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The static polarizabilities of a series of zigzag single walled carbon nanotubes [( m, 0) with m ≠ 3 n for m = 7-25] were computed by the coupled perturbed Kohn-Sham (CPKS) coupled method recently implemented in the periodic CRYSTAL code. This method makes it possible to compute both the longitudinal and transverse polarizabilities with the same approach including the local field depolarization effects for the whole series of the investigated nanotubes. To quantify these effects, the unscreened longitudinal and transverse polarizabilities were also calculated within the linear response theory using the Sum Over States (SOS) method. Our results confirmed the inverse-square dependence on the bandgap of the longitudinal polarizabilities which are found weakly affected by the crystalline orbital relaxation. On the other hand, the comparison between the screened (CPKS) and unscreened (Sum Over States) transverse polarizabilities permit to calibrate more precisely the classical correction of Benedict et al. (Phys Rev B, 1995, 52, 8541) which must be applied to include the local field effect at the SOS level. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011 [ABSTRACT FROM AUTHOR]

Methyliminopropadienone MeN=C=C=C=O 1a was generated by flash vacuum thermolysis from four different precursors and isolated in solid argon. The matrix-isolation infrared spectrum is dominated by unusually strong anharmonic effects resulting in complex fine structure of the absorptions due to the NCCCO moiety in the 2200 cm-1 region. Doubling and tripling of the corresponding absorption bands are observed for phenyliminopropadienone PhN=C=C=C=O 1b and bis(phenylimino)propadiene PhN=C=C=C=NPh 9, respectively. Anharmonic vibrational frequency calculations allow the identification of a number of overtones and combination bands as the cause of the splittings for each molecule. This method constitutes an important tool for the characterization of reactive intermediates and unusual molecules by matrix-isolation infrared spectroscopy. [ABSTRACT FROM AUTHOR]

We present first-principle calculations of core-level binding energies for the study of insulating, bulk phase, compounds, based on the Slater-Janak transition state model. Those calculations were performed in order to find a reliable model of the amorphous LixPOyNz solid electrolyte which is able to reproduce its electronic properties gathered from X-ray photoemission spectroscopy (XPS) experiments. As a starting point, Li2PO2N models were investigated. These models, proposed by Du et al. on the basis of thermodynamics and vibrational properties, were the first structural models of LixPOyNz. Thanks to chemical and structural modifications applied to Li2PO2N structures, which allow to demonstrate the relevance of our computational approach, we raise an issue concerning the possibility of encountering a non-bridging kind of nitrogen atoms (=N-) in LixPOyNz compounds. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations are performed on the pure silica zeolite silicalite (MFI framework code), maintaining via a new force field both framework flexibility and realistic account of electrostatic interactions with adsorbed water. The force field is similar to the well-known “BKS” model [B. W. H. van Beest et al., Phys. Rev. Lett. 64, 1955 (1990)], but with reduced partial atomic charges and reoptimized covalent bond potential wells. The present force field reproduces the monoclinic to orthorhombic transition of silicalite. The force field correctly represents the hydrophobicity of pure silica silicalite, both the adsorption energy, and the molecular diffusion constants of water. Two types of adsorption, specific and weak unspecific, are predicted on the channel walls and at the channel intersection. We discuss molecular diffusion of water in silicalite, deducing a barrier to crossing between the straight and the zigzag channels. Analysis of the thermal motion shows that at room temperature, framework oxygen atoms incurring into the zeolite channels significantly influence the dynamics of adsorbed water. [ABSTRACT FROM AUTHOR]

Abstract: Various defects – either bright or dark triangular defects – are observed on the titanium disulfide surface (001) by ultra high vacuum scanning tunneling microscopy. The experimental interpretations of the images available in the literature suggest that a fraction of Ti atoms could be displaced from the octahedral site they occupied to vacant sites of the crystal structure, leading to more or less correlated defects. In this paper, we have performed ab initio periodic LCAO-B3LYP calculations on a (5×5) biperiodic supercell to model the electronic and geometrical involvements of Ti vacancies and to generate the theoretical STM images within the Tersoff–Hamann approximation. The relaxed structure of the Ti vacancy shows an inward movement of the neighboring sulfur atoms at the surface. However, the occupied vacancy electronic states at the Fermi level are mainly developed on the atomic orbitals of the first S neighbors at the surface, leading to bright triangular zones on the simulated image. [Copyright &y& Elsevier]

