bluuebunny/crossref_metadata_2025 huggingface dataset:
Solid-to-liquid phase change and fragmentation in C60
Authors: Serra S. , Sanguinetti S. , Colombo L.
Published: July 2002

We present a study of the thermodynamical properties of C60 in the microcanonical ensemble. Solidlike and metastable liquidlike form can be identified in the low energy and in the high energy range, respectively. The transition between the two phases is characterized by a finite energy range, in agreement with general theories of cluster melting. In particular, we have observed that the melting is preceded by a highly isomerized transition region where a sizeable atomic mobility is achieved via hopping between different isomer structures.
Analytical energy gradients and geometry optimization in the divide-and-conquer method for large molecules
Authors: Zhao Qingsheng , Yang Weitao
Published: July 2002

Based on the divide-and-conquer method in the density-functional theory, an efficient approach is developed to compute analytically the energy gradients with respect to the nuclear coordinates. Tests performed show that both energy gradients and optimized molecular geometry converge to the corresponding results of the Kohn–Sham method when the nearest neighbor contributions are increased.
Monte Carlo simulations of chain molecules in confined environments
Authors: van Giessen A. E. , Szleifer I.
Published: July 2002

The conformational and thermodynamic properties of lattice chain molecules in confined environments have been studied by Monte Carlo simulations. In the case of homopolymers with purely repulsive interactions with the walls the scaling laws proposed by Daoud and deGennes for the free energy of confinement, from three dimensions to one dimension and for two dimensions to one dimension, have been confirmed. The number of self-avoiding walks (SAW) of a chain with n segments for each film thickness d has been found to follow an effective relationship of the form NSAW(d)∝zeffn (d)nγ (d)−1, where zeff (d) is a thickness dependent effective lattice coordination number and γ (d) is the d dependent enhancement exponent. In the case of triblock copolymers in which the segments of the end blocks have attractions with the walls of the confined media it was found that for strong enough attractive interactions there is a minimum in the effective interactions between parallel walls. The distance at which the interaction potential has its minimum corresponds to the bulk end-to-end distance of the middle (nonattractive) block. This turns out to also be the distance at which the chains have the maximum number of bridging configurations between the two surfaces.
Towards an analytical model of water: The octupolar model
Authors: Blum L. , Vericat F. , Bratko D.
Published: July 2002

A simple potential for the water intermolecular potential, consisting of hard spheres with a point dipole and a potential well with the symmetry of a tetrahedral octupole was proposed by Bratko, Blum, and Luzar some time ago. This structural model was formally solved by Blum, Cummings, and Bratko, and explicit solutions have been recently obtained. We show here that this very simple model agrees surprisingly well with the experimental pair correlation functions of Soper and Phillips. The agreement with the gOH(r) and gHH(r) functions is quite good. For the gOO(r) the agreement is not as good, but this has to do with the hard core nature of the potential. Analytical solutions for soft spherical cores exist.
Quantum scattering calculations for vibrational and rotational excitation of CO by hot hydrogen atoms
Authors: Green Sheldon , Pan Baiyu , Bowman Joel M.
Published: July 2002

Collision cross sections were calculated for vibrational and rotational excitation of CO by H atoms at collisional kinetic energies from 0.7 to 1.9 eV. The BBH [J. M. Bowman, J. S. Bittman, and L. B. Harding, J. Chem. Phys. 85, 911 (1986)] potential energy surface was used and collision dynamics were treated within the quantum coupled states approximation, which is shown to be quite accurate for this system, and also using the infinite order sudden approximation for the rotational degree of freedom, which is shown to be less accurate than expected. Results are compared with experimental data and with quasiclassical trajectory values.
A theoretical study on the ionization of tetrachloroethylene with analysis of vibrational structure of the photoelectron spectra
Authors: Takeshita Kouichi
Published: July 2002

Ab initio calculations have been performed to study the molecular structures and the vibrational levels of the low-lying eight ionic states (2B2u, 2B2g, 2Au, 2B3u, 2B1g, 2Ag, 2B1u, and 2B3g) of tetrachloroethylene. The equilibrium molecular structures and vibrational modes of these states are presented. The theoretical ionization intensity curves including the vibrational structures of the low-lying eight ionic states are also presented and compared with the photoelectron spectrum. Some new assignments of the photoelectron spectra are proposed.
CH4 dissociation on Ni(100): Comparison of a direct dynamical model to molecular beam experiments
Authors: Luntz A. C.
Published: July 2002

This paper makes an extensive comparison of a dynamical model for a mechanism of direct dissociation to the detailed molecular beam experiments of CH4 dissociation on a Ni(100) surface reported in the previous paper. When a PES incorporating an ‘‘exit channel’’ barrier is used in the model and steric (multidimensional) aspects are included approximately via a ‘‘hole’’ approximation, excellent agreement is achieved between the model and experiments. This strengthens the qualitative mechanistic conclusions of Holmblad, Wambach, and Chorkendorff [J. Chem. Phys. 102, 8255 (1995)].
VB resonance theory in solution. I. Multistate formulation
Authors: Bianco Roberto , Hynes James T.
Published: July 2002

