bluuebunny/crossref_metadata_2025 huggingface dataset:
An extended hybrid density functional (X3LYP) with improved descriptions of nonbond interactions and thermodynamic properties of molecular systems
Authors: Xu Xin , Zhang Qingsong , Muller Richard P. , Goddard William A.
Published: January 2005

We derive here the form for the exact exchange energy density for a density that decays with Gaussian-type behavior at long range. This functional is intermediate between the B88 and the PW91 exchange functionals. Using this modified functional to match the form expected for Gaussian densities, we propose the X3LYP extended functional. We find that X3LYP significantly outperforms Becke three parameter Lee–Yang–Parr (B3LYP) for describing van der Waals and hydrogen bond interactions, while performing slightly better than B3LYP for predicting heats of formation, ionization potentials, electron affinities, proton affinities, and total atomic energies as validated with the extended G2 set of atoms and molecules. Thus X3LYP greatly enlarges the field of applications for density functional theory. In particular the success of X3LYP in describing the water dimer (with Re and De within the error bars of the most accurate determinations) makes it an excellent candidate for predicting accurate ligand–protein and ligand–DNA interactions.
Theory of photon statistics in single-molecule Förster resonance energy transfer
Authors: Gopich Irina , Szabo Attila
Published: January 2005

We present the theory for the distribution of the number of donor and acceptor photons detected in a time bin and the corresponding energy-transfer efficiency distribution obtained from single-molecule Förster resonance energy-transfer measurements. Photon counts from both immobilized and freely diffusing molecules are considered. Our starting point is the joint distribution for the donor and acceptor photons for a system described by an arbitrary kinetic scheme. This is simplified by exploiting the time scale separation between fast fluorescent transitions and slow processes which include conformational dynamics, intersystem conversion to a dark state, and translational diffusion in and out of the laser spot. The fast fluorescent transitions result in a Poisson distribution of the number of photons which is then averaged over slow fluctuations of the local transfer efficiency and the total number of photons. The contribution of various processes to the distribution and the variance of the energy-transfer efficiency are analyzed.
Rheology of gelling polymers in the Zimm model
Authors: Löwe Henning , Müller Peter , Zippelius Annette
Published: January 2005

In order to study rheological properties of gelling systems in dilute solution, we investigate the viscosity and the normal stresses in the Zimm model [B. H. Zimm, J. Chem. Phys. 24, 269 (1956)]. for randomly cross-linked monomers. The distribution of cluster topologies and sizes is assumed to be given either by Erdős–Rényi random graphs or three-dimensional bond percolation. Within this model the critical behavior of the viscosity and of the first normal stress coefficient is determined by the power-law scaling of their averages over clusters of a given size n with n. We investigate these scaling relations numerically and conclude that the scaling exponents are independent of the hydrodynamic interaction strength. The numerically determined exponents agree well with experimental data for branched polymers. However, we show that this traditional model of polymer physics is not able to yield a critical divergence at the gel point of the viscosity for a polydisperse dilute solution of gelation clusters. A generally accepted scaling relation for the Zimm exponent of the viscosity is thereby disproved.
Quantum and dynamical effects of proton donor-acceptor vibrational motion in nonadiabatic proton-coupled electron transfer reactions
Authors: Soudackov Alexander , Hatcher Elizabeth , Hammes-Schiffer Sharon
Published: January 2005

This paper presents a general theoretical formulation for proton-coupled electron transfer (PCET) reactions. The solute is represented by a multistate valence bond model, and the active electrons and transferring proton(s) are treated quantum mechanically. This formulation enables the classical or quantum mechanical treatment of the proton donor-acceptor vibrational mode, as well as the dynamical treatment of the proton donor-acceptor mode and the solvent. Nonadiabatic rate expressions are presented for PCET reactions in a number of well-defined limits for both dielectric continuum and molecular representations of the environment. The dynamical rate expressions account for correlations between the fluctuations of the proton donor-acceptor distance and the nonadiabatic PCET coupling. The quantities in the rate expressions can be calculated with a dielectric continuum model or a molecular dynamics simulation of the full system. The significance of the quantum and dynamical effects of the proton donor-acceptor mode is illustrated with applications to model PCET systems.
Energetics of hydrogen coverage on group VIII transition metal surfaces and a kinetic model for adsorption/desorption
Authors: Faglioni Francesco , Goddard William A.
Published: January 2005

We determined the binding energy of hydrogen to the closest packed surface for all nine group VIII transition metals as a function of surface coverage using quantum mechanics (density functional theory with the generalized gradient approximation) with periodic boundary conditions. The study provides a systematic comparison of the most stable surfaces of the nine group VIII transition metals, leading to results consistent with available surface science studies. We then use these to develop a simple thermodynamic model useful in estimating the surface coverage under typical heterogeneous catalysis conditions and compare these results to temperature programmed desorption experiments.
Dynamics of circular hydrogen bond array in calix[4]arene in a nonpolar solvent: A nuclear magnetic resonance study
Authors: Lang Jan , Deckerová Veronika , Czernek Jiřı́ , Lhoták Pavel
Published: January 2005

