bluuebunny/crossref_metadata_2025 huggingface dataset:
The theory of semipermeable vesicles and membranes: An integral-equation approach. I. General formalism and application to a hard-sphere mixture
Authors: Zhou Yaoqi , Stell George
Published: July 2002

An exact integral-equation formalism for a system of a binary hard-sphere mixture interacting with a spherical semipermeable vesicle (SPV) and plane semipermeable membrane (SPM) is derived by using the Ornstein–Zernike (OZ) equation with appropriate closures. The Percus–Yevick (PY) closure or the hypernetted chain (HNC) closure, in which the bulk correlation is obtained by the PY approximation, are considered as examples. We refer to these as the PY/PY and HNC/PY approximations, respectively. The mixture contains solvent particles, which are permeable to the membrane, and solute particles (‘‘protein’’ or ‘‘polymer’’ particles), which can not pass through the membrane. We develop an exact general formalism for this problem and as an illustration of its use give quantative results for solvent and solute particles modeled as hard spheres of different diameters. An analytical expression for the density ratio in the PY/PY and HNC/PY approximations between two sides of a plane SPM is obtained. Results obtained from these expressions agree very well with results obtained by equating chemical potentials in the region of interest. It turns out that the protein–membrane direct correlation function can be given by a simple analytic expression for the limit of a point solvent in the PY/PY approximation. The osmotic pressure and density profiles for the system containing an ideal spherical SPV or plane SPM in the PY/PY approximation are evaluated. Extension to the nonlocal density-functional closures previously introduced by Blum and Stell is discussed. Finally, we note that certain impenetrable-wall problems previously considered elsewhere can be regarded as semipermeable membrane problems treated via McMillan–Mayer formalism in the continuum–solvent approximation.
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o analytic polarizability, first and second hyperpolarizabilities of large conjugated organic molecules: Applications to polyenes C4H6 to C22H24
Authors: Hurst Graham J. B. , Dupuis Michel , Clementi Enrico
Published: July 2002

The static dipole polarizability and second hyperpolarizability tensors are calculated for polyene systems via ab initio coupled-perturbed Hartree–Fock theory. The effect of basis set augmentation on the calculated properties is explored for C4H6 and example basis sets are used to calculate the polarizability and second hyperpolarizability for the longer polyenes: C6H8, C8H10, C10H12, C12H14, C14H16,C16H18, C18H20, C20H22, C22H24. Results for the finite polyenes are extrapolated to predict the unit-cell polarizability and second hyperpolarizability of infinite polyacetylene. The working equations which take advantage of the 2n+1 theorem of perturbation theory for calculating up to the second hyperpolarizability are given, and their implementation is briefly discussed. In particular it is shown that the implementation is readily amenable to parallel processing.
An extended x-ray absorption fine structure study of aqueous rare earth perchlorate solutions in liquid and glassy states
Authors: Yamaguchi Toshio , Nomura Masaharu , Wakita Hisanobu , Ohtaki Hitoshi
Published: July 2002

Extended x-ray absorption fine structure (EXAFS) measurements were performed for concentrated aqueous rare earth perchlorate solutions (R=28; R is the moles of water per mole of salt) in the liquid state at room temperature and in the glassy state at liquid nitrogen temperature. The quantitative analysis of the EXAFS data has revealed that the hydration number changes from about nine for light rare earth ions to about eight for heavy rare earth ions through the intermediate ions of Sm3+ ∼Eu3+ in both liquid and glassy states. The average Ln3+ –OH2 distances were determined and they are in agreement with previously reported values from x-ray and neutron diffraction. The Debye–Waller factor of the average Ln3+ –OH2 bonds for the light rare earth ions was larger than that for the heavy ions, suggesting that the hydration shell of the light rare earth ions is statically disordered, consisting of different Ln3+ –OH2 bonds.
Background and bridge functions for the homonuclear hard diatomic fluid
Authors: Lomba E. , Lombardero M. , Abascal J. L. F.
Published: July 2002

The Monte Carlo method has been used to compute the coefficients of the spherical harmonic expansion of the function y=g exp(βu) for a hard diatomic fluid. The ‘‘series’’ function S(12) is also computed from MC data by means of an integral equation procedure. Thus, the Bklm(r) terms of the harmonic series of the bridge function B(12) can be easily obtained. The spherical harmonic expansion has proved an efficient tool to deal with these angular functions since the series is very rapidly convergent. We also have investigated the Percus–Yevick approximation both for the S(12) and the B(12), and a remarkable qualitative agreement with our MC data is found.
Absolute infrared transition moments for open shell diatomics from J dependence of transition intensities: Application to OH
Authors: Nelson David D. , Schiffman Aram , Nesbitt David J. , Yaron David J.
Published: July 2002

A general approach to the determination of the dipole moment function and of the absolute vibrational transition moments for diatomic molecules is presented. This method utilizes the variation of intensity with J within a vibrational transition, together with permanent dipole moment information, to extract the absolute transition moments. An essential feature of the model is its use of algebraic expressions for calculating vibration–rotation line intensities. These expressions can be rapidly evaluated in a least squares fit which determines the dipole moment function. This approach is general in that it is not limited to 1Σ state molecules, nor to the simplest of Hund’s case couplings of spin, orbital and mechanical angular momentum. It is also not limited to molecules with essentially linear dipole moment functions. The model is successfully applied to the OH molecule which violates each of these restrictions. In the accompanying work we report experimental measurements of relative infrared absorption intensity measurements for OH v=1←0 transitions and the extraction of an experimental μ(r) using the approach presented here.
The bonding of acetonitrile and CH2CN on Ag(110) determined by near edge x-ray absorption fine structure: Evidence for π-donor bonding and azimuthal ordering
Authors: Stevens P. A. , Madix R. J. , Stöhr J.
Published: July 2002