A theory for the description of electronic structure in solution for solution phase chemical reactions is formulated in the framework of a dielectric continuum solvent model which takes solute boundary effects into account. This latter feature represents a generalization of the Kim–Hynes theory, in which the solute boundary was treated in the dielectric image approximation. The electronic structure of the molecular solute, embedded in a cavity of the dielectric, is described by a manifold of orthogonalized diabatic—e.g., valence bond (VB)—states. The polarization of the dielectric solvent is partitioned into an electronic (fast) and an orientational (slow) component. The formulation encompasses both nonequilibrium and equilibrium regimes of the orientational polarization with respect to the solute charge distribution. The analysis is carried out in the general case of quantized solvent electronic polarization, but with reference to two limits in terms of which the general results can be most readily comprehended: with the electronic polarization much slower than the solute electronic motions and equilibrated to a delocalized solute charge distribution—the self-consistent limit; with the electronic polarization fast enough to equilibrate to components of the solute electronic distribution rather than to the average distribution—the Born–Oppenheimer limit. The general results depend on the relative time scales of the resonant interconversion between the VB states and the solvent electronic polarization. With the ansatz that the nonequilibrium orientational polarization is a linear combination of equilibrium terms with nonequilibrium coefficients, the solute–solvent system free energy is obtained together with a nonlinear Schrödinger equation for the solute electronic structure. A procedure is given for the natural definition of the set of solvent coordinates which describe the nonequilibrium regime necessary for the treatment of chemical reactions, and convenient matrix forms for the free energy and the Hamiltonian matrix elements are provided.
Molecular dynamics study of water clusters containing ion pairs: From contact to dissociation
Authors: Laria Daniel , Fernández-Prini Roberto
Published: July 2002

We have studied the potential of mean force between pairs of monovalent ions immersed in water clusters composed of up to 64 molecules at 200 K using constrained molecular dynamics techniques. Two different Hamiltonians for the water particles were investigated: one has fixed-point charges while the other has induced atomic dipoles which explicitly introduce effects due to fluctuations in the electronic density of the molecules. The qualitative behaviors of both models present similarities. For the case of pairs of equally charged ions, the solvent reactive field introduces a net attraction between the ions that prevents the dissociation of the clusters over a wide range of interionic distances. Similar binding effects are found for neutral ion pairs where the solvent reinforces the ionic attraction when the interionic distance attains values comparable to the cluster size. The correct thermodynamic interpretation of the calculated averages is restricted to small interionic distances; beyond this range proper sampling of all relevant fluctuations is not possible. Polarization effects in the water Hamiltonian introduce significant changes in the equilibrium structures: the clusters exhibit less structure and present a lower degree of ionic solvation.
Classical analysis of diatomic dissociation dynamics in intense laser fields
Authors: Thachuk Mark , Wardlaw David M.
Published: July 2002

The dissociation of a diatomic ion in an intense laser field is studied using a one-dimensional model with a Morse function representing the nuclear interaction potential, and coupling to a linear dipole moment representing the interaction with the laser field. A perturbative treatment is generally not possible because the field strengths employed are large enough to significantly distort the potential surface. Instead, classical trajectories are used to investigate some qualitative features of the dissociation process, with the goal of introducing some simple models to explain these features. A modified barrier suppression model is proposed which predicts the field strength at which trajectories first start to dissociate, and a ‘‘wagging tail’’ model is proposed which predicts the maximum kinetic energy of the dissociation products. Both these models provide physical insight into the dissociation process, and can be used to qualitatively understand experimental results.
A kinetic theory of suspensions. III. Hard spheres and lubrication dynamics
Authors: Jorquera Héctor , Dahler John S.
Published: July 2002

Numerical estimates are presented for the shear viscosity coefficient of a hard-sphere suspension. Our calculations are based on a previously reported theory which uses a Fokker–Planck equation to describe the evolution of the solute many-body distribution function. The hydrodynamic interactions of the solute particles are modeled by two-body forces which include the short-ranged, singular contributions due to lubrication. This produces estimates of the viscosity that agree significantly better with experiment than did our earlier calculations which neglected the effects of lubrication. Our predictions are compared with those of other theories, with results of computer simulations, and with experimental data.
Vaporization of DyBr3(s) and thermochemistry of the dimer homocomplex (DyBr3)2(g)
Authors: Hilpert K. , Miller M. , Ramondo F.
Published: July 2002

The vaporization of DyBr3(s) was investigated in the temperature range between 803 and 1053 K by Knudsen effusion mass spectrometry. The ions Dy+, DyBr+, DyBr+2, DyBr+3, Dy2Br+5, and Br+ were detected and their appearance potentials determined leading to an upper bound for the atomization energy of DyBr3(g). The species DyBr3(g) and (DyBr3)2(g) were identified in the vapor and their partial pressures determined using quantitative vaporization. Enthalpies and entropies of sublimation resulted according to the second-law and third-law methods. The enthalpy and entropy changes of the dissociation reaction (DyBr3)2(g) = 2 DyBr3(g) were obtained as ΔdH0298=220.8±6.6 kJ mol−1 and ΔdS0298=163.7±8.5 J mol−1 K−1.
Effect of polar additives on dielectric properties and charge transport in a molecularly doped polymer: A test of dielectric polarization models
Authors: Young Ralph H. , Fitzgerald John J.
Published: July 2002

