bluuebunny/crossref_metadata_2025 huggingface dataset:
Lattice model for polymer propagation in confined media
Authors: Tribet C. , Provata A. , Nicolis G.
Published: July 2002

Self-avoiding walks in 2-d and 3-d lattices are implemented as models of polymer propagation in confined media like vesicle bilayers and micelles. Simulations conducted on such microreactors of various sizes (total number of monomers Ntot between 80 and 106) and shapes show that the propagation rate essentially depends on the ratio of the degree of polymerization (D.P.) and Ntot. A scaling function is proposed giving excellent agreement with the simulations: it features a decrease of the rate of propagation arising from a direct boundary effect, related to the contact between the radical and the impermeable boundaries of the lattice, and from an exponential factor, accounting for the confinement effect. Taking into account this rate dependence on the D.P., length and mass distributions are shown to be increasing functions of the D.P. when the rate of termination (by transfer) is small enough. In this limit, these distributions present a sharp increase in the vicinity of a D.P. about Ntot. This accumulation of polymers at a D.P. close to Ntot entails a significant decrease of the polydispersity.
Reorientational dynamics in supercooled m-tricresyl phosphate: Its relation to main and secondary relaxation—31P nuclear magnetic resonance study of relaxation, line shape, and stimulated echo
Authors: Rössler Ernst , Eiermann Peter
Published: July 2002

Supercooled m-tricresyl phosphate has been studied by different nuclear magnetic resonance (NMR) methods using the 31P spin probe (180–400 K). Combining measurements of the spin-lattice relaxation at two Larmor frequencies, of the line shape and of the stimulated echo decay, a reorientational correlation function F2(t) is probed over the entire supercooled regime. The analysis of the stimulated echo is described in detail. The long-time behavior of F2(t) (α process) does not change its shape below 260 K and is approximated by a stretched exponential, however, some deviations are observed at short times. Above 260 K a decrease of the stretching of F2(T) is found. Reorientational correlation times τrot are reported in the range 10−11 s <τrot<10 s. The times scale τrot and the one of viscous flow separate only weakly upon supercooling, if at all, whereas translational diffusion and viscous flow clearly decouple below 270 K (∼1.26 Tg). These findings are taken as an indication of a change of transport mechanism in the supercooled liquid. For T≳Tg a change of spectral density is revealed by the relaxation experiments, and it is attributed to the short-time behavior of F2(t), i.e., to the β process and to peculiarities of the α process itself. The β process has a small amplitude which decreases upon cooling.
Adsorption energies of NH3 and NH+4 in zeolites corrected for the long-range electrostatic potential of the crystal
Authors: Teunissen E. H. , Jansen A. P. J. , van Santen R. A. , Orlando R. , Dovesi R.
Published: July 2002

The calculation of adsorption energies in zeolites in the cluster approximation has the advantage of a reduced computational effort compared to that of the periodic approach. However, in the cluster approximation, the long-range electrostatic effects of the infinite crystal are ignored and there are boundary effects. In order to remove a part of the disadvantages of the cluster approach, we developed a method to embed a cluster in a zeolite crystal by imposing an electrostatic potential on it. This potential adds the long-range electrostatic effects of the crystal and subtracts the electrostatic potential of the boundary of the cluster. It is calculated from the charge distribution of the crystal obtained with an ab initio calculation using the crystal program. We calculated the adsorption energies of NH3 and NH+4 on three different clusters embedded in the potential of a chabazite crystal, and we have compared them with the adsorption energies of NH3 and NH+4 in the crystal. If a cluster is used that has dangling bonds four, or more, bonds away from the adsorbate, i.e., the adsorption site is described well covalently, embedding the cluster reduces the error in adsorption energies (relative to the crystal calculations) from ≊20 to ≊2–3 kJ/mol.
Molecular dynamics simulations of reactions on metal surfaces
Authors: Sellers Harrell
Published: July 2002

We present a molecular dynamics method for the simulation of reactions on metal surfaces. The method employs reactive potentials obtained from our modified version of the bond order conservation method of Shustorovich. Although we have used these potentials in solving classical equations of motion, quantum mechanical calculations could also employ them. Example calculations in four and six dimensions are given for the dissociation of H2 over Ni(111) and H–SCH3 over Au(111). It is demonstrated that the calculated quantities representing reaction rate constants show the correct Arrhenius behavior. We show that the coupling of the reaction coordinate to ‘‘rotational’’ degrees of freedom contains important effects in agreement with existing work in the literature.
Fully quantum study of the 3D diffractive scattering of He from isolated CO adsorbates on Pt(111)
Authors: Carré Marie-Noëlle , Lemoine Didier
Published: July 2002

We present a three-dimensional, fully quantum study of the diffractive scattering of He from isolated CO adsorbates on Pt(111). The nonspecular scattered He distributions are dominated by quantum interference effects such as Fraunhofer diffraction and rainbows. The determination of the final scattering distributions hence requires an exact treatment of the translational motion of the helium atom. Our wave packet simulations are free of dynamical approximations in the gas phase and provide the whole diffractive scattering distributions. One important motivation of this work was to assess the adequacy of a reduced dimensionality treatment to derive the geometrical parameters of the adsorption. It is found that, even if the interaction is characterized by azimuthal symmetry, a two-dimensional Fourier treatment is not sufficient to give reasonable estimates of both the size of the adsorbate and its position above the surface. We further investigate the scattering distributions as a function of the collision energy and discuss the appropriateness of a pure Fraunhofer model to interpret the results and derive geometrical parameters.
Modeling of the optical properties of multicomponent films on small isotropic spheres
Authors: Xu Muliang , Mamiche-Afara Suzane , Dignam Michael J.
Published: July 2002

