ROTOR-VIBRATOR COUPLINGS IN PARTIALLY DEUTERATED TOLUENES

Fluorescence excitation spectra covering the origin-band region of the two partially methyl-deuterated toluenes (alpha-deuterotoluene, C6H5C H2D and alpha-dideuterotoluene, C6H5CHD2) are reported. The spectra, o bserved in a cold supersonic beam, show transitions between low methyl -rotor levels, and are analyzed in terms of vibrationally adiabatic to rsional potentials with two-, four-, and six-fold periodicity. To inte rpret the resulting potential parameters, nb initio vibrational force field calculations are carried out at the four stationary structures o f each isotopomer. The calculations yield accurate values for the adia batic torsional barriers and qualitatively correct results for the vib rationally adiabatic potentials. Specifically, they show that the twof old potential term dominates in the partially methyl-deuterated isotop omers and that this component has the same magnitude but opposite sign in the mono- and dideuterocompounds, The potential parameters are int erpreted in terms of nonbonded interactions between a CH bond of the m ethyl group and the phenyl ring; specifically, they represent the diff erence between the interactions of a methyl-CH and a methyl-CD bond wi th the ortho-CH bonds of the phenyl ring. Modeling of this interaction as a function of Ii-II separation in terms of a simple analytical exp ression shows that the repulsive potential is very soft and that indiv idual H-H interactions are of the same order of magnitude as vibration al frequencies. (C) 1997 American Institute of Physics.