bluuebunny/crossref_metadata_2025 huggingface dataset:
Quenching of OH (A 2Σ+, v′=0) by H2, N2O, and hydrocarbons at elevated temperatures
Authors: Smith Gregory P. , Crosley David R.
Published: July 2002

Rate constants for collisional quenching of OH (A 2Σ+, v′=0) have been measured for H2, N2O, and ten hydrocarbon molecules at temperatures near 1200 K. A laser pyrolysis/laser fluorescence technique was used, in which a pulsed CO2 laser heats a gas mixture and a pulsed ultraviolet laser excites the OH. The rate constants are obtained from the pressure dependence of the direct time decay of the fluorescence. The resulting thermally and rotationally averaged cross sections are lower than those at room temperature, where comparisons are available; this is in accord with a picture of quenching involving attractive interactions between OH and the quencher. The bigger hydrocarbons such as butane and especially butenes have particularly large cross sections, which a simple attractive forces theory cannot explain.
Spectra of isotopically mixed benzene dimers: Details on the interaction in the vdW bond
Authors: Börnsen K. O. , Selzle H. L. , Schlag E. W.
Published: July 2002

Isotopically mixed jets of benzene produced the various possible isotopic benzene dimers in a supersonic jet. Mass coincidence in time-of-flight multiphoton mass spectra separated the spectra due to the various dimers produced into distinguishable spectra even in the presence of overlap. From the many different isotopic spectral shifts a detailed model of interaction between the two halves of the dimer can be established revealing detailed new information on this interaction. In particular a splitting in the 0–0 transition is reported for the first time indicating an exciton interaction in the dimer of isotropic identical benzene molecules in the gas phase.
Late stage spinodal decomposition of a binary polymer mixture. I. Critical test of dynamical scaling on scattering function
Authors: Hashimoto Takeji , Itakura Masahiko , Hasegawa Hirokazu
Published: July 2002

Unmixing kinetics of quenched critical polymer mixture of polystyrene and poly(vinyl methyl ether) was investigated by time-resolved light scattering technique. In the late stage of spinodal decomposition τ>60, τ being a reduced time, a simple dynamic scaling law was found on the scattering function I(q,t); I(q,t)∼ξ(t)3S̃[qξ(t)] with ξ(t)∼ta, where q is the scattering vector and ξ(t)=q−1m(t) is the correlation length of the periodic unmixed structure at time t which is measured by the position qm(t) of the scattering maximum at time t. The universal scaling functions (or the scaled structure function) F[qξ(t)] experimentally determined from the scattering functions for the lower quench depths (i.e., unmixing at 96.8, 97.3, and 98.2 °C, the spinodal temperature Ts being 95.8 °C) are in good agreement with that predicted by Furukawa, S̃(x)∼x2/(3+x8) for the critical mixtures (percolation regimes) in three dimensions [x=qξ(t)], although the scaling functions F(x) experimentally determined for the higher quench depths (i.e., unmixing at 102.5 and 106.8 °C) are much broader than the predicted function. Thus the decomposition of the polymer mixtures is proposed to obey universality found for metallic alloys, inorganic glasses, and small-molecular liquids in that the decomposed structures obtained at different time scale in the late stage has the self-similarity and unmixing dynamics is scaled with a single length parameter ξ(t).
Curve fitting analysis of near-edge core excitation spectra of free, adsorbed, and polymeric molecules
Authors: Outka D. A. , Stöhr J.
Published: July 2002

The quantitative analysis of the near-edge region of K-shell excitation spectra of free, chemisorbed, condensed, and polymeric molecules is undertaken using curve fitting procedures. The deconvolution of both x-ray absorption (NEXAFS) and electron impact near-edge excitation spectra is considered. Among the topics discussed are the line shape of resonances in the near-edge region, the line shape of the continuum steps, physical interpretations for the line shapes, and parameters describing core excitation spectral features, and background corrections for NEXAFS spectra. The goal of the work is to establish systematic procedures for analyzing near-edge spectra which allow peak positions to be consistently determined, and enable peak areas to be reliably obtained so that, e.g., the orientation of molecules can be determined from NEXAFS spectra. A more quantitative understanding of the features in near-edge spectra will also aid development of an understanding of the physical events underlying a near-edge spectrum.
Long-time self-diffusion coefficient and zero-frequency viscosity of dilute suspensions of spherical Brownian particles
Authors: Cichocki B. , Felderhof B. U.
Published: July 2002

We study the long-time self-diffusion coefficient and the zero-frequency effective viscosity for a suspension of spherical Brownian particles. The correction to first order in the volume fraction to the self-diffusion coefficient and the correction to second order in the volume fraction to the effective viscosity are given by integral expressions derived by Batchelor. Using exact series expansions of the hydrodynamic interaction functions in powers of the inverse distance between centers of a pair of particles we obtain accurate values for the correction terms. We consider hard spheres with mixed slip-stick boundary conditions as well as spherical liquid droplets with a viscosity different from the bulk.
Comparison of time-resolved fluorescence Stokes shift measurements to a molecular theory of solvation dynamics
Authors: Maroncelli Mark , Fleming Graham R.
Published: July 2002

