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 (k2a+k2c)=(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.