bluuebunny/crossref_metadata_2025 huggingface dataset:
Nonlocal inflationThis paper was prsented at the Theory CANADA 4 conference, held at the Centre de Recherches Mathématiques at the Université de Montréal, Québec, Canada on 4–7 June 2008.
Authors: Barnaby Neil
Published: July 2009

We consider the possibility of realizing inflation in nonlocal field theories containing infinitely many derivatives. Such constructions arise naturally in string field theory and also in a number of toy models, such as the p-adic string. After reviewing the complications (ghosts and instabilities) that arise when working with high-derivative theories, we discuss the the initial value problem and perturbative stability of theories with infinitely many derivatives. Next, we examine the inflationary dynamics and phenomenology of such theories. Nonlocal inflation can proceed even when the potential is naively too steep and generically predicts large non-Gaussianity in the cosmic microwave background.
Quasi-particles in conformal field theoryThis paper was presented at the Theory CANADA 4 conference, held at the Centre de Recherches Mathématiques at the Université de Montréal, Québec, Canada on 4–7 June 2008.
Authors: Mathieu P.
Published: July 2009

After recalling some basing features of conformal field theory, we present an elementary introduction to the description of the states in the irreducible modules of the minimal models. The characters, which encode the state content of each module, are easily constructed from the representation theory of the Virasoro algebra and they take the form of infinite alternating sums. On the other hand, the relation between the minimal models and particular statistical models has led to the discovery of alternative expressions for the characters, which are positive definite. These entail a (yet to be fully worked out) quasi-particle description of the space of states, via a filling process with restrictions akin to the exclusion principle. The simplest class of positive characters, pertaining to the ℳ(2,p) models, is presented, and its underlying combinatorial aspects are spelled out. A natural conformal field theoretical interpretation of its quasi-particles is presented.
D-term chaotic inflation in supergravity and leptogenesisThis paper was prsented at the Theory CANADA 4 conference, held at the Centre de Recherches Mathématiques at the Université de Montréal, Québec, Canada on 4–7 June 2008.
Authors: Yamaguchi Masahide , Kadota Kenji , Kawano Teruhiko
Published: July 2009

We review a model of D-term dominated chaotic inflation in supergravity. The F-flat direction present in this model is lifted by the dominant D-term, which leads to chaotic inflation and subsequent reheating. The leptogenesis scenario via the inflaton decay in this D-term chaotic inflation scenario is also discussed.
Mott scattering in strong laser fields revisited
Authors: Manaut B. , Attaourti Y. , Taj S. , Elhandi S.
Published: July 2009

In this work, we review and correct the first Born differential cross section for the process of Mott scattering of a Dirac–Volkov electron, namely, expression (26) derived by Szymanowski et al. (Phys. Rev. A, 56, 3846 (1997)). In particular, we disagree with the expression of (dσ/dΩ) that they obtained and we give the exact coefficients multiplying the various Bessel functions appearing in the scattering differential cross section. Comparison of our numerical calculations with those of Szymanowski et al. shows qualitative and quantitative differences when the incoming total electron energy and the electric-field strength are increased particularly in the direction of the laser propagation. Such corrections are very important since the relativistic electronic dressing of any Dirac–Volkov charged particle gives rise to these coefficients that multiply the various Bessel functions, and the relativistic study of other processes (such as excitation, ionization, etc….) depends strongly upon the correctness and reliability of the calculations for this process of Mott scattering in the presence of a laser field. Our work has been accepted, Attaourti and Manaut (Phys. Rev. A, 68, 067401 (2003)) but only as a comment. In this paper, we give full details of the calculations as well as a clear explanation of the large discrepancies that their results could cause when working in the ultra relativistic regime and using a very strong laser field corresponding to an electric field strength ε = 5.89 au.
Time-dependence and holography in the c=1 matrix model This paper was presented at the Theory CANADA 4 conference, held at Centre de recherches mathématiques, Montréal, Québec, Canada on 4–7 June 2008.
Authors: Karczmarek Joanna L.
Published: July 2009

Ideas related to the study of time-dependence in two dimensional Liouville string theory using the c=1 matrix model are reviewed. Following an introduction to Liouville string theory, the matrix model and the relationship between the two, an example of an exact quantum mechanical time-dependent solution is given. There is a brief discussion of the holographic issues complicating the construction of the exact spacetime interpretation of such solutions. An attempt is made to include sufficient background material to make the presentation self-contained and accessible to a non-expert.
Pancharatnam phase for the generalized Jaynes–Cummings model with a nonlinear Kerr cavity
Authors: Aouachria M. , Chetouani L.
Published: July 2009

The formalism of path integral is used to treat the influence of Stark shift on the atomic population inversion (API) and Pancharatnam phase in the Jaynes–Cummings model with a nonlinear Kerr cavity. The propagators are given explicitly as perturbation series. In the case of a quantized wave interacting with a two-level system, these are summed up exactly, and the corresponding Pancharatnam phase as well as atomic population inversion, and energy spectrum with corresponding wave functions are deduced.
Perils and promises of heterotic standard modelsThis paper was presented at the Theory CANADA 4 conference, held at Centre de recherches mathématiques, Montréal, Québec, Canada on 4–7 June 2008.
Authors: Bouchard Vincent
Published: July 2009

