bluuebunny/crossref_metadata_2025 huggingface dataset:
The kinetic theory of the effect of chemical reaction on diffusion of a trace gas
Authors: Nowakowski Bogdan , Popielawski Jan
Published: July 2002

Simultaneous diffusion and chemical reaction of a trace gas is studied by means of the Boltzmann–Lorentz kinetic equation. It is assumed that the reaction term is relatively small and can be treated as a perturbation. The procedure is outlined applying the Resibois method for derivation of corrections to the classical result for the diffusion coefficient, as well as to the reaction rate constant. The effect of chemical reaction on diffusion is calculated for the model of hard spheres reacting chemically with a finite activation energy. This influence can be appreciable if the molecular masses of the diffusing gas and the carrier gas are disparate.
Structure of mono- and multilayer Langmuir–Blodgett films from Cd arachidate and Ca arachidate
Authors: Kinzler M. , Schertel A. , Hähner G. , Wöll Ch. , Grunze M. , Albrecht H. , Holzhüter G. , Gerber Th.
Published: July 2002

Langmuir–Blodgett (LB) films of Cd arachidate and Ca arachidate with thicknesses of 1, 3, 9, and 25 layers were investigated using near edge x-ray absorption fine structure (NEXAFS spectroscopy), small angle x-ray scattering (SAXS), and reflected high energy electron diffraction (RHEED). The results from the x-ray diffraction experiments on layer thickness and interface roughness complement the structural information from NEXAFS on the average tilt angle of the alkyl chains in real space. The RHEED measurements confirm these findings and give additional information on the lateral order in these organic films. We find that the monolayer film shows a substantial degree of disorder, while for three and more LB layers, the molecular orientation is virtually identical to that of the corresponding fatty acid salts, i.e., the hydrocarbon chains are oriented perpendicular to the substrate surface in an all-trans conformation. Furthermore, a simplified and more robust analysis scheme for NEXAFS data obtained for hydrocarbon chains is presented.
Static hyperpolarizability of N2
Authors: Archibong Edet F. , Thakkar Ajit J.
Published: July 2002

An extensive investigation is reported of the effects of one-particle basis set selection, electron correlation, and molecular vibrations on the static dipole hyperpolarizability of N2. Our vibrationally averaged values of the mean static polarizability ᾱ/e2a20E−1H=11.87±0.12, the static polarizability anisotropy Δα/e2a20E−1H=4.70±0.09, and the mean, static, electronic hyperpolarizability γ̄/e4a40E−3H=956±50 are in good agreement with experiment. The mean, static, vibronic hyperpolarizability is predicted to be γ̄v=89e4a40E−3H.
The fraternal twins of quartet O+4
Authors: Lindh Roland , Barnes Leslie A.
Published: July 2002

Eleven stationary geometries of quartet O+4 have been studied by ab initio methods. The geometries were optimized at the complete active space self-consistent field (CASSCF) level of theory and the energies were calculated by the multiconfigurational second order pertubation method (CASPT2), using double-ζ plus polarization (DZP), triple-ζ plus double polarization (TZ2P), average atomic natural orbital (ANO) [5s4p2d] and average ANO [6s5p3d2f] basis sets. The rectangular and trans-planar structures are found to be the most stable, with an energy barrier to conversion between the two at the threshold of dissociation. Both have a delocalized hole and are stable relative to separated O2 and O+2 by 11.0 and 11.5 kcal/mol for the rectangular and the trans-planar structure, respectively, compared with the experimentally deduced energy in the range of 9.2 to 10.8 kcal/mol. The adiabatic ionization potentials of O4 and O2 are computed to be 11.67 and 12.21 eV, while experimental values are 11.66 and 12.07 eV, respectively. The vibrational frequencies have been computed for all degrees of freedom at the CASSCF level of theory. Symmetry breaking is found to be a particular problem in the computation of the antisymmetric stretch frequency for the delocalized structures at the CASSCF level of theory. Attempts to rectify these problems using the restricted active space self-consistent field (RASSCF) method leads to additional difficulties, but further analysis yields insight into the symmetry breaking and problems with earlier calculations. Finally, a nonorthogonal configuration interaction (CI) calculation based on the interaction of localized CASSCF wave functions using the complete active space state interation (CASSI) method leads to a balanced treatment of the antisymmetric stretch which is free from symmetry breaking. The study explains the four most prominent absorption frequencies observed in the partially unassigned IR spectrum of O+4 isolated in solid neon as the antisymmetric OO stretch, and the combination band of the symmetric and antisymmetric OO stretch of both the rectangular and trans-planar structures.
Quantum effects in Ar+HF rotationally inelastic scattering: A semiclassical interpretation
Authors: Barrett James J. C. , Mayne Howard R. , Keil Mark
Published: July 2002

