann Elimination
• The Hofmann and Curtius Rearrangements
• The Cope Elimination
• Protecting Groups for Amines - Carbamates
• The Strecker Synthesis of Amino Acids
• Introduction to Peptide Synthesis
• Reactions of Diazonium Salts: Sandmeyer and Related Reactions
• Amine Practice Questions

24 Carbohydrates

• D and L Notation For Sugars
• Pyranoses and Furanoses: Ring-Chain Tautomerism In Sugars
• What is Mutarotation?
• Reducing Sugars
• The Big Damn Post Of Carbohydrate-Related Chemistry Definitions
• The Haworth Projection
• Converting a Fischer Projection To A Haworth (And Vice Versa)
• Reactions of Sugars: Glycosylation and Protection
• The Ruff Degradation and Kiliani-Fischer Synthesis
• Isoelectric Points of Amino Acids (and How To Calculate Them)
• Carbohydrates Practice
• Amino Acid Quizzes

25 Fun and Miscellaneous

• Organic Chemistry GIFS - Resonance Forms
• Organic Chemistry and the New MCAT
• A Gallery of Some Interesting Molecules From Nature
• The Organic Chemistry Behind "The Pill"
• Maybe they should call them, "Formal Wins" ?
• Intramolecular Reactions of Alcohols and Ethers
• Planning Organic Synthesis With "Reaction Maps"
• Organic Chemistry Is Shit
• The 8 Types of Arrows In Organic Chemistry, Explained
• The Most Annoying Exceptions in Org 1 (Part 1)
• The Most Annoying Exceptions in Org 1 (Part 2)
• Reproducibility In Organic Chemistry
• Screw Organic Chemistry, I'm Just Going To Write About Cats
• On Cats, Part 1: Conformations and Configurations
• On Cats, Part 2: Cat Line Diagrams
• The Marriage May Be Bad, But the Divorce Still Costs Money
• Why Do Organic Chemists Use Kilocalories?
• What Holds The Nucleus Together?
• 9 Nomenclature Conventions To Know
• How Reactions Are Like Music

Comment section

82 thoughts on “ infrared spectroscopy: a quick primer on interpreting spectra ”.

This has really been helpful for my studies in chemistry

I am glad you find it helpful Cirona!

This is very helpful

Glad you find it helpful Anand

I love this analysis very much impressive thanks 👍

Glad you find it helpful!

A lifesaver if there was ever one. Infrared Spectroscopy was so confusing for me in undergrad,and post grad had me even more muddled.One look at this article on the morning of the test was enough to make me take my test confidently and do it well! The way you simplified it while highlighting important points is crazy. I was trying to remember all the values given from the typical IR frequency table which wasn’t working at all and was leaving me anxious. Tongues and Swords made it so simple and memorable. Thanks for all that you do and more! This is Monica Rao all the way from India!

Glad to hear you found it useful! I had a similar experience in undergraduate and glad that this simplified things for you!

i love you, you just saved my life

explanation is very easy to understand. thank you

Best Explanation so far. Really helpful

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• Pingback: IR Spectroscopy and FTIR Spectroscopy: How an FTIR Spectrometer Works and FTIR Analysis - Technology Networks - Mubashir Khan.
• Pingback: IR spectroscopy - Easy To Calculate

This is so helpful thank you

Best Review on IR

Thank you Sagar.

Hi James, Thank you for your very clear tutorials on interpreting IR spectra. They have been really helpful to me.

I have a few questions regarding a compound with an unknown structure, which I am trying to decipher using FTIR. Would you be happy to have a look at this for me and confirm whether or not I have done it right, based on the information on your tutorials?

Thanks I am newbie and this finally made a pathway in my grey cells :)

Thanks a lot. This has really helped me I understood everything in it

Thank you so much for this great work. I have one problem: I used to work with polymers (in my particular case I am working with PVC films). Firstly, I do an FTIR spectrum of the “as received” PVC film. Next, I carry out a thermal treatment of the PVC film (below its Tg) and repeat the FTIR. The peaks have not change, however the intensity of them is different. I have tried to figure out an explanation for this phenomenon (searching in bibliography), but I didn´t found an answer. Do you have any idea of why this happen?

thank you very much.

You are absolutely amazing. I feel so happy and satisfied reading this. Your style of presenting the context is so good. Thank You for your hard work for us.

Thank you so much. Two month i have struggled about this topic. Full of detail in simple words with various example. Thank you again

I am really grateful this lesson is really awesome.

Thank you so much!! Your post really helped understangding IR :)

Thank you so much for this great information sir

Thank you!! This is so easy explained and helpful. I have one question: How much can I trust in my software suggestions? the software of my FTIR instrumen has some libraries included.

I’m not sure. There can be considerable variability between samples of the same molecule, depending on how the sample is prepared (thickness of film) and the amount of water present (which affects hydrogen bonding). The libraries are a good starting point but not a magic bullet, good when part of a more holistic approach to combine with other information (e.g. HRMS data)

Thanks Paul – I was unaware of the overtones in that region. Very helpful, thank you!

One thing you didn’t mention is the carbonyl overtone peaks, which result when the molecule absorbs two photons of IR light. These show up as weak peaks at 2 x the carbonyl frequency, so are in that 3300 – 3500 range.

It’s important know about this because beginning students very often assign those peaks to NH stretches. And it’s not crazy, because NH stretches in monosubstituted amides can be relatively weak, so it can be difficult to distinguish them.

This isn’t perfect, but, from an instructor perspective, my advice is to avoid the urge to assign them to amine or amide. If you see a carbonyl, expect to see that overtone and don’t call it an NH stretch. Now, this means you might miss an amide, but that alone is not sufficient to conclude it is amide. You would need to verify it by other means. As noted, amide C=O stretches tend to be lower energy than other functional groups, but even then I’d be careful about putting too fine a point on it (absorptions usually come in ranges, not in specific spots – the C=O is 1680: does that mean it’s amide? Could be, but it could also be a ketone at the edge of its range; it’s consistent with both). Now, if you have a mass spectrum that indicates the presence of a N (by having an odd molecular mass), so you know N is present, then sure, it could be NH stretching. But absent other information that indicates an amide, my advice is don’t go that direction.

