The isomers of ionized dimethyl sulfoxide (C2H6OS+·) and their CH3OS+ fragments. An ab initio and multiple‐stage mass spectrometric (MSn) study

Abstract

AbstractThe relative stabilities, isomerizations and dissociations of ionized dimethyl sulfoxide (DMSO), its three C2H6OS+· isomers and of all their 14 conceivable CH3OS+ fragments (1–14), have been investigated by ab initio calculations at the MP2/6–31G(d,p)//6–31G(d,p) + ZPE and G2 levels of theory, and by multiple‐stage two‐ and three‐dimensional mass spectrometry performed in a pentaquadrupole instrument. The ab initio relative energies of the isomers, their connecting transition states, and their dissociation thresholds were used to elaborate potential energy surface diagrams that precisely corroborate and unify several previously divergent experimental observations on these systems. The most kinetically favorable isomerization of (CH3)2SO+· (I) to its aci‐form CH2S(OH)CH (II) displays a transition state considerably lower in energy than the threshold for its direct dissociation by CH3⋅ loss. Therefore, low‐energy, long‐lived metastable ions I are predicted to isomerize to II, and to dissociate in turn to CH2S+OH (2) upon CH3⋅ loss. Ions I excited a few electronvolts above the threshold are, on the other hand, predicted to dissociate directly to CH3S+O (1). Isomerization of I to the most stable C2H6OS+· ion, that is CH3SOCH (III), displays the most energetic transition state and should be negligible, whereas III is predicted to dissociate by CH3⋅ loss preferentially to 1 rather than 5 (CH3OS+). Nine CH3OS+ isomers, that is 1, 2, (HCS…OH2)+ (3), +CH2S(O)H (4), 5, HC(SH)OH+ (9), CH2OSH+ (10), CH2+OSH (11), and (HCO…SH2)+ (14), were found as true minima on the RHF/6–31G(d,p) potential‐energy surface, and some of their isomerization barriers and dissociation thresholds were estimated. Tandem and multiple‐stage (MS3) mass spectrometric experiments show that non‐dissociating DMSO+· ions produce, upon collision‐induced dissociation (CID), a mixture of approximately 40% of 1 and 60% of 2, whereas the CID chemistry of 1 and 2 is affected considerably by the collision energies employed. Both the experimental and theoretical results on 1 and 2 allow a detailed interpretation of their complex dissociation chemistry, which clarifies the nature of most of their indirect fragments. Such fragments are proposed to be formed via the common isomerization/dissociation sequences 1 → 2 → 9 ⇌ 3 → HCS+ (m/z 45) + H2O, and 1 → 2 → [7, HOCH2S+] → S + CH2OH+ (m/z 31). These processes are favored at lower collision energies, whereas direct dissociation of 1 to CH3+ (m/z 15) and SO+· (m/z 48), and of 2 to CH2S+· (m/z 46) occurs to greater extents at higher collision energies.