Invoking a semiclassical approach to state-to-state rotationally inelastic scattering, this study seeks to determine the origin of several novel features observed in earlier quantum mechanical calculations [Rawluk et al., Chem. Phys. Lett. 202, 291 (1993)]. These features were absent from comparable classical trajectory calculations. The semiclassical (classical path) method used here treats the relative motion of Ar and HF classically, while the HF rotation is treated quantum mechanically by expansion in a rigid rotor basis set. This semiclassical approach reproduces the exact quantum results very well. The time dependence of the classical path allows a detailed study of the role played by the potential energy surface governing the dynamics. In particular, the behavior of the expansion coefficients 〈Yjm‖Ψ(t)〉 in the complex plane is very revealing. From this analysis, it is clear that the quantum effects stem from a balance between the attractive and repulsive parts of the potential.
On the reproducibility of glasses
Authors: Speedy Robin J.
Published: July 2002

The reproducibility of glasses of 500 hard spheres is examined. Most amorphous packings are shown to tend to the limiting density z=0.870±0.008 (relative to the close packed crystal). When the fluid is compressed through the glass transition, in a molecular dynamics experiment, fast enough to avoid crystallites, the equation of state of the glasses, PV/NkT=2.67/(1−z/0.8754), and the statistical entropy, obtained by computing the configuration integral, are reproducible. The pressure of the reproducible glasses does not diverge as fast as that of the crystal, with the result that the excess entropy of the glass (relative to the crystal) diverges. It is argued that the glass transition in hard spheres is a consequence of the geometry of the configuration space, and not simply a kinetic phenomenon.
Scattering of Ne from the liquid–vapor interface of glycerol: A molecular dynamics study
Authors: Benjamin Ilan , Wilson Michael , Pohorille Andrew
Published: July 2002

A model potential for the scattering of Ne off liquid glycerol is developed. The model is based on a nine-site description of glycerol which takes into account torsional flexibility and hydrogen bonding. This model is used to carry out molecular dynamics calculations of the scattering as a function of collision energy. The results for the sticking probability and energy transfer are in good agreement with experiments. The model predicts a wide angular distribution of the scattered atoms with a mild decrease in the energy transfer as a function of exit angle for a fixed incident angle. The model also provides insight into the importance of the corrugated nature of the surface and the types of liquid modes that play a major role in the energy transfer process.
Spin–orbit coupled excited states in transition metal complexes: A configuration interaction treatment of HCo(CO)4
Authors: Ribbing Carl , Daniel Chantal
Published: July 2002

A direct spin–orbit coupled configuration interaction method is presented. The basis functions are Slater determinants expressed as combinations of alpha and beta strings. The one and two electron operators are expressed in second quantization form. Spin–orbit coupling is described as a one electron effective operator. The structure of the configuration expansion is single and double excitations from a limited full CI space. Calculations of the Ms=0 component of the three lowest excited states 1,3E and 3A1 in HCo(CO)4 have been performed in order to evaluate the spin–orbit interactions responsible for the 1E→3E and the 1E→3A1 radiationless transitions. A large difference is found in favor of the 1E→3E path.
Inhomogeneities and relaxation in supercooled liquids
Authors: Mohanty U.
Published: July 2002

