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 geometry and the vibrational frequencies of three hydrogen/deuterium (d0, d1, d2) isotopomers of nitromethane. The Raman gas-phase spectra of these compounds are also presented. They compare well with the infrared ones and the theoretical prediction. The calculated overall zero-point vibrational energy of both d1 and d2 isotopomers presents an angular dependence of the form V2 cos(2theta) + V4 cos(4theta). It contributes significantly 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 experimental results shows that theoretical calculations can be a convenient alternative to analyze the lone CH or CD stretching spectra of the partially methyl groups.