The Theory and Application of IR Spectroscopy: A Quick Overview

IR Spectroscopy Tutorial IR spectroscopy is widely used in labs for compound identification, QA/QC, and process monitoring. This interactive tutorial gives a quick overview of the theory and application of this popular tool. IR spectroscopy uses a sample to absorb frequencies of radiation and translates those signals into a molecular fingerprint. This information can quickly tell you the identity of a chemical species. Theory Infrared spectroscopy is used to analyse the vibrations of molecules. This allows information to be obtained about the structure and composition of a substance. It can also be used to identify different functional groups in chemical compounds. IR spectroscopy works by measuring the transmission of an infrared beam through a sample and a reference solution. The resulting spectrum is then recorded using detectors. The absorbed frequencies are displayed on a graph. A variety of different IR spectra can be obtained depending on the conditions under which they are measured. Different vibrational modes of a molecule produce different peaks in the IR spectrum. Those with polar bonds have a dipole moment that causes them to absorb infrared radiation at certain wavelengths. This means that their IR spectra will be broad, while those of non-polar molecules have sharp peaks. The molecule’s vibrational energy and rotational state determines the spectral characteristics. For example, carbon tetrachloride, acetone and methylene chloride are polar solvents while methyl acetate, pyrrolidine and hexanol are non-polar solvents. Parts of an IR spectrum IR spectroscopy is one of the main techniques that chemists use to identify different functional groups in molecules. It works by identifying the various vibrations that a covalent bond can take, which will then correspond to specific frequencies of absorbed radiation. Those frequencies are usually measured in cm-1, and the bands observed correlate with certain vibrational modes. The higher frequencies tend to show the most predominant bands and are related to stretching vibrations, whereas lower ones indicate bending, wagging and twisting vibrations. A molecule’s IR spectrum can be acquired by irradiating the sample with a broad range of frequencies and measuring the reflected or transmitted intensity as a function of frequency. The absorption spectrum may then be reconstructed from the information obtained. IR can be used on solid, liquid or gaseous samples. Usually, the FTIR method is employed for obtaining the spectra since it is faster and simpler than the dispersive technique. However, there are some exceptions to this. IR peaks Over time organic chemists have recorded the types and locations of the IR absorptions produced by a variety of functional groups. Consequently it is very easy to reference these tables of IR absorptions, arranged by the group, and compare a sample’s spectrum to its expected one. The frequencies on these charts are displayed as a frequency range rather than as a number of cycles per second (Hz) because the number of wave cycles in a centimeter is more manageable. The peaks in an IR spectrum show the energy associated with different bond stretches. For example, the C=O bond stretch gives a sharp peak around 1700 cm-1. This peak is diagnostic for carbonyls, such as ketones, aldehydes, esters and carboxylic acids. IR can be used to detect samples of solids, liquids and solutions, but only in the case where the solvent doesn’t absorb IR radiation in the same region. This is why IR spectra are most often compared to the spectra of a known compound. Interpretation When a beam of infrared light passes through a sample, different chemical species in the sample will absorb portions of the signal. This causes bonds in the sample to vibrate in a variety of ways, and these vibrations give rise to characteristic peaks in the IR spectrum. Each peak identifies a specific type of bond and type of movement. Interpreting IR spectra involves correlating the observed frequencies of these absorptions with the known absorption frequencies for different types of bonds. Students are encouraged to become familiar with the frequency and wavelength units displayed in IR spectral tables, as well as the notation for signal intensity (weak, medium or strong) and shape (narrow or broad). The first step toward interpretation is to identify a sample’s dominant peak, also known as its “fingerprint region.” For example, signals below 3000 cm-1 identify saturated carbons; those above it indicate unsaturation; and the presence of a broad peak at 2800 cm-1 indicates exchangeable protons found in alcohols, amines and carboxylic acids. Continue for more insights…