The mobility of holes in a molecularly doped polymer (tri-p-tolylamine doped polystyrene) is dramatically diminished by the presence of a few wt % of a highly polar additive (t-amylphthalonitrile, TAP). Thermal and dielectric data indicate that the TAP is molecularly dissolved and held rigidly by the matrix up to approximately the glass transition temperature (Tg). The dielectric constant increases with increasing TAP concentration to a degree that is small except above Tg. Two proposed mechanisms for the effect of a polar additive on charge-carrier mobility, involving enhanced dielectric polarization of the medium surrounding a charge carrier, are tested. The observed increases in dielectric constant in the presence of TAP are too small for these mechanisms to explain the large decreases in hole mobility and increases in its activation energy.
Gaussian-type orbitals basis sets for the calculation of continuum properties in molecules: The differential photoionization cross section of Li2 and LiH
Authors: Cacelli Ivo , Moccia Roberto , Rizzo Antonio
Published: July 2002

The differential cross section of the one-photon ionization of the valence shell of Li2 and LiH is computed ab initio adopting the static-exchange approximation (SEA) and the random phase approximation (RPA). Using large L2 basis sets of polynomial spherical Gaussian-type orbitals (PSGTO) the matrix elements involving the final states in the electronic continuum are computed by a K-matrix based technique which also allows to obtain the phase shifts of the partial wave channels. The results reported here suggest that, with the methods proposed, the generally employed GTO’s could advantageously be exploited to evaluate reliable values of the properties of the continuum.
Simulation studies of shear viscosity time-correlation-functions
Authors: Stassen Hubert , Steele William A.
Published: July 2002

Computer simulations of the Green–Kubo time-correlation-functions (tcfs) for the shear viscosity are reported for argon (Lennard-Jonesium) at several state points. The principal function of interest is the contribution that involves the intermolecular forces. This particular tcf is the sum of a two-body, a three-body, and a four-body term. The time-dependencies and magnitudes of these individual tcfs have been evaluated over a wide range of density at 151 K and for a single state point near the argon triple point. It is shown that all of these component tcfs are significant; that their time-dependencies at moderately short times are quite different from one another; and that the individual functions display pronounced long-time-tails at high density. However, a tail does not appear in the total tcf because of cancellation in the summation. That is, the long-time-tails appear in the partial tcfs in the ratio 2-body:3-body:4-body=1:−2:1.
Structure and thermochemistry of sulfur fluorides SFn (n=1–5) and their ions SF+n (n=1–5)
Authors: Irikura Karl K.
Published: July 2002

Ab initio calculations, including the GAUSSIAN−2 procedures, have been performed to determine the molecular structure and thermochemistry of the sulfur fluorides SFn (n=1–5) and sulfur fluoride cations SF+n (n=1–5) in the gas phase. Based upon a comparison between calculated and experimental heats of formation of SF6, standard enthalpies of formation are derived for the ten title species. Values for SF3, SF5, and SF+5 differ from literature values by more than 50 kJ/mol. Bond energies are also derived (uncertainties are 2σ): D°0(SF5–F)=444±6, D°0(SF4–F)=159±7, D°0(SF3–F)=398±7, D°0(SF2–F)=227±6, D°0(SF–F)=374±6, and D°0(S–F)=345±6 kJ/mol for the neutral species, and D°0(SF+4–F)=371±6, D°0(SF+3–F)=56±6, D°0(SF+2–F)=400±6, D°0(SF+–F)=372±6, and D°0(S+–F)=367±8 kJ/mol for the ions. Adiabatic ionization energies are calculated to be 10.13±0.20, 10.15±0.19, 8.36±0.18, 11.90±0.16, and 9.71±0.16 eV for SF through SF5, respectively. Calculated geometries, vibrational frequencies, entropies, integrated heat capacities, and vertical ionization energies are also reported. Energy levels for low-frequency vibrations are determined variationally.
Ab initio variational transition state theory calculations for the O+NH2 hydrogen abstraction reaction on the 4A′ and 4A″ potential energy surfaces
Authors: Duan Xiaofeng , Page Michael
Published: July 2002

We report high level ab initio calculations that characterize the quartet potential energy surfaces 4A″ and 4A′ for the O+NH2 direct hydrogen abstraction reaction. Minimum energy reaction pathways have been computed on both surfaces at the full-valence complete active space self-consistent field level using a correlation consistent polarized valence double zeta basis set. Energies along these reaction pathways have been further refined by multireference configuration interaction calculations with a correlation-consistent polarized valence triple zeta basis set. Canonical variational transition state theory calculations using the ab initio potential energy surface information as input and incorporating tunneling through the ground state vibrationally adiabatic potential energy curves indicate that below about 2000 K, the O+NH2 reaction is dominated by addition/(isomerization)/dissociation pathways, and for temperatures below about 1000 K, even the OH+NH product channel is dominated by the addition/isomerization/dissociation route.
The onset of instability in nanostructures: The role of nonlinear resonance
Authors: Sumpter Bobby G. , Noid Donald W.
Published: July 2002

