2-Benzylindane radical cations in the gas phase (Part I): Substituent effects on a stereoselective McLafferty reaction and related hydrogen transfer processes

Abstract

In the present and the accompanying article, the unimolecular fragmentation of the radical cations of 2-benzylindane and eleven derivatives bearing meta- or para-substituents at the benzylic moiety has been studied with a special focus on the hydrogen exchange that precedes the McLafferty reaction. Standard EI mass spectra, low-energy (11eV) mass spectra and mass-analyzed ion kinetic energy (MIKE) spectra were recorded to probe the excitation-energy and ion-lifetime dependence of the fragmentation and the hydrogen exchange. Density-functional theory calculations were used to rationalize the substituent effects on the competition between the benzylic cleavage and the McLafferty reaction and to clarify the role of the distonic arenium ion-type intermediate formed by the γ-hydrogen-transfer step of the McLafferty reaction. The ipso-protonolysis of the benzyl residue giving rise to the loss of substituted benzenes specifically from most of the para-isomers was also found to reflect the hydrogen exchange.