Thione S-methylide (TSM), the parent species of the thiocarbonyl ylide family, is a 1,3-dipolar, planar species on the
[C2SH4] potential energy surface (PES), which has not shared the richness of studies dedicated to its isomers, the
cyclic thiirane (THI), and the keto-enol pair vinyl thiol (VTH)/thioacetaldehyde (THA). While the conrotatory ring
closure reaction toward THI was studied in the ‘90s, no complete analysis of the PES is available in the literature. In
the present paper, we report a computational study of the reaction scheme linking all species on that PES. We employ
several levels of calculation, ranging from density functional theory (DFT), through CCSD(T) based composite
schemes, to CASSCF/CASPT2 multi-reference procedures, to find the best description of TSM, its isomers, and the
transition states (TSs) ruling their interconversion. Fragmentation of TSM, THA and THI were investigated and
compared to the available experimental information. We found that the B2PLYP-D3 functional, contrary to M06-2XD3 or B97X-D, describes well the geometry of both TSM and the transition state connecting it to THI. The reverse
barrier, from THI to TSM, amounts to 52.2 kcal mol-1
(to be compared to 17.6 kcal mol-1
for the direct one), thus
explaining why, in general, thiocarbonyl ylides cannot be prepared from thiiranes. Conversion of THI to VTH implies
also a large barrier, explaining why the reaction has been observed only at high temperatures. The fragmentation of
THI to S(3P) or S(1D) and ethylene was also explored, together with the decomposition to H2S plus acetylene. Open
species, both in triplet and singlet states, were identified as intermediates in the fragmentations, and their energies
were found to be lower than the transition state for the isomerization of THI to VTH, thus explaining the preference
for fragmentation over isomerization at relatively low temperatures.
Computational Study of PmHMGR Thiohemiacetal Breakdown Transition State