Studies Of Fragmentation Behaviors Of Organic Compounds Induced By Proton Or Lithium Ion In ESI-MS

Electrospray ionization mass spectrometry (ESI-MS) is an ideal tool to study the gas-phase chemistry of organic compounds and has been widely applied in the structural analysis of organic and biologic compounds. However, unlike the cases in EI-MS, the fragmentation reactions of protonated or cationized molecules in ESI-MS have not yet been well investigated. As a result, it is very important and necessary to explore and summarize the dissociation mechanisms. This study tries to find some universal rules for the gas-phase dissociation reactions in ESI-MS" based on the studies of several kinds of model compounds, including the following five parts:1. Competitive proton transfer reactions:In mass spectrometry of the model compound, Ph-CO-CH=CH-N(CH3)2, the added proton transfers from the thermodynamically favored site at the carbonyl oxygen to the dissociative protonation site at ipso-position of the phenyl ring or the double bond carbon atom adjacent to the carbonyl leading to the loss of C6H6or C4H9N, respectively. It was found that the electron-donating groups were in favor of losing the C6H6, whereas the electron-attracting groups favored the loss of C4H9N. The results of the D-labeling experiments indicate that the H/D exchange takes place between the added proton and all the protons of the phenyl ring and the proton is floating in the whole molecular ion.2. Various reactions mediated by reactive intermediates:In mass spectrometry of protonated N-phenylcinnamides, the proton transfers from the carbonyl oxygen to the dissociative protonation site at the amide nitrogen atom or the α-carbon atom, leading to the formations of important reactive intermediates, rather than the final fragment ions. Many interesting reactions such as proton transfer and losses of phenyl isocyanate, benzene and ketene were mediated by these reactive intermediates.3. Elimination of benzene induced by benzyl cation transfer: In mass spectrometry of protonated N-benzylindoline, elimination of benzene was observed besides the formation of odd-electron ion. D-labeling experiments and accurate mass determinations of the product ions confirm that the elimination reaction is initiated by benzyl cation transfer rather than proton transfer. The cleavage of the C-N bond induced by protonation leads to the formation of ion/neutral complex (INC). Within the INC, benzyl cation could attack one of the sp2-hybridized carbon atoms in the indoline core and subsequent proton transfer reaction leads to the elimination of benzene. Density functional theory (DFT)-based calculations were performed and the computational results also support the proposed mechanism.4. Competitive losses of LiOH and H2O:In mass spectrometry of lithiated N-phenylcinnamides, the loss of LiOH was discovered and this is similar to the dissociations of lithiated2-(4,6-dimethoxypyrimidin-2-ylsulfanyl)-N-phenylbenzamides and N-phenylbenzamides. Besides, the Li+could acitivate the hydrogen of the double bond carbon atom adjacent to the carbonyl and subsequent proton transfer to the oxygen leads to the loss of H2O. The presence of electron-donating substituents was found to facilitate elimination of LiOH, while the loss of H2O was expediated by electron-withdrawing substituents. H/D exchange between the proton on the nitrogen and the proton of the phenyl ring was witnessed by D-labeling experiments.5. Competitive proton transfer and lithium ion transfer:In mass spectrometry of the model compound, Ph-CH=CH-CO-S-Ph, the lithium ion could acitivate the hydrogen of the double bond carbon atom adjacent to the carbonyl and subsequent proton transfer to the sulfer results in the loss of PhSH. Besides, the lithium ion could also transfer to the sulfer leading to the loss of PhSLi. Although the physical and chemical properties of lithium ion and proton are different, they are both transferable in gas phase.