Rotor-vibrator couplings in partially deuterated toluenes

Abstract

Fluorescence excitation spectra covering the origin–band region of the two partially methyl-deuterated toluenes (α-deuterotoluene, C₆H₅CH₂D and α-dideuterotoluene, C₆H₅CHD₂) are reported. The spectra, observed in a cold supersonic beam, show transitions between low methyl–rotor levels, and are analyzed in terms of vibrationally adiabatic torsional potentials with two-, four-, and sixfold periodicity. To interpret the resulting potential parameters, ab initio vibrational force field calculations are carried out at the four stationary structures of each isotopomer. The calculations yield accurate values for the adiabatic torsional barriers and qualitatively correct results for the vibrationally adiabatic potentials. Specifically, they show that the twofold potential term dominates in the partially methyl-deuterated isotopomers and that this component has the same magnitude but opposite sign in the mono- and dideuterocompounds. The potential parameters are interpreted in terms of nonbonded interactions between a CH bond of the methyl group and the phenyl ring; specifically, they represent the difference between the interactions of a methyl-CH and a methyl-CD bond with the ortho-CH bonds of the phenyl ring. Modeling of this interaction as a function of H–H separation in terms of a simple analytical expression shows that the repulsive potential is very soft and that individual H–H interactions are of the same order of magnitude as vibrational frequencies.