We present an overview of recent progress in obtaining phenomenologically viable four-dimensional low-energy limits of the E8 × E8 heterotic string compactified on smooth non-simply connected Calabi–Yau threefolds. We briefly review the main steps of the construction and present two particular geometric models that lead to interesting four-dimensional phenomenology.
Gravitationally coupled magnetic monopole and conformal symmetry breakingThis paper was presented at the Theory CANADA 4 conference, held at Centre de recherches mathématiques, Montréal, Québec, Canada on 4–7 June 2008.
Authors: Edery Ariel , Fabbri Luca , Paranjape M. B.
Published: July 2009

We consider a Georgi–Glashow model conformally coupled to gravity. The conformally invariant action includes a triplet of scalar fields and SO(3) non-Abelian gauge fields. However, the usual mass term μ2ϕ2 is forbidden by the symmetry, and this role is now played by the conformal coupling of the Ricci scalar to the scalar fields. Spontaneous symmetry breaking occurs via gravitation. The spherically symmetric solutions correspond to localized solitons (magnetic monopoles) in asymptotically anti-de Sitter (AdS) spacetime and the metric outside the core of the monopole is found to be Schwarzschild–AdS. Though conformal symmetry excludes the Einstein–Hilbert term in the original action, it emerges in the effective action after spontaneous symmetry breaking and dominates the low-energy–long-distance regime outside the core of the monopole.
Two-temperature theory in generalized magneto-thermo-viscoelasticity
Authors: Ezzat Magdy A. , El Bary Alaa Abd , El Karamany Ahmed S.
Published: July 2009

A one-dimensional model of the two-temperature generalized magneto-viscoelasticity with two relaxation times in a perfect conducting medium is established. The state space approach is adopted for the solution of one-dimensional problems for any set of boundary conditions. The resulting formulation together with the Laplace transform techniques are applied to a specific problem of a half-space subjected to thermal shock and traction-free surface. The inversion of the Laplace transforms is carried out using a numerical approach. Numerical results are given and illustrated graphically for the problem. Some comparisons have been shown in figures to estimate the effects of the temperature discrepancy and the applied magnetic field.
Pionization at AGS and SPS energies ― degree of coherence and its target excitation dependence
Authors: Ghosh Dipak , Deb Argha , Dutta Srimonti
Published: July 2009

The target excitation dependence of the higher order cumulant correlation of pions is studied in the present work in the framework of void probability scaling. The data set of pions produced for 16O–AgBr interactions at 2.1 AGeV and 32S–AgBr interactions at 200 AGeV was divided into three sets depending upon the number of grey tracks ng. The different sets of ng correspond to different degrees of target excitation. Analysis was carried out for each set in different pseudorapidity intervals. For the lower projectile energy the void probability scaling is observed only for low target excitation for all pseudorapidity intervals. For the comparatively higher projectile energy, scaling is observed for all pseudorapidity intervals and all target excitation. In other words, for higher projectile energy, scaling is almost independent of the target excitation. The results are interpreted from the point of view of the two source model of particle production.
Periodic giant persistent current in sharp pulses on a ring
Authors: He Y. Z. , Bao C. G.
Published: July 2009

We show here a mesoscopic device based on a narrow ring containing an electron. In the device, an amount of energy is stored in advance. Similar to a pendulum, an exact periodic motion of the electron is thereby initiated. The motion appears as a series of sharp pulses and is in nature different from the well-known Aharonov–Bohm (A–B) oscillation. In particular, the current pulses can be tuned to be very strong (say, more than two orders of magnitude stronger than the usual A–B current). Related theory and numerical results are presented.
Crystal structure of semiorganic antimony thiourea bromide monohydrate crystals: a redetermination
Authors: Upadhya Mahesha K. , Udayashankar N. K.
Published: July 2009

The growth of antimony thiourea bromide monohydrate (ATBM) single crystals by evaporation technique at room temperature is reported here. Antimony thiourea bromide monohydrate crystallizes in the orthorhombic space group Cmca with unit cell parameters a = 12.320(7) Å, b = 11.668(7) Å, c = 18.615(11), and z = 8. The structure has been solved by direct methods and refined to a final R value of 0.0239.
Analysis of wave motion in transversely isotropic elastic material with voids under a inviscid liquid layer
Authors: Kumar Rajneesh , Kumar Rajeev
Published: July 2009

The present investigation is to study the surface wave propagation in a semi-infinite transversely isotropic elastic material with voids under a homogeneous inviscid liquid layer. The frequency equation is derive after developing the mathematical model. The dispersion curves giving the phase velocity and attenuation coefficients versus wave numbers are plotted graphically to depict the effects of voids for (i) a transversely isotropic elastic half-space with voids under a homogeneous inviscid liquid layer and (ii) a transversely isotropic elastic half-space with voids. The particle path is also obtained for Rayleigh wave propagation in a transversely isotropic elastic half-space with voids, i.e., case (ii). The amplitudes of the displacements, the volume fraction field, and the normal stresses in both the media are obtained and are shown graphically for a particular model to depict the voids and anisotropy effects. Some special cases are also deduced from the present investigation.
Parisi–Wu stochastic quantization for relativistic particles in a weak gravitational plane wave
Authors: Haouat S. , Chine N. , Chetouani L.
Published: July 2009