The model developed recently by us for calculating the optical properties of a small isotropic sphere, with a radially uniaxial coating of molecules, is extended here to include multicomponent coatings in an approximation in which the molecules, treated as point polarizable, are assumed to be randomly distributed over spherical shells of close to the same radius. Calculations are performed to determine the effect of coadsorption of Xe on the infrared absorption properties of CO2 physisorbed on silica particles with the molecular axes oriented radially. The calculations confirm an earlier suggestion by one of us that the effect of coadsorption of an ‘‘inert molecule,’’ such as Xe, on the infrared absorption spectrum of an adsorbed species can be used to provide information on the orientation of the species with respect to the local surface normal.
Water above its boiling point: Study of the temperature and density dependence of the partial pair correlation functions. I. Neutron diffraction experiment
Authors: Postorino P. , Ricci M. A. , Soper A. K.
Published: July 2002

Neutron diffraction data on water, employing the technique of hydrogen/deuterium isotope substitution, are reported at three thermodynamic states above the boiling point. The structural information is analyzed in terms of the partial radial distribution functions, OO, OH, and HH, which are extracted from the neutron data. It is found that temperature affects mainly the medium and longer range order in the liquid, while density plays a significant role in controlling the degree of hydrogen bonding. To understand the structure of water obtained from these data it appears that many-body cooperative interactions have to be correctly accounted for.
Thermal and photo-induced dissociation of (C2H5)2Zn to yield C2H5 on the Pd(100) surface
Authors: Kovács Imre , Iost Norbert , Solymosi Frigyes
Published: July 2002

The adsorption and dissociation of (C2H5)2Zn was studied on Pd(100) surface. The methods used included photoelectron spectroscopy (XPS, UPS), electron energy loss spectroscopy (EELS) in the electronic range, temperature-programmed desorption (TPD), and work function measurements. Diethyl zinc adsorbs predominantly irreversibly on Pd surface. At submonolayer it dissociates even at 90 K. Thermal dissociation of adsorbed monolayer occurs at 170–230 K. Illumination of molecularly adsorbed layer at 90–95 K enhances the extent of the dissociation. The primary products of the thermal and photodissociation, C2H5 and Zn, were characterized by a C(1s) binding of 284.0 eV and by Zn(2p3/2) binding energy of 1021.0 eV. The C2H5 reacted to form C2H4 and C2H6. Coupling reactions to C4H10 and C4H8 were also observed. On the basis of He i, He ii, and EELS data, energy levels of electron orbitals for (C2H5)2Zn–Pd(100) system have been calculated.
Potential energy curves of ground and excited states of tetra halomethanes and the negative ions
Authors: Roszak S. , Kaufman J. J. , Koski W. S. , Vijayakumar M. , Balasubramanian K.
Published: July 2002

A large scale multireference configuration interaction study (involving up to 2.3 million configurations) of potential energy curves for ground and excited states for dissociative electron attachment are presented for CCl4/CCl−4, CBr4/CBr−4, and CI4/CI−4 systems. The spin–orbit coupling in CI4/CI−4 is also calculated using the relativistic configuration interaction approach. The results agree reasonably well with the available experimental data for electron affinities, giving a confidence in the overall quality of potential surfaces. The study leads to a better understanding of the reaction CX4+e→[CX4]−→CX3+X−, which is believed to be an important step in biological processes involving halocarbons.
Molecular desorption of methyl halides from GaAs(110): The role of lateral dipole–dipole interaction between adsorbates
Authors: Lu Ping-He , Lasky P. J. , Yang Qing-Yun , Wang Yubo , Osgood R. M.
Published: July 2002

Temperature programmed desorption (TPD) spectra of CH3X (X=Cl, Br, I) from GaAs(110) were recorded with a heating rate of 5 K/s for coverages from less than 0.1 to 2 ML, where methyl halides desorb molecularly, i.e., without dissociation. The shapes of the TPD spectra are strongly coverage dependent with the peak temperature of desorption decreasing with increasing coverage, especially for the submonolayer region. A model incorporating dipole–dipole repulsive interactions between the adsorbed molecules and which was previously proposed for the desorption of adsorbed molecules from metal surfaces has been employed to fit our TPD spectra. The fit gives the activation energy, effective dipole moment, and effective polarizability for the adsorbate. The resulting dipole moment and polarizability are less than the gas-phase molecular values, suggesting the importance of an ordered adsorption orientation.
Simulated annealing using the classical density distribution
Authors: Ma Jianpeng , Straub John E.
Published: July 2002