Carbon K edge near edge x-ray absorption fine structure (NEXAFS) spectra of adsorbed acetonitrile and reactive intermediates derived therefrom on Ag(110) were studied in order to determine their orientation and bonding to the surface. For both the multilayer and the adsorbed monolayer there is excellent agreement between the spectra of the adsorbed acetonitrile and the gas phase species. For the multilayer coverage the orientation of the acetonitrile molecules is not completely random; the molecules prefer an orientation with their bond axes closer to parallel than perpendicular to the surface. At monolayer coverage acetonitrile bound to the surface is unrehybridized. It is bound with its linear molecular skeleton parallel to the surface and randomly oriented azimuthally. Annealing molecular CH3 CN to 325 K in the presence of coadsorbed atomic oxygen produces a linear CH2 CN surface intermediate, the molecular axis of which is parallel to the surface and perpendicular to the close-packed direction. The C–C and C–N bond lengths are essentially unperturbed from their values in gas phase CH3 CN. Bonding appears to occur between the methylene carbon and the metal, as expected. A broadening of the π* resonance for transition into the π* orbital perpendicular to the surface is interpreted in terms of π-donor bonding.
Fracture of polymer chains in extensional flow: Experiments with DNA, and a molecular-dynamics simulation
Authors: Reese Howard R. , Zimm Bruno H.
Published: July 2002

The behaviors of polymer chains in simple shear flow and in flows with a large component of extension are now considered to be qualitatively different [R. B. Bird, C. F. Curtiss, R. C. Armstrong, and O. Hassager, Dynamics of Polymeric Liquids, 2nd ed. (Wiley, New York, 1987), Vol. 2, Chap. 13]. To examine the extensional case, we have fractured DNA molecules in solution in steady sink (primarily extensional) flow. DNA was chosen for this study because it can be obtained as monodisperse, unique-sequence material, and because the size distribution of the fracture fragments can be obtained by gel electrophoresis. Dilute monodisperse T7 DNA (Mw=26×106) solutions were recirculated through a 0.13 mm orifice in a flat plate. The flow field upstream of the orifice closely approximated an ideal sink flow, being free of vortices under the conditions used, DNA concentrations less than 12.5 μg/mℓ and flow rates less than 0.045 cm3/s. The kinetics of the fracture at low flow rates showed an initial lag period followed by a period of first-order rate; the lag period disappeared at higher flow rates while the first-order period persisted. The fracture rate increased exponentially with the flow rate. Contrary to the classic theory by Frenkel, our experimental results showed fracture products broadly distributed in size; this anomaly was explained by a bead-spring molecular-dynamics computer simulation. The simulation showed that just prior to chain fracture the chain was aligned parallel to the flow but contained many folds, so that points of maximum stress were not usually at the molecular center. These results suggest that the residence time in the converging flow was too short for the chain to reach complete extension.
On the internal energy at lattice polymer interfaces
Authors: Madden William G.
Published: July 2002

A recent lattice theory for polymer–solvent interfaces by Szleifer and Widom contains two independent energy scales. This feature is shared by several other lattice theories in the literature. It is shown that the nearest-neighbor lattice model for polymers requires that microstates be weighted only by the exchange energy Δε. All polymer–monomer interfaces on the lattice, including the melt-vacuum interface, can be made rigorously isomorphic by an appropriate rescaling of temperature. The Szleifer–Widom theory, like most theories for the interfaces of lattice polymer and solvent, contains a Flory–Huggins term for the local contribution to the spatially varying energy. A modification which incorporates more accurate expressions for the local contribution to the energy is suggested. Direct comparisons with the results of computer simulation shows that the revision offers a substantial improvement over theories with a Flory–Huggins–like local term. However, some discrepancies remain, and it is unclear whether they arise from the remaining inadequacies in the local term or from uniquely polymeric nonlocal contributions.
Perturbative approaches to highly excited molecular vibrations of H2O, D2O, and HDO
Authors: McCoy Anne B. , Silbert Edwin L.
Published: July 2002

Molecular vibrations of water are studied using Van Vleck perturbation theory. In these calculations, the OH stretches are expressed in terms of the Morse coordinate, yi =[1−exp(−αri)]/α, and its conjugate momentum, while the bend is treated in a traditional manner. Nearly degenerate perturbative calculations are presented for bond and normal-mode representations. The eigenvalues agree well with those obtained variationally. The coefficients of the perturbatively derived, effective Hamiltonians are compared to those of spectroscopic Hamiltonians with similar structural forms. The former are derived from the potential of Halonen and Carrington [J. Chem. Phys. 88, 4171 (1988)], whereas the latter are treated as spectral fitting parameters. The fit parameters are accurately reproduced.
Nonhydrodynamic molecular motions in a complex liquid: Temperature dependent dynamics in pentylcyanobiphenyl
Authors: Deeg F. W. , Greenfield S. R. , Stankus John J. , Newell Vincent J. , Fayer M. D.
Published: July 2002

