Comparison of the electronic structure of the P-O and P-S bonds

The electronic structures of the P-O and P-S bonds were compared in PO₃^-, HPO₃, H₂PO₄^-, H₃PO₄ and their monosubstituted sulfur analogues. Plots of the charge difference density around the P-X (XO or S) bond support a polar structure for both the oxygen and the sulfur anions. The proton affinities and the binding of a single water molecule at the oxygen and sulfur sites are comparable, illustrating the polarity of both the P-O and the P-S bond. The energy surface for the phosphate and the phosphorothioate anions as a function of the torsional angle for the orientation of the O-H bonds also demonstrates the polarity of the two bonds. Difference density plots comparing the neutral and ionic phosphates suggest that this bond is also similar in both neutral and ionic systems. An ionic P-O and P-S bond provides a better representation of the structure than a double bond, even for the neutral molecules.