Hydroxyl groups on the lower rim of calix[4]arene form a circular array of four equivalent hydrogen bonds. The rate constants of reversal of the array in the temperature range of 221–304 K were determined by means of the NMR measurements of quaternary C13 nuclear spin transverse relaxation dependence on the effective radio frequency field. The flip-flop rate constants are in the range of 1.4×102–4.2×104 s−1, the activation enthalpy is 36.8 kJ/mol, the activation entropy is −36 J mol−1 K−1. This process was found uncorrelated with conformational transition cone–inverted cone, which is about thousand times slower. Molecular tumbling of calix[4]arene measured using C13 spin relaxation was found isotropic with correlation times lying in the range of 0.1–3 ns and with the activation energy of 21 kJ/mol. In order to assess relaxation of C13 aromatic nuclei, their principal components of chemical shift tensor were calculated using the density functional theory approach.
A bond-order analysis of the mechanism for hydrated proton mobility in liquid water
Authors: Lapid Hadas , Agmon Noam , Petersen Matt K. , Voth Gregory A.
Published: January 2005

Bond-order analysis is introduced to facilitate the study of cooperative many-molecule effects on proton mobility in liquid water, as simulated using the multistate empirical valence-bond methodology. We calculate the temperature dependence for proton mobility and the total effective bond orders in the first two solvation shells surrounding the H5O2+ proton-transferring complex. We find that proton-hopping between adjacent water molecules proceeds via this intermediate, but couples to hydrogen-bond dynamics in larger water clusters than previously anticipated. A two-color classification of these hydrogen bonds leads to an extended mechanism for proton mobility.
Ion-molecule reactions in He4 droplets: Flying nano-cryo-reactors
Authors: Fárnı́k Michal , Toennies J. Peter
Published: January 2005

Ion-molecule reactions are studied inside large (≈104 atoms) very cold (0.37 K) superfluid He4 droplets by mass spectrometric detection of the product ions. He+ ions initially formed inside the droplets by electron impact ionization undergo charge transfer with either embedded D2, N2, or CH4. For D2 this charge transfer process was studied in detail by varying the pickup pressure. For either N2 or CH4 the reagent ions were formed by this charge transfer and the reaction pathways of the secondary reactions N2++D2, CH4++D2, and CH3++D2 each with an additionally embedded D2 molecule were also determined from the pickup pressure dependencies. In several cases, notably He⋅N2+ and CH3D2+ reaction intermediates are observed. The analysis is facilitated by the tendency for molecular ion products to appear without (or with only very few) attached He atoms whereas the atomic ion products usually appear in the mass spectra with several attached He atoms, e.g., Hem⋅D+ ions with up to m=19.
Photoinduced charge transfer reaction at surfaces. III. (HF)2⋯Nan/LiF(001)+hν(640 nm)→HFF−Nan+/LiF(001)+H(g)
Authors: Dobrin Sergey , Giorgi Javier B. , Naumkin Fedor Y. , Polanyi John C.
Published: January 2005

A sub-monolayer of atomic sodium was deposited on a LiF(001) surface at 40 K. The adsorbed sodium exists at the surface as single atoms and clusters. The surface was dosed with 1 L of HF, to form adsorbed (HF)2⋯Nan (n=1,2,3,…) complexes, which were then irradiated by 640 nm laser light, to induce charge-transfer reaction. The reaction-product atomic H(g) was observed leaving the surface by two-color Rydberg-atom time-of-flight (TOF) spectroscopy. The TOF spectrum of the desorbed H atoms contained two components; a “fast” component with a maximum at ≈0.85 eV, and a “slow” component with a maximum at 0.45 eV. These two components were attributed to photoreaction on adsorbed single atoms and clusters of sodium, respectively. The fast component exhibited a structure (48±17 meV spacing) near the high-energy end of spectrum. This structure was attributed to vibration of NaFHF photoproduct residing on the surface. The cross section of the harpooning event in the Na⋯(HF)2 adsorbed complex was determined as (9.1±2.0)×10−19 cm2. To interpret the experimental vibrational structure and the relative energies of the fast and slow components of the TOF spectrum, high-level ab initio calculations were performed for reactants Nan⋯(HF)m (n,m=1,2) and reaction products NanFmHm−1. The calculated NaF–HF and Na–Na(HF)2 bond dissociation energies indicated that photoexcitation of the precursor complexes led not only to ejection of H atoms, but also to dissociation of the Nan⋯(HF)2 (n=1,2) species through cleavage of the NaF–HF and Na–Na(HF)2 bonds.
A molecular dynamics computer simulation study of room-temperature ionic liquids. II. Equilibrium and nonequilibrium solvation dynamics
Authors: Shim Y. , Choi M. Y. , Kim Hyung J.
Published: January 2005

