The Thermal Decomposition of Thiirane: A Mechanistic Study by Ab Initio MO Theory

Abstract

Using high-level ab initio MO methods, we have identified two reaction pathways with different thermodynamic and kinetic properties for the thermal decomposition of the three-membered heterocycle thiirane (C2H4S) and related derivatives. A homolytic ring opening, followed by attack of the generated diradical on another thiirane molecule, and subsequent elimination of ethene in a fast radical chain reaction results in the formation of disulfur molecules in their triplet ground state (3S2) and requires activation enthalpies of deltaH#(298) = 222 kJ mol(-1) and deltaG#(298) = 212 kJ mol(-1). This reaction mechanism would result in a first-order rate law in agreement with one reported gas-phase experiment but does neither match the experimental activation energy nor does it explain the observed retention of the stereochemical configuration in the thermal decomposition of certain substituted thiiranes. Alternatively, sulfur atoms can be transferred from one thiirane moleculeto another with the intermediate formation of thiirane 1-sulfide (C2H4S2). This molecule can either decompose unimolecularly to ethene and disulfur in its excited singlet state (1S2) or, by means of spin crossover, S2 in its triplet ground state may be formed. On the other hand, the thiirane 1-sulfide may react with itself and transfer one sulfur atom from one molecule to another with formation of thiirane 1,1-disulfide (C2H4S3), which is an analogue of thiirane sulfone; thiirane is formed as the second product. The 1,1-disulfide may then decompose to ethene and S3. In still another bimolecular reaction, the thiirane 1-sulfide may react with itself in a strongly exothermic reaction to give S4 and two equivalents of ethene. This series of reactions results in a second-order rate law and requires activation enthalpies of deltaH#(298) = 109 kJ mol(-1) and deltaG#(298) = 144 kJ mol(-1) for the formation of thiirane 1-sulfide, while the consecutive reactions require less activation enthalpy. Elemental sulfur (S8) is eventually formed by oligomerization of either S2, S3, or S4 in spin-allowed reactions. These findings are in agreement with most experimental data on the thermal desulfurization of thiirane and its substituted derivatives. Thiirane 1-persulfide (C2H4S3) with a linear arrangement of the three sulfur atoms as well as zwitterions and radicals derived from thiirane are not likely to be intermediates in the thermal decomposition of episulfides.

Encapsulation and convex‐face thiozonolysis of triatomic sulfur (S3) with carbon nanotubes

“…Compound 7 has S1-S2 and S2-S3 bond lengths of 1.94 Å and a structure analogous to a thiirane sulfone. [81] As expected, the 1,3-dipolar addition of S 3 at the 6,6 site was preferred compared with the [2 + 2] cycloaddition and chelotrope paths. The computed barriers of paths A-C were 5.6 kcal/mol, 28.0 kcal/mol, and 42.6 kcal/mol, respectively.…”

“…Path C involves formation of the C1–S2 and C2–S2 bonds of TS4/7 to give C 2v symmetric thiirane‐1,1‐disulfide 7 . Compound 7 has S1–S2 and S2–S3 bond lengths of 1.94 Å and a structure analogous to a thiirane sulfone …”

5-Iodo-4-thio-2′-Deoxyuridine as a Sensitizer of X-ray Induced Cancer Cell Killing

“…Using the same model as before (with explicitly added water molecules), we obtained the barrier for unstable oxathiirane formation equal to 24.5 kcal/mol and the subsequent barrier-free opening to IOSU with an overall thermodynamic stimuli of −4.9 kcal/mol, as shown in Scheme 2. This product was then ready to extrude the sulfur, giving IU in the final step, as suggested in the literature (Scheme 2) [36,37,38,39,40].…”

Reactive Site Model of the Reduction of SO2 on Graphite

“…1.30 (N 2 ) 21.94 (CO 2 ) ) for the thermolysis of thiirane C 2 H 4 S is much lower than the enthalpy of the hypothetical sulfur extrusion reaction (58 kcal mol −1 ), 72 and therefore, the reaction does not proceed through a monomolecular mechanism, eq 3, but through several bimolecular reactions as was shown by theoretical calculations. 73 However, such a mechanism is not acceptable for reactions taking place on rigid carbon matrixes.…”