The problem of relativistic particles moving in the background of a weak gravitational plane wave is solved by the use of Parisi–Wu stochastic quantization method. After having solved the corresponding Langevin equations, we have calculated exactly the correlation product of two fields known as a Green's function at the thermal-equilibrium limit for the fictitious time. By addition of an infinitesimal imaginary part of the mass, the existence of the limit of the correlation function at the equilibrium is assured. Analytical and exact expressions of the wave functions are obtained both for Klein–Gordon (KG) and Dirac particles.
Exact solutions to a massive charged scalar field equation in the magnetically charged stringy black-hole geometry and Hawking radiation
Authors: Sakalli I. , Al-Badawi A.
Published: July 2009

Exact solutions of a massive complex scalar field equation in the geometry of a Garfinkle–Horowitz–Strominger (stringy) black hole with magnetic charge is explored. The separated radial and angular parts of the wave equation are solved exactly in the nonextreme case. The angular part is shown to be an ordinary spin-weighted spheroidal harmonics with a spin-weight depending on the magnetic charge. The radial part is achieved to reduce a confluent Heun equation with a multiplier. Finally, based on the solutions, it is shown that the Hawking temperature of the magnetically charged stringy black hole has the same value as that of the Schwarzschild black hole.
Analysis of the complex effective permittivity of a heterogeneous sample by the finite-difference time-domain method
Authors: Martín Juan Carlos
Published: July 2009

Numerical results of the complex effective permittivity and DC conductivity of heterogeneous samples with random constituents’ distribution are analyzed to search for a suitable mixture law. The calculation method consists of the simulation of a time-domain reflectometry experiment by means of the finite-difference time-domain (FDTD) algorithm. The permittivity and conductivity of several series of samples with their constituents distributed at random are calculated. Numerical results, as a function of the volume fractions of the mixture’s constituents, are compared with analytical mixture laws. For lossless binary media, it is found that both the Bruggeman law and a weighted geometric mean provide a good agreement. The last one can be generalized to composites with more than two constituents, with good results. On the other hand, for lossy binary mixtures it is shown that the weighted geometric mean of the constituents’ complex permittivities matches the numerical results obtained for the effective complex permittivity of the mixture.
Raman investigation of hydrogen-implanted and DC hydrogen-plasma-treated Cz Si wafers
Authors: Saad A. M.
Published: July 2009

The general goal of this work is to demonstrate that the buried defect layer created by hydrogen implantation can serve as a gettering region for hydrogen introduced from a DC plasma and to study the efficiency of this gettering affected by the implantation regimes. Standard n-type 4.5 Ω⋅cm Cz Si wafers were implanted by hydrogen ions with an energy of 100 keV and different doses of 1 × 1014, 1 × 1015, or 5 × 1015 atoms/cm2 at temperatures of 150, 300, 400, or 500 °C. After implantation, hydrogen was introduced to the wafers from the DC plasma at 150 °C. For a comparative estimation of the hydrogen concentration in the wafers implanted in different regimes Raman spectroscopy was used. The peaks of the Raman spectra associated with molecular hydrogen (H2), vacancy-hydrogen (V–H), and silicon–hydrogen (Si–H) complexes were studied depending on the implantation conditions. It is demonstrated that peaks of Raman spectra depend significantly on the dose and temperature of implantation. This means that the concentration of hydrogen in the wafers could be determined from the concentration and type of defects formed by hydrogen implantation. Maximum peaks associated with H2, Si–H, and V–H complexes were observed for the samples implanted at a temperature of 500 °C.
Optical properties of nematic liquid crystal (C21H27NO2S) under AC/DC electric fields
Authors: Emek Mehriban , Besli Nurettin , Yildirim Ahmet , Yilmaz Suleyman
Published: July 2009

In this study, the effects of the phase transition on the optical transmittance of the nematic liquid crystal C21H27NO2S, 4′-isothiocyanatophenyl-4-pentylbicyclo[2,2,2]octane-1-carboxylate are investigated in terms of temperature variation and rotational angle of the polarizer through electro-optical methods under AC / DC electric fields. It is observed that the domain structure of the material is affected considerably by the applied electric field as the temperature changes. Under applied electric fields, the crystal-nematic (CN) phase-transition point changes and the behaviour of the liquid crystal in the phase-transition region shows some differences. The intensity of the light passing through the system under a DC electric field increases as the electric field rises. Nevertheless, the intensity of the transmitted light under an AC electric field increases at the beginning and then decreases as the electric field rises to a temperature of more than 355 K. These results can be explained through the formation of a domain structure during the phase-transition process and the light scattering caused by these structures.
Fresnel’s formulae and the Minkowski momentum
Authors: Hirose A. , Dick R.
Published: July 2009

It is shown that the momentum of an electromagnetic wave in a dielectric medium can be uniquely determined to be the Minkowski momentum by considering oblique incidence of electromagnetic wave on a flat dielectric boundary. The Minkowski momentum is consistent with the Fresnel’s formulae and satifies the energy and momentum conservation laws.
Influence of physical and geometrical parameters on three-dimensional load transfer mechanism at tunnel face
Authors: Gomes Ricardo A.M.P. , Celestino Tarcisio B.
Published: July 2009

