Reactions of H3O+(H2O)n and H+(H2O)n(CH3COCH3)m with CH3SCH3

We have measured the rate constants and product branching fractions for the reactions of water clusters and mixed water/acetone clusters, H3O+(H2O)(n) and H+(H2O)(n)(CH3COCH3)(m), reacting with dimethylsulfide, CH3SCH3. The proton hydrates react rapidly. For n = 0-2, the reactions proceed at the collision rate within experimental error, while the reaction efficiency decreases slightly for n = 3 and further for n = 4 and n 5. The product distributions for the higher clusters are mainly switching one H2O for the CH3SCH3. The product distributions appear to be affected by thermal dissociation at higher temperatures. The reactions of H+(H2O)(n)(CH3COCH3)(m) with CH3SCH3 are also rapid and proceed by switching both a CH3COCH3 and a H2O out of the primary ion. The rate constant for H+(CH3COCH3)(2) is very slow, although the addition of two H2O ligands increases reaction by a factor of 13 and 21, respectively. The reactivity of H+(H2O)(CH3COCH3)(3) decreases to a value close to that of H+(CH3COCH3)(2). These results indicate that H3O+(H2O)(n) ions can be used as chemical ionization agents for CH3SCH3 detection in the atmosphere and that use of the mixed cluster ions, H+(H2O)(n) (CH3COCH3)(m), would be problematic. (C) 1998 Elsevier Science B.V.