Mercury Isotopes in Earth and Environmental Chemistry

Аннотация

In Earth and environmental chemistry magnetic isotopes are the universal means to identify reaction mechanisms. Mass-independent fractionation of isotopes as a signature of mechanism occurs by two ways: first, via magnetic isotope effect (MIE), which is controlled by magnetic, or hyperfine, coupling between unpaired electrons and magnetic nuclei in paramagnetic species (in radicals, particularly), and, second, via nuclear volume effect (NVE), which is induced by the difference in volumes of isotopic nuclei. MIE is the dependence of the reaction rates on the nuclear magnetic moment of reactants and fractionates magnetic and nonmagnetic isotopes; NVE fractionates isotopes with different nuclear volumes. Both effects, MIE and NVE, are supposed to coexist in condensed phases. Decisive test for their differentiation is illustrated by example of radical pairs with mercury nuclei: if isotope fractionation is controlled by MIE, the ratio Δ²⁰¹Hg/Δ¹⁹⁹Hg is expected to be in the limits 1.05–1.25 for isotopic enrichment and 0.80–0.92 for impoverishment. If isotope fractionation is controlled by NVE, this ratio is estimated to be in the range 0.50–0.62. In contrast to MIE-induced two-directional fractionation, which is controlled by direction of coherent spin conversion of radical pair (triplet–singlet or vice versa), the NVE induces one-directional, universal isotope fractionation, almost independent on the reaction mechanism.