Intermolecular hydrogen bonding and low-wave-number vibrational spectra of formamide, N-methylformamide, and N-methylacetamide in the liquid state
✍ Scribed by Hajime Torii; Mitsuo Tasumi
- Publisher
- John Wiley and Sons
- Year
- 1998
- Tongue
- English
- Weight
- 263 KB
- Volume
- 70
- Category
- Article
- ISSN
- 0020-7608
No coin nor oath required. For personal study only.
✦ Synopsis
Relationship between intermolecular hydrogen bonding and features in
Ž .
Ž . the low-wave-number infrared IR and Raman spectra of liquid formamide FA , Ž . Ž . N-methylformamide NMF , and N-methylacetamide NMA is studied by performing ab initio molecular orbital calculations on clusters of these molecules. Strongly Ramanactive modes are calculated at ; 200 and ; 100 cm y1 for the FA hexamer consisting of two antiparallel linear trimers, but only at ; 100 cm y1 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 y1 region. The strong Raman band of liquid FA observed at ; 200 cm y1 is therefore characteristic of two-dimensional hydrogen bonding. The origin of the spectral features is examined by calculating the IR Ž . 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.