Column: IR Spectral Interpretation Workshop

Articles

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.

Library Searching

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.

The Grand Review I: Why Do Different Functional Groups Have Different Peak Positions?

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

Organic Nitrogen Compounds X: Nitro Groups, An Explosive Proposition

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

Organic Nitrogen Compounds IX: Urethanes and Diisocyanates

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.

Organic Nitrogen Compounds VIII: Imides

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.

Organic Nitrogen Compounds, VII: Amides—The Rest of the Story

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.

Organic Nitrogen Compounds VI: Introduction to Amides

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.

Organic Nitrogen Compounds V: Amine Salts

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

Organic Nitrogen Compounds IV: Nitriles

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

Organic Nitrogen Compounds III: 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.

Organic Nitrogen Compounds II: Primary Amines

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.

Organic Nitrogen Compounds, Part I: Introduction

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.

The C=O Bond, Part VIII: Review

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.

The C=O Bond, Part VII: Aromatic Esters, Organic Carbonates, and More of the Rule of Three

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

The C=O Bond, Part VI: Esters and the Rule of Three

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.

The Carbonyl Group, Part V: Carboxylates—Coming Clean

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.

The C=O Bond, Part IV: Acid Anhydrides

How to spot carboxylic acids in your IR spectra

The C=O Bond, Part III: Carboxylic Acids

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.

The C=O Bond, Part II: Aldehydes

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