WAGGING AND TORSION VIBRONIC STRUCTURE IN THE T-1 [- S-0 ELECTRONIC-SPECTRUM OF ACETALDEHYDE

The T-1 <-- S-0 electronic band spectrum in acetaldehyde was simulated from RHF and UHF/MP2/6-311G(d,p) ab initio calculations for the two e lectronic states. The torsion-wagging energy levels were evaluated by the variational method using free rotor basis functions expressed as s ymmetrized double-Fourier expansions. A comparison of the calculated b and spectrum to the long-path absorption spectrum allowed for the assi gnment of a number of clearly defined bands and placed the origin at 2 7 240 cm(-1). The calculated height of the barrier to internal rotatio n for the lower S-0 state of 415 cm(-1) yielded levels in excellent ag reement with the torsional levels derived from far infrared measuremen ts. The calculated T-1 excited-state barrier heights for the pure tors ion and wagging motions were 647 and 968 cm(-1). The corresponding val ues in the S-1 state are 709 and 776 cm(-1).