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Ab initio conformational study of caffeic acid

VanBesien, E.; Marques, M. P. M.
Fonte: Universidade de Coimbra Publicador: Universidade de Coimbra
Tipo: Artigo de Revista Científica Formato: aplication/PDF
ENG
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86.27%
A complete conformational analysis of caffeic acid, a phenolic derivative with well known antioxidant properties, was carried out by ab initio calculations, at the density funtional theory (DFT) level. Fourteen different conformers were obtained, the most stable ones being planar, as the conformational preferences of this molecule were found to be mainly determined by the stabilising effect of [pi]-electron delocalisation. Harmonic vibrational frequencies, as well as potential energy profiles for rotation around several bonds within the molecule, were also calculated.; http://www.sciencedirect.com/science/article/B6TGT-485XKBT-1/1/4810698c8fb05c4b7002fb1111592b3c

Ab initio calculations on some transition metal heptoxides by using effective core potentials

Amado, Ana M.; Ribeiro-Claro, Paulo J. A.
Fonte: Universidade de Coimbra Publicador: Universidade de Coimbra
Tipo: Artigo de Revista Científica Formato: aplication/PDF
ENG
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86.32%
The ab initio molecular structures and vibrational frequencies for several transition metal heptoxides X2O7n- (n=0, 2, 4) were calculated using effective core potentials at the HF and DFT (B3LYP) levels. The relative merits of different valence basis set arrangements were tested by comparison with experimental results available, in particular with gas-phase Re2O7 molecular structure and vibrational frequencies. The calculations were then extended to other heptoxides of the VB, VIB and VIIB transition metal groups. The results indicate that a staggered geometry (either D3d or C2) is the energy minimum for most of the heptoxides studied. The only exceptions are Mn2O7, which clearly prefers an eclipsed C2v(syn) configuration, and Tc2O7, for which C2 and C2v(syn) geometries have nearly the same energy.; http://www.sciencedirect.com/science/article/B6TGT-3X6B8H7-P/1/83f93f7b8e1c991d62c7833f9d9ae1ac

Edge effects in bilayer graphene nanoribbons: Ab initio total-energy density functional theory calculations

Lima, Matheus Paes; Fazzio, Adalberto; Silva, Antonio Jose Roque da
Fonte: AMER PHYSICAL SOC Publicador: AMER PHYSICAL SOC
Tipo: Artigo de Revista Científica
ENG
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76.32%
We show that the ground state of zigzag bilayer graphene nanoribbons is nonmagnetic. It also possesses a finite gap, which has a nonmonotonic dependence with the width as a consequence of the competition between bulk and strongly attractive edge interactions. All results were obtained using ab initio total-energy density functional theory calculations with the inclusion of parametrized van der Waals interactions.; FAPESP; CNPq

Barrier-free substitutional doping of graphene sheets with boron atoms: Ab initio calculations

Pontes, Renato Borges; Fazzio, Adalberto; Dalpian, Gustavo Martini
Fonte: AMER PHYSICAL SOC Publicador: AMER PHYSICAL SOC
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
86.27%
Using ab initio methods, we propose a simple and effective way to substitutionally dope graphene sheets with boron. The method consists of selectively exposing each side of the graphene sheet to different elements. We first expose one side of the membrane to boron while the other side is exposed to nitrogen. Proceeding this way, the B atoms will be spontaneously incorporated into the graphene membrane without any activation barrier. In a second step, the system should be exposed to a H-rich environment, which will remove the CN radical from the layer and form HCN, leading to a perfect substitutional doping.; UFABC; FAPESP; CNPq

Full-dimensional analytical ab initio potential energy surface of the ground state of HOI

OLIVEIRA-FILHO, Antonio G. S. de; AOTO, Yuri A.; ORNELLAS, Fernando R.
Fonte: AMER INST PHYSICS Publicador: AMER INST PHYSICS
Tipo: Artigo de Revista Científica
ENG
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86.27%
Extensive ab initio calculations using a complete active space second-order perturbation theory wavefunction, including scalar and spin-orbit relativistic effects with a quadruple-zeta quality basis set were used to construct an analytical potential energy surface (PES) of the ground state of the [H, O, I] system. A total of 5344 points were fit to a three-dimensional function of the internuclear distances, with a global root-mean-square error of 1.26 kcal mol(-1). The resulting PES describes accurately the main features of this system: the HOI and HIO isomers, the transition state between them, and all dissociation asymptotes. After a small adjustment, using a scaling factor on the internal coordinates of HOI, the frequencies calculated in this work agree with the experimental data available within 10 cm(-1). (C) 2011 American Institute of Physics. [doi: 10.1063/1.3615545]; Fundacao de Amparo a Pesquisa do Estado de Sao Paulo (FAPESP); CNPq

