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International audience; A periodic ab initio Hartme-Fock method has been used to evaluate a number of electronic and structural properties of MgHs. The calculated quantities are the equilibrium lattice parameters, the cohesive energy, the elastic constants, the energy-band struo ture, the density of' states, the electronic charge distribution and the Compton profiles. For the two former properties correlation effects are taken into account and improve the agreement with the experimental data.The strongly but not fitlly ionic character of MgHs is confirmed by the present study.

This study investigates the electronic configuration of titanium nitride-nickel (TiN-Ni) nanocomposites, in order to explain the high stability and activity of this hydrogenolysis catalyst. TiN-Ni is compared to a titanium oxide-nickel reference (TiO2-Ni). Strong metal support interactions are observed between the TiN and Ni. Scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDS) illustrate that the Ni distributes more homogeneously on the nitride support. Computational comparison of TiN Ni and TiO2-Ni provides evidence of preferential Ni adsorption onto nitrogen sites of the nitride support. DFT calculations also predict a charge polarization between Ti and Ni atoms. X-ray photoelectron spectroscopy (XPS) corroborates computational analysis by revealing a suppression of surface nitride species upon deposition of Ni onto TiN. Shifts in Ti 2p and Ni 2p binding energy positions are also evident, which indicate an electronic perturbation between TiN and Ni. We conclude that the presence of nitrogen in the nitride support influences the electronic and structural properties of TiN-Ni and is partly responsible for the beneficial catalytic properties reported for this nanocomposite.

Here we report the solid-state experimental and computational analysis of naphtho[2,3-c][1,2,5]thiadiazole (1) and naphtho[2,3- c][1,2,5]selenadiazole (2) showing that the three different crystal phases of polymorphic 1 and the sole crystal phase of nonpolymorphic 2 are characterized by predominance of some intermolecular motifs over the others. In particular, the intermolecular interactions present in the compounds, π···π, C−H···π, C−H···N, S···N, and Se···N, are hierarchically ranked, in such a way that in 1 polymorphism is observed, with the three different phases showing different combinations of the intermolecular interactions, while in 2 only the packing that maximizes the strongest intermolecular interaction by far, i.e., chalcogen bond, is observed. Moreover we also show that different packings produce different responses in the solid-state topochemical reactivity of the crystalline compounds, spanning from nonreactive packings to packings producing butterfly dimers. In order to rationalize the different responses, the relevance of the transverse parallel shift of molecules is suggested, as an important factor, in addition to the known Schmidt’s rules for solid-state topochemical reactions.

The present study investigates the coal char combustion by a combination of thermochemical and X-ray photoemission spectroscopy (XPS) analyses. Thermoanalytical methods (differential thermogravimetry, differential scanning calorimetry, and temperature-programmed desorption) are used to identify the key reactive steps that occur upon oxidation and heating of coal char (chemisorption, structural rearrangement and switchover of surface oxides, and desorption) and their energetics. XPS is used to reveal the chemical nature of the surface oxides that populate the char surface and to monitor their evolution throughout thermochemical processing. XPS spectra show the presence on the carbon surface of three main components. It is shown that the most abundant oxygen functionality in the raw char is epoxy. It decreases with preoxidation at 300 degrees C and even more at 500 degrees C, where carboxyl and ether oxygen functionalities prevail. The rearrangement of epoxy during preoxidation goes together with activation of the more stable and less reactive carbon sites. Results are in good agreement with semi-lumped kinetic models of carbon oxidation, which include (1) formation of "metastable" surface oxides, (2) complex switchover, and (3) desorption into CO and CO2.

The sole crystal structure generally observed for a chemical compound (actual crystal structure) corresponds to only one of the many minima present in the crystal energy landscape of that compound. A challenging problem of crystallography is to explain why the many other possible crystal structures (in terms of energy and density) present in the landscape (virtual structures) are not observed as polymorphs. In the present paper, with specific reference to semicarbazides of substituted benzoic acids, the concept of actual and virtual structures is extended and it is proven that optimization of a molecule in a different crystal packing of another molecule (interchangeability of molecules and packings) can lead to low minima of the lattice energy and to a high lattice density also in the case of molecules significantly different in shape (para/meta substituted compounds). This suggests that a crystal structure can be viewed as a sort of elastic system that can be deformed to fit molecules of different electronic character and shape. Surprisingly, in many cases, this deformation is accomplished at low cost of energy and density. Our results also suggest that the small number of polymorphs cannot be explained only by inspection of the crystal energy landscape, which would otherwise suggest many, but also, and perhaps mostly, by the analysis of the initial steps of crystal formation even before nucleation, when many different molecular clusters corresponding to different packings are in competition with each other.

Imines obtained by condensation of 4-hydroxybenzohydrazide with aliphatic ketones are a rare example of a class of compounds showing a remarkable tendency to crystallize in acentric polar space groups (Pna21 or Cc). In fact, all of the (seven) compounds studied up to now show at least one polar polymorph. In some cases, polymorphism was detected, and a nonpolar centrosymmetric phase was also identified (P21/c or P21/n space group). With the aim to disclose the conditions that can favor the formation of acentric structures in molecular crystals, we report, in this paper, a theoretical analysis (ab initio density functional theory with periodic boundary) of the lattice energy and density of all the packing modes observed in the whole set of imines. The computational analysis has been performed by optimizing each compound in its own experimental packings (actual crystal structures) and also in the packings of the other compounds of the class (virtual structures). The experimental crystallographic data and the theoretical analysis suggest that two conformers, basically differing for the orientation of the phenolic H atom in the plane of the phenyl ring, compete, in solution, for the formation of polar or centrosymmetric packings. The transitions between polar and centrosymmetric polymorphs are of diffusive type, and single crystals are not preserved, while the transitions between different polar polymorphs can be of single-crystal-to-single-crystal type.

