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EPR data of the positively charged N2 defect (N+2) are reported from all-electron calculations based on the B3LYP hybrid functional using a local (Gaussian type) basis set, within a supercell scheme, as implemented in the CRYSTAL code. The Fermi contact Aiso term, the components of the hyperfine coupling tensor, , and the electric field gradient (only P‖ is available) are all in excellent agreement with two independent sets of experimental data, with errors of less than 4%. The equilibrium geometry, electronic structure, and IR and Raman spectra are also computed, where the two spectra are compared with those of the neutral N2 and double positive N++2, both of which have closed shell ground states. In particular the IR and Raman spectra of the three species exhibit important differences. Information on the charge, spin and geometry of defective species derived from experimental EPR data are based mostly on relatively simple models; here they are obtained directly from rigorous quantum mechanical simulations. It is shown that the extrapolation from the experimental EPR data can lead to relatively serious errors.

The electronic structure and the equilibrium geometry of the VN3 defect in diamond are computed by using a quantum mechanical approach, the main ingredients being an all electron gaussian type basis set, a global hybrid functional, the supercell scheme (216 atoms) and the CRYSTAL code. For the first time the Fermi contact and the hyperfine coupling tensor B of the open shell structure (Sz = 1/2) are evaluated and compared with available experimental EPR (electron paramagnetic resonance) data. The agreement is excellent. The infrared (IR) spectrum is also computed. It results to be very close to the one of the so called B defect (VN4), and with similarities to the one of the other members of the VNx (x = 0, 1, 2) family, so that the attribution of specific experimental peaks to one of the five mentioned compounds is difficult and questionable. The peaks measured by Sutherland (Nature, 174, 901, 1954) and attributed to the B center might, at least in part, come from both VN3 and VN4. Being the ground state of the latter a closed shell, and then EPR inactive, a combined use of the two spectroscopies (IR and EPR) is mandatory for the identification of the relative weight of the two defects.

The amplification of microscopic molecular motions so as to produce a controlled macroscopic body effect is the main challenge in the development of molecular mechanical devices. That amplification requires the coherent and ordered movement of each molecule of a whole macroscopic set, such as that taking place in a single-crystal-to-single-crystal transition. Actually, single-crystal-to-single-crystal transitions in molecular crystals can produce a variety of mechanical effects potentially useful in the development of smart materials. A challenging issue in these dynamic crystals, propedeutic to many possible applications in devices, is the gaining of a strict control over the mechanical effects associated with the transition. Here we report an example in which the control of the mechanical effects was successfully obtained. The compound studied undergoes a reversible single-crystal-to-single-crystal transition at 71 °C, from a planar stacked to a herringbone type packing. To this transition, a reversible macroscopic self-reshaping of the crystal is associated. Depending on the morphology, the crystal specimen undergoes a reversible longitudinal expansion of about 20% or a reversible transverse expansion of 20%, the other two dimensions of the crystal specimen being substantially unchanged. The amount of the macroscopic reshaping effect (20%) fully matches the relative variation of the sole unit cell parameter that changes during the transition (from 8.139 to 9.666 Å) in a sort of scale-invariant process. This represents striking evidence of controlled translation of sub-nanometer molecular motions up to the macroscopic scale of body motion.

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.

Recent advances in the soot studies have shown experimental evidences of π-radicals and cross-linked structures among the molecular constituents of just-nucleated soot particles. π-radicals could have an important role in particle nucleation by increasing the binding energy between polycyclic aromatic hydrocarbons with respect to pure van der Waals interactions. In this work we use density functional theory by Grimme D3 dispersion correction (DFT-D3) with hybrid functional and localized Gaussian basis set (B3LYP/6-31G**) to analyze and classify the clustering behaviors of two aromatic radicals visualized experimentally by atomic force microscopy (Commodo et al. Combust. Flame 205: 154–164, 2019). These aromatic radicals have different topological structures and delocalization of the unpaired electron. The binding energy and energy bandgap characteristics of the clusters are calculated. The theoretical results show a different clustering behavior for the two aromatic radicals. The one with a partial localization of the unpaired electron tends to form a σ-dimer; conversely, the radical with a greater delocalization of the unpaired electron leads to π-stacking formation with a slight overbinding of few kcal mol−1 with respect to pure van der Waals interactions and a marked lowering of the energy bandgap. The formation of π-stacking induced by delocalized π-radicals could in part explain some spectroscopic evidences observed during soot nucleation. © 2020 Elsevier Ltd

This paper reports an interpretation of the FTIR and Raman spectra of the HKUST-1 MOF in light of an ab initio theoretical analysis of the vibrational spectra of the framework. Several models, with different degrees of complexity have been compared in terms of their capability of reproducing structural parameters and vibrational behavior. Satisfactory simulations have been obtained only by considering a complete periodic model accounting for the totality of the structural features. Simpler models limited to a single paddlewheel motif are found to be too inaccurate for an effective vibrational analysis. A number of signals have been identified that are characteristic of framework defects. These were properly simulated by introducing specific functional groups in the framework, thus confirming the chemical nature of the defects and identifying spectroscopic signatures for their recognition. The complete vibrational analysis reported herein revises a number of literature assignments relevant for the study of host/guest molecular interactions.