The molecular dynamics (MD) simulation study of solvation structure and free energetics in 1-ethyl-3-methylimidazolium chloride and 1-ethyl-3-methylimidazolium hexafluorophosphate using a probe solute in the preceding article [Y. Shim, M. Y. Choi and H. J. Kim, J. Chem. Phys. 122, 044510 (2005)] is extended to investigate dynamic properties of these liquids. Solvent fluctuation dynamics near equilibrium are studied via MD and associated time-depedent friction is analyzed via the generalized Langevin equation. Nonequilibrium solvent relaxation following an instantaneous change in the solute charge distribution and accompanying solvent structure reorganization are also investigated. Both equilibrium and nonequilibrium solvation dynamics are characterized by at least two vastly different time scales—a subpicosecond inertial regime followed by a slow diffusive regime. Solvent regions contributing to the subpicosecond nonequilibrium relaxation are found to vary significantly with initial solvation configurations, especially near the solute. If the solvent density near the solute is sufficiently high at the outset of the relaxation, subpicosecond dynamics are mainly governed by the motions of a few ions close to the solute. By contrast, in the case of a low local density, solvent ions located not only close to but also relatively far from the solute participate in the subpicosecond relaxation. Despite this difference, linear response holds reasonably well in both ionic liquids.
Quantum and quasiclassical studies of the O(3P)+HCl→OH+Cl(2P) reaction using benchmark potential surfaces
Authors: Xie Tiao , Bowman Joel , Duff J. W. , Braunstein M. , Ramachandran B.
Published: January 2005

We have performed quantum mechanical (QM) dynamics calculations within the independent-state approximation with new benchmark triplet A″ and A′ surfaces [B. Ramachandran et al., J. Chem. Phys. 119, 9590 (2003)] for the rovibronic state-to-state measurements of the reaction O(3P)+HCl(v=2,j=1,6,9)→OH(v′j′)+Cl(2P) [Zhang et al., J. Chem. Phys. 94, 2704 (1991)]. The QM and experimental rotational distributions peak at similar OH(j′) levels, but the QM distributions are significantly narrower than the measurements and previous quasiclassical dynamics studies. The OH(low j′) populations observed in the measurements are nearly absent in the QM results. We have also performed quasiclassical trajectory with histogram binning (QCT-HB) calculations on these same benchmark surfaces. The QCT-HB rotational distributions, which are qualitatively consistent with measurements and classical dynamics studies using other surfaces, are much broader than the QM results. Application of a Gaussian binning correction (QCT-GB) dramatically narrows and shifts the QCT-HB rotational distributions to be in very good agreement with the QM results. The large QCT-GB correction stems from the special shape of the joint distribution of the classical rotational/vibrational action of OH products. We have also performed QM and QCT calculations for the transition, O+HCl(v=0,T=300 K)→OH(v′j′)+Cl from threshold to ∼130 kcal mol−1 collision energy as a guide for possible future hyperthermal O-atom measurements. We find in general a mixed energy release into translation and rotation consistent with a late barrier to reaction. Angular distributions at high collision energy are forward peaked, consistent with a stripping mechanism. Direct collisional excitation channel cross sections, O+HCl(v=0,T=300 K)→O+HCl(v′=1), in the same energy range are large, comparable in magnitude to the reactive channel cross sections. Although the A″3 state dominates most collision processes, above ∼48 kcal mol−1, the A′3 state plays the major role in collisional excitation.
Epitaxial silicide formation on recoil-implanted substrates
Authors: Hashimoto Shin , Egashira Kyoko , Tanaka Tomoya , Etoh Ryuji , Hata Yoshifumi , Tung R. T.
Published: January 2005

An epitaxy-on-recoil-implanted-substrate (ERIS) technique is presented. A disordered surface layer, generated by forward recoil implantation of ∼0.7−3×1015cm−2 of oxygen during Ar plasma etching of surface oxide, is shown to facilitate the subsequent epitaxial growth of ∼25–35-nm-thick CoSi2 layers on Si(100). The dependence of the epitaxial fraction of the silicide on the recoil-implantation parameters is studied in detail. A reduction in the silicide reaction rate due to recoil-implanted oxygen is shown to be responsible for the observed epitaxial formation, similar to mechanisms previously observed for interlayer-mediated growth techniques. Oxygen is found to remain inside the fully reacted CoSi2 layer, likely in the form of oxide precipitates. The presence of these oxide precipitates, with only a minor effect on the sheet resistance of the silicide layer, has a surprisingly beneficial effect on the thermal stability of the silicide layers. The agglomeration of ERIS-grown silicide layers on polycrystalline Si is significantly suppressed, likely from a reduced diffusivity due to oxygen in the grain boundaries. The implications of the present technique for the processing of deep submicron devices are discussed.
A loss mechanism study of a very high Q silicon micromechanical oscillator
Authors: Liu Xiao , Vignola J. F. , Simpson H. J. , Lemon B. R. , Houston B. H. , Photiadis D. M.
Published: January 2005

The room-temperature quality factors of silicon micromechanical oscillators have been investigated by scanning laser vibrometry. One of the flexural modes has very little attachment loss to its environment, which enables us to study internal loss mechanisms. After several consecutive annealing steps up to 800°C, the quality factor Q has increased from 8×104 to 6.0×105. However, the Q decays to 1.4×105 over six months in air. We conclude that near-surface lattice defects caused by reactive-ion etching and surface adsorbates are the main source of internal loss while surface adsorbates are responsible for the time dependence. We also discuss the thermoelastic limit in terms of Zener’s theory and flexural modal components of thin plates with vibratory volume change, and compare it with our results.
Refractive indices of cubic-phase MgxZn1−xO thin-film alloys
Authors: Chen N. B. , Wu H. Z. , Xu T. N.
Published: January 2005

