N-Centered Odd-Electron Ions Formation from Collision-Induced Dissociation of Electrospray Ionization Generated Even-Electron Ions: Single Electron Transfer via Ion/Neutral Complex in the Fragmentation of ProtonatedN,N′-Dibenzylpiperazines and ProtonatedN-Benzylpiperazines

Abstract

Single electron transfer (SET) via ion/neutral complex (INC) was proposed and confirmed to be the key step in the formation of N-centered odd-electron ions from fragmentation of protonated even-electron ions in the present study. Upon collisional activation, the model compounds, protonated N,N′-dibenzylpiperazine and protonated N-benzylpiperazines initially dissociated to form intermediate INCs consisting of N-benzylpiperazine (or piperazine) and benzyl cation. In these ion/neutral complexes, SET reaction and direct separation as well as other reactions were observed and characterized experimentally and theoretically. Density functional theory calculations demonstrated that the energy requirement for homolysis of the precursor ion was so large that it could not be achieved, whereas the heterolytic dissociation followed by electron transfer via INC was energetically preferred. The SET process occurred only when the radical products were more stable than the separation products. The energy barrier for SET in the compounds studied was roughly estimated by comparison with other competing reactions. When the INC contained electron donor with lower ionization energy and electron acceptor with higher electron affinity, the SET reaction was more efficient.

The ESI CAD fragmentations of protonated 2,4,6‐tris(benzylamino)‐ and tris(benzyloxy)‐1,3,5‐triazines involve benzyl–benzyl interactions: a DFT study

“…The applications of electron ionization and electrospray ionization (ESI) mass spectrometry to various substituted triazines are well known in the literature . Tandem mass spectrometric experiments have shown that the dissociation of protonated molecules containing more than one benzyl groups yield product ion of m/z 181 having molecular formula C 14 H 13 + due to benzyl‐benzyl interactions . Examples include study of the protonated dibenzyl esters of dicarboxylic acid generated by chemical ionization and N , N ′‐dibenzylpiperzines produced by ESI .…”

“…The proposed mechanism for the benzyl–benzyl interactions involves ion/neutral complexes (INCs) based on experimental data. Recently DFT calculations, using B3LYP/6‐31G(d) level of theory, have been utilized to substantiate the involvement of INCs . The 2,4,6‐tris(benzylamino)‐ s ‐triazines ( 1–5 ) and 2,4,6‐tris(benzyloxy)‐ s ‐triazines ( 6 and 7 ) offers a unique opportunity to investigate the gas‐phase behavior of compounds containing three benzyl groups and to compare the dissociations of compound in which the benzyl groups are attached to N‐atom and O‐atom.…”

Gas-Phase Chemistry of Benzyl Cations in Dissociation ofN-Benzylammonium andN-Benzyliminium Ions Studied by Mass Spectrometry

“…The ionic and neutral components of an INC show reactivities similar to those expected for the isolated species in the bimolecular reactions. In previous studies, we found dissociation of protonated N-benzyl amines gave rise to [benzyl cation/amine] complex and then nucleophilic aromatic substitution [8], electron transfer [23], or hydride transfer [24] could occur within the complex in different cases. In the present study, we systematically investigated the gas-phase reactions of benzyl cations with neutral N-containing species through INC intermediates in collision-induced dissociation of N-benzylated cations in ESI-MS. With these efforts, we try to figure out a more integrated landscape of the gas-phase reactivity of benzyl cations.…”

“…In electrospray ionization mass spectrometry (ESI-MS), fragmentation of benzylated cations prefers to generate benzyl cations [7,8,[20][21][22][23][24][25][26][27][28][29][30], though a few exceptions have been reported that tropylium ion can be co-produced in the fragmentation of benzylpyridinium ions [18,31,32]. Besides the formation of benzyl cations, other interesting ions resulting from hydride transfer [24,25] or electrophilic aromatic substitution reactions [20][21][22] via the nonconventional ion/neutral complex (INC) can also be generated in the dissociation of benzylated cations.…”

“…Consequently, a need for other examples of INC-mediated reactions is evident in order to better understand the fragmentation reactions in the contemporary ESI mass spectrometry, a method that has become one of the most popular analytical methods. Thus, renewed interest in studies on INC-mediated fragmentation reactions has emerged in recent years [8,[20][21][22][23][24][25][39][40][41][42][43][44][45][46][47].…”