Stationary Conditions of the Electron Density Along the Reaction Path: Connection with Conceptual DFT and Information Theory.

The Kohn-Sham density functional theory (DFT) formalism has been used to investigate the influence of the stationary behavior of the electron density (ρ(r⃗;s)) along a minimum energy path on the corresponding stationary conditions observed in the total potential energy of the reactive system, information theory measures (Shannon information entropy and Onicescu information energy), and chemical reactivity indexes (the chemical hardness). The theoretical treatment presented in this work, combined with DFT calculations on 3 different test reactions: H' + H2, H' + CH4 and H- + CH4, suggest that for any reactive system, properties that can be cast as a functional of the electron density, must exhibit stationary points along the IRC path modulated by the corresponding stationary behavior of the electron density.

A density functional theory study of the topology of the charge density of complexes of 8-hydroxyquinoline with Mn(III), Fe(III), and Co(III).

By use of the quantum theory of atoms in molecules, it was found that the electronic charge distribution rho(r) of the metal atoms in Mn(III), Fe(III), and Co(III) complexes of 8-hydroxyquinoline (8HQ) showed eight nonbonded concentrations in their valence shell that were located at the corners of a cube and a depletion region was located in each of its six faces. Coordination was such that regions of charge concentration of the ligands matched the depletion ones of the metal. The O- and N-metal bonds showed low rho(r(c)) values at the bond critical point r(c) and low and positive ones for its Laplacian indicating that they were dative bonds of close shell type with a degree of covalency. Most changes in rho(r) were located around the N and O atoms of 8HQ directly involved in dative bonds. By use of the delocalization index delta(C(A),C(B)) only for C-C bonds, it was found that coordination increased the aromaticity of most of them. The most important changes in rho(r) were found in the C-H bonds were a noticeable increase in bond strength was obtained upon coordination.

Dipole orientation and surface cluster size effects on chemisorption-induced magnetism: a DFT study of the interaction of gold-thiopolypeptide.

A nanosystem formed by a high electric dipole moment thiopolypeptide alpha-helix, consisting of eight l-glycine units, chemisorbed on the (111) surface of Au23 and Au55 clusters, with the S as the linking atom, was studied using the wave function broken symmetry UDFT method. We have found a strong correlation between the orientation of the electric dipole of the alpha-helix and charge transfer and the magnetic behavior of the adsorbate-cluster system. Upon chemisorption, dipole moments may be quenched or enhanced, with respect to the gas phase value, with the strongest reduction corresponding to the magnetic state. A reduction of the alpha-helix's electric dipole with the net charge transfer from the Au surface was obtained for the more stable state. In this state description, it may happen that the calculated spin densities of the chemisorbed alpha-helix and its free radical form are similar. The magnetic properties are strongly dependent on the size of the Au cluster and on its electronic structure with respect to nuclei positions. In general, the localized spin density per atom increases and the magnetization of the extended system decreases with cluster size, a trend found experimentally for organic monolayers with a similar type of adsorbate we consider here.

Topology of charge density of flucytosine and related molecules and characteristics of their bond charge distributions.

The molecular charge distribution of flucytosine (4-amino-5-fluoro-2-pyrimidone), uracil, 5-fluorouracil, and thymine was studied by means of density functional theory calculations (DFT). The resulting distributions were analyzed by means of the atoms in molecules (AIM) theory. Bonds were characterized through vectors formed with the charge density value, its Laplacian, and the bond ellipticity calculated at the bond critical point (BCP). Within each set of C=O, C-H, and N-H bonds, these vectors showed little dispersion. C-C bonds formed three different subsets, one with a significant degree of double bonding, a second corresponding to single bonds with a finite ellipticity produced by hyperconjugation, and a third one formed by a pure single bond. In N-C bonds, a decrease in bond length (an increase in double bond character) was not reflected as an increase in their ellipticity, as in all C-C bonds studied. It was also found that substitution influenced the N-C, C-O, and C-C bond ellipticity much more than density and its Laplacian at the BCP. The Laplacian of charge density pointed to the existence of both bonding and nonbonding maxima in the valence shell charge concentration of N, O, and F, while only bonding ones were found for the C atoms. The nonbonding maxima related to the sites for electrophilic attack and H bonding in O and N, while sites of nucleophilic attack were suggested by the holes in the valence shell of the C atoms of the carbonyl groups.