The T-2 phase in the Nb-Si-B system studied by ab initio calculations and synchrotron X-ray diffraction

Joubert, J. -M.; Colinet, C.; Rodrigues, G.; Suzuki, P. A.; Nunes, C. A.; Coelho, G. C.; Tedenac, J. -C
Fonte: ACADEMIC PRESS INC ELSEVIER SCIENCE; SAN DIEGO Publicador: ACADEMIC PRESS INC ELSEVIER SCIENCE; SAN DIEGO
Tipo: Artigo de Revista Científica
ENG
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The solid solution based on Nb5Si3 (Cr5B3 structure type, D8(l), tl32, 14/mcm, No140, a=6.5767 angstrom, c=11.8967 angstrom) in the Nb-Si-B system was studied from the structural and thermodynamic point of view both experimentally and by ab initio calculations. Rietveld refinement of powder X-ray synchrotron data allowed to determine the boron to silicon substitution mechanism and the structural parameters. Ab initio calculations of different ordered compounds and selected disordered alloys allowed to obtain in addition to the enthalpy of formation of the solution, substitution mechanism and structural parameters which are in excellent agreement with the experimental data. The stability of the phase is discussed. (C) 2012 Elsevier Inc. All rights reserved.; Brazilian Synchrotron Light Laboratory (LNLS); Brazilian Synchrotron Light Laboratory (LNLS) [D12A-XRD1-1697]; FAPESP agency [1997/06348-4]; FAPESP Agency

Determinação da estrutura molecular do ciclooctano por métodos ab initio e difração de elétrons na fase gasosa

De Almeida,Wagner B.
Fonte: Sociedade Brasileira de Química Publicador: Sociedade Brasileira de Química
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/10/2000 PT
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The determination of the molecular structure of molecules is of fundamental importance in chemistry. X-rays and electron diffraction methods constitute in important tools for the elucidation of the molecular structure of systems in the solid state and gas phase, respectively. The use of quantum mechanical molecular orbital ab initio methods offer an alternative for conformational analysis studies. Comparison between theoretical results and those obtained experimentally in the gas phase can make a significant contribution for an unambiguous determination of the geometrical parameters. In this article the determination of the molecular structure of the cyclooctane molecule by electron diffraction in the gas phase and ab initio calculations will be addressed, providing an example of a comparative analysis of theoretical and experimental predictions.

Policabornitrile: a semiemperical ab initio and de density functional study of molecular stability

Del Nero, Jordan; Laks, Bernardo; Custodio, Rogerio
Fonte: Inmetro - Instituto Nacional de Metrologia, Qualidade e Tecnologia Publicador: Inmetro - Instituto Nacional de Metrologia, Qualidade e Tecnologia
Tipo: Artigo de Revista Científica
ENG
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76.42%
2 p. : il., tab; The theoretical literature data with respectt o the electronic properties of this compound is quite scarce and makes use of the planar alltransStructure as the most stable for the calculations In this work semiempirical (AM1 and PM3), ab initio (at the Hartree-Fock level)and density functional theory (using the correlation functional of Vosko, Wilk and Nussair) were used to anaIyse the conformationalstability of the all-trans and all-cis dimers, t rimers and tetramers of polycarbonitrile.The semiempirical and ab initio calculations at theHartree-Fock level showed in general that the all-trans structure with respect to other conformers is the most unstable s tructure. TheInclusion of electronic correlation energy through t he MP2 calculations or the VWN functional method suggest that the trans structure isthe most stable. The relative energies calculated at the correlated level presented diferences around 2 kcal/mol among the differentconformers . While the all-cis compounds presented a planar structure for any of the three methods the all-trans polymer showed a strongdeviation of planarity with a set of local minima in its energy surface.These results suggestth that further calculations on the electronic properties of this polymer can be significantly different of those actually available in the literature.