The isotropic phase of nematogenic substances above the nematic–isotropic phase transition temperature Tni is characterized by short range orientational correlation of the molecules and the existence of pseudonematic domains. This microscopic structure has a strong influence on reorientational dynamics in the liquid. We present time resolved transient grating optical Kerr effect measurements of pentylcyanobiphenyl between 35 and 120 °C, which allow us to monitor dynamics on timescales from subpicoseconds to tens of nanoseconds. The well-known collective component characterizing the reorientational dynamics of the pseudonematic domains follows Landau–deGennes behavior up to 30–35 degrees above Tni where the theoretical correlation length ξ≊three molecular lengths. Additionally fast relaxation channels (with time constants between 1 and 300 ps) are observed which are associated with local reorientational and intermolecular dynamics within the pseudonematic domains. The local dynamics are temperature independent in the range where the pseudonematic domains dominate the liquid structure, demonstrating a decoupling of the molecular motions from the bulk viscosity. Only at high temperatures, T≳70 °C, where the pseudonematic domains cease to exist, is a viscosity dependence observed. At these elevated temperatures, the slowest contribution to the fast molecular dynamics approximately follows hydrodynamic theory.
Molecular surface structure of tetracene mapped by the atomic force microscope
Authors: Overney R. M. , Howald L. , Frommer J. , Meyer E. , Güntherodt H.-J.
Published: July 2002

The atomic force microscope has been used to record molecular structure on free-standing organic crystals. A crystal of tetracene has been imaged with molecular resolution which allows the assignment of lattice parameters to the surface layer. The intermolecular spacings on the surface of tetracene correspond remarkably closely with those in the bulk. It is even possible to distinguish between the two translationally inequivalent molecules of the unit cell. The mechanism for using force microscopy to distinguish between different molecular orientations is discussed.
Rotational relaxation of nitrogen in ternary mixtures N2–CO2–H2O: Consequences in coherent anti-Stokes Raman spectroscopy thermometry
Authors: Bonamy J. , Bonamy L. , Robert D. , Gonze M. L. , Millot G. , Lavorel B. , Berger H.
Published: July 2002

The influence of CO2 and H2O on the rotational relaxation processes of N2 in ternary mixtures N2–CO2–H2O is investigated. The efficiency of these perturbers is responsible for significant modifications of the state-to-state relaxation rates and broadening coefficients. Flame data are well reproduced by taking into account these modifications. The role of these minor species in the determination of temperatures in premixed flames is analyzed. The present relaxation model allows us to understand why the discrepancy between observed and calculated coherent anti-Stokes Raman spectroscopy (CARS) spectra in flames is sometimes resolved by empirically adding a dephasing component to pure nitrogen linewidths. Moreover, this model improves the accuracy of CARS temperature measurements.
Comment on: ‘‘Self-consistency correction to homogeneous nucleation theory’’
Authors: Girshick Steven L.
Published: July 2002

In a previous paper [S. L. Girshick and C.-P. Chiu, J. Chem. Phys. 93, 1273 (1990)] a modified expression for the homogeneous nucleation rate was obtained, which multiplied the classical expression by an often large, temperature-dependent factor. This result had been derived as an extension of kinetic nucleation theory [J. L. Katz and H. Wiedersich, J. Colloid Interface Sci. 61, 351 (1977)]. It is shown that the same result could be derived directly from the classical theory, and that the modification in the nucleation rate is a consequence of writing the expression for the equilibrium cluster distribution self-consistently, regardless of whether the classical or the kinetic theories are adopted. An interesting paradox is also discussed: The self-consistency correction does not affect the expression for the evaporation coefficient, yet it affects the nucleation rate.
Low energy (<1 eV) electron transmission through condensed layers of water
Authors: Jo Sam K. , White J. M.
Published: July 2002

The intensity and energy distribution of photogenerated low energy electrons transmitted through amorphous ice condensed on Pt(111) have been measured. As predicted by recent theoretical calculations, elastic scattering dominates.
Sticky electrolyte mixtures in the Percus–Yevick/mean spherical approximation
Authors: Herrera J. N. , Blum L.
Published: July 2002

The analytical solution of the sticky-electrolyte model is obtained in the mean spherical/Percus–Yecvick approximation. This model can be used to represent chemical association in electrolytes. As in the case of the simple mean spherical approximation (MSA), the general solution is given in terms of a single scaling parameter Γ, which is now also a function of the stickiness parameters. We obtain analytic expressions for the thermodynamic functions ΔA, ΔP, and ΔEex. These expressions give the correct limits when the charge is zero (Barboy–Tenne) and when the sticky parameter vanishes. A particularly simple form of the thermodynamic functions is obtained in which the expressions are identical to those of the MSA, but with a different value of Γ, which now depends on the sticky interactions.
Catalytic decomposition of formic acid on Ru(001): Transient measurements
Authors: Sun Y.-K. , Weinberg W. H.
Published: July 2002

The kinetics of decomposition of formic acid on Ru(001) have been investigated with thermal desorption mass spectrometry following the adsorption of DCOOH and HCOOH at both 130 and 305 K. Formic acid chemisorbs dissociatively on the perfect (001) surface via O–H bond cleavage to form a formate and a hydrogen adatom, and at defect (step) sites on the surface via C–O bond cleavage to form CO, a hydrogen adatom and a hydroxyl. The saturation fractional coverage of the dissociatively chemisorbed formic acid is approximately 0.33 following adsorption at 130 K, and approximately 0.49 following adsorption at 305 K. Thermal desorption spectra of CO2 are a direct probe of the kinetics of C–H bond cleavage of the formate and are characterized by a kinetic isotope effect (C–H vs C–D bond cleavage), a progressively narrowing peak width, and an upward shift in peak temperature with increasing surface coverage of the formate.

