Intramolecular bonding and charge distributions in XO4 (X = Si, P, S, Cl and Ge, As, Se, Br) oxyanions from topological analyses of the electron density

Intramolecular X-O bonds and charge distributions in XO4 (X = Si, P, S, Cl, and Ge, As, Se, Br) oxyanions were investigated with topological analyses of the electron density in the frameworks of the theories of atoms in molecules (AIM) and of the electron localization function (ELF). The optimized geometries and wave functions of all oxyanions were obtained at the B3LYP/6-311+G(3dt) level. AIM analyses recover a significant concentration of electrons at (3, -1) bond critical points increasing from SiO44- (0.12 au) to ClO41- (0.37 au) and from GeO44- (0.14 au) to BrO41- (0.27 au). Perchlorate is the only oxyanion exhibiting true shared interactions in the sense of AIM with a large and negative Laplacian of the electron density at the bond critical point. All other oxyanions are intermediate to shared and closed-shell interactions with positive values of the Laplacian of the electron density. ELF analyses tend to support the conclusions of AIM by localizing disynaptic basins between 3rd row X atoms and O with electron populations ranging from 1.56e (SiO44-) to 1.84e (CIO41-). The X-O bonds are also shown to be of donor-acceptor type. Fourth row oxyanions have highly delocalized and small disynaptic basin populations ranging from 0.27e (ASO(4)(3-)) to 0.30e (BrO41-) with GeO44- as the only oxyanion without any formally defined disynaptic basin. The attractive forces in these oxyanions are thus mostly of electrostatic nature. All ELF disynaptic basins have strong cross-contributions with the lone pair monosynaptic valence basins of O. These latter basins contain about three electron pairs (6.12e) for ClO41- to about four (7.99e) for GeO44-. Finally, the Merz-Kollman scheme was used to determine point charges that can reproduce the electrostatic potential of the oxyanions. Their values are however notably different than those obtained by AIM.