Series of ZnII complexes with coordinated pyridine and a O,N,O tridentate core are reported. Four species of the general formula ZnLiPy2 were synthesized, where Li is a substituted ((aroylhydrazono)methyl)-3-hydroxyphenyl-4-(octyloxy)benzoate. The structure of the four mononuclear complexes and of dinuclear Zn2L24Py2 was solved by single crystals X-ray analysis. By TGA-DSC analysis and optical observation a nematogenic phase behavior was detected for ligands only and not for complexes. Photoluminescence spectra were recorded both in solution and in solid state Only the complexes showed a noteworthy photoluminescence in the solid state with tunable wavelength by varying the aroyl group. Medium to very high PL quantum yields were recorded on the crystalline complexes obtained in their mononuclear coordination environment. In the case of 4-nitrobenzoyl substituent a dinuclear complex with formula Zn2L24Py2 showed an unprecedent exceptionally high PLQY value. Both periodic and molecular DFT calculations were performed to rationalize the absorbance of these complexes at “one particle” level.

Reaction of N-rich conjugated bis(3,4-diamino-1,2,4-triazole)s with dilute hydrochloric acid affords bis(3,4-diamino-1,2,4-triazol-2-ium)chlorides. These compounds form layered structures in which planar layers of molecules are parallelly stacked. The π-stacking distance of the layers is relatively short, as compared with all-carbon-containing aromatic compounds, ranging between 3.00 and 3.22 Å at 173 K, and several contacts shorter than the sum of van der Waals radii are observed in the crystal structures. The features of the crystal structures are discussed in terms of the high nitrogen content of the compounds and of the H-bonding patterns. Periodic ab initio theoretical calculations of the crystal structures have allowed decomposing the lattice energy into various contributions in order to put up the relevance of van der Waals interactions for the π-stacking. In particular, it is found that van der Waals interactions account for about 10% of the total lattice energy and about 50% of the stacking energy (interlayer energy).

cited By 0; International audience; The VN3H defect in diamond (a vacancy surrounded by three nitrogen and one carbon atoms, the latter being saturated by a hydrogen atom) is investigated quantum-mechanically by use of a periodic supercell approach, an all-electron Gaussian-type basis set, "hybrid" functionals of density functional theory, and the Crystal program. Three fully optimized structural models (supercells containing 32, 64, and 128 atoms) are considered to investigate the effect of defect concentration. The electronic configuration of the defect is reported along with a description of its structural features. In particular, the influence of the lone-pair electrons of the three nitrogen atoms on the C-H bond is discussed. A thorough characterization of the vibrational spectroscopic features of the VN3H defect is also presented, where the anharmonicity of the most relevant normal modes is discussed. The infrared and Raman spectra show specific peaks, which allow for the identification of this particular defect among the many defects that are commonly present in both natural and irradiation-damaged diamonds. In particular, the main feature of the spectral fingerprint of the defect (i.e. the C-H stretching mode), experimentally observed at 3107 cm-1, is here computed at 3094 cm-1 with the B3LYP "hybrid" functional (with an anharmonic redshift of 157 cm-1 with respect to its harmonic value). The role played by the three nitrogen atoms on the spectral features of the defect is clearly identified through the redshift due to the 14N → 15N isotopic substitution. © 2017 the Owner Societies.

The adsorption of TiCl4 on the surfaces of MgCl2 crystals has been investigated by means of state-of-the-art periodic hybrid DFT methods, as the first step of a comprehensive study aiming to elucidate the structure of the active species in industrial MgCl2-supported Ziegler-Natta catalysts for ethene and propene polymerization. A first distinctive feature of the approach was the thorough evaluation of dispersion forces, crucial because the binding of TiCl4 on MgCl2 surfaces turned out to be essentially dispersion-driven. Also important was a careful investigation of the effects of different choices on basis set and density functional (DF) on the quantitative aspects of the results; this allowed us to trace the unusually large disagreement in the previous literature and identify unambiguous trends. In particular, three full sets of calculations were run adopting the B3LYP(-D), PBE0(-D) and M06 approximations: to the best of our knowledge, the last represents the first case of M06 functional implementation in a periodic code (CRYSTAL) of widespread use. The results consistently indicated that the adsorption of TiCl4 on well-formed MgCl2 crystals under conditions relevant for catalysis can only occur on MgCl2(1 1 0) or equivalent lateral faces, whereas the interaction with MgCl2(1 0 4) - for decades claimed as the most important surface in stereoselective catalysts - is too weak for the formation of stable adducts. The implications of these findings for catalysis are discussed. (C) 2011 Elsevier Inc. All rights reserved.

The present paper studies MX crystals in rock-salt structure (M: Li, Na, K; X: F, Cl, Br, I). They are often described as being formed by ions. Pictures based on quantum mechanical calculations sustain and quantify it. The tools used are (i) the Quantum Theory of Atoms in Molecules, (ii) the Electron Localization Function, and (iii) the maximization of the probability to find in a spatial domain a number of electrons equal to that of the ion under consideration. The present paper shows that the images provided by these three different tools to analyze the quantum mechanical calculations yield, for these systems, very similar results, in the sense that the spatial domains and probability distributions are close. While results for the first two methods are already present in the literature, the last of the methods is applied for the first time to these systems, and details about the method of calculation and program are also given.

