Hydrogen bonding and complex formation of dimethylamine. Infrared investigations on the NH stretching vibration bands

Abstract

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°.

A Low-temperature Infrared Study of Self-association in Thiolic Acids

“…The frequency is only slightly changed. These systems seem to behave similarly to the dimethylamine N H stretching band, as described by Wolff and Gamer (7). (For a more elaborate discussion of the effect of "inert" solvents on X-H stretching bands (X = O,N), see ref.…”

S N Ar Reactions of Amines in Aprotic Solvents

“…This is specially important for reactions in aprotic solvents in which solvation is not as clear as in the network encountered in protic solvents. It is well known 112,114,145,146,157 that amines may undergo auto-association in aprotic media giving rise to aggregates of various stoichiometry. The dominating aggregate is a dimer with typical formation constant K ¾ D 0.1 M 1 (the value for cyclohexylamine in cyclohexane) 145 .…”

“…Amine aggregates are known 145,157 to be affected inversely by temperature, hence in S N Ar rates, then, overall negative energies of activation can be observed in reactions where a pre-equilibrium, such as 2B K ! B:B , exists.…”

A dihydrogen bond between a bridging hydride and the NH proton of a coordinated dimethylamine: solid state, solution and theoretical characterization

“…The presence of a hydrogen bond usually causes a red-shift and a strong broadening of the stretching absorption of the X-H hydrogen bond donor. In this case, the m(NH) IR band of the coordinated amine of 2 appears at 3262 cm À1 , approximately a 100 cm À1 shift to lower wavenumbers with respect to the literature value for pure DMA in a solvent not able to act as proton acceptor (3361 cm À1 in n-hexane) [13]. An hydrogen bonding interaction is therefore shown to be present in 2, even if it is rather weak.…”