Three-dimensional discretizations used in numerical analyses of tunnel construction normally include excavation step lengths much shorter than tunnel cross-section dimensions. Simulations have usually worked around this problem by using excavation steps that are much larger than the actual physical steps used in a real tunnel excavation. In contrast, the analyses performed in this study were based on finely discretized meshes capable of reproducing the excavation lengths actually used in tunnels, and the results obtained for internal forces are up to 100% greater than those found in other analyses available in the literature. Whereas most reports conclude that internal forces depend on support delay length alone, this study shows that geometric path dependency (reflected by excavation round length) is very strong, even considering linear elasticity. Moreover, many other solutions found in the literature have also neglected the importance of the relative stiffness between the ground mass and support structure, probably owing to the relatively coarse meshes used in these studies. The analyses presented here show that relative stiffness may account for internal force discrepancies in the order of 60%. A dimensionless expression that takes all these parameters into account is presented as a good approximation for the load transfer mechanism at the tunnel face.
Response of buried steel pipelines subjected to relative axial soil movement
Authors: Wijewickreme Dharma , Karimian Hamid , Honegger Douglas
Published: July 2009

The performance of buried steel pipelines subjected to relative soil movements in the axial direction was investigated using full-scale pullout testing in a soil chamber. Measured axial soil loads from pullout testing of pipes buried in loose dry sand were comparable to those predicted using guidelines commonly used in practice. The peak values of axial pullout resistance observed on pipes buried in dense dry sand were several-fold (in excess of 2 times) higher than the predictions from guidelines; the observed high axial pullout resistance is primarily due to a significant increase in normal soil stresses on the pipelines, resulting from constrained dilation of dense sand during interface shear deformations. This reasoning was confirmed by direct measurement of soil stresses on pipes during full-scale testing and numerical modeling. The research findings herein suggest that the use of the coefficient of lateral earth pressure at-rest (K0) to compute axial soil loads, employing equations recommended in common guidelines, should be undertaken with caution for pipes buried in soils that are likely to experience significant shear-induced dilation.
Fracture network analysis of the St-Eustache quarry, Quebec, Canada, for groundwater resources management
Authors: Lemieux Jean-Michel , Kirkwood Donna , Therrien René
Published: July 2009

A detailed structural survey has been conducted on a fractured sedimentary rock formation in the St-Eustache quarry, Quebec, Canada, to supplement a hydrogeological study. The two main types of discontinuities in the quarry are horizontal bedding planes and vertical joints. The fracture network is classified as a stratabound network that could be considered as an equivalent porous medium (EPM) for groundwater flow for a volume of rock of 25 to 100 m3. Using detailed statistical data of the fracture network, a geometric model is used to infer a range of hydraulic conductivity values for the low permeability fractures not directly measured with hydraulic tests and treated as EPM for their interpretation. This analysis shows that the vertical and horizontal fractures have about the same permeability in the rock mass, except for a few high hydraulic-conductivity bedding planes. Hydraulic conductivity of single fractures ranges between 1.7 × 10−3and 1.7 × 10−1m/s. The porosity of the bulk rock mass inferred from fracture spacing measurements was found to be between 0.03% and 0.3%. Because vertical boreholes provide limited information on vertical fractures, the interpretation of hydraulic tests at the site was greatly improved by the structural surveys.
Development and validation of a low-cost electrical resistivity tomographer for soil process monitoring
Authors: Damasceno Victor M. , Fratta Dante , Bosscher Peter J.
Published: July 2009

This paper presents the development and validation of a low-cost electrical resistivity tomography (ERT) system for the monitoring of process in soils. The ERT technique is capable of evaluating the distribution of electrical properties within a cross section of soil specimens by measuring the applied electrical current and monitoring the voltage distribution along boundary electrodes. This new ERT system consists of (i) a two-dimensional (2D) acrylic testing cell with strip copper electrodes, (ii) an acquisition system and control software, and (iii) a third-party reconstruction software. The tomographer is developed to collect low-noise data at moderate acquisition speeds. To validate the quality of the proposed ERT system, noise levels were evaluated with and without the testing cell and imaging tests were conducted to calibrate the system, evaluate the sensitivity of the instrument to locate a low-resistivity inclusion, and to monitor chemical diffusion in a saturated Ottawa sand specimen. Results show that the proposed low-noise, low-cost ERT acquisition system can be successfully used, with proper interpretation, to locate inclusions and obtain images of diffusion processes in soils. These validation tests show the applicability of the developed acquisition system in monitoring time-dependent processes that do not require real-time acquisition and reconstruction.
Combined tight-binding/DFT investigation of the electronic structure of triimine-platinum(II)/TCNQ extended stacks,
Authors: Rabaâ Hassan , Cundari Thomas R. , Omary Mohammad A.
Published: July 2009

. The [Pt(tbtrpy)(X)][TCNQ] (X = OH or SH) complexes form sandwich stacks with nitrile acceptors leading to extended-chain supramolecular assemblies, tbtrpy = 4,4′,4″-tert-Bu3-2,2′:6′,2″-terpyridine. Calculations with the extended Hückel tight-binding (EHTB) method are performed upon crystalline {[Pt (tbtrpy)(X)][TCNQ]}∞ species to analyze their electronic structure and consequent properties, TCNQ = 7,7,8,8-tetracyanoquinodimethane. The donor/acceptor extended chains in the solid state are predicted to exhibit metallic behavior with a large contribution from π and π* bands of TCNQ to the valence and conduction bands, respectively. Moreover, the valence band moves upward (i.e., to a less negative energy) for X = SH as compared to X = OH. Density functional theory (DFT) calculations suggest that this is due to large thiolate character in the HOMO of the square-planar donor complex, which also supports the experimental assignment of the electronic absorption bands and redox potentials. Calculations of infrared (νCN bands of TCNQ) and structural (CC bond lengths within TCNQ) data explain the metallic behavior of the stacks in terms of charge delocalization, leading to fractionally-charged species of the form [Pt(tbtrpy)X](1+δ)+[TCNQ](1+δ)- with δ > 0 and a greater δ value for X = SH vs OH.
Structure and stability of aquotetrafluorouranyl(VI) in the solid state — Density functional study of [UO2F4(H2O)][NMe4]2·2H2O,
Authors: Bühl Michael
Published: July 2009