The relative stability of nine different well defined CeO2 surfaces has been studied by periodic density functional calculations using GGA + U and B3LYP exchange-correlation functional. Both methods consistently predict that CeO2(111) is the most stable surface and also provide a consistent picture of the most stable surfaces which indeed are in agreement with previous studies based on empirical interatomic potentials. The facility of ceria surfaces to undergo a redox process has been investigated by forcing spin-polarized solutions, which lead to the occupancy of Ce 4f orbitals. These calculations provide evidence that surfaces with low-coordinated Ce cations are likely to be reduced more easily than regular low-index Miller surfaces. Fil: Branda, Maria Marta. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Física del Sur. Universidad Nacional del Sur. Departamento de Física. Instituto de Física del Sur; Argentina Fil: Ferullo, Ricardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Química del Sur. Universidad Nacional del Sur. Departamento de Química. Instituto de Química del Sur; Argentina Fil: Causá, Mauro. Università degli Studi di Napoli Federico II; Italia Fil: Illas, Francesc. Universidad de Barcelona; España

PAH-based models, with an even or odd number of unsaturated carbon atoms and pi electrons (even and odd PAHs for short), are selected to investigate, by molecular and periodic methods, their electron distribution and border reactivity toward ozone, and also to represent local features and edge reactivity of even or odd soot platelets. These results will contrast those previously collected for the internal positions of similar even (J. Phys. Chem. A 2005, 109, 10929.) or odd systems (J. Phys. Chem. A 2008, 112, 973.). Topologically different peripheral positions, representative of armchair and zigzag borders, exhibit different reactivity right from the beginning. Ozone attacks start off either to give primary ozonides by concerted addition or, nonconcertedly, to first produce trioxyl intermediates. Then, a variety of pathways are described, whose viability depends on both model and position. They can open the way to the possible formation of epoxide, aldehyde, and phenol groups (all entailing O(2) production) or ether (+CO(2)), lactone (+H(2)CO), and ketone functionalities. To sum up, functionalization, regardless of how achieved, can give a number of groups, most of which actually observed in PAH ozonization experimental studies. This picture can be matched up to the results on internal sites of our preceding papers, for which epoxidation was the only outcome. Most interestingly, formation of a ketone group may turn an even system into an odd one (and conversely) while involving production of HOO(center dot).

In this work, the GLOB model, an effective and reliable computational approach well suited for ab initio and QM/MM molecular dynamics simulations of complex molecular systems in solution, has been applied to study two representative open-shell systems, the cobalt(II) ion and the glycine radical in aqueous solution, with special reference to their structural and magnetic properties. The main structural features of the solvent cage around the cobalt ion and the hydrogen bonding patterns around the neutral and zwitterionic forms of the glycine radical have been investigated in some detail. The general good agreement with experiments supports the use of the present model to investigate more challenging and biological/technological relevant open-shell systems.

"Classical" MgCl2-supported Ziegler-Natta catalysts (ZNCs) continue to dominate the industrial prodn. of isotactic polypropylene. There is a growing awareness of the inherent competitive edge of these low-cost systems over single-center (primarily metallocene) catalysts and of the potential for further improvement, particularly if deeper insight into the structure of the catalytic surfaces and the mechanisms of their modification by electron donors can be achieved. In the framework of a project ultimately aiming at the implementation of ZNCs with known and controlled surface structures, we are revisiting this whole area by using a combination of advanced computational (periodic DFT) and spectroscopic (high-resoln. magic-angle-spinning 1H NMR spectroscopy) tools. In this article, we report on the neat MgCl2 matrix and on model MgCl2/electron-donor adducts. The (104) surface, with five-coordinate Mg cations, is the dominant lateral termination in well-formed large crystals, and in highly activated MgCl2 samples prepd. by ball-milling. In the latter case, a minor fraction of surface Mg sites with a higher extent of coordinative unsatn. [e.g., four-coordinate Mg cations on (110) edges and/or at crystal corners or other defective locations] also appear to be present. RMe2Si(OMe) (R=octadecyl) binds to both types of Mg sites, albeit with different strengths resulting in different mobilities. The less-electron-donating RMeSi(OMe)2, in contrast, binds to the more unsatd. Mg sites only. The approach described herein is currently being extended to MgCl2/TiCln systems, and to their adducts with internal and external donors of different natures, strengths, and steric demands.

Activated MgCl2 nanocrystals were prepared by controlled dealcoholation of the MgCl2 center dot 6CH(3)OH adduct, mimicking the routinely adopted methods to synthesize industrial heterogeneous Ziegler Natta catalysts. The effect of the alcohol in driving the morphology of MgCl2 crystals, i.e., the type and extension of the exposed surfaces, was investigated by integrating a detailed structural, morphological, and surface characterization with a state-of-the-art computational modeling. FT-IR spectroscopy of CO adsorbed at 100 K emerged as a feasible, simple, and powerful method to characterize the surface of structurally disordered MgCl2 and MgCl2-based Ziegler Natta catalysts. Our computational morphological analysis revealed that the (012), (015), and (110) surfaces are highly stabilized donor, especially at the temperature typically adopted in the preparation of the precatalysts. FT-IR spectroscopy of adsorbed CO allows distinguishing these surfaces from the other penta-coordinated ones and provides a clear experimental evidence that TiCl4 binds to the (110) and (015) surfaces. The (015) surface was never considered in the past and is characterized by an unusual flexibility in the presence of adsorbates, which detach the Mg cations from the Cl underneath, leaving a coordination vacancy available for the binding of asymmetric titanium sites. Since the recent literature identified a tetra-coordinated Mg as a site of election for the deposition of the Ti species relevant in olefin polymerization, the presence of two eligible Mg sites for Ziegler Natta catalysis is highly interesting.

Graphite, and particularly defective graphite, is chosen to model soot particles. Quantum mechanical calculations are first carried out on molecular polycyclic-aromatic-hydrocarbon-type systems and then extended to a periodic representation of one graphite layer. The features of the interaction of H, HO, NO, NO2, and NO3 with these model systems are examined, with the aim of defining a suitable representation of the atmospheric or combustion gas−solid interactions by which functionalization reactions can take place. The more interesting interactions with small reactive molecules regard the edge of the graphene sheet and the in-plane carbon vacancies. While these interactions can be well described by sufficiently extended molecular models, periodic models are necessary to describe accurately the equilibrium geometries because they introduce the necessary geometric constraints. The ability of a graphene sheet to easily accommodate unpaired electrons in σ or π orbitals is the basis for its interesting intera...

