THEORETICAL DETERMINATION OF THE TORSION-WAGGING STRUCTURE OF THE S1[-S0 ELECTRONIC-SPECTRUM OF ACETALDEHYDE

The molecular structure and the potential energy hypersurfaces of acet aldehyde for the S0 state were determined by CISD/RHF/4-31G* ab initi o molecular orbital calculations and CISDT/ROHF/4-31G* for the S1 sta te. The torsional-wagging energy levels were evaluated by the variatio nal method using free rotor basis functions expressed as symmetrized d ouble Fourier expansions. The relative strengths of the Herzberg-Telle r vibronically induced transitions and the direct Franck-Condon electr onically allowed transitions were calculated from the transition dipol e moments. The torsional-wagging spectrum simulate from the two-dimens ional calculation was compared to the S1 <-- S0, n --> pi jet-cooled fluorescence excitation spectrum.