A strong effect of hydrogen bonding upon delta(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 delta NC3(A(1)) frequency reaches about 30% for st rong hydrogen bonds. The integrated intensity of corresponding IR band incr eases about 9 times. The ab initio calculations performed for some model TM A complexes confirm the tendency observed in the experiments, the maximum e ffects being predicted for the homoconjugated (CH3)(3)NH+N(CH3)(3) cation. The optimized structure of the cation, resulting from the calculations, imp lies the double-well profile of the potential surface and splitting of inte rnal TMA frequencies. The doublet structure of delta(A(1)) and other CNC ba nds 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. (C) 1999 Elsevier Science B.V. All rights reserved.