A Density Functional Theory- and Atoms in Molecules-based Study of NiNTA and NiNTPA Complexes toward Physical Properties Controlling their Stability. A New Method of Computing a Formation Constant

The log K1 value of analytical quality was obtained for the NiNTPA complex using the density functional theory (DFT)-computed (at the B3LYP/6-311þþG(d,p) level of theory in solvent, CPCM/UAKS) G(aq) values of the lowest-energy conformers of the ligands, nitrilotriacetic acid (NTA) and nitrilotri-3-propanoic acid (NTPA), and the Ni(II) complexes (NiNTA and NiNTPA). The described mathematical protocol is of a general nature. The topological analysis, based on the quantum theory of atoms in molecules (QTAIM) of Bader, was used to characterize coordination bonds, chelating rings, and additional intramolecular interactions in the complexes. The topological data, but not the structural analysis, explained the observed difference in stability of the NiNTA and NiNTPA complexes. It was found that the structural H 3 3 3 H contacts (classically regarded H-clashes, a steric hindrance destabilizing the complex) are in fact the H-H bonds contributing to the overall stability of NiNTPA. Also a CH-O bond was found in NiNTPA. The absence of intramolecular bonds between the atoms that fulfill a distance criterion in NiNTPA is explained by the formation of adjacent intramolecular rings that have larger electron density at the ring critical points when compared with the rings containing these atoms. It is postulated that the strength of a chelating ring (a chelating effect) can be measured by the electron density at the ring critical point. It was found that the strain energy, Es, in the as-in-complex NTPA ligand (Es is significantly lowered by the presence of the intramolecular bonded interactions found by QTAIM) is responsible for the decrease in strength of NiNTPA; the Es ratio (NTPA/NTA) of 1.9 correlates well with the experimental log K1 ratio (NTA/NTPA) of 1.98.Financial support of the National Research Foundation of South Africa and the University of Pretoria

Morphology-Tuned Synthesis of Single-Crystalline V5Si3 Nanotubes and Nanowires

We report the morphology-tuned synthesis of single-crystal line V5Si3 nanotubes and nanowires. Free-standing hexagonal V5Si3 nanostructures are grown on a vanadium foil substrate placed on Si powder by a chemical vapor deposition method. By controlling the reaction time and substrate temperature, we have succeeded in morphology tuning Of V5Si3 nanotubes and nanowires. When the downstream temperature is fixed at 950 degrees C, V5Si3 nanotubes are formed with the reaction time of 5 min and V5Si3 nanowires are obtained with the reaction time of 15 min. With the downstream temperature of 950 degrees C and the reaction time of 15 min, when Si powder is mixed with carbon powder to the Si composition of 10-50%, both nanotubes and nanowires are simultaneously formed on the vanadium foil along the temperature gradient. Single-crystalline V5Si3 nanotubes thus synthesized are the first metal silicide nanotubes reported so farclose141