An analysis of the methyl rotation and aldehyde wagging dynamics in the S (X̃ 1A′) and T1 (ã 3A″) states of thioacetaldehyde from pyrolysis jet spectra

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, ã 3A″← 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.

Isomerization and Fragmentation Reactions on the [C2SH4] Potential Energy Surface: The Metastable Thione S-Methylide Isomer

“…Contrary to its oxygen analog, THA is an unstable species in the gas phase (half-life of about 10 s), which polymerizes readily, usually forming cyclic trimers. The monomeric species was isolated and characterized in 1974 by photoelectron spectroscopy, followed by several other spectroscopic studies, − often combined with quantum chemical calculations. − THA is a C s molecule with eclipsed methyl and carbonyl groups, whose thermal stability and unimolecular reaction pathways were studied by Ding et al…”

Isomerization and Fragmentation Reactions on the [C2SH4] Potential Energy Surface. The Metastable Thione-S-Methylide Isomer

“…Contrary to its oxygen analog, THA is an unstable species in the gas phase (half-life of about 10 s), 35 which polymerizes readily, usually forming cyclic trimers. The monomeric species was isolated and characterized in 1974 by photoelectron spectroscopy, 35 followed by several other spectroscopic studies, [38][39][40][41][42] often combined with quantum chemical calculations. [43][44][45][46] THA is a planar (Cs) molecule with eclipsed methyl and carbonyl groups, whose thermal stability and unimolecular reaction pathways were studied by Ding et al 47 We reported previously a speculative path to TSM 48,49 based on a detailed computational study of the H-abstraction path in the oxidation of DMS by OH radicals under the hypothesis of high hydroxyl concentration.…”