INTERMOLECULAR HYDROGEN-BONDING AND LOW-WAVE-NUMBER VIBRATIONAL-SPECTRA OF FORMAMIDE, N-METHYLFORMAMIDE, AND N-METHYLACETAMIDE IN THE LIQUID-STATE

Relationship between intermolecular hydrogen bonding and features in t he low-wave-number infrared (IR) and Raman spectra of liquid formamide (FA), N-methylformamide (NMF), and N-methylacetamide (NMA) is studied by performing ab initio molecular orbital calculations on clusters of these molecules. Strongly Raman-active modes are calculated at simila r to 200 and similar to 100 cm(-1) for the FA hexamer consisting of tw o antiparallel linear trimers, but only at similar to 100 cm(-1) for t he NMF linear tetramer and the NMA linear trimer. These calculated res ults are consistent with spectral features observed in the liquid stat e. The IR spectra calculated for these cluster species are also in agr eement with experimental results. By contrast, no strongly Raman-activ e mode is calculated for the FA linear hexamer in the 250-150 cm(-1) r egion. The strong Raman band of liquid FA observed at similar to 200 c m(-1) is therefore characteristic of hue-dimensional hydrogen bonding. The origin of the spectral features is examined by calculating the LR and Raman intensities arising from translations, rotations, and methy l torsion(s) of each molecule. It is clarified that the Raman intensit ies in the low-wave-number region mainly originate from rotational mot ions in the out-of-plane direction and is explained by anisotropy of t he polarizability tensor of each molecule. The IR intensities in the l ow-wave-number region mainly arise from rotations of the permanent dip ole moment of each molecule, but other factors also have some contribu tions. (C) 1998 John Wiley & Sons, Inc.