Three algorithms for global energy minimization based on the simulated annealing of the classical density distribution are presented. The first is based on annealing the classical density distribution directly in temperature and is the classical analog of imaginary time quantum dynamics. Another two algorithms are based on the approximate solution of the classical Liouville equation for the dynamics of a system coupled to a heat bath using a rigid temperature constraint and Fokker–Planck dynamics. These three methods are compared with standard simulated annealing based on molecular dynamics. The results for a series of Lennard-Jones clusters demonstrate that by annealing the continuous density distribution (representing a volume of phase points) the likelihood of finding the global minimum is dramatically enhanced.
Transport properties of semiconducting ternary vanadate glasses
Authors: Ghosh A.
Published: July 2002

Measurements are reported for the electrical dc conductivity of the semiconducting vanadium tellurite glasses containing P2O5, Sb2O3, or Bi2O3 in the temperature range 80–450 K. The experimental data have been analyzed in the light of existing theoretical models. It has been observed that the general behavior of the electrical conductivity is similar for all glass compositions. The high temperature conductivity data are consistent with Mott’s models of phonon-assisted polaronic hopping in the nonadiabatic regime, while at low temperatures variable range hopping is valid. The small polaron hopping model of Schnakenberg is the best model to interpret the temperature dependence of the activation energy and the conductivity in the entire temperature range. The percolation model applied to the polaron hopping regime is also found to be consistent with the temperature dependence of the conductivity data at high and low temperatures. The various parameters obtained by fitting these models to the experimental data are found to depend on glass forming oxides other than the vanadium oxide.
Atomic photofragmentv–jcorrelation: Dissociation of Cl2 at 355 nm
Authors: Wang Yifei , Loock Hans-Peter , Cao Jianying , Qian Charles X. W.
Published: July 2002

The photodissociation of Cl2 at 355 nm was studied by measuring the angular distribution of the chlorine fragments in their electronic ground state (2P3/2). Resonance-enhanced multiphoton ionization and time-of-flight (TOF) techniques were combined to obtain state-selective photofragment TOF profiles. It was observed that the shape of these photofragment TOF profiles depends sensitively on the polarizations of both the photolysis and the probe lasers. The dependence on the photolysis laser polarization reveals the angular distribution of the photofragments whereas the dependence on the probe laser polarization can be understood in terms of the atomic photofragment v–j correlation. Since the ground state Cl fragment has an electronic angular momentum greater than 1/2 ℏ, it can be aligned along the direction of fragment recoil. A theoretical treatment based on the adiabatic approximation is presented here to explain the observed polarization dependence of the photofragment TOF profiles.
Computer simulations of vapor–liquid phase equilibria of n-alkanes
Authors: Smit Berend , Karaborni Sami , Siepmann J. Ilja
Published: July 2002

For petrochemical applications knowledge of the critical properties of the n-alkanes is of interest even at temperatures where these molecules are thermally unstable. Computer simulations can determine the vapor–liquid coexistence curve of a large number of n-alkanes ranging from pentane (C5) through octatetracontane (C48). We have compared the predicted phase diagrams of various models with experimental data. Models which give nearly identical properties of liquid alkanes at standard conditions may have critical temperatures that differ by more than 100 K. A new n-alkane model has been developed by us that gives a good description of the phase behavior over a large temperature range. For modeling vapor–liquid coexistence a relatively simple united atom model was sufficient to obtain a very good agreement with experimental data; thus it appears not necessary to take the hydrogen atoms explicitly into account. The model developed in this work has been used to determine the critical properties of the long-chain alkanes for which experiments turned out to be difficult and contradictory. We found that for the long-chain alkanes (C8–C48) the critical density decreases as a function of the carbon number. These simulations were made possible by the use of a recently developed simulation technique, which is a combination of the Gibbs-ensemble technique and the configurational-bias Monte Carlo method. Compared with the conventional Gibbs-ensemble technique, this method is several orders of magnitude more efficient for pentane and up to a hundred orders of magnitude for octatetracontane. This recent development makes it possible to perform routinely phase equilibrium calculations of complex molecules.
A comparison of single reference methods for characterizing stationary points of excited state potential energy surfaces
Authors: Stanton John F. , Gauss Jürgen , Ishikawa Naoto , Head-Gordon Martin
Published: July 2002

The accuracy of geometries, vibrational frequencies and dipole moments of stationary points on excited state potential energy surfaces is assessed for three single reference excited state theories—configuration interaction (CIS), a perturbative doubles correlation correction to CIS, termed CIS(D), and equation-of-motion coupled cluster theory with single and double substitutions (EOM-CCSD). Two groups of systems are studied: the diatomic molecules H2, BH, BF, C2, CO, and N2; and the lowest singlet excited states of ammonia, formaldehyde and acetylene. The calculations demonstrate that CIS systematically underestimates bond lengths and overestimates frequencies and dipole moments, a pattern often associated with the Hartree–Fock method for ground states. CIS(D) fails to provide a systematic improvement to CIS for all geometries and frequencies, often overestimating correlation corrections. EOM-CCSD, by contrast, performs significantly better than CIS for all properties considered.
Percus–Yevick-like integral equation for random sequential addition
Authors: Boyer D. , Tarjus G. , Viot P. , Talbot J.
Published: July 2002