Nonexponential relaxation in glass forming liquids has been attributed by Robertson and Donth to inhomogeneous distribution of small local regions. We show, based neither on free-volume nor on configurational entropy theories that the correlation volume V of such inhomogeneous regions isR [ΔH* (1−x)/RT]2{kBT4gΔκTg/〈 Δ2 ln τ〉}, where Δh* is the enthalpy of activation near the glass transition temperature Tg, x is the Narayanaswamy–Gardon nonlinear parameter, ΔκTg is the change in thermal conductivity at Tg, 〈Δ2 ln τ〉, describes how wide is the spectrum of relaxation times, and kB and R are the Boltzmann and the gas constants, respectively. The correlation length does not diverge at Tg. In fact, the correlation length at Tg for B2O3, glycerol, and PVAc are found to be approximately 1.27, 0.91, and 1.53 nm, respectively. Our results indicate, in agreement with Moynihan and Schroeder, that characteristics of nonexponential relaxation in glass forming liquids may be due to inhomogeneous domains whose size are in the nanometer length scale.
Improved KrF(B) and KrF(X) state potentials
Authors: Lo Glenn , Setser D. W.
Published: July 2002

The KrF(B–X) emission spectra were simulated to obtain improved models of the electronic state potentials and transition dipole moment functions that are also consistent with recently published photoassociation spectra. The spectrum from a 300 K vibrational distribution of KrF(B) molecules was used to determine the nature of the potentials near Re′; the transition dipole function was mainly based on the emission spectra from highly excited KrF(B,v′) distributions. The upper state potential was represented by a truncated Rittner potential with ωe′ = 330 cm−1; the lower state potential was represented by an exponential function at a short internuclear distance plus a −c6/R6 function at a large distance. The electronic transition dipole moment function μel(R) was represented by a linear combination of Gaussian functions, with a maximum near the classical inner turning points of the v′=5, 6 levels. Comparison is made with the KrF(X) potential deduced from molecular beam work. The well depth assigned by Aquilanti et al. from the scattering data is larger than the value obtained from the −c6/R6 function, and additional experimental work is needed to define De″.
Circumventing the Heisenberg principle: A rigorous demonstration of filter-diagonalization on a LiCN model
Authors: Neuhauser Daniel
Published: July 2002

In a previous paper [J. Chem. Phys. 93, 2611 (1990)] a new method, filter diagonalization, was introduced for extracting highly excited rovibrational states from an arbitrary Hamiltonian, in any desired energy range. In the method, an arbitrary initial wave packet is propagated for a short time and during the propagation a ‘‘short time filter’’ of the wave packet is accumulated at various energies in any desired ‘‘window,’’ yielding a small set of functions which span the eigenfunctions of the Hamiltonian in the desired range. A small Hamiltonian matrix is then evaluated in the filtered-functions basis, to yield the eigenvalues in the desired range. The combination of the time-dependent (TD) propagation with the small matrix diagonalization eliminates the uncertainty-relation limitation associated with a pure TD approach and the large-matrix diagonalization necessary in a purely time-independent approach. In this paper we give the first demonstration of the power of filter diagonalization for a molecular Hamiltonian exhibiting accidental near degeneracies, thereby supplying a stringent test of our approach. A two-dimensional model of LiCN (J=0) is used. Good agreement is established with previous results for high-energy states. For a further check of consistency, we perform a large-scale direct diagonalization of the Hamiltonian, and verify a very-high accuracy even for nearly degenerate levels. Extraction of these levels by a purely TD approach would have necessitated ≊700-fold increase in propagation time.
Anharmonic oscillator modeling of nonlinear susceptibilities and its application to conjugated polymers
Authors: Takahashi Akira , Mukamel Shaul
Published: July 2002

Molecular optical susceptibilities are calculated by deriving equations of motion for the single electron reduced density matrix, and solving them using the time dependent Hartree–Fock (TDHF) approximation. The present approach focuses directly on the dynamics of the charges in real space and completely avoids the tedious summations over molecular eigenstates. It further maps the system onto a set of coupled harmonic oscillators. The density matrix clearly shows the electronic structures induced by the external field, and how they contribute to the optical response. The method is applied to calculating the frequency-dispersed optical susceptibility χ(3) of conjugated linear polyenes, starting with the Pariser–Parr–Pople (PPP) model. Charge density wave (CDW) like fluctuations and soliton pair like local bond-order fluctuations are shown to play important roles in the optical response of these systems.
Quantum simulations of nonlinear resonance and torsional dynamics
Authors: Collins Michael A. , Schranz Harold W.
Published: July 2002