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.

Recent advances in the literature have shown experimental evidences of a consistent amount of ?-radicals and cross-linked structures among the molecular constituents of just-nucleated soot particles. The inclusion of such experimental observations in the mechanism of soot particle formation will improve the comprehension of the physics and chemistry involved during the evolution of soot particles in flames. In this regards ?-radical could have a fundamental role, particularly at the onset of soot formation, when gas-phase molecules convert to a condensed-phase material. According to the literature of highly delocalized ?-radicals, two type of dimerization process can be taken into account. These are: a ?-stacking mechanism that occurs by forming multi-electron/multi-center delocalized bonds, thus covalent-like, with shorter intermolecular distances and stronger interactions with respect to van der Waal interactions; a conventional ?-bond formation, with longer distance and weaker strength than regular C-C bond. In this work we use density functional theory (DFT) with hybrid functional and localized Gaussian basis set (B3LYP/6-31G**) to analyze and classify two radicals having different topological structures in term of localization/delocalization of the unpaired electron, and in turn binding energy and optical band gap characteristics of the clustering of two different molecular radicals. The ambition of this study can be represented by the investigation of the two polycyclic aromatic, labeled as radical 1 (R1), and radical 2 (R2), experimentally determined by Atomic Force Microscopy (AFM) studies and calculated by ab-initio quantum investigations. As results the different clustering of the two polycyclic aromatic hydrocarbons with a couple of unpaired electron delocalized, in different spin multiplicity (Sz=1, 3), leads to a different reductions of the optical gap.

The structural, electronic and vibrational properties of two common defects in diamond, CHN and CHB, describing the case in which a carbon C1 atom is substituted by a nitrogen atom, or by a boron atom, breaking a C1–C2 bond, followed by the saturation of the dangling bond of C2 by a hydrogen atom, are investigated at the quantum mechanical level, by using a periodic supercell approach, hybrid DFT functionals and a local Gaussian-type basis set as implemented in the CRYSTAL code. The effect of concentration of the defects has been explored, by considering two supercells containing 64 and 216 atoms (S64 and S216). Formation and hydrogenation energies, geometries, Mulliken charges and the band structure of both defects are reported. The vibrational features of the defects have been investigated, by generating the IR and Raman spectra, and by analyzing graphically and through the isotopic substitution (H → D, 11B → 10B and 14N → 15N) the nature of the most relevant modes related to the defects. The computed C–H stretching mode of CHN, once corrected for anharmonicity (3408 cm $$^{-1}$$ ), falls to wavenumbers very close to the experimental peak observed at 3394 cm $$^{-1}$$ , which can then be reasonably attributed to this specific defect. The present manuscript is included in a special volume in honor and memory of Janos Angyan. Although he did not study in particular the kind of defects discussed in the present manuscript, the many methodological contributions he introduced in computational science have inspired many of the tools we have been using here. One of the present authors, RD, in particular, is grateful to Janos for the illuminating discussions they had in Paris, Nancy and Torino.

The VN

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.

The ability of thermosalient solids, organic analogues of inorganic martensites, to move by rapid mechanical reconfiguration or ballistic event remains visually appealing and potentially useful, yet mechanistically elusive phenomenon. Here, with a material that undergoes both thermosalient and non-thermosalient phase transitions, we demonstrate that the thermosalient effect is preceded by anomalous thermal expansion of the unit cell. The crystal explosion occurs as sudden release of the latent strain accumulated during the anisotropic, exceedingly strong expansion of the unit cell with αa = 225.9 × 10−6 K−1, αb = 238.8 × 10−6 K−1 and αc = −290.0 × 10−6 K−1, the latter being the largest negative thermal expansivity observed for an organic compound thus far. The results point out to the occurence of the thermosalient effect in phase transitions as means to identify new molecular materials with strong positive and/or negative thermal expansion which prior to this work could only be discovered serendipitously.

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.

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).

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...

