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

✍ Scribed by Athula B. Attygalle; Josef Ruzicka; Deepu Varughese; Jason B. Bialecki; Sayed Jafri

• Publisher: John Wiley and Sons
• Year: 2007
• Tongue: English
• Weight: 419 KB
• Volume: 42
• Category: Article
• ISSN: 1076-5174
• DOI: 10.1002/jms.1252

✦ Synopsis

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 •H 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 α 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. Copyright © 2007 John Wiley & Sons, Ltd.