This is the best review I have ever seen-splendid!

This is an excellent resource on IR for a newbie…love to give this to my students for reading. Looking for posts on mass spectrometry..

Thanks Anju – appreciate it. This is what I wish someone told me when I was learning how to interpret IR spectra.

Very clear, lots of examples and well thought out instructions. I feel so much more confident! Thank you soo much!!!

Great! So glad you feel more confident!

Symply excellent. Please, we need MOC Text book.

Not happening! But thank you

Seriously it is the best of all explanation I have seen ,it really helpful 💖

Very helpful. I can understand the materials much better

Thanks for the wonderful lecture, my question is how can one identify aromatic or the benzene ring absorption. Please I also need your email address

Look for the C-H bond stretch below 3000 cm-1. It is not specific for the aromatic ring but at least points to an sp2 hybridized carbon bonded to H.

saved my life honestly.

Honestly? Awesome!

Best explanation ever ! The only one I understood .. Thank you a lot!

Thanks Olivia! Glad you found it helpful!

I was completely lost at lecture on IR but after reading this, i realized its simple things made difficult. You saved me a failure.

So glad to hear it Josan.

Thanks! This article saved me. Recommended this to all my friends.

Thanks for letting me know Harshit!

Wow! Thanks – you will never know how much time this saved me.

So glad to hear it Freeman.

Excellent explanation! Thank you for all the hard work.

Thanks Zeke!

Thank you! you just saved my life

If it made IR less painful, that’s awesome Alejandra!

I see nothing about <500 cm-1 which is what I need to know

Really? I wish I had a better answer for you. The region below 500 cm-1 is an “enduring mystery” for many of us. https://amphoteros.com/2019/01/18/an-enduring-mystery/

Loved this!!!!!

THANK YOU!!!! THIS SAVED MY LIFE!!!!!!

I know your faculty plans did not work out, but you are so better than many professors! Thank you! Never stop chasing your dreams!

Beautifully explained Sir !!

Best explanation of IR spectra I’ve came across. Waiting for your next post :)

I completely agree with the above posts. You should Youtube as well my friend. Great job!

I do have a Youtube channel but it has been quite neglected!

I COMPLETELY AGREE 100% with the previous praises and comments – you have been a SAVING grace in my organic chemistry understanding and I appreciate your approach in simplifying the most complex things. I have honestly spent 4+hrs in attempting 2 problems in figuring out the structures and feel so much better moving forward. THANK YOU! Keep up the phenomenal job!

Thank you Maribel!

Thanks for such a great focused article. It’s really very helpful.

Tried to make it useful. If it succeeded, great!

This is very clear and understandable even to a layman. Thanks a lot

You’re welcome!

why alkenes group (3000 -3100) & alkyl halides (500 -539) are added to NORYL (PPE + PS) plastic? which properties are affected?

How do you know the peak in the 3000-3100 isn’t from the styrene?

Thank you so much for this guide! Very thorough approach and great explanation.

Meg – so glad you’ve found it helpful. Put a lot of work into it!

Great work! best I could find in all these years in fact.

Never using another website or youtube vid (unless its yours) for help again. You’re amazing

Beautifully explained!

This is the best review for IR Spectroscopy out there!

THIS IS SO HELPFUL!! so many different examples were used and I understand everything now! Will there be a quick tutorial for carbon and proton NMR as well?

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• Analytical Chemistry
• Infrared Spectroscopy

IR Spectroscopy

IR spectroscopy (which is short for infrared spectroscopy) deals with the infrared region of the electromagnetic spectrum, i.e. light having a longer wavelength and a lower frequency than visible light. Infrared Spectroscopy generally refers to the analysis of the interaction of a molecule with infrared light.

The IR spectroscopy concept can generally be analyzed in three ways: by measuring reflection, emission, and absorption. The major use of infrared spectroscopy is to determine the functional groups of molecules, relevant to both organic and inorganic chemistry.

Table of Contents

What is ir spectroscopy, samples in infrared spectroscopy, ir spectroscopy instrumentation.

• Frequently Asked Questions – FAQs

An IR spectrum is essentially a graph plotted with the infrared light absorbed on the Y-axis against. frequency or wavelength on the X-axis. An illustration highlighting the different regions that light can be classified into is given below.

IR Spectroscopy detects frequencies of infrared light that are absorbed by a molecule. Molecules tend to absorb these specific frequencies of light since they correspond to the frequency of the vibration of bonds in the molecule.

The energy required to excite the bonds belonging to a molecule, and to make them vibrate with more amplitude, occurs in the Infrared region. A bond will only interact with the electromagnetic infrared radiation, however, if it is polar.

The presence of separate areas of partial positive and negative charge in a molecule allows the electric field component of the electromagnetic wave to excite the vibrational energy of the molecule.

The change in the vibrational energy leads to another corresponding change in the dipole moment of the given molecule. The intensity of the absorption depends on the polarity of the bond. Symmetrical non-polar bonds in N≡N and O=O do not absorb radiation, as they cannot interact with an electric field.

Check  ⇒ NMR Spectroscopy

Regions of the Infrared spectrum

Most of the bands that indicate what functional group is present are found in the region from 4000 cm -1 to 1300 cm -1 . Their bands can be identified and used to determine the functional group of an unknown compound.

Bands that are unique to each molecule, similar to a fingerprint, are found in the fingerprint region, from 1300 cm -1 to 400 cm -1 . These bands are only used to compare the spectra of one compound to another.

The samples used in IR spectroscopy can be either in the solid, liquid, or gaseous state.

• Solid samples can be prepared by crushing the sample with a mulling agent which has an oily texture. A thin layer of this mull can now be applied on a salt plate to be measured.
• Liquid samples are generally kept between two salt plates and measured since the plates are transparent to IR light. Salt plates can be made up of sodium chloride , calcium fluoride, or even potassium bromide.
• Since the concentration of gaseous samples can be in parts per million, the sample cell must have a relatively long pathlength, i.e. light must travel for a relatively long distance in the sample cell.

