Collision‐induced dissociation processes of protonated benzoic acid and related compounds: competitive generation of protonated carbon dioxide or protonated benzene

• Published in: Journal of mass spectrometry. Vol. 52; no. 4; pp. 230 - 238
• Main Authors: Xu, Sihang, Pavlov, Julius, Attygalle, Athula B.
• Format: Journal Article
• Language: English
• Published: England Wiley Subscription Services, Inc 01.04.2017
• Subjects: Benzene; Benzoic acid; Carbon; Carbon dioxide; Carbon monoxide; Fragmentation; Spectra; Terephthalic acid
• ISSN: 1076-5174; 1096-9888
• DOI: 10.1002/jms.3920

Upon activation in the gas phase, protonated benzoic acid (m/z 123) undergoes fragmentation by several mechanisms. In addition to the predictable water loss followed by a CO loss, the m/z 123 ion more intriguingly eliminates a molecule of benzene to generate protonated carbon dioxide (H ‐ O+ ═ C ≡ O, m/z 45), or a molecule of carbon dioxide to yield protonated benzene (m/z 79). Experimental evidence shows that the incipient proton ambulates during the fragmentation processes. For the CO2 or benzene loss, protonated benzoic acid transfers the charge‐imparting proton initially to the ortho position and then to the ipso position to generate a transient species which dissociates to form an ion‐neutral complex between benzene and protonated CO2. The formation of the m/z 45 ion is not a phenomenon unique to benzoic acid: spectra from protonated isophthalic acid, terephthalic acid, trans‐cinnamic acid and some aliphatic acids also displayed a peak for m/z 45. However, the m/z 45 peak is structurally diagnostic only for certain benzene polycarboxylic acids because the spectra of compounds with two carboxyl groups on adjacent ring carbons do not produce a peak at m/z 45. For the m/z 79 ion to be formed, an intramolecular reaction should take place in which protonated CO2 within the ion‐neutral complex acts as the attacking electrophile to transfer a proton to benzene. Copyright © 2017 John Wiley & Sons, Ltd.