Refractive indices for cubic-phase MgxZn1−xO (0.55⩽x⩽1) thin-film alloys were determined from transmission measurements and Manifacier envelope method. The refractive index of cubic MgxZn1−xO decreases with the Mg fraction increase, such as at the wavelength of 400nm the refractive index decreases from 1.89 to 1.73 as x increases from 0.57 to 1.0. The refractive index dispersion relation of cubic MgxZn1−xO in the visible-light region (400–800nm) follows the first-order Sellmeier equation and decreases with the increase of Mg fraction.
Diamond growth in solid substrate from gaseous source
Authors: Hirakuri Kenji K. , Ohyama Manabu , Komura Nobuo , Sudo Kazuyuki , Sato Keisuke
Published: January 2005

Diamond growth on iron-based materials is extremely important for mechanical and electrical applications because the iron-based materials are widely used in domestic science. In our previous study, chemical-vapor deposition diamond nucleation and subsequent film deposition have been precisely controlled by the residence time of the source gas, which is an essential parameter during chemical-vapor deposition diamond growth. In order to investigate the mechanism of diamond growth on the iron-based materials, the inside of the stainless-steel substrates has been investigated by observing the cross-sectional images after the substrates are fractured by halves. The diamond grains were confirmed in the inside of the stainless-steel substrates. At an optimum condition, diamond growth with high crystalline quality in stainless-steel substrates was identified by Raman spectroscopy and scanning electron microscopy.
Mechanical dissipation in tetrahedral amorphous carbon
Authors: Czaplewski D. A. , Sullivan J. P. , Friedmann T. A. , Carr D. W. , Keeler B. E. N. , Wendt J. R.
Published: January 2005

We have fabricated micromechanical oscillators from tetrahedrally coordinated amorphous carbon (ta-C) in order to study mechanical dissipation mechanisms in this material. Cantilever oscillators with either in-plane or out-of-plane dominant transverse vibrational modes and free-free beam oscillators with in-plane modes were fabricated with critical dimensions ranging from 75nm to over 1mm. The resonant frequency and quality factor were measured for all oscillators. The resonant frequencies ranged from a few kilohertz to several megahertz, while the quality factor remained nearly constant at approximately 2–4×103. Possible dissipation mechanisms were evaluated for these oscillators, and it was found that the observed dissipation was not limited by mechanical clamping losses, air damping, thermoelastic dissipation, or dissipation due to phonon-mechanical vibration interactions. However, an extrinsic dissipation mechanism in which dissipation is limited by a spectrum of defects in ta-C was found to be consistent with the observed behavior. Assuming that the mechanical relaxation associated with the dissipative defects is thermally activated, we derive a defect distribution that is relatively flat with activation energies ranging from about 0.35 to over 0.55eV.
Ferroelectric phase stability studies in potassium nitrate: Polyvinylidene fluoride composite films
Authors: Kumar Neeraj , Nath R.
Published: January 2005

The structural and thermal properties of potassium nitrate (KNO3): polyvinylidene fluoride composite films have been studied. Differential scanning calorimetry has been employed to characterize the phase transitions in the composite and x-ray diffraction has been used to quantify the crystalline phases. The ferroelectric phase (III) of KNO3 is stabilized relative to the paraelectric phase (II) at room temperature at concentration ⩽50wt%, with the largest proportion at 50wt%. The enthalpies and transition temperature width are also affected in the composite. The polarization hysteresis loops also show that the remanent polarization is largest at 50wt% of KNO3. The capacitance–voltage characteristics exhibit strong hysteresis with pronounced peaks and some asymmetry. The switching current behavior of the composite film was also investigated and supports the presence of ferroelectric phase in the composite film at room temperature.
Evolution of in-plane texture in reactively sputtered CrN films
Authors: Zhao Z. B. , Rek Z. U. , Yalisove S. M. , Bilello J. C.
Published: January 2005

The microstructure and texture of chromium nitride films reactively sputtered on silicon substrates were investigated using x-ray scattering, pole figures, transmission electron microscopy, and atomic force microscopy. Under the given deposition geometry, the CrN films were shown to develop a in-plane texture. The three preferred crystallographic orientations of the CrN films approximately coincided with the characteristic directions associated with the deposition geometry. There appear to be two regimes that govern the microstructural evolution and texture development for reactively sputtered chromium films. The first one involves the deposition conditions that lead to the formation of a single, stable phase such as stöichiometric CrN (above certain level of nitrogen partial pressure). In this regime, the film growth appears to be controlled by local epitaxy in individual columns, competitive grain growth, and kinetic roughening. The film characteristics resulted from this regime include the development of the in-plane texture, well-organized microstructures with relatively coarse grains, increased surface roughness, and large tensile stress. The second regime involves the transitional region prior to formation of the stable phase CrN in which significant microstructural refinements take place. This transitional region is associated with the thermodynamically metastable phase CrNx or the presence of multiple phases. The continuous renucleations during film growth disrupt the local epitaxy and impede kinetic roughening. This leads to film characteristics manifested by weakened or no texture, ultrafine microstructure (e.g., nanocrystalline structures), reduced surface roughness, and a tendency for residual stress to transit from tensile to compressive.
The extrapolation of one-electron basis sets in electronic structure calculations: How it should work and how it can be made to work
Authors: Schwenke David W.
Published: January 2005

