THEORETICAL AND SPECTROSCOPIC STUDY OF ASYMMETRIC METHYL ROTOR DYNAMICS IN GASEOUS PARTIALLY DEUTERATED NITROMETHANES

Ab-initio calculations have been performed at the HF/6-31G* and HF/6- 31++G(d,p) levels to study the conformational dependence of the geomet ry and the vibrational frequencies of three hydrogen/deuterium (d0, d1 , d2) isotopomers of nitromethane. The Raman gas-phase spectra of thes e compounds are also presented. They compare well with the infrared on es and the theoretical prediction. The calculated overall zero-point v ibrational energy of both d1 and d2 isotopomers presents an angular de pendence of the form V2 cos(2theta) + V4 cos(4theta). It contributes s ignificantly to the methyl torsion potential and causes two different equilibrium positions for the d1 (eclipsed) and d2 (staggered) methyl groups. The vibrational origin of the V2 and V4 pseudopotential terms of the rotational potential of the partially deuterated methyl groups is thus confirmed. The close correspondence between calculated and exp erimental results shows that theoretical calculations can be a conveni ent alternative to analyze the lone CH or CD stretching spectra of the partially methyl groups.