Classical trajectory methods are used to examine the vibrational dynamics of carbon nanotubes. The results clearly demonstrate an integral relationship between the diameter and length of a nanotube and its positional stability: tubes having diameters smaller than 0.7 nm undergo large-amplitude motion. The origin of this motion is due to strong coupling(s) between the longitudinal (vibration along the length) and a ring breathing mode (vibration about the axis of the cylinder). It is shown that the vibrational frequency of these modes follow a simple scaling law: ωc∝1/C, ωL∝1/L, where C is the contour length around the end of the tube and L is the length of the tube along its axis. This law should be applicable to any isotropic material with a cylindrical shape and provides an analytical equation for predicting mechanical stability: When the frequencies have small integer ratios with one another, in particular a 1:2 ratio, instability will occur on a short time scale (this phenomena represents a nonlinear resonance controlled by the geometry of the system).
Computational studies of the potential energy surface for O(3P)+H2S: Characterization of transition states and the enthalpy of formation of HSO and HOS
Authors: Goumri A. , Laakso Dianna , Rocha John-David R. , Smith C. E. , Marshall Paul
Published: July 2002

Structures and vibrational frequencies for minima and 11 transition states on the O(3P)+H2S potential energy surface have been characterized at the MP2=FULL/6-31G(d) level. GAUSSIAN-2 theory was employed to calculate ΔHf,298 for HSO and HOS of −19.9 and −5.5 kJ mol−1, respectively. The kinetics of HSO=HOS isomerization are analyzed by Rice–Ramsperger–Kassel–Marcus theory. Transition state theory analysis for O+H2S suggests OH+HS is the dominant product channel, with a rate constant given by 1.24×10−16 (T/K)1.746 exp(−1457 K/T) cm3 molecule−1 s−1. Kinetic isotope effects and the branching ratio for H+HSO production are also analyzed. The other possible products H2+SO and H2O+S do not appear to be formed in single elementary steps, but low-barrier pathways to these species via secondary reactions are identified. No bound adducts of O+H2S were found, but results for weakly bound triplet HOSH are presented. The likely kinetics for the reactions OH+SH→S(3P)+H2O, OH+SH→cis and trans 3HOSH, cis 3HOSH→HOS+H, and HSO and HOS+H→H2+3SO are discussed.
Λ-doublet and spin-doublet population distributions in the products of photofragmentation via coupled electronic channels: H2O(B̃ 1A1)→H+OH(X 2Π)
Authors: Dixon Richard N.
Published: July 2002

The consequences of the photofragmentation of a triatomic molecule on several coupled potential energy surfaces are considered with specific attention to branching ratios over the fine structure components of an open-shell product with orbital degeneracy. Attention is limited to the cases where the parent molecule dissociates on singlet surfaces. It is shown how the components of the outgoing wave function on the different surfaces interfere in determining these ratios. The theory is applied to the dissociation of H2O through its second B̃ 1A1 excited state, for which the coupling between three singlet states of the parent (B̃, Ã, and X̃) leads to two sets of dissociation products, H+OH(X 2Π) and H+OH(A 2Σ+). The variation with N of the Λ-doublet ratios in the OH(X) state is found to be a very sensitive function of the quantum state of the parent molecule, of the energy, and of the exit channel potentials.
How robust are molecular properties? A stability criterion for eigenstates
Authors: Schmid Günter M. , Coy Stephen L. , Field Robert W. , Silbey Robert J.
Published: July 2002

We propose a stability criterion applicable to eigenstates of approximate Hamiltonians. We define a ‘‘robustness’’ of the physical properties of the underlying system under a variety of internal and external conditions that can be deduced from stability properties of the eigenstates of the approximate Hamiltonian. Stability properties are assigned to an ensemble of eigenstates according to the statistics of its response to an ensemble of random perturbations of given magnitude. The stability criterion is explicitly formulated for the eigenstates of polyatomic molecules exhibiting a normal to local mode transition. As an example, a stability analysis for the water molecule is carried out and experiments in the low density gas phase (rotational energy transfer), high density gas phase (collision-induced spectra), and condensed phase are suggested.
Efficient molecular numerical integration schemes
Authors: Treutler Oliver , Ahlrichs Reinhart
Published: July 2002

New grids for three-dimensional numerical integration are introduced. They include a new mapping for radial integration of the Gauss–Chebyshev type which seems to surpass in accuracy the existing integration schemes as proposed by Becke [J. Chem. Phys. 88, 2547 (1988)], Murray et al. [Mol. Phys. 78, 997 (1993)], or Gill et al. [Chem. Phys. Lett. 209, 506 (1993)]. Lebedev grids are employed for spherical integration. Open ended quadrature schemes are presented using the efficient Lobatto formula for the θ integration. These grids are employed for self-consistent density functional calculations using local approximation and nonlocal corrections and are implemented into the program package turbomole. The results of grid tests and demonstrative applications of energy and especially analytical gradient calculations are given.
Rules for developing basis sets for the accurate computation of hyperpolarizabilities: Applications to He, H2, Be, Ne, F−, and HF
Authors: Papadopoulos M. G. , Waite J. , Buckingham A. D.
Published: July 2002

Various procedures for designing basis sets to be used in the computation of high-quality wave functions have been considered. The most satisfactory results for the polarizability and hyperpolarizabilities of He, H2, Be, Ne, F−, and HF were obtained using the core basis sets of van Duijneveldt, augmented by 7f1(l+1)5f2(l+2), where f1 and f2 are polarization Gaussian-type functions which are energy optimized and l defines the symmetry of the highest occupied subshell of the atom. Subsequently, simple rules, like replacing the 2s and 2f functions having the largest exponents with an equal number of orbitals of the same symmetry, the exponents of which form a geometric or even-tempered sequence with the two most diffuse exponents, have been used. Correlation effects are taken into account using fourth-order Mo/ller–Plesset perturbation theory.
Electronic spectra of jet-cooled tropolone–Mn (n=1,2) clusters. Microscopic solvent effects on proton tunneling in the S1 state
Authors: Sekiya Hiroshi , Hamabe Hidenori , Ujita Hiroki , Nakano Naohiko , Nishimura Yukio
Published: July 2002