The latter indicates an increasing apparent activation energy for the C–H bond cleavage reaction with increasing formate coverage, which results in self-accelerating decomposition kinetics that cause the observed sharp peak in the production of CO2. Although the activation energy for the C–H bond cleavage reaction varies strongly with the formate coverage, the relative yields of CO2 and CO are independent of the formate coverage and are in a ratio of 1:1. This observation, together with the observation that the formation of HDO and D2O, which results from C–O bond cleavage of the formate, appears at approximately the same temperature as that of CO2 formation following saturation adsorption of DCOOH at both 130 and 305 K, prove that cleavage of the C–D and C–O bonds of the formate are not two independent reaction pathways.

A novel mechanism for the reaction is postulated which involves the hydrogen from the C–H bond cleavage reaction in one formate (producing carbon dioxide) reacting with an adjacent formate to form a hydroxyl and formyl. The latter react to form the other observed products: carbon monoxide, and a mixture of water and molecular hydrogen, with some chemisorbed oxygen remaining on the surface. The increase in the activation energy for decomposition of the formate with increasing formate coverage is due to the fact that a chemisorbed formate is stabilized by the presence of another nearby formate.
Nonlocal pseudopotentials and diffusion Monte Carlo
Authors: Mitáš Luboš , Shirley Eric L. , Ceperley David M.
Published: July 2002

We have applied the technique of evaluating a nonlocal pseudopotential with a trial function to give an approximate, local many-body pseudopotential which was used in a valence-only diffusion Monte Carlo (DMC) calculation. The pair and triple correlation terms in the trial function have been carefully optimized to minimize the effect of the locality approximation. We discuss the accuracy and computational demands of the nonlocal pseudopotential evaluation for the DMC method. Calculations of Si, Sc, and Cu ionic and atomic states and the Si2 dimer are reported. In most cases ∼90% of the correlation energy was recovered at the variational level and excellent estimations of the ground state energies were obtained by the DMC simulations. The small statistical error allowed us to determine the quality of the assumed pseudopotentials by comparison of the DMC results with experimental values.
Kinetic theory and rheology of dilute, nonhomogeneous polymer solutions
Authors: Bhave Aparna V. , Armstrong Robert C. , Brown Robert A.
Published: July 2002

A phase-space kinetic theory of dilute polymer solutions is developed to account for the effects of nonhomogeneous velocity and stress fields. The theory allows the configuration distribution function to depend on spatial location and explicitly treats the polymer molecule as an extended object in space. Constitutive equations for the mass flux vector and stress tensor are derived that predict polymer migration induced by stress gradients and nonuniform velocity gradients. In addition, the constitutive equation for stress contains a diffusive term in stress, and hence the model does not fall within the class of simple fluids. Simple shear flow between parallel plates is solved to illustrate the features of the constitutive equations. Asymptotic analysis and numerical calculations show the formation of boundary layers in stress, velocity gradient, and polymer concentration that arise near solid walls as a result of preferential orientation of the polymer molecules there. The thickness of these layers scales as λHDtr/L2, where λH is the relaxation time of the macromolecule modeled as a Hookean dumbbell, Dtr is its translational diffusivity in solution, and L is the characteristic length scale of the macroscopic flow. The presence of these layers causes only a small change in the shear stress measured in typical rheometers, but can have a profound effect on the macroscale flow of polymer solutions in complex geometries by causing apparent fluid slip near solid boundaries.
Rotation–vibration interactions in highly excited states of SO2 and H2CO
Authors: McCoy Anne B. , Burleigh Darin C. , Sibert Edwin L.
Published: July 2002

Canonical Van Vleck perturbation theory (CVPT) is used to investigate rotation–vibration mixing of highly excited vibrational states of SO2 and H2CO. For SO2 we find a nearly complete separation of the rotational and vibrational degrees of freedom, even for J=12 and Evib=11 000 cm−1. In contrast, for H2CO we observe extensive mixing between rotational and vibrational degrees of freedom at similar rotational excitation but with Evib=8000 cm−1. Although a-axis Coriolis coupling is pronounced, b- and c-axis Coriolis couplings play an important additional role in mixing states with different Ka quantum numbers. The implementation of CVPT, the choice of internal coordinates, and the convergence of the results are discussed in detail.
Stress relaxation in polymer melts and concentrated solutions
Authors: Fixman M.
Published: July 2002

