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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 nature of the H-bonds between the human protein HLA-DR1 (DRB*0101) and the hemagglutinin peptide HA306-318 has been studied using the Quantum Theory of Atoms in Molecules for the first time. We have found four H-bond groups: one conventional CO··HN bond group and three nonconventional CO··HC, π··HC involving aromatic rings and HN··HC aliphatic groups. The calculated electron density at the determined H-bond critical points suggests the follow protein pocket binding trend: P1 (2,311) >> P9 (1.109) > P4 (0.950) > P6 (0.553) > P7 (0.213) which agrees and reveal the nature of experimental findings, showing that P1 produces by a long way the strongest binding of the HLA-DR1 human protein molecule with the peptide backbone as consequence of the vast number of H-bonds in the P1 area and at the same time the largest specific binding of the peptide Tyr308 residue with aromatic residues located at the binding groove floor. The present results suggest the topological analysis of the electronic density as a valuable tool that allows a non-arbitrary partition of the pockets binding energy via the calculated electron density at the determined critical points.

The nature of the electron density localization in two MoS2 nanoclusters containing eight rows of Mo atoms, one with 100% sulphur coverage at the Mo edges (n8-100S) and the other with 50% coverage (n8-50S) was studied using a localized-electron detector function defined in the local moment representation. For n8-100S, pairs of neighboring S2 dimers cover the edges and the electron density localization function analysis shows the presence of a local triangular-shaped ring zone of highly delocalized electrons along these edges, which corresponds to a good metallic conductor zone. The optimized geometry analysis shows that the Mo-S2 bond length is much longer than that of the Mo-S bonds inside the cluster. The removal of one S atom from each sulphur dimer to create a cluster with 50% coverage produces a general compressive stress on the cluster optimized geometry, which shortens the Mo-S bond length, particularly at the edge. The electron density location function analysis shows that close to the cluster corners, a zone of highly delocalized electron zones with a characteristic semiconductor pattern and broken one-dimensional metallic ring was generated. These results suggest that the Mo-S2 bond elongation produced by the sulphur dimers is similar to a MoS2 monolayer under tensile strain and is the origin of the one-dimensional metallic sites at the Mo-edges. In general, the present findings show excellent agreement with the key features of the reported ambient pressure X-ray photoemission spectra and the corresponding simulated scanning tunneling microscopy images.

En este trabajo, la distribucion de las moleculas de 1-butanol y 2-butanol en los sistemas agua/n-octano y agua/SDS/n-octano fue determinada usando las herramientas gmx-density y gmx-densmap del programa gromacs con la finalidad de complementar a nivel computacional el comportamiento experimental estos co-surfactantes cuando estan localizados en la region interfacial de estos sistemas. Los modelos de energia potencial GROMOS53A6 y SPC fueron utilizados para describir a las moleculas de 1-butanol, 2-butanol, SDS y agua, respectivamente. Estos modelos fueron capaces de predecir las propiedades interfaciales del sistema agua/n-octano y el area por molecula del Dodecil Sulfato de Sodio en la interfaz agua/n-octano de forma consistente. Finalmente, los perfiles y mapas de densidad demuestran que las moleculas de alcohol y SDS coexisten en la region interfacial del sistema agua/n-octano favoreciendo la estabilidad de la monocapa de surfactante y la pelicula interfacial.

Interfacial properties of water/SDS+1-butanol/n-octane and water/SDS+2-butanol/n-octane systems have been determined using MD simulations with the aim of studying the molecular interactions between alcohols and SDS when are located at the water/n-octane interface. Also, interfacial properties of water/1-butanol/n-octane, water/2-butanol/n-octane and water/SDS/n-octane systems were determined to validate the used force fields. The relations of SDS/alcohol equal to 16:25, 4:5, 1:1 and 6:5 were used. The g(r)s were evaluated to determine effective interaction between the SDS and alcohol molecules at the interface. The g(r)s demonstrate that –OSO3− group present an effective interaction with the hydroxyl group of the alcohols. This indicate that hydroxyl groups can be located in the gap that exist between the –OSO3− groups of the SDS molecules. Therefore, interfacial activity of the SDS/alcohol monolayer is better, favoring the stability of the protective film and the reduction of the interfacial tension. This help to the formation and stability of the microemulsion.