Thus, samples of multiple physical states can be used in Infrared Spectroscopy.

Principle Of Infrared Spectroscopy

The IR spectroscopy theory utilizes the concept that molecules tend to absorb specific frequencies of light that are characteristic of the corresponding structure of the molecules. The energies are reliant on the shape of the molecular surfaces, the associated vibronic coupling, and the mass corresponding to the atoms.

For instance, the molecule can absorb the energy contained in the incident light and the result is a faster rotation or a more pronounced vibration.

The instrumentation of infrared spectroscopy is illustrated below. First, a beam of IR light from the source is split into two and passed through the reference and the sample respectively.

Now, both of these beams are reflected to pass through a splitter and then through a detector. Finally, the required reading is printed out after the processor deciphers the data passed through the detector.

Graph of the IR spectrum

Given below is a sample of typical Infrared Absorption Frequencies.

Frequently Asked Questions – FAQs

Can we use water in ir spectroscopy.

Because water has two high infrared absorption peaks, it cannot be employed as a solvent for IR spectroscopy. Also, water is a polar solvent that dissolves alkali halide disks, which are extensively employed in IR.

How sensitive is IR spectroscopy?

Infrared spectroscopy can now identify samples as small as 1 to 10 grams. Almost all organic and certain inorganic molecules can be analysed using infrared spectroscopy. It can be used in both qualitative and quantitative analysis and has a wide range of applications.

What is necessary condition for IR spectroscopy?

The change in the electric dipole moment of the functional group present in a molecule or a sample during the vibration based on the selection rule for IR transitions is a necessary requirement for a molecule or sample to show infrared spectrum.

Which lamp is used in IR spectroscopy?

For infrared spectroscopy, a Globar is employed as a thermal light source. It’s a silicon carbide rod with a diameter of 5 to 10 mm and a length of 20 to 50 mm that’s been electrically heated to 1,000 to 1,650°C (1,830 to 3,000 degrees Fahrenheit).

Which solvent are used in IR spectroscopy?

Carbon Tetrachloride (CCl4) and Carbon Disulfide (CD) are the most prevalent solvents (CS2). Solvents for polar materials include chloroform, methylene chloride, acetonitrile, and acetone. Solids reduced to fine particles can be analysed as a thin paste or mull.

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can you tell me how IR spectroscopy is useful for pollution control

Infrared spectroscopy is one of many common analytical methods used for the detection and measurement of atmospheric pollution. Much of the fundamental research on atmospheric pollution used spectroscopic techniques, as this technology helps us to comprehend various threats to both public health and the environment.

how can you distinguish alkana and alkene by IR spectroscopy?

Stretching vibrations of the –C=C–H bond are of higher frequency (higher wavenumber) than those of the –C–C–H bond in alkanes. This is a very useful tool for interpreting IR spectra. Only alkenes and aromatics show a C-H stretch slightly higher than 3000 cm-1. Compounds that do not have a C=C bond show C-H stretches only below 3000 cm-1.

Why we are using ccl4 as a solvent

Carbon tetra chloride is used as a solvent in IR spectroscopy because it absorbs at a shallow frequency below 1600 cm−1. Moreover, carbon tetrachloride is a non-polar solvent, i.e. it does interfere with the absorption spectra.

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Infrared: Interpretation

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• Page ID 1846

Infrared spectroscopy is the study of the interaction of infrared light with matter. The fundamental measurement obtained in infrared spectroscopy is an infrared spectrum, which is a plot of measured infrared intensity versus wavelength (or frequency) of light.

Introduction

In infrared spectroscopy, units called wavenumbers are normally used to denote different types of light. The frequency, wavelength, and wavenumber are related to each other via the following equation(1):

These equations show that light waves may be described by their frequency, wavelength or wavenumber. Here, we typically refer to light waves by their wavenumber, however it will be more convenient to refer to a light wave's frequency or wavelength. The wavenumber of several different types of light are shown in table 1.

Table 1. The Electromagnetic spectrum showing the wavenumber of several different types of light.

When a molecule absorbs infrared radiation, its chemical bonds vibrate. The bonds can stretch, contract, and bend. This is why infrared spectroscopy is a type of vibrational spectroscopy. Fortunately, the complex vibrational motion of a molecule can be broken down into a number of constituent vibrations called normal modes. For example, when a guitar string is plucked, the string vibrates at its normal mode frequency. Molecules, like guitar strings, vibrate at specfic frequencies so different molecules vibrate at different frequencies because their structures are different. This is why molecules can be distinguished using infrared spectroscopy. The first necessary condition for a molecule to absorb infrared light is that the molecule must have a vibration during which the change in dipole moment with respect to distance is non-zero. This condition can be summarized in equation(2) form as follows:

Vibrations that satisfy this equation are said to be infrared active. The H-Cl stretch of hydrogen chloride and the asymmetric stretch of CO 2 are examples of infrared active vibrations. Infrared active vibrations cause the bands seen in an infrared spectrum.

The second necessary condition for infrared absorbance is that the energy of the light impinging on a molecule must equal a vibrational energy level difference within the molecule. This condition can be summarized in equation(3) form as follows:

If the energy of a photon does not meet the criterion in this equation, it will be transmitted by the sample and if the photon energy satisfies this equation, that photon will be absorbed by the molecule.(See Infrared: Theory for more detail)

As any other analytical techniques, infrared spectroscopy works well on some samples, and poorly on others. It is important to know the strengths and weaknesses of infrared spectroscopy so it can be used in the proper way. Some advantages and disadvantages of infrared spectroscopy are listed in table 2.

Table 2. The Advantage and Disadvantage of Infrared Spectroscopy

Origin of Peak Positions, Intensities, and Widths

Peak positions.

The equation(4) gives the frequency of light that a molecule will absorb, and gives the frequency of vibration of the normal mode excited by that light.