It is a widespread assumption that the backbone stress is dominant in polymer melts and concentrated solutions. This work suggests on the basis of Brownian simulations that the assumption is incorrect. The excluded volume stress seems to be much larger in entangled solutions of arbitrarily thin chains. A restriction to backbone stresses in the popular reptation model leads to the necessity for a rigid primitive path. To explain the persistence and eventual relaxation of the excluded volume stress, it suffices that entanglements provide elasticity on the distance scale of the radius of gyration.
Time-dependent small-angle light scattering of shear-induced concentration fluctuations in polymer solutions
Authors: van Egmond Jan W. , Werner Douglas E. , Fuller Gerald G.
Published: July 2002

A small-angle light-scattering experiment probing the time-dependent behavior of a semidilute polymer solution under simple shear is presented. Specifically, this paper investigates time-dependent anisotropic growth and orientation of concentration fluctuations in a semidilute solution of polystyrene in dioctyl phthalate above the quiescent cloud-point temperature. Two flow cells, which permitted investigation of the structure and dynamics of concentration fluctuations in both the flow-shear and the flow-vorticity planes, were used. Concentration fluctuations were found to be enhanced anisotropically by the shear flow in both planes. In the flow-vorticity plane, shear-enhanced concentration fluctuations orient along the vorticity direction, while in the flow-shear plane, the axis of average orientation is at an angle between 135° and 45° to the direction of flow. Qualitative trends of this anisotropic growth of concentration fluctuations are in agreement with the predictions of the Helfand–Fredrickson model in the flow-shear plane. Furthermore, the Onuki–Doi theory relating the scattering dichroism to the structure factor has been verified experimentally by comparing scattering dichroism calculated from scattering patterns with previous experimentally measured dichroism. Scattering dichroism, both calculated from structure factors and experimentally determined over a range of shear rates and temperatures, is demonstrated to depend only on the Weissenberg number. Relaxation of shear-enhanced fluctuations on cessation of flow allowed for the calculation of a diffusion coefficient.
Molecular graphs, point groups, and fullerenes
Authors: Manolopoulos David E. , Fowler Patrick W.
Published: July 2002

A general and convenient method is described by which one can obtain the point group, the 13C NMR pattern, and the number of IR- and Raman-active vibrations of any given fullerene structure directly from its molecular graph. The method is based on certain ‘‘topological’’ atomic coordinates, the Cartesian components of which in a topological principal axis system are proportional to the components of three selected eigenvectors of the adjacency matrix of the graph. Its practical utility lies in combination with existing theoretical results, including analytical molecular-orbital rules which predict two distinct families of closed-shell fullerenes and a systematic search which generates all known Cn fullerene graphs. The overall strategy is illustrated in an experimentally relevant application to the 24 distinct isolated-pentagon fullerene isomers of C84.
Oxidation of CO by oxygen on a stepped platinum surface: Identification of the reaction site
Authors: Szabó András , Henderson Michael A. , Yates John T.
Published: July 2002

The coadsorption of oxygen and carbon monoxide on the stepped Pt(112) surface has been studied using electron stimulated desorption–ion angular distribution (ESDIAD), temperature programmed desorption (TPD), and low energy electron diffraction (LEED). It has been possible to preferentially adsorb different isotopic CO molecules on step and terrace sites, respectively, following oxygen adsorption on step sites to partial coverage. Transient kinetic experiments show that below ∼200 K, isotopic CO present exclusively on terrace sites is more effectively involved in CO2 production, compared to less reactive CO on the step sites. Above ∼200 K, site exchange between step and terrace CO species prevents the measurement of the relative reactivity of the two kinds of chemisorbed CO. The results show that the elementary step producing CO2 from adsorbed CO and adsorbed oxygen is structure sensitive, even though the overall catalytic reaction between CO and O2 is generally classed as a structure insensitive reaction.
Periodic Hartree–Fock studies of the properties of the Cl2/MgO (001) interface
Authors: McCarthy Maureen I. , Hess Anthony C.
Published: July 2002

The properties of the Cl2/MgO (001) interface were investigated using the ab initio periodic Hartree–Fock LCAO method. The calculations treated a system that was periodic in two dimensions with the adsorbate aligned perpendicular to a three-layer slab of MgO (001). The total energy was calculated as the periodic net of Cl2 molecules approached either an oxygen or magnesium site on the surface. The finite thickness of the slab was set such that the interlayer atoms resembled those found in the bulk. The basis set for the solid has an optimized split valence form with 8–61G on Mg and 8–51G on O. The binding energy is reported as a function of Cl2 surface coverage, with a 6–21G* basis set on Cl, in the range from 1:1 to 1:8. At the most dilute coverage (1:8), the Cl2 molecule is bound over an oxygen with a binding energy of 4.1 kcal/mol. The energetics of Cl2 approaching a magnesium show a purely repulsive interaction at all coverages. The interfacial properties are probed with calculations of the band structure, total and projected density of states, density deformation maps, Mulliken analyses, and electrostatic potential maps. The charge density and density deformation maps indicate that the binding of the adsorbate involves very little charge transfer (only ∼0.03 ‖e‖) between the surface and the molecule. The calculated electrostatic potential and the electric field maps reveal that the interaction is dominated by the electrostatic interaction between the molecule and the surface. A simple electrostatic description of the attractive forces in the binding energy can account for the preference of the adsorbate to bind over oxygen. The total and projected density of states indicate that the higher energy valence states in the slab are dominated by contributions from the 2s and 2p orbitals centered on the oxygens. These states overlap and mix with the states resulting from the 3s and 3p orbitals of the Cl2 net. This mixing occurs to a much smaller extent between chlorine and magnesium orbitals.
Molecular dynamics simulations of Langmuir monolayers: A study of structure and thermodynamics
Authors: Karaborni S. , Toxvaerd S.
Published: July 2002

