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 c arried 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 isomer s include the thioacetyl radical (2), thiiranyl radical (4), 1-thiovinyl ra dical (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 radical s. 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, wi th 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-f0 values w ithin 200 kJ mol(-1) of that of 2(+) include 8(+), 4(+), 5(+), 1,(+) (tripl et state of 1(+)), and S-protonated ethynylthiol (16(+)). Reactions involvi ng 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 th e 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 pa thways. The Delta H-f0 values for various species calculated in this work a re in good accord with available experimental measurements. Furthermore, th e kinetics data reported here for reactions involving [C2H3S](+) cations ar e consistent with the results obtained in low-energy collision activated di ssociation experiments.