Even‐electron ions: a systematic study of the neutral species lost in the dissociation of quasi‐molecular ions

Abstract

The collision-induced dissociations of the even-electron [M + H](+) and/or [M - H](-) ions of 121 model compounds (mainly small aromatic compounds with one to three functional groups) ionized by electrospray ionization (ESI) or atmospheric pressure chemical ionization (APCI) have been studied using an ion trap instrument, and the results are compared with the literature data. While some functional groups (such as COOH, COOCH(3), SO(3)H in the negative ion mode, or NO(2) in both the positive and negative ion modes) generally promote the loss of neutrals that are characteristic as well as specific, other functional groups (such as COOH in the positive ion mode) give rise to the loss of neutrals that are characteristic, but not specific. Finally, functional groups such as OH and NH(2) in aromatic compounds do not lead to the loss of a neutral that reflects the presence of these substituents. In general, the dissociation of [M + H](+) and [M - H](-) ions generated from aliphatic compounds or compounds containing an aliphatic moiety obeys the even-electron rule (loss of a molecule), but deviations from this rule (loss of a radical) are sometimes observed for aromatic compounds, in particular for nitroaromatic compounds. Thermochemical data and ab initio calculations at the CBS-QB3 level of theory provide an explanation for these exceptions. When comparing the dissociation behaviour of the even-electron [M + H](+) and/or [M - H](-) ions (generated by ESI or APCI) with that of the corresponding odd-electron [M](+) ions (generated by electron ionization, EI), three cases may be distinguished: (1) the dissociation of the two ionic species differs completely; (2) the dissociation involves the loss of a common neutral, yielding product ions differing in mass by one Da, or (3) the dissociations lead to a common product ion.

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

“…The formation of OE ions from protonated tertiary amines also has been observed by others [5,8]. In the present study, we focus on the mechanism for formation of OE ions from protonated Nbenzyl compounds and provide a novel perspective.…”

“…When these closed-shell ions are subjected to CID, they commonly dissociate to daughter closed-shell ions and neutral fragments based on 'even-electron rule' [1]. It is rare that an electrospray ionization generated even-electron (EE) ion directly dissociates to form an odd-electron (OE) ion, though exceptions are more common in electron ionization (EI) [1][2][3][4] than in ESI mass spectrometry [5][6][7][8][9][10][11][12]. It is significant and desirable to investigate the underlying mechanism of formation of OE ions from EE ions because such considerations are important for a better understanding of ESI mass spectrometry.…”

Gas‐phase dissociation of 1,4‐naphthoquinone derivative anions by electrospray ionization tandem mass spectrometry

“…The fragmentation pathways of [M − H] − are usually applied to molecules with acid groups, such as carboxylic acids and phenols. [16,17] Recently, Levsen and coworkers [18] have applied CID to protonated and deprotonated molecules generated by ESI or atmospheric pressure chemical ionization (APCI [18] which has furnished important data for MS/MS users.…”

The Protonation Site of para-Dimethylaminobenzoic Acid Using Atmospheric Pressure Ionization Methods

“…For protonated p-DMABA, the fragmentations of Oand N-protonated species are expected to produce different product ions (Scheme 2). Loss of methyl radical from the Nprotonated N-methyl compounds is commonly observed in mass spectrometry [37,38]. Fragmentation of N-protonated p-DMABA gives rise to a radical cation at m/z 151.…”