A simple model of the vibrational dynamics of ABBA type sequentially bonded tetra-atomic molecules is investigated by quantum mechanical methods. The model Hamiltonian excludes bond stretching and asymmetric bending but includes the kinematic coupling between the torsional motion and symmetric bond bending which results in nonlinear resonances. The effect of this coupling on energy levels and the timescale of intramolecular energy transfer is evaluated and discussed in terms of both resonant and nonresonant effects.
Evidence for the stabilization of rectangular O+4 in solid neon
Authors: Jacox Marilyn E. , Thompson Warren E.
Published: July 2002

The reassignment of the 1320 and 2949 cm−1 absorptions of O+4, previously attributed to the trans isomer, is considered. A satisfactory least-squares force constant fit of the isotopic data for these two absorptions, assuming a rectangular structure, has been obtained. In accord with the results of ab initio calculations by Lindh and Barnes, it is concluded that, as previously proposed, the 1164 and 2808 cm−1 absorptions should be assigned to the trans ground state of O4+O, but that the rectangular isomer, which they calculated to lie at only slightly higher energy, may contribute the 1320 and 2949 cm−1 absorptions.
Hydrogen bonding in supercritical water
Authors: Chialvo Ariel A. , Cummings Peter T.
Published: July 2002

We study the hydrogen bonding structure of water models at supercritical conditions by molecular dynamics to directly compare with recent microstructural data obtained by neutron diffraction with isotopic substitution (NDIS) experiments. We also study the angular dependence of the hydrogen–oxygen pair distribution function to gain insight into the hydrogen bonding mechanism in the molecular models for water. The simulation results suggest that the angle-averaged radial distribution function gOH(r) measured by NDIS experiments may not provide a complete picture of the degree of hydrogen bonding.
Electronic decay processes following the resonance excitation of the B 1s core electron in BF3
Authors: Ueda K. , Chiba H. , Sato Y. , Hayaishi T. , Shigemasa E. , Yagishita A.
Published: July 2002

Electron spectroscopy has been carried out to investigate spectator and participant resonance Auger decay processes following the B 1s→2a2″ excitation in gaseous BF3 molecules using monochromatized synchrotron radiation. The resonance-enhanced satellite bands corresponding to the spectator Auger electron emission form six broad peaks showing good correspondence with the normal Auger bands. Resonance enhancement of the photolines corresponding to the participant Auger electron emission occurs only for the bonding orbitals having B character.
Nuclear-spin relaxation in paramagnetic complexes in the slow-motion regime for the electron spin: The anisotropic pseudorotation model for S=1 and the interpretation of nuclear magnetic relaxation dispersion results for a low-symmetry Ni(II) complex
Authors: Larsson Tomas , Westlund Per-Olof , Kowalewski Jozef , Koenig Seymour H.
Published: July 2002

This work presents the anisotropic pseudorotation (APR) model, which is a new dynamic model for the interpretation of experimental nuclear spin-lattice relaxation times in paramagnetic (S=1) complexes of low symmetry. It comprises two dynamic processes active in modulating the zero-field splitting interaction (ZFS). Reorientation of the complex modulates a static zero-field splitting, defined as a measure of the asymmetry in the equilibrium geometry at the paramagnetic site. Local motions of the ligands surrounding the paramagnetic site further contribute a rapidly fluctuating (transient) zero-field splitting interaction. This dynamic model is evaluated within a general theoretical framework capable of dealing with the electron-spin system in the low- and high-magnetic field limits for both Redfield and slow-motion cases, i.e., where the motions inducing electron-spin relaxation and the electron-spin relaxation itself are characterized by the same time scale. The dynamic model is characterized and discussed by calculating results for a large number of parameter sets. The obtained results are compared with the traditional theory, the Solomon–Bloembergen–Morgan equations (SBM), by least-squares fitting the SBM equations to the APR model. Results show that in most cases the SBM model can fit the nuclear magnetic relaxation dispersion (NMRD) profiles from the APR model at the expense of using a different parameter set. For both models, a restricted fit to experimental NMRD data, from bis(2,2,6,6-tetramethyl-3,5-heptanedionato)Ni(II)(aniline-d5)2 (abbreviated Ni(dpm)2(aniline-d5)2 or simply NIDPM) in solution, has been performed. The parameters obtained suggest that NIDPM is a slow-motion case comprising a static contribution to its zero-field splitting, so that the SBM model is inapplicable.
Theoretical investigation of nonstatistical dynamics, energy transfer, and intramolecular vibrational relaxation in isomerization reactions of matrix-isolated HONO/Xe
Authors: Agrawal Paras M. , Thompson Donald L. , Raff Lionel M.
Published: July 2002

