A comparison of the reactions of the oxide radical anion (o(-.)) with simple aromatic compounds at atmospheric and at reduced pressure conditions.

The reactions of oxide radical anions (O(-.)) with benzene and toluene under atmospheric pressure (APCI) and conventional chemical ionization (CI) conditions were compared. Hydrogen radical (H(-)) displacement by oxygen, yielding [M - H + O](-), was observed in both the APCI and the CI source. However, the product, [M - 2H](-.), derived from dihydrogen radical ion (H2 (+.)) transfer which was observed in the CI spectra, was consistently absent under APCI conditions. This behavior is rationalized in terms of the higher pressures and chemical equilibrium associated with the APCI source. In addition to the formation of the a priori expected phenoxide isomers, the reaction of O(-.) with toluene to yield the [M - H + O](-) product generates a benzyloxide anion. Tandem mass spectrometry data from collision-induced dissociation and isotopic labeling with deuterium support a reaction mechanism initiated by α hydrogen abstraction for both the H(.) and the H2 (+.) transfer pathways.