Using periodic first principles simulations we investigate the interaction of oxygen molecules with both regular Al(111) and Al(001) surfaces as well as a stepped Al(111) substrate. The limitation of this approach is the use of thin metallic slabs with a limited range for their coverage by adsorbed oxygen. The advantage is the detailed modeling that is possible at an atomic level. On the regular Al(111) surface, we have been able to follow the oxidation process from the approach of O 2 molecules to the surface, through the chemisorption and absorption of O atoms, up to the formation of first Al 2 O 3 formula units. An energetically feasible mechanism for the formation of these Al 2 O 3 ‘molecules’ is proposed but their aggregation to Al 2 O 3 growth nuclei can only be surmised. On the Al(001) surface, absorption of oxygen atoms occurs more readily without any restrictions on the density of their surface overlayer, in agreement with the failure to observe a distinct chemisorption stage for O on Al(001) experimentally. The stepped Al(111) surface contains both {111} and {001} microfacets: the latter are obviously preferred for penetration of the oxygen adatoms into the subsurface space of the substrate. Before considering the O/Al interfaces the computational method is tested thoroughly by simulations on bulk Al and close-packed aluminum surfaces.

Chemical looping combustion of solid fuels has recently received considerable attention as a promising capture-ready combustion technology. An alternative method to overcome the inherent limitations of carrier-based looping combustion of solid carbons was proposed under the name of CarboLoop process [1,2] which was based on the well established role played by surface oxides as reaction intermediates in the combustion of solid carbons [3,5]. An interesting development of this concept in the future energetic scenario could be in the direction of chemical looping gasification. The present work addresses the chemistry and the thermicity of oxygen chemisorption on coal chars followed by desorption of CO and CO2 at higher temperature. The experimental work relies on traditional thermoanalytical and calorimetric techniques, but additionally takes advantage of modern spectroscopic analyses, in particular XPS with syncrotrone radiation, to disclose the evolution of functional groups present on the surface of char throughout oxidation and thermal desorption..

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.

In this work we propose a general strategy to calculate accurate He-surface interaction potentials. It extends the dispersionless density functional approach recently developed by Pernal et al. [Phys. Rev. Lett. 103, 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 coupled cluster singles and doubles and perturbative triples (CCSD(T)) level via the method of increments [H. Stoll, J. Chem. Phys. 97, 8449 (1992)]. The performance of the composite approach is tested on (4)He/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 (4)He droplets with a single graphene sheet. It is found that dispersion effects play a key role in the fast spreading of the (4)He 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.

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 concepts of atoms and bonds in molecules which appeared in chemistry during the nineteenth century are unavoidable to explain the structure and the reactivity of the matter at a chemical level of understanding. Although they can be criticized from a strict reductionist point of view, because neither atoms nor bonds are observable in the sense of quantum mechanics, the topological and statistical interpretative approaches of quantum chemistry (quantum theory of atoms in molecules, electron localization function and maximum probability domain) provide consistent definitions which accommodate chemistry and quantum mechanics.

The present study discusses the main features of the two programs CRYSTAL and EMBED developed for the ab initio study of the electronic properties of perfect periodic structures and of crystals with local defects, respectively. After a brief historical introduction, the structure of CRYSTAL is outlined and some specific aspects are discussed in detail: the use of local basis functions, the way of dealing with the Coulomb and the exchange series, the exploitation of point symmetry, the possibility to adopt either the Hartree–Fock approach or one among a variety of Kohn–Sham Hamiltonians. The present capabilities of the program are illustrated by a survey of selected applications from existing literature. Information is provided concerning work in progress aimed at removing some of the limitations of the code and improving its performance. The characteristics of EMBED are analyzed with emphasis given to the critical aspects of the method: limits of validity of the fundamental approximation on which the embedding technique relies, problems of convergence of the self-consistent procedure; and the delicate issue of the estimate of the defect formation energy. Again, a critical survey of applications clarifies the capabilities of the code. The envisaged improvements to be introduced in a forthcoming release of EMBED are presented. The present and prospective role of the two programs in the field of computational studies of condensed matter problems is outlined. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 77: 1032–1048, 2000

The p(1×1) adsorption of atomic oxygen on the fcc and hcp three-fold hollow site of a Pt(111) surface has been investigated by a periodic slab model, varying the number of layers from two to four. ...

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.

