Formation of C7H7+ ions from ethylbenzene and o-xylene ions: Fragmentation versus isomerization

The C7H7+ ion, existing in three major isomeric forms (tropylium, benzylium and tolylium), is one of the most fascinating species in organic mass spectrometry. Yet the properties of the three isomers are still under some debate. In a recent experiment we have measured the rate constants for the formation of C7H7+ from energy-selected ethylbenzene and o-xylene ions [M. Malow, M. Penno, K.M. Weitzel, J. Phys. Chem. A, 107, (2003) 10625]. In the current work we present an extended kinetic analysis taking into account competition of isomerization and fragmentation in both ethylbenzene and o-xylene ions. The comparison of the kinetic model with experimental data indicates that isomerization of the precursors to a common intermediate (the methyl-cycloheptatriene ion) prior to fragmentation does not play an important role in the threshold region. Most likely the effective barriers for isomerization of the precursors are significantly higher than the thermochemical onset of fragmentation.