O-protonated sulfine CH2=S=OH+. Its generation from ionized dimethylsulfoxide by delayed dissociation

Mass spectrometry based experiments show that dimethylsulfoxide (DMSO) radical cations, CH3S(=O)CH+.3, dissociating with rate constants < 105 s−1 form not CH3S=O+, 1, but rather the more stable isomer CH2=S=OH+, 2, via the aci-tautomer CH2=S(OH)CH+.3. It is proposed that the previously measured appearance energy for loss of CH3 from DMSO corresponds to 2, leading to ΔHf(2)= 736 kJ/mol. Ab initio calculations indicate that ΔHf(1) may need to be revised upwards to about 815 kJ/mol. Previous labelling results on DMSO are in excellent agreement with the formation of 2.