Coordinated dissociative proton transfers of external proton and thiocarbamide hydrogen: MS experimental and theoretical studies on the fragmentation of protonated S-methyl benzenylmethylenehydrazine dithiocarboxylate in gas phase

The dissociation chemistry of the protonated S-methyl benzenylmethylenehydrazine dithiocarboxylate, PhCHNNHC(S)SCH3, has been investigated by collision-induced dissociation (CID) mass spectrometry experiments in combination with density functional theory (DFT) calculations. Eliminations of H2S, CH3SH and (NSC)SCH3 were the three fragmentation reactions observed in the tandem mass spectra, witnessed by the MS/MS analysis of native 34S-isotopic ion and the D-labeling CID-MS experiment. Of the three fragmentations, both the added proton and the internal thiocarbamide hydrogen shift to the fragment ion (m/z 106) in the dissociation of losing (NSC)SCH3, while both of them shift to the neutral fragment H2S to generate the minor product ion at m/z 177. In the case of the feasible fragmentation process of CH3SH elimination, one of the proton/the thiocarbamide hydrogen migrates to the fragment ion at m/z 163, and the other migrates to the neutral specie. Calculated results show that thiocarbamide sulfur (S5) is the most thermodynamically favored position for protonation. The mechanisms of these reactions were postulated according to the theoretical results, and the reaction energy profiles were also constructed. These results indicated that fragmentation of the protonated molecule was viewed as a result of the coordinated migration of both the external proton and the thiocarbamide hydrogen.

The added proton and the thiocarbamide hydrogen have different transferring directions in the fragmentations: fragment ion, neutral product, or both.Figure optionsDownload high-quality image (70 K)Download as PowerPoint slide