The results of time-resolved Stokes shift measurements are compared to a molecular theory of ionic solvation dynamics recently solved by Rips, Klafter, and Jortner [J. Chem. Phys. 88, 3246 (1988)]. Although the theoretical predictions show only semiquantitative agreement with experiment, this molecularly based model provides a satisfying framework for understanding the large departures observed from the behavior predicted by simple continuum treatments of solvation dynamics.
An improved algorithm for reaction path following
Authors: Gonzalez Carlos , Schlegel H. Bernhard
Published: July 2002

A new algorithm is presented for obtaining points on a steepest descent path from the transition state of the reactants and products. In mass-weighted coordinates, this path corresponds to the intrinsic reaction coordinate. Points on the reaction path are found by constrained optimizations involving all internal degrees of freedom of the molecule. The points are optimized so that the segment of the reaction path between any two adjacent points is given by an arc of a circle, and so that the gradient at each point is tangent to the path. Only the transition vector and the energy gradients are needed to construct the path. The resulting path is continuous, differentiable and piecewise quadratic. In the limit of small step size, the present algorithm is shown to take a step with the correct tangent vector and curvature vector; hence, it is a second order algorithm. The method has been tested on the following reactions: HCN→CNH, SiH2+H2→SiH4, CH4+H→CH3+H2, F−+CH3F→FCH3+F−, and C2H5F→C2H4+HF. Reaction paths calculated with a step size of 0.4 a.u. are almost identical to those computed with a step size of 0.1 a.u. or smaller.
Relaxation of conformers and isomers in seeded supersonic jets of inert gases
Authors: Ruoff R. S. , Klots T. D. , Emilsson T. , Gutowsky H. S.
Published: July 2002

We have studied the relaxation of conformers and the formation/relaxation of isomeric, weakly bonded dimers in pulsed supersonic expansions of seeded inert gases (He, Ne, Ar, Kr). The relaxation was determined from the intensity of a rotational transition for the higher energy species as a function of carrier gas composition, using the Balle/Flygare Fourier transform microwave spectrometer. Of thirteen molecules with rotational conformers which we examined, those with barriers to internal rotation greater than 400 cm−1 did not relax significantly in any of the carriers. The higher energy forms of ethyl formate, ethanol, and isopropanol, with smaller barriers, were not relaxed by He; those of ethanol and isopropanol were somewhat relaxed by Ne; and all were completely relaxed by as little as 5 to 20 mole percent of Ar or Kr in He or Ne. The relaxation in He or Ne is first order in the concentration of added Ne, Ar, or Kr as well as in the concentration of the high energy conformer. The pseudo first-order rate constants (larger in Ne than in He) increase sharply with Z of the rare gas, roughly in a 0:1:2:4 progression for He, Ne, Ar, and Kr, suggesting that the relaxation involves relatively long-range polarization effects. Similar behavior was found in the formation/relaxation of the weakly bonded dimer pairs: linear OCO–HCN, T-shaped HCN–CO2; linear FH–NNO and bent NNO–HF; and bent HF–DF and DF–HF. The case of the HCN/CO2 dimers is particularly striking. The T-shaped dimer was found first, using Ar as the carrier gas. Five years later the linear form was found with first run neon as carrier, but it could not be detected at all with Ar as the carrier. These results show that in favorable cases high energy species can be studied in supersonic expansions by freezing out a ‘‘high-temperature’’ concentration with a nonrelaxing carrier gas.
The melting behavior of organic materials confined in porous solids
Authors: Jackson Catheryn L. , McKenna Gregory B.
Published: July 2002

The solid–liquid phase transition temperatures and heats of fusion ΔHf of nonpolar organic solids confined in the pores of controlled pore glasses were measured by differential scanning calorimetry. The pore diameters d were in the range of 40–730 Å and the organics studied were cis-decalin, trans-decalin, cyclohexane, benzene, chlorobenzene, naphthalene, and heptane. In accordance with previous reports on studies of primarily inorganic materials, the melting point of the pore solid T(d) decreased with decreasing pore diameter. In addition, a large reduction in the bulk enthalpy of fusion ΔHf of the pore solid was measured, which apparently has not been studied in detail by other workers. A linear correlation was found between the melting point depression (ΔTm) and the reciprocal diameter, as predicted by theories of solidification in a capillary. The calculated values of the solid–liquid interfacial energy σsl were in reasonable agreement with values reported in the literature based on other methods of measurement.
The dipole moment of carbon monoxide
Authors: Scuseria Gustavo E. , Miller Michael D. , Jensen Frank , Geertsen Jan
Published: July 2002

The dipole moment of CO has been calculated with many-body perturbation theory (MBPT) and coupled cluster (CC) methods using basis sets which have been optimized at the MBPT-2 level. It is demonstrated that triple excitations as well as g-type functions in the basis set are crucial to obtain satisfactory agreement with experiment. The most reliable prediction (0.125 D) is obtained at the CCSD(T) (coupled cluster including all single, double, and connected triple excitations, perturbatively) level of theory using a 10s9p4d2f1g basis set (160 basis functions). This result is in excellent agreement with the experimental value of 0.122 D.
The chemical nature of the nitrogens in polypyrrole and polyaniline: A comparative study by x-ray photoelectron spectroscopy
Authors: Tan K. L. , Tan B. T. G. , Kang E. T. , Neoh K. G.
Published: July 2002