Only two variables in equation(4) are a chemical bond's force constant and reduced mass. Here, the reduced mass refers to (M 1 M 2 )/(M 1 +M 2 ) where M 1 and M 2 are the masses of the two atoms, respectively. These two molecular properties determine the wavenumber at which a molecule will absorb infrared light. No two chemical substances in the universe have the same force constants and atomic masses, which is why the infrared spectrum of each chemical substance is unique. To understand the effect of atomic masses and force constant on the positions of infrared bands, table 3 and 4 are shown as an example, respectively.

The reduced masses of C- 1 H and C- 2 D are different, but their force constants are the same. By simply doubling the mass of the hydrogen atom, the carbon-hydrogen stretching vibration is reduced by over 800cm -1 .

Table 4. An Example of an electronic Effect

When a hydrogen is attached to a carbon with a C=O bond, the C-H stretch band position decrease to ~2750cm -1 . These two C-H bonds have the same reduced mass but different force constants. The oxygen in the second molecule pulls electron density away from the C-H bond so it makes weaken and reduce the C-H force constant. This cause the C-H stretching vibration to be reduced by ~250cm -1 .

The Origin of Peak Intensities

The different vibrations of the different functional groups in the molecule give rise to bands of differing intensity. This is because \( \frac{\partial \mu}{\partial x}\) is different for each of these vibrations. For example, the most intense band in the spectrum of octane shown in Figure 3 is at 2971, 2863 cm -1 and is due to stretching of the C-H bond. One of the weaker bands in the spectrum of octane is at 726cm -1 , and it is due to long-chain methyl rock of the carbon-carbon bonds in octane. The change in dipole moment with respect to distance for the C-H stretching is greater than that for the C-C rock vibration, which is why the C-H stretching band is the more intense than C-C rock vibration.

Another factor that determines the peak intensity in infrared spectra is the concentration of molecules in the sample. The equation(5) that relates concentration to absorbance is Beer's law,

The absorptivity is the proportionality constant between concentration and absorbance, and is dependent on ( ¶ µ/ ¶ x) 2 . The absorptivity is an absolute measure of infrared absorbance intensity for a specific molecule at a specific wavenumber. For pure sample, concentration is at its maximum, and the peak intensities are true representations of the values of ¶ µ/ ¶ x for different vibrations. However, in a mixture, two peaks may have different intensities because there are molecules present in different concentration.

The Orgins of Peak Widths

In general, the width of infrared bands for solid and liquid samples is determined by the number of chemical environments which is related to the strength of intermolecular interactions such as hydrogen bonding. Figure 1. shows hydrogen bond in water molecules and these water molecules are in different chemical environments. Because the number and strength of hydrogen bonds differs with chemical environment, the force constant varies and the wavenumber differs at which these molecules absorb infrared light.

Figure 1. Hydrogen Bonding in water molecules

In any sample where hydrogen bonding occurs, the number and strength of intermolecular interactions varies greatly within the sample, causing the bands in these samples to be particularly broad. This is illustrated in the spectra of ethanol(Fig7) and hexanoic acid(Fig11). When intermolecular interactions are weak, the number of chemical environments is small, and narrow infrared bands are observed.

The Origin of Group Frequencies

An important observation made by early researchers is that many functional group absorb infrared radiation at about the same wavenumber, regardless of the structure of the rest of the molecule. For example, C-H stretching vibrations usually appear between 3200 and 2800cm -1 and carbonyl(C=O) stretching vibrations usually appear between 1800 and 1600cm -1 . This makes these bands diagnostic markers for the presence of a functional group in a sample. These types of infrared bands are called group frequencies because they tell us about the presence or absence of specific functional groups in a sample.

Figure 2. Group frequency and fingerprint regions of the mid-infrared spectrum

The region of the infrared spectrum from 1200 to 700 cm -1 is called the fingerprint region. This region is notable for the large number of infrared bands that are found there. Many different vibrations, including C-O, C-C and C-N single bond stretches, C-H bending vibrations, and some bands due to benzene rings are found in this region. The fingerprint region is often the most complex and confusing region to interpret, and is usually the last section of a spectrum to be interpreted. However, the utility of the fingerprint region is that the many bands there provide a fingerprint for a molecule.

Spectral Interpretation by Application of Group Frequencies

Organic compounds.

One of the most common application of infrared spectroscopy is to the identification of organic compounds. The major classes of organic molecules are shown in this category and also linked on the bottom page for the number of collections of spectral information regarding organic molecules.

Hydrocarbons

Hydrocarbons compounds contain only C-H and C-C bonds, but there is plenty of information to be obtained from the infrared spectra arising from C-H stretching and C-H bending.

In alkanes, which have very few bands, each band in the spectrum can be assigned:

In alkenes compounds, each band in the spectrum can be assigned:

In alkynes, each band in the spectrum can be assigned:

In aromatic compounds, each band in the spectrum can be assigned:

Figure 6. shows the spectrum of toluene.

Figure 6. Infrared Spectrum of Toluene

Functional Groups Containing the C-O Bond

Alcohols have IR absorptions associated with both the O-H and the C-O stretching vibrations.

The carbonyl stretching vibration band C=O of saturated aliphatic ketones appears:

If a compound is suspected to be an aldehyde, a peak always appears around 2720 cm -1 which often appears as a shoulder-type peak just to the right of the alkyl C–H stretches.

The carbonyl stretch C=O of esters appears:

The carbonyl stretch C=O of a carboxylic acid appears as an intense band from 1760-1690 cm -1 . The exact position of this broad band depends on whether the carboxylic acid is saturated or unsaturated, dimerized, or has internal hydrogen bonding.

Organic Nitrogen Compounds

Organic Compounds Containing Halogens

Alkyl halides are compounds that have a C–X bond, where X is a halogen: bromine, chlorine, fluorene, or iodine.

For more Infrared spectra Spectral database of organic molecules is introduced to use free database. Also, the infrared spectroscopy correlation tableis linked on bottom of page to find other assigned IR peaks.

