Mass Spectrometric Fragmentation Reaction Mediated By Ion-Neutral Complexes And Dissociative Protonation

Ion-neutral complexes, as a kind of important intermediate in gas phase reaction, are usually suggested to explain a number of otherwise puzzling reactions of ions in the gas phase. In ESI-MS/MS experiment, ion-neutral complexes often result from dissociative protonation. In order to study ion-neutral complex resulting from dissociative protonation, two series ofα-aromaticmethyl benzyl ether compounds, namely,α-furanylmethyl p-substituted-benzyl ethers and 4-N, N-dimethylbenzyl p-substituted-benzyl ethers, are investigated by ESI-MS/MS. The loss of CH₂O during mass spectrometry in the two series of compounds is particularly interesting. The fragmentation mechanism is proposed to involve an ion-neutral complex-mediated pathway. Specifically, before the formation of an ion-neutral intermediate, the proton is transferred from the thermodynamically favored site at either the ether oxygen atom or the nitrogen atom to the dissociative protonation site at Cαposition in either the furyl group or the 4-N, N-dimethylphenyl group. This transfer has been clarified via computational studies and isotopically labeled experiments. In addition, the decomposition of the intermediate may be affected by the substituent groups on the phenyl ring. This conclusion is indicated by the reasonably good correlation between ln[([M+H-CH₂O]⁺)/([M+H-CH₂O-C₆H₅R]⁺)] and the substituent constants.