We consider the extrapolation of the one-electron basis to the basis set limit in the context of coupled cluster calculations. We produce extrapolation coefficients that produce much more accurate results than previous extrapolation forms. These are determined by fitting to accurate benchmark results. For coupled cluster singles doubles energies, we take our benchmark results from the work of Klopper that explicitly includes the interelectronic distance. For the perturbative triples energies, our benchmark results are obtained from large even-tempered basis set calculations.
Anharmonic vibrational properties by a fully automated second-order perturbative approach
Authors: Barone Vincenzo
Published: January 2005

This paper describes the implementation of a fully automated code for the building of anharmonic force constants and their use in a second-order perturbative evaluation of vibrorotational parameters. Next, a number of test applications are discussed, which show the strengths and limits of various computational levels.
The aromatic fluctuation index (FLU): A new aromaticity index based on electron delocalization
Authors: Matito Eduard , Duran Miquel , Solà Miquel
Published: January 2005

In this work, the aromatic fluctuation index (FLU) that describes the fluctuation of electronic charge between adjacent atoms in a given ring is introduced as a new aromaticity measure. This new electronic criterion of aromaticity is based on the fact that aromaticity is related to the cyclic delocalized circulation of π electrons. It is defined not only considering the amount of electron sharing between contiguous atoms, which should be substantial in aromatic molecules, but also taking into account the similarity of electron sharing between adjacent atoms. For a series of rings in 15 planar polycyclic aromatic hydrocarbons, we have found that, in general, FLU is strongly correlated with other widely used indicators of local aromaticity, such as the harmonic-oscillator model of aromaticity, the nucleus independent chemical shift, and the para-delocalization index (PDI). In contrast to PDI, the FLU index can be applied to study the aromaticity of rings with any number of members and it can be used to analyze both the local and global aromatic character of rings and molecules.
Short-range exchange and correlation energy density functionals: Beyond the local-density approximation
Authors: Toulouse Julien , Colonna François , Savin Andreas
Published: January 2005

We propose approximations which go beyond the local-density approximation for the short-range exchange and correlation density functionals appearing in a multideterminantal extension of the Kohn–Sham scheme. A first approximation consists of defining locally the range of the interaction in the correlation functional. Another approximation, more conventional, is based on a gradient expansion of the short-range exchange-correlation functional. Finally, we also test a short-range generalized-gradient approximation by extending the Perdew–Burke–Ernzerhof exchange-correlation functional to short-range interactions.
Density-functional theory-symmetry-adapted intermolecular perturbation theory with density fitting: A new efficient method to study intermolecular interaction energies
Authors: Heßelmann A. , Jansen G. , Schütz M.
Published: January 2005

The previously developed DFT-SAPT approach, which combines symmetry-adapted intermolecular perturbation theory (SAPT) with a density-functional theory (DFT) representation of the monomers, has been implemented by using density fitting of two-electron objects. This approach, termed DF-DFT-SAPT, scales with the fifth power of the molecular size and with the third power upon increase of the basis set size for a given dimer, thus drastically reducing the cost of the conventional DFT-SAPT method. The accuracy of the density fitting approximation has been tested for the ethyne dimer. It has been found that the errors in the interaction energies due to density fitting are below 10−3 kcal/mol with suitable auxiliary basis sets and thus one or two orders of magnitude smaller than the errors due to the use of a limited atomic orbital basis set. An investigation of three prominent structures of the benzene dimer, namely, the T shaped, parallel displaced, and sandwich geometries, employing basis sets of up to augmented quadruple-ζ quality shows that DF-DFT-SAPT outperforms second-order Møller-Plesset theory (MP2) and gives total interaction energies which are close to the best estimates infered from combining the results of MP2 and coupled-cluster theory with single, double, and perturbative triple excitations.
Theoretical fine spectroscopy with symmetry adapted cluster–configuration interaction general-R method: First-row K-shell ionizations and their satellites
Authors: Kuramoto Kei , Ehara Masahiro , Nakatsuji Hiroshi
Published: January 2005

Molecular core ionization spectra and their satellites were studied by the symmetry adapted cluster–configuration interaction (SAC-CI) general-R method. The core-electron binding energies of C, N, O, and F atoms of 22 molecules were calculated with an average deviation of 0.11 eV from the experimental values. The energy splittings between K-shell gerade and ungerade states were calculated and discussed in relation to the bond length. The satellite spectra of the C 1s and N 1s core ionizations of methane and ammonia were investigated. The SAC-CI general-R method gave many shake-up states with moderate intensities, reproducing the general feature of the experimental spectra, and thus enabling the detailed understanding and assignments of the core-electron ionization spectra.
Semiclassical statistical mechanics of two-dimensional hard-body fluids
Authors: Karki Shanker S. , Karki Bimal P. , Dey Tarun K. , Sinha Suresh K.
Published: January 2005

The problem of calculating the thermodynamic properties of two-dimensional semiclassical hard-body fluids is studied. Explicit expressions are given for the first-order quantum corrections to the free energy, equation of state, and virial coefficients. The numerical results are calculated for the planar hard dumbbell fluid. Significant features are the increase in quantum corrections with increasing η and increasing L*=L/σ0.
The slow-scale stochastic simulation algorithm
Authors: Cao Yang , Gillespie Daniel T. , Petzold Linda R.
Published: January 2005