Theoretical molecular dynamics studies of cis–trans isomerization, intramolecular vibration relaxation (IVR), and vibrational relaxation rates to lattice phonon modes of HONO isolated in a face-centered cubic (fcc) xenon matrix at 12 K are reported. The effect of the matrix environment upon the dynamics is obtained by comparison with the corresponding gas-phase results. Questions related to statistical vs nonstatistical behavior and the effect of lattice imperfections are also addressed. It is found that both cis→trans and trans→cis isomerization rates are significantly enhanced by the presence of the matrix in spite of the steric effects produced by the environment. It is shown that this result occurs because the matrix opens a (vibration→lattice phonon modes→rotation→torsional vibration) energy transfer path. The calculated isomerization rate coefficients indicate significant nonstatistical dynamics. The IVR rates in the matrix and in the gas phase are slow relative to the isomerization rates. Consequently, the isomerization cannot be statistical. The calculated cis→trans and trans→cis ratio is found to be significantly less than previously reported measurements indicate. Vibrational relaxation rates to the lattice phonon modes are found to be almost independent of the initial energy partitioning. It is suggested that this may be a result of the transfer rates approaching their limiting values determined by the Debye frequency of the lattice. The presence of lattice vacancies is found to exert a profound influence upon the dynamics. When the percentage of lattice vacancies approaches 20%, the calculated dynamics in the matrix are found to approach the gas-phase results.
Monte Carlo simulations of the adsorption of rare gases at high pressures
Authors: Vermesse J. , Levesque D.
Published: July 2002

The excess surface adsorption of rare gases on the surface of a graphite crystal is calculated, at room temperature, by numerical simulations for thermodynamic states of densities covering the full domain of their fluid phases. A comparison with the measured excess surface adsorption on active carbon, in a domain of pressures from 5 to 600 MPa, shows that the experimental data are in qualitative agreement with the simulation results. When the pressure is increased, the excess surface adsorption first goes through a maximum, then decreases towards a shallow minimum, and subsequently stays constant or increases slightly up to the solid phase of rare gases in both the experiments and the simulations.
Measurement of vibrational energy redistribution rates in molecular clusters by time-resolved stimulated emission pumping
Authors: Hineman Max F. , Bernstein Elliot R. , Kelley David F.
Published: July 2002

Time-resolved stimulated emission pumping has been used to measure the vibrational energy redistribution rate in jet cooled aniline (N2)1 clusters, following 11 excitation. An IVR time of 200±50 ps is obtained, which is consistent with earlier time correlated single photon converting measurements.
Structure, internal mobility, and spectrum of the ammonia dimer: Calculation of the vibration–rotation-tunneling states
Authors: Olthof E. H. T. , van der Avoird A. , Wormer P. E. S.
Published: July 2002