Random sequential addition is a process that generates nonequilibrium configurations of hard objects. The corresponding spatial pair correlations are investigated via a Percus–Yevick (PY)-like integral equation. Numerical solutions are obtained in one, two, and three dimensions. Comparison with exact results in one dimension and with Monte Carlo data in higher dimensions shows that the PY-like integral equation provides an accurate description of the structure, except close to the jamming limit, where the logarithmic divergence of the correlation function at contact is not reproduced. Using diagrammatic expansions, we show that in one dimension, contrary to its equilibrium counterpart, this equation is only exact up to the second order in density.
Far-infrared permanent and induced dipole absorption of diatomic molecules in rare-gas fluids. II. Application to the CO–Ar system
Authors: Roco J. M. M. , Medina A. , Hernández A. Calvo , Velasco S.
Published: July 2002

The general theory of far-infrared absorption of diatomic molecules in rare-gas fluids reported in Paper I is applied to the case of the CO–Ar system. The experimental absorption profiles of CO in Ar at three different densities (low-density gas, high-density gas, and liquid) are theoretically reproduced by considering the permanent dipole contribution and the four leading electrostatic multipole-induced dipole contributions. From these results an estimation of the quadrupole, ‖Θ‖, octupole, ‖Ω‖, and hexadecapole, ‖Φ‖, moments of CO is given.
Ab initio determination of the energy barriers and rate constants for HOBO+HF→FBO+H2O and OBBO+HF→FBO+HBO
Authors: Linder Douglas P. , Page Michael
Published: July 2002

We report ab initio calculations of features of the potential energy surfaces for two reactions of potential importance for the combustion of boron in fluorine-containing environments; HF+HOBO→H2O+FBO and HF+OBBO→FBO+HBO. Both reactions proceed through four-center transition states and yield the product FBO, a stable molecule that appears to play a similar role for B/F/O/H combustion as the isoelectronic CO2 does for hydrocarbon combustion. Multireference configuration interaction calculations with valence triple-zeta, double polarization basis sets yield energy barriers of 27.2 and 46.2 kcal/mol, respectively, for these two reactions. Transition state theory calculations based on the ab initio potential energy surface information yield the following three-parameter fits for the temperature-dependent rate coefficients. kHOBO(T)=1.436×102 T2.71 exp(−11 917/T) cm3/mol s and kOBBO(T)=4.819×104 T2.38 exp(−22 028/T) cm3/mol s. Both reactions are predicted to be orders of magnitude slower than the estimated rates used in recent combustion modeling efforts.
A variational centroid density procedure for the calculation of transmission coefficients for asymmetric barriers at low temperature
Authors: Messina Michael , Schenter Gregory K. , Garrett Bruce C.
Published: July 2002

The low temperature behavior of the centroid density method of Voth, Chandler, and Miller (VCM) [J. Chem. Phys. 91, 7749 (1989)] is investigated for tunneling through a one-dimensional barrier. We find that the bottleneck for a quantum activated process as defined by VCM does not correspond to the classical bottleneck for the case of an asymmetric barrier. If the centroid density is constrained to be at the classical bottleneck for an asymmetric barrier, the centroid density method can give transmission coefficients that are too large by as much as five orders of magnitude. We follow a variational procedure, as suggested by VCM, whereby the best transmission coefficient is found by varying the position of the centroid until the minimum value for this transmission coefficient is obtained. This is a procedure that is readily generalizable to multidimensional systems. We present calculations on several test systems which show that this variational procedure greatly enhances the accuracy of the centroid density method compared to when the centroid is constrained to be at the barrier top. Furthermore, the relation of this procedure to the low temperature periodic orbit or ‘‘instanton’’ approach is discussed.
On the Feynman path centroid density as a phase space distribution in quantum statistical mechanics
Authors: Hernandez Rigoberto , Cao Jianshu , Voth Gregory A.
Published: July 2002

The phase space formulation of quantum statistical mechanics using the Feynman path centroid density offers an alternative perspective to the standard Wigner prescription for the classical-like evaluation of equilibrium and/or dynamical quantities of statistical systems. The use of this formulation has been implicit in recent work on quantum rate theories, for example, in which the centroid density distribution replaces the classical Boltzmann distribution. In order to further understand the approximations involved in this and similar transcriptions, the present work elaborates and clarifies the issue of operator ordering in a rigorous centroid-based formulation. In particular, through the use of the Weyl correspondence, a precise definition of the centroid symbol of operators and their products is presented. Though we fall short of finding the algebraic structure tantamount to that found in the Weyl symbols—of which the Wigner distribution is an example— the resulting expressions have internal consistency and are amenable to approximate evaluation through cumulant expansions.
Competitive adsorption of xenon and argon in zeolite NaA. 129Xe nuclear magnetic resonance studies and grand canonical Monte Carlo simulations
Authors: Jameson Cynthia J. , Jameson A. Keith , Lim Hyung-Mi
Published: July 2002

