A Comparison of the Methyl Torsion and Aldehyde Wagging Dynamics in the Ground and First Excited States of Acetaldehyde and Thioacetaldehyde

Abstract

The vibronic fine structure in the S0 → S1 and S0 → T1 electronic spectra of acetaldehyde and thioacetaldehyde which results from n → π* excitation can be attributed to the activity of the methyl torsion and the aldehyde hydrogen out -of - plane wagging modes. The band patterns in these two spectra were calculated from ab initio calculations and were compared to the jet cooled laser induced fluorescence (LIF) and laser induced phosphorescence (LIP) spectra. For this purpose, the kinetic and potential energy surfaces were mapped out for the aldehyde wagging (α) and the methyl torsion (Θ) internal coordinates with RHF, ROHF and UHF procedures and were corrected for electron correlation with CI. The energy values, as well as the kinetic parameters were fitted to double Fourier expansions as functions of the rotational angles. The solutions were developed on the basis of the symmetry eigenvectors of the G6 nonrigid group, which factorize the Hamiltonian matrix into boxes. Intensities were obtained from the calculated electronic transition moment variation and the nuclear statistical weights and were combined with the transition energies to predict the spectra.