Secondary hydrogen isotope effects on simple cleavage reactions in the gas phase. The .alpha.-cleavage of tertiary amine cation radicals

Abstract

Intramolecular secondary hydrogen isotope effects on the ..cap alpha..-cleavage reaction of 14 unsymmetrically deuterium labeled tertiary amine cation radicals have been determined in order to probe the variation of the isotope effect with the distance to the rupturing carbon-carbon bond, with the structure of the radical lost, and with the internal energy of the reactant. The low-energy molecular ions exhibit normal isotope effects (i.e., predominant loss of the unlabeled alkyl radical), which are highest for ..beta..-labeled ions (1.30 per deuterium) and decrease with increasing distance between the isotopic label and the point of rupture; significant effects are observed even for deuterium substitution four bonds removed (1.04 per deuterium for epsilon-labeled ions). The isotope effects are relatively small and normal for molecular ions with high internal energy, with one exception: the secondary isotope effect on the fast (ion-source) reactions of the delta-labeled N-methyldipentylamine is inverse (0.98 per deuterium). The structure of the alkyl radical lost seems not have a pronounced influence on the secondary isotope effects.