The chemical nature of the nitrogens corresponding to various intrinsic redox states of polypyrrole (PPY) and polyaniline (PAN) has been critically compared using x-ray photoelectron spectroscopy (XPS) as a primary tool. Proton modifications of nitrogens in PPY give rise to a number of intrinsic redox states analogous to those observed in PAN. The behavior of the corresponding oxidation states in both polymers towards oxidation/reduction, deprotonation/reprotonation, or charge–transfer interactions with electron acceptors are grossly similar. However, the nitrogens of the two oxidized polymer complexes do differ in their thermal degradation behavior which suggests that the oxidized pyrrolylium nitrogens are more susceptible to deprotonation than their oxidized counterparts.
Fluctuations and mass action law breakdown in statistical thermodynamics of small systems
Authors: Blumenfeld L. A. , Grosberg A. Yu. , Tikhonov A. N.
Published: July 2002

The statistical equilibrium of chemical reaction of the type PQ⇄P+Q, taking place within small but macroscopic closed vesicle, is considered using statistical physics approach. It is shown, that mass action law, being the result of mean-field-type approximation, breaks down for sufficiently small vesicle volume and/or equilibrium constant of reaction, when mean number of free ‘‘P’’ particles, 〈p〉, within the vesicle becomes of order one or less. At the same time, the Nernst equation is shown to be applicable for systems of arbitrary volume and it gives the relation ΔF∼−ln〈p〉 for free energy ‘‘payment’’ for one ‘‘P’’ particle liberation from the vesicle. Due to fluctuations the true 〈p〉 and (−ΔF) values are essentially lower than corresponding mass action law predictions. The same effect of fluctuations leads to essential random inhomogeneity in the ensemble of vesicles, prepared under equivalent macroscopic conditions, and to non-Gaussian distributions of these vesicles over total number of particles within them or over ΔF value. Estimations show the effects of fluctuations to be essential for vesicles with sizes of order 102–103 Å, which is just the typical order of magnitude for many biological vesicles. Corresponding possible explanations of some experimental results in bioenergetics are discussed briefly.
Formulas for the analysis of surface sum-frequency generation spectrum by CH stretching modes of methyl and methylene groups
Authors: Hirose Chiaki , Akamatsu Naotoshi , Domen Kazunari
Published: July 2002

Derivation has been made of the expressions of surface vibrational sum-frequency generation(SVSFG). The results include (a) the vibrational selection rule, (b) the expressions of the susceptibility tensors at the surfaces composed of functional groups with C3v and C2v local symmetry, (c) the formulas of the SFG tensor components which apply specifically to the vibrational resonance with the CH stretching modes of methyl and methylene groups, and (d) the interference effect by the nearby Fermi resonance band. Changes introduced by the slight distortion of C3v symmetry into Cs symmetry have also been discussed. Sample calculations have been made of the SVSFG of the CH stretch modes of oriented CH3 groups.
Density-functional thermochemistry. II. The effect of the Perdew–Wang generalized-gradient correlation correction
Authors: Becke Axel D.
Published: July 2002

In an earlier paper [A. D. Becke, J. Chem. Phys. 96, 2155 (1992)], Kohn–Sham density-functional calculations of the total atomization energies of the 55 molecules of the Gaussian-1 database of Pople and co-workers [J. Chem. Phys. 90, 5622 (1989); 93, 2537 (1990)] were reported. We found that the local-spin-density exchange-correlation approximation with a ‘‘gradient correction’’ for exchange gave an average deviation from experiment of only 3.7 kcal/mol. In the present work we assess the role of gradient corrections for dynamical correlation, and we enlarge our earlier survey to include 42 atomic and molecular ionization potentials and 8 proton affinities as well. We conclude that gradient corrections for correlation do not improve atomization energies, but are vitally important in electron nonconserving processes such as ionization.
Mo/ller–Plesset perturbation investigation of the He2 potential and the role of midbond basis functions
Authors: Tao Fu-Ming , Pan Yuh-Kang
Published: July 2002

The Mo/ller–Plesset perturbation theory up to the complete fourth order is applied to calculate the ab initio van der Waals energy potential of He2. A scheme of constructing a very efficient basis set is proposed based on theoretical considerations. Such a basis set contains a set of Gaussian functions (usually of low angular momentum) centered at the midbond of the van der Waals molecule, which have been proven very efficient to reproduce the intersystem correlation interaction energy normally to be achieved by use of the nucleus-centered high angular momentum polarization functions. A nuclear-centered moderate set [5s4p2d] augmented by a set of midbond functions such as {3s3p2d} produces a value of 10.04 K for the well depth of He2 at the complete fourth-order (MP4) theory. Results from a series of other basis sets suggest that this value is nearly saturated with the further increase of basis, which is in agreement with the estimate of the complete basis-set limit. Calculations of the potential at the minimum and the other distances (3.0–10.0 a0) confirm that the MP4 method, together with the use of midbond functions, is a reliable and convenient choice for the accurate calculations.
Pathways and kinetic energy disposal in the photodissociation of nitrobenzene
Authors: Galloway Douglas B. , Bartz Jeffrey A. , Huey L. Gregory , Crim F. Fleming
Published: July 2002

