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The process of equilibration of the tetradecane-water interface in the presence of sodium hexadecane-benzene sulfonate is studied using intensive atomistic molecular dynamics simulations. Starting as an initial point with all of the surfactants at the interface, it is obtained that the equilibration time of the interface (several microseconds) is orders of magnitude higher than previously reported simulated times. There is strong evidence that this slow equilibration process is due to the aggregation of surfactants molecules on the interface. To determine this fact, temporal evolution of interfacial tension and interfacial formation energy are studied and their temporal variations are correlated with cluster formation. To study cluster evolution, the mean cluster size and the probability that a molecule of surfactant chosen at random is free are obtained as a function of time. Cluster size distribution is estimated, and it is observed that some of the molecules remain free, whereas the rest agglomerate. Additionally, the temporal evolution of the interfacial thickness and the structure of the surfactant molecules on the interface are studied. It is observed how this structure depends on whether the molecules agglomerate or not.

The electron density localization in insulator and semiconductor elemental cubic materials with diamond structure, carbon, silicon, germanium, and tin, and good metallic conductors with face centered cubic structure such as α-Co, Ni, Cu, Rh, Pd, Ag, Ir, Pt, and Au, was studied using a localized electrons detector defined in the local moment representation. Our results clearly show an opposite pattern of the electron density localization for the cubic ceramic and transition metal materials. It was found that, for the elemental ceramic materials, the zone of low electron localization is very small and is mainly localized on the atomic basin edges. On the contrary, for the transition metals, there are low-valued localized electrons detector isocontours defining a zone of highly delocalized electrons that extends throughout the material. We have found that the best conductors are those in which the electron density at this low-value zone is the lowest.

The topology of the electron density for face-centered cubic transition metals (β-Co, Ni, Cu, Rh, Pd, Ag, Ir, Pt, and Au) was studied. All of the electron density critical points in the unit cell w...

The interaction of a CO molecule with the (100) surface of the fcc d-block transition metals is analyzed by the topology of the Laplacian of the electronic charge density −∇ 2 ρ . This analysis shows that the atomic graph for the top atoms is an octahedron that exposes, outside the (100) surface, a vertex V top with a local maximum of charge, and four faces F top with a local minimum. The corresponding graph for the second-layer atoms is also an octahedron that exhibits a vertex V sec outside the surface. Atomic graphs for the CO molecule show that the carbon atom has a non-bonded vertex C nb , and a torus C torus of charge depletion perpendicular to the CO bond direction. Laplacian topology predicts CO perpendicular orientation in bridge- or top-sites. The bridge-site involves attractive interactions with two atoms of the surface: face (metal top atom)–vertex (C atom)–face (metal top atom). The top-site involves the attractive interaction vertex (C atom)–face (metal top atom). Attractive interaction between the surface and only one atom of the CO molecule rules out dissociation and predicts molecular adsorption.

This paper demonstrates that the Laplacian of the electron density, the quantity ∇2ϱ(r), can be used to determine the conditions required for activation of a surface atom and thus guides one in determining surface adsorption sites. It also predicts the geometry of approach of the substrate relative to the surface site and whether the interaction will correspond to physi- or chemisorption. The topology of the electron density and its Laplacian distribution for bulk MgO, its (100), (110) and (140) surfaces, as well as of embedded clusters that model these and other structural features of the surface, are studied to determine the conditions required for chemisorption of carbon monoxide. The unreactive nature of five-coordinated oxygen in the (100) surface is retained when its coordination is decreased to four in a step or to three in a corner. It is shown that electronic excitation can activate a surface oxygen atom towards reaction with a Lewis base through the creation of a “hole” in its valence shell of charge concentration, as defined by the topology of the Laplacian distribution. The results determine the site responsible for the formation of the CO−2 radical anion on the surface of thermally activated MgO in the presence of CO gas. It also shown how a neighbouring activated oxygen atom in a stepped surface can lead to the formation of carbonate ion.

The virial field V(r) is defined by the local statement of the quantum mechanical virial theorem, as the trace of the Schrodinger stress tensor. This field defines the electronic potential energy density of an electron at r and integrates to minus twice the electronic kinetic energy. It is the most short-ranged description possible of the local electronic potential energy and it exhibits the same transferable behavior over bounded regions of real space (corresponding to the functional groups of chemistry) as does ρ(r). This article establishes a structural homeomorphism between −V(r) and ρ(r), showing that the two fields are homeomorphic over all of the nuclear configuration space. The stable or unstable structure defined by the gradient vector field Δρ(r; X) for any configuration X of the nuclei can be placed in a one-to-one correspondence with a structure defined by the field −ΔV(r; X′). In particular, a molecular graph for ρ(r) defining a molecular structure is mirrored by a corresponding virial graph for V(r) and the lines of maximum density linking bonded nuclei in the former field are matched by a set of lines of maximally negative potential energy density in the latter. The homeomorphism is also geometrically faithful, an equilibrium geometry in general, exhibiting equivalent structures in the two fields. The demonstration that the virial field, whose integrated value equals twice the total energy, is essentially just a locally scaled version of the electron density is suggestive of possible new approaches in density functional theory. © 1996 John Wiley & Sons, Inc.

An ab initio SCP MO study of the molecular electrostatic potential was performed on a series of monocyclic and bicyclic azines. Connections between the chemical reactivity of the azines and the molecular electrostatic potential were explored. Good correlations between the minima of the electrostatic potential, at the distance of the nitrogen van der Waals radius and at the position of the nitrogen nucleus, and the azines experimental basicity were found.