Reactions in real chemical systems often take place on vastly different time scales, with “fast” reaction channels firing very much more frequently than “slow” ones. These firings will be interdependent if, as is usually the case, the fast and slow reactions involve some of the same species. An exact stochastic simulation of such a system will necessarily spend most of its time simulating the more numerous fast reaction events. This is a frustratingly inefficient allocation of computational effort when dynamical stiffness is present, since in that case a fast reaction event will be of much less importance to the system’s evolution than will a slow reaction event. For such situations, this paper develops a systematic approximate theory that allows one to stochastically advance the system in time by simulating the firings of only the slow reaction events. Developing an effective strategy to implement this theory poses some challenges, but as is illustrated here for two simple systems, when those challenges can be overcome, very substantial increases in simulation speed can be realized.
Towards formation of buckminsterfullerene C60 in quantum chemical molecular dynamics
Authors: Zheng Guishan , Irle Stephan , Morokuma Keiji
Published: January 2005

We present follow-up studies on the formation mechanism of fullerene molecules from random ensembles of C2 molecules using quantum chemical molecular dynamics. Two possible roadmaps are investigated as to how buckminsterfullerene C60 and higher fullerenes could be formed. In a “size-up” scenario, fullerenes of the cage size of C72–C96 were found to form directly from high concentrations of C2 molecules at 2000 K with periodic supply of batches of additional C2’s. In a “size-down” approach, smaller fullerenes are sometimes formed by losing carbon fragments in “fall-off” or “pop-out” annealing processes under prolonged heating of giant fullerenes, which were self-assembled at initial stages from C2’s with lower concentrations. Both roadmaps are found to provide explanations for the appearance of C60 and larger fullerenes in combustion and carbon arc experiments.
Intermolecular effect in molecular electronics
Authors: Liu Rui , Ke San-Huang , Baranger Harold U. , Yang Weitao
Published: January 2005

We investigate the effects of lateral interactions on the conductance of two molecules connected in parallel to semi-infinite leads. The method we use combines a Green function approach to quantum transport with density functional theory for the electronic properties. The system, modeled after a self-assembled monolayer, consists of benzylmercaptane molecules sandwiched between gold electrodes. We find that the conductance increases when intermolecular interaction comes into play. The source of this increase is the indirect interaction through the gold substrate rather than direct molecule-molecule interaction. A striking resonance is produced only 0.3 eV above the Fermi energy.
Influence of Ni on the structural and magnetic properties of NixFe73.5−xSi13.5B9Nb3Cu1 (⩽x⩽25) alloys
Authors: Agudo P. , Vázquez M.
Published: January 2005

A systematic investigation about the influence of Ni content on structure and magnetic properties of melt-spun NixFe73.5−xSi13.5B9Nb3Cu1 (0⩽x⩽25) alloys has been performed. The study has been done in amorphous ribbons as well as after thermal treatments leading to structure relaxation, to partial devitrification into nanocrystalline state, and finally to full structural crystallization. Different characterization techniques have been employed. Structure has been investigated by x-ray diffraction, differential scanning calorimetry, and Mössbauer technique, while magnetic properties as coercivity, saturation magnetostriction constant, or Curie temperature have been studied by induction methods. Two main behaviors have been distinguished depending on the Ni content of the alloys: (i) Low Ni content (up to x=15), when Ni atoms diffuse towards Fe sites in the disordered bcc (Fe,Ni)3Si structure originating reduced structural distortion and small changes in magnetic properties as coercivity and (ii) High Ni content (x=20,25) for which drastic changes are observed in structure (appearance of a tetragonal Fe3NiSi1.5 phase) and magnetic properties (coercivity increases up to two orders of magnitude) particularly after annealing at those temperatures for which optimum nanocrystalline structure is achieved for x=0.
Vibrationally resolved rate coefficients and branching fractions in the dissociative recombination of O2+
Authors: Petrignani Annemieke , van der Zande Wim J. , Cosby Philip C. , Hellberg Fredrik , Thomas Richard D. , Larsson Mats
Published: January 2005

We have studied the dissociative recombination of the first three vibrational levels of O2+ in its electronic ground X 2Πg state. Absolute rate coefficients, cross sections, quantum yields and branching fractions have been determined in a merged-beam experiment in the heavy-ion storage ring, CRYRING, employing fragment imaging for the reaction dynamics. We present the absolute total rate coefficients as function of collision energies up to 0.4 eV for five different vibrational populations of the ion beam, as well as the partial (vibrationally resolved) rate coefficients and the branching fractions near 0 eV collision energy for the vibrational levels v=0, 1, and 2. The vibrational populations used were produced in a modified electron impact ion source, which has been calibrated using Cs–O2+ dissociative charge transfer reactions. The measurements indicate that at low collision energies, the total rate coefficient is weakly dependent on the vibrational excitation. The calculated thermal rate coefficient at 300 K decreases upon vibrational excitation. The partial rate coefficients as well as the partial branching fractions are found to be strongly dependent on the vibrational level. The partial rate coefficient is the fastest for v=0 and goes down by a factor of two or more for v=1 and 2. The O(1S) quantum yield, linked to the green airglow, increases strongly upon increasing vibrational level. The effects of the dissociative recombination reactions and super elastic collisions on the vibrational populations are discussed.
Mass spectrometric and computational study of SnPb in the gas phase
Authors: Gigli G. , Meloni Giovanni , Carrozzino M.
Published: January 2005