Vacuum-ultraviolet photoionization molecular-beam mass spectrometry is a means of identifying primary photodissociation products and determining their recoil energies. At several photolysis wavelengths between 220 and 320 nm, we have observed three primary photodissociation pathways for nitrobenzene. Two of the pathways are C6H5NO2 →C6H5+NO2 and C6H5NO2→C6H5NO+O. The third pathway produces NO by one or both of the processes C6H5NO2→C6H5O+NO and C6H5NO2→C5H5+CO+NO. The relative yield of the pathways producing NO2 and NO varies strongly with the photolysis wavelength. The production of NO2 exceeds that of NO by about 50% for the 280 nm photolysis, but increases to almost a sixfold excess in 222 nm dissociation. The second pathway has a threshold energy that is about 0.50 eV greater than the thermodynamic limit for the formation of nitrosobenzene (C6H5NO) and an oxygen atom from nitrobenzene, probably reflecting the energy required to produce triplet nitrosobenzene and, perhaps, a barrier to dissociation on the triplet surface. The distribution in arrival times for a fragment provides an estimate of the recoil energy at each photolysis wavelength in these experiments. The channel producing nitric oxide (NO) radicals releases a relatively large amount of kinetic energy. Assuming the channel producing nitric oxide (NO) also produces phenoxy (C6H5O), we calculate a linear increase in kinetic energy from 0.29 eV at 320 nm to 1.1 eV at 220 nm. By contrast, the other two channels release only a small amount of kinetic energy (≊0.1 eV) at all wavelengths. An impulsive model does not describe the observed kinetic energy release for these low energy channels, suggesting that the energy release is more nearly statistical. The recoil energy predicted by an impulsive model for the channel producing nitric oxide and phenoxy radicals is closer to the observed kinetic energy release.
Collision-induced intersystem crossing of CH2 fromã 1A1 toX̃ 3B1: A case study of the mixed-state model
Authors: Bley U. , Temps F.
Published: July 2002

The rate coefficients for collision-induced intersystem crossing (CIISC) of methylene from the ã 1A1 first excited to the X̃ 3B1 ground electronic state, CH2 (ã 1A1)+M→CH2 (X̃ 3B1)+M, were investigated within the framework of the mixed-state mechanism [see, e.g., K. F. Freed, in Potential Energy Surfaces, edited by K. P. Law (Wiley, New York, 1980)]. Accordingly, the overall electronic relaxation was assumed to proceed via a sequence of rotational transitions within the ã manifold and allowed transitions from the ã to the X̃ manifold originating via ‘‘gate’’ states of ã which are states that contain some triplet character due to spin–orbit coupling with nearby X̃ rovibrational states. The perturbed ã and perturbing X̃ levels and relevant interaction matrix elements were identified from the available spectroscopic data. Rate coefficients for rotational relaxation processes were obtained from collision broadening measurements of CH2 (X̃) far-infrared laser magnetic resonance (FIR-LMR) transitions. Taking these data, thermal CIISC rate constants corresponding to experimental results for the overall depletion of rotationally thermalized CH2 (ã) and build up of (X̃) by M were evaluated for twelve interesting collision partners (M=He, Ne, Ar, Kr, Xe, N2, SF6, H2, D2, CH4, C2H6, H2O), taking into account every single CH2 (ã) rotation vibration state with energies up to Evr≤900 cm−1. The results were found to be in good agreement with reported room-temperature experimental data. Temperature dependencies, which were predicted for M=Ar, N2, and CH4, also agree with measured values.
Structure and hydrogen bond dynamics of water–dimethyl sulfoxide mixtures by computer simulations
Authors: Luzar Alenka , Chandler David
Published: July 2002

We have used two different force field models to study concentrated dimethyl sulfoxide (DMSO)–water solutions by molecular dynamics. The results of these simulations are shown to compare well with recent neutron diffraction experiments using H/D isotope substitution [A. K. Soper and A. Luzar, J. Chem. Phys. 97, 1320 (1992)]. Even for the highly concentrated 1 DMSO : 2 H2O solution, the water hydrogen–hydrogen radial distribution function, gHH(r), exhibits the characteristic tetrahedral ordering of water–water hydrogen bonds. Structural information is further obtained from various partial atom–atom distribution functions, not accessible experimentally. The behavior of water radial distribution functions, gOO(r) and gOH(r) indicate that the nearest neighbor correlations among remaining water molecules in the mixture increase with increasing DMSO concentration. No preferential association of methyl groups on DMSO is detected. The pattern of hydrogen bonding and the distribution of hydrogen bond lifetimes in the simulated mixtures is further investigated. Molecular dynamics results show that DMSO typically forms two hydrogen bonds with water molecules. Hydrogen bonds between DMSO and water molecules are longer lived than water–water hydrogen bonds. The hydrogen bond lifetimes determined by reactive flux correlation function approach are about 5 and 3 ps for water–DMSO and water–water pairs, respectively, in 1 DMSO : 2 H2O mixture. In contrast, for pure water, the hydrogen bond lifetime is about 1 ps. We discuss these times in light of experimentally determined rotational relaxation times. The relative values of the hydrogen bond lifetimes are consistent with a statistical (i.e., transition state theory) interpretation.
Accuracy of the Boys and Bernardi function counterpoise method
Authors: Gutowski Maciej , van Duijneveldt-van de Rijdt Jeanne G. C. M. , van Lenthe Joop H. , van Duijneveldt Frans B.
Published: July 2002