Inorganic Compounds

Generally, the infrared bands for inorganic materials are broader, fewer in number and appear at lower wavenumbers than those observed for organic materials. If an inorganic compound forms covalent bonds within an ion, it can produce a characteristic infrared spectrum.

Main infrared bands of some common inorganic ions:

Chracteristic infrared bands of diatomic inorganic molecules: M(metal), X(halogen)

Characteristic infrared bands(cm -1 ) of triatomic inorganic molecules:

1388, 1286 3311 2053 714, 784 327

667 712 486, 471 380 249

2349 2049 748 2219 842

Bent Molecules H 2 O O 3 SnCl 2

3675 1135 354

1595 716 120

3756 1089 334

Identification

There are a few general rules that can be used when using a mid-infrared spectrum for the determination of a molecular structure. The following is a suggested strategy for spectrum interpretation: 2

• Infrared Spectral Interpretation by Brian Smith, CRC Press, 1999
• Infrared Spectroscopy: Fundamentals and Applications by Barbara Atuart, John Wiley&Sons, Ltd., 2004
• Interpretation of Infrared Spectra, A Practical Approach by John Coates in Encyclopedia of Analytical Chemistry pp. 10815-10837, John Wiley&Sons Ltd, Chichester, 2000

Outside Links

• Spectral Database for Organic Compounds SDBS: http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology, date of access)
• Infrared Spectroscopy Correlation Table: en.Wikipedia.org/wiki/Infrared_spectroscopy_correlation_table
• FDM Reference Spectra Databases: http://www.fdmspectra.com/index.html
• www.cem.msu.edu/~reusch/Virtu...d/infrared.htm
• Fermi resonance : en.Wikipedia.org/wiki/Fermi_resonance

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Home > Books > Infrared Spectroscopy - Perspectives and Applications

IR Spectroscopy in Qualitative and Quantitative Analysis

Submitted: 03 June 2022 Reviewed: 18 July 2022 Published: 07 September 2022

DOI: 10.5772/intechopen.106625

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Infrared Spectroscopy - Perspectives and Applications

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The infrared technique is one of the oldest techniques; it deals with the frequencies of bond vibration in a molecule. The main uses of this technique are to identify and determine components in various organic or inorganic compounds. In this technique, a part of the incident infrared radiation is absorbed by the molecules of the sample and the other is transmitted. The favorite method of infrared spectroscopy is FTIR (Fourier transform infrared). There have been many developments in using IR technique in qualitative and quantitative analyses, including the first and second derivatives of the infrared spectrum. IR rays do not damage the exposed skin like other rays such as ultraviolet light. It must be mentioned that the IR technique was used in hyphenated techniques (instead of the detector in chromatographic device), for example, after separation by gas chromatography detected by IR. Also, this chapter contains essential information about Raman spectroscopy. Infrared spectroscopy is a technique that has acceptable accuracy and sensitivity to be one of the most important analytical techniques used in the qualitative analysis, and also, it is used in the quantitative estimation of compounds through measuring the transmitted or absorption intensity of the active groups.

• first and second derivatives
• qualitative and quantitative estimations
• hyphenated techniques

Author Information

Nabeel othman *.

• Chemistry Department, College of Science, University of Mosul, Mosul, Iraq

*Address all correspondence to: [email protected]

1. Introduction

The introduction included the followings below:

1.1 Infrared spectroscopy

Spectroscopy is the branch of science contracts with learning about the interaction of the radiation of electromagnetic rays with substances.

Electromagnetic Radiation (EMR) is a type of energy that is around us and taking various forms, these types included radio waves, microwaves, infrared, visible light, ultraviolet X-rays, and gamma-rays. Sunlight is also considered a form of EMR, with Vis light only a minor share of the EM spectrum, which covers a wide range of wavelengths. Visible light has high energy compared with IR light [ 1 , 2 ].

Infrared Spectroscopy (IRS) deals with the frequencies of bond vibration in a molecule. The main use is to identify the functional groups in many samples. The most covalently bonded compounds, whether organic or inorganic compounds, absorb electromagnetic radiation in the region of infrared. This IR region lies between the visible light and the microwaves region. IR radiation mainly considers thermal energy, in covalent bonds it gives stronger vibrations to molecules. Near-IR can be used in direct determination (nondestructively) of protein present in feeds, and this type of IR region is increasingly used in analytical chemistry for quantitative analysis of various compounds [ 1 ].

Near-Infrared (NIR, 0.78~3.0 μm).

Mid-Infrared (MIR, 3.0~50.0 μm)

Far-Infrared (FIR, 50.0~1000.0 μm) [ 3 ].

In UV and Vis. of the spectrum, the unite of wavelength is nanometer (nm), while in the infrared region wavenumbers are used, and cm −1 is the unit [ 2 , 4 ]. The IR spectrum is drawn via a plot of absorbed or transmittance% (T%) against the wavenumber ( Figures 1 and 2 ).

The spectrum of an absorption mode.

The spectrum of T% mode.

1.2 Fourier transform infrared spectroscopy (FTIR)

The favorite method of IRS is FTIR (Fourier Transform infrared), in IRS the infrared radiation is passed through the investigated sample. A part of the incident infrared radiation is absorbed by the sample and the other is transmitted. The resulting spectrum represents the absorption molecules. FTIR spectrophotometers have many advantages when compared with the older techniques IR, the FTIR instruments are more accurate, and more sensitive, all frequencies of functional groups are estimated simultaneously compared with an individual estimation of functional groups in IR, and they are fast in performance as was in the case of older IR instruments.

1.3 Classification of IR bands

Figure 3 shows the main three types of IR bands they classified according to their relative intensities in the IR spectrum.

The types of IR bands according to their relative intensities.

An increase in the dipole moment according to the increase in the distance between atoms caused an increase in the intensity of the absorption peak [ 5 ].

1.4 IR peaks shapes

Two main types of IR band shapes are narrow (thin and pointed) and broad (wide and smoother). An example for broad is the O-H peak in alcohols and carboxylic acids, as shown below in Figure 4 [ 5 ].