The S1←S0 fluorescence excitation spectra of jet-cooled tropolone (TRN)–Mn (M=Ar, Kr, Xe, N2, CH4/CD4, C2H6, C3H8, CCl4; n=1,2) clusters have been measured in the wavelength region near the electronic origin to investigate the effects of van der Waals interactions on proton tunneling in the electronic excited S1 state. The solvation of TRN with the rare gas atom(s) has small effect on the 000 tunneling splitting, while the solvation with the molecule(s) considerably decreases the tunneling splitting. The decrease in the tunneling splittings of the TRN clusters has been explained by strong coupling of intermolecular vibration with intramolecular vibration of TRN, increasing the effective potential barrier height and/or tunneling distance. The anisotropy in the intermolecular interactions, and the configuration and number of solvent molecules are suggested to be important factors for the changes in the tunneling splitting.
Numerical solution of the hypernetted chain equation for a solute of arbitrary geometry in three dimensions
Authors: Beglov Dmitrii , Roux Benoît
Published: July 2002

The average solvent distribution near complex solid substrates of arbitrary geometry is calculated by solving the hypernetted chain (HNC) integral equation on a three-dimensional discrete cubic grid. A numerical fast Fourier transform in three dimensions is used to calculate the spatial convolutions appearing in the HNC equation. The approach is illustrated by calculating the average solvent density in the neighborhood of small clusters of Lennard-Jones particles and inside a periodic array of cavities representing a simplified model of a porous material such as a zeolite. Molecular dynamics simulations are performed to test the results obtained from the integral equation. It is generally observed that the average solvent density is described accurately by the integral equation. The results are compared with those obtained from a superposition approximation in terms of radial pair correlation functions, and the reference interaction site model (RISM) integral equations. The superposition approximation significantly overestimates the amplitude of the density peaks in particular cases. Nevertheless, the number of the nearest neighbors around the clusters is well reproduced by all approaches. The present calculations demonstrate the feasibility of a numerical solution of HNC-type integral equations for arbitrarily complex geometries using a three-dimensional discrete grid.
Thermodynamics and local structure of vinyl polymer melts
Authors: Yethiraj Arun , Curro John G. , Rajasekaran John J.
Published: July 2002

Monte Carlo simulation results are reported for the site-site pair correlations and equation of state of model vinyl polymer melts. The molecules are freely jointed hard chains with a hard sphere side-group attached to every other backbone bead. The local structure and pressure are investigated as a function of the diameter of the side group for melt-like densities. The intramolecular correlation functions are well represented by a single chain model where excluded volume interactions are included for beads separated by four bonds or less and neglected otherwise. The intermolecular correlation functions show interesting packing effects. The side group shields the backbone beads from approaching each other, to a degree that increases with increasing diameter of the side group. The polymer reference interaction site model integral equation theory is in good agreement with the simulation results for the pair correlation functions. At fixed volume fraction, the pressure is found to be a non-monotonic function of the size of the side group.
The interpretation of the c 1Π←a 1Δ excitation spectra of the ArNH complex
Authors: Yang Moonbong , Alexander Millard H. , Chuang Cheng-Chi , Randall Robert W. , Lester Marsha I.
Published: July 2002

We use slightly modified versions of earlier ab initio potential energy surfaces for Ar+NH(a 1Δ) and Ar+NH(c 1Π) to investigate the bend–stretch levels of ArNH complexes, both within an adiabatic bender model and in full variational calculations. The positions of these levels are used to reinterpret the fluorescence excitation spectra reported previously [R. W. Randall, C.-C. Chuang, and M. I. Lester, Chem. Phys. Lett. 200, 113 (1992)]. The first 15 observed bands can be unambiguously assigned, with near-quantitative accuracy. The weak anisotropy in the lower electronic state [ArNH(a 1Δ)] complicates the rotational structure, since each band involves overlapped transitions out of several, nearly degenerate bend–stretch levels of the complex.
Quantum theory of activated rate processes: A maximum free energy approach
Authors: Pollak Eli
Published: July 2002

A quantum theory of activated rate processes applicable to nonlinear potentials of interaction is developed. The central premise is that the rate is determined by the point of maximal quantum free energy separating reactants and products. The quantum free energy is defined in terms of a quantum centroid potential. The resulting rate expressions reduce to known limits for generalized Langevin equations and their Hamiltonian representation. They also reduce in the classical limit to previous results derived using an optimal planar dividing surface classical variational transition-state theory. A saddle-point estimate of the quantum rate leads to a generalization of Wolynes’ high temperature rate expression valid for nonlinear system bath interactions and anharmonic baths. Maximizing the free energy leads to a quantum friction function. Application to realistic systems demands the computation only of centroid densities.
Thermodynamics of fluids in random microporous materials from scaled particle theory
Authors: Ford David M. , Thompson Aidan P. , Glandt Eduardo D.
Published: July 2002