Structural characteristics, interfacial distribution and molecular interactions of the components of the CO2(gas)/SDS/water/SDS/CO2(gas) systems as a function of the CO2(gas)/water interface coverage by the SDS surfactant to different amounts of the CO2 were studied with molecular dynamics simulations and the NVT ensemble. Initially, the repulsive nonbonding parameter between the water oxygen and CO2 oxygen was adjusted to improve the prediction of the solvation free energy, solubility of the CO2 gas in water and the behavior of the CO2(gas)/SDS/water/SDS/CO2(gas) systems at molecular level. Our results show that the stability of the studied foams can be improved incrementing of the vapor/water interface coverage with the SDS surfactant and the amount of CO2 in the system. With the highest interface coverage, the sulfate group has a molecular array more compact at the interface. Furthermore, CO2 gas have a reduction of the diffusion across of the hydrocarbon chains to the water layer with an increment of the number of CO2 molecules in the system, indicating a behavior more hydrophobic of the CO2 gas. The tendencies obtained of the simulations are consistent with the reported experimental results.

The nature of the electron density localization in two MoS2 nanoclusters containing eight rows of Mo atoms, one with 100% sulphur coverage at the Mo edges (n8_100S) and the other with 50% coverage (n8_50S) was studied using a localized-electron detector function defined in the local moment representation. For n8_100S, pairs of neighboring S2 dimers cover the edges and the electron density localization function analysis shows the presence of a local triangular-shaped ring zone of highly delocalized electrons along these edges, which corresponds to a good metallic conductor zone. The optimized geometry analysis shows that the Mo-S2 bond length is much longer than that of the Mo-S bonds inside the cluster. The removal of one S atom from each sulphur dimer to create a cluster with 50% coverage produces a general compressive stress on the cluster optimized geometry, which shortens the Mo-S bond length, particularly at the edge. The electron density location function analysis shows that close to the cluster corners, a zone of highly delocalized electron zones with a characteristic semiconductor pattern and broken one-dimensional metallic ring was generated. These results suggest that the Mo-S2 bond elongation produced by the sulphur dimers is similar to a MoS2 monolayer under tensile strain and is the origin of the one-dimensional metallic sites at the Mo-edges. In general, the present findings show excellent agreement with the key features of the reported ambient pressure X-ray photoemission spectra and the corresponding simulated scanning tunneling microscopy images.

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 effect of the pre-adsorption of sulfur on the hexagonal-Mo_{2}C-001 surface in the hydrodechlorination HDC reaction of chlorobenzene was studied using density functional theory DFT calculations. The results related with the coordination modes and the adsorption energies of the aromatic molecule suggest that the main effect of the sulfur incorporation into the surface is to lead towards a weaker chlorobenzene adsorption that will benefit the continuity of the catalytic cycle and therefore to avoid the carbonization and chlorination of the catalyst. The study of the HDC mechanism was also performed and two different reaction paths were considered. The calculated energy barriers indicate that both mechanisms may occur at the normal reaction temperature 350°C. These latter approaches involve new roles of the superficial sulfur as atomic or radical hydrogen scavenger for the S-Hformation and as a precursor of the σ chlorobenzene coordination, necessary for effective hydrogenation in the proposed mechanisms.

Using molecular dynamics simulations a systematic study of the binding energy per cross sectional area for the water/n-alkane (hexane, octane, decane, dodecane and tetradecane) interfaces was performed. The effect of the Sodium p-(1-heptylnonyl) benzene sulfonate surfactant, on the adhesion forces of the water/n-hydrocarbon (decane, undecane, dodecane, and tetradecane) interfaces was studied. Scanning of the binding energy per area against n-alkanes shows that the magnitude of this parameter for the surfactant tail-alkane interactions at the interface systematically increases with the chain length of the alkane, whereas it shows a maximum at undecane for the water-surfactant head interactions at the interface. This maximum of head adhesion forces thus agrees with the reported minimum value of the interfacial tension at undecane for the p-(1-heptylnonyl) benzene sulfonate, suggests that for the water/alkane interface it is this trend in surfactant head adhesion at the interface that defines that interfacial tension minimum value.

