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.