Structural and dynamic aspects of hydration of HAsO4(-2): an ab initio QMCF MD simulation

An ab initio quantum mechanical charge field simulation has been carried out in order to obtain molecular level insight into the hydration behavior of HAsO4(-2), one of the major biologically active components of As(V) oxoanion in neutral to slightly alkaline aqueous medium. Moreover, a geometrical definition of hydrogen bonding has been used to probe and characterize both solute-solvent and solvent-solvent hydrogen bonding present in the system. The asymmetry of the anion induced by the protonation of one of the oxygens of the arsenate anion causes rather irregular hydration structure. The nonprotonated oxygen atoms preferably form relatively stable hydrogen bonds with two to three water molecules in their vicinity, while the protonated oxygen forms one or two hydrogen bonds, weaker than water-water hydrogen bonds. The two types of As-O distances obtained from the simulation (1.68 and 1.78 A for the protonated and nonprotonated oxygens, respectively) are in good agreement with the experimental data. The two types of As-O stretching frequencies obtained from the simulation (855 and 660 cm(-1) reproduce well the experimental ATR-FTIR results (859 and 680-700 cm(-1)).