Magnetic isotope effect and theory of atomic orbital hybridization to predict a mechanism of chemical exchange reactions
ABSTRACT A novel approach is suggested to investigate the mechanisms of chemical complexation reactions based on the results of Fujii with co-workers; they have experimentally observed that several metals and metalloids demonstrate mass-independent isotope fractionation during the reactions with the DC18C6 crown ether using solvent-solvent extraction. In this manuscript, the isotope fractionation caused by the magnetic isotope effect is used to understand the mechanisms of chemical exchange reactions. Due to the rule that reactions are allowed for certain electron spin states, and forbidden for others, magnetic isotopes show chemical anomalies during these reactions. Mass-independent fractionation is suggested to take place due to the hyperfine interaction of the nuclear spin with the electron spin of the intermediate product. Moreover, the sign of the mass-independent fractionation is found to be dependent on the element and its species, which is also explained by the magnetic isotope effect. For example, highly negative mass-independent isotope fractionation of magnetic isotopes was observed for reactions of DC18C6 with SnCl(2) species and with several Ru(III) chloro-species, and highly positive for reactions of this ether with TeCl(6)(2-), and with several Cd(II) and Pd(II) species. The atomic radius of an element is also a critical parameter for the reaction with crown ether, particularly the element ions with [Kr]4d(n)5s(m) electron shell fits the best with the DC18C6 crown ring. It is demonstrated that the magnetic isotope effect in combination with the theory of orbital hybridization can help to understand the mechanism of complexation reactions. The suggested approach is also applied to explain previously published mass-independent fractionation of Hg isotopes in other types of chemical exchange reactions.
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ABSTRACT: Mercury (Hg) is assumed to be predominantly methylated by microorganisms in the environment. However, the mechanisms and extent of abiotic methylation are poorly appreciated. The understanding of the mecha-nisms leading to abiotic methylation and demethylation in the aquatic environment is of special concern since methylmercury (MeHg) biomagnifies in the food web. Bioaccumulating organisms have also been found to preserve specific Hg isotopic signatures that provide direct insight into aquatic Hg transformations. In this study we investigated the influence of chloride on the magnitude of Hg isotope fractionation during abiotic methylation of inorganic Hg (Hg(II)) using methylcobalamin as methyl donor compound. Coupling of gas chromatography with multi-collector inductively coupled plasma mass spectrometry has allowed to determine simultaneously isotopic ratios of inorganic and methyl-Hg species. Kinetic experiments demon-strated that the presence of chloride not only slowed the chemical alkylation of Hg(II) by methylcobalamin, but also decreased the extent of the methylation, which it is especially significant under visible light condi-tions due to the enhancement of MeHg photodecomposition. Abiotic methylation of Hg(II) by methylcobalamin in the presence of chloride caused significant Hg mass-dependent isotope fractionation (MDF) for both Hg(II) substrate (δ 202 Hg(II) from − 0.74‰ to 2.48‰) and produced MeHg (δ 202 MeHg from − 1.44‰ to 0.38‰) both under dark and visible light conditions. The value of this MDF under such saline con-ditions was higher than that previously reported (δ 202 MeHg from − 0.73‰ to 0.09‰) in the absence of chlo-ride and appeared mainly related to inorganic Hg speciation in solution, which is predominantly mercuric chloro-complexes (i.e. HgCl 4 2−). Different isotopic signatures were observed for the different Hg species at the same time of reaction for either dark or visible light (450–650 nm wavelengths) conditions. However, no significant mass-independent fractionation (MIF) was induced under any conditions within the analytical uncertainties (−0.17 ± 0.31 b Δ 201 Hg b 0.17 ± 0.28‰), suggesting that photo-induced demethylation does not always involve MIF. These results also suggest that methylation by methylcobalamin can be an experi-mental model to study Hg isotope fractionation extent during elementary reaction of methyl transfer in biotic systems.Chemical Geology 01/2013; 336:26. DOI:10.1016/j.chemgeo.2012.08.029 · 3.48 Impact Factor
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ABSTRACT: The isotopic composition of species-specific atmospheric mercury (Hg) was investigated in the coastal environment of Grand Bay, Mississippi, USA. Atmospheric mercury species (Hg0(g), HgII(g) and Hg(p)) were collected individually, and analyzed for isotopic composition. Gaseous elemental Hg (Hg0(g)) displayed δ202Hg ranging from − 3.88‰ to − 0.33‰. Aerosol Hg (Hg(p)) displayed intermediate δ202Hg ranging from − 1.61‰ to − 0.12‰, while reactive gaseous Hg (HgII(g)) displayed positive δ202Hg ranging from + 0.51‰ to + 1.61‰. Significant positive mass-independent fractionation (MIF) was observed in Hg(p) (∆199Hg = + 0.36‰ to + 1.36‰), while Hg0(g) displayed negative MIF (∆199Hg = − 0.41‰ to − 0.03‰) and HgII(g) displayed intermediate MIF (∆199Hg = − 0.28‰ to 0.18‰). Positive MIF of 199Hg and 201Hg measured in Hg(p) is consistent with significant in-aerosol photoreduction. Significant MIF of 200Hg was observed in all Hg species with Hg0(g) displaying negative ∆200Hg values of − 0.19‰ to − 0.06‰ while HgII(g) and Hg(p) displayed positive ∆200Hg values of + 0.06‰ to + 0.28‰, which are similar to ∆200Hg values reported by Gratz et al. (2010). These results suggest that isotope tracing of each atmospheric Hg species may be feasible during important atmospheric processes such as wet and dry deposition.Chemical Geology 01/2013; 336:37–49. DOI:10.1016/j.chemgeo.2012.10.007 · 3.48 Impact Factor
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ABSTRACT: Isotope fractionation between the common Cu species present in solution (Cu+, Cu2+, hydroxide, chloride, sulfide, carbonate, oxalate, and ascorbate) has been investigated using both ab initio methods and experimental solvent extraction techniques. In order to establish unambiguously the existence of equilibrium isotope fractionation (as opposed to kinetic isotope fractionation), we first performed laboratory-scale liquid–liquid distribution experiments. Upon exchange between HCl medium and a macrocyclic complex, the 65Cu/63Cu ratio fractionated by −1.06‰ to −0.39‰. The acidity dependence of the fractionation was appropriately explained by ligand exchange reactions between hydrated H2O and Cl− via intramolecular vibrations. The magnitude of the Cu isotope fractionation among important Cu ligands was also estimated by ab initio methods. The magnitude of the nuclear field shift effect to the Cu isotope fractionation represents only ∼3% of the mass-dependent fractionation. The theoretical estimation was expanded to chlorides, hydroxides, sulfides, sulfates, and carbonates under different conditions of pH. Copper isotope fractionation of up to 2‰ is expected for different forms of Cu present in seawater and for different sediments (carbonates, hydroxides, and sulfides). We found that Cu in dissolved carbonates and sulfates is isotopically much heavier (+0.6‰) than free Cu. Isotope fractionation of Cu in hydroxide is minimal. The relevance of these new results to the understanding of metabolic processes was also discussed. Copper is an essential element used by a large number of proteins for electron transfer. Further theoretical estimates of δ65Cu in hydrated Cu(I) and Cu(II) ions, Cu(II) ascorbates, and Cu(II) oxalate predict Cu isotope fractionation during the breakdown of ascorbate into oxalate and account for the isotopically heavy Cu found in animal kidneys.Geochimica et Cosmochimica Acta 06/2013; 110:29–44. DOI:10.1016/j.gca.2013.02.007 · 4.25 Impact Factor