Infrared (IR) spectroscopy is a powerful tool in chemistry that allows us to identify and analyze the functional groups in a molecule based on the vibrations of its bonds. In this IR cheat sheet, we will cover the basics of IR spectroscopy, including its principles, instrumentation, and interpretation of spectra. We will also discuss some common functional groups and their characteristic IR frequencies.
Principles of IR Spectroscopy
IR spectroscopy is based on the fact that molecules absorb light at specific frequencies that correspond to the energy required to excite specific vibrations of their bonds. When IR radiation is passed through a sample, some of the radiation is absorbed by the molecules, resulting in a spectrum that shows the frequencies of the absorbed radiation as a function of the intensity of the absorption.
The intensity of the absorption is proportional to the concentration of the absorbing species, the path length of the radiation through the sample, and the strength of the absorbing bond. Thus, IR spectroscopy can be used for qualitative and quantitative analysis of compounds in solution or as a solid.
Instrumentation for IR Spectroscopy
IR spectroscopy requires a source of infrared radiation, a sample holder, and a detector to measure the intensity of the radiation after it has passed through the sample. The source of infrared radiation is typically a heated filament or a laser, which emits radiation in the mid-infrared (MIR) or near-infrared (NIR) regions of the spectrum.
The sample holder is usually a small cell made of a material that is transparent to the infrared radiation, such as sodium chloride or potassium bromide. The sample is placed in the cell, which is then inserted into the IR spectrometer.
The detector used in IR spectroscopy is typically a thermocouple or a photoconductive device that measures the intensity of the radiation after it has passed through the sample. The detector converts the radiation into an electrical signal, which is then processed to produce the IR spectrum.
Interpretation of IR Spectra
Interpretation of IR spectra requires an understanding of the characteristic frequencies of the functional groups present in the molecule. The frequency at which a bond absorbs IR radiation depends on the bond strength, the atomic masses of the atoms in the bond, and the geometry of the molecule.
The most important regions of the IR spectrum are the fingerprint region (400-1500 cm-1), which contains a complex pattern of absorption bands that are unique to each molecule, and the functional group region (1500-4000 cm-1), which contains the characteristic absorption frequencies of the functional groups.
The functional group region can be divided into several subregions, each of which contains characteristic absorption frequencies for specific functional groups. Some of the most important subregions and their associated functional groups are:
The O-H stretch region (3200-3600 cm-1), which contains the characteristic absorption frequency of alcohols, phenols, and carboxylic acids.
The C-H stretch region (2800-3000 cm-1), which contains the characteristic absorption frequency of alkanes, alkenes, and alkynes.
The C=O stretch region (1630-1850 cm-1), which contains the characteristic absorption frequency of carbonyl groups in aldehydes, ketones, and carboxylic acids.
The N-H stretch region (3100-3500 cm-1), which contains the characteristic absorption frequency of amides, amines, and nitro groups.
The C-O stretch region (1000-1300 cm-1), which contains the characteristic absorption frequency of ethers, esters, and carboxylic acids.
In addition to these functional group regions, there are also characteristic absorption frequencies for other functional groups, such as halogens, sulfides, and phosphates, which can be used to identify these groups in a molecule.
Common Functional Groups and Their Characteristic IR Frequencies
Alkanes
Alkanes are hydrocarbons that contain only single bonds between carbon atoms. The C-H stretch region of the IR spectrum for alkanes is typically a broad, featureless band centered around 2920 cm-1. The fingerprint region of the IR spectrum for alkanes is relatively simple, with a few sharp absorption bands in the 1000-1500 cm-1 region.
Alkenes
Alkenes are hydrocarbons that contain at least one double bond between carbon atoms. The C-H stretch region of the IR spectrum for alkenes is similar to that of alkanes, but with a more intense absorption band at around 3010 cm-1 due to the presence of the C-H bond adjacent to the double bond. The C=C stretch region is typically a sharp absorption band at around 1650 cm-1.
Alkynes
Alkynes are hydrocarbons that contain at least one triple bond between carbon atoms. The C-H stretch region of the IR spectrum for alkynes is similar to that of alkenes, but with a more intense absorption band at around 3300