The performance of the Boys and Bernardi function counterpoise (FCP) method in eliminating the basis set superposition error (BSSE) is studied for He2, at R=5.6 a.u., within the supermolecular coupled electron pair approximation (CEPA-1) method. A series of one-electron Gaussian basis sets is designed that allows a systematic approach to the basis set limit value of the interaction energy. Every basis set contains a part suitable to reproduce the atomic correlation energy and a second part optimized for the dispersion interaction in He2. BSSE-free correlated first-order interaction energies [E(1)], calculated using perturbation theory, are reported for each of these sets. Extrapolation to the basis set limit yields a new value of 33.60±0.02 μH for E(1) at R=5.6 a.u. Extending previous work, the supermolecular CEPA-1 interaction energies for each set are then compared to the total of E(1) and the BSSE-free Mo/ller–Plesset second-order dispersion energy reported previously. While for some basis sets the uncorrected ΔE values deviate up to 43 K from the perturbation estimate, the FCP-corrected results always agree within 0.4 K. A virtuals-only counterpoise procedure is considered as well, but fails badly. The remaining discrepancies in the FCP results are ascribed to a failure of the Mo/ller–Plesset approach to precisely model the dispersion energy at the CEPA level. This problem is removed in a further, more stringent test where supermolecular EintCEPA-intra results, in which only the intra-atomic correlation (at the CEPA-1 level) is taken into account, are directly compared to the BSSE-free E(1) values. In this test the FCP-corrected supermolecular results agree, for the larger sets, to within 0.001 K with the results expected on the basis of E(1). These findings demonstrate, for the first time, that at least in He2 the FCP recipe yields interaction energies that correspond precisely (to machine precision) to the basis set and correlation method at hand.
Angle-resolved photoemission spectroscopy of ultrathin films of H2-phthalocyanine on MoS2 surfaces
Authors: Ueno Nobuo , Suzuki Katsumi , Hasegawa Shinji , Kamiya Koji , Seki Kazuhiko , Inokuchi Hiroo
Published: July 2002

The angle-resolved ultraviolet photoelectron spectra were measured for ultrathin films of H2–phthalocyanine deposited on cleaved MoS2 surfaces using synchrotron radiation. The take-off angle (θ) dependence of photoelectron intensity from the highest π band showed a sharp maximum at θ=34°. The observed angular distribution could be explained excellently by an angular distribution calculated for flat-lie orientation of the molecules using independent-atomic-center approximation combined with molecular orbital calculation. The results confirm that the calculation method is useful to describe photoelectron angular distribution from thin films of large and complex organic molecules and H2-phthalocyanine molecules lie flatly on MoS2 surface.
Relativistic regular two-component Hamiltonians
Authors: Lenthe E. van , Baerends E. J. , Snijders J. G.
Published: July 2002

In this paper, potential-dependent transformations are used to transform the four-component Dirac Hamiltonian to effective two-component regular Hamiltonians. To zeroth order, the expansions give second order differential equations (just like the Schrödinger equation), which already contain the most important relativistic effects, including spin–orbit coupling. One of the zero order Hamiltonians is identical to the one obtained earlier by Chang, Pelissier, and Durand [Phys. Scr. 34, 394 (1986)]. Self-consistent all-electron and frozen-core calculations are performed as well as first order perturbation calculations for the case of the uranium atom using these Hamiltonians. They give very accurate results, especially for the one-electron energies and densities of the valence orbitals.
Topological analysis of magnetically induced molecular current distributions
Authors: Keith Todd A. , Bader Richard F. W.
Published: July 2002

The topology of the first-order current density J(1)(r) induced in a molecule by an applied magnetic field is analyzed and classified in terms of the properties of its critical points, as determined by the 3×3 coefficient matrix of the asymmetric tensor ∇J(1). The eigenvalues of this tensor yield the topological indices for classifying the possible critical points in the J(1)(r) field. The phase portraits describing the current flow associated with these critical points and their role in determining the structure of a molecular current distribution are illustrated. A molecular current distribution is a fully three-dimensional vector field. In addition to closed loops of current, it exhibits one- and two-dimensional sources and sinks which generate surfaces, spirals, and single lines of current. The nonisolated critical points lie on stagnation paths which, along with the isolated critical points, fully characterize the current distribution. The antisymmetric component of ∇J(1) is the curl of J(1) which defines the vorticity of the current distribution. Whether a region of current flow is diamagnetic or paramagnetic depends on the location of its critical point relative to the atomic shell structure exhibited by the vorticity field. The group theoretical classification of the induced current is described.
Analytical derivatives for molecular solutes. I. Hartree–Fock energy first derivatives with respect to external parameters in the polarizable continuum model
Authors: Cammi R. , Tomasi J.
Published: July 2002

Analytical expressions for the derivatives of the free energy of solution of molecular solutes with respect to the dielectric constant and to a parameter defining the size of the cavity are here presented. These derivatives refer to a version of the model that describes solutes at ab initio Hartree–Fock level (as well as at the classical level) placed within a cavity reproducing the molecular shape. Attention is paid to the appropriate description of the boundary conditions which determine the integral value of the apparent charge spread on the cavity surface.
Large fluctuations and optimal paths in chemical kinetics
Authors: Dykman M. I. , Mori Eugenia , Ross John , Hunt P. M.
Published: July 2002