Investigation of competitive adsorption is carried out using the Xe–Ar mixture in zeolite NaA as a model system. The Xen clusters are trapped in the alpha cages of this zeolite for times sufficiently long that it is possible to observe individual peaks in the NMR spectrum for each cluster while the Ar atoms are in fast exchange between the cages and also with the gas outside. The 129Xe nuclear magnetic resonance spectra of 12 samples of varying Xe and Ar loadings have been observed and analyzed to obtain the 129Xe chemical shifts and the intensities of the peaks which are dependent on the average argon and xenon occupancies. The detailed distributions, f(XenArm), the fractions of cages containing n Xe atoms and m Ar atoms cannot be observed directly in this system, that is, individual peaks for XenArm mixed clusters are not observed in the NMR spectrum. This information is, however, convoluted into the observed 129Xe chemical shifts for the Xen peaks and the distributions Pn, the fraction of cages containing n Xe atoms, regardless of the number of Ar atoms, obtained from their relative intensities. Grand canonical Monte Carlo (GCMC) simulations of mixtures of Xe and Ar in a rigid zeolite NaA lattice provide the detailed distributions and the average cluster shifts, as well as the distributions Pn. The agreement with experiment is reasonably good for all 12 samples. The calculated absolute chemical shifts for the Xen peaks in all samples at 300 K range from 75 to 270 ppm and are in good agreement with experiment. The GCMC results are compared with a strictly statistical model of a binary mixture, derived from the hypergeometric distribution, in which the component atoms are distinguishable but equivalent in competition for eight lattice sites per cage under mutual exclusion. The latter simple model introduced here provides a limiting case for the distributions, with which both the GCMC simulations and the properties of the actual Xe–Ar system are compared.
Mass-resolved multiphoton ionization spectroscopy of jet-cooled Cl2. I. Bound-free-bound spectroscopy
Authors: Al-Kahali Mohamed S. N. , Donovan Robert J. , Lawley Kenneth P. , Min Zhiyuan , Ridley Trevor
Published: July 2002

Spectroscopic constants, obtained using two-color optical double resonance via repulsive intermediate states, are presented for four ion-pair states of Cl2; i.e., the E(0+g), β(1g), f(0+g), and G(1g) states. One-color excitation, also via a repulsive intermediate state, has been used to further extend the vibrational data for the β(1g) state. The same pumping scheme has been used to extend a vibrational progression in the [2Π1/2]c4s; 1g Rydberg state. The absence of perturbations when the [2Π1/2]c4s; 1g Rydberg and the β(1g) ion-pair states cross, together with the key role played by the intermediate C(1u) state in accessing both singlet and triplet final states, are discussed in terms of the changes in spin–orbital coupling schemes that are required on bond stretching.
Small angle neutron scattering near the wetting transition: Discrimination of microemulsions from weakly structured mixtures
Authors: Gradzielski M. , Langevin D. , Sottmann T. , Strey R.
Published: July 2002

The wetting transitions of water, n-alkane, and n-alkyl polyglycol ether (CiEj) systems are examined in order to locate the transition between weakly structured mixtures and microemulsions. Using small angle neutron scattering (SANS) we determine the local structure and relate it to the phase behavior and wetting transitions observed by macroscopic measurements. We measure the SANS of the mixtures across the transition along two different experimental paths. One path begins with well-structured mixtures, and the effective chain length of the surfactant combination C6E2/C4E1 is decreased by increasing the C4E1 fraction. The other path starts with equal amounts of water and oil mixed by the strong amphiphile C8E3. The local structure of these ‘‘good’’ microemulsions is weakened by increasing the temperature and concomitantly the oil/water volume ratio approaching the upper critical endpoint. As in previous studies analyzing the scattering experiments quantitatively permits determination of the amphiphilicity factor which is a measure of the strength of the surfactant. We confirm predictions that the amphiphilicity factor measured at the wetting transition becomes more negative as the temperature interval between the transition and the critical endpoint decreases.
Static and dynamic structural analysis of a saturated solution of ZnBr2 in water: Anomalous x-ray diffraction and molecular dynamics simulations
Authors: Löffler G. , Mager Th. , Gerner Ch. , Schreiber H. , Bertagnolli H. , Steinhauser O.
Published: July 2002

The supposedly very simple system of a saturated solution of ZnBr2 in water exhibits unusually complex and therefore interesting structural behavior. Motivated by this, Mager did a detailed x-ray diffraction study (Th. Mager, PhD. thesis, Universität Würzburg, 1991), and we performed a long molecular dynamics (MD) simulation—using potential parameters from the general purpose GROMOS force field—of such a solution, which can be grossly characterized by the formula ZnBr2⋅3H2O. We start by calculating those properties that are directly accessible through the experiment from the MD simulation, in order to validate the physical relevance of the simulation. Seeing that the simulation delivers results that are compatible with those of the experiment, we proceed by analyzing the MD simulation in much more detail according to the static and dynamic structure of the system, thereby gaining insight into the structural behavior of ZnBr2⋅3H2O that is very difficult, if at all possible, to get from experimental studies. To this end we use the Voronoi algorithm to define coordination shells around atoms and ions in ZnBr2⋅3H2O. We study the time averaged as well as the time-resolved geometry and composition of these coordination shells and find that octahedral coordination of Zn2+ ions is the dominant geometry in ZnBr2⋅3H2O, and that these octahedra are remarkably stable (after 1 ns only 10% decayed). We further find evidence for polymerlike Zn2+ chains, where O atoms of water and Br− ions connect the Zn2+ ions.
The influence of solvent dynamics on the lifetime of solute–solvent hydrogen bonds
Authors: Benigno Andrea J. , Ahmed Ednan , Berg Mark
Published: July 2002

