A strong effect of hydrogen bonding upon δ(NC3) bands in the IR spectra of trimethylamine

Remarkable effects of hydrogen-bond formation and its strengthening on the NC3 bending modes of trimethylamine (TMA) are reported. The relative value of the blue shift of the δNC3(A1) frequency reaches about 30% for strong hydrogen bonds. The integrated intensity of corresponding IR band increases about 9 times. The ab initio calculations performed for some model TMA complexes confirm the tendency observed in the experiments, the maximum effects being predicted for the homoconjugated (CH3)3NH+N(CH3)3 cation. The optimized structure of the cation, resulting from the calculations, implies the double-well profile of the potential surface and splitting of internal TMA frequencies. The doublet structure of δ(A1) and other CNC bands with frequencies close to those predicted for the homoconjugated cation have been detected for a TMA–pentachlorophenol complex of 2:1 composition in chloroform solution.