Computing the properties of the copper thioarsenite complex, CuAsS(SH)(OH).

In aqueous solutions in equilibrium with the mineral assemblage CuS (covellite)-Cu(1.8)S (digenite)-Cu(3)AsS(4) (enargite), a copper thioarsenite species, CuAsS(SH)(OH), makes major contributions to both Cu and As solubility (Clarke, M. B.; Helz, G. R. Environ. Sci. Technol. 2000, 34, 1477-1482. The structure and energetics of this complex have been calculated quantum mechanically (Tossell, J. A. Environ. Sci. Technol. 2000, 34, 1483-1488), confirming its high stability and establishing the presence of a novel direct bond between Cu (formally Cu(I)) and As (which is formally As(III) and retains its lone pair). To provide further evidence for the existence of this complex, it would be desirable to concentrate it and to measure its spectral properties. To assist in the confirmation of its identify, we have calculated a number of different spectral properties for this complex, including its vibrational, visible/UV, X-ray absorption near edge (XANES), and Cu and As NMR spectra. The visible/UV spectrum has been calculated using both Hartree-Fock and density functional theory based methods, which have been tested against the known properties of the gas-phase CuCl molecule. We calculate distinctive stretching vibrations of the three-membered Cu-As-S ring around 440-500 cm(-1), an absorption in the visible at around 2.4 eV, and an absorption at low energy in XANES (compared to that calculated for Cu(SH)(2)(-)). The calculated Cu and As NMR properties of CuAsS(SH)(OH) are also distinctive, but both Cu and As are quadrupolar nuclides and their calculated quadrupole coupling constants in CuAsS(SH)(OH) are very large, so their NMR signals may not be observable. Mulliken population analyses, natural bond orbital analyses, and contour plots of HOMO and LUMO electron densities are also used to characterize the bonding within the copper thioarsenite complex. We have also calculated the hydration energy of the complex using polarized dielectric continuum methods, confirming its low degree of stabilization in water.