Features of the thioformaldehyde potential energy surfaces

Abstract

The structures of seven minima and five transition states of the S0 and T1 potential energy surfaces of thioformaldehyde have been located at the 3-21G* SCF level. Further calculations have been carried out to determine harmonic vibrational frequencies and to examine the effects of larger basis sets and of configuration interaction on energy differences. The molecular dissociation limit of H2 and CS is thermodynamically accessible at the energy of the lowest n,π* excited states and the singlet thiohydroxymethylenes lie only slightly too high. However, there are large barriers of ~85 to 90 kcal mol−1 to the molecular dissociation or to the 1,2-hydrogen shifts from thioformaldehyde to the thiohydroxymethylenes. The dissociation to H and HCS requires ~85.4 kcal mol−1 on the ground singlet and faces a barrier of several kcal mol−1 relative to products on the triplet surface. Any unimolecular photochemistry of thioformaldehyde is likely to require excitation to higher excited states than the lowest n,π* states.

Basis sets and restricted Hartree-Fock instabilities in thiocarbonyls

“…Table III presents results obtained optimizing the molecular geometry for ground state thioformaldehyde. Many of the results reported here for the smaller basis sets were, as in the case of Table 11, also reported previously by others [3][4][5]7]. The possibility of optimizing to a C , structure was permitted, but in each case the optimized structure became C,, .…”

“…Pope, Hillier, and Guest [3]* have reported very comprehensive studies using three basis sets, two including polarization functions, optimized RHF structures, and single-point CI calculations of 21 important extrema on the H,CS and H2CS+ potential surfaces and have carefully integrated discussion of these extrema with previous theoretical and experimental studies. Goddard's most recent contribution to thioformaldehyde [4] considers many of the same extrema characterized by Pope, Hillier, and Guest with a similar degree of rigor. De Mare [5] has reported RHF and CI optimized geometries using the 3-21G basis set for the ground and lower excited states of thioformaldehyde.…”

Thioamide spectroscopy: long path length absorption and quantum chemical studies of thioformamide vapour, CHSNH2/CHSND2

“…he magnitude of this lengthening is probably somewhat overestimated (19). However, the thiocarbonyl stretch, v;, also should be active in the spectrum.…”

Quantum chemical studies of alkyldinitrogen species implicated in nitrosamine carcinogenesis

“…The geometry optimization procedure is the Berny (gradient method) (17) and the Hessian matrices were examined in order to ensure that all the eigenvalues are positive and, as such, the stationary point is a true minimum. SCF calculations with split valence or larger basis sets are known usually to predict accurate geometries (16)(17)(18)(19). The net atomic charges are calculated via the Mulliken Population Analysis.…”