Structures, energetics, and reactions of [C₂H₃S] radicals and [C₂H₃S]⁺ ions : A Gaussian-2 ab initio study

A detailed computational study on the structures, energetics, and reactions of the isomers of the [C₂H₃S] radical and the [C₂H₃S]⁺ cation has been carried out. The computational models used are slightly modified versions of the ab initio Gaussian-2 method. Ten [C₂H₃S] isomers have been identified. Among them, the thioformylmethyl radical (1) has the lowest energy, with a Delta;H_f0 value of 214 kJ mol^-1. Other more chemically important isomers include the thioacetyl radical (2), thiiranyl radical (4), 1-thiovinyl radical (8), and 2-thiovinyl radical (11, 12, 13, 14); the Delta;H_f0 values for these isomers are 251, 301, 314, and 342-347 kJ mol^-1, respectively. In addition, we have also studied various reactions involving these radicals. For instance, it is found that isomerization reactions of 1 to 2, 4, 8, and 12 have barriers ranging from 123 to 227 kJ mol^-1. On the other hand, reaction 2 → 3 proceeds via a dissociation/recombination mechanism: 2 → CS + CH₃ → 3. The dissociation process is the rate-determining step, with a barrier of 187 kJ mol^-1. On the [C₂H₃S]⁺ potential energy surface; 12 isomers have been found. Among them, 2⁺ has the lowest energy, with a Delta;H_f0 value of 891 kJ mol^-1. Other isomers with Delta;H_f0values within 200 kJ mol^-1 of that of 2⁺ include 8⁺, 4+, 5⁺, 1,⁺ (triplet state of 1⁺), and S-protonated ethynylthiol (16⁺). Reactions involving these cations studied in this work include: 4⁺ → 2⁺ (barrier being 116 kJ mol^-1) and 4⁺ → 8⁺ (152 kJ mol^-1), both having 1⁺ as the transition structure; 1,2-H shift reaction 4⁺ → 5⁺ (219 kJ mol^-1); 1,3-H shift reaction 8⁺ → 16⁺ (232 kJ mol^-1); 8⁺ → 2⁺/4⁺ (162 kJ mol^-1); and 8⁺ → 5⁺ (171-175 kJ mol^-1) with two distinct pathways. The Delta;H_f0 values for various species calculated in this work are in good accord with available experimental measurements. Furthermore, the kinetics data reported here for reactions involving [C₂H₃S]⁺ cations are consistent with the results obtained in low-energy collision activated dissociation experiments. © 2000 American Chemical Society.