Periodic density functional computations have been performed for solid [UO2F4(H2O)][NMe4]2·2H2O at the BLYP level. A model with disordered fluoro and aquo ligands in the uranyl anion is significantly lower in energy than one with a symmetrical assignment of these sites, which was favored in the original X-ray crystallography study. According to optimized energies and Car–Parrinello molecular dynamics (CPMD) simulations, the [UO2F4(H2O)]2− ion in the solid is stable with respect to loss of the coordinated water molecule. In contrast, CPMD simulations had found this ligand to be unbound in aqueous solution. The role of the counterions in stabilizing the higher coordination number in the crystal is discussed.
Bonding analysis for sterically uncongested simple aurocarbons CnAum
Authors: Zaleski-Ejgierd Patryk , Pyykkö Pekka
Published: July 2009

The title series of simple model aurocarbons, CAu4, C2Au2, C2Au4, C2Au6, and C6Au6, is investigated using density functional theory with TPSS and B3LYP functionals and the second-order Møller–Plesset perturbation theory, with the latest Karlsruhe basis sets. The vibrational frequencies and the 0 K thermodynamical properties are calculated. The bonding mechanism is also investigated. For C2Aun the calculated MP2 formation energy suggests increased stabilization with increasing n, due to the Au–Au aurophilic pair interactions.
Theoretical investigation of CO adsorption on Rhn(n= 3–13) clusters
Authors: Shetty Sharan , Strych Sebastian , Jansen A. P.J. , van Santen Rutger A.
Published: July 2009

The adsorption of CO onto Rhn(n = 3–13) clusters has been investigated using the density functional approach. Stable active sites for CO adsorption such as top, bridge, and hollow have been identified on these clusters. Our results show that CO mostly prefers the bridge or top site, except on the Rh4and Rh11clusters where it prefers hollow sites. Rh6demonstrates two different active sites of almost equal energies for CO adsorption. Highly stable clusters show weak CO adsorption behavior. We also observe that the magnetic moment of the clusters is usually reduced after the CO adsorption. The preference of the active sites for CO adsorption has been analyzed using the charge density difference plots.
The nuclear quadrupole moment of69Ga and115In
Authors: Yakobi Hana , Eliav Ephraim , Kaldor Uzi
Published: July 2009

Electric field gradients at the nuclei of gallim and indium are determined by finite field calculations of the atomic energies as functions of the nuclear quadrupole moments. The four-component Dirac–Coulomb–Gaunt Hamiltonian serves as framework, and all electrons are correlated by Fock-space coupled cluster with single and double excitations or by single reference coupled cluster with approximate triples. Large, converged basis sets (e.g., 28s24p20d13f5g4h for In) and virtual spaces are used. Together with experimental nuclear quadrupole coupling constants, known with high precision, the calculated electric field gradients yield the nuclear quadrupole moments. For69Ga, we get Q = 174(3) mb, in agreement with the earlier 171(2) mb obtained from molecular calculations. The115In moment is Q = 772(5) mb, considerably lower than the previously accepted 810 mb, and in good agreement with the recent molecular value of 770(8) mb.
Ethene trimerization at CrI/CrIII — A density functional theory (DFT) study
Authors: Budzelaar Peter H.M.
Published: July 2009

Catalytic ethene trimerization at a chromium(I) indolate-AlR2Cl centre has been studied by density functional theory (DFT) methods. The reaction is found to follow the standard metallacycle mechanism. At most stages of the reaction, coordination of Cr to the pyrrole ring of the indolate is preferred. In all 13-e intermediates, coordination of the Al-bound chloride to Cr provides additional stabilization: the chloride behaves as a hemilabile ligand. Benzene is found to compete with ethene for coordination to CrI. The final hexene-forming step involves direct Cβ → Cα′ hydrogen transfer; reductive elimination from a possible (hydride)(hexenyl) intermediate is more difficult. The kinetic isotope effect calculated for the direct hydrogen transfer (4.1) agrees well with the experimental value for a bis(phosphino)amide chromium catalyst. Side products obtained in such systems (methylenecyclopentane, methylcyclopentane) can plausibly be explained through routes not involving any (hydride)(alkyl) reductive elimination. Our results indicate that a CrI/CrIII trimerization cycle is possible for some chromium trimerization catalysts, and also suggest that direct hydrogen transfer is most likely a general feature of trimerization at Cr centres.
Homolytic bond-dissociation enthalpies of tin bonds and tin–ligand bond strengths — A computational study
Authors: Whittleton Sarah R. , Boyd Russell J. , Grindley T. Bruce
Published: July 2009