Molecular dynamics (MD) simulations have been performed on Langmuir monolayers of single chain surfactants at the air–water interface using a new anisotropic united atom model (AUA) for chain–chain interactions and a dipolar potential for head–head repulsions. Water–surfactant interactions are modeled using an external potential that does not fix the head group positions. The forces of the skeletal chains involved intramolecular effects of angle bending, and rotation among quartets of adjacent segments. Several molecular dynamics simulations have been performed on monolayers with densities ranging from 18 to 30 Å2/molecule. The results show two transitions in the monolayer. The first phase transition is a melting from a triangular lattice state maintained by the carbon chains to a fluidlike state with chain diffusion and lattice defects. The second transition is characterized by a change in molecular conformation, but with no change in lattice defects.
Proton spin-lattice relaxation dispersion of liquid crystal polymers: Characterization of local and collective motions
Authors: Zeuner U. , Dippel T. , Noack F. , Müller K. , Mayer C. , Heaton N. , Kothe G.
Published: July 2002

This paper reports on studies of the longitudinal proton spin relaxation dispersion T1Z(ω) of a nematic main chain liquid crystal polymer (M̄n=30 000) over a very broad Larmor frequency range (1 kHz≤ω/2π≤120 MHz). Analysis of the experiments is achieved in terms of a density operator treatment employing the Redfield approximation. The results show that collective motions contribute to the proton spin relaxation process in the kilohertz regime, as found for low molar mass liquid crystals, whereas the conventional megahertz range is dominated by reorientation of individual molecules. The intramolecular motions consist of trans–gauche isomerization and phenyl ring flips. These motions are the fastest in the hierarchy of time with correlation times of 10−10 s in the nematic melt of the polymer at T≊460 K. The intermolecular (whole molecule) motions are interpreted as rotational diffusion in an orienting potential. They exhibit a T1Z(ω)∼ω0.65 dispersion in the megahertz range and have correlation times ranging from 10−9 to 10−7 s at this temperature. The slowest motions affecting longitudinal spin relaxation can be assigned to nematic order director fluctuations characterized by a broad distribution of thermally activated modes. Analysis of the dispersion profiles in the kilohertz regime provides the viscoelastic parameters of the main chain liquid crystal polymer. At T=460 K, an average elastic constant of K=8×10−11 N and an effective viscosity of η=1×103 Pa s have been determined. Using the experimentally accessible value for the short wavelength cutoff of the elastic modes, one obtains the mean-square amplitude of the director fluctuations <θ02≳=0.02, corresponding to a director order parameter of SOF=0.98. Thus, the contributions of the collective chain motions to the measured order parameters are negligible.
Angle-resolved velocity distributions of excited Rh atoms ejected from ion-bombarded Rh{100}
Authors: Bernardo Dan N. , El-Maazawi Mohamed , Maboudian Roya , Postawa Zbigniew , Winograd Nicholas , Garrison Barbara J.
Published: July 2002

The distributions of metastable excited state (4F7/2) and ground state (4F9/2) Rh atoms ejected from Ar+-bombarded Rh{100} are experimentally determined as a function of ejection velocity and angle. Corresponding theoretical predictions are made by incorporating a nonradiative deexcitation model into molecular dynamics simulations of the bombardment process. There is good agreement between the experimental and theoretical distributions. The simulations show that a fraction of the ejected atoms are excited via collisions 1–20 Å above the surface, and that these atoms make a significant contribution to the excited atom yield at low ejection velocities.
The vibrational Stark effect
Authors: Bishop David M.
Published: July 2002

Expressions for the frequency and intensity of the fundamental-vibrational transitions of a polyatomic molecule in the presence of a uniform or nonuniform electric field are reported. They have been developed on the basis of perturbation theory and take into account, individually, first-order electrical and mechanical anharmonicity. They are more general than previously-used expressions, since terms which are quadratic in the field strength are included. As an example of their use and of their interpretative capability, they are applied to CO.
Difference between nuclear spin relaxation and ionic conductivity relaxation in superionic glasses
Authors: Ngai K. L.
Published: July 2002

Tatsumisago, Angell, and Martin [J. Chem. Phys. 97, 6968 (1992)] have compared conductivity relaxation data and 7Li nuclear spin lattice relaxation (SLR) data measured on a lithium chloroborate glass and found pronounced differences in the most probable relaxation times. The electrical conductivity relaxation (ECR) time, τ*σ, at some temperature occurs on a time scale shorter by some two orders of magnitude than the 7Li spin lattice relaxation correlation time, τ*s, and has a significantly lower activation energy. SLR and ECR monitor the motions of ions through different dynamic variables and correlation functions. Using this fact and the coupling model, I am able to explain quantitatively all aspects of the difference between SLR and ECR, and to establish relations between their different relaxation characteristics. The large difference between the observed activation energies of SLR and ECR alone should have implications on the validity of any proposed theory of the dynamics of ionic transport.
Ab initio study of the potential energy surface of CH4-H2O
Authors: Szczȩśniak M. M. , Chal/asiński G. , Cybulski S. M. , Cieplak P.
Published: July 2002