The SnPb molecule has been identified in a Knudsen effusion mass spectrometry experiment. The direct dissociation reaction and two isomolecular exchange reactions involving the Sn2 and Pb2 molecules have been studied, in the 1426–1705 K range of temperatures, using both second and third law procedures. The D0∘(SnPb,g) has been derived, for the first time, as (122.6±4.0) kJ mol−1. Density functional and ab initio calculations up to the coupled clusters level of theory were also performed. In addition, the anion dissociation energy D0∘(SnPb−,g) of (179.2±4.2) kJ mol−1 was determined using the D0∘(SnPb,g) mass spectrometric value derived in this investigation and literature data.
Gaining mechanistic insight from closed loop learning control: The importance of basis in searching the phase space
Authors: Langhojer Florian , Cardoza David , Baertschy Mark , Weinacht Thomas
Published: January 2005

This paper discusses different routes to gaining insight from closed loop learning control experiments. We focus on the role of the basis in which pulse shapes are encoded and the algorithmic search is performed. We demonstrate that a physically motivated, nonlinear basis change can reduce the dimensionality of the phase space to one or two degrees of freedom. The dependence of the control goal on the most important degrees of freedom can then be mapped out in detail, leading toward a better understanding of the control mechanism. We discuss simulations and experiments in selective molecular fragmentation using shaped ultrafast laser pulses.
Structure of amorphous poly-(ethylmethacrylate): A wide-angle x-ray scattering study
Authors: Wind M. , Graf R. , Renker S. , Spiess H. W. , Steffen W.
Published: January 2005

The local, temperature dependent structure of poly-(ethylmethacrylate) was studied with wide-angle x-ray scattering. The results were set into context to recent wide-angle neutron scattering results as well as to the results from a multitude of studies of the dynamics of this polymer. The temperature dependence of the wide angle x-ray results point to the development of local order which is backed by the neutron scattering results and which is connected to characteristic temperatures of the relaxation dynamic Tg and Tc. The poly-(ethylmethacrylate) was studied in its predominantly syndiotactic as well as predominantly isotactic state displaying vastly different local structures as manifested in the x-ray results.
Sub -T g relaxations due to dipolar solutes in nonpolar glass-forming solvents
Authors: Shahin Md. , Murthy S. S. N.
Published: January 2005

It is well known that rigid dipolar solutes (in smaller quantity) dispersed in a nonpolar glassy matrix exhibit a sub-Tg (or βs) relaxation due to the solute often designated as Johari–Goldstein (JG) relaxation, which is intermolecular in nature. In this article, we report the results of our study of such a sub-Tg process in a wide variety of dipolar solutes in different glassy systems using dielectric spectroscopy over a frequency range of 20–106 Hz down to a temperature of 77 K. The Tg of these solutions are determined using differential scanning calorimetry. The solvents used in this study are o-terphenyl (OTP), isopropylbenzene (IPB), and methylcyclohexane. In the case of rigid molecular solutes, like mono-halogen benzenes, the activation energy (ΔEβ) of the βs process is found to increase with decreasing Tg of the solvent, with a corresponding decrease in the magnitude of the βs process. In the case of more symmetrical molecular solute, for example, tert-butylchloride, the change in ΔEβ is not very appreciable. These results emphasize the importance of the size of the cage of the host matrix in the relaxation of the solute molecules. We have also studied the sub-Tg relaxation(s) due to some flexible molecular solutes, viz., 1butylbromide, 1hexylbromide, 1butylacetate, and benzylacetate. These solutes in IPB matrix exhibit only one relaxation, whereas in OTP matrix they exhibit an additional sub-Tg process, which may be identified with a JG type of relaxation. These observations lead us to the conclusion that the β process observed in the glassy states of these pure solutes is predominantly intramolecular in nature.
Dynamics of ferrocene in AlPO4-5 single crystals probed by nuclear forward scattering
Authors: Asthalter T. , Garibay J. Villanueva , Olszowka V. , Kornatowski J.
Published: January 2005

We have investigated the dynamics of ferrocene in the channels of single crystals of AlPO4-5, using nuclear forward scattering. Around 150 K, rotational dynamics sets in, which is strongly anisotropic up to 235 K. Assuming planar rotation of the ferrocene molecules within the channels we obtained an activation energy of (8.2±1.0) kJ mol−1. Between 235 K and room temperature, the rotation becomes isotropic.
Infrared absorption spectrum of matrix-isolated noble-gas hydride molecules: Fingerprints of specific interactions and hindered rotation
Authors: Khriachtchev Leonid , Lignell Antti , Juselius Jonas , Räsänen Markku , Savchenko Elena
Published: January 2005