The eikonal approximation (instanton technique) is applied to the problem of large fluctuations of the number of species in spatially homogeneous chemical reactions with the probability density distribution described by a master equation. For both autocatalytic and nonautocatalytic reactions, the analysis of the distribution about a stable stationary state and of the transitions between coexisting stable states comes, to logarithmic accuracy, to the analysis of Hamiltonian dynamics of an auxiliary dynamical system. The latter can be done explicitly in a few cases, including one-species systems, systems with detailed balance, and systems close to the bifurcation points where the number of the stable states changes. In the last case, the fluctuations display universal features, and, for saddle-node bifurcation points, the logarithm of the probability of escape from the metastable state (per unit time) is proportional to the distance to the bifurcation point (in the parameter space) raised to the power 3/2. We compare the eikonal approximation for the stationary distribution of a master equation to Monte Carlo numerical solutions for two chemical two-variable systems with multiple stationary states, where none of the cited restrictions exists. For one of the systems in the pattern of optimal paths we observe caustics emanating from the saddle point.
Rate expressions for excitation transfer. II. Electronic considerations of direct and through–configuration exciton resonance interactions
Authors: Harcourt Richard D. , Scholes Gregory D. , Ghiggino Kenneth P.
Published: July 2002

The electronic interactions which promote singlet–singlet and triplet–triplet electronic excitation (energy) transfer (EET) are investigated in detail. Donor and acceptor locally excited configurations, ψ1(A*B) and ψ4(AB*), respectively, are each allowed to mix with bridging ionic configurations, ψ2(A+B−) and ψ3(A−B+) to form the new donor and acceptor wave functions ΨR=ψ1+λψ2+μψ3 and ΨP=ψ4+μψ2+λψ3. Use of the latter wave functions leads to the establishment of the matrix element TRP= 〈ΨR‖H−E1‖ΨP〉≊T14−(T12T24+T 13T34)/A, with Tij=〈ψi‖H−E1‖ψj〉 and A=E2−E1, as the exciton resonance interaction term for EET. Introduction of the Mulliken approximation shows that the ‘‘direct’’ exciton resonance interaction term (T14) contributes primarily a Coulombic interaction, for singlet–singlet EET, while the ‘‘through–configuration’’ exciton resonance interaction term [−(T12T24+T13T34)/A] replaces the Dexter exchange integral (which is a component of H14) as the primary source of short-range orbital overlap-dependent EET. The origins of ‘‘Dexter-type’’ energy transfer are thus shown to be quite different from that originally outlined.
Molecular solvent model for a cryptate solution in acetonitrile: A hypernetted chain study
Authors: Fries P. H. , Kunz W. , Calmettes P. , Turq P.
Published: July 2002

Hypernetted chain calculations are reported for pure liquid acetonitrile and for a 0.39 M acetonitrile solution of cryptate K+ 222, ClO−4. The obtained molecular correlations between solvent and solute particles are discussed in detail. It is found that the acetonitrile correlations are very similar in the pure liquid and in the solution. The solvation geometries around the cations and the anions are surprisingly unsymmetric and hence far from the picture which chemists currently use to describe ionic solvation. The theoretical results are compared to available wide-angle neutron scattering data for the pure liquid and to small-angle neutron scattering data for the cryptate solution. To our knowledge, this is the first integral equation study of molecular correlations in a real nonaqueous electrolyte solution.
Molecular magnetic properties within continuous transformations of origin of the current density
Authors: Zanasi R. , Lazzeretti P. , Malagoli M. , Piccinini F.
Published: July 2002

A new method for the calculation of molecular magnetic susceptibility and nuclear magnetic shielding is presented. It is based on continuous transformations of origin of the current density which make the transverse part of the paramagnetic current vanish. For any molecule all the components of the nuclear magnetic shielding tensor provided by the new method are independent on the origin of the gauge, whereas the components of the magnetizability tensor are translationally invariant only for center-symmetric molecules (they, in general, show a linear dependence on the shift of origin). This method, termed CTOCD-PZ, has been implemented for the theoretical determination of molecular magnetic properties via numerical integration techniques and, from preliminar results obtained for methane and carbon dioxide molecules, reveals reliable.
Ab initio molecular dynamics simulation of the solvation and transport of hydronium and hydroxyl ions in water
Authors: Tuckerman M. , Laasonen K. , Sprik M. , Parrinello M.
Published: July 2002

Charge defects in water created by excess or missing protons appear in the form of solvated hydronium H3O+ and hydroxyl OH− ions. Using the method of ab initio molecular dynamics, we have investigated the structure and proton transfer dynamics of the solvation complexes, which embed the ions in the network of hydrogen bonds in the liquid. In our ab initio molecular dynamics approach, the interatomic forces are calculated each time step from the instantaneous electronic structure using density functional methods. All hydrogen atoms, including the excess proton, are treated as classical particles with the mass of a deuterium atom. For the H3O+ ion we find a dynamic solvation complex, which continuously fluctuates between a (H5O2)+ and a (H9O4)+ structure as a result of proton transfer. The OH− has a predominantly planar fourfold coordination forming a (H9O5)− complex. Occasionally this complex is transformed in a more open tetrahedral (H7O4)− structure. Proton transfer is observed only for the more waterlike (H7O4)− complex. Transport of the charge defects is a concerted dynamical process coupling proton transfer along hydrogen bonds and reorganization of the local environment. The simulation results strongly support the structural diffusion mechanism for charge transport. In this model, the entire structure—and not the constituent particles—of the charged complex migrates through the hydrogen bond network. For H3O+, we propose that transport of the excess proton is driven by coordination fluctuations in the first solvation shell (i.e., second solvation shell dynamics). The rate-limiting step for OH− diffusion is the formation of the (H7O4)− structure, which is the solvation state showing proton transfer activity.
Gas–liquid nucleation in two-dimensional fluids
Authors: Zeng X. C.
Published: July 2002

