A theoretical study of the large Hg-Hg spin-spin coupling constants in Hg₂²⁺, Hg₃²⁺, and Hg₂²⁺-crown ether complexes

Nuclear spin-spin coupling constants ¹J(Hg-Hg) in the systems Hg₂²⁺ and Hg₃²⁺ represent the largest coupling constants so far observed in NMR experiments. We have performed a computational study on these ions, on Hg₂²⁺ complexes with 18-crown-6 and 15-crown-5, and on Hg₃²⁺ with solvent molecules and counterions. The results obtained with our recently developed program for the density functional computation of heavy nucleus spin-spin coupling constants are in good agreement with experiments. The data reveal that the bare ions Hg₂²⁺ and Hg₃²⁺ would afford much larger coupling constants than those experimentally observed, with an upper limit of approximately 0.9 MHz for Hg₂²⁺. This limit is much larger than that previously estimated by Hückel theory. It is demonstrated that in solution or due to complexation the experimentally determined values are much smaller than the free ion's coupling constants. With the help of intuitive MO arguments, it is illustrated how the environment strongly reduces the coupling constants in Hg₂²⁺ and Hg₃²⁺. The two-bond coupling constant ²J(Hg-Hg) in Hg₃²⁺ is also examined.