| Abstract: | The p-ethyltoluene molecular ion fragments by loss of either the methyl group attached to the ring or the β-methyl of the ethyl group. Using specific isotopic labeling and charge exchange techniques the relative importance of the losses of the two methyl groups has been studied as a function of internal energy from metastable ions to ions of 7 eV internal energy. The ratio of loss of the β-methyl to loss of the ring-methyl increases from 3.4 for metastable ions to 6.7 for ions of 7 eV internal energy. This variation is interpreted in terms of an energy-dependent competition between fragmentation and hydrogen migration in a dimethylcy-cloheptatriene intermediate. In variable energy collision-induced dissociation studies the ratio of loss of the β-methyl to loss of the ring-methyl decreases with increasing collision energy. It is speculated that this different behavior is related to the differences in preparation of the molecular ions. Those subjected to collisional activation are prepared initially as ground-state ethyltoluene ions and are vibrationally excited upon collision and may fragment largely from the original structure. By contrast, in the charge exchange and electron impact experiments the molecular ions with sufficient energy to fragment are probably formed initially in electronically excited states and the rearrangement to the cycloheptatriene structure may be more facile in these excited states or during the internal conversion processes leading to vibrationally excited ground-state ions. |
