n Rearrangement in Molecular Ions of nzenes : Appearance Potentials an Substituent Effects

The mechanism of the formation of [C,Hs]' ions by hydrogen rearrangement in the molecular ions of 1-phenylpropane and 1,3-diphenylpropane has been investigated by looking at the effects of CH,O and CF, substituents in the meta and para positions on the relative abundances of the corresponding ions and on the appearance energies. The formation of [C,H,]' ions from 1,3-diphenylpropane is much enhanced at the expense of the formation of [C,H,]' ions by benzylic cleavage, due to the losalized activation of the migrating hydrogen atom by the y phenyl group. A methoxy substituent in the 1,3-diphenylpropane, exerts a site-specific influence on the hydrogen rearrangement, which is much more distinct than in I-phenylpropane and related 1-phenylalkanes, the rearrangement reaction being favoured by a meta methoxy group. The mass spectrum of 1-(3-methoxyphenyl)-3-(4-trideuteromethox~henyl)-propane shows that this effect is even stronger than the effect of para methoxy groups on the henzylic cleavage. From measurements of appearance potentials it is concluded that the substituent effect is not due to a stabilization of the [C,H,X]! product ions. Whereas the [C7H7Ii ions are formed directly from molecular ions of 1-phenylpropane and 1,3dipheuylpropane, the [C,H,]' ions arise by a two-step mechanism in which the u complex type ion intermediate can either return to the molecular ion or fragment to [C,H,]' by allylic bond cleavage. Obviously the formation of this (I complex type ion, is iduenced by electron donating substituents in specific positions at the phenyl group. This is borne out by a calculation of the AH, values of the various species by thermochemical data. Thus, the relative abundances of the fragment ions are determined by an isomerization equilibrium of the molecular ions, preceding the fragmentation reaction.