Electron correlation on torsional and inversion barrier heights of acetaldehyde and acetone in the S 1(n,π ∗) state: CASSCF and CAS-MP2 study

Abstract

The torsional and inversion barrier heights of acetaldehyde and acetone in the S 1 state were studied using CASSCF and CAS-MP2 wavefunctions, where the geometries of the conformers were optimized at the CASSCF level and the dynamical electron correlation energies were evaluated by the multiconfigurational second-order perturbation theory (CAS-MP2). The CASSCF wavefunction largely overestimates the inversion barrier height, while it slightly underestimates the torsional barrier height. The second-order correction to the torsional barrier height is positive, but that to the inversion barrier height is negative. The results of this study suggest that the nondynamical electron correlation, especially the correlation of π and 3p x (O) electrons for the torsional barrier and the dynamical electron correlation for the inversion barrier, is important. Calculated torsional and inversion barrier heights agreed reasonably with experimental results.