Thermodynamic model for arsenic speciation in sulfidic waters: A novel use of ab initio computations     

G.R. Helz  J.A. Tossell 《Geochimica et cosmochimica acta》2008,72(18):4457-4468

Recent discoveries demonstrate that the chemistry of arsenic in sulfidic waters is much more complex that previously believed. One implication is that all earlier thermodynamic data on stabilities of As thioanions require revision. Previously used experimental approaches for determining As thioanion stabilities may be inadequate to deal with the full range of complexity. Here we use computational as well as empirical information to construct a provisional model for equilibrium As thioanion distributions in sulfidic waters. Whereas previous authors have argued for either As(III) or As(V) thioanions, the new model predicts that both are important and can occur simultaneously under commonly encountered pH and ΣS^−II conditions. At the order of magnitude level, the model reasonably predicts the solubility of As₂S₃ in sulfidic solutions, provides tentative peak assignments for published Raman spectroscopic data and plausibly accounts for how sulfide modifies the bacterial toxicity of As. The model yields a thermodynamic justification for how sulfide, which is usually regarded as a reducing agent, can counter-intuitively drive oxidation of As(III) to As(V), as has been observed both in the laboratory and in the field. Despite its uncertain accuracy, the model serves as a useful source of new, testable hypotheses about As geochemistry and highlights crucial experimental data needs.  相似文献   

First principles quantum mechanical calculation of the electric dipole polarizabilities of the borate,carbonate, nitrate and related ions     

J. A. Tossell 《Physics and Chemistry of Minerals》1990,17(2):133-141

Electric dipole polarizabilities have been calculated from first principles of quantum mechanics for the BO ₃ ^3? , CO ₃ ^2? , NO ₃ ^? series and for NO ₂ ^? and LiNO₃(g). Calculated trends in average polarizability and polarizability anisotropy in the BO ₃ ^3? -NO ₃ ^? series are in agreement with experiment and can be qualitatively interpreted in terms of the varying energies of the a₁′, a₂″ and e′ symmetry unoccupied MO's of the oxyanions. Embedding a CO ₃ ^2? ion in a D_3h symmetry array of divalent cations reduces both the average polarizability and its anisotropy, particularly when diffuse s and p functions are included in the calculation. Calculations on the gas phase LiNO₃ molecule and on the free NO ₃ ^? ion in the distorted geometry found in LiNO₃(g) allow us to separate polarizability contributions internal to the NO ₃ ^? and Li⁺ ions from those which arise from the Li⁺-NO ₃ ^? interaction. The Li⁺-NO ₃ ^? interaction term so obtained is much smaller than the NO ₃ ^? contribution but is in turn larger than the Li⁺ contribution, suggesting that the inclusion of this interaction term is essential for obtaining accurate results for ion pairs. Although static polarizabilities are in reasonable agreement with experiment for NO ₃ ^? the wavelength dispersion of the polarizability is underestimated by about a factor of two, apparently as a result of inadequacies in the quantum mechanical method. Calculated values are also presented for ¹⁴N NMR shieldings in the nitrogen oxyanions but these are in only qualitative agreement with the experimental values. Similarly, calculated values of magnetic susceptibility are in only qualitative agreement with experiment although trends along the BO ₃ ^3? -NO ₃ ^? series are properly reproduced.  相似文献   

Ab initio molecular orbital calculations for boron isotope fractionations on boric acids and borates   总被引：1，自引：0，他引：1  

Yun Liu  John A. Tossell 《Geochimica et cosmochimica acta》2005,69(16):3995-4006

Ab initio quantum chemistry calculations have been performed on the isotopic exchange reaction between B(OH)₃ and B(OH)₄⁻. Several calculation methods have been carefully compared and evaluated. The “water-droplet” method is chosen to investigate this isotope exchange reaction using cluster models with up to 34 water molecules surrounding the solute. HF/6-31G* level calculations coupled with a 0.920 scaling factor are used for the frequency calculations. A larger K value (1.027) is obtained from this study compared to the commonly used 1.0194 (Kakihana et al., 1977).The fractionations for several boric acid polymers and boron minerals are also studied. Our results suggest that assuming the BO₄ bonding in B(OH)₄⁻ is identical to that in borosilicates is wrong. Tetrahedral boron in silicates has a significantly smaller reduced isotopic partition function ratio (RPFR) and hence will be much isotopically lighter than in B(OH)₄⁻.The new theoretical curve of pH vs. δ¹¹B composition of B(OH)₄⁻ using our calculated 1.027 can be used to predict pH values for equilibrium cases such as incorporation into inorganic calcite. We also find that the shape of this curve is very sensitive to both K and pK_a value, giving the possibility of also predicting salinity from the different shapes of the curve.  相似文献   