The thermodynamic properties of fluids confined to disordered porous solids are studied using a scaled particle theory approach. For simple hard sphere fluids in matrices of hard spheres, this method is of comparable accuracy to those previously introduced. In past studies of such systems, a strong thermodynamic similarity between the partly quenched and fully annealed cases has been evident; an exception to this behavior occurs when the diameters are nonadditive. For adsorbed polymeric molecules, on the other hand, the scaled particle theory introduced in this paper is the only route to the thermodynamics yet presented. The partition coefficients for polymers at infinite dilution in matrices of various porosities agree well with simulation over many orders of magnitude. For bulk polymers, the scaled polymer predictions are in much closer agreement with simulation than those of the traditional pressure and compressibility equations.
Stability of the CH3Cl dication
Authors: Duflot D. , Robbe J-M. , Flament J-P.
Published: July 2002

The chloromethane cation and dication have been studied using the complete active space self-consistent field method followed by a multireference perturbative configuration interaction. The 2E ground state of CH3Cl+ presents a weak Jahn–Teller distortion, the 2A′ and 2A″ components being separated by only 3.4 kcal/mol with very similar geometries. Both states are equilibrium structures. Contrary to previous calculations, the lowest singlet and triplet states of CH3Cl++ are calculated to be stable. While the 3A2 state keeps the neutral C3v symmetry, the 1E singlet state shows a large Jahn–Teller effect, with a splitting of 32.7 kcal/mol in favor of the 1A′ state, which is the ground state of the molecule. The planar 1A′ state of CH2ClH++ is found to be the most stable isomer, lying 41.4 kcal/mol below the corresponding state of CH3Cl++. The 3A″ state, which lies 22.59 kcal/mol above the 1A′ state, has a nonplanar Cs-symmetry geometry. Finally, the CH3Cl++ (1A′)→CH2ClH++ (1A′) isomerization takes place via a C1-symmetry transition state, with an energy barrier of 32.85 kcal/mol.
Phase behavior of random copolymers in quenched random media
Authors: Chakraborty Arup K. , Shakhnovich E. I.
Published: July 2002

In this paper, we consider the behavior of random heteropolymers in a quenched disordered medium. We develop a field theory and obtain a mean-field solution that allows for replica symmetry breaking. The presence of an external disorder leads to the formation of compact states; a homopolymeric effect. We compute the phase diagram for two classes of problems. First, we consider the situation wherein the bare heteropolymer prefers like segments to segregate, and second, we examine cases where the bare heteropolymer prefers unlike segments to mix. For the first class of systems, we find a phase diagram characterized by a replica symmetry broken phase that exists below a particular temperature. This temperature grows with the strength of the external disorder. In the second class of situations, the phase diagram is much richer. Here we find two replica symmetry broken phases with different patterns separated by a reentrant phase. The reentrant phase and one of the two replica symmetry broken phases are induced by interactions with the external disorder. The dependence of the location of the phase boundaries on the strength of the external disorder are elucidated. We discuss our results from a physical standpoint, and note the testable experimental consequences of our findings.
Theoretical study of the unimolecular dissociation HO2→H+O2. I. Calculation of the bound states of HO2 up to the dissociation threshold and their statistical analysis
Authors: Dobbyn Abigail J. , Stumpf Michael , Keller Hans-Martin , Schinke Reinhard
Published: July 2002

This is the first of a series of papers in which we investigate the unimolecular dissociation of hydroperoxyl. Using the DMBE IV potential energy surface [Pastrana et al., J. Phys. Chem. 94, 8073 (1990)], in the present study 726 bound states of HO2(X̃) up to the H+O2 dissociation threshold are calculated in an attempt to access the extent of the coupling between the modes of the system. The first approach involves an analysis of the nodal structure of the wave functions. While the wave functions for the lowest states are regular and assignable, the degree of mixing and complexity rapidly increases with energy. The wave functions close to the dissociation threshold are mostly irregular without any clear cut nodal structure and fill the entire coordinate space available. Nevertheless, a small number of regular states, that are associated with large excitation in the O2 stretching coordinate and no or only little excitation in the other modes, are found even at high energies. The second approach used to study the degree of intramolecular coupling is an analysis of the energy spectrum. The nearest neighbor level spacing distribution, which probes the short-range correlation, as well as the Σ2 and Δ3 statistics, which are sensitive to the long-range correlations in the spectrum, are investigated and compared to the distributions predicted for regular and irregular spectra. Both of these approaches indicate that the system is almost totally irregular with a Brody parameter of about 92%. In addition, the sum of states at a particular energy, which is extremely important in all statistical models for unimolecular dissociation, is approximately calculated from the volume of classical phase space and found to be in excellent agreement with the exact quantum mechanical result.
A re-examination of the S→S1 excitation spectrum of dimethylaniline
Authors: Weersink Robert A. , Wallace Stephen C. , Gordon Robert D.
Published: July 2002

