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IR Spectrum

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Molecules with 100% transmittance do not absorb energy, so only molecules with lower transmittance values produce infrared spectra.

A compound's IR spectrum is the plot of intensity or percentage transmittance versus wavenumber, where percentage transmittance is the ordinate and wavenumber is the abscissa.

Downward signals on a spectrum indicate absorption bands at specific frequencies in wavenumbers. For instance, in decane, peaks around 3000, 1465, and 1375 cm^-1, signify energy absorption at these frequencies.

Signals on the left indicate high energy, high-frequency radiations, whereas those on the right denote low energy, low-frequency radiations.

Absorption bands corresponding to most functional groups lie above 1500 cm^-1 in the diagnostic region.

The region below 1500 cm^-1 is the fingerprint region, where each molecule shows a distinctive pattern. For example, 1-butanol and 2-butanol have similar functional groups but different absorption bands in this region.

Overview

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When infrared (IR) radiation passes through a molecule, the bonds stretch or bend by absorbing the radiation. This absorption creates the molecule's absorption spectrum, which is the plot of its percentage transmittance versus wavenumber.

Transmittance is defined as the ratio of the radiant power passing through a sample to that from the radiation's source. Multiplying the transmittance by 100 gives the percent transmittance (%T), which varies between 100% (no absorption) and 0% (complete absorption). When IR radiation passes through a molecule, 100% transmittance indicates that the molecule has not absorbed any energy. Conversely, lower percentage transmittance values suggest that the molecule has absorbed some energy, allowing us to obtain the IR spectrum. Each downward-pointing peak in the spectrum is called an absorption band. Usually, the position of the absorption band in the IR spectrum is reported using wavenumber or frequency values. In the IR spectrum, wavenumber values range from 400 to 4000 cm⁻¹.

In the IR spectrum, signals on the left side correspond to higher-energy and higher-frequency radiations, whereas those on the right side denote low-energy, low-frequency radiations. Based on shape and intensity, these signals are categorized as strong, medium, weak, broad, or sharp.

The IR spectrum can be divided into two main regions. The functional group region or diagnostic region is the left two-thirds portion of the spectrum. The absorption bands corresponding to most functional groups can be observed in this area. The right-hand side of the spectrum is referred to as the fingerprint region. Similar to the fingerprint of individuals, this region is unique to each compound. The diagnostic region of an infrared spectrum is dominated by bond stretching vibrations, while the fingerprint region is characterized by more complex vibrations like bond rocking, bending, and twisting. Although two compounds may contain the same functional group, they will show a unique pattern in the fingerprint region due to the differences in their chemical environment.

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Infrared Spectrum

Percent Transmittance

Absorption Bands

Wavenumber Range

Functional Group Region

Fingerprint Region

Bond Stretching

Bond Vibrations

Chemical Environment