A new variational algorithm called adaptive vibrational configuration interaction (A-VCI) intended for the resolution of the vibrational Schrödinger equation was developed. The main advantage of this approach is to efficiently reduce the dimension of the active space generated into the configuration interaction (CI) process. Here, we assume that the Hamiltonian writes as a sum of products of operators. This adaptive algorithm was developed with the use of three correlated conditions, i.e., a suitable starting space, a criterion for convergence, and a procedure to expand the approximate space. The velocity of the algorithm was increased with the use of a posteriori error estimator (residue) to select the most relevant direction to increase the space. Two examples have been selected for benchmark. In the case of H2CO, we mainly study the performance of A-VCI algorithm: comparison with the variation-perturbation method, choice of the initial space, and residual contributions. For CH3CN, we compare the A-VCI results with a computed reference spectrum using the same potential energy surface and for an active space reduced by about 90%. [ABSTRACT FROM AUTHOR]

A new off–on fluorescent hemicryptophane probe for acetylcholine has been designed. This hemicryptophane, made fluorescent via an extension of the conjugation of its cyclotriveratrylene C3-symmetry part, exhibits improved fluorescence properties compared with fluorescent hemicryptophanes previously described. Indeed, both the excitation and emission wavelengths are red-shifted and the quantum yield is increased. Moreover, this hemicryptophane is able to bind acetylcholine with a high association constant of 3.2 × 104 M−1. This recognition process is accompanied by an increase in the brightness of the capsule. Surprisingly, contrary to what is commonly observed with cyclotriveratrylene-based hosts, the quaternary ammonium of the guest interacts with the tris(2-aminoethyl)amine south part of the hemicryptophane instead of the cyclotriveratrylene north part. This unusual binding mode is supported by both proton NMR experiments and density functional theory calculations. [ABSTRACT FROM AUTHOR]

• Asphaltene aggregate stability in crude oils and the association with dipolar moment. • The behavior of asphaltene subfractions A1 and A2 and the impact of their structure in aggregate formation and stability. • Direct consequence of the zero dipolar moment of the aggregates with their stability in solution. The study of the behavior of asphaltenes in solution has focused on their tendency to form aggregates and less importance has been given to describing and understanding what allows the formation of asphaltene aggregates that remain stably dispersed. Using molecular dynamics simulations, we calculated that the dipole moment of models for asphaltene aggregate tends to zero when the number of molecules in the aggregate tends to large values. In other words, such dipole moment cancellation would be an important contributor to the dispersion of these aggregates. [ABSTRACT FROM AUTHOR]

• Asphaltene aggregate stability in crude oils and the association with dipolar moment. • The behavior of asphaltene subfractions A1 and A2 and the impact of their structure in aggregate formation and stability. • Direct consequence of the zero dipolar moment of the aggregates with their stability in solution. The study of the behavior of asphaltenes in solution has focused on their tendency to form aggregates and less importance has been given to describing and understanding what allows the formation of asphaltene aggregates that remain stably dispersed. Using molecular dynamics simulations, we calculated that the dipole moment of models for asphaltene aggregate tends to zero when the number of molecules in the aggregate tends to large values. In other words, such dipole moment cancellation would be an important contributor to the dispersion of these aggregates. [ABSTRACT FROM AUTHOR]

The presence of metalloporphyrins in crude oil has been known by many years now but their role on the physical-chemical properties of petroleum fluids, such as the aggregation of the high-molecular weight phases, remains unknown. In this paper, these properties are studied using different molecular modeling techniques (Molecular Dynamics, Semi-empirical PM7 and Density Functional Theory). This combined methodology allowed us characterizing the nature of these interactions, how it dominates the electronic structure of the stacked molecules and what is their participation on the formation of the nano-, micro- and macro-aggregates. [ABSTRACT FROM AUTHOR]

LiNi x Mn y Co 1 − x − y O 2 compounds (NMC) are layered oxides widely used in commercial lithium-ion batteries at the positive electrode. Nevertheless surface reactivity of this material is still not well known. As a first step, based on first principle calculations, this study deals with the electronic properties and the surface reactivity of LiMO 2 (M = Co, Ni, Mn) compounds, considering the behavior of each transition metal separately in the same R 3 ̅ m α -NaFeO 2 -type structure, the one of LiCoO 2 and NMC. For each compound, after a brief description of the bare slab electronic properties, we explored the acido-basic and redox properties of the (110) and (104) surfaces by considering the adsorption of a gaseous probe. The chemisorption of SO 2 produces both sulfite or sulfate species associated respectively to an acido-basic or a reduction process. These processes are localized on the transition metals of the first two layers of the surface. Although sulfate species are globally favored, a different behavior is obtained depending on both the surface and the transition metal considered. We conclude with a simple scheme which describes the reduction processes on the both surfaces in terms of formal oxidation degrees of transition metals. [ABSTRACT FROM AUTHOR]