Density functional theory and second-order Møller–Plesset perturbation theory with effective core potentials have been used to calculate homolytic bond-dissociation enthalpies, D(Sn–X), of organotin compounds, and their performance has been assessed by comparison with available experimental bond enthalpies. The SDB-aug-cc-pVTZ basis set with its effective core potential was used to calculate the D(Sn–X) of a series of trimethyltin(IV) species, Me3Sn–X, where X = H, CH3, CH2CH3, NH2, OH, Cl, and F. This is the most comprehensive report to date of homolytic Sn–X bond-dissociation enthalpies (BDEs). Effective core potentials are then used to calculate thermodynamic parameters including donor–acceptor bond enthalpies, [Formula: see text], for a series of tin-ligand complexes, L2SnX4 (X = Br or Cl, L = py, dmf, or dmtf), which are compared with previous experimental and nonrelativistic computational results. Based on computational efficiency and accuracy, it is concluded that effective core potentials are appropriate computational methods to examine bonding in organotin systems.
The affinity of HGGG, GHGG, GGHG, and GGGH peptides for copper(II) and the structures of their complexes — An ab initio study
Authors: Barry Stephen D. , Rickard Gail A. , Pushie M. Jake , Rauk Arvi
Published: July 2009

The structures and relative free energies in aqueous solution of the Cu(II) complexes of the “histidine walk” peptides, AcHGGGNH2, AcGHGGNH2, AcGGHGNH2, and AcGGGHNH2, were determined as a function of pH. Numerous structures of each species were found by gaseous- and solution-phase geometry optimization at the B3LYP/6–31G(d) level, and the effect of solvation estimated by the IEFPCM continuum solvation model. Free energies of solvation of the ionic species are large and favour structures with an extended peptide chain. In all Cu(II)–peptide complexes, deprotonation of two amide groups occurs readily at or below pH 7. In each system, the most abundant species at pH 7 is a neutral 1:1 complex with N3O1 coordination pattern. Binding in the forward direction toward the C terminus is preferred. The results are compared to recent experimental spectroscopic and potentiometric studies on related systems. Alternative explanations are offered for some of the experimental observations.
Mapping approach for quantum-classical time correlation functions
Authors: Nassimi Ali , Kapral Raymond
Published: July 2009

The calculation of quantum canonical time correlation functions is considered in this paper. Transport properties, such as diffusion and reaction rate coefficients, can be determined from time integrals of these correlation functions. Approximate quantum-classical expressions for correlation functions, which are amenable to simulation, are derived. These expressions incorporate the full quantum equilibrium structure of the system but approximate the dynamics by quantum-classical evolution where a quantum subsystem is coupled to a classical environment. The main feature of the formulation is the use of a mapping basis where the subsystem quantum states are represented by fictitious harmonic oscillator states. This leads to a full phase space representation of the dynamics that can be simulated without appeal to surface-hopping methods. The results in this paper form the basis for new simulation algorithms for the computation of quantum transport properties of large many-body systems.
Localized-orbital locator (LOL) profiles of transition-metal hydride and dihydrogen complexes,
Authors: Jacobsen Heiko
Published: July 2009

A bond descriptor based on the kinetic-energy density, the localized-orbital locator (LOL), is used to characterize the nature of the chemical bond in transition-metal hydride and dihydrogen complexes. Cationic complexes of the iron triad [MH3(PMe3)4]+(M = Fe, Ru, Os) serve as model compounds for transition-metal hydrogen bonding, since these complexes not only present examples for hydride as well as dihydrogen complexes, but for certain representatives, the two different types of metal–hydrogen bonds are realized within the same molecule. Both types of ligands show characteristic LOL profiles: (3,–3) Γ attractors in close vicinity to the H-atom for hydride ligands, and (3,–3) Γ attractors located between the two atoms for a dihydrogen ligand with νΓ-values of 0.8 and 0.9, respectively. In-between structures combine elements of the hydride and dihydrogen ligands. Relativistic effects on the relative stability of various isomers for the set of model compounds have been evaluated.
Competition between mixing and segregation in bimetallic AgnRbnclusters (n= 2–10),
Authors: Fournier René , Zamiruddin Shaima , Zhang Min
Published: July 2009

We found the minimum-energy structures of AgnRbn(n = 2–10) clusters by a combination of density functional theory (DFT) and taboo search global optimization. The global minimum geometry is mixed for n ≤ 4 and segregated, with a core-shell arrangement, for n > 4. There is a change in the nature of the bonding, from ionic to metallic, between n = 4 and n = 5. Although metallic bonding dominates at n > 4, large atomic charges (in the order of ±0.5) persist. These atomic charges (negative on the interior Ag atoms, positive on the surface Rb atoms) make AgnRbnclusters analogous to Zintl compounds and could prevent them from coalescing. This makes them intriguing potential building blocks for cluster-assembled materials. Ag4Rb4is relatively stable compared with other AgnRbnclusters; it has a nearly cubic shape, a large HOMO–LUMO gap (2 eV), and a highly ionic character with atomic charges equal to roughly ±1 au.
A time-dependent density functional theory (TDDFT) interpretation of the optical spectra of zinc phthalocyanine π-cation and π-anion radicals
Authors: Rosa Angela , Ricciardi Giampaolo
Published: July 2009