Maximum Probability Domain (MPD) analysis has been recently applied to pure covalent and ionic crystals. The present study is devoted to a first MPD analysis of semi ionic crystals, Silicon Oxide, Aluminum Oxide and Titanium Oxide. These crystals are involved in important catalytic and photo-catalytic processes occurring on their surfaces. For this reason the study has been performed on bulk crystal and on surface slab models. Also surface neutral oxygen vacancy, the F-0 surface defect, has been considered. The Electron Localization Function (ELF) analysis has also been performed, due to its holistic approach to electronic structures. (C) 2015 Published by Elsevier BM.

Coal chars, like most solid carbons, have a pronounced tendency to chemisorb oxygen at low and moderate temperatures. Characterization by XPS of surface oxides on carbon has been accomplished with the aim of providing a better atomistic insight into: (a) reactions involved in molecular oxygen adsorption on coals and (b) the relations between the nanostructure of solid carbons and the chemistry of oxidation. High-resolution C 1s and O 1s core level and valence band XPS spectra effectively reflected the oxidative functionalization of different types of coals and synthetic carbons upon oxidation in air at moderate temperatures (300 and 500 °C). More specifically, analysis of C 1s and valence band spectra could be directed to monitor the structural evolution of the carbons in terms of extension of sp2 versus sp3 conjugation, carbon vacancies and oxidized carbon. Comparison of the O 1s spectra, on the other hand, provided a tool to characterize the nature of oxygen bonding on carbon and to determine the relative abundance of carbon-oxygen species. Results underline the important role of epoxy groups in the early stages of oxidation, providing a mechanistic framework for the identification of the stable and metastable intermediates in the heterogeneous oxidation of coal by molecular oxygen. © 2015 Elsevier Ltd. All rights reserved.

5 pags.; 2 figs.; 2 tabs., In this work we propose a general strategy to calculate accurate He-surface interaction potentials. It extends the dispersionless density functional (dlDF) approach recently developed by Pernal et al. [Phys. Rev. Lett. 109, 263201 (2009)] to adsorbate-surface interactions by including periodic boundary conditions. We also introduce a scheme to parametrize the dispersion interaction by calculating two- and three-body dispersion terms at CCSD(T) level via the method of increments [Stoll, J. Chem. Phys. 97, 8449 (1992)]. The performance of the composite approach is tested on 4He/graphene by determining the energies of the low-lying selective adsorption states, finding an excellent agreement with the best available theoretical data. Second, the capability of the approach to describe dispersionless correlation effects realistically is used to extract dispersion effects in time-dependent density functional simulations on the collision of 4He droplets with a single graphene sheet. It is found that dispersion effects play a key role in the fast spreading of the 4He nanodroplet, the evaporation-like process of helium atoms, and the formation of solid-like helium structures. These characteristics are expected to be quite general and highly relevant to explain experimental measurements with the newly developed helium droplet mediated deposition technique. © 2014 AIP Publishing LLC, This work has been supported by Grants Nos. CCG08-CSIC/ESP-3680 from CSIC-CM, FIS2011-29596-C02-01 and FIS2011-28617- C02-01 from DGI, Spain (FEDER), and 2009SGR1289 from Generatitat de Catalunya.

The role of surface oxygen vacancies in the optical properties of tin dioxide nanobelts is investigated in this paper. Using a first-principles approach, based on the density functional theory combined to a very accurate exchange correlation functional, we characterize SnO(2) (101), that is the nanobelt largest surface. We show that the presence of surface oxygen vacancies leads to the appearance of (i) occupied states located at about 1 eV above the valence band and (ii) unoccupied states lying in resonance with the conduction band. Photoluminescence characterization performed on samples of SnO(2) nanobelts at low temperature shows that the basic spectral features of luminescence are in excellent agreement with theoretical predictions.

By using the CRYSTAL95 program, ab initio periodic Hartree-Fock (PHF) calculations with the full potential and 6-21G* basis set are applied to interpretation of scanning tunneling microscope (STM) images on hexagonal graphite. Our results show asymmetry similar to the experimental and previous pseudopotential calculations. The dominant feature is the deep hollow in the middle of the carbon hexagon. The three carbon atoms that do not have a neighbor in the second layer appear as small hills, the other three with neighbors in the second layer simply appear as saddle points. Our calculation has been successfully used to reproduce experimental features such as the effect of increasing the magnitude of the bias voltage and the effect of increasing the tip-to-surface separation.

Vibrational spectroscopy and Density Functional Theory (DFT) quantum chemical calculations, have been used to investigate structure and chemical properties of some fluorinated ethers. A good agreement between experimental and theoretical results has been obtained. Interesting conformational behaviour has been observed and interpreted for C-H bonds in difluoromethyl end groups. These effects help in understanding the chemical behaviour of these compounds, since C-H equilibrium bond length and bond dissociation energy (BDE) can be correlated with vibrational properties. (C) 1998 Elsevier Science S.A. All rights reserved.

Four neutral complexes were synthesized by reaction of tridentate N-salicylidene-N′-aroylhydrazine ligands with zinc(II) acetate and pyridine as a further ligand. The O,N,O-chelating molecules N-4- [octyloxy(phenoxy)]salicylidene-N′-fluorobenzoylhydrazine (L1), N-4-[octyloxy(phenoxy)]salicylidene-N′-(benzothiazolyl)hydrazine (L 2), N-4-[octyloxy(phenoxy)]salicylidene-N′- cyanobenzoylhydrazine (L3), and N-4-[octyloxy(phenoxy)]salicylidene- N′-nitrobenzoylhydrazine (L4) gave dinuclear O-bridged complexes Zn2Li2Py2n (n = 1 or 2 in a special case). The ligands and complexes were characterized by mass spectrometry and 1H NMR, UV/Vis, photoluminescence (PL), and FTIR spectroscopy. The structures of the complexes were solved by single-crystal X-ray analysis. Nematogenic phase behavior was detected for some ligands by thermogravimetric analysis/differential scanning calorimetry (TGA-DSC) and optical observations. The solution and solid-state photoluminescence spectra were recorded and show that the ligands are roughly nonfluorescent, whereas all of the complexes show noteworthy emission in widely different spectral regions. The photoluminescence maxima of the complexes in the solid state are dependent on the acceptor group, and medium-to-high PL quantum yields were recorded for all of the crystalline complexes. Both periodic and molecular DFT calculations were performed to rationalize the absorbance of these complexes at the "one-particle" level. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Semicarbazides of p-substituted benzoic acids are a class of simple molecules endowed with H-bonding donor and acceptor groups capable of forming H-bonded rows along three linearly independent directions (orthogonal H-bonding synthons). When the acceptor group at the para position is strong (–NO2, –CN, –N=) the same crystal packing is observed, in which three linearly independent chains are formed, and the lattice parameters are easily predictable. In the case of halogen atoms at the para position (–F, –Cl) two different packings are observed in which only one or two H-bonded chains are present. Through ab initio periodic DFT-LCAO computations, we have calculated the lattice energy and density of each semicarbazide in each of the three packings, constructing the matrices Uij and ρij of the lattice energy and density. It is found that all of the different modes of packing observed experimentally for single members of the class correspond to minima of the lattice energy and to acceptable lattice densities for every member of the class.