A nonclassical theory of nucleation, based on the density-functional (DF) approach, is developed for the gas–liquid transitions of two-dimensional (2D) Lennard-Jones (LJ) fluids. The methods of Weeks–Chandler–Andersen perturbation theory are used to approximate the LJ potential with a temperature-dependent hard-disk diameter plus an attractive tail. The resulting free energy functional is then used to calculate the free energy barrier to nucleation. We find that the curvature of the 2D nucleus is not important to the rate of nucleation (in contrast to the 3D counterpart). The effect of curvature is readily inferred from the ratio of nucleation rate from classical Becker–Döring theory to that from DF theory. Our calculation suggests that classical nucleation theory actually works reasonably well for 2D LJ fluids in predicting the temperature-dependence of the nucleation rate (whereas for 3D LJ fluids it fails badly).
The effect of spin decoupling on line shapes in solid-state nuclear magnetic resonance
Authors: Sachleben Joseph R. , Caldarelli Stefano , Emsley Lyndon
Published: July 2002

Experimental and theoretical aspects of carbon-13 line shapes in static solids are described for on-resonance spin decoupling conditions. A relatively simple theoretical approach is provided for describing line shapes in static solids based on an operator representation of static second-order perturbation theory and theoretical line shapes in I2S and InS systems are calculated. The line shapes are predicted to comprise a single center line and ‘‘decoupling sidebands’’ on each side of the center line which move outward and diminish in amplitude as the decoupling field is increased. The predicted behavior is confirmed by experiments on an isolated seven spin system, where the decoupling sidebands are observed directly, and some organic solids in which the decoupling sidebands are not observed directly but in which their presence can be deduced from the behavior of the center line. A comparison is made between the theoretical predictions based on a complete quantum mechanical treatment and the predictions made using classical approximations in the model for the line shape. We conclude, based on our experimental results, that the line shape has a character which reflects the quantum nature of the spin system, even in organic solids, and that on-resonance terms appear to dominate experimental line shapes.
Rotational isomeric state models for polyoxyethylene and polythiaethylene on a high coordination lattice
Authors: Doruker Pemra , Rapold Roland F. , Mattice Wayne L.
Published: July 2002

The mapping of rotational isomeric state (RIS) models for chain molecules onto a high coordination lattice may permit efficient simulations of the self-assembly of block copolymers. Recently the second nearest neighbor diamond (2nnd) lattice was introduced for this purpose, and the RIS model for polyethylene was successfully mapped onto that lattice. Here the mapping procedure is extended to the two examples of poly (A–A–B) with A=CH2, B=O or S in order to ascertain whether the mapping can produce reasonable values for the mean square dipole moment, as well as the mean square end-to-end distance, and also to provide mapped RIS chains for construction of block copolymers. Simulations of single chains of polyoxyethylene (POE) and polythiaethylene have been performed on the 2nnd lattice by incorporating short range interactions through an extended RIS formalism. The overall dimensions of the chains, specifically of POE, are in agreement with the unperturbed dimensions of the classical RIS model. The characteristic ratio of the mean square dipole moments for long POE chains is close to the values reported in experiments and earlier RIS calculations. The ratio of the mean square radius of gyration for cyclic to acyclic POE chains of the same degree of polymerization approaches the long chain limit of 1/2. Simulations of single chains have also been performed in varying solvent environments, by implementing long range interactions via the lattice formulation of the second virial coefficient. As a result of these simulations, the strength of attractive and repulsive long range interactions on the 2nnd lattice can be determined for attaining theta solvent conditions.
Microwave spectrum of the FS2 radical in the ground electronic X̃ 2A″ state
Authors: Tang Jian , Saito Shuji
Published: July 2002

The FS2 radical has been detected in the glow discharge of the SF6 gas by microwave spectroscopy. Both a-type and b-type rotational transitions with the fine and hyperfine structure were observed in the millimeter-wave range of 120–185 GHz. The least-squares analysis for the 440 line frequencies led to the determination of the rotational constants, the centrifugal distortion constants, the spin-rotation interaction constants with the centrifugal distortion corrections, and the hyperfine interaction constants. Especially, the off-diagonal constants (εab+εba)/2 and Tab were determined in good precision because some nearly degenerate energy levels were mixed strongly by the off-diagonal spin-rotation interaction, which separated the energy levels up to 3 GHz and even induced the forbidden transitions to be observed strongly. The spin density on the fluorine atom and the chemical bond moments were derived for FS2, and the possible signs of Tab and (εab+εba)/2 were discussed. The behaviors of the FS2 radical and other related transient species in the SF6-discharge system were compared by using the corresponding rotational transitions observed in the microwave spectra.
Effect of the attractive interactions on the phase behavior of the Gay–Berne liquid crystal model
Authors: de Miguel Enrique , Martín del Rio Elvira , Brown Julian T. , Allen Michael P.
Published: July 2002