We have obtained a potential for (NH3)2 by calculating the six-dimensional vibra- tion–rotation-tunneling (VRT) states from a model potential with some variable parameters, and adjusting some calculated transition frequencies to the observed far-infrared spectrum. The equilibrium geometry is strongly bent away from a linear hydrogen bonded structure. Equivalent minima with the proton donor and acceptor interchanged are separated by a barrier of only 7 cm−1. The barriers to rotation of the monomers about their C3 axes are much higher. The VRT levels from this potential agree to about 0.25 cm−1 with all far-infrared frequencies of (NH3)2 observed for K=0, ‖K‖=1, and ‖K‖=2 and for all the symmetry species: Ai=ortho–ortho, Ei=para–para, and G=ortho–para. Moreover, the dipole moments and the nuclear quadrupole splittings agree well with the values that are observed for the G states. The potential has been explicitly transformed to the center-of-mass coordinates of (ND3)2 and used to study the effects of the deuteration on the VRT states. The observed decrease of the dipole moment and the (small) changes in the nuclear quadrupole splittings are well reproduced. It follows from our calculations that the ammonia dimer is highly nonrigid and that vibrational averaging effects are essential. Seemingly contradictory effects of this averaging on its properties are the consequence of the different hindered rotor behavior of ortho and para monomers.
Vacuum ultraviolet Fourier transform spectroscopy of the δ(0,0) and β(7,0) bands of NO
Authors: Murray J. E. , Yoshino K. , Esmond J. R. , Parkinson W. H. , Sun Y. , Dalgarno A. , Thorne Anne P. , Cox G.
Published: July 2002

The combined band oscillator strength of the δ(0,0) and β(7,0) bands of nitric oxide has been measured in absorption by utilizing a vacuum ultraviolet Fourier transform spectrometer to resolve the rotational line profiles. We give accurate rotational line positions of the δ(0,0) and β(7,0) bands, and improved term values up to J=19.5 for the C 2Π(v=0) and J=8.5 for the B 2Π(v=7) levels, measured from the ground state. A deperturbation procedure has been used on the mixed levels, and the calculated molecular constants are in good agreement with previous work.
Solvation effects on association reactions in microclusters: Classical trajectory study of H+Cl(Ar)n
Authors: Schmidt Burkhard , Gerber R. Benny
Published: July 2002

The role of solvent effects in association reactions is studied in atom-cluster collisions. Classical trajectory studies of the systems H+Cl(Ar)n (n=1,12) are used to investigate the influence of size, structure, and internal energy of the ‘‘microsolvation’’ on the H+Cl association reaction. The following effects of solvating the chlorine in an Arn cluster are found. (1) In the H+ClAr system there is a large ‘‘third body’’ effect. The single solvent atom stabilizes the newly formed HCl molecule by removing some of its excess energy. The cross section found at low energies is a substantial fraction of the gas-kinetic cross section. The molecule is produced in highly excited vibrational-rotational states. (2) Some production of long-lived HCl...Ar complexes, with lifetimes of 1 ps and larger, is found for the H+ClAr collisions. Weak coupling stemming from the geometry of the cluster is the cause for long life times. These resonance states decay into HCl+Ar. (3) At low collision energy (E=10 kJ/mol) for H+Cl(Ar)12, the H+Cl association shows a sharp threshold effect with cluster temperature. For temperatures T≥45 K the cluster is liquidlike, and the reaction probability is high. For T≤40 K the cluster is solidlike, and there is no reactivity. This suggests the potential use of reactions as a signature for the meltinglike transition in clusters. (4) At high collision energies (E=100 kJ/mol) H atoms can penetrate also the solidlike Cl(Ar)12 cluster. At this energy, the solid–liquid phase change is found not to increase the reaction probability.
Diffusion-controlled reactions. II. An approach based on a generalized diffusion equation
Authors: Dong W. , Andre J. C.
Published: July 2002

In this work we present a new theoretical approach for diffusion-controlled reactions which generalizes the classic theory of Smoluchowski and that of Collins and Kimball. In this approach, the non-Markovian effect is taken into account in the framework of a generalized diffusion equation with a time-dependent diffusion coefficient. When Smoluchowski’s absorbing boundary condition is considered, we have found the exact analytical solution of the generalized diffusion equation despite the presence of a time-dependent diffusion coefficient. Our generalized Smoluchowski theory removes the unphysical singularity in reaction rate, k(t), at t=0 in the classic Smoluchowski theory. This allows to evaluate the initial value of reaction rate. The result shows that the initial reaction rate is overestimated for collision-induced reactions. This is due to the inappropriateness of the absorbing boundary condition for describing the collision-induced reactions. To take into account more properly these kinds of reactions, Collins–Kimball’s boundary condition is considered. In this case, a perturbation method and an approximation ansatz are developed to find an approximate solution. The approximation for reaction rate contains an adjustable parameter. A scheme is prescribed for the optimal choice of this parameter in short and long time regions. With the help of this scheme, we obtain an approximation of the reaction rate which has the virtue to give simultaneously the exact asymptotic behavior at long time and the right initial value of the reaction rate.
Semiclassical analysis of hierarchical spectra
Authors: Sepúlveda Miguel Angel , Heller Eric J.
Published: July 2002

