Low‐energy collision‐induced fragmentation of negative ions derived from ortho‐, meta‐, and para‐hydroxyphenyl carbaldehydes, ketones, and related compounds

Abstract

Collision-induced dissociation (CID) mass spectra of anions derived from several hydroxyphenyl carbaldehydes and ketones were recorded and mechanistically rationalized. For example, the spectrum of m/z 121 ion of deprotonated ortho-hydroxybenzaldehyde shows an intense peak at m/z 93 for a loss of carbon monoxide attributable to an ortho-effect mediated by a charge-directed heterolytic fragmentation mechanism. In contrast, the m/z 121 ion derived from meta and para isomers undergoes a charge-remote homolytic cleavage to eliminate an žH and form a distonic anion radical, which eventually loses CO to produce a peak at m/z 92. In fact, for the para isomer, this two-step homolytic mechanism is the most dominant fragmentation pathway. The spectrum of the meta isomer on the other hand, shows two predominant peaks at m/z 92 and 93 representing both homolytic and heterolytic fragmentations, respectively.18 O-isotope-labeling studies confirmed that the oxygen in the CO molecule that is eliminated from the anion of meta-hydroxybenzaldehyde originates from either the aldehydic or the phenolic group. In contrast, anions of ortho-hydroxybenzaldehyde and 2-hydroxy-1-naphthaldehyde, both of which show two consecutive CO eliminations, specifically lose the carbonyl oxygen first, followed by that of the phenolic group. Anions from 2-hydroxyphenyl alkyl ketones lose a ketene by a hydrogen transfer predominantly from the a position. Interestingly, a very significant charge-remote 1,4-elimination of a H 2 molecule was observed from the anion derived from 2,4-dihydroxybenzaldehyde. For this mechanism to operate, a labile hydrogen atom should be available on the hydroxyl group adjacent to the carbaldehyde functionality.

Generation and characterization of distonic dehydrophenoxide radical anions under electrospray and atmospheric pressure chemical ionizations

“…The CID experiments with 11 and 12 yielded comparable spectra as those observed in the ESI-ISD experiments. The results from the CID experiments agree well with those reported by Attygalle and coworkers [25].…”

“…They observed that the deprotonated precursor molecules readily lost either the HCO radical (may be via two steps) or the RCO radical to produce the phenoxide radical anions at m/z 92. The proposed mechanisms were studied using isotope labeling experiments [25].…”

Loss of benzene to generate an enolate anion by a site‐specific double‐hydrogen transfer during CID fragmentation of o‐alkyl ethers of ortho‐hydroxybenzoic acids

“…1,2 In practical terms, such appears to be extracted randomly from several available positions. 4 In our explorations on collision-induced fragmentation of low-molecular-weight anions, 5,6 we noted a site-specific double-hydrogen migration which enables unequivocal differentiation of ortho isomers of alkyloxybenzoic acids from those of meta and para isomers. Moreover, this unique fragmentation enables facile generation of gaseous enolate anions which are known to act as intermediates in photochemical smog cycles, combustion reactions and many organic syntheses of complex compounds.…”

“Meta Elimination,” a Diagnostic Fragmentation in Mass Spectrometry

“…Recent investigations reveal that collision-induced fragmentation spectra of evenelectron ions of substituted aromatic compounds also show peaks diagnostic of the ortho-substitution position [13][14][15][16][17][18][19][20].…”