En este trabajo, usando dinámica molecular se determinaron las propiedades interfaciales y el comportamiento del Dodecil Sulfato de Sodio (SDS) ubicado en la región interfacial de los sistemas vacío/agua y agua/n-octano. La tensión interfacial fue estimada con el modelo propuesto por Kirkwood y Buff [23]. A su vez, los espesores de película interfacial fueron determinados usando los criterios 10-90 y 90-90. Además, el área por molécula fue estimado con la variación de la presión superficial en función de la concentración del surfactante. En los sistemas vacío/SDS/agua, el área por molécula del SDS fue obtenida con dos procedimientos diferentes. Los valores fueron 53.3 Å2 y 54.3 Å2, respectivamente. Para los sistemas agua/n-octano y agua/SDS/n-octano, los espesores de película interfacial aumentan en función del número de moléculas de surfactantes presentes en la región interfacial. Los resultados obtenidos son consistentes con datos medidos por experimentación.

Intermetallics AlTi, Al_{3}Ti and AlTi_{3} have been characterized in terms of the electron density topology. Atomic properties such as atomic charge and atomic volume were determined using the theory of atoms in molecules AIM. Our calculations indicate a good match between the electron density difference, Δρ, and enthalpy of formation of each intermetallic, as well as the electric work function EWF. We also found an excellent correlation between the EWF, deduced from the topological charges of the intermetallics, and EWF determined experimentally, demonstrating the predictive scope of our model.

This paper describes the characterization of structural, energetic, and electric properties of the molecular complexes between Orinoco belt resins through the application of computational molecular mechanics (MM), semi-empirical parametrization (PM6), and density functional theory (DFT) (PW91 and HCTH) in conjunction with the double numeric and polarized (DNP) basis set. The resin sources for the studied molecules are Orinoco belt vacuum residues (Carabobo, Hamaca, Merey, Merey-Mesa, and Zuata). Molecular structures of these compounds were proposed from analysis of experimental characterization. The study of molecular interactions has shown that these resins are able to form stable van der Waals complexes, where their computed stability has a large dependence upon the applied theory level. A qualitative description of the formation of these complexes could be obtained with these methodologies. In particular, MM overestimates resin interaction energies when compared to PM6 and DFT (PW91 and HCTH) results. ...

The nature of small nickel sulfide clusters was explored using the atoms in molecules theory and a based-electrostatic potential methodology. It was found that Ni$_{3}$S$_{4}$, whose structure corresponds to a C$_{3V}$ {7, 9, 3} pyramid, should be (at 0 K) the most stable of the studied nickel sulfide clusters. Electrostatic potential results suggest that the Lewis acidity of the studied cluster is much larger (and therefore much more HDS reactive) than the respective acidity of the practically inactive Ni$_{3}$S$_{2}$ (111) surfaces. Atomistic thermodynamic calculations have shown that at typical HDS working conditions very small particles of non supported nickel sulphide should be mainly present as Ni$_{3}$S$_{4}$ cluster.

MoS$_{2}$ bulk packing was studied using the Atoms in Molecules Theory. The network of bond paths describing the atomic connectivity has shown that the crystal graph of this material results from the packing of three types of polyhedra containing cage critical points localized at d, f and a Wyckoff position. Alternatively, the crystal packing can be described by means of the basin of the Mo and S atoms with $D_{3h}$ and $C_{3v}$ symmetry, respectively. These basins display the full local point-group symmetry at the nuclear sites, and fill the space without overlapping. S-S secondary bonds in the interlayer region of this layered material were also determined. The presence of these bonds is fundamental for the found packing of the MoS$_{2}$ bulk.