Sistemas Nanoestruturados: Heteroestruturas Quasi-Periódicas de Nitretos e Cálculos Ab initio em Polimorfos CaCO3; Sistemas Nanoestruturados: Heteroestruturas Quasi-Periódicas de Nitretos e Cálculos Ab initio em Polimorfos CaCO3

Medeiros, Subênia Karine de
Fonte: Universidade Federal do Rio Grande do Norte; BR; UFRN; Programa de Pós-Graduação em Física; Física da Matéria Condensada; Astrofísica e Cosmologia; Física da Ionosfera Publicador: Universidade Federal do Rio Grande do Norte; BR; UFRN; Programa de Pós-Graduação em Física; Física da Matéria Condensada; Astrofísica e Cosmologia; Física da Ionosfera
Tipo: Tese de Doutorado Formato: application/pdf
POR
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76.46%
The physical properties and the excitations spectrum in oxides and semiconductors materials are presented in this work, whose the first part presents a study on the confinement of optical phonons in artificial systems based on III-V nitrides, grown in periodic and quasiperiodic forms. The second part of this work describes the Ab initio calculations which were carried out to obtain the optoeletronic properties of Calcium Oxide (CaO) and Calcium Carbonate (CaCO3) crystals. For periodic and quasi-periodic superlattices, we present some dynamical properties related to confined optical phonons (bulk and surface), obtained through simple theories, such as the dielectric continuous model, and using techniques such as the transfer-matrix method. The localization character of confined optical phonon modes, the magnitude of the bands in the spectrum and the power laws of these structures are presented as functions of the generation number of sequence. The ab initio calculations have been carried out using the CASTEP software (Cambridge Total Sequential Energy Package), and they were based on ultrasoft-like pseudopotentials and Density Functional Theory (DFT). Two di®erent geometry optimizations have been e®ectuated for CaO crystals and CaCO3 polymorphs...

Ab initio potential-energy surface for the reaction Ca+HCl --> CaCl+H

Verbockhaven, Gilles; Sanz, Cristina; Groenenboom, Gerrit C.; Roncero, Octavio; Avoird, Ad van der
Fonte: American Institute of Physics Publicador: American Institute of Physics
Tipo: Artículo Formato: 209679 bytes; application/pdf
ENG
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76.42%
12 pages, 9 figures, 7 tables.-- PACS nrs.: 31.15.Ar; 82.20.Kh; 33.15.Bh; 34.20.Gj; 31.15.Dv; 33.20.Ea; 33.20.Vq.; The potential-energy surface of the ground electronic state of CaHCl has been obtained from 6400 ab initio points calculated at the multireference configuration-interaction level and represented by a global analytical fit. The Ca+HCl --> CaCl+H reaction is endothermic by 5100 cm–1 with a barrier of 4470 cm–1 at bent geometry, taking the zero energy in the Ca+HCl asymptote. On both sides of this barrier are potential wells at linear geometries, a shallow one due to van der Waals interactions in the entrance channel, and a deep one attributed to the H(–)Ca(++)Cl(–) ionic configuration. The accuracy of the van der Waals well depth, ≈ 200 cm–1, was checked by means of additional calculations at the coupled-cluster singles and doubles with perturbative triples level and it was concluded that previous empirical estimates are unrealistic. Also, the electric dipole function was calculated, analytically fitted in the regions of the two wells, and used to analyze the charge shifts along the reaction path. In the insertion well, 16 800 cm–1 deep, the electric dipole function confirmed the ionic structure of the HCaCl complex and served to estimate effective atomic charges. Finally...

Ab initio vibrational predissociation dynamics of He-I2(B) complex

Valdés, Álvaro; Prosmiti, Rita; Villarreal, Pablo; Delgado Barrio, Gerardo; Lemoine, Didier; Lepetit, Bruno
Fonte: American Institute of Physics Publicador: American Institute of Physics
Tipo: Artículo Formato: 517578 bytes; application/pdf
ENG
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76.47%
7 pages, 5 figures, 5 tables.-- PACS nrs.: 31.15.Ar; 31.15.Dv; 33.80.Gj; 33.20.Tp; 33.70.Ca.; Three-dimensional quantum mechanical calculations on the vibrational predissociation dynamics of HeI2 B state complex are performed using a potential energy surface accurately fitted to unrestricted open-shell coupled cluster ab initio data, further enabling extrapolation for large I2 bond lengths. A Lanczos iterative method with an optimized complex absorbing potential is used to determine energies and lifetimes of the vibrationally predissociating He,I2(B,v') complex for v' ≤ 26 of I2 vibrational excitations. The calculated predissociating state energies agree with recent experimental results within 0.5 cm−1. This excellent agreement is remarkable since no adjustment was made with respect to the experiments. The present ab initio approach, however, shows its limitations in the fact that the computed lifetimes that are highly sensitive to subtle details of the potential energy surfaces such as anisotropy, are a factor of 1.5 larger than the available experimental data.; The Centro de Cálculo (IMAFF), CTI (CSIC), CESGA, and MareNostrum (BSC) are acknowledged for allocation of computer time. One of the authors (A.V.) would like to thank CICYT...

