An analysis of the methyl rotation and aldehyde wagging dynamics in the S0(X̃¹A′) and T1(ã³A″) states of thioacetaldehyde from pyrolysis jet spectra

Authors:

• D.C. Moule,
• H.A. Bascal,
• Y.G. Smeyers,
• D.J. Clouthier,
• Jerzy Karolczak,
• A. Niño

Abstract

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, ã³A″ ← X̃ ¹A′, results from an n → π* 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 (α) and the torsional (Θ) 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 α and Θ 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. © 1992 American Institute of Physics.