Investigation of hydrogen bonding and self-association in neat HCONH2 and the binary mixture (HCONH2 þ CH3OH) by concentration dependent Raman study and ab initio calculations

Raman spectra of neat formamide (HCONH2) and its binary mixture (HCONH2 þ CH3OH) with hydrogen donor solvent, methanol (CH3OH) were investigated using a sensitive scanning multichannel detection scheme, which is simultaneously more precise also, especially when the observed Raman line profile has multiple component bands. The spectra in the two regions, namely 1200–1500 and 1500– 1800 cm21 were recorded with varying mole fractions of the reference molecule, HCONH2, from 0.1 to 0.9. The spectra in the region 1200– 1500 cm21 show a broad band at ,1312 cm21, which shows a peculiar concentration dependence, and a relatively sharp peak at ,1392 cm21, whose peak position is not influenced by concentration. The spectra in the region 1500–1800 cm21 also show two peaks, one at ,1593 cm21 and the other one at ,1668 cm21 which are assigned to NH2 bending and n(CyO) stretching vibrations, respectively. Both these Raman bands show an appreciable upshift of ,15–20 cm21 and the one at ,1668 cm21 has also a distinct asymmetry towards higher wavenumber. The optimized geometries and vibrational wavenumbers of various normal modes for neat formamide as well as its hydrogenbonded complexes were also calculated using ab initio theory at the MP2 level. The results have been used to understand and explain the concentration dependent changes in the spectral features in terms of hydrogen bonding and self-association. q 2003 Elsevier B.V. All rights reserved.