Interpreting the infrared spectrum of dimethylamine (N-methylmethanamine)

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 dimethylamine

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

Interpretation of the infrared spectrum of dimethylamine

The most prominent infrared absorption lines of dimethylamine

At wavenumbers ~3300 to 3300 cm^-1 is a broad band for N-H bond stretching vibrations,  characteristic of secondary amines.

Secondary aliphatic amines have a highly polar bond (^δ-N-H^δ+) and the intermolecular forces are increased by permanent dipole - permanent dipole attractions including hydrogen bonding (N-H^δ+llll^δ-N-H) between the dimethylamine molecules (diagram below).

The hydrogen bonding interferes with the N-H stretching vibrations producing the broad band peaking at around ~3400 cm^-1.

Around 3000 to 2800 cm^-1 are absorptions due to C-H stretching vibrations - they overlap with the N-H stretching vibrations of dimethylamine.

There are also characteristic bands due to N-H vibrations at wavenumbers ~1650 to 1580 cm^-1, often sharp, so do not confuse with the C=O vibrations from carbonyl compounds.

Absorption due to C-N vibrations, characteristic of C-N bonds in aliphatic amines like dimethylamine occur at 1250 to 1020 cm^-1.

There are other N-H vibration absorptions at wavenumbers ~910 to 665 cm^-1,

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