Mercury Magnetic Isotope Effect: A Plausible Photochemical Mechanism

Large mass-independent fractionation signatures in Hg have been observed in the laboratory and the environment, prompting deep questions about the chemical reasons behind these signatures. Since the relative lack of mechanistic information about Hg chemistry in the environment has precluded explanations of these isotope effects, the present study uses high-level electronic structure methods to evaluate the possible photochemical mechanisms of mass-independent isotope effects (MIEs) in HgX₂ and CH₃HgX (X = Cl, Br, I, and SCH₃). The results show that spin-orbit coupling wipes out the potential of MIEs for Hg bound to Br or I, but that complexes involving lighter elements, HgX₂ and CH₃HgX (X = Cl and SCH₃), have relatively small spin-orbit couplings upon photolysis. This unexpected finding shows that magnetic isotope fractionation due to hyperfine coupling is possible, depending on the identity of the Hg complex. By examination of the photolysis potential energy profiles, this study shows that HgX₂ complexes can have a positive or a negative MIE (depending on reaction conditions), while CH₃HgX complexes exclusively result in a positive MIE. These findings agree with MIE recorded in natural samples, demonstrating a plausible mechanism for the surprising mass-independent fractionation of Hg in the environment.