AN ANALYSIS OF THE METHYL ROTATION AND ALDEHYDE WAGGING DYNAMICS IN THE SO((X)OVER-TILDE1A') AND T1((A)OVER-TILDE3A'') STATES OF THIOACETALDEHYDE FROM PYROLYSIS JET SPECTRA

Jet-cooled, laser induced phosphorescence (LIP) excitation spectra of thioacetaldehyde (CH3CHS, CH3CDS, CD3CHS, and CD3CDS) have been observed in the 15 800-17 300 cm-1 region in a continuous pyrolysis jet. The responsible electronic transition, T1 <-- S0, a3A" <-- X1A', results from an n --> pi* electron promotion and gives rise to a pattern of vibronic bands that can be attributed to activity of the methyl torsion and the aldehyde hydrogen out-of-plane wagging modes. Potential and kinetic energy surfaces were mapped out for the aldehyde wagging (alpha) and the torsional (THETA) internal coordinates by using 6-31G* Hartree-Fock calculations in which the structural parameters were fully relaxed. The potential and kinetic energy data points were fitted to double Fourier expansions in alpha and THETA and were incorporated into a two-dimensional Hamiltonian operator. The spectrum was simulated from the transition energies and the Franck-Condon factors and was compared to the observed jet cooled LIP spectra. It was concluded that while the RHF procedure gives a good description to the ground state dynamics, the triplet state surface generated by the UHF method is too bumpy and undulating.