Department of Chemistry, University of Michigan, 930 N. University Avenue, Ann Arbor, Michigan 48109, United States.
J Phys Chem A. 2020 May 14;124(19):3711-3719. doi: 10.1021/acs.jpca.0c00661. Epub 2020 May 4.
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 HgX and CHHgX (X = Cl, Br, I, and SCH). 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, HgX and CHHgX (X = Cl and SCH), 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 HgX complexes can have a positive or a negative MIE (depending on reaction conditions), while CHHgX 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.
已经在实验室和环境中观察到汞的大质量独立分馏特征，这引发了对这些特征背后化学原因的深入质疑。由于环境中汞化学的机制信息相对较少，无法解释这些同位素效应，本研究使用高水平的电子结构方法来评估 HgX 和 CHHgX（X = Cl、Br、I 和 SCH）中质量独立同位素效应（MIE）的可能光化学反应机制。结果表明，自旋轨道耦合消除了与 Br 或 I 结合的 Hg 发生 MIE 的可能性，但涉及较轻元素的配合物，HgX 和 CHHgX（X = Cl 和 SCH）在光解时具有相对较小的自旋轨道耦合。这一意外发现表明，由于超精细耦合引起的磁同位素分馏是可能的，这取决于 Hg 配合物的特性。通过对光解势能曲线的检查，本研究表明 HgX 配合物可以具有正或负的 MIE（取决于反应条件），而 CHHgX 配合物则仅导致正 MIE。这些发现与在自然样本中记录的 MIE 一致，为环境中汞的惊人质量独立分馏提供了合理的机制。
