Elementary reactions of the SCl radical. Part1.—Rate constants and mechanisms of the reaction Cl + C2H4S → SCl + C2H4, SCl + SCl → products and SCl + Cl2→ SCl2+ Cl

Abstract

The reaction of Cl atoms with ethylene sulphide, C₂H₄S, has been used as a source of SCl radicals in a discharge-flow system with detection of free-radical and molecular species by molecular-beam-sampling mass spectrometry. The rate constant for the overall reaction of Cl with C₂H₄S Cl + C₂H₄S → products (1) was found to be k₁=(6.6 ± 0.9)× 10^–11 cm³ molecule^–1 s^–1(1σ) at 292 K from the decay of C₂H₄S in an excess of Cl. By measuring the absolute yield of C₂H₄ from this reaction the branching ratio for the formation of SCl and C₂H₄ by the direct sulphur abstraction channel (1a) was found to be 0.70 ± 0.08 (1 σ): Cl + C₂H₄S → SCl + C₂H₄. (1a) A minor hydrogen abstraction channel could be identified. The rate constant and branching ratio for the bimolecular self-reaction SCl + SCl → products (2) was also determined. This reaction proceeds mainly by disproportionation to S₂Cl + Cl (2a) with ⩽ 11 ± 4% forming the disproportionation products SCl₂+ S(2b). The four-centre reaction (2c) forming S₂+ Cl₂ may also occur to some extent. A rate constant (k_2a+k_2c)=(6.0 ± 2.4)× 10^–11 cm³ molecule^–1 s^–1(1σ) was measured at 295 K, where k is defined by –d[SCl]/dt= 2k[SCl]². The results are compared with the analogous reactions of the SF, SH and oxyhalide XO radicals (X = F, Cl, Br). In addition, the rate constant was estimated for the reaction SCl + Cl₂→ SCl₂+ Cl; k²⁹⁵₃≈ 7 × 10^–14 cm³ molecule^–1 s^–1. (3) This value of k₃ was used, together with available thermochemical data, to calculate an upper limit for the rate constant for the reverse reaction SCl₂+ Cl → SCl + Cl₂(3′) of 1 × 10^–18 cm³ molecule^–1 s^–1 at 295 K.