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The infrared spectra of dimethylamine in the fundamental and in the first overtone range of the NH stretching vibration have been investigated as a function of concentration and of temperature. In the fundamental range, in CCb and n-hexane solutions bands of hydrogen-bonded and of free NH groups have been observed. A reduction of the frequency of the bonded groups in CC14 solution relative to the frequency in hexane solution is attributed to the increase in proton-donor ability of the amino group due to complexation with the solvent. A significant frequency decrease in hexane with increasing amine concentration is explained by an increase of proton-donor ability due to self-association. In the overtone range, except at the lowest temperatures, only the band of free groups has been detected. A decrease of its frequency in hexane with increase of amine concentration is attributed to the change from amino groups of monomers to the free groups of self-associates. The rather slight frequency shift in CC14 is explained by the transition from the free groups of CCI4 complexes of monomers to the free groups of self-associates. From localization of the associate overtone at wave numbers 60 to 100 cm-1 lower than the free groups overtone, it follows that the decrease of mechanical anharmonicity caused by hydrogen bonding does not have the significance assumed previously. The enthalpy of association is determined to be about -1900 cal/mol in hexane and about -1100 cal/mol in CC14. The fraction of the free NH groups in the pure compound is calculated to be about 60% at +20°a nd about 40% at -20°.

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