Trends in the frequencies of ν(AsOxHx-1) [x=2-4] in selected As(V)-containing compounds investigated using quantum chemical calculations

The application of computational chemistry to studies in geochemistry is increasingly becoming invaluable in explaining experimentally observed trends for surface interactions of pollutants with sorbents ubiquitous in the environment. We report computational results on factors that affect the force constant of AsQ(x) bonds in As(V)-containing compounds relevant to geochemical environments. Geometries, atomic charges, and stretching frequencies of -AsOxHx-1 (x = 2-4) moieties in these molecules were calculated using semi-empirical methods (PM3) and density functional theory (B3LYP) for both isolated (gas phase) molecules and hydrated complexes in which the molecules are surrounded by four water molecules. We found that the number of organic substituents has a relatively smaller effect on the force constant of AsOx bonds than protonation. The increase in resonance effect with deprotonation causes As-O bond lengths to increase, and the decrease in resonance in fully deprotonated species with increasing organic substitution causes As-O bond lengths to decrease. In the absence of the resonance effect in fully protonated species, As-O bond lengths increase with more organic substituents. Also, increasing organic substitution causes the charge on the central arsenic atom to decrease. Charges on oxygen atoms in As-OH bonds are more sensitive to deprotonation than to resonance relative to other oxygen atoms in As-O bonds. As expected, frequencies of nu(AsOx) show an inverse relationship with As-O bond lengths upon deprotonation and organic substitution. Our results have implication for the interpretation of infrared and X-ray absorption spectra of adsorbed As(V)-containing compounds.