The wave functions of boron, aluminium and gallium phosphides have been calculated at both the Hartree - Fock and local density approximation levels using the ab initio linear combination of atomic orbitals method implemented in the program CRYSTAL. The Mulliken populations, bandstructure, electron charge densities, and structure factors are evaluated and compared. The chemical bonds are mainly covalent, as revealed by the difference electron charge-density maps obtained by subtracting from the total densities the superposition of the spherical atomic densities. A charge-density accumulation between the nearest neighbours slightly shifted towards the phosphorus appears in the AlP and GaP compounds, whereas an opposite charge transfer from phosphorus to boron is observed in boron phosphide from the Mulliken population analysis.

In carbon materials the mobile π electrons are situated in topologically different circumstances at edge sites, and their π electronic states, essentially controlled by the network structure of sp 2 carbon, may be significantly affected. In this work, we derived topological indications about the reactivity of carbon nanotubes and fullerenes with the hydroxyl radical (OH•), the most important oxidizing species in the troposphere. For each molecular structure, we computed the local softness, the Mulliken charges of the reacting carbons of (n,n) and (n,0) clusters, and their Huckel-type aromaticity rules, as an index to determine topologically independent sites and predicting a certain grade of reactivity of the nanotube and fullerenic carbon atoms. Using local softness, closely related to the energy gap, it was possible to separate the periodical nanotubes in three families according to their reactivity. A connection between the reactivity index ΔE and the topology was established by means of the Fukui integrated function. It resulted that for (n,0) clusters, odd n implies aromaticity, whereas even n, non-aromaticity; (n,n) clusters are in any case non-aromatic. For a better understanding of some experimental results, we also discussed how edge effects can influence topological reactivity due to the increment of the number of benzene rings in some cluster arrangements.

Our purpose is to identify a computational level sufficiently dependable and affordable to assess trends in the interaction of a variety of radical or closed shell unsaturated hydro-carbons A adsorbed on soot platelet models B. These systems, of environmental interest, would unavoidably have rather large sizes, thus prompting to explore in this paper the performances of relatively low-level computational methods and compare them with higher-level reference results. To this end, the interaction of three complexes between non-polar species, vinyl radical, ethyne, or ethene (A) with benzene (B) is studied, since these species, involved themselves in growth processes of polycyclic aromatic hydrocarbons (PAHs) and soot particles, are small enough to allow high-level reference calculations of the interaction energy ΔEAB. Counterpoise-corrected interaction energies ΔEAB are used at all stages. (1) Density Functional Theory (DFT) unconstrained optimizations of the A-B complexes are carried out, using the B3LYP-D, ωB97X-D, and M06-2X functionals, with six basis sets: 6-31G(d), 6-311 (2d,p), and 6-311++G(3df,3pd); aug-cc-pVDZ and aug-cc-pVTZ; N07T. (2) Then, unconstrained optimizations by Møller-Plesset second order Perturbation Theory (MP2), with each basis set, allow subsequent single point Coupled Cluster Singles Doubles and perturbative estimate of the Triples energy computations with the same basis sets [CCSD(T)//MP2]. (3) Based on an additivity assumption of (i) the estimated MP2 energy at the complete basis set limit [EMP2/CBS] and (ii) the higher-order correlation energy effects in passing from MP2 to CCSD(T) at the aug-cc-pVTZ basis set, ΔECC-MP, a CCSD(T)/CBS estimate is obtained and taken as a computational energy reference. At DFT, variations in ΔEAB with basis set are not large for the title molecules, and the three functionals perform rather satisfactorily even with rather small basis sets [6-31G(d) and N07T], exhibiting deviation from the computational reference of less than 1 kcal mol(-1). The zero-point vibrational energy corrected estimates Δ(EAB+ZPE), obtained with the three functionals and the 6-31G(d) and N07T basis sets, are compared with experimental D0 measures, when available. In particular, this comparison is finally extended to the naphthalene and coronene dimers and to three π-π associations of different PAHs (R, made by 10, 16, or 24 C atoms) and P (80 C atoms).

Two new perylene diimide derivatives N,N′-bis(5-tridecyl-1,3,4- thiadiazol-2-yl)perylene-3,4,9,10-tetracarboxylic 3,4:9,10-diimide (PDI-T1) and N,N′-bis[5-(1-hexyl)nonyl-1,3,4-thiadiazol-2-yl]perylene-3,4,9, 10-tetracarboxylic 3,4:9,10-diimide (PDI-T2), achieved by functionalizing the basic perylene molecular core at imide nitrogen with 1,3,4-thiadiazole rings, have been synthesized. Both these compounds make possible the fabrication of n-type organic thin-film transistors able to work in air, even when bare SiO2 surfaces are utilized as gate dielectric. As active channels of transistors in the bottom-contact bottom-gate configuration, PDI-T1 evaporated films exhibited a maximum mobility of 0.016 cm2/V s in vacuum. For evaporated PDI-T2 films, instead, mobility values were found to be more than one order of magnitude lower, because of their reduced degree of crystalline order. However, PDI-T2 films can be also deposited by solution techniques and field-effect transistors were fabricated by spin-coating, displaying mobility values ranging between 10-6 and 10-5 cm2/V s. Similar to what previously found for other perylene diimide derivatives, our experimental work also demonstrates that the electrical response of both PDI-T1 and PDI-T2 transistors under ambient conditions can be improved by increasing the level of hydrophobicity of the dielectric surface. © 2012 Elsevier B.V. All rights reserved.