The lifetimes of the hydrogen bonds formed between resorufin and the solvents ethanol, ethylene glycol, and 1,3-butanediol are measured as a function of temperature. The results show that the hydrogen-bond breaking reaction is strongly influenced by the dynamics of the solvent, in violation of traditional transition-state theory. The relevant solvent dynamics are not well described by the viscosity of the solvent, but do correlate with the dielectric relaxation time. We propose a model in which the hydrogen bond initially breaks to form a poorly solvated, ‘‘dangling’’ hydrogen bond, which has a high probability of geminate recombination. The product is not stabilized until the solvent hydrogen-bond structure can reorganize to incorporate the ‘‘dangling’’ bond. The same reorganization determines the dielectric relaxation time.
Phase behavior of the Widom–Rowlinson mixture
Authors: Shew Chwen-Yang , Yethiraj Arun
Published: July 2002

The Widom–Rowlinson mixture is a two component fluid where like species do not interact and unlike species interact via a hard core repulsion. As the density is increased this fluid phase separates. Using semigrand ensemble simulations and finite size scaling we accurately locate the critical density at ρσ3=0.762±0.016 where ρ is the total number density and σ is the hard sphere diameter. This density is much higher than previous estimates for this model. We implement several integral equation theories, none of which are accurate for the critical point or coexistence curve.
Role of collective modes in vibrational polarizabilities and hyperpolarizabilities of polyacetylene and other quasilinear polymers
Authors: Kirtman Bernard , Champagne Benoît , André Jean-Marie
Published: July 2002

It is shown that the low frequency collective modes of quasilinear polymers can contribute to the vibrational polarizability and hyperpolarizability despite the fact that their infrared or Raman intensity vanishes in the infinite chain limit. The most significant of these modes are characterized and the dynamic vibrational nonlinear optical (NLO) properties are obtained, at the doubly harmonic Hartree–Fock level of approximation, from static field calculations on polyacetylene as well as polyyne, polysilane, and polyethylene. Comparison is made to a pair of unit cell modes, one Raman-active and the other infrared-active, that are expected to generate a large vibrational hyperpolarizability. The combined effect of the collective and unit cell vibrations is potentially of considerable importance for the enhancement of NLO properties. Further studies are suggested to exploit this potential.
A unified framework for quantum activated rate processes. I. General theory
Authors: Cao Jianshu , Voth Gregory A.
Published: July 2002

It is shown that several existing quantum reaction rate theories can be unified around a single mathematical framework. These theories include the high temperature parabolic barrier approximation, the instanton approach, and the path integral quantum transition state theory. In particular, it is shown that the quantum reactive flux can be approximated as a product of a simple frequency factor times the imaginary part of the barrier partition function. The latter term arises from the steepest descent solution to the partition function in the barrier region, while the prefactor can be interpreted as the frequency of the stationary barrier trajectory. Importantly, the analysis introduces the imaginary time Feynman path centroid variable into the expression for the quantum activated rate constant in an a priori way. The present theoretical framework also allows for an analytical treatment of the quantum activated rate problem in the vicinity of the crossover temperature, and for a generalization of the saddle point analysis in an extended-dimensional quantum mechanical space of the Fourier path modes.
Ab initiocomputation of semiempirical π-electron methods. V. Geometry dependence ofHν π-electron effective integrals
Authors: Martin Charles H. , Freed Karl F.
Published: July 2002

The ab initio effective valence shell Hamiltonian (ℋν) provides ab initio analogs of the correlated π-electron integrals which should appear in the traditional Pariser–Parr–Pople (PPP) semiempirical π-electron theory. In our continuing studies of the ab initio basis of an improved PPP theory, we examine the geometry dependence of the correlated ℋν π-electron effective integrals (also called parameters) for the linear polyenes, ethylene, the allyl radical, trans-butadiene, and hexatriene, and the cyclic polyenes, cyclobutadiene and benzene. We find particularly interesting features for each of the true π-electron parameters corresponding to the PPP αi, βi,j, and γi,j integrals. First, the one-electron, two-center resonance integrals βi,j differ from the so-called ‘‘theoretical’’ values by roughly a constant shift of 0.3–0.4 eV for nearest neighbors i and j and not at all for more distant neighbors. Second, the correlated αi parameters conform to the standard point charge model fairly well, except the slopes and intercepts lack the transferability typically ascribed to them. A more accurate PPP model therefore must model the one-center, one-electron interactions more carefully. Finally, the effective Coloumb interactions γi,j follow the standard Mataga–Nishimoto distance dependence quite well for the linear polyenes, although there is a small breakdown of transferability due to long range correlation effects. For instance, the hexatriene γ1,2 is 0.5 eV smaller than the ethylene γ1,2 even when the C1=C2 bond lengths are identical. Additionally, the set of γi,j for the cyclic polyenes is not even a single function of Ri,j, a feature reflecting the subtle contributions of electron correlation to the ab initio γi,j. However, plots of γ−1i,j vs Ri,j display some unforeseen regularity which may prove useful in improving current semiempirical models for cyclic polyenes.
Variance reduced simulations of polymer dynamics
Authors: Melchior Martin , Öttinger Hans Christian
Published: July 2002

