INTERMOLECULAR COUPLING IN LIQUID AND CRYSTALLINE STATES OF TRANS-N-METHYLACETAMIDE INVESTIGATED BY POLARIZED RAMAN AND FT-IR SPECTROSCOPIES

The isotropic and anisotropic Raman spectra of neat N-methylacetamide (NMA) at different temperatures between -10 and 60 degrees C and NMA i n acetonitrile were measured in order to spectroscopically compare and characterize the crystallized (T < 28 degrees C) and liquid states. T hese plus infrared data were subjected to a self-consistent component band analysis. We found that the amide I band is composed of two subba nds 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 coup ling interactions associated with the A(g) and B-2g species of the cry stal unit cell. Depolarization ratio measurements suggest a departure from strict Du, symmetry. The three subbands in the liquid phase refle ct different aggregate structures. The lowest frequency band at 1634 c m(-1) results from an NMA oligomer exhibiting a structure similar to t hat observed in the ordered crystal phase. The most intense subband sh ows a significant negative noncoincidence effect, its isotropic compon ent appearing at 1650 cm(-1) and its anisotropic part at 1655 cm(-1). This subband is interpreted as resulting from locally ordered short ol igomeric hydrogen-bonded structures. The third subband is at 1675 cm(- 1) and results from isolated nonhydrogen-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 li quid phase (2 cm(-1)), which is also assignable to transition dipole c oupling between adjacent molecules in a locally ordered environment. T he Raman bands arising from the symmetric bending modes of the two met hyl-groups are significantly affected by crystallization; the CCH3 sym metric bending mode becomes depolarized and less intense while the NCH 3 symmetric bending mode gains intensity and becomes polarized. Ab ini tio calculations of torsional distortions of the CH3 groups, caused by interactions between adjacent non-hydrogen-bonded NMA molecules in th e crystal, qualitatively reproduce these effects.