Density functional theory (DFT) calculations combined with surface thermodynamic arguments and the Gibbs-Curie-Wulff equilibrium morphology formalism have been employed to explore the effect of the reaction conditions, temperature (T), and gas-phase partial pressures (p h2 and p h2s ) on the stability of low Miller index ruthenium sulfide (RuS 2 ) surfaces. The calculated thermodynamic surface stabilities and the resulting equilibrium morphology models suggest that unsupported RuS 2 nanoparticles in HDS conditions are like to a polyhedron with six, eight, and twelve (100), (111), and (102) faces, respectively. The area of these faces covers about 40%, 37%, and 23% of the total particle, respectively. The atomic basins of the outermost individual atoms of the exposed surfaces were determined using the quantum theory of atoms in molecules methodology. Direct visualization of these basins has shown that a hole just at the middle of the outermost sulfur basins provides access to uncovered metal sites. Analysis of the electrostatic potential mapped onto a selected electron density isocontour (0.001 au) on the exposed surface reveals a very high potential reactivity of these holes toward electrodonating reagents. Consequently, the high attraction between these uncovered sites and S atoms coming from reagent polluting molecules makes these kinds of particles quite active for HDS catalysis.

The nature of the active sites on the NiMoS catalyst was explored using the combination of the atoms in molecules theory and an electrostatic potential based methodology. Direct visualization of the basin of the outermost atoms of the studied NiMoS edge models has shown that the Mo atoms are practically covered by the outermost S atoms while the promoter Ni are the most accessible atoms to the exterior of the edges. Electrostatic potential mapping on the atomic border exposed to the exterior of the edge and adsorption of pyridine as the probe molecule have shown that the adsorption of a molecule containing the pollutant atoms strongly depends on two factors: the Lewis acidity of the site and the repulsive interaction of the incoming molecule with sulfur atoms impeding the free access to the site. In this sense, the main role of the promoter is to produce sulfur uncovered Lewis acid sites. Extraction of the outermost S atom creates a very strong Lewis acid site locate at uncovered Mo atoms.

The nature of the MoS 2 catalyst edge was studied using the atoms in molecules theory. The network of bond paths describing the atomic connectivity has shown that the exposed graph of this edge results from the packing of polyhedra, with six vertexes and four faces, located at the Mo edges and polyhedra, with five vertexes and three faces, located at both sides of the S edge. The former polyhedra are bonded by a much bigger electron density suggesting a relationship between this fact and the smaller necessary energy to create a sulfur vacancy on the S edges than the Mo edges. Additionally, we have found that potential active sites on the catalyst surface can be localized visualizing the outermost interatomic surfaces.

The synergism of mixtures of anionic and non-ionic surfactants adsorbed at the water/alkane interface was studied using mesoscopic Dissipative Particle Dynamic simulations. The effect of the mixtures of sodium m, p-exylbenzenesulfonate and sodium m, p-octylbenzenesulfonate with the nonionic surfactant lauryl alcohol polyoxyethylene (9) ether on the surface tension of the water/alkane interface was explored. Alkane scanning curves for n-alkanes from hexane to tetradecane are reported. Our results suggest that dissolution of lauryl alcohol polyoxyethylene (9) ether in the hydrocarbon phase is the origin of this behavior. For the sodium m, p-octylbenzenesulfonate case, the mixture with lauryl alcohol polyoxyethylene (9) ether induces a synergism for alkanes with low carbon number. A moderate lipophilic behavior of the mixture was observed for hexane and as a result 95% of the mixture stays at the interface forming a really compact layer at the interface. Additionally, increase on the carbon number alkane produces an increase in the hydrophilic behavior of lauryl alcohol polyoxyethylene (9) ether inducing higher interfacial tension values.

The energy of formation and the Lewis acid strength of sulfur vacancies or coordinative unsaturated sites on the MoS2 edges were studied using density functional theory for periodic systems and an electrostatic potential-based methodology. The results suggest that the more energetically favorable sites are located on the sulfur edges; however, their Lewis acid strength is considerably smaller than the site acidity at the molybdenum edges. The acid strength for the reported most hydrodesulfurization active site of RuS2 was also determined. In general, the Lewis acid for the site on RuS2 is 100% smaller than the sites on the Mo edges and around 20% larger than the most favorable site on the S edges of MoS2. Binding of the pyridine molecule in the eta1 adsorption configuration on the considered sites has corroborated the trend of Lewis acidity suggested by the electrostatic potential methodology.

