Intermolecular Coupling in Liquid and Crystalline States of trans-N-Methylacetamide Investigated by Polarized Raman and FT-IR Spectroscopies

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 1998-01, Vol.102 (1), p.118-127
• Main Authors: Schweitzer-Stenner, Reinhard, Sieler, Guido, Mirkin, Noemi G, Krimm, Samuel
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp972408x

The isotropic and anisotropic Raman spectra of neat N-methylacetamide (NMA) at different temperatures between −10 and 60 °C and NMA in acetonitrile were measured in order to spectroscopically compare and characterize the crystallized (T < 28 °C) and liquid states. These plus infrared data were subjected to a self-consistent component band analysis. We found that the amide I band is composed of two subbands in the solid phase and three in the liquid phase. For the former, the subbands at 1633 and 1656 cm-1 arise from transition dipole coupling interactions associated with the Ag and B2g species of the crystal unit cell. Depolarization ratio measurements suggest a departure from strict D 2 h symmetry. The three subbands in the liquid phase reflect different aggregate structures. The lowest frequency band at 1634 cm-1 results from an NMA oligomer exhibiting a structure similar to that observed in the ordered crystal phase. The most intense subband shows a significant negative noncoincidence effect, its isotropic component appearing at 1650 cm-1 and its anisotropic part at 1655 cm-1. This subband is interpreted as resulting from locally ordered short oligomeric hydrogen-bonded structures. The third subband is at 1675 cm-1 and results from isolated non-hydrogen-bonded NMA molecules or from amide I modes of the terminal groups of the above oligomers. Amide III shows a small but detectable positive noncoincidence effect in the liquid phase (2 cm-1), which is also assignable to transition dipole coupling between adjacent molecules in a locally ordered environment. The Raman bands arising from the symmetric bending modes of the two methyl groups are significantly affected by crystallization; the CCH3 symmetric bending mode becomes depolarized and less intense while the NCH3 symmetric bending mode gains intensity and becomes polarized. Ab initio calculations of torsional distortions of the CH3 groups, caused by interactions between adjacent non-hydrogen-bonded NMA molecules in the crystal, qualitatively reproduce these effects.