Stochastic simulation techniques play an important role in investigating polymer dynamics. Such simulations can be made more efficient by applying variance reduction methods. We present and compare various variance reduction schemes, based either on importance sampling strategies or on the use of control variates. In order to demonstrate that variance reduced simulations are a powerful tool in polymer kinetic theory, we show that for Hookean dumbbells with hydrodynamic interaction up to two orders in computer time can be saved uniformly in time.
An advanced continuum medium model for treating solvation effects: Nonlocal electrostatics with a cavity
Authors: Basilevsky M. V. , Parsons D. F.
Published: July 2002

The Born–Kirkwood–Onsager (BKO) model of solvation, where a solute molecule is positioned inside a cavity cut into a solvent, which is considered as a dielectric continuum, is studied within the bounds of nonlocal electrostatics. The nonlocal cavity model is explicitly formulated and the corresponding nonlocal Poisson equation is reduced to an integral equation describing the behavior of the charge density induced in the medium. It is found that the presence of a cavity does not create singularities in the total electrostatic potential and its normal derivatives. Such singularities appear only in the local limit and are completely dissipated by nonlocal effects. The Born case of a spherical cavity with a point charge at its centre is investigated in detail. The corresponding one-dimensional integral Poisson equation is solved numerically and values for the solvation energy are determined. Several tests of this approach are presented: (a) We show that our integral equation reduces in the local limit to the chief equation of the local BKO theory. (b) We provide certain approximations which enable us to obtain the solution corresponding to the preceding nonlocal treatment of Dogonadze and Kornyshev (DK). (c) We make a comparison with the results of molecular solvation theory (mean spherical approximation), as applied to the calculation of solvation energies of spherical ions.
The hydroperoxyl anion HO−2: Ab initio potential energy surface and vibrational splittings for proton transfer
Authors: Chan Wai-To , Hamilton I. P.
Published: July 2002

We calculate over 1500 ab initio points for the HO−2 ground state at the QCISD(T)/6-311++G(2df,pd) level for a wide range of geometries. We fit these points to a 120-parameter analytic function to obtain a potential energy surface (PES) valid for large amplitude hydrogen motion. We then calculate and assign vibrational energy levels for this PES. There is intramolecular proton transfer when the hydrogen atom tunnels through a T-shaped transition state separating the two equivalent equilibrium geometries. The O–O distances are very different for the equilibrium and transition state geometries and the proton transfer is truly multidimensional. When there is nuclear tunneling the vibrational levels are split and we focus on levels with significant splitting. The barrier to proton transfer is 6058 cm−1 (4752 cm−1 with zero-point correction). Significant splittings are observed for relatively low-lying vibrational levels that may be experimentally accessible. Results are presented for H–16O–16O and H–17O–17O. For H–16O–16O the first three levels with splitting greater than 10 cm−1 are, in order of increasing energy, (0,2,3), (0,1,5), and (0,3,2) where ν2 is the O–O–H bend quantum number and ν3 is the O–O stretch quantum number. The (0,4,0) level lies between the (0,2,3) and (0,1,5) levels but the splitting is only 6 cm−1 showing that although H–O–O bend excitation is essential, O–O stretch excitation greatly facilitates proton transfer. This is not because it permits tunneling at a smaller O–O distance than that for the equilibrium geometry. Rather, it is because it permits tunneling at the larger O–O distance corresponding to the transition state geometry.
Order fluctuations in the mesophase of polydiethylsiloxane as studied by the dipolar-correlation effect on the stimulated echo
Authors: Grinberg F. , Kimmich R. , Möller M. , Molenberg A.
Published: July 2002

Order fluctuations in polydiethylsiloxane (PDES) mesophase are studied using the dipolar-correlation effect on the stimulated echo. The attenuation of the stimulated echo is described on the basis of a two-site slow exchange model. The conclusion is that collective chain fluctuations result in temporarily and locally ordered and temporarily and locally defect-enriched states. The local order parameter is inhomogeneously distributed in the samples as indicated by the absence of any oscillatory modulation of the dipolar-correlation effect. The mean exchange time between the two mobility states varies in the temperature range of the mesophase between 0.1 and 1 s. It turned out to be an exponential function of the reduced temperature relative to the isotropization temperature. Apart from this implicit function no explicit dependence on the molecular weight could be detected. This suggests that the exchange phenomenon and the type of the order fluctuations are features universal for the PDES mesophase.
Inelastic collision dynamics of vibrationally excited I2(X)
Authors: Lawrence W. G. , Van Marter T. A. , Nowlin M. L. , Heaven M. C.
Published: July 2002

