Gas-phase reactivity of SO+center dot: a selected ion flow tube study

We present a systematic study of the reactions of SO+.(X (2)Pi(r)), an impo rtant ion in space plasmas, with organic molecules of interstellar interest . A selected ion flow tube has been used to investigate the reactions of SO +. with CH4, C2H6, C3H8, C2H2, C2H4, C3H4 (allene), n-C3H6, CH3OH, C2H5OH, CH3OCH3, OCS, CH2O, CH3CHO, CH3C(O)CH3, HCO2H, and HCO2CH3, and additionall y the reactions of S-2(+.) with C2H2 and O-2(+.) with CH4, C2H2, C3H4 (alle ne), n-C3H6, CH3OCH3, and HCO2H at 294.5 +/- 2.5 K, With just a few excepti ons the reactions proceed at or near their theoretical collisional capture rates. Apart from the smaller and more saturated hydrocarbons and OCS, the reactions of SO+. are dominated by heterogenic abstractions of R- (R = H, O H, CH3, OCH3). Charge transfer, where it is exothermic, occurs in competiti on with the abstraction channels. Hydride abstraction is particularly preva lent, forming the thioperoxy radical, HSO., or its structural isomer, SOH.. Hydroxide abstraction to form the hydroxysulfinyl radical, HOSO., occurs i n some of the reactions with oxygen-bearing molecules. Where neutral, the a bstraction products are inferred from the calculated reaction energetics; h owever, they are frequently detected directly in their protonated forms. Th is suggests a two-step reaction mechanism whereby competition for a proton occurs between leaving partners in the exit channel of the activated comple x. In the reaction of SO+. with HCO,CH,, the protonated methoxysulfinyl rad ical, CH3OSOH+., is observed for the first time. The reactions of SO+. with the smaller unsaturated hydrocarbons are more complex, and largely involve rupture of the S-O bond and a C-C bond to form products containing CS and C-O bonds. The SO+. reactions are discussed in terms of their mechanisms, p roduct formation, thermodynamics, and interstellar significance, and are co mpared with the related reactions of S-2(+.) and O-2(+.). (C) 2000 Elsevier Science B.V.