Structural and electronic evolution of the As(OH)3 molecule in high temperature aqueous solutions: An x-ray absorption investigation.

The geometrical and electronic structure of the arsenious acid molecule As(OH)₃ in aqueous solutions has been investigated by x-ray absorption spectroscopy (XAS) within extended x-ray absorption spectroscopy (EXAFS) and x-ray absorption near edge structure (XANES), using realistic first-principle calculations in the latter case. This investigation was performed on aqueous solutions of arsenious acid from ambient to supercritical conditions (P=250 and 600 bars, T≤500 °C) using a new optical cell. The analysis of the XAS spectra is consistent with (1) a constant As-O distance, (2) an opening of the O-As-O angles within the C_3V pyramidal structure in the range 30–200 °C. This structural evolution comes along with a small decrease of the partial charges of the atoms in the As(OH)₃ molecule. The explanation invoked for both structural and electronic modifications observed is the weakening of the interactions, through hydrogen bonds, between the As(OH)₃ complex and water molecules. This is a fingerprint of the similar weakening of hydrogen bonding interactions in the solvent itself. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]