Columns | Column: IR Spectral Interpretation Workshop

In the second installment of “The Big Review,” we discuss the physical mechanism behind how molecules absorb infrared (IR) radiation. Because light can be thought of as a wave or a particle, we have two equivalent pictures of IR absorbance. We also discuss the quantum mechanics behind IR absorbance, and how this leads to the different peak types observed in IR spectrum.

I have been writing this column for almost 10 years, and we have reached a watershed moment. We have covered what I call the beginning and intermediate topics. However, before we venture off into advanced topics, a review is in order. I will spend the next four columns summarizing what we have learned so far so that the advanced topics will make more sense.

We wrap up our discussion of the mid-infrared spectra of inorganic compounds by looking at the spectra of silicates, nitrates, and phosphates. We will see that silicates have complex surface chemistry, and that infrared spectroscopy can tell us something about this. We will note that, of the five families of inorganics examined, the wavenumber ranges for the polyatomic anion stretching peaks in several of these functional groups overlap. However, polyatomic anion bending vibration peaks can be used to distinguish the five types of inorganics studied.

This column discusses the spectra of different inorganic functional groups, such as sulfates, carbonates, nitrates, silicates, and phosphates, with special attention being paid to the stretching and bending vibrations of the polyatomic anions in these compounds.

The author dispels the notion that inorganics do not have mid-infrared spectra by providing an overview of the general characteristics of their infrared spectra.

An examination of halogenated compounds and some common halogenated polymers is explored here.

The structure and infrared spectra of polyurethanes and polymeric amides are similar, but there are several key differences worth calling attention to.

Slip agents are commonly used amide small molecules that lubricate the molds in injection molding processes. By studying the spectra of a primary amide, we can see how they can contaminate the spectra of polymeric materials.

We study the spectra of organic nitrogen polymers with a particular focus on polyamides.

We study the spectrum of polymethyl methacrylate (PMMA)—otherwise known as plexiglass—and the spectra of PMMA mixtures and copolymers.

We continue our survey of the spectra of carbonyl-containing polymers by looking at cellulose acetate and the economically important polycarbonate Lexan.

An exploration of polyethylene terephthalate (PET), one of the most important polymers, is presented.

We continue our survey of the infrared (IR) spectra of polymers with a look at the spectra of polymers that contain carbonyl or C=O bonds. Our long-term goal is to examine the spectra of polymers that contain ketone, carboxylic acid, ester, and carbonate linkages. Studying these spectra is vital, because these molecules are important economically and are ubiquitous in society.

An examination of polymers with C-O bonds, focusing on the spectra of polyvinyl alcohol, cellulose, starch, and copolymers.

Epoxies are perhaps the most economically important functional group of polymers. We explain how to interpret their spectra.

Many rubbery polymers contain C=C bonds, which means they are alkenes. Thus, we can identify natural and synthetic rubbers by examining the spectra of cis-, trans-, and tri-substituted alkenes.

Infrared (IR) spectroscopy can be used to distinguish the spectra of hydrocarbon polymers from one another.

Here, we finish a discussion of the spectrum of polyethylene (PE) and explore how different PE syntheses produce materials with different physical and spectroscopic properties.

Polymers contain practically every infrared functional group imaginable. Now that we understand infrared functional groups from previous articles, it is time to look at the spectra of polymers in greater detail.

Determining the components in a mixture can be a significant challenge in infrared spectroscopy, but spectral subtraction can help. We show the proper way to perform spectral subtraction, and the pitfalls to avoid.

One of the biggest practical limitations of infrared spectroscopy is its difficulty in analyzing mixtures. Infrared library searching can help, but it must be done right.

Integrating peak position, height, and width information is important to properly interpret infrared spectra. But do you understand why peak heights and widths vary? We explain.

Articles in this column have addressed five main areas: theory, functional groups containing the C-H bond, those containing the C-O bond, those with the C=O bond, and those with organic nitrogen compounds. Here, we review the concepts.

We review one final organic nitrogen functional group, representing explosive compounds. This is the nitro group, which requires a different set of interpretation rules.

Here we delve into the interpretation of another organic nitrogen compound, known as polyurethane, a ubiquitous polymer used for wood finishes and foam rubber.

Here, we continue our examination of the infrared (IR) spectra of organic nitrogen compounds with imides, which are a common chemical intermediate. IR can be used not only to identify imides, but also to distinguish between straight chain and cyclic imides. We explain how.

The N-H stretching and bending peaks can be used to distinguish primary, secondary, and tertiary amides and to ascertain protein structure. Here’s how.

Amides are an important functional group found extensively in polymers and proteins. There are three different families of amides. Here, is explained how to distinguish them using infrared spectroscopy.

The amine salt functional group contains ionic bonding, and is extremely polar, giving rise to a number of intense and uniquely placed peaks that are easy to identify for primary, secondary, and ter tiary amines.

Nitriles are easy to spot because of the unusual intensity and position of the C≡N stretching peak.

A detailed guide to interpreting the infrared spectra of organic nitrogen compounds, including secondary and tertiary amines.

Amines, which contain carbon, hydrogen, and nitrogen, come in six varieties, but using the N-H stretching peak positions by themselves will distinguish between all the different amine types.

So far, we have restricted our discussion to organic functional groups that contain carbon, hydrogen, and oxygen, with past columns addressing the theory of infrared spectral interpretation of C-H bonds, C-O bonds, and the C=O functional group. We now turn our attention to interpretation involving organic nitrogen compounds.

