How to Interpret Infrared Spectra

An infrared (IR) spectrum is a chart that shows the intensity of light absorbed at different infrared wavelengths by a substance. These absorptions correspond to specific vibrations in the molecule, such as the stretching and bending of bonds. Interpreting an infrared spectrum requires an understanding of the various components of the chart and a basic knowledge of the correlation between absorption peaks and specific types of chemical bonds and functional groups.

 

Understanding the Axes

1. X-Axis

It represents the wavenumber (usually in cm^−1 units), with the high wavenumber side typically on the right (generally representing shorter wavelength, high-energy light).

 

2. Y-Axis

It displays the absorption intensity or transmittance, usually expressed as a percentage. Areas with high absorption intensity indicate that the infrared light of this wavenumber (energy) is strongly absorbed by the sample.

 

Identifying Functional Groups

1. Determining the Wavenumber Region

(1) 4000 - 1500 cm^−1: This is typically the functional group region, which is used to identify various functional groups in the molecule (e.g., OH, NH, C=O, C=C, C≡N, etc.).

(2) 1500 - 400 cm^−1: This is the fingerprint region, which usually contains many overlapping peaks and can be used to differentiate between very similar compounds.

 

2. Locating and Identifying Absorption Peaks

(1) The shape of the absorption peaks (such as sharp or broad) can provide additional information. For example, a broad O-H absorption peak may indicate the presence of a hydroxyl group.

(2) The intensity of the absorption peaks (i.e., the height of the peaks) can provide clues about the concentration of specific functional groups.

 

Analyzing the Fingerprint Region

1. The lower wavenumber region of an infrared spectrum (approximately below 1500 cm⁻¹) is called the "fingerprint region" because the complex absorption patterns here are unique, much like a fingerprint. The absorption peaks in the fingerprint region are usually caused by complex vibrations within the molecule.

2. Absorption peaks in the fingerprint region are particularly useful for identifying specific compounds, as they provide highly specific information.

 

Comparing with Known Spectra

Comparing the infrared spectrum of the sample with the standard spectral library of known substances can help confirm the composition of the sample.