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

Abstract

Thione S-methylide, parent species of the thiocarbonyl ylide family, is a 1,3dipolar species on the [C 2 SH 4 ] potential energy surface, not so much studied as its isomers, thiirane, vinyl thiol, and thioacetaldehyde. The conrotatory ring-closure reaction toward thiirane was studied in the 90s, but no complete analysis of the potential energy surface is available. In this paper, we report a computational study of the reaction scheme linking all species. We employed several computational methods (density functional theory, CCSD(T) composite schemes, and CASSCF/CASPT2 multireference procedures) to find the best description of thione S-methylide, its isomers, and transition states. The barrier from thiirane to thione Smethylide amounts to 52.2 kcal mol −1 (against 17.6 kcal mol −1 for the direct one), explaining why thiocarbonyl ylides cannot be prepared from thiiranes. Conversion of thiirane to vinyl thiol implies a large barrier, supporting why the reaction has been observed only at high temperatures. Fragmentations of thiirane to S( 3 P) or S( 1 D) and ethylene as well as decomposition to hydrogen sulfide plus acetylene were also explored. Triplet and singlet open-shell species were identified as intermediates in the fragmentations, with energies lower than the transition state between thiirane and vinyl thiol, explaining the preference of the latter at low temperatures.

Exploring the Maze of Cycloserine Conformers in the Gas Phase Guided by Microwave Spectroscopy and Quantum Chemistry

“…This level of theory is hereafter denoted as rDSD. Improved electronic energies for the most significant low-energy minima and the transition states ruling their interconversion were obtained by exploiting explicitly correlated CCSD­(T)-F12 computations within the so-called CCSD­(T)-F12/CBS+CV composite scheme , (hereafter CCF12). Briefly, while interested readers can find a detailed description of this model in ref and in Supporting Information (SI), we mention that CC12 recovers basis-set truncation errors and accounts for core-correlation effects.…”

Dipolar 1,3‐cycloaddition of thioformaldehydeS‐methylide (CH2SCH2) to ethylene and acetylene. A comparison with (valence) isoelectronicO3,SO2,CH2OOandCH2SO

“…The parent compound of the thiocarbonyl ylide family, thioformaldehyde S ‐methylide (TSM), was initially recognized by Knott in 1955 18 and later studied by several research groups using different spectroscopic and theoretical methods 11–15,19,20 . However, after the work by Sustmann et al 20 in 2003 concerning the mechanism of cycloaddition of TSM, the only available theoretical studies are those performed in the last 2 years by our groups 21–23 . In particular, we started by proposing an original path leading to TSM based on a detailed computational study of the H‐abstraction path in the oxidation of dimethyl sulfide (DMS) by OH radicals, under the hypothesis of high OH concentration 21,22 .…”

“…We identified a reaction channel that could be open for the transformation of DMS into a closed shell planar structure where two methylene groups are bound to the central sulfur, namely TSM. Next, we discussed the structural details in the context of the stationary points ruling the [SC 2 H 4 ] potential energy surface (PES) 23 . Further studies on the reactivity of TSM are of current interest, and in this paper, we analyze in detail its behavior in 1,3‐dipolar cycloadditions, with specific reference to the well‐known process of ozonolysis.…”

“…[11][12][13][14][15]19,20 However, after the work by Sustmann et al 20 in 2003 concerning the mechanism of cycloaddition of TSM, the only available theoretical studies are those performed in the last 2 years by our groups. [21][22][23] In particular, we started by proposing an original path leading to TSM based on a detailed computational study of the H-abstraction path in the oxidation of dimethyl sulfide (DMS) by OH radicals, under the hypothesis of high OH concentration. 21,22 We identified a reaction channel that could be open for the transformation of DMS into a closed shell planar structure where two methylene groups are bound to the central sulfur, namely TSM.…”

“…Next, we discussed the structural details in the context of the stationary points ruling the [SC 2 H 4 ] potential energy surface (PES). 23 Further studies on the reactivity of TSM are of current interest, and in this paper, we analyze in detail its behavior in 1,3-dipolar cycloadditions, with specific reference to the well-known process of ozonolysis.…”