The UV–visible and near-IR spectra of the zinc phthalocyanine π-cation and π-anion radicals, [ZnPc(–1)]•+ and [ZnPc(–3)]•–, are investigated by time-dependent density functional theory (TDDFT) calculations using the pure, asymptotically correct, statistical average of (model) orbital potentials (SAOP) functional. The nature and intensity of the main spectral features are highlighted and interpreted on the basis of the ground-state electronic structure of the complexes. Similarities and differences with previous TDDFT/B3LYP results are discussed. TDDFT/SAOP results for the π-anion radical prove to be in excellent agreement with the solution spectra and generally in line with deconvolution analyses of solution absorption and magnetic circular dichroism (MCD) spectra. On the basis of these results a novel interpretation of the Q-band system is proposed. For the π-cation radical TDDFT/SAOP calculations provide a satisfactory description of the UV region of the spectrum. However, they do not reproduce accurately the energy and intensity of the Q band observed at 825 nm. The description of the Q-band region appears to be complicated by the presence of spurious non-Gouterman transitions. Furthermore, the calculations, either in the gas phase or in solution, do not account for the broad absorption near 500 nm that has been suggested to arise from a nondegenerate, z-polarized 2A2g excited state. Theory and experiment can be reconciled if the presence of an axial ligand such as CN– is explicitly considered in the calculations. TDDFT/SAOP results for the axially ligated [ZnPc(–1)(CN)]• species indicate that the 500 nm feature is related to the axial ligation induced symmetry lowering of the π-cation radical and this band is assigned to a z-polarized transition associated with the hole in the 2a1u.
Nonlinear time-dependent density functional theory studies of the ionization of CO2 by ultrashort intense laser pulses
Authors: Fowe Emmanuel Penka , Bandrauk André Dieter
Published: July 2009

Time-dependent density functional theory (TDDFT) studies of the ionization of CO2 by intense laser pulses (3.50 × 1014, 1.40 × 1015, 2.99 × 1015, and 1.25 × 1016 W/cm2) at 800 nm (ω = 0.0584 au) are presented in the nonlinear nonpertubative regime. Special emphasis is placed on elucidating molecular orbital orientation and various peak-intensities effects on the ionization processes. The results reveal that molecular orbital ionizations are strongly sensitive to their symmetry and the laser intensities. Most notably, we found that with a proper choice of the laser intensity (3.5 × 1014 W/cm2), the sensitivity is strong enough such that the nature and symmetry of the highest occupied molecular orbital (HOMO) can be directly probed and visualized from the angular dependence of laser-induced ionization. At higher intensities, ionization is found to occur also from inner orbitals, thus complicating the imaging of simple orbitals. A time-dependent electron-localization function (TDELF) is used to get a visual insight on the time evolution process of the electron density.
An investigation of (C5H5)Fe(C5H4–C(OBF3)–CH3),
Authors: Ogini Francis O. , Elder Philip J. W. , Britten James F. , Vargas-Baca Ignacio
Published: July 2009

Acetylferrocene readily forms an adduct with BF3. The product is a neutral model for protonated acetylferrocene, a species that is a crucial intermediate in the formation of 1,3,5-trisferrocenyl-benzene and other conjugated heterometallic derivatives by cyclocondensation. The title compound’s structure was determined in the solid state by X-ray diffraction and its spectroscopic properties were examined in detail, particularly the electronic excitation spectrum, which confirms the previous identification of the in situ generated protonated ketone.
Computational design of ruthenium hydride olefin-hydrogenation catalysts containing hemilabile ligands,
Authors: Rowley Christopher N. , Woo Tom K.
Published: July 2009

Three ruthenium hydridocarbonyl complexes containing bidentate hemilabile ligands have been evaluated as possible catalysts for the H2 hydrogenation of olefins. Our previous investigations of the mainstay hydridoruthenium catalyst, RuHCl(CO)(PR3)2 (1), indicated that the rate-limiting olefin-insertion barrier was increased by the need for an H2 molecule to act as a stabilizing two-electron donor. Using density functional theory (DFT) calculations, we have determined that a P,N phosphane-oxazoline would be suitable for stabilizing the metal center of the complex RuHCl(CO)(PiPr3)(DZ), where DZ is a neutral bidentate hemilable ligand. In the reaction catalyzed by this complex, the olefin insertion occurs before the coordination of H2, avoiding the entropic penalty of coordinating H2. We predict that the phosphane-oxazoline containing catalyst will have higher activity than 1 and a different rate-law, zero-order in [H2]. The oxazoline increases the strength of coordination of the ethylene due to stronger back-donation from destabilized Ru d-orbitals to the ethylene π* molecular orbital.
Impacts of repeated fertilization on fine roots, mycorrhizas, mesofauna, and soil chemistry under young interior spruce in central British Columbia
Authors: Berch Shannon M. , Brockley Robert P. , Battigelli Jeff , Hagerman Shannon
Published: July 2009

