ELECTRON CORRELATION ON TORSIONAL AND INVERSION BARRIER HEIGHTS OF ACETALDEHYDE AND ACETONE IN THE S-1(N,PI-ASTERISK) STATE - CASSCF AND CAS-MP2 STUDY

The torsional and inversion barrier heights of acetaldehyde and aceton e 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 b y the multiconfigurational second-order perturbation theory (CAS-MP2). The CASSCF wavefunction largely overestimates the inversion barrier h eight, while it slightly underestimates the torsional barrier height. The second-order correction to the torsional barrier height is positiv e, but that to the inversion barrier height is negative. The results o f this study suggest that the nondynamical electron correlation, espec ially the correlation of pi and 3p(x)(O) electrons for the torsional b arrier and the dynamical electron correlation for the inversion barrie r, is important. Calculated torsional and inversion barrier heights ag reed reasonably with experimental results. (C) 1997 Elsevier Science B .V.