A STRUCTURAL AB-INITIO STUDY OF THE T-1 TRIPLET-STATE OF ACETALDEHYDE- THE EFFECTS OF ELECTRON CORRELATION AND ADDITIONAL FUNCTIONS IN THEBASIS-SET

The UMP2(full)/6-311G(d,p) basis set has been used for the computation of the fully relaxed T-1 potential energy surface of acetaldehyde as a function of the methyl torsion and aldehyde hydrogen wagging coordin ates. The structure for the triplet state was very different from the ground electronic state in that a hydrogen from the methyl group was f ound to antieclipse the oxygen atom while the aldehyde hydrogen distor ted from the frame by 39.7 degrees. The calculation of the inversion a nd torsional barriers at the all electrons UMP4(SDTQ) level yielded re sults similar to the full UMP2 method. The use of the enhanced UMP2(fu ll)/6-311G(2d,p) and UMP2(full)/6-311+G(d,p) basis set did not signifi cantly improve the description of the potential energy surface. Eviden ce is presented that supports the hypothesis that the changes in the b arrier heights to inversion and torsion are the result of steric hindr ance.