Most spectroscopic parameters are influenced by nuclear dynamics and by the chemical environment. However, proper inclusion of these effects still represents a challenge in computational spectroscopy studies. In many cases, a route coupling satisfactory accuracy with reasonable computational costs consists in the integration of DFT-based methods to compute spectroscopic parameters, with ab initio molecular dynamics simulations to sample from the classical phase space of the system. Here, we discuss the application of this approach in two case studies of remarkable practical interest, namely the simulation of the absorption spectrum of 9-methyladenine, an adenine nucleoside model; and the prediction of electron spin resonance parameters for nitroxyl radicals, the prototypical spin probes. In both cases, the accuracy of the results increases significantly when the subtle interplay of intra-molecular dynamics and solvent effects is introduced.

The hydrate crystal lithium hydroxide monohydrate LiOH.H2O has been studied by ab initio periodic Hartree-Fock calculations. The influence of the crystalline environment on the local molecular properties (molecular geometry, atomic charges, electron density, molecular vibrations and deuterium quadrupole coupling constants) of the water molecule, the lithium and hydroxide ions has been calculated. A number of crystalline bulk properties are also presented, optimized crystalline structure, lattice energy and electronic band structure. The optimized cell parameters from calculations with a large basis set of triple-zeta quality differ by only 1-3% from the experimental neutron-determined cell, whereas the STO-3g basis set performs poorly (differences of 5-10%). With the triple-zeta basis also the atomic positions and intermolecular distances agree very well with the experiment. The lattice energy differs by approximately 8% from the experimental value, and by at most 3% when a density-functional electron correlation correction is applied. Large electron-density rearrangements occur in the water molecule and in the hydrogen bond and are in qualitative and quantitative agreement with experimental X-ray diffraction results. The quadrupole-coupling constants of the water and hydroxide deuterium atoms are found to be very sensitive to the O-H bond length and are in good agreement with experimental values when the calculation is based on the experimental structure. The anharmonic O-H stretching vibrations in the crystal are presented and found to be very close to results from calculations on molecular clusters. The electronic band and density-of-states spectra are discussed. Model calculations on a hydrogen fluoride chain were used to rationalize the results.

Calculations are reported of the enthalpies of mixing of MnO/MgO and NiO/MgO based on temperature-dependent lattice statics and dynamics and ab initio periodic Hartree-Fock methodology with a posteriori corrections for electron correlation derived from density-functional theory. For MnO/MgO both methodologies predict enthalpies of mixing that are close to those reported by Hahn and Muan (J. Phys. Chem. Solids, 1961, 19, 338). For NiO/MgO, on the other hand, classical simulations predict positive deviations from ideality in contrast to Hartree-Fock calculations which predict small negative deviations in agreement with the experimental data of Davies and Navrotsky (J. Solid State Chem., 1981, 38, 264).

Regions of space are defined to maximize the probability to find two electrons in it. They can be interpreted as regions where Lewis' electron pairs are most likely to be found. These maximum probability domains (MPDs) provide information similar to that obtained from the electron localization function (ELF), but is not identical to it. The elements in the diamond structure provide examples for a comparison.

The effect of the Madelung field corresponding to formal point charges ± e arranged according to the fcc lattice of LiF upon CO, N2 and NO molecules is studied in order to isolate the purely electrostatic interaction from the other forces acting in the related physisorption processes. The polarization which is produced in those molecules by the Madelung field is analyzed in terms of dipole and quadrupole moments, as well as in terms of electron charge distributions. The influence of the electron correlation is taken into account by using the QCISD technique, which requires the semi-infinite lattice being simulated with a suitable cluster of point charges.

The 50 nm-thick polystyrene (PS) film, involved in some innovative memory devices, is sandwiched between two Al electrodes and contains 8-hydroxyquinoline (8HQ) molecules and gold nanoparticles. A periodic DFT study of the molecular adsorption of two different tautomers of the 8HQ on an Al (111) surface has been performed. The changes of the metal work function under molecular adsorption have been calculated to determine the possible effect at the interface on the electron and hole injection barriers. The strong adsorption of the 8HQ(II) tautomer makes the work function change up to 1.2 eV with respect to the bare Al (111) surface. Hence, the stabilization of the 8HQ(II) at the interface could play a key role in the switching mechanism of the device.

The electronic structure of different magnetic states of alpha, beta and gamma modifications of TiCl(3) has been computed employing the density functional theory with periodic boundary conditions and localized Gaussian basis sets. The analysis of the density of the electronic states (DOS) and of the spin density makes it possible to classify these halides as Mott-Hubbard insulators, where the band gap appears a result of large on-site Coulomb interaction. For each crystalline phase, the relative stability of different magnetic states has been analyzed in terms of exchange mechanisms. The electronic population data along with the spin density maps support the assumption of a d(1) Titanium ion in a distorted octahedral crystal field, notwithstanding the not fully ionic character of TiCl(3) modifications. Dispersion forces are particularly important for this material: a classical correction (of the type f(R)/R(6)) has been added to the DFT energies and gradients, providing a good agreement with structural data.