Noble-gas hydride molecules with the general formula HNgY (Ng denotes noble-gas atom and Y denotes electronegative fragment) are usually prepared in solid noble gases. In many cases, the matrix-isolated HNgY molecules show a characteristic structure of the H–Ng stretching absorption: A close doublet as the main spectral feature and a weaker satellite at higher energy. This characteristic band structure is studied here for matrix-isolated HXeBr and HKrCl molecules. Based on the experimental and theoretical results, we suggest a model explaining the common features of the band structure of the HNgY molecules in noble-gas matrices. In this model, the main doublet bands are attributed to matrix sites where the splitting is caused by specific interactions of the embedded molecule with noble-gas matrix atoms in certain local morphology. The weaker blueshifted band is probably a fingerprint of hindered rotation (libration) of the embedded molecule in the lattice. This librational band has a mirror counterpart at lower energies appearing at higher matrix temperatures. Our present ab initio calculations for the one-to-one Xe⋯HXeBr complexes and the simulation of hindered rotation in a matrix support this image.
A full-dimensional quantum dynamical approach to the vibrational predissociation of Cl2–He2
Authors: Garcı́a-Vela A.
Published: January 2005

A full-dimensional, fully coupled wave packet method is proposed and applied to investigate the vibrational predissociation dynamics of the Cl2(B,v′)–He2 complex. Simulations are carried out for the resonance states associated with the v′=10–13 initial vibrational excitations of Cl2, and the results are compared with the available experimental data. A good agreement with experiment is achieved for the resonance lifetimes (typically within experimental error) and the Cl2 fragment rotational distributions. The mechanism of dissociation of the two He atoms is found to be dominantly sequential, through the Δv′=−2 channel. The probabilities obtained for the Δv′=−1 dissociation channel are, however, overestimated due to the use of absorbing boundary conditions combined with finite grid effects. It is suggested that a mechanism of energy redistribution through the couplings between the van der Waals modes of the two weak bonds takes place in the Δv′=−1 dissociation. This mechanism is consistent with the resonance lifetimes and Cl2 rotational distributions predicted. The favorable comparison with most of the experimental data supports the reliability of the potential used to model Cl2(B,v′)–He2, at least in the present range of v′ levels.
Quantum studies of H atom trapping on a graphite surface
Authors: Sha Xianwei , Jackson Bret , Lemoine Didier , Lepetit Bruno
Published: January 2005

The trapping and sticking of H and D atoms on the graphite (0001) surface is examined, over the energy range of 0.1–0.9 eV. For hydrogen to chemisorb onto graphite, the bonding carbon must pucker out of the surface plane by several tenths of an angstrom. A quantum approach in which both the hydrogen and the bonding carbon atoms can move is used to model the trapping, and a potential energy surface based on density functional theory calculations is employed. It is found, for energies not too far above the 0.2 eV barrier to chemisorption that a significant fraction of the incident H or D atoms can trap. The forces on the bonding carbon are large, and it can reconstruct within 50 fs or so. After about 100 fs, most of the trapped H atoms scatter back into the gas phase, but the 5%–10% that remain can have lifetimes on the order of a picosecond or more. Calculations of the resonance eigenstates and lifetimes confirm this. An additional lattice degree of freedom is included quantum mechanically and is shown to significantly increase the amount of H that remains trapped after 1 ps. Further increasing the incident energy destabilizes the trapped state, leading to less H remaining trapped at long times. We estimate that for a full dissipative bath, the sticking probabilities should be on the order of 0.1.
Vibron-polaron critical localization in a finite size molecular nanowire
Authors: Falvo C. , Pouthier V.
Published: January 2005

The small polaron theory is applied to describe the vibron dynamics in an adsorbed nanowire with a special emphasis onto finite size effects. It is shown that the finite size of the nanowire discriminates between side molecules and core molecules which experience a different dressing mechanism. Moreover, the inhomogeneous behavior of the polaron hopping constant is established and it is shown that the core hopping constant depends on the lattice size. However, the property of a lattice with translational invariance is recovered when the size of the nanowire is greater than a critical value. Finally, it is pointed out that these features yield the occurrence of high energy localized states in which both the nature and the number are summarized in a phase diagram in terms of the relevant parameters of the problem (small polaron binding energy, temperature, lattice size).
A density functional study of NO adsorption and decomposition on Ni(211) and Pd(211) surfaces
Authors: Orita Hideo , Nakamura Isao , Fujitani Tadahiro
Published: January 2005

The adsorption and decomposition of NO have been investigated by using density functional theory method at the generalized gradient approximation level. We have performed calculations on adsorption energies and structures of NO on Ni(211) and Pd(211) surfaces with full-geometry optimization and compared them with the experimental data. The most favorite adsorption on both surfaces occurs at the bridge site parallel to step edge (sb), while the energy difference from the second favorite site of a threefold hollow site near step edge is less than 0.1 eV. Decomposition pathways have been investigated with transition state search. The decomposition pathway, where NO leans toward the step, is most probable for both surfaces. The overall activation energy for decomposition is 0.39 and 1.26 eV for Ni(211) and Pd(211), respectively. The present results clearly show that the NO molecules on Pd(211) are less activated than those on Ni(211). We have studied also reorganization of NO on Pd(211) at higher coverages up to 1/3 ML (monolayer) [three NO molecules in a (3×1) unit cell]. The site occupation is not in a sequential manner as the NO coverage is increased, and a reorganization of NO adsorbates occurs (the NO molecule at sb becomes tilting up at higher coverage), which can interpret the experimental data of Yates and co-workers very well.