Toluene and isotoluene radical cations: Alkoxymethyl substituents as a probe to study the formation, fragmentation and isomerization

The unimolecular reactions of toluene radical cations and tautometric isoto luene radical rations (methylenecyclohexadiene radical cations) substituted by an ROCH2 group (R = CH3, C2H5, n-C3H7, i-C3H7) were studied by mass-ana lyzed ion kinetic energy (MIKE) spectrometry and collisional activation mas s spectrometry. The alkoxymethyl-substituted toluene radical cations were o btained directly by 70 eV electron ionization, while the substituted isotol uene radical cations were prepared by elimination of an aldehyde or ketone molecule from ionized bis(alkoxymethyl)benzenes. It is shown that the fragm entation reactions induced by the alkoxymethyl side-chain in the toluene ra dical cations and isotoluene radical rations can be used conveniently to di stinguish unambiguously between the tautomers. The substituted toluene radi cal cations eliminate abundantly the alkoxy group of the sidechain as aldeh yde or ketone by a well known fragmentation mechanism, Typical of the subst ituted isotoluene radical cations is an abundant elimination of alcohol fro m the side-chain of metastable ions, which with the exception of some para- substituted ions surpasses the abundance of the loss of aldehyde/ketone. Fu rther, the substituted toluene radical cations typically fragment by loss o f the methyl group of the aromatic ring, The analysis of the MIKE spectra r eveals that <10% of the isotoluene racial cations are isomerized into the t oluene isomers if these ions are generated by energetically favorable proce sses from the bis(alkoxymethyl)benzenes. Otherwise, extensive tautomerizati on between toluene and isotoluene-like ions takes place, Thus, (substituted ) toluene and isotoluene radical cations are stable species which can be de tected br tandem mass spectrometry. However, the activation barrier between the tautomeric ions cannot be very large, and energetically excited isotol uene radical rations are converted into the more stable toluene isomer. Cop yright (C) 1999 John Wiley & Sans, Ltd.