The potential energy surface of CH4-H2O is calculated through the fourth-order Mo/ller–Plesset perturbation theory. In an attempt to obtain basis-set saturated values of interaction energies the extended basis sets are augmented by bond functions which simulate the effects of high-symmetry polarization functions. The absolute minimum occurs for the configuration involving the C–H-O hydrogen-bond in which O-H points toward one of the faces of the CH4 tetrahedron. The equilibrium C–O separation is equal to 6.8 a0 which corresponds to the bond energy of 0.83 kcal/mol. Due to basis set unsaturation of the dispersion energy the bond energy may still be underestimated by about 0.05 kcal/mol. The secondary minimum involving the C-H–O hydrogen-bond is some 0.2 kcal/mol less stable, and the corresponding C–O distance is longer by 0.6 a0. The anisotropy of the potential energy surface is analyzed via the perturbation theory of intermolecular forces. The binding in CH4-H2O is chiefly due to the dispersion energy which sets the general trend for the anisotropy of the surface. A more detailed examination, however, indicates that the anisotropy of the surface results from a complex interplay of various factors, including electrostatics, exchange repulsion, and to a lesser degree, the deformation effects. Analysis of various exchangeless perturbation approximations to the deformation effect indicates that the neglect of exchange component of deformation may lead to an incorrect description of the van der Waals region. The analytical potential for the CH4-H2O interaction is provided.
Ultrafast underdamped solvation: Agreement between computer simulation and various theories of solvation dynamics
Authors: Roy Srabani , Bagchi Biman
Published: July 2002

A theoretical analysis of the three currently popular microscopic theories of solvation dynamics, namely, the dynamic mean spherical approximation (DMSA), the molecular hydrodynamic theory (MHT), and the memory function theory (MFT) is carried out. It is shown that in the underdamped limit of momentum relaxation, all three theories lead to nearly identical results when the translational motions of both the solute ion and the solvent molecules are neglected. In this limit, the theoretical prediction is in almost perfect agreement with the computer simulation results of solvation dynamics in the model Stockmayer liquid. However, the situation changes significantly in the presence of the translational motion of the solvent molecules. In this case, DMSA breaks down but the other two theories correctly predict the acceleration of solvation in agreement with the simulation results. We find that the translational motion of a light solute ion can play an important role in its own solvation. None of the existing theories describe this aspect. A generalization of the extended hydrodynamic theory is presented which, for the first time, includes the contribution of solute motion towards its own solvation dynamics. The extended theory gives excellent agreement with the simulations where solute motion is allowed. It is further shown that in the absence of translation, the memory function theory of Fried and Mukamel can be recovered from the hydrodynamic equations if the wave vector dependent dissipative kernel in the hydrodynamic description is replaced by its long wavelength value. We suggest a convenient memory kernel which is superior to the limiting forms used in earlier descriptions. We also present an alternate, quite general, statistical mechanical expression for the time dependent solvation energy of an ion. This expression has remarkable similarity with that for the translational dielectric friction on a moving ion.
Benchmark calculations with correlated molecular wave functions. III. Configuration interaction calculations on first row homonuclear diatomics
Authors: Peterson Kirk A. , Kendall Rick A. , Dunning Thom H.
Published: July 2002

Using correlation consistent basis sets from double through quintuple zeta quality, potential energy functions have been calculated for the electronic ground states of the first row homonuclear diatomic molecules B2, C2, N2, O2, and F2 using single and double excitation configuration interaction (HF+1+2, GVB+1+2, and CAS+1+2) wave functions. Spectroscopic constants have been calculated for each species and compared to experiment. The dependence of the calculated spectroscopic constants on systematic extensions of the one-particle basis set are, in general, found to be very regular. By fitting the directly calculated values with a simple exponential function, accurate estimates of the complete basis set (CBS) limit for Ee, De, and re have been obtained for each level of theory. The estimated CBS limits are compared to the available experimental results, and the intrinsic errors associated with each theoretical method are discussed. In addition, the accuracy of the internally contracted CAS+1+2 method is compared to conventional uncontracted calculations using large basis sets. For B2, a full CI calculation have been carried out for De with the correlation consistent double zeta basis set and is compared to the CAS+1+2 method using both a supermolecule and separated atom approach for the dissociated limit.
Vibrational relaxation of M(CO)6 (M=Cr, Mo, W): Effect of metal mass on vibrational cooling dynamics and non-Boltzmann internal energy distributions
Authors: King Jason C. , Zhang Jin Z. , Schwartz Benjamin J. , Harris Charles B.
Published: July 2002