We review the group wavenumbers of the many carbonyl-containing functional groups we have examined this year and discuss how to distinguish these functional groups from each other.

Aromatic esters follow the ester Rule of Three, but each of these three peak positions is different for saturated and aromatic esters, which makes them easy to distinguish. Organic carbonates are structurally similar to esters and follow their own Rule of Three.

Esters are a common and economically important functional group made by reacting an alcohol and a carboxylic acid.

Carboxylates are made by reacting carboxylic acids with strong bases such as inorganic hydroxides. Carboxylates contain two unique carbon–oxygen “bond and half” linkages that coordinate with a metal ion to give two strong infrared peaks, which make them easy to see.

Acid anhydrides are unique in that they have two carbonyl groups in them. The intensity and position of their IR peaks can be used to determine which of the four types of anhydride exist in a sample.

How to spot carboxylic acids in your IR spectra

Aldehydes feature a unique “lone hydrogen” atom, giving rise to unique C-H stretching and bending peaks, making them easy to spot. In this installment, a new feature is also presented, “IR Spectral Interpretation Review,” where key concepts from past columns are presented for those new to the column and for readers who need a refresher.

An introduction to the IR spectroscopy of the carbonyl group, exploring why the peak is intense and showing how to apply that knowledge to the analysis of the spectra of ketones

Two unrelated discussions are presented: carbohydrates and alkynes.

We will discuss three different types of ether, which are characterized by the type of carbons attached to the central oxygen.

In addition to primary alcohols there exist secondary and tertiary alcohols.

We now turn our attention to the C-O bond, how to detect its presence in a sample from an infrared (IR) spectrum, and a study of the functional groups that contain this bond. In this first installment on the topic, we study the spectra of alcohols and learn to distinguish primary, secondary, and tertiary alcohols from each other based on their infrared spectra.

Now that we have completed our discussion of benzene rings and the infamous “benzene fingers,” the next topic on our hydrocarbon hit parade are carbon-carbon double and triple bonds. C=C bonds, otherwise known as alkenes, come in six different structural isomer types, while triple bonds, known as alkynes, come in two varieties. This column provides you with all the tools you need to distinguish all of these different types of molecules from each other.

With the theoretical background of benzene analysis laid out in part 1 of this series, we now know what fundamental, overtone, and combination bands look like. Here, I show that the benzene fingers are a series of overtone and combination bands that can be used to distinguish substituted benzene rings from each other when other methods do not work. I review the benzene finger patterns for mono-, ortho-, meta-, and para- substituted benzene rings, and describe an easy mnemonic in which you use your fingers to help you remember the patterns.

This installment begins with a needed discussion on the theory behind the three different types of infrared bands, how to recognize them, and how to use them to help you interpret spectra. Continuing on from the last column, this knowledge is used to help better distinguish mono- and di-substituted benzene rings from each other.

Following up on the last installment, we examine the infrared spectra of mono- and di-substituted benzene rings. We will examine numerous example spectra and learn how the position of C-H wagging peaks, and the presence or absence of a ring-bending peak, allow one to distinguish between mono-, ortho-, meta-, and para-substituted rings most of the time.

Continuing our theme of investigating the infrared spectra of hydrocarbons, we look at the nature of aromatic bonding and why aromatic rings have unique structures, bonding, and infrared spectra. Then we examine, in detail, the spectra of mono- and di-substituted benzene rings, and learn that infrared spectroscopy easily distinguishes between ortho-, meta-, and para- structural isomers. 

We wrap up our introduction to the theory of infrared spectral interpretation with a discussion of the correct process to follow when interpreting spectra. The author has developed this 12-step system over many years of interpreting spectra, and finds it gives him the best results. The process includes knowing how a spectrum was measured, systematically identifying peaks, and the proper use of infrared spectral interpretation aids. The answer to last column’s quiz is also disclosed.

Identity testing is used in the pharmaceutical, food, and dietary supplement industries (amongst others) to ensure raw materials and final products have the correct chemical composition by answering the spectral question: Are these two samples the same? The first part of this installment instructs readers on the correct way to perform identity testing. The interpretation portion of the installment wraps up our discussion of straight chain alkanes by discussing how to determine chain length from infrared spectra. We also go over the answer to the problem from the last installment.

Continuing the theory and practice themes from previous columns, the theory portion of this column will be a discussion of the proper way of handling the infrared spectral interpretation of mixtures. In my opinion, mixtures are the biggest obstacle to interpreting infrared spectra, and I will share with readers five tried-and-true techniques for dealing with them. The practice portion of the column will give the answer to the last installment’s problem, and complete the spectral analysis of straight chain alkanes.

Interpreting infrared spectra is fun, but to do it properly one must be grounded in theory, which might be not so enjoyable for some. To cover theory and interpretation judiciously, this installment (and the next several installments) will begin with a section on theory and end with coverage of interpreting spectra. Here, we introduce the theory behind light and spectral units and the interpretation of methyl and methylene groups contained in straight alkane chains.

There is a continuing need for Fourier transform infrared (FT-IR) users to receive training in how to interpret the infrared spectra they measure. This new column will provide practical advice about how to do this. This first installment will present why this type of column is important, discuss some basic IR theory, and lay out a blueprint for future installments.