Gas‐phase formation of protonated benzene during collision‐induced dissociation of certain protonated mono‐substituted aromatic molecules produced in electrospray ionization

Protonated benzene, C₆H, has been studied extensively to understand the structure and energy of a protonated organic molecule in the gas phase. The formation of C₆H is either through direct protonation of benzene, i.e., chemical ionization, or through fragmentation of certain radical cations produced from electron ionization or photon ionization. We report a novel observation of C₆H as a product ion formed in the collision‐induced dissociation (CID) of protonated benzamide and related molecules produced via electrospray ionization (ESI). The formation of C₆H from these even‐electron precursor ions during the CID process, which has not been previously reported, is proposed to occur from the protonated molecules via a proton migration in a five‐membered ring intermediate followed by the cleavage of the mono‐substituent C? C bond and concurrent formation of an ion‐molecule complex. This unique mechanism has been scrutinized by examining some deuterated molecules and a series of structurally related model compounds. This finding provides a convenient mean to generate C₆H, a reactive intermediate of considerable interest, for further physical or chemical investigation. Further studies indicate that the occurrence of C₆H in liquid chromatography/electrospray ionization tandem mass spectrometry (LC/ESI‐MS/MS) appears to be a rather common phenomenon for many compounds that contain ‘benzoyl‐type’ moieties. Hence, the observation of the C₆H ion in LC/ESI‐MS/MS can be used as an informative fragmentation pathway which should facilitate the identification of a great number of compounds containing the ‘benzoyl‐type’ and similar structural features. These compounds are frequently present in food and pharmaceutical products as leachable impurities that require strict control and rapid elucidation of their identities. Copyright © 2010 John Wiley & Sons, Ltd.