Matrix photolysis of 2-pyrazinyl azides/tetrazolo[1,5-a]pyrazines generates nitrile ylides 15 via pyrazinylnitrenes 13 and triazacycloheptatetraenes 14. The nitrile ylides 15 are characterized by IR spectroscopy in conjunction with harmonic and anharmonic vibrational frequency calculations. The nitrile ylides exist in the matrices in the Z,Z-conformations in which they are born. Substitution on the nitrile carbon of nitrile ylides has a profound effect on their structure. Even different conformers of the same molecule can have differences up to 200 cm-1 in the IR absorptions of the ylide moieties. Nitrile ylides 15a and 15b (R = H or Cl, R' = H) have allenic structures (15 Allenic). Nitrile ylide 15c (R = R' = CH3) has a distinctly propargylic structure (15 Propargylic) in the experimentally observed Z,Z-conformation. [ABSTRACT FROM AUTHOR]

Abstract: Optical properties such as the dynamic dielectric function, reflectance, and energy-loss function of beryllium oxide (BeO) in its ambient and high-pressure phases are reported for a wide energy range of 0–50eV. The calculations of optical properties employ first-principles methods based on all-electron density functional theory together with sum over states and finite-field methods. Our results show subtle differences in the calculated optical properties of the wurtzite, zincblende, rocksalt and CsCl phases of BeO, which may be attributed to the higher symmetry and packing density of these phases. For the wurtzite phase, the calculated band gap of 10.4eV corresponds well with the experimental value of 10.6eV and the calculated (average) index of refraction of 1.70 shows excellent agreement with the experimental value of 1.72. [Copyright &y& Elsevier]

Nanostructuration is one of the most promising strategies to develop performant electrode materials for energy storage devices, such as hybrid supercapacitors. In this work, we studied the influence of precipitation medium and the use of a series of 1-alkyl-3-methylimidazolium bromide ionic liquids for the nanostructuration of β(III) cobalt oxyhydroxides. Then, the effect of the nanostructuration and the impact of the different ionic liquids used during synthesis were investigated in terms of energy storage performances. First, we demonstrated that forward precipitation, in a cobalt-rich medium, leads to smaller particles with higher specific surface areas (SSA) and an enhanced mesoporosity. Introduction of ionic liquids (ILs) in the precipitation medium further strongly increased the specific surface area and the mesoporosity to achieve well-nanostructured materials with a very high SSA of 265 m2/g and porosity of 0.43 cm3/g. Additionally, we showed that ILs used as surfactant and template also functionalize the nanomaterial surface, leading to a beneficial synergy between the highly ionic conductive IL and the cobalt oxyhydroxide, which lowers the resistance charge transfer and improves the specific capacity. The nature of the ionic liquid had an important influence on the final electrochemical properties and the best performances were reached with the ionic liquid containing the longest alkyl chain. [ABSTRACT FROM AUTHOR]

• Experimental-theoretical study: XPS, gaseous probe adsorption, DFT calculations. • The periodicity of the potential energy is not broken by the stacking fault. • Stacking faults do not impact the surface electronic structure. • Surface reactivity is governed by the morphology and the manganese environments. This paper investigates the role of the stacking fault (5%, 20% and 50%) and the morphology of Li 2 MnO 3 lamellar materials, issued from coprecipitation method with three annealing temperatures, on the surface reactivity. The structure and the morphology have been characterized by XRD, SEM and TEM. We studied the surface reactivity of these materials by combining X-ray photoemission spectroscopy (XPS), gaseous adsorption and first-principle calculations. An evolution of the reactivity toward the SO 2 acid gaseous probe has been observed for the three materials, from pure redox mechanism toward mixed acid-base/redox mechanisms, respectively for 5% and 50% of stacking faults. We demonstrated that the electronic structure of Li 2 MnO 3 being not modified by stacking faulted. Thus, the surface reactivity of faulted Li 2 MnO 3 is not linked to the SF rate but only governed by the accessible crystalline surfaces and the manganese environments at the surface atomic layer. The formation of (0 0 1)-Li surface according to the Li-overstoichiometry on the extreme surface and the random particles shape of the more faulted materials are responsible of the reactivity tuning. [ABSTRACT FROM AUTHOR]