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

Abstract

AbstractHydrogen migrations in the molecular ions of 1,3‐diphenylpropane, preceding the fragmentations to [C7H7]+ and [C7H8]+ ions, have been investigated by use of deuterated derivatives. By comparing the distribution of deuterium labels in the [C7(H, D)8]+ 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 γ‐methylene group and at the ortho positions of the phenyl group: (ii) exchange between hydrogen isotopes at the ortho and orthó positions in the intermediate. In these mechanisms the eight hydrogen isotopes at both benzylic positions and both the ortho and orthó positions of 1,3‐diphenylpropane participate in a mutual exchange. A statistical equipartition of the hydrogen isotopes at these eight positions is not reached in metastable molecular ions, however. The distribution pattern of [C7(H, D)8]+ 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 n = 5–8 for metastable ions. Therefore, the hydrogen exchange in the molecular ion of 1,3‐diphenylpropane is a rather slow process. These results firmly establish the isomerization reaction involving the conversion of the molecular ion of 1,3‐diphenylmethane to the intermediate and hence to the molecular ion of 7‐(2‐phenylethyl)‐5‐methylene cyclohexa‐1,3‐diene and preceding the fragmentations. The postulated intermediate is a true one which corresponds to a s̀‐complex type ion and which fragments to [C7H8]+ ions. Surprisingly, no isomerizations of the intermediate by hydrogen shifts within the protonated aromatic system (‘ring walks’) are observed.