The broad peak of the hydroxyl group.

1.5 Range of IR absorption

The typical IR absorption range to covalent bonds in molecules is from 600 to 4000 cm −1 . The graph shows the regions of the spectrum where the following types of bonds normally absorb. For example, the sharp band around 2200–2400 cm −1 would designate the possibility of the presence of a C-N or a C-C triple bond, and other ranges in IR-absorption for other types of bounds.

1.6 Overtones and combination bands

When a molecule absorbed electromagnetic radiation in the IR region, then the molecule is promoted from the ground state to the second, third, or even fourth vibrational excited state. These bands are known as Overtones. The intensity of these bands is very weak. It is helpful in the characterization of aromatic compounds.

When two fundamental vibrational frequencies (ν1 + ν2) in a molecule couple give rise to a new vibrational frequency within the molecule, it is known as a combination band.

1.7 Coupled vibrations

The coupled vibrations are observed in groups such as –CH 2 , NH 2 , etc. In these groups, the same atoms are attached to the central atom. When –CH 2 undergoes vibration, vibrational frequencies for the –CH 2 group are observed at 2950 cm −1 (asymmetric stretching) and 2860 cm −1 (symmetric stretching). A number of molecules contain the same functional group and show a similar peak above 1500 cm −1 , but they show a different peak in the fingerprint region. Therefore, we can say that each and every molecule has a unique peak or band, which is observed in the fingerprint region; it is just like the fingerprint of a human.

1.8 The functional groups and fingerprint regions

IR spectrum can be separated mainly into two regions. Most of the functional groups show absorption bands at the wavelength (4000–1200 cm −1 ) region, which is called the functional group region. Will the second region from 1200 to 400 cm −1 is called the fingerprint region. Fingerprint region is characteristic of the compound as a whole. An example is 2-pentanol and 3-pentanol, the two compounds with similar absorption in the functional group region. However, their fingerprint regions are different, because the two compounds differ, and to accurately identify the compound by comparing the fingerprint area with the fingerprint area of a standard or known sample of this compound [ 6 ].

1.9 Factors affecting the vibrational frequency

Conjugation: As the conjugation increases, stretching frequency decreases, because force content decreases due to conjugation.

Inductive effect and resonance effect: Oxygen is more electronegative than nitrogen; therefore, nitrogen easily donates electron or ion pair of nitrogen undergoes delocalization with a C=O bond. Due to delocalization double bond of a C=O change into a partial double bond, therefore force constant decreases, which decreases the C=O stretching frequency.

Hydrogen bonding: Intermolecular hydrogen bonding weakens the O-H bond, thereby shifting the band to a lower frequency. For example, in a clear solution O-H stretching vibration of phenol was observed in the range from 3400 to 3300 cm −1 . When the solution is diluted the O-H frequency shifted toward a higher frequency at 3600 cm −1 . Whereas in the case of methyl salicylate, intramolecular hydrogen bonding lowers the stretching frequency of O-H at 3200 cm −1 . Intramolecular hydrogen bonding does not change its frequency even in a very dilute solution because upon dilution structure of the compound does not change.

Ring strain: As the size of the ring decreases, the vibrational frequency of C=O increases. For example [ 5 ]:

An increase in wavenumber of the carbonyl group

1.10 General uses of IR

One of the most important uses of infrared rays is for military purposes, and one of these uses is in binoculars for night vision in case of difficulty in seeing and observing hostile targets.

Use in remote sensing, astronomy, and space in planetary detection, radio communications, spectroscopy, and weather forecasting.

Infrared radiation, which is the oldest technique used in wireless communication, and is used in remote control and TV or recorder, as it is used in calculators, one of disadvantages is the speed offered is slow compared with other wireless technologies.

Spectroscopy Infrared is a widely used technique to help identify carbon-containing organic compounds. Only the polar molecules are active because they have a permanent dipole moment.

Infrared therapy numerous studies have been described that IR can recover the healing of skin wounds, relieve pain, psychiatric disorders, and cardiac stem cells. There are two types of treatments:

Low-level light therapy (LLLT) using light of low power intensity and the effects are not a response to heat but to the light. The popular light sources used are low-power lasers.

Photobiomodulation (PBM) therapy uses non-ionizing types of light sources, including lasers, it is a non-thermal process.

It is now approved that the PBM therapy is an extra accurate and exact term for the therapeutic application of low-level light compared with “LLLT.” A basic principle called the biphasic dose-response included that the large doses of light were found to be less actual than smaller doses. The human skin is reliably exposed to environmental IR radiation, which indirectly or directly stimulates the manufacture of free radicals or reactive oxygen species( ROS). 8~12 μm IR radiation is almost used on full-thickness skin wound therapeutic in rats.

IR light crosses the outer layers of the skin and reaches the tissues of the body. The good thing about using infrared light in therapy is that IR rays do not damage the exposed skin like other rays such as ultraviolet light. An advantage of exposure to IR ray that it improves the circulation of blood and promotes cell regeneration [ 8 , 9 , 10 , 11 ].

1.11 Raman spectroscopy

Raman scattering firstly was observed by Raman and Krishnan (Indian physicists) in 1928. It is an analytical technique where the scattered light is used to measure the vibrational energy styles of molecules. Raman spectroscopy can offer chemical structural information, as well as identify the substances to be studied through their characteristic Raman “fingerprint.” Raman spectroscopy extracts the information over the detection of Raman scattering from the investigated sample. After the light is scattered via molecule, the oscillating electromagnetic field of the photon persuades a polarization of the molecular electrons cloud. The photon is transported to the molecule, due to the formation of a very short-lived complex (photon-molecule), and it is called commonly the virtual state. It is not stable and the photon can be re-emitted immediately as scattered light. Approximately 1/10 million photons Raman scattering occurs. The transfer of energy between the scattered photon and molecule and if the molecules gain energy from the photon according to the scattering (an excitation to a higher vibration level) and after that, the scattered photon loses energy, and this phenomena is called Stokes Raman, included an increase in wavelength. If the molecule loses energy by transferring to a lower vibrational level the scattered photon gains energy, inversely, the wavelength decreases, which is called Anti-Stokes Raman. Finally, if most of the molecules are in the ground vibrational level (Boltzmann distribution) and as a result, the Stokes Raman scatter is a continuously more probable process and intense than the anti-Stokes; for this reason, it is approximately always the Stokes Raman scatter used in Raman spectroscopy.

