Interpreting the infrared spectrum of bromomethane (methyl bromide)

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK IB KS5 A/AS GCE advanced A level organic chemistry students US K12 grade 11 grade 12 organic chemistry courses involving molecular spectroscopy analysing infrared spectra of bromomethane

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Spectra obtained from a liquid film of bromomethane. The right-hand part of the of the infrared spectrum of bromomethane, wavenumbers ~1500 to 400 cm^-1 is considered the fingerprint region for the identification of bromomethane and most organic compounds. It is due to a unique set of complex overlapping vibrations of the atoms of the molecule of bromomethane.

Interpretation of the infrared spectrum of bromomethane

The most prominent infrared absorption lines of bromomethane

Several moderately strong absorption bands at wavenumbers 2975 to 2845 cm^-1 for C-H stretching vibrations - collectively give the impression of strong absorption in the infrared spectrum of bromomethane.

Several strong absorption bands from C-H deformation vibrations at wavenumbers 1470 to 1370 cm^-1.

Several strong absorption bands from 750 to 550 cm^-1 due to C-Br stretching vibrations of bromomethane.

Note the extra sharpness of the peaks using vapourised bromomethane compared to the broader bands you get with liquid film infrared spectra.

The absence of other specific functional group bands will show that a particular functional group is absent from the bromomethane molecular structure.