TORSIONAL POTENTIAL-ENERGY SURFACE OF ACETONE IN THE S-1 (N,PI(ASTERISK)) STATE

Examination has been made of the torsional potential energy surface of acetone in the S-1 (n,pi) state in order to clarify methyl-methyl in teraction, based on CASSCF calculations for two models (fully-relaxed and partially-relaxed). Skeletal changes during methyl torsion motion are 7 degrees for the C=O out-of-plane wagging angle, 0.003 Angstrom f or the C=O bond length, 2.7 degrees for the C-C-C bond angle, and 0.00 5 Angstrom for the C-C bond length. Skeletal flexing introduces the te rms sin 3(theta(1) - theta(2)) and sin 3 theta(1) - sin 3 theta(2) in the torsional potential function, where theta(1) and theta(2) are the torsional angles of the two methyl tops. Change in the C=O wagging ang le during torsion motion is shown to be a geometric relaxation with a decrease in steric hindrance rather than an increase in attractive int eraction. The large change in the C=O out-of-plane wagging angle indic ates that the torsional mode is coupled with the C=O out-of-plane wagg ing mode. Calculated frequencies for low-lying levels on the torsional potential are shown to be in good agreement with those observed.