A qualitative molecular orbital study of the stability of polyanions in mineral structures     

J. A. Tossell 《Physics and Chemistry of Minerals》1983,9(3-4):115-123

The existence and structures of various polyanion species, A _k ^n? ,occurring in inorganic compounds and minerals are rationalized using qualitative molecular orbital (MO) theory. Polyanions which may be stabilized by high pressure or lattice defects are discussed and compounds likely to show such species are identified. Previously observed high pressure properties of Fe riebeckite and MgO are interpreted in terms of O - O bond formation at high pressure.  相似文献   

Electronic structures of sulfide minerals — Theory and experiment     

David J. Vaughan  John A. Tossell 《Physics and Chemistry of Minerals》1983,9(6):253-262

The sulfide minerals exhibit a rich diversity in sturctural chemistry and in electrical, magnetic and other physical properties. Models based on molecular orbital theory and incorporating some elements of band theory can be developed to describe the diverse valence electron behavior in these minerals. Qualitative models can be proposed on the basis of observed properties, and the models can be tested and refined using experimental data from X-ray emission and X-ray photoelectron spectroscopy and quantum mechanical calculations performed on cluster units which form the basic building blocks of the crystals. This approach to chemical bonding in sulfide minerals is illustrated for binary non-transition metal sulfides (ZnS, CdS, HgS, PbS), binary transition metal sulfides (FeS₂, CoS₂, NiS₂, CuS₂ ZnS₂) and more complex sulfides (CuFeS₂, Cu₂S, Ag₂S, CuS, Co₃S₄, CuCo₂S₄, Fe₃S₄). The relationship between qualitative and quantitative theories is reviewed with reference to the pyrite-marcasite-arsenopyrite-loellingite series of minerals. Application of the models to understanding structure-determining principles, relative stabilities, solid solution limits and properties such as color, reflectance and hardness are discussed.  相似文献   

Calculation of the UV Absorption Spectra of As(III) Oxo- and Thio- Acids and Anions in Aqueous Solution and of PF3 in the Gas-phase     

J. A. Tossell 《Aquatic Geochemistry》2001,7(4):239-254

Changes in the UV spectra of As(OH)₃ solutions with variations in pH and temperature have recently been used to determine the temperature dependence of the pK_a of the acid. In previous studies I used quantum mechanical techniques to study changes in structure and vibrational spectra as a function of pH for arsenites and thioarsenites. I previously calculated UV spectra for ``molecular' minerals, like realgar As₄S₄. Here I use a number of different quantum mechanical methods, both Hartree-Fock and density functional theory based, to calculate the UV spectra for both a related simple well-characterized gas-phase molecule PF₃ and for As(OH)₃ and As(SH)₃ and their conjugate anions and some neutral and anionic oligomers in aqueous solution. For the monomeric species small numbers of water molecules have been explicitly included, in a supermolecule or microsolvation approach. I find that UV absorption energies accurate to a few tenths of an eV can be obtained both for gas- phase PF₃ and for neutral arsenious acid in aqueous solution, for which the UV absorption maximum is calculated to occur around 6.5 eV, consistent with experiment. Accurate calculation of the UV energies for arsenite anions in aqueous solution is much more difficult, since basis set size and solvation effects are considerably larger than for the neutral molecules, but fairly reliable results can still be obtained. Deprotonation is found to reduce the lowest calculated UV transition energy by about half an eV. Oligomerization also reduces the lowest calculated UV energy by at least half an eV. Replacement of one or all the –OH groups by –SH groups reduces the lowest calculated UV energies by about 2 eV. UV excitation energies have been calculated for oligomeric species as large as As₃E₃(EH)₃ and As₄E₆, where E = O, S, and may be useful for identifying such species in solution.  相似文献   