Nucleophilic attack towards the C atom of the CN group in CH3CN, NH2CN and FCN is analyzed on the basis of the laplacian of the molecular charge density ϱ(r) obtained from ab initio calculations. The relative reactivity of the C atom towards a nucleophilic attack, predicted by the values of the laplacian of ϱ(r) at the (3, +1) critical points of the valence shell, is in accordance with experiment. The connection between the electron-withdrawing ability of the substituent in aliphatic nitrites and nucleophilic attack toward the C atom was also obtained.

The dividing surfaces defining the regions where a variationally determined virial theorem holds can be taken as the surfaces through which the flux of the gradient of a scalar function that is different from the electronic density vanishes. Alternatively, real space can be partitioned through zero-flux surfaces of the gradient of the electronic density, a process which also renders fragments where the virial theorem holds. In the present study we examined the question of whether these two criteria are the same for very simple model wavefunctions for both homonuclear and heteronuclear diatomic molecules within the Born-Oppenheimer approximation. It was found that in the homonuclear case, which is essentially governed by the symmetry of the problem, the two criteria agree closely, the difference being in the curvature of the gradient contour lines. However, in the heteronuclear model, significant differences were found in both the curvature of the gradient contour lines and the shape of the dividing surface. The chemical and physical implications of these differences were analysed.

From ab initioSCF-MO calculation using a 6—31G** basis set, the relationship between the topology of the charge distribution and the electric field gradient (EFG) at the nuclei of the two-coordinated N atoms was determined in imines with R = H2C , HFC , NCHC , and H3CHC ; di-imides with R = H, F, and CN; and the monocyclic azines with one and two N atoms in it. It was found that the N nuclear quadrupole coupling constant was determined by the nonbonded concentration of N valence shell while the asymmetry parameter has contributions from this shell and also from the rest of the molecule.

The interaction of CH 4 with Mg 2+ O 2− and Li + O − adsorption model sites was studied using ab initio calculations and the topology of the charge distribution (Bader's formalism). Results indicate that the Li + O − site is the most active one because its charge density is more conveniently located and presents a lower activation barrier with respect to the Mg 2+ O 2− site. These results give an explanation of the catalytic enhancement which was observed experimentally in the activation of methane over an MgO catalyst with Li added as promoter.

The correlation energy for various diatomic molecules is calculated employing the expressions recently proposed (R.L. Boada and V.V. Karasiov, Chem. Phys. Lett., 175 (1990) 641; R.L. Boada, J. Maldonado and V.V. Karasiov, J. Mol. Struct. (Theochem), 287 (1994) 11). The calculated correlation energies are compared with results obtained from other methods.

It was found that the topology of the Laplacian of the molecular charge distribution can be used in the evaluation of the reactivity of model active sites such as those involved in the gas phase H abstraction from CH 4 by MgO and LiO molecules. The improvement of the catalytic activity of the surfaces of these solid oxides based in this result is also discussed

Ab-initio MO calculations using a 4-31G basis were performed for R-CNs with R = H, Cl, CCH, CN, NO, CH 3 , NH 2 and COH. It was found that substitution produced larger changes in σ than in π bonds of CN and that the C of that group exhibited a complicated two-way charge transfer with both R and N. Good correlation was found between changes in π population of CN and resonance parameters but not between the changes in sigma population and inductive parameters. The changes upon substitution in σ, and in σ plus π populations showed a good correlation with the difference in electronegativity (Δ EN ) between R and CN thus indicating that those changes are determined by Δ EN . It was also found that the electronegativity scale based on the analysis of the population at H in XH molecules produced better correlations with changes in population than other scales.

The relationship between the principal component of the electric field gradient (eqN) and the stretching force constant (kCN) obtained by Salem for diatomic molecules is extended to polyatomic molecules with linear fragments of type RCN. It is found that for nitriles there is a linear correlation between kCN and eqN Values, as predicted by theory, and that such correlation depends on the degree of conjugation between R and CN orbitals.

The force field for the in-plane vibrations of TCNQ and TCNQ-d 4 , has been calculated by the iterative self consistent method of Koptev et al. (Opt. Spektrosk., 25 (1968) 350). With the new set of force constants an improved matching was obtained between the observed and calculated vibration frequencies in TCNQ and TCNQ-d 4 .

The force field for the in-plane vibrations of TCNQ and TCNQ-d 4 , has been calculated by the iterative self consistent method of Koptev et al. (Opt. Spektrosk., 25 (1968) 350). With the new set of force constants an improved matching was obtained between the observed and calculated vibration frequencies in TCNQ and TCNQ-d 4 .

The interaction of a CO molecule with the Fe(100) surface is analyzed by the topology of the Laplacian of the electronic charge density −∇2ρ. This analysis shows that the atomic graph of the top atoms of the Fe(100) surface is a cube that exposes a face, Ftop, with a “hole” of charge or (3,+1) critical point of −∇2ρ, whereas the corresponding graph for the second layer atoms is an octahedron that exhibits a vertex, Vsec, protruding above the Fe(100) surface, with a “peak” of charge or (3,−3) critical point of −∇2ρ. Atomic graphs determined for the CO molecule show that the carbon atom has a non-bonded vertex, CVnb, and a torus, Ctorus, of charge depletion perpendicular to the C–O bond direction. As the CO molecule approaches the surface, electron transfer towards the tilted CO in turn induces two non-bonded critical points, OVnb, on the O atom. In accordance with experimental and theoretical results, the topological theory of the Laplacian suggests that the preferred pathway for CO dissociation corresponds to the tilted orientation on the surface, in which the attractive interaction Ctorus–Vsec is enhanced by the interaction of the two OVnb with the Ftop on two Fe top atoms of the (100) surface.