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

Abstract

We present a systematic study of the reactions of SO +·( X 2Π r ), an important 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 CH 4, C 2H 6, C 3H 8, C 2H 2, C 2H 4, C 3H 4 (allene), n-C 3H 6, CH 3OH, C 2H 5OH, CH 3OCH 3, OCS, CH 2O, CH 3CHO, CH 3C(O)CH 3, HCO 2H, and HCO 2CH 3, and additionally the reactions of S 2 +· with C 2H 2 and O 2 +· with CH 4, C 2H 2, C 3H 4 (allene), n-C 3H 6, CH 3OCH 3, and HCO 2H at 294.5 ± 2.5 K. With just a few exceptions 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, OH, CH 3, OCH 3). Charge transfer, where it is exothermic, occurs in competition with the abstraction channels. Hydride abstraction is particularly prevalent, forming the thioperoxy radical, HSO ·, or its structural isomer, SOH ·. Hydroxide abstraction to form the hydroxysulfinyl radical, HOSO ·, occurs in some of the reactions with oxygen-bearing molecules. Where neutral, the abstraction products are inferred from the calculated reaction energetics; however, they are frequently detected directly in their protonated forms. This suggests a two-step reaction mechanism whereby competition for a proton occurs between leaving partners in the exit channel of the activated complex. In the reaction of SO +· with HCO 2CH 3, the protonated methoxysulfinyl radical, CH 3OSOH +·, 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 C–S and C–O bonds. The SO +· reactions are discussed in terms of their mechanisms, product formation, thermodynamics, and interstellar significance, and are compared with the related reactions of S 2 +· and O 2 +·.