INTERNAL DYNAMICS CONTRIBUTIONS TO THE CH STRETCHING OVERTONE SPECTRAOF GASEOUS MONOHYDROGENATED NITROMETHANE NO2CHD2

The methyl CH-stretching overtone spectra of gaseous monohydrogenated nitromethane NO2CHD2 have been recorded with conventional near infrare d spectroscopy in the Delta upsilon (CH)=1 to 4 regions and by intraca vity laser photoacoustic spectroscopy in the Delta upsilon(CH)=5 and 6 regions. They all exhibit a complex structure with, at Delta upsilon( CH)=1 and 2, a characteristic asymmetric top vibration-rotation profil e which vanishes as vibrational energy increases. These spectra have b een analyzed with a theoretical model which takes into account in the adiabatic approximation the coupling between the anharmonic CH stretch described by a Morse potential and the quasifree internal rotation of the methyl group. All the parameters of this model (the zero point en ergy, the CH stretch frequency, the expansion coefficients of the dipo le moment function) and their variation with the internal rotation coo rdinate have been determined from HF/6-31G* ab initio calculations, T his simple calculation, which contains no adjustable parameters, succe ssfully describes the relative intensity and frequency of each peak wi thin a given overtone and accounts for the variation of the dipole mom ent function as the vibrational energy increases. Owing to the relativ e localization of the wave functions within the effective potential we lls, the spectral features can be assigned to particular pseudoconform ers. The outer bands correspond to rovibrational transitions associate d with the parallel and perpendicular conformation of the CH bond vers us the molecular plane, the central band is generated by ''free rotor' ' rovibrational transitions. Fermi resonance phenomena lead to no size able IVR until the fifth overtone. The CH/CD interbond coupling shifts the overtone spectra toward high frequency and is responsible for som e additional weaker features in the high overtone spectra(Delta upsilo n=5 and 6). (C) 1995 Americn Institute of Physics.