Mechanistic inferences from deuterium isotope effects on competing metastable decompositions of organic ions

Abstract

Substantial changes in relative abundances of competing metastable transitions from organic ions in the mass spectrometer are shown to occur in certain cases on partial or complete deuteriation. In the case of benzene, primary isotope effects occur upon the loss of H and H₂ from the molecular ion. The occurrence of a primary isotope effect on the loss of Cl from the molecular ion of p-chloroethylbenzene is demonstrated by this method and this isotope effect strongly implicates a rearrangement (probably to a 7-membered-ring isomer) before loss of Cl. A quasi-equilibrium theory calculation is carried out to illustrate that a change in relative metastable abundance by a factor of 2·4 is consistent with a primary isotope effect upon one of the reactions. A rearrangement is also inferred from isotope-effect data for the elimination of Br from p-bromoethylbenzene.