Structure and unimolecular chemistry of protonated sulfur betaines, (CH3)2S+(CH2)nCO2H (n = 1 and 2)

Abstract

The fixed charge zwitterionic sulfur betaines dimethylsulfonioacetate (DMSA) (CH₃)₂S⁺CH₂CO₂⁻ and dimethylsulfoniopropionate (DMSP) (CH₃)₂S⁺(CH₂)₂CO₂⁻ have been synthesized and the structures of their protonated salts (CH₃)₂S⁺CH₂CO₂H⋯Cl⁻ [DMSA.HCl] and (CH₃)₂S⁺(CH₂)₂CO₂H⋯Pcr⁻ [DMSP.HPcr] (where Pcr = picrate) have been characterized using X-ray crystallography. The unimolecular chemistry of the [M+H]⁺ of these betaines was studied using two techniques; collision-induced dissociation (CID) and electron-induced dissociation (EID) in a hybrid linear ion trap Fourier transform ion cyclotron resonance mass spectrometer. Results from the CID study show a richer series of fragmentation reactions for the shorter chain betaine and contrasting main fragmentation pathways. Thus while (CH₃)₂S⁺(CH₂)₂CO₂H fragments via a neighbouring group reaction to generate (CH₃)₂S⁺H and the neutral lactone as the most abundant fragmentation channel, (CH₃)₂S⁺CH₂CO₂H fragments via a 1,2 elimination reaction to generate CH₃S⁺CH₂ as the most abundant fragment ion. To gain insights into these fragmentation reactions, DFT calculations were carried out at the B3LYP/6-311++G(2d,p) level of theory. For (CH₃)₂S⁺CH₂CO₂H, the lowest energy pathway yields CH₃S⁺CH₂via a six-membered transition state. The two fragment ions observed in CID of (CH₃)₂S⁺(CH₂)₂CO₂H are shown to share the same transition state and ion-molecule complex forming either (CH₃)₂S⁺H or (CH₂)₂CO₂H⁺. Finally, EID shows a rich and relatively similar fragmentation channels for both protonated betaines, with radical cleavages being observed, including loss of ˙CH₃.