The vibrational relaxation dynamics following the dissociation of C≡O from M(CO)6 (M=Cr, Mo, W) have been studied with picosecond transient absorption spectroscopy. After dissociation of C≡O, the pentacarbonyl species forms a complex with a solvent molecule. The cooling of these solvated pentacarbonyl complexes was monitored from 1 ps to 1 ns and different rates for vibrational relaxation were found for each of these three compounds. The W(CO)5⋅S (S=cyclohexane) vibrationally relaxes in 35 ps, whereas Mo(CO)5⋅S relaxes twice as quickly, 18 ps. This result is surprising because the higher density of states in W(CO)5⋅S would be expected to lead to faster cooling of the hot solvated complex. The primary cooling of Cr(CO)5⋅S is completed in 18 ps just as in Mo(CO)5⋅S, but a slower cooling of approximately 150 ps is also present. This component is assigned to vibrational relaxation of the C≡O stretching mode. From comparisons with other studies, it appears that the existence of this slower cooling component is present only in first row transition metal carbonyls.
Field ion microscopic studies of the CO oxidation on platinum: Field ion imaging and titration reactions
Authors: Gorodetskii V. , Drachsel W. , Block J. H.
Published: July 2002

Elementary steps of the CO oxidation—which are important for understanding the oscillatory behavior of this catalytic reaction—are investigated simultaneously on different Pt-single crystal surfaces by field ion microscopy. Due to preferential ionization probabilities of oxygen as imaging gas on those surface sites, which are adsorbed with oxygen, these sites can be imaged in a lateral resolution on the atomic scale. In the titration reaction a COad-precovered field emitter surface reacts with gaseous oxygen adsorbed from the gas phase or, vice versa, the Oad-precovered surface with carbon monoxide adsorbed from the gas phase. The competition of the manifold of single crystal planes exposed to the titration reaction at the field emitter tip is studied.

The surface specificity can be documented in the specific reaction delay times of the different planes and in the propagation rates of the reaction-diffusion wave fronts measured on these individual planes during the titration reaction with a time resolution of 40 ms. At 300 K the COad-precovered surfaces display the {011} regions, precisely the {331} planes as the most active, followed by {012}, {122}, {001}, and finally by {111}. Reaction wave fronts move with a velocity of 8 Å/s at {012}, with ≊0.8 Å/s at {111}, and have a very fast ‘‘switch-on’’ reaction at the (001) plane with 500 Å/s. At higher temperature, T=350 K, an acceleration of reaction rates is combined with shorter delay times.

The titration reaction of a precovered Oad surface with COgas at T=373 K shows the formation of CO islands starting in the {011} regions with a quickly moving reaction front into the other surface areas without showing particular delay times for different surface symmetries. The two reverse titration reactions have a largely different character. The titration of COad with oxygen adsorbed from the gas phase consists of three different steps, (i) the induction times, (ii) the highly surface specific reaction, and (iii) different rates of wave front propagation. The reaction of COgas with a precovered Oad layer on the other hand starts with nucleating islands around the {011} planes from where the whole emitter surface is populated with COad without pronounced surface specifity.
Field ion microscopic studies of the CO oxidation on platinum: Bistability and oscillations
Authors: Gorodetskii V. , Drachsel W. , Ehsasi M. , Block J. H.
Published: July 2002

The oscillating CO oxidation is investigated on a Pt-field emitter tip by using the field ion mode of surface imaging of Oad sites with O2 as imaging gas. Based on data of the titration reactions [V. Gorodetskii, W. Drachsel, and J. H. Block, J. Chem. Phys. 100, C. E. UPDATE (1994)], external control parameters for the regions of bistability and of self-sustained isothermal oscillations could be found. On a field emitter tip, oscillations can be generated in a rather large parameter space. The anticlockwise hysteresis of O+2 ion currents in temperature cycles occurs in agreement with results on single crystal planes. Unexpected regular oscillation sequences could occasionally be obtained on the small surface areas of a field emitter tip and measured as function of the CO partial pressure and of the temperature. Different stages within oscillating cycles were documented by field ion images. Oscillations of total ion currents are correlated with variations in the spatial brightness of field ion images. In the manifold of single crystal planes of a field emitter {331} planes around the {011} regions are starting points for oscillations which mainly proceed along [100] vicinals. This excludes the {111} regions from autonomous oscillations. With slightly increased CO partial pressures fast local oscillations at a few hundred surface sites of the Pt(001) plane display short-living CO islands of 40 to 50 Å diameter. Temporal oscillations of the total O+2 ion current are mainly caused by surface plane specific spatial oscillations. The synchronization is achieved by diffusion reaction fronts rather than by gas phase synchronization.
Nuclear quadrupole interaction of highly polarized gas phase 131Xe with a glass surface
Authors: Butscher R. , Wäckerle G. , Mehring M.
Published: July 2002

We report nuclear magnetic resonance (NMR) experiments on 131Xe (I=3/2) gas-phase atoms which exhibit nuclear quadrupole interaction with the surface of the sample cell. Nuclear quadrupole coupling constants and quadrupole relaxation rates are obtained from the time-domain signal of the freely precessing nuclear magnetization in weak magnetic fields. The nuclear spin species is polarized by spin-exchange collisions with optically pumped ground-state spins of Rb gas atoms. The Rb atoms also present in the sample are used as a magnetometer to probe the free-induction decay of the nuclear-spin ensemble. The temperature dependence of both the effective quadrupole splittings and the relaxation rates are explained by a model for the surface interactions of a Xe atom adsorbed on the glass surface. The desorption is thermally activated with an activation energy of EA=0.12 eV. The surface diffusion of an adsorbed atom is characterized by an activation energy ED for thermally activated hopping between neighboring surface sites. Both energies enter the spectral density function governing wall-induced nuclear quadrupole relaxation. Our experimental results lead to the conclusion that they are on the same order of magnitude.