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

Abstract

The unimolecular reactions of toluene radical cations and tautomeric isotoluene radical cations (methylenecyclohexadiene radical cations) substituted by an ROCH2 group (R=CH3 , C2H5 , n -C3H7 , i-C3 H7) were studied by mass-analyzed ion kinetic energy (MIKE) spectrometry and collisional activation mass spectrometry. The alkoxymethyl-substituted toluene radical cations were obtained directly by 70 eV electron ionization, while the substituted isotoluene radical cations were prepared by elimination of an aldehyde or ketone molecule from ionized bis(alkoxymethyl)benzenes. It is shown that the fragmentation reactions induced by the alkoxymethyl side-chain in the toluene radical cations and isotoluene radical cations can be used conveniently to distinguish unambiguously between the tautomers. The substituted toluene radical cations eliminate abundantly the alkoxy group of the side-chain as aldehyde or ketone by a well known fragmentation mechanism. Typical of the substituted isotoluene radical cations is an abundant elimination of alcohol from the side-chain of metastable ions, which with the exception of some para-substituted ions surpasses the abundance of the loss of aldehyde/ketone. Further, the substituted toluene radical cations typically fragment by loss of the methyl group of the aromatic ring. The analysis of the MIKE spectra reveals that <10% of the isotoluene racial cations are isomerized into the toluene isomers if these ions are generated by energetically favorable processes from the bis(alkoxymethyl)benzenes. Otherwise, extensive tautomerization between toluene and isotoluene-like ions takes place. Thus, (substituted) toluene and isotoluene radical cations are stable species which can be detected by tandem mass spectrometry. However, the activation barrier between the tautomeric ions cannot be very large, and energetically excited isotoluene radical cations are converted into the more stable toluene isomer. Copyright © 1999 John Wiley & Sons, Ltd.