MgCl(2) is the preferred support for the industrial Ziegler-Natta catalysts, and is believed to act as a template for the epitactic chemisorption of the active Ti species. As the first step of a thorough computational modeling of these systems, we studied the bulk and surface structure of the ordered alpha and beta phases of MgCl(2) by means of periodic DFT (B3LYP) methods using localized basis sets. The layer structure of both phases was reproduced satisfactorily with the inclusion of a (small) empirical dispersion correction ("DFT-D") as a practical method to describe the attraction between the layers. Surface models were studied on slabs with adequate thickness. It appears that various surfaces exposing 5-coordinated Mg are very similar in energy and are the lowest non-trivial surfaces. Cuts exposing 4-coordinated Mg are significantly less stable; both kinetic and equilibrium models of crystal growth indicate that they should normally not be formed to a significant extent. "Nano-ribbons" of single, flat chains of MgCl(2), sometimes proposed as components of the disordered delta phase, were also evaluated, but are predicted to be unstable to rearrangement. Implications for the role of MgCl(2) as catalyst support are discussed.

Extensive DFT calculations are reported for the carbon centered glycine radical embedded in a-glycine crystal. Combined use of periodic and embedded cluster models leads to accurate structural and magnetic properties. Environmental effects are not negligible for all properties, whereas vibrational averaging effects connected to inversion at the radical center are significant only for C-alpha and H-alpha atoms. The good agreement between computed and experimental ESR parameters confirms the nature of one of the radicals obtained upon irradiation of a-glycine crystals and the reliability of DFT computations properly including both vibrational averaging and environmental effects. (c) 2007 Elsevier B.V. All rights reserved.

PAHs made from an odd number of unsaturated carbon atoms and pi electrons (odd PAHs) have been detected in flames and flank the more familiar even PAHs, having approximately the same quantitative importance, particularly for PAHs containing more than 25 carbon atoms. Similarly, soot platelets containing an odd number of carbon atoms can be reasonably assumed to form during combustion. PAHs are intended here as small models for the investigation of some, of their local features. To this end, quantum mechanical calculations were also carried out on periodic models. The spin density patterns were found to be highly dependent on the PAH size and shape. PAHs and soot, once released in the environment, can undergo several oxidation processes. Ozone is then taken as a probe of the reactivity properties of some internal exposed portions of a platelet. A primary ozonide (PO) corresponds to an energy minimum, but the relevant concerted addition pathway does not exist, because a PO-like saddle point is second-order. The reaction begins with a nonconcerted attack that produces a trioxyl radical (TR). Subsequent O-2 loss from the TR leaves either an epoxide with a pi-delocalized electron or a pi-delocalized oxepine, by cleavage of the ring carbon-carbon bond. The initial doublet spin multiplicity thus provides a description of the reaction surface unlike that for the internal reactivity of the closed-shell even systems investigated in a previous work, even though the final functionalization is the same.

The adsorption of TiCl 4 on the surfaces of MgCl 2 crystals has been investigated by means of state-of-the-art periodic hybrid DFT methods, as the first step of a comprehensive study aiming to elucidate the structure of the active species in industrial MgCl 2-supported Ziegler-Natta catalysts for ethene and propene polymerization. A first distinctive feature of the approach was the thorough evaluation of dispersion forces, crucial because the binding of TiCl 4 on MgCl 2 surfaces turned out to be essentially dispersion-driven. Also important was a careful investigation of the effects of different choices on basis set and density functional (DF) on the quantitative aspects of the results; this allowed us to trace the unusually large disagreement in the previous literature and identify unambiguous trends. In particular, three full sets of calculations were run adopting the B3LYP(-D), PBE0(-D) and M06 approximations; to the best of our knowledge, the last represents the first case of M06 functional implementation in a periodic code (CRYSTAL) of widespread use. The results consistently indicated that the adsorption of TiCl 4 on well-formed MgCl 2 crystals under conditions relevant for catalysis can only occur on MgCl 2(1 1 0) or equivalent lateral faces, whereas the interaction with MgCl 2(1 0 4) - for decades claimed as the most important surface in stereoselective catalysts - is too weak for the formation of stable adducts. The implications of these findings for catalysis are discussed

In magnetic resonance imaging (MRI), the degree of contrast can be improved by using suitable contrast agents. The diethylenetriaminepentaacetic acid (DTPA) complexes of paramagnetic ions have been proposed for this purpose. This paper deals with extensive simulations of the distribution of species in blood plasma when solutions of manganese(II), copper(II), iron(III) and gadolinium(III) ions (as their soluble salts and as DTPA complexes) are injected. The various interaction equilibria, for which formation constants are known, are considered in order to assess toxic side-effects associated with their use in MRI. The data obtained support from the thermodynamic point of view, the use of GdDTPA, and suggest that the administration of a slight excess of ligand would guarantee complete coordination of the toxic gadolinium ion, and only minor interaction with the metal ions naturally present in blood plasma.

The electron density, the electrostatic potential and the electric field of the MgO (100) surface, both regular and containing an oxygen vacancy (F s center), are compared in order to understand the modifications induced in the surface-absorbate interaction by the presence of the defect, with particular attention to the metal-oxide case. The spin-density for a gold atom absorbing on the most characteristic sites of the regular and F s -defected surface is also shown. It is found that in the defected surface the electron pair in the vacancy protrudes appreciably out of the surface, thus shifting the electrostatic potential to negative values (but producing a similar electric field) and being able to chemically interact with neighboring absorbed species. These results rationalize the rotational invariance and double frustration effects previously described for the metal/F s -defected MgO (100) surface.

The potentiality of the time dependent density functional theory (TDDFT) for the description of core excitation spectra (XAS) in transition metal oxides is analyzed, considering the rutile form of TiO(2) as a test case. Cluster models are adopted to mimic the bulk, embedded within an array of point charges to simulate the Madelung potential. All of the edges, titanium and oxygen K and titanium L edges, are considered, and the TDDFT results are compared with the experimental data in order to assess the performance of the theoretical approach in dealing with this complex class of compounds. Satisfactory results have been obtained for the Ti and O K edges, while in the case of the Ti L edge some discrepancies with the experiment are still present. The configuration mixing explicitly included in the TDDFT model strongly influences the distribution of the 2p metal oscillator strength. The origin of the spectral features is investigated with the help of the partial density of the virtual states (PDOS) calculated for each core hole considered, which can be qualitatively compared with the theoretical spectra calculated in the Kohn-Sham one-electron approach.