Structures and cohesive properties of hydroxides and fluorides calculated using the modified electron gas ionic model     

J. A. Tossell 《Physics and Chemistry of Minerals》1981,7(1):15-19

Molecular cluster calculations using the purely theoretical modified electron gas (MEG) version of the ionic model predict M-OH and M-F distances with an average error of less than 0.03 Å for a wide range of M cations. Such accuracy compares favorably with that obtained using the more expensive ab initio self-consistent-field molecular orbital (MO) method. The frequencies of totally symmetric stretching vibrations are also predicted with an average error of about 25% using the MEG method but cohesive energies are often in error by large amounts. Cohesive energy errors are highly correlated with electronegativity differences, suggesting that they should be ascribed to ovalent bonding effects. The increase of M-X distance with increasing M coordination number can be simply interpreted in terms of the magnitude and number of M-X attractive and X-X repulsive interactions within the MEG model.  相似文献   

Molecular orbital studies of geometries and spectra of minerals and inorganic compounds     

J. A. Tossell  G. V. Gibbs 《Physics and Chemistry of Minerals》1977,2(1-2):21-57

Extended Hückel molecular orbital theory (EHT) and simple, approximate Self-Consistent-Field MO methods are employed to explain the geometries of nontransition metal bearing minerals and inorganic compounds. The spectra of such minerals and the electronic structure of transition metal oxidic minerals are explained using the Self-Consistent-Field X α MO method. EHT provides an objective algorithm for rationalizing and correlating bond length and angle data for insular and polymerized TO ₄ ^?n tetrahedral oxyanions where T=Be, B, Al, Si, P, S, Ge, As and Se. Calculated bond overlap populations n(T-O), correlate linearly with the observed T-O bond lengths with shorter bonds tending to involve larger n(T-O) values. Such calculations show that n(T-O) is strongly dependent upon the average of the three O-T-O angles associated with a common bond, larger n(T-O) values involving wider angles. Calculations of n(T-O) as a function of the T-O-T angles in T ₂O ₇ ^?n ions, indicate that the n(T-O) values for the bonds to the bridging oxygen atoms increase nonlinearly with increasing T-O-T angle whereas those the nonbridging oxygens decrease slightly as the angle widens. In agreement with the experimental data, these results predict that shorter T-O bonds should involve wider O-T-O and T-O-T angles. The SCF-X α MO cluster model is then applied to silica and FeO. The calculations yield a satisfactory interpretation of the visible, UV and X-ray emission and X-ray photoelectron spectra of these materials. Theoretical and empirical MO diagrams are constructed and the electronic structures of the materials are discussed.  相似文献   

大兴安岭中南段中生代成矿物质的深部来源与背景   总被引：19，自引：3，他引：16  

邵济安  牟保磊  朱慧忠  张履桥 《岩石学报》2010,26(3):649-656

大兴安岭是我国北方一个重要的多金属成矿带。本文从成矿的物质来源和构造作用两方面讨论大兴安岭的成矿系统。Sr、Nd、O、Pb同位素的研究显示大兴安岭成矿物质的深部来源,大兴安岭晚中生代壳幔混熔花岗质岩石组成及其构造环境的研究,表明它们是在板内非造山的伸展环境下形成的A型花岗岩。与南岭花岗岩及其成矿作用的对比研究,将加深对大兴安岭中生代成矿特征的认识。深部构造特征也进一步印证了大兴安岭的成岩-成矿的背景。  相似文献   

Interpretation of K X-ray emission spectra and chemical bonding in oxides of Mg,Al and Si using quantitative molecular orbital theory     

J.A Tossell 《Geochimica et cosmochimica acta》1973,37(3):583-594

Quantitative molecular orbital calculations are reported for Mg, Al and Si in tetrahedral and octahedral coordination with oxygen. These calculations are employed to assign and interpret the ME_α, ME_β and OK_α X-ray emission spectra of the corresponding oxides. The interpretation of the MK_β spectrum reproduces the observed trends in main peak and satellite energies with variation of metal, ligand and geometry. The splitting of the main K_β peak, observed in many oxides, is found to be a result of interaction between adjacent metal atoms. The calculations also reproduce the observed trends in OK_α spectra. The electronic structures of the various oxides are discussed briefly.  相似文献