Hydrogen rearrangement in molecular ions of alkyl benzenes: Mechanism and time dependence of hydrogen migrations in molecular ions of 1, 3-diphenylpropane and deuterated analogues

Hydrogen migrations in the molecular ions of 1,3-diphenylpropane, preceding the fragmentations to [C7H,]' and [C,H& ions, have been investigated by use of deuterated derivatives. By comparing the distribution of deuterium labels in the [C,(H, D)Jt products from metastable molecular ions with the distribution patterns calculated for various exchange models, it is shown that the H migrations occur by two processes linked by a common intermediate: (i) exchange between hydrogen isotopes at the y-methylene group and at the ortbo positions of the phenyl group: (ii) exchange between hydrogen isotopes at the ortbo and ortbo' positions in the intermediate. In these mechanisms the eight hydrogen isotopes at both benzylic positions and both the ortbo and ortbo' positions of 1,3-diphenylpropane participate in a mutual exchange. A statistical eqoipartition of the hydrogen isotopes at these eight positions is not reached in metastable molecular ions, however. The distribution pattern of [C,(H, D)$ ions from the deuterium labelled compounds as a function of the mean number n of exchange cycles has been calculated according to this reaction model and compared with experimental results for unstable molecular ions, generated by 70 eV and 12 eV electrons, respectively, and metastable molecular ions. Good agreement is obtained for all compounds and n=0.4-0.8 for unstable molecular ions and It = 5-8 for metastable ions. Therefore, the hydrogen exchange in the molecular ion of 1,3-diphenylpropane is a rather slow process. These results 6rmly establish the isomerhation reaction involving the conversion of the molecular ion of 1,3-diphenylmethane to the intermediate and kence to the molecular ion of 7-(2-phenylethyl)-5-methylene cyclohexa-l,3-diene and preceding the fragmentations. The postulated intermediate is a true one which corresponds to a a-complex type ion and which fragments to [C7HJf ions. Surprisingly, no isomerizations of the intermediate by hydrogen shifts within the protonated aromatic system ('ring walks') are observed.