Kinetic isotope effects in solution Reactions of muonium atoms as H-isotopes

Abstract

Muonium (Mu) and protium (¹H) differ by only 0.43% in reduced mass but by a factor of 8.8 in atomic mass, so they constitute an excellent pair of atoms with which to examine kinetic isotope effects. When these atoms react with solutes in water, the ratio of observed rate constants (k_M/k_H) ranges from 10³ to 10⁻². Many of these effects can be accounted for by differences in diffusion, zero-point energies, quantum tunneling and steric hindrance when they involve a single reaction channel to form isotopomeric products. But there are also isotope effects which differ in kind not just in degree, where different reactions are involved for Mu and H leading to different reaction products. In general, Mu falls between H-atoms and hydrated electrons in its reactivity. Examples where Mu and H take different paths include Mu behaving as a nucleophilic addition reagent towards substituted benzenes, and where Mu adds to the carbonyl of a ketone while H abstracts a hydrogen from an adjacent alkyl group.

This is certainly not an exhaustive review of Mu vs. H kinetic isotope effects in solution. Rather, it is a selective look at some 25 solutes in water—which includes most of those showing subtle or unusual effects, and those which have emerged as a result of the unique mass difference and the potential quantum character of these two H-isotopes.