Ab initio potential energy surface and spectrum of the B(3Π) state of the HeI2 complex

Valdés, Álvaro; Prosmiti, Rita; Villarreal, Pablo; Delgado Barrio, Gerardo; Werner, Hans-Joachim
Fonte: American Institute of Physics Publicador: American Institute of Physics
Tipo: Artículo Formato: 514216 bytes; application/pdf
ENG
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76.44%
9 pages, 6 figures, 4 tables.-- PACS nrs.: 31.15.Ar; 31.50.-x; 33.50.Dq; 33.15.Bh; 31.15.Dv; 33.70.Jg.; The three-dimensional interaction potential for I2(B 3Π0(u)+)+He is computed using accurate ab initio methods and a large basis set. Scalar relativistic effects are accounted for by large-core relativistic pseudopotentials for the iodine atoms. Using multireference configuration interaction calculations with subsequent treatment of spin-orbit coupling, it is shown for linear and perpendicular structures of the complex that the interaction potential for I2(B 3Π0(u)+)+He is very well approximated by the average of the 3A' and 3A'' interaction potentials obtained without spin-orbit coupling. The three-dimensional 3A' and 3A'' interaction potentials are computed at the unrestricted open-shell coupled-cluster level of theory using large basis sets. Bound state calculations based on the averaged surface are carried out and binding energies, vibrationally averaged structures, and frequencies are determined. These results are found to be in excellent accord with recent experimental measurements from laser-induced fluorescence and action spectra of HeI2. Furthermore, in combination with a recent X-state potential, the spectral blueshift is obtained and compared with available experimental values.; This work has been supported by DGICYT...

Cluster-surface and cluster-cluster interactions: ab initio calculations and modelling of Van der Waals forces

Botti, Silvana; Castro, Alberto; Andrade, Xavier; Rubio Secades, Ángel; Marques, Miguel A. L.
Fonte: American Physical Society Publicador: American Physical Society
Tipo: Artículo Formato: 216577 bytes; application/pdf
ENG
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96.32%
10 pages, 7 figures.-- PACS nrs.: 34.35.+a, 34.20.Gj, 31.15.ee; We present fully ab initio calculations of van der Waals coefficients for two different situations: (i) the interaction between hydrogenated silicon clusters and (ii) the interactions between these nanostructures and a nonmetallic surface (a silicon or a silicon carbide surface). The methods used are very efficient and allow the calculation of systems containing hundreds of atoms. The results obtained are further analyzed and understood with the help of simple models. These models can be of interest for molecular-dynamics simulations of silicon nanostructures on surfaces, where they can give a very fast yet sufficiently accurate determination of the van der Waals interaction at large separations.; The authors were partially supported by the EC Network of Excellence NANOQUANTA (Grant No. NMP4-CT-2004–500198). S. Botti acknowledges financial support from French ANR (Grant No. JC0546741). A. Castro acknowledges financial support from the Deutsche Forschungsgemeinschaft within Grant No. SFB 658. X. Andrade acknowledges partial support from Marie Curie Action through the EU Programme (Grant No. HPMT-CT-2001–00368). M.A.L. Marques acknowledges partial support by the Portuguese FCT through Project No. PTDC/FIS/73578/2006. X. Andrade and A. Rubio acknowledge financial support from the Spanish Ministry of Education (Grant No. FIS2007–65702-C02–01) and Grupos Consolidados UPV/EHU of the Basque Country Government (2007).; Peer reviewed

Ordered arrays of quantum wires through hole patterning : ab initio and empirical electronic structure calculations

Rurali, Riccardo; Suñé, Jordi,
Fonte: Universidade Autônoma de Barcelona Publicador: Universidade Autônoma de Barcelona
Tipo: Artigo de Revista Científica Formato: application/pdf
Publicado em //2007 ENG
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We propose an approach to the fabrication of one-dimensional nanostructures, based on the design of a pattern of channels onto a semiconductor surface. The feasibility of this approach is demonstrated by means of ab initio and empirical electronic structure calculations. When the channel diameter is sufficiently larger than the interstitial space, the resulting pillars constitute an ordered array of electronically independent, though mechanically connected nanowires. In the opposite regime a tunable metamaterial results. The proposed method provides a path to the realization of uniform quantum wires—both in size and doping characteristics—while easing electrical contacting.

