Torsional potential energy surface of acetone in the S 1 (n, [formula omitted]) state

Abstract

Examination has been made of the torsional potential energy surface of acetone in the S 1 (n, π ∗ ) state in order to clarify methyl-methyl interaction, based on CASSCF calculations for two models (fully-relaxed and partially-relaxed). Skeletal changes during methyl torsion motion are 7 ° for the CO out-of-plane wagging angle, 0.003 Å for the CO bond length, 2.7 ° for the CCC bond angle, and 0.005 Å for the CC bond length. Skeletal flexing introduces the terms sin 3( θ 1 − θ 2) and sin 3 θ 1 − sin 3 θ 2 in the torsional potential function, where θ 1 and θ 2 are the torsional angles of the two methyl tops. Change in the CO wagging angle during torsion motion is shown to be a geometric relaxation with a decrease in steric hindrance rather than an increase in attractive interaction. The large change in the CO out-of-plane wagging angle indicates that the torsional mode is coupled with the CO out-of-plane wagging mode. Calculated frequencies for low-lying levels on the torsional potential are shown to be in good agreement with those observed.