The nature of the interaction of water molecules with the surface of a set of experimentally well-studied alkane−amidethiols adsorbed on a gold(111) surface and its effect on the water contact angle have been studied by carrying out a systematic determination of the topology of electrostatic potential using ab initio density functional theory methods for periodic systems. The obtained results have shown that the water contact angle decreases systematically as the number of minima by surface cell unit and the electrostatic potential magnitude at these minima, and consequently the adsorption strength, increases. Thus, this electrostatic potential magnitude can be used as a measure of the efficiency and ability of a chemical group for wetting of a surface by water.

Binding between water and models of poly(ethylene oxide), (CH2−CH2−O)n, n = 2−40, has been studied using the topographic features of the electrostatic potential, V(r), and standard density functional theory methods. It was found that, in general, the contour around the minima of the oxygen atoms overlap forming a negative-valued spiral coiled around a positive-valued helix. The positive zone defines a helical groove in the O−C−C−O units where minima lone pairs critical points are located. Topological analysis of the water molecule has also suggested that the attractive electrostatic effect between the positive water O−H zone and the negative PEO lone pairs plays an important role in the hydrogen bonding of the PEO−water system. Thus, the V(r) topology predicts a coil of water molecules around the PEO chain forming hydrogen bonding with two sites of ether oxygens. This coil is formed in such a way that more water molecules accumulate on the cavities surrounding the poly(ethylene oxide)'s oxygen atoms where...

In this work, the topology of the electrostatic potential using density functional theory for periodic systems was used to study the nature of the interaction of water with laponite surfaces; an uncharged sheet model was also used. The topological analysis predicts that for uncharged surfaces the adsorption mode is such that the water molecules are adsorbed almost parallel to the surface. For laponite surfaces, where there is a net charge, the adsorption mode involves electrostatic repulsion between the negative lone pairs on the water molecules and the ones on the surface oxygen atoms. As a consequence, the water molecules bind to the surface in a perpendicular and tilted approach, minimizing the repulsive interactions. The advantage of using the topology of the electrostatic potential as an efficient method to describe the electrostatic interactions between adsorbates and surfaces is also discussed.

To study the effect of the surface Ru sulfur coordination number and of the surface S−H and Ru−H species into the thiophene adsorption on RuS2, a topologic study of the Laplacian of the electronic density of selected (100) and (111) surfaces was carried out. It was found that a nonbonded local charge concentration on the S atom of the thiophene interacts with a local minimum on the outermost Ru atoms of the surfaces. This interaction is strongly affected by a nonbonded local charge concentration located on the outermost S atoms of the surface. Both interactions combine in such way that the strength of the thiophene adsorption on the unhydrogenated surfaces shows small changes. The main role of the S−H bond is to move away the surface sulfur local charge concentrations from the Ru atoms while the Ru−H species favors the hydride attacks to a local minimum located at the Cα of the thiophene molecule.

The “atoms in molecules” theory (AIM) is very useful to extract significant information from the electronic density, ρ( r ) . In the present paper, we show the implementation in WIEN 97 of some algorithms to determine the main elements of this novel theory (critical points of ρ( r ) , bond paths, crystal graphs, interatomic surfaces, atomic basins and atomic properties). WIEN 97 program is one successful implementation of the full-potential linearized augmented-plane wave method (FPLAPW), which allows most accurate calculations of the electronic density of crystals and surfaces using density functional theory. In order to illustrate those algorithms, they were applied to Pt and PtS bulk as two selected examples.

