Structural determination of [C7H7O]+ ions in the gas phase by ion cyclotron resonance spectrometry

The [C₇H₇O]⁺ ions from a series of compounds have been studied using ion cyclotron resonance spectrometry. The techniques employed in this gas phase ion structure determination of [C₇H₇O]⁺ were photodissociation, ion-molecule reactions, and collisionally activated dissociation using a Fourier transform mass spectrometer (FTMS-CAD). In addition to the low energy FTMS-CAD results, high energy CAD data obtained with a sector mass spectrometer is also provided. Evidence was found for five unique [C₇H₇O]⁺ structures, including the hydroxybenzyl ion, the hydroxytropylium ion, the protonated benzaldehyde ion, the methylaryloxy ion and the phenyl methylene ether ion. Ion-molecule reactions, invovling both proton transfer and methylene transfer, provided the most unambiguous results and yielded qualitative and quantitative evidence for the five structures. However, a combined approach using the three techniques was necessary to identify all of the structures. The tropylium form of [C₇H₇O]⁺ was found to absorb strongly at 305 nm, while the protonated benzaldehyde ion was found to have a strong absorption band at 305 nm and a weak band at 370 nm. The proton affinity of 2,4,6-cycloheptatrienone was determined to be 918±8 kJ mol^?1, which is considerably lower than a previously reported value. In addition, deprotonation reactions of the methylaryloxy ion yielded a proton affinity of 871±14 kJ mol^?1 for 4-methylenecyclohexa-2,6-diene-1-one.