The desorption mechanism for oxygenated functionalities on soot is investigated by quantum mechanical calculations on functionalized polycyclic aromatic hydrocarbon PAH models and compared with recently published temperature programed desorption-mass spectrometry results. Substituents on PAHs of increasing size up to 46 carbon atoms in the parent PAH are chosen to reproduce the local features of an oxidized graphenic soot platelet. Initially, the study is carried out on unimolecular fragmentation extrusion, in some cases processes producing HO, CO, or CO2, in model ketones, carboxylic acids, lactones, anhydrides, in one aldehyde, one peroxyacid, one hydroperoxide, one secondary alcohol, and one phenol. Then, a bimolecular process is considered for one of the carboxylic acids. Furthermore, some cooperative effect which can take place by involving two vicinal carboxylic groups derived from anhydride hydrolysis is investigated for other four bifunctionalized models. The comparison between the computed fragmentation desorption barriers for the assessed mechanisms and the temperature at which maxima occur in TPD spectra for HO, CO, or CO2 desorption offers a suggestion for the assignment of these maxima to specific functional groups, i.e., a key to the description of the oxidized surface. Notably, the computations suggest that 1 the desorption mode from a portion of a graphenic platelet functionalized by a carboxylic or lactone groups is significantly dependent from the chemical and geometric local environment. Consequently, we propose that 2 not all carboxylic groups go lost at the relatively low temperatures generally stated, and 3 lactone groups can be identified as producing not only CO2 but also CO. © 2006 American Institute of Physics.

The ozonization mechanism for polycyclic aromatic hydrocarbons (PAHs) and soot is investigated by quantum mechanical calculations carried out on molecular and periodic systems. PAHs, interesting per se, serve also to model the local features of the graphenic soot platelets, for which another model is provided by a periodic representation of one graphenic layer. A concerted addition leads to a primary ozonide, while a nonconcerted attack produces a trioxyl diradical (in which one of the two unpaired electrons is pi-delocalized). Easy loss of (i) (1)O(2) or (ii) (3)O(2) from either intermediate, with spin conservation, would yield stable (i) singlet or (ii) triplet pi-delocalized species which carry an epoxide group. The trioxyl diradical pathway is estimated to be preferred, in these systems. An intersystem crossing, taking place in the trioxyl diradicals, can be invoked to allow the even easier loss of a ground-state oxygen molecule with the formation of a ground-state epoxide in a more exoergic and less demanding step. We propose that soot ozonization can take place by such a process, with ultimate functionalization of the graphenic platelets by epoxide groups.

In order to give a deeper insight into the cracking of the Verneuil corundum boules, several samples of different colors and from different industrial sources have been analyzed by means of white beam synchrotron radiation X-ray diffraction topography (WB-SR-XRDT) and PBC analysis (Hartman and Perdock theory). The cracking occurs along an irregular surface containing the elongation axis of boules, and the WB-SR-XRDT analysis shows that this surface is nearly parallel to the { 1 2 0 } crystallographic planes. Following Hartman and Perdock theory, these planes form F-planes with a high density of PBC directions: 3 PBC directions—[2 1 1], [2 1 0] and [ 4 2 1 ]—have been recognized as parallel to the 1 2 0 plane. The 1 02 planes that are normal to the cracking surface of boules are F-planes too, but they show a lower bond density per unit area with respect to the 1 2 0 planes. Ab initio calculations show that these planes have lower surface energies with respect to other hk 0 planes, such as {1 0 0} and {0 1 0}.

The reaction rate constants of a hydroxyl radical with some hydrofluorocarbons and hydrofluoroethers have been calculated using “transition state theory”. A new computational method has been developed for transferring along the reaction coordinate the very high accuracy corrections, obtained at G2 level of theory, from small models (CH 4 and CF 3 H) to larger molecules (halogenated ethanes and methyl ethers). The substituent effects on the CH bond strengths and on the reaction rate constants have been examined, as well as the geometrical and electronic features of the transition states.

cited By 48; International audience; We present the results of a first principles study on the ordered Ge 0. 50C 0.50 and Sn 0.50C 0.50 cubic alloys. A linear combination of atomic orbitals approach in the framework of density functional theory is employed for total energy calculations in the zincblende phase. A fitting of the energy surface to the equation of state yields the lattice constant of 4.61 and 5.17 Å and the bulk modulus of 181 and 119 GPa for GeC and SnC, respectively. Analysis of band structure suggests a crossover of the nature of the band gap from indirect to direct in going from SiC to GeC to SnC. Although both alloys predicted to be unstable with respect to their elemental components at zero pressure and temperature, GeC appears to become stable at higher pressure. It appears that both the lattice constant and bulk modulus of the ordered alloys do not follow Végard's linear rule, though the calculated dielectric constant of the cubic alloys is approximately the average of the dielectric constant of their elemental components.

Bulk platinum and platinum slabs one to four (111) layers thick have been studied using the HF and DFT CO-LCAO (Hartree-Fock and density functional theory crystalline-orbital and linear-combination-of-atomic-orbitals) CRYSTAL program. The objective of this paper is to test the suitability of slabs of different thicknesses for modelling the (111) surface of platinum. The cohesive properties of the bulk (lattice constant, bulk modulus, cohesive energy) are reported and the electronic structure of the platinum bulk is discussed. The surface and relaxation energies as well as the relaxation distances of the slabs were evaluated. The electronic structure of the slabs is discussed in terms of Mulliken population analysis, the density of states, the band structure and electron-density difference maps. `Initial-state' surface-atom core-level shifts are reported, as well as atomic dipole and quadrupole moments perpendicular to the surface.