The main differences between IR and Raman scattering are listed in Table 1 .

The main differences between IR and Raman spectroscopy.

As a common rule included that everything that does not seem in the IRS is taken in Raman (bond of molecule either be with Raman active or be IR active but it not with be both). H 2 or CCL 4 doesn’t have spectrum in IR; but they give spectra in Raman. Also nitrogen-nitrogen, carbon-carbon, and sulfur-sulfur bonds have a change in polarizability, the incident photons interact with these models, these are examples of bonds that give rise to Raman active spectrum bands, but it is difficult to get spectrum in FTIR [ 12 , 13 ].

Raman spectroscopy has several applications, such as the identification of materials and identification of different minerals ranging from iron oxy (hydroxides) to rare minerals. Study of the crystallinity , the composition, and uniformity, and also measurement of local temperature and stress. Raman spectroscopy is nondestructive, and the technique has a good resolution [ 14 ].

Recently, Raman spectroscopy has been used in blood identification and distinguishing between human and nonhuman blood using a portable Raman spectrometer, which can be used at a crime division, and the bloodstain of human could be distinguished from the non-human ones via using a principal component analysis, and also this analysis is useful for forensic [ 15 ].

2. Application

2.1 qualitative analysis.

FTIR spectroscopy is the most reliable tool for identifying bone types and can also be widely used in forensic medicine. Identification of human and non-human skeletal remains unknown to investigators and is of great interest in forensic and anthropological procedures. Especially when the traditional morphological methods for diagnosing and differentiating between these types of bones took a long time. Therefore, the use of infrared spectroscopy and chemical measurement methods to determine the spectral differences between these two types of bones, human, and non-human bones (such as pigs, goats, and cows). The results showed that pig bone is not suspicious of human bone in the study of changes after death because it is more sensitive to environmental conditions than human bone [ 16 ].

The micro-FTIR technique was used to characterize the components of a dye painted on the walls of a church in Cyprus. The product was copper-based and the dye contained hydrated copper oxalate. Reflective imaging of the localization sites for the presence of copper and calcium oxalate within the layers of the plate. We conclude from this study that imaging calcium oxalate within different layers of paint samples is very important for studying copper-based pigments in general, and in particular for analyzing pigments used in coatings on different external surfaces [ 17 ].

Different heterocyclic compound derivatives have been synthesized via the reaction of ortho-Carboxybenzaldehyde with various aromatic amines (using six amino compounds) to produce Schiff bases ( Figure 5 ).

The reaction of Schiff base preparation.

The Schiff bases compounds gave FTIR spectra with an absorption appeared at wavenumber between 1602 and 1614 cm −1 this peak belongs to the new C=N group, and also carbonyl of carboxyl group gave absorption appeared at (1741–1766) cm −1 , and the absorption at (3306–3462) cm −1 for OH group of carboxylic acid. The authors noticed that the carbonyl of aldehyde disappeared, therefore our conclusion that FTIR proves the suggested mechanism and helps to suggest the structure of the product using the absorption of selective functional groups ( Table 2 , Figure 6 ) [ 18 ].

The aromatic part of amine.

FTIR spectrum of the product resulted from the reaction of p- toluidine with ortho-carboxybenzaldehyde [from reference 18 ].

2.2 Quantitative analysis

Fourier transform infrared (FTIR) is used in numerous areas of industrial pharmacy with satisfactory results. The technique’s characteristic and nature tolerate unequivocally bright forecasts for quantitative analysis. FTIR is considered a green analytical chemistry technique. It is very easy, fast to work by a temperately knowledgeable technician, covers a large range of spectra to analyze the pharmaceutical formulations, the main advantages are that it has a good resolution and is considered nondestructive device, and it is also friendly to the environment because in procedure no use of a dangerous organic solvent or any harmful reagents is required for the analysis. Many attempts were suggested for using derivative IR in determination diclofenac sodium in its formulations, but the results indicated that the first derivative spectra are the best technique for determination of diclofenac sodium. The first derivative spectra deleted IR band overlapping with the band understudies and increased sensitivity without any interference of the other band’s [ 19 ].

Abdulhameed and Nabil (2022) developed a simple and rapid method for the determination of ketoprofen. The method is based on normal and infrared derivative (first derivative) spectroscopy. The results of the study found that the method is accurate and there is the possibility of its application in quality control to determine ketoprofen in pharmaceutical formulations. Ketoprofen was quantified in a range of estimation from 1000 to 4000 μg/ml. This range was based on measuring the T% of the normal spectrum and its first derivative spectrum versus the concentration of ketoprofen in the solution ( Figure 7 ). The results prove the validity of the method, as the relative errors were +4.33% and 4.78% and the RSD% values were 1.15% and 1.37%, respectively, and since the values ​​are less than ±5%, the method is considered accurate and precise. The research also included the application of the two methods to estimate the compound under study in its different pharmaceutical preparations with a comparison of the results obtained with the results obtained via using high-performance liquid chromatography technique and calculating the t-student and F tests at P = 0.05.

The first derivative and the normal spectra of two standard ketoprofen from two companies Erbil and Turkey [from reference 20 ].

Figure 7 shows the derivative spectra of Standard, Erbil, and Turkey ketoprofen solutions, CCl4 was the solvent used. The two individual peaks of carbonyl groups at 1718 cm −1 as a positive peak and at 1705 cm −1 as a negative peak, and these peaks gave two calibration curves as various concentrations analyses, there is a reverse proportional relationship between the concentration and the percentage of transmittance(T%) ( Figure 8 ) and the other indirect proportion. The reverse proportional relationship is according to decreases in the transmittance% of the solution with an increase in concentration (as shown in Figure 8 ), will in Figure 9 there is a direct proportion or positive relationship for the first derivative IR according to the peak chosen (peaks of carbonyl groups at 1718 cm −1 as a positive peak) [ 20 ].