A topological analysis of the Laplacian of the electronic charge density of bulk 3d transition metals has been performed. This analysis shows that the atomic graph for the atoms of the fcc metals (Ni and Cu) is an octahedron with six vertexes (corresponding to local charge concentrations) linked by twelve edges and bounded by eight faces (corresponding to local charge minima). For the bcc case (V, Cr, and Fe), the corresponding graph is a cube with eight vertexes, twelve edges, and six faces. For the early hcp metals (Sc and Ti), the graph is a trigonal bipyramid with five vertexes, nine edges, and six faces, whereas for Co, it is a trigonal prism with six vertexes, nine edges, and five faces. Despite the different kinds of the graphs obtained, we have been able to correlate the value of the Laplacian at the local minima with the experimental heats of adsorption (which show a systematic decrease) for the O2 and CO molecules on the 3d transition metals. This result suggests that the bonds formed by these m...

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.

A numerical method to analyze the topology of the electronic density regardless of how it was obtained (analytically or numerically) was implemented for the Extreme 94 program. The method allows th...

A numerical method that analyzes the topology of the Laplacian of the electronic density regardless of how it was obtained (analytically, numerically, or even experimentally) was implemented for the BUBBLE program. The method allows the study of a complex system where the electronic correlation is important, by density functional theory regardless the kind of basis set used, and allows the analysis of the Laplacian of the resulting charge density with the topological theory of Bader. The method was applied to CO and NO molecules and the Cu13, Cu45, and Fe21 clusters modeling the (100) Cu and (001) Fe surfaces, respectively.

The ab initio molecular charge density ρ(r) of substituted aziridines was calculated at the MP2 level for R = -H, -CH3, -CF3, -Cl, -NO2, -CN, -OH, -NO, and -F. The use of the topology of the Laplacian of ρ(r) allowed the analysis of the electric field gradient (EFG) at the nitrogen nucleus directly in terms of its valence shell charge concentration. The EFG changed sign and the orientation of its y- and z-axes with respect to aziridine when R = -Cl, -NO2, -CN, -OH, -NO, and -F. The changes in the EFG in these aziridines upon substitution were correlated with those calculated in the valence charge concentrations along the direction of the N—R bond. The sign of the qzz component and the orientation of the principal axes of the EFG tensor were found to be determined by the relative value of the contributions from the charge concentrations at the points in the valence shell along the N—C and N—R bond directions. Key words: molecular charge distribution, aziridines, Laplacian of the charge density, electric field gradient.

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.

The solubility parameters, δ, of several asphaltene models were calculated by mean of an atomistic NPT ensemble. Continental and archipelago models were explored. A relationship between the solubility parameter and the molecule structure was determined. In general, increase of the fused-rings number forming the aromatic core and the numbers of heteroatoms such as oxygen, nitrogen, and sulfur produces an increase of the solubility parameter, while increases of the numbers and length of the aliphatic chains yield a systematic decrease of this parameter. Molecules with large total carbon atom number at the tails, n(c), and small aromatic ring number, n(r), exhibit the biggest values of δ, while molecules with small n(c) and large n(r) show the smallest δ values. A good polynomial correlation δ = 5.967(n(r)/n(c)) - 3.062(n(r)/n(c))(2) + 0.507(n(r)/n(c))(3) + 16.593 with R(2) = 0.965 was found. The solubilities of the asphaltene models in toluene, heptane, and amphiphiles were studied using the Scatchard-Hildebrand and the Hansen sphere methodologies. Generally, there is a large affinity between the archipelago model and amphiphiles containing large aliphatic tails and no aromatic rings, while continental models show high affinity for amphiphiles containing an aromatic ring and small aliphatic chains.

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.

MCSCF-CAS and SCF ab initio calculations were carried out for the abstraction of H from CH[sub 4] by MgO and LiO molecules. Effects of the electron correlation on internuclear distances, reaction energies, and Laplacian of the charge density [del][sup 2][rho](r) of the O valence shell were studied. An excellent correlation between the values of -[del][sup 2][rho](r) at an external saddle point of this shell and the activation barrier for the H abstraction from CH[sub 4] already reported was obtained. In this way, the use of -[del][sup 2][rho](r) has been extended to include the prediction of the chemical reactivity of a reaction involving a homolytic break of a pair of bonding electrons. The changes in the charge redistribution in going from the reactants to the transition state and to the products was also determined in terms of variations of -[del][sup 2][rho](r) at the critical points of the valence shells. 22 refs., 7 figs., 5 tabs.