Intermolecular hydrogen bonding and low-wave-number vibrational spectra of formamide,N-methylformamide, andN-methylacetamide in the liquid state

Relationship between intermolecular hydrogen bonding and features in the 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 ∼200 and ∼100 cm−1 for the FA hexamer consisting of two antiparallel linear trimers, but only at ∼100 cm−1 for the NMF linear tetramer and the NMA linear trimer. These calculated results are consistent with spectral features observed in the liquid state. The IR spectra calculated for these cluster species are also in agreement with experimental results. By contrast, no strongly Raman-active mode is calculated for the FA linear hexamer in the 250–150 cm−1 region. The strong Raman band of liquid FA observed at ∼200 cm−1 is therefore characteristic of two-dimensional hydrogen bonding. The origin of the spectral features is examined by calculating the IR and Raman intensities arising from translations, rotations, and methyl torsion(s) of each molecule. It is clarified that the Raman intensities in the low-wave-number region mainly originate from rotational motions in the out-of-plane direction and is explained by anisotropy of the polarizability tensor of each molecule. The IR intensities in the low-wave-number region mainly arise from rotations of the permanent dipole moment of each molecule, but other factors also have some contributions.