Calibration curve via normal IR method via first derivative IR.

Calibration curve [from reference 20 ]. method [from reference 20 ].

Michael et al (1995) used second derivative IR spectroscopy as a non-destructive tool to assess the purity and structural integrity of various samples such as proteins. Spectroscopy using second derivative infrared is a fast, easy, reproducible, cost-effective, and nondestructive method for assessing the purity of samples of some proteins (water-soluble) extracted from a diversity of sources. The 2ed IR spectra were calm under the lab-proven conditions of aqueous (D 2 O) solutions of seven different commercial samples for the same enzyme, porcine pancreatic elastase (2.0–3.8 mg protein/100 μl D 2 O, pD = 5.4–9.1). , the amide at the region defined by I (1700–1620 cm −1 ) from the IR spectra using the 2ed derivative for each of the seven elastase samples displays a characteristic pair of bands: one of them is very weak showing intensities near to 1684 cm −1 ; the other is close to 1633 cm −1 is moderate to strong. While one of the 7 samples under study shows a striking decrease in the noted density of amide I bands relative to the 1516 cm absorbance, along with the appearance of a new strong band at 1614 cm −1 . That the seventh sample is of much lower quality than the other samples and sure contains a quintile of the protein present in the non-native state. In addition, the apparent slight changes in the relative location, and intensity of a section of the separate amide I band among the seven spectra indicate slight differences in the formation of the amount of the peptide support of the samples under study. From the results of two samples, it seems that these few changes, sample purity, and identification of non-protein contaminants [ 21 ].

3. Hyphenated techniques

During the past five decades, hyphenated techniques developed rapidly and seemed to dominate many analyzes by introducing them to solve many problems related to complex analyzes, as they were widely used in the pharmaceutical industries from the stage of discovery to human use and the study of its concentration in living body fluids. Accuracy and high sensitivity, and one of the most important disadvantages are the high costs of the devices, and they need maintenance and accurate knowledge while working on the device. Liquid chromatography-mass spectroscopy (HPLC -MS) is one of the most widely applied hyphenated techniques because MS is more compatible with high performance-liquid chromatography (HPLC), and has good sensitivity compared with nuclear magmatic resonance (NMR) or IR. It is also possible to connect infrared spectrometers with thermal analyzers, the methods used by thermal analysis give information about the important temperature to study the physical properties of different materials. However, it is not always possible to obtain information about the chemical changes that occur as a result of changes in temperature through the literature. We note that it is possible to link the thermal analyzer with an infrared spectrometer in order to obtain information about the chemical and physical changes that occur at different and more appropriate temperatures. More suitable is the connection between thermogravimetric analysis (TGA) and FTIR spectroscopy However, there are limitations in its analytical use. The more advantages of the hyphenated technique include sensitivity, accuracy, speed, and applicability [ 7 , 22 ].

3.1 Gas chromatography–infrared (GC-IR)

3.1.1 difficulties in the combination of gc-ir.

In the development of joining the IR technique with GC, the speed of the IR must be changed to a high speed so that the unknown components can enter at the same speed from the GC column, in this case, there is a loss of efficacy and the results are not complete. The best way to solve the connecting problem is that the condensation of the gas that comes out from the column and the process is not easy, it must collect all gas eluted because it contains the component and the gas is collected in a cooled part that converts the gas into a liquid because the infrared technique deals with the liquid solutions. Reentry GC technique combined with Fourier transform infrared to give faster and more accurate technique.

3.1.2 Application

Salerno, et al (2020) suggest an accurate method for determination of illicit drugs via gas chromatography–Fourier transform infrared spectroscopy. According to the increasing number of synthetic molecules that can be used in the illicit drug market, correspondingly they require strong separation and sophisticated analytical techniques. It can be achieved by spectroscopic measurements, using firstly a gas chromatography (GC) technique as the separation device. Then the GC is coupled with FTIR to give a powerful tool. In the current study, the efficacy of GC-FTIR, in achieving elucidation of the structure of 1-pentyl-3(1-naphthyl) indole, known as JWH-018, a synthetic cannabinoid whose components have been identified as being a component of non-incense “incense blends” have been demonstrated in the current study. Moreover, it was quantified with an estimation range on the nano-gram scale. It was obtained in the range of 20–1000 ng, the detection limit and the quantification limit were evaluated to be 4.3 and 14.3 ng, respectively. Finally, the new technique was applied to quantify the activity in the “ST” sample." A real drug seized by law enforcement officers, consisted of a herbal collection containing four types of industrial cannabis belonging to the JWH class. Correct estimation of this type of compound showed that they are chemically similar to each other. The usefulness of the proposed method of analysis using related techniques. It obtains reliable results for complex mixtures of illegal drugs and is a widely applicable alternative to measurement using mass spectrometry [ 23 ].

4. Conclusions

Infrared is an important technique and its main application at the beginning to identify polar organic compounds that have a dipole moment. The infrared device has been developed, and we have obtained Fourier transform infrared (FTIR) technique, which is characterized by high accuracy, high sensitivity, and speed of analysis of the compound as a whole. The uses of the technique in the qualitative analysis are identifying the effective groups and the type of bonds between the different atoms constituting the molecule. The technique is used in the quantitative analysis through measurement of absorption or percentage of transmittance (proportional with concentration). The researchers used the first and second derivatives of the infrared spectrum in quantification research and also linked the infrared device with separation devices (for example, GC) to form a new technique called hyphenated techniques, and used in many studies with high sensitivity and precision compared with using each technique individually.

Acknowledgments

I must appreciate thanks and gratitude to the researchers who preceded us for their efforts to reach the information that enriches the subject of infrared techniques, which is the topic of the chapter.

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