SELECTED-ION FLOW TUBE STUDIES OF THE REACTIONS OF H3O-2(+) WITH SOMEORGANOSULFUR MOLECULES(, NO+ AND O)

We report the results of a selected ion flow tube (SIFT) study of the reactions of H3O+ NO+ and O-2(+) with eight organosulphur molecules, M . If we assume that all the exothermic proton transfer reactions of H( 3)O(+)occur at the collision rate (i.e. the rate coefficients, k, are equal to the collisional rate coefficient, k(c)), it is seen that the k for most of the NO+ and O-2(+) reactions also are equal to or close to k(c). However, the reaction of H3O+ with one of the chosen reactant s, CS2, is known from our previous SIFT study to be slightly endotherm ic, and therefore it proceeds more slowly than the collisional rate. T he ionic product of seven of the H3O+ reactions is only the protonated parent molecule MH+, but in one reaction, that of thiolacetic acid CH 3COSH, elimination of an H2S molecule occurs in a fraction of the coll isions. The NO+ reactions mostly proceed via charge transfer, producin g the parent molecular ion M+, but the reaction with ethanthiol is unu sually complicated, with four processes occurring in parallel (charge transfer, H- and SH- transfer, and ion-molecule association), and the reaction with thiolacetic acid occurs exclusively and rapidly via SH- transfer. The O-2(+) reactions proceed by dissociative charge transfer , often with the production of three or more ionic fragments of the pa rent molecular ion. Sample mass spectra obtained using our SIFT method of analysis of the vapours emitted by cut onion and crushed garlic ar e presented and discussed, noting the presence of some organosulphur m olecules and other organic species. (C) 1998 Elsevier Science B.V.