A new assignment for the S0→S1 transition of N,N-dimethylaniline (DMA) and related derivatives is presented. The low frequency bands and long Franck–Condon envelope observed in DMA-h6 and DMA-d6 are assigned to the coupled methyl torsion mode of the amino group, not to torsion of the amino substituent about the C–N bond. This new assignment is consistent with the change in frequency of the excitation bands upon deuteration of the methyl groups and the strong origin transitions observed in the excitation spectra of other alkyl anilines. The assignment was confirmed by simulations of the excitation spectra of DMA-h6 and DMA-d6, with parameters of the calculated potential energy surface determined to be V3=148.0±0.5 cm−1, V+=−31.6±0.5 cm−1, V−=8.5±0.5 cm−1, and V6=−15±0.5 cm−1. By Franck–Condon analysis, it was determined that the weak origin transition is due to the shifting of the S1 torsion minimum by 40° along the gearing coordinate relative to the corresponding minimum in the ground state.
The universal repulsive-core singularity and Yang–Lee edge criticality
Authors: Lai Sheng-Nan , Fisher Michael E.
Published: July 2002

In 1984 Poland proposed that lattice and continuum hard-core fluids are characterized by a singularity on the negative fugacity axis with an exponent, here called φ(d), that is universal, depending only on the dimensionality d. We show that this singularity can be identified with the Yang–Lee edge singularity in d dimensions, which occurs on a locus of complex chemical potential above a gas-liquid or binary fluid critical point (or in pure imaginary magnetic fields above a ferromagnetic Curie point) and, hence, with directed lattice animals in d+1 dimensions and isotropic lattice animals or branched polymers in d+2 dimensions. It follows that φ=3/2 for d≥6 while power series in ε=6−d can be derived for φ(d) and for the associated correction-to-scaling exponent θ(d) with θ(1)=1 and θ(2)=5/6. By examining the two-component primitive penetrable sphere model for d=1 and d=∞ and long series for the binary Gaussian-molecule mixture (GMM) for all d, we conclude that the universality of φ(d) and θ(d) extends to continuum fluid mixtures with hard and soft repulsive cores [the GMM having Mayer f functions of the form −exp(−r2/r20)]. The new estimates φ(3)=1.0877(25) and θ(3)=0.622(12) are obtained with similar results for d=4 and 5.
A density functional analysis of the effects of surface correlations in simple models of polymer blends
Authors: Wong Ka-Yiu , Trache Maria , McMullen William E.
Published: July 2002

We analyze the long-wavelength features of near-surface composition profiles of monomers in a binary, symmetric blend using a recently developed density functional formalism for polymers in nonuniform environments. Relative to simpler theories of polymer–surface adsorption, our theory predicts a small increase of the interfacial width. We illustrate a derivation of a surface–boundary condition on the composition profile directly from the monomer–surface Hamiltonian. This boundary condition, when combined with our density functional analysis, leads to first-order wetting transitions.
The ethylene–carbon dioxide complex: A double internal rotor
Authors: Bemish R. J. , Block P. A. , Pedersen L. G. , Miller R. E.
Published: July 2002

A rotationally resolved near-infrared spectrum has been obtained for the ethylene–CO2 van der Waals complex. The structure obtained for this complex has the carbon dioxide lying above the plane of the ethylene molecule, with the CO2 axis parallel to this plane. The individual subbands observed in the spectrum are shifted from the expected rigid rotor positions, indicative of internal rotation of the two monomer units within the complex. A double rotor model is presented that accounts for the shifts and provides an estimate of the barrier to internal rotation. The results indicate that the complex is completely delocalized along the internal rotational coordinate. Ab initio calculations are also presented that are in good agreement with this picture.
Theoretical investigation of weakly-bound complexes of B with H2
Authors: Alexander Millard H. , Yang Moonbong
Published: July 2002

Continuing an earlier study [M. H. Alexander, J. Chem. Phys. 99, 6014 (1993)] of the B(2P)...H2 complex, we report here a further investigation into the adiabatic→diabatic transformation for this system, characterized by three potential energy surfaces (PES’s) which become asymptotically degenerate. The diabatic PES’s are used to determine the energies of the lowest bend–stretch levels of complexes of B(2P) with either o- or pH2. The predicted dissociation energies (D0) are 27.9 and 38.6 cm−1 for the complexes with pH2 and oH2, respectively, and 37.3 and 48.5 cm−1 for the complexes with oD2 and pD2, respectively. The motion of the oH2 moiety within the cluster cannot be described accurately using a single electronic potential energy surface. In addition, new ab initio calculations are reported for the complex of H2 with B in its first electronic excited state (...2s23s). The PES for this B(2S)...H2 complex is repulsive and nonreactive, at least in the region sampled by vertical electronic excitation of the B(2P)...H2 complex.
Consistent treatment of correlation effects in molecular dissociation studies using randomly chosen configurations
Authors: Greer J. C.
Published: July 2002

A means of performing configuration interaction calculations by randomly generating expansion vectors has recently been proposed and referred to as Monte Carlo configuration interaction (MCCI). To check the ability of the approach to accurately describe molecular dissociation, a comparison with benchmark full configuration interaction (FCI) calculations is presented. The results of the comparison indicate that the MCCI approach can consistently treat correlation effects in molecular dissociation studies.
Fluorescence depletion spectroscopy of the CH/D–Ne B 2Σ−–X 2Π transition
Authors: Basinger William H. , Lawrence W. G. , Heaven Michael C.
Published: July 2002