Rotational and vibrational energy transfer rate constants have been measured for excited rovibrational levels of I2(X). Stimulated emission pumping was used to excite the levels v=23, J=57, and v=38, J=49 via the B–X transition. Laser induced fluorescence from the D–X system was used to follow the collision dynamics. Energy transfer processes induced by collisions with He,Ar,N2,O2,Cl2,I2, and H2O were investigated. Rotational energy transfer was found to be efficient for all collision partners. In accordance with classical models, the total rotational transfer rate constants were proportional to the collision momentum (except for H2O). The total transfer rate constants and the distributions of rotational levels populated by collisions were not dependent on the initial vibrational state. For colliders that are not good quenchers of I2(B), the rotational energy transfer dynamics of the X and B states were found to be very similar. For colliders that are good quenchers, comparisons of the X and B state dynamics show that quenching competes with rotational energy transfer in the B state. Vibrational energy transfer was characterized for all collision partners with the exception of I2, which appears to have a low vibrational transfer efficiency. Vibrational transfer was dominated by Δv=−1 steps. Multiquantum vibrational transfer was not observed. The dependence of the vibrational transfer rate constants on the initial vibrational state appeared to be weaker than the linear scaling predicted by the Landau–Teller model. Vibrational deactivation of I2(X) plays an important role in chemically driven oxygen–iodine lasers. Effective deactivation rate constants have been derived from the vibrational transfer rate constants. Estimates for the deactivation rate constants for O2 and H2O differ from those currently in use by almost an order of magnitude.
Phase coexistence properties of polarizable Stockmayer fluids
Authors: Kiyohara Kenji , Gubbins Keith E. , Panagiotopoulos Athanassios Z.
Published: July 2002

We report the phase coexistence properties of polarizable Stockmayer fluids of reduced permanent dipoles |m0*|= 1.0 and 2.0 and reduced polarizabilities α*= 0.00, 0.03, and 0.06, calculated by a series of grand canonical Monte Carlo simulations with the histogram reweighting method. In the histogram reweighting method, the distributions of density and energy calculated in Grand Canonical Monte Carlo simulations are stored in histograms and analyzed to construct the grand canonical partition function of the system. All thermodynamic properties are calculated from the grand partition function. The results are compared with Wertheim’s renormalization perturbation theory. Deviations between theory and simulation results for the coexistence envelope are near 2% for the lower dipole moment and 10% for the higher dipole moment we studied.
Dynamics of hydrogen atom abstraction in the O−+CH4 reaction: Product energy disposal and angular distributions
Authors: Carpenter M. A. , Farrar J. M.
Published: July 2002

Energy and angular distributions for the hydrogen abstraction reaction O−+CH4→OH−+CH3, exothermic by 0.26 eV, and a prototype ionic pathway for methane oxidation in hydrocarbon flames have been studied in a crossed molecular beam experiment at collision energies of 0.34, 0.44, and 0.64 eV. At the two lower collision energies, two mechanisms contribute to the differential cross section: In the first, low impact parameter rebound collisions form sharply backward-scattered products, while in the second, larger impact parameter collisions produce a broad distribution of forward scattered products. We suggest that the first group of products is formed by collisions with hydrogen atoms oriented essentially along the relative velocity vector and proceeding through a near-collinear O⋯H⋯CH3 geometry, while the second group corresponds to collisions with one of the three off-axis hydrogens. The products are formed on average with 65% of the total available energy in product internal excitation. The product kinetic energy distribution shows structure that correlates with excitation of the ν2 umbrella bending mode of CH3. At the highest collision energy, the product angular distribution shifts entirely to the forward direction, suggesting that the low impact parameter collisions are no longer important in the reactive process. At this energy, the average product internal excitation corresponds to 59% of the total available energy. The data suggest that the majority of product internal excitation resides in the ν2 umbrella bending mode of CH3, with OH in its ground vibrational state.
Dissociative adsorption of H2 on Cu(100): A four-dimensional study of the effect of rotational motion on the reaction dynamics
Authors: Mowrey R. C. , Kroes G. J. , Wiesenekker G. , Baerends E. J.
Published: July 2002

The reaction of H2 on Cu(100) is investigated using a four-dimensional (4D) quantum dynamical fixed-site model to assess the influence of molecular rotation on dissociation over the most reactive (the bridge) site. The potential energy surface (PES) is a fit to the results of density functional calculations performed using a generalized gradient approximation treating a Cu slab with a periodic overlayer of H2. Dissociation probabilities for molecules with “helicoptering’’ (mj=j) and “cartwheeling’’ (mj=0) rotational motions are here found to be comparable because of the strong corrugation in the azimuthal coordinate. The calculations indicate that reaction is accompanied by significant rotationally inelastic scattering. Surprisingly, vibrational excitation is also found to be an efficient process in collisions with the reactive bridge site. In these collisions, the molecular axis is tilted away from the orientation parallel from the surface. Considering the approximate nature of the 4D model used, the calculated reaction probabilities are in good agreement with experiment, indicating that the PES that was used is accurate.
Symmetry-adapted perturbation theory of three-body nonadditivity in Ar trimer
Authors: Lotrich Victor F. , Szalewicz Krzysztof
Published: July 2002

The symmetry-adapted perturbation theory has been used to compute the three-body nonadditivity of Ar3 interaction. Several components which had not been considered before have been taken into account. A complete nonadditive potential surface has been fitted to 36 computed points. It is found that all of the dispersion components considered, including the famous Axilrod–Teller–Muto term, are significantly damped at short to intermediate interatomic separations. Functional forms describing this damping are obtained from fits to computed values. It is also seen that the leading intramonomer correlation correction to the dispersion energy cannot be ignored as it typically provides about 15% contribution to this energy. The fourth-order dispersion energy is similarly important at intermediate distances. Both components have not been considered before and appear to be necessary for accurate description of Ar3 interaction. The nonadditivity of the more complicated exchange energies has been examined through second-order including an analysis of the expansion in powers of the interatomic overlap integrals.