Speciation analysis of mercury dimethylmercury

Speciation analysis of mercury, rather than determination of its total content, is critical. In addition, studying the binding of different mercury species to biomolecules can help understand their biotransformation, bioaccumulation toxicity, and detoxification effects. This chapter aims to provide an overview of sample preparation and analytical methods for speciation analysis and the binding of mercury speciation to biomolecules. Direct observation of both isotopes of cadmium is relatively easy, with 113Cd having a minor advantage in receptivity. Direct measurements of 199Hg using multicore FT systems have produced a large amount of data in recent years, some of which are summarized in Table 18. The most relevant problem for 199Hg observations is excessive linewidth. The 199Hg resonance at 4.7 T was found to be severely broadened by shielding anisotropic relaxation. Many researchers use external pure dimethylmercury as a chemical shift reference or a concentrated sample as a preliminary search for 199Hg reference. In fact, dimethylmercury has several inherent properties well suited for 199Hg experiments (fast and accurate measurement, strong coupling to protons), but is highly toxic (LD50 MeHg, 0.1 ml). For this purpose, mercury perchlorate has also been reported in the literature as a reference. The chemical shift relative to dimethylmercury at a concentration of 0.1 mol l−1 in perchloric acid solution is -2250 ppm. Linear organomercury provides a convenient basis for studying the effects of substitution of organic moieties on metal chemical shifts. In the Hg-CH3 derivative, an increase in shielding of ~150 ppm was observed each time H was substituted with Me, while a smaller decrease (~30 ppm) in shielding was observed when β-Hs was substituted. Cadmium alkyls show similar behavior, but for silylmercury compounds substitutions at the α and β positions lead to deshielding.
The first ionization potential of [HgCl(R)] is mainly related to the σ-HgC bond, showing a dependence on δ(199Hg). However, increasing the number of β-carbon atoms decreases the ionization potential, which increases the electron density on the mercury and decreases ΔE, both of which lead to deshielding, not shielding. Electron-withdrawing substituents generally increase shielding. When substituent effects are transported through –CC– in Hg(CCR)2, the shielding is roughly consistent with inductive and mesoscopic effects, with electron-rich groups such as tert-butyl giving the strongest deshielding resonances.
Among late transition metals, 199Hg has been the subject of most spin-spin coupling reports.
Table 19 reports typical ranges for some coupling constants involving Cd and Hg. Among alkylmercury compounds, 3J(199Hg13C) shows a Karplus-type dependence on dihedral angle. For a series of [Hg(L)CH2C(OMe)Me2], the changes of 1J(199Hg,13C) and 2J(199Hg,1H) closely follow the calculated mutual polarizability, and show that the change of 6 s has a significant effect on the Hg-C bond It is the main reason for the magnitude change of 1J(199Hg,13C). The increase of 2J(199Hg,1H) and 1J(199Hg,13C) in organic dimethylmercury with solvent donor ability is usually explained by coordination. The same order is observed for both poor and intermediate mercury acceptors, and so far the latter does not correspond to the behavior of δ(199Hg). In difluoroalkylmercury derivatives, 2J(199Hg,19F) decreases with the increase of solvent donor ability.

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