Fluorescence depletion techniques were used to test vibronic and rotational assignments for the B 2Σ−–X 2Π transition of CH–Ne. Previous vibronic assignments [W. H. Basinger, U. Schnupf, and M. C. Heaven, Faraday Discuss. 97, 351 (1994)] were confirmed, and observations of transitions to dissociation continua provided accurate dissociation energies for the B and X states. Errors in the rotational assignments were discovered. Re analysis of the rotational structure yielded ground state parity splittings and improved rotational constants. Adiabatic model calculations were used to determine approximate angular potential energy curves for the B and X states. These calculations also accounted for the prominent optical activity of internal rotation in the spectrum.
Electronic control of the spin–orbit branching ratio in the photodissociation and predissociation of HCl
Authors: Liyanage Rohana , Yang Yung-an , Hashimoto Satoshi , Gordon Robert J. , Field Robert W.
Published: July 2002

The fine structure branching ratio of chlorine atoms produced in the photodissociation and predissociation of HCl was measured for excitation energies between 51 800 and 83 800 cm−1. The branching ratio between 60 000 and 70 000 cm−1 is in good agreement with the calculations of Alexander et al. [J. Chem. Phys. 99, 1752 (1993)], converging to the statistical limit at high energy. Predissociation of electronically excited bound states display a surprisingly rich behavior, demonstrating that the nature of the predissociating continuum state has a large effect on the atom fragment branching ratio.
Generalized overlap amplitudes using the extended Koopmans’ theorem for Be
Authors: Morrison Robert C. , Ayers Paul W.
Published: July 2002

Approximate generalized overlap amplitudes (GOAs), also called Feynman–Dyson amplitudes, have been calculated from a full configuration interaction (CI) wave function for the ground state of beryllium using the extended Koopmans’ theorem (EKT). The GOAs were also calculated from the ground state CI wave function for Be and the ground state and excited state CI wave functions for Be+. The EKT GOAs are nearly identical to the corresponding CI GOAs for the lower 2S states of Be+ and for the Be+ 1s2s2 state which has a large GOA occupation number. There are many CI GOAs for which there is not a corresponding EKT GOA. This may be due in part to the limited size of the basis set and to the inability to include natural spin orbitals with small occupation numbers in the EKT calculations.
Correlated reflection and conductivity measurements on (2,5-dimethyl-dicyanoquinonediimine)2Cu crystals and films
Authors: Karutz F.-O. , Wachtel H.
Published: July 2002

We present correlated temperature dependent reflection and conductivity measurements on crystals and films of the undeuterated (h8) and the partly deuterated (d6) (2,5-dimethyl- dicyanoquinonediimine)2Cu radical anion salt. Using thin optical fibers, it was possible to study individual needle like crystals of a diameter of only a few μm. The temperature dependence of the reflectivity of single (d6) needles in the visible and the near-infrared region shows a ‘‘dip’’, which coincides with the phase transition in the conductivity. To interpret our data, we extended the commonly used models for the conductivity from Drude and Epstein, so that we are able to deal with reflection and conductivity in terms of one consistent model. For the (d6) films, a metal–insulator phase transition has been proved by reflection and conductivity. With the assumption of a broad distribution of different phase transition temperatures for the crystallites of the films, the observed spectra of films can also be treated with the model used for the crystals.
High resolution electronic spectroscopy of MgCH3
Authors: Rubino Rossana , Williamson James M. , Miller Terry A.
Published: July 2002

The MgCH3 radical was produced by a laser ablation/photolysis technique in a cold supersonic free-jet expansion and probed by laser induced fluorescence. Rotationally resolved spectra for both spin-orbit components of the Ã 2E←X̃ 2A1 electronic transition have been recorded. The analysis of these spectra yields the rotational constants of MgCH3 and therefrom a structure for the radical is proposed. A comparison is made among a series of alkyl organometallic radicals.
Chaotic scattering: An invariant fractal tiling of phase space
Authors: Tiyapan Ampawan , Jaffé Charles
Published: July 2002

The existence of an invariant fractal tiling of phase space for unbound Hamiltonian systems is demonstrated. The fractal properties of this partitioning of phase space is intimately related to the redistribution of energy among the various modes of the system. The existence of this tiling enables one to express the expectation values of physical observables as infinite sums over all of the tiles. Furthermore, knowledge of the scaling laws associated with the tiling then enables one to evaluate these sums.
Quantum dynamics of the Mu+H2(HD,D2) and H+MuH(MuD) reactions
Authors: Tsuda Ken-ichiro , Moribayashi Kengo , Nakamura Hiroki
Published: July 2002

Quantum mechanically accurate calculations are carried out for the following reactions involving muonium atom (Mu) using the hyperspherical coordinate approach: Mu+H2→MuH+H, Mu+D2→MuD+D, Mu+HD→MuH(MuD)+D(H), H+MuH→MuH+H, and H+MuD ↔MuH+D. The initial vibrational state is restricted to the ground state (vi=0) and the collision energies considered are up to ∼1.2 eV. The various aspects of the dynamics, such as the isotope effects, the initial rotational state (ji) dependence, and the final rotational state (jf) distribution are analyzed for a wide range of ji and jf. Some of the isotope effects can be interpreted in terms of the variations in reaction barrier and endothermicity. The following two intriguing features are also found: (1) strong enhancement of reaction by initial rotational excitation, and (2) oscillation of integral cross section as a function of collision energy in the case of the Mu-transfer reactions.