The generation, stability, dissociation and ion/molecule chemistry of sulfinyl cations in the gas phase

Abstract

Sulfinyl cations [R–S⁺O (R = CH₃, Ph, Cl, CH₃O and C₂H₅O)] have been demonstrated by MO calculations in conjunction with pentaquadrupole multidimensional (2D and 3D) MS² and MS³ mass spectrometric experiments to be stable and easily accessible gas phase species, and their dissociation and ion/molecule chemistry have been studied. Potential energy surface diagrams indicate that the sulfoxides (CH₃)₂SO, Ph₂SO, Cl₂SO, (CH₃O)₂SO and (C₂H₅O)₂SO do not undergo rearrangement upon dissociative ionization, yielding the corresponding sulfinyl cations as primary fragments. Ph(CH₃)SO⁺˙, on the other hand, is predicted to isomerize to CH₃–S–O–Ph⁺˙via a four-membered ring transition state, yielding upon further CH₃˙ loss the isomeric ion SO⁺–Ph. The sulfinyl cations were found by ab initio calculations to be much more stable than their SO⁺–R isomers, hence isomerization via[1,2-R] shifts is not expected. Direct cleavage of the R–SO⁺ bonds and/or processes that are preceded by isomerization dominate the low-energy collision dissociation chemistry of the sulfinyl cations, thus providing limited structural information. On the other hand, a general and structurally diagnostic ion/molecule reaction with 2-methyl-1,3-dioxolane occurs for all the sulfinyl cations yielding abundant net oxirane (C₂H₄O) addition products. The reaction probably occurs via a transketalization-like mechanism that leads to cyclic 2-thia-1,3-dioxolanylium ions. This reactivity parallels that of several acylium (R–C⁺O) and thioacylium ions (R–C⁺S), and is not shared by the isomeric ions SO⁺–Ph and CH₂S⁺–OH. While the corresponding acylium ions react extensively with isoprene by [4 + 2⁺] cycloaddition, only the phenylsulfinyl cation Ph–S⁺O yields an abundant cycloadduct.