We present in this paper a computer simulation study of the phase behavior of the Gay–Berne liquid crystal model. The effect of the anisotropic attractive interactions on stabilizing orientationally ordered phases is analyzed by varying the anisotropy parameter κ′ at fixed values of the molecular elongation parameter κ. Molecular dynamics simulations have been performed at constant density and temperature along several isotherms and approximate transition densities are reported. It is found that, for a given value of the molecular elongation κ=3, smectic order is favored at lower densities as κ′ increases. When κ′ is lowered, the smectic phase is preempted by the nematic phase. As a result, the nematic phase becomes increasingly stable at lower temperatures as κ′ is decreased. Additionally, we have studied the liquid–vapor coexistence region for different values of κ′ by using Gibbs ensemble and Gibbs–Duhem Monte Carlo techniques. We have found evidence of a vapor–isotropic–nematic triple point for κ′=1 and κ′=1.25. For temperatures below this triple point, we have observed nematic–vapor coexistence as is found for many liquid crystals in experiments.
Coarsening mechanisms of droplet spinodal decomposition in binary fluid mixtures
Authors: Tanaka Hajime
Published: July 2002

We demonstrate here the experimental results indicating the existence of interdroplet interactions between droplets even in the late stage of droplet spinodal decomposition. On the basis of these results, we discuss a few possible coarsening mechanisms of droplet spinodal decomposition. A droplet of finite curvature has long-range concentration fields around it to satisfy the boundary condition given by the Gibbs–Thomson relation. For a pair of neighboring droplets, thus, the closed isoconcentration lines including the two droplets are generally formed in the matrix phase in the late stage of spinodal decomposition. Thus, the droplet can feel its neighboring droplet via the concentration gradient produced by the latter. The excess bulk and gradient energy associated with the diffuse concentration changes around droplets may cause a long-range attractive interaction between the droplets, which induces the interdroplet collisions. We call this mechanism ‘‘gradient-induced-coupling mechanism.’’ The gradient force associated with an interface is likely responsible for the motion of droplets under the incompressibility condition. In fluid mixtures, this coupling of concentration fields can further be enhanced by the violation of the local equilibrium due to the quick hydrodynamic coalescence of colliding droplets. In such a case, there is a possibility that the concentration diffusion cannot follow this quick geometrical coarsening. This interface quench effect probably leads to the phenomenon of ‘‘collision-induced collision via diffusion’’: A droplet experiencing collision, which accompanies strong diffusion fields, has a higher probability of the subsequent collisions. The hydrodynamic flow induced by droplet coalescence also leads to the subsequent collisions, which we call ‘‘collision-induced collision via flow.’’ Further, the geometrical configuration of droplets also leads to ‘‘geometrical collision-induced collision.’’ All the above mechanisms likely make the coarsening of droplet spinodal decomposition faster than the prediction of Brownian-coagulation mechanism.
Quantum dynamics of the Walden inversion reaction Cl−+CH3Cl→ClCH3+Cl−
Authors: Clary David C. , Palma Juliana
Published: July 2002

Quantum scattering calculations on the SN2 reaction Cl−+CH3Cl→ClCH3+Cl− are reported. The rotating bond approximation (RBA) has been adapted so that three degrees of freedom including the C–Cl stretching vibration and the CH3 umbrella mode are treated explicitly. The calculations have been done with minor modifications of a potential due to Vande Linde and Hase. It is found that initial excitation of the C–Cl vibration has a large effect on the reaction probabilities, while excitation of the CH3 umbrella vibration is less significant. The reaction is dominated by scattering resonances with lifetimes ranging from 0.1 to 10 ps. It is found that the length of the C–Cl bond at the transition state of the reaction has a particularly pronounced effect on the reaction probabilities. The magnitude of the quantum reaction probabilities compares quite well with those calculated using the quasiclassical trajectory method.
Optimized cluster theory of structurally symmetric polymer blends
Authors: Melenkevitz J. , Curro J. G.
Published: July 2002

The thermodynamic properties of model structurally symmetric polymer blends were calculated using a recently developed theory that is a generalization of the optimized cluster theories for atomic mixtures and single component molecular systems. Within this approach, cluster expansions for the Helmholtz free energy and pair correlation functions were developed for a mixture of polymer chains modeled using the interaction site formalism. A series of topological reductions was then performed to render an approximate expression for these quantities. The theory was previously shown to be equivalent to a set of diagrammatically proper integral equations. The critical temperature was determined to scale linearly with chain length in agreement with neutron scattering measurements and computer simulations. The critical temperature was also found to be suppressed with respect to the mean-field value due to composition fluctuations. The effective interaction parameter, χs, at the critical composition was well represented by the functional form, χs=A/T+B, where A and B are constants that depend on the chain length and total segment density. Deviations from random mixing were found to extend well beyond monomeric length scales and increased substantially as the critical temperature was approached.
Basis-set convergence of correlated calculations on water
Authors: Helgaker Trygve , Klopper Wim , Koch Henrik , Noga Jozef
Published: July 2002

The basis-set convergence of the electronic correlation energy in the water molecule is investigated at the second-order Mo/ller–Plesset level and at the coupled-cluster singles-and-doubles level with and without perturbative triples corrections applied. The basis-set limits of the correlation energy are established to within 2 mEh by means of (1) extrapolations from sequences of calculations using correlation-consistent basis sets and (2) from explicitly correlated calculations employing terms linear in the interelectronic distances rij. For the extrapolations to the basis-set limit of the correlation energies, fits of the form a+bX−3 (where X is two for double-zeta sets, three for triple-zeta sets, etc.) are found to be useful. CCSD(T) calculations involving as many as 492 atomic orbitals are reported.