The capabilities of the CRYSTAL14 program are presented, and the improvements made with respect to the previous CRYSTAL09 version discussed. CRYSTAL14 is an ab initio code that uses a Gaussian-type basis set: both pseudopotential and all-electron strategies are permitted; the latter is not much more expensive than the former up to the first-second transition metal rows of the periodic table. A variety of density functionals is available, including as an extreme case Hartree-Fock; hybrids of various nature (global, range-separated, double) can be used. In particular, a very efficient implementation of global hybrids, such as popular B3LYP and PBE0 prescriptions, allows for such calculations to be performed at relatively low computational cost. The program can treat on the same grounds zero-dimensional (molecules), one-dimensional (polymers), two-dimensional (slabs), as well as three-dimensional (3D; crystals) systems. No spurious 3D periodicity is required for low-dimensional systems as happens when plane-waves are used as a basis set. Symmetry is fully exploited at all steps of the calculation; this permits, for example, to investigate nanotubes of increasing radius at a nearly constant cost (better than linear scaling!) or to perform self-consistent-field (SCF) calculations on fullerenes as large as (10,10), with 6000 atoms, 84,000 atomic orbitals, and 20 SCF cycles, on a single core in one day. Three versions of the code exist, serial, parallel, and massive-parallel. In the second one, the most relevant matrices are duplicated, whereas in the third one the matrices in reciprocal space are distributed for diagonalization. All the relevant vectors are now dynamically allocated and deallocated after use, making CRYSTAL14 much more agile than the previous version, in which they were statically allocated. The program now fits more easily in low-memory machines (as many supercomputers nowadays are). CRYSTAL14 can be used on parallel machines up to a high number of cores (benchmarks up to 10,240 cores are documented) with good scalability, the main limitation remaining the diagonalization step. Many tensorial properties can be evaluated in a fully automated way by using a single input keyword: elastic, piezoelectric, photoelastic, dielectric, as well as first and second hyperpolarizabilies, electric field gradients, Born tensors and so forth. Many tools permit a complete analysis of the vibrational properties of crystalline compounds. The infrared and Raman intensities are now computed analytically and related spectra can be generated. Isotopic shifts are easily evaluated, frequencies of only a fragment of a large system computed and nuclear contribution to the dielectric tensor determined. New algorithms have been devised for the investigation of solid solutions and disordered systems. The topological analysis of the electron charge density, according to the Quantum Theory of Atoms in Molecules, is now incorporated in the code via the integrated merge of the TOPOND package. Electron correlation can be evaluated at the Moller-Plesset second-order level (namely MP2) and a set of double-hybrids are presently available via the integrated merge with the CRYSCOR program. (C) 2014 Wiley Periodicals, Inc.

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.

All-electron density-functional calculations are performed to study atomic structure and electronic properties of the nonpolar surfaces, namely zinc blende (110) and wurtzite (101-|Am0) of AlN. Both surfaces are modeled using a two-dimensional periodic slab allowing the relaxation of the first two surface layers in the calculations. The results predict a small layer rotation angle accompanied by a contraction of Al-N bond length for both surfaces. These results do not follow the well-accepted rotation-relaxation model that predicts large layer rotation angles (\ensuremath{\sim}28\ifmmode^\circ\else\textdegree\fi{}) with no change in the bond length for most of the III-V semiconductor surfaces. Analysis of the relaxed configurations of the AlN surfaces in terms of atomic geometry, density of states, and charge density plots shows a presence of partial double-bond character in the surface Al-N bond. A similarity of these results with an earlier study on GaN nonpolar surfaces [J. E. Jaffe, R. Pandey, and P. Zapol, Phys. Rev. B 53, R4209 (1996)] led us to suggest the contraction-relaxation model where the relaxation proceeds via strengthening of the surface bond. The primary driving force of such a type of relaxation appears to be the ability of nitrogen to form a double bond that facilitates redistribution of the charge density associated with anion dangling bond to the surface bond.

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.

In this article, the results of a recently implemented DFT a posteriori and Kohn-Sham (KS) linear combination of atomic orbital computational scheme for solids are presented. The equilibrium lattice parameters, bulk moduli, and lattice energies are calculated for eight crystallized systems. Local density approximation (LDA) and generalized gradient approximation (GCA) functionals and potentials are used. The maps of the Hartree-Fock (HF) and Ks electronic densities and band structures are depicted. The KS results confirm the trend of the a posteriori scheme. Very good agreement between calculated and experimental lattice energies has been found for GGA potentials. © 1995 John Wiley & Sons, Inc.

Electron Localization Function (ELF) and Maximum Probability Domain (MPD) analyses have been applied to model metal–porphyrins and show compatible and complementary results. ELF basins are quite different from MPDs, but are a necessary starting point for optimizing them. The analyses of the bond between the metal and porphyrin do not show significant differences between non-transition and transition metals. In all the cases considered, we find signatures characteristic of essentially ionic bonds.

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.