Chemical pressure-chemical knowledge: squeezing bonds and lone pairs within the valence shell electron pair repulsion model.

The valence shell electron pair repulsion (VSEPR) model is a demanding testbed for modern chemical bonding formalisms. The challenge consists in providing reliable quantum mechanical interpretations of how chemical concepts such as bonds, lone pairs, electronegativity, or hypervalence influence (or modulate) molecular geometries. Several schemes have been developed thus far to visualize and characterize these effects; however, to the best of our knowledge, no scheme has yet incorporated the analysis of the premises derived from the ligand close-packing (LCP) extension of the VSEPR model. Within the LCP framework, the activity of the lone pairs of the central atom and ligand-ligand repulsions constitute the two key features necessary to explain certain controversial molecular geometries that do not conform to the VSEPR rules. Considering the dynamical picture obtained when electron local forces at different nuclear configurations are evaluated from first-principles calculations, we investigate the chemical pressure distributions in a variety of molecular systems, namely, electron-deficient molecules (BeH2, BH3, BF3), several AX3 series (A: N, P, As; X: H, F, Cl), SO2, ethylene, SF4, ClF3, XeF2, and nonequilibrium configurations of water and ammonia. Our chemical pressure maps clearly reveal space regions that are totally consistent with the molecular and electronic geometries predicted by VSEPR and provide a quantitative correlation between the lone pair activity of the central atom and electronegativity of ligands, which are in agreement with the LCP model. Moreover, the analysis of the kinetic and potential energy contributions to the chemical pressure allows us to provide simple explanations on the connection between ligand electronegativity and electrophilic/nucleophilic character of the molecules, with interesting implications in their potential reactivity. NH3, NF3, SO2, BF3, and the inversion barrier of AX3 molecules are selected to illustrate our findings.

The removal of some rare earth elements from their aqueous solutions on by-pass cement dust (BCD).

The sorption behavior of yttrium (Y(3+)), neodymium (Nd(3+)), gadolinium (Gd(3+)), samarium (Sm(3+)) and lutetium (Lu(3+)) from their aqueous solutions by by-pass cement dust (BCD) has been investigated using a batch technique. The sorption on BCD was studied as a function of pH, shaking time, initial concentration, mass of BCD and temperature. It was found that the sorption capacity of BCD had the order of Lu(3+) > Sm(3+) > Y(3+) > Gd(3+) ≈ Nd(3+) following Freundlich isotherm at the determined optimum conditions. The results also demonstrated that the sorption data fit well the pseudo-second-order kinetic model. Thermodynamic parameters such as ΔH°, ΔS° and ΔG° indicated that the sorption of the investigated REEs on BCD was endothermic, favored at high temperature and spontaneous in nature.