Are pi-ligand exchange reactions of thiirenium and thiiranium ions feasible? An ab initio investigation

Solling, Theis; Wild, Stanley (Bruce); Radom, Leo
Fonte: Wiley-VCH Verlag GMBH Publicador: Wiley-VCH Verlag GMBH
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
86.27%
Ab initio calculations at the G2 level have been employed to investigate the hitherto unobserved π-ligand exchange reactions of thiirenium and thiiranium ions. Two pathways for exchange and one pathway for insertion are presented. The lower energy exchan

Substituent Effects in Xanthate-Mediated Polymerization of Vinyl Acetate: Ab Initio Evidence for an Alternative Fragmentation Pathway

Coote, Michelle; Radom, Leo
Fonte: American Chemical Society Publicador: American Chemical Society
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
86.32%
High-level ab initio calculations have been performed for the addition and fragmentation steps in reversible addition-fragmentation chain transfer (RAFT) polymerization of vinyl acetate. The RAFT agents considered were a series of xanthates of the form S=C(O-Z′)S-R, where Z′ = methyl, ethyl, iso-propyl, and tert-butyl and R = CH2OCOCH3 and CH3. The results indicate that increasing substitution within the Z′ group stabilizes the RAFT adduct radical, thereby reducing the rate of fragmentation of the S-R bond. For the model vinyl acetate system, there is an additional substantial reduction in rate for the bulkier iso-propyl and tert-butyl substituents (compared with methyl and ethyl) associated with a sterically induced conformational change in the transition structures. However, the calculated S-R fragmentation rate for Z′ = tert-butyl is still not low enough to explain the experimentally observed rate retardation in this system. Instead, the rate retardation appears to be the result of the preferred fragmentation of the O-C bond in the tert-butoxy group of the RAFT adduct. This fragmentation pathway is not normally favored in RAFT polymerization. However, in this particular system, the combination of the vinyl acetate radical being a poor homolytic leaving group...

Ab Initio Study of the Addition-Fragmentation Equilibrium in RAFT Polymerization: When is Polymerization Retarded?

Coote, Michelle
Fonte: American Chemical Society Publicador: American Chemical Society
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
86.27%
High level ab initio calculations of the addition-fragmentation equilibrium constants at 333 K were performed for model RAFT reactions: R· + S=C(Z)SCH3 → R-SC·(Z)SCH3, for all combinations of Z = CH3 phenyl, or benzyl with R = CH3, benzyl, CH2COOCH3,

The kinetics of addition and fragmentation in reversible addition fragmentation chain transfer polymerization: an ab initio study

Coote, Michelle
Fonte: American Chemical Society Publicador: American Chemical Society
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
86.27%
High-level ab initio calculations of the forward and reverse rate coefficients have been performed for a series of prototypical reversible addition fragmentation chain transfer (RAFT) reactions: R· + S=C(Z)SCH3 → R-SC·(Z)SCH3, for R = CH 3, with Z = C

Ab initio calculation of structural and electronic properties of alloys

López-Apreza,E.; Arriaga,J.; Olguín,D.
Fonte: Sociedad Mexicana de Física Publicador: Sociedad Mexicana de Física
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/06/2010 EN
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76.37%
Using the density functional theory (DFT) with the generalized gradient approximation (GGA), we calculated the structural and electronic properties of wurtzite AlN, GaN, InN, and their related alloys, . We have performed accurate ab initio total energy calculations using the full-potential linearized augmented plane wave (FP-LAPW) method to investigate their structural and electronic properties. We found that in both alloys the fundamental parameters do not follow Vergard's law. The lattice parameters, a, c, and u, for the alloy are found to exhibit positive bowing parameters, while for there is a negative bowing for the a and c parameters and a positive bowing for the internal parameter, u. We calculated as well the nearest neighbor and next nearest neighbor distances, as a function of the concentration, and we obtained a good agreement with experimental results. Furthermore, we found that for both alloys, the band gap does not follows the Vegard law. As a by-product of our electronic band structure calculations, the effective masses of the binary compounds and their alloys were calculated. All the calculated properties show good agreement with most of the previously reported results. Finally...

Magnetic properties of Fe nanoclusters: ab initio calculations of FeN N = 9,15, 27, 51, and 59

López Laurrabaquio,G.; Pérez Alvarez,M.; Montejano-Carrizales,J.M.; Aguilera-Granja,F.; Morán-López,J.L.
Fonte: Sociedad Mexicana de Física Publicador: Sociedad Mexicana de Física
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/08/2006 EN
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The magnetic properties of iron clusters (FeN, N = 9, 15, 27, 51, and 59 atoms) at T = 0 K with bcc-like structure and bulk parameters are studied using ab initio methods. In these studies we consider the spin-orbit coupling and applied external magnetic fields. The basis set includes wave functions of the s, p, d, and f valence electrons. An analysis of the spin and orbital magnetic moments for every shell of the different cluster sizes is performed. The results obtained in the present work agree with other results reported in the literature and call for additional experiments.