The formation of the sulfur halides SX4 from SX2 and X-2: Reaction enthalpies, transition states, and activation energies for X = F and Cl

High-level ab initio MO calculations by different methods demonstrate that the reaction of SF2 with F-2 to form SF4 is strongly exothermic and exergonic [CCSD(T)/6-311+G(2df)//MP2/6-311+G*: Delta H(298)degrees = -445 kJ mol(-1), Delta G(298)degrees = -398 kJ mol(-1)] and proceeds via a very weakly bonded intermediate 2 with C-s symmetry. The structure of 2 corresponds to a donor-acceptor complex F2S-->F-2 with a considerable transfer of charge (0.69 e). The transition state (TS1) between 2 and SF4 has a similar structure to 2 but with C-1 symmetry. The energy of the adduct and of TS1 is almost the same as the combined energy of the separate molecules SF2 and F-2. Therefore, the overall activation energy for the reaction of SF2 with F-2 to form SF4 is practically negligible (6 kJ mol(-1) at 298 K). The analogous reaction of SCl2 with Cl-2 to form the hypothetical molecule SCl4 is endothermic and endergonic (Delta H(298)degrees = +32 kJ mol(-1), Delta G(298)degrees = +74 kJ mol(-1)). The ground-state geometry of SCl2 has C-2v symmetry at the MP2/6-311+G* level and the coordination geometry at sulfur is pseudo-trigonal-bipyramidal. The transition state (TS2) of this reaction is ionic ([SCl3+]Cl-) and has C-1 symmetry. The activation energy at 0 K for the chlorination of SCl2 amounts to 203 kJ mol(-1) at the CCSD(T)/6-311+G(2df) level; it is 43 kJ mol(-1) lower than the experimental bond dissociation energy of Cl-2. The proposed formation of the analogous tetrathiasulfuranes S(SR)(4) from (RS)(2)S and RSSR is discussed in connection with the various interconversion reactions of polysulfur compounds. [References: 56]

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