Energy‐dependent reversal of secondary isotope effects on simple cleavage reactions: Tertiary amine radical cations with deuterium at remote positions

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Abstract
Deuterium substitution at remote sites gives rise to inverse secondary kinetic isotope effects on the α‐cleavage of a number of tertiary amines in the ion source, but to normal isotope effects on reactions occurring in the field‐free regions. The change from normal to inverse secondary isotope effects when the internal energy of the reacting ions increases is consistent with transition‐state models that involve slight lowering of vibrational frequencies also for bonds well removed from the site of cleavage. The isotope effect on the reactions of ions of high internal energy is caused by the influence that even small changes of isotope‐dependent frequencies have on the state sums (a statistical weight effect favouring loss of the labelled radical), whereas the behaviour of low‐energy ions is determined by the zero‐point energy changes which favour loss of the unlabelled fragment.

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