We assessed soil biota and soil chemistry in a 20-year-old interior spruce ( Picea glauca × engelmannii) stand in the interior of British Columbia 10 years after initiation of annual fertilization. Treatments included an unfertilized control, ON1 (having received per hectare 800 kg nitrogen (N), 350 kg phosphorus (P), 400 kg potassium (K)), and ON2 (having received per hectare 1400 kg N, 350 kg P, 400 kg K). Micronutrients were added periodically. There was more mineralizable N and NO3-N in the forest floor and upper mineral soil of ON2 plots than in the controls or ON1. Forest floor NH4-N increased with each level of fertilizer application. Total forest floor N, total sulphur, and exchangeable magnesium were higher with both levels of fertilization than with the unfertilized control. For the forest floor, available P and exchangeable K were highest in ON1. Forest floor densities of Acari, Prostigmata, and Oribatida were greater in fertilized plots than unfertilized control plots. In mineral soil, only Oribatida increased significantly with fertilization. Fertilization increased fine root length in ON1 relative to the control and ON2, but ectomycorrhizal colonization did not differ among treatments. These results contrast with previously reported responses to similar fertilizer treatments under lodgepole pine ( Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) in which negative responses of fine roots and soil biota to repeated nutrient additions were observed.
The influence of riparian vegetation on leaf litter inputs to Boreal Shield streams: implications for partial-harvest logging in riparian reserves
Authors: Muto Elisa A. , Kreutzweiser David P. , Sibley Paul K.
Published: July 2009

Litter inputs to headwater streams were measured to characterize and describe input patterns to the streams and to quantify the relationships between leaf litter inputs and surrounding riparian characteristics in Boreal Shield forests. Our goal was to provide information on litter inputs to streams in the Boreal Shield to determine if partial-harvest logging of commercial trees within riparian reserves has the potential to cause significant changes to litter inputs. Total leaf litter comprised 87% deciduous leaves and 13% conifer needles during the June to October periods. Commercial deciduous tree species accounted for approximately 50% of total deciduous leaf litter inputs. Distance-weighted deciduous basal area was the best predictor of overstorey deciduous litter (r2 = 0.597, P = <0.001), indicating that the size, abundance, and proximity of deciduous trees to streams are important to deciduous litter inputs. Leaf litter inputs to Boreal Shield streams are strongly influenced by surrounding riparian vegetation and can be effectively predicted by stem basal area. A linear regression model based on commercial deciduous tree basal area is presented that can be useful to determine the number of trees to retain within a riparian area to maintain leaf litter inputs at or near preharvest levels.
Variation in foliar nitrogen and aboveground net primary production in young postfire lodgepole pine
Authors: Turner Monica G. , Smithwick Erica A.H. , Tinker Daniel B. , Romme William H.
Published: July 2009

Understanding nutrient dynamics of young postfire forests may yield important insights about how stands develop following stand-replacing wildfires. We studied 15-year-old lodgepole pine stands that regenerated naturally following the 1988 Yellowstone fires to address two questions: (1) How do foliar nitrogen (N) concentration and total foliar N vary with lodgepole pine density and aboveground net primary production? (2) Is foliar N related to litter production and to rates of gross production, consumption, and net production of soil NH4+ and NO3–? Foliar N concentration of new lodgepole pine needles averaged 1.38%; only stands at very high density (>80 000 trees·ha–1) approached moderate N limitation. Foliar N concentration in composite (all-age) needles averaged 1.08%, varied among stands (0.87%–1.39%), and declined with increasing tree density. The foliar N pool averaged 48.3 kg N·ha–1, varied among stands (3.6–218.4 kg N·ha–1), and increased with aboveground net primary production. Total foliar N was not related to laboratory estimates of net production of NH4+ or NO3– in soils. Lodgepole pine foliage is a strong N sink, and N does not appear to be limiting at this early successional state. The initial spatial patterns of postfire tree density strongly influence landscape patterns of N storage.
Site carbon storage along productivity gradients of a late-seral southern boreal forest
Authors: Kranabetter J.M.
Published: July 2009

The quantity and distribution of carbon (C) storage in old-growth forests is a fundamental parameter needed to more accurately predict management effects on landscape C. C accounting on a regional or national level is generally based on zonal ecosystems, but total ecosystem C can vary widely with soil productive capacity within landscapes. To illustrate this, I compared old-growth forests of contrasting plant associations reflecting typical soil productivity gradients of the southern boreal forest in British Columbia, Canada. Total ecosystem C of zonal sites (medium–Huckleberry plant association) averaged 309 Mg C·ha–1, while less and more productive forest types ranged from 120 to 725 Mg C·ha–1, respectively. On average, 62% of ecosystem C was in live trees, 20% in mineral soils (0–50 cm), 9% in forest floors, and 9% in coarse woody debris and snags. Positive linear correlations between total ecosystem C and soil nitrogen availability or asymptotic stand height confirmed the strong influence of site productive capacity on C storage. The results demonstrate how ecological site classification or direct measures of stand productivity could refine estimates of the upper limits in potential C storage.
ADVERTISING AND IRREVERSIBLE OPINION SPREADING IN COMPLEX SOCIAL NETWORKS
Authors: CANDIA JULIÁN
Published: July 2009

Irreversible opinion spreading phenomena are studied on small-world and scale-free networks by means of the magnetic Eden model, a nonequilibrium kinetic model for the growth of binary mixtures in contact with a thermal bath. In this model, the opinion of an individual is affected by those of their acquaintances, but opinion changes (analogous to spin flips in an Ising-like model) are not allowed. We focus on the influence of advertising, which is represented by external magnetic fields. The interplay and competition between temperature and fields lead to order–disorder transitions, which are found to also depend on the link density and the topology of the complex network substrate. The effects of advertising campaigns with variable duration, as well as the best cost-effective strategies to achieve consensus within different scenarios, are also discussed.