Studies Of Gas-phase Ion Reactions Of N-containing Organic Compounds In ESI Mass Spectrometry

Electrospray mass spectrometry is an important tool for studying organic chemistry and reactive organic ions. In recent years, the rapid development and application of mass spectrometry technique has opened up a new way for organic reactive intermediates research. But the use of mass spectrometry techniques to study organic reactive intermediates is still in the initial stage. This work tries to study carbenium ions, carbanions, benzyl cations, and phenylnitrenium ions from nitrogen containing compounds by electrospray mass spectrometry, unimolecular reaction and ion/molecule reactions. It specifically includes the following four parts:1. Unexpected methyl transfer occurs in the fragmentation of protonated pyridyl carbamates during tandem mass spectrometry. Here we found that methyl transfers take place at the meta, ortho, and para positions of substituted analogues. Ethyl and n-butyl transfers occur via similar fragmentation routes. According to our conventional understanding of the reaction mechanism, the proton is dissociative in protonated compounds. However, the present results suggest that the proton is localized and the alkyl cation transfer occurs at a charge-remote site. The configuration of the alkyl cation would invert to facilitate a lower-energy-barrier channel for its transfer to the oxygen. Our research not only provides a vital supplement to studies on alkyl cation transfer in the gas phase, but also helpful to understand the proton transfer mechanism of amides and pyridine analogues.2. Deprotonated N-benzylidene-2-hydroxylanilines with different functional groups were studied by negative ion ESI-MS/MS, and the proposed dissociation pathways were elucidated. The proton from the CH=N transfers to the phenol oxygen with the loss of benzonitrile. Proton abstraction occurs within the intermediate formed from nucleophilic addition reaction, resulting in the formation of benzene and benzoxazole anion. The presence of electron-withdrawing groups on ring B is in favor of forming phenyl anion, whereas the electron-donating groups strongly favor the proton abstraction reaction to generate benzoxazole anion. Compounds that contain heterocycles or substitutions on ring A also present the corresponding ions in the CID experiments. Density functional theory calculations were invoked to investigate the mechanisms behind these reactions.3. The fragmentation of protonated imine resveratrol analogues with different functional groups were studied by ESI-MS/MS, and the proposed dissociation pathways were elucidated. The specific elimination of the quinomethane neutral, CH2=C6H4=O via the corresponding ion-neutral complexes was observed. The fragmentation pathway for the related meta-isomer ion, para-methyl analogue ion, and the other congeners, was not observed. Para- and ortho-hydroxylbenzyl cation can form an ion-neutral complex and the following rotation of the hydroxylbenzyl cation leads to the elimination of CH2=C6H4=O neutral. It enriches the knowledge of gas-phase chemistry of benzyl cations, ortho hydroxyl effect and ion-neutral complex-mediated reactions.4. Gas-phase phenylnitrenium ions were produced from readily available anilines with convenient electrospray ionization tandem mass spectrometry.Taking advantage of the new method reported herein and ion-molecule reaction, the first examination of the gas-phase acidity, hydride affinity, electronic states and other fundamental properties of the phenylnitrenium ion was achieved. It’s indicated that the phenylnitrenium ion, especially the triplet one, is not a weak acid in its deprotonation to the phenylnitrene. Hydride transfer, double-hydrogen transfer, electron transfer, and gas-phase synthesis of nitrogen-containing compounds were carefully examined.In addition, this paper describes the mass spectrometry technologies used in the unimolecular reaction, gas-phase ion/molecule reactions, and some theoretical basis and applications.