Magnetic isotopes as a means to elucidate Earth and environmental chemistry

In Earth and environmental chemistry, magnetic isotopes provide a universal means to identify reaction mechanisms. Mass-independent fractionation of isotopes as a signature of a mechanism occurs by two ways: first, via the magnetic isotope effect (MIE), which is controlled by magnetic, or hyperfine, coupling between unpaired electrons and magnetic nuclei in paramagnetic species (particularly, in radicals), and, second, via the nuclear volume effect (NVE), which is induced by the volume difference between isotopic nuclei. The MIE is the dependence of the reaction rates on the nuclear magnetic moment of reactants and fractionates magnetic and nonmagnetic isotopes, whereas NVE fractionates isotopes with different nuclear volumes. Both effects, MIE and NVE, are supposed to coexist in condensed phases. A decisive test for their differentiation is illustrated by the example of radical pairs with mercury nuclei. Namely, if isotope fractionation is controlled by MIE, the ratio $\Delta^{201}\mathrm{Hg}/\Delta^{199}\mathrm{Hg}$ is expected to be in the range of 1.05–1.25 for isotopic enrichment and 0.80–0.92 for depletion. If isotope fractionation is controlled by NVE, this ratio is estimated to be in the range of 0.50–0.62. In contrast to MIE-induced bidirectional fractionation controlled by the direction of coherent spin conversion of the radical pair (triplet–singlet or vice versa), the NVE induces unidirectional, universal isotope fractionation, which is almost independent of the reaction mechanism. In contrast to MIE which exhibits inversion of the fractionation sign depending on the spin multiplicity of reactants, NVE is incompatible with the inversion of the fractionation sign. The MIE is an unambiguous indicator of the radical mechanisms and dominates in chemical reactions, whereas NVE prevails in nonchemical processes. Chemical scenarios of MIE-induced oxygen, sulfur, iron, silicon, tin, mercury, germanium and uranium isotope fractionation in photostimulated and dark reactions are analyzed in terms of reaction mechanisms including reactions in living organisms. In conclusion, some restrictions, uncertainties and problems in Earth and environmental chemistry are discussed.
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