The semiclassical techniques developed in the previous paper are applied to the understanding of the hierarchical structure underlying the spectra. This organization, as analyzed by Davis with statistical models, is revealed by continuously changing the energy resolution of the spectra and noting the branching pattern of the peaks. We argue that the greater part of this hierarchical organization can be understood with classical events in the time domain.
Molecular dynamics simulations of a flexible molecule in a liquid crystalline solvent
Authors: Alejandre J. , Emsley J. W. , Tildesley D. J. , Carlson P.
Published: July 2002

Molecular dynamics simulations of a hexane molecule in isotropic, nematic, and smectic phases are reported. The interactions between the solvent molecules is modeled using the Gay–Berne potential and the hexane–solvent potential is modeled as an explicit site–site interaction. The conformational distributions are reported of the hexane molecule at a fixed temperature but at different densities in isotropic, nematic, and smectic phases, and these are compared with the results from Monte Carlo simulations on an isolated molecule at the same temperature. The positions and orientations of the hexane molecule at each time step in the molecular dynamics simulations are used to calculate interproton dipolar coupling constants. These data are used to test the mean field models which have been applied with dipolar couplings obtained previously from nuclear magnetic resonance (NMR) experiments to yield conformational distributions.
Mode selective chemistry in the reactions of OH with HBr and HCl
Authors: Clary David C. , Nyman Gunnar , Hernandez Ramon
Published: July 2002

Quantum scattering calculations are reported for the reactions OH+HCl→H2O+Cl and OH+HBr→H2O+Br. The rotating bond approximation is used. This involves the explicit treatment of the bending vibration and local OH stretching vibration in H2O together with the vibration of HX (X=Cl,Br) and rotation of OH. Simple potential energy surfaces for the reactions are used which contain an accurate potential for H2O. The transition state of the potential for the OH+HCl reaction agrees quite well with ab initio data. The most likely product vibrational state of H2O is the ground state for the OH(j=0)+HCl reaction, and the combination band that has one quantum of energy in the H2O bending mode and one quantum in the local OH stretching mode of H2O for the OH(j=0)+HBr reaction. The reaction cross sections are found to depend on (2j+1)−1, where j is the initial rotational quantum number of OH. This results in a T−1/2 dependence in the rate constant for the OH+HBr reaction at low temperatures, in agreement with experiment.
The complex of N2 with H2O, D2O, and HDO: A combined ab initio and diffusion Monte Carlo study
Authors: Sandler P. , Jung J. oh , Szczȩs̀niak M. M. , Buch V.
Published: July 2002

We present an analysis of the complex of water with N2 which includes (a) ab initio calculations of the potential energy surface; (b) ab initio analysis of the different contributions to the interaction energy (exchange, electrostatic, dispersion, induction); (c) a diffusion Monte Carlo study of the vibrational ground state and a calculation of vibrationally averaged spectroscopic constants for the different isotopic species; (d) construction of an N2⋅⋅H2O potential function by fitting to ab initio points followed by adjustment against the experimental rotational and quadrupole coupling constants of Leung et al. [J. Chem. Phys. 90, 700 (1989)].
Comment on ‘‘Mean force potential for the calcium–chloride ion pair in water’’ [J. Chem. Phys. 99, 4229 (1993)]
Authors: Dang Liem X. , Smith David E.
Published: July 2002

The interionic potential of mean force (pmf) for the Ca++–Cl− ion pair in water is computed using the molecular dynamics computer simulation technique. The calculated pmf indicates a stable contact pair (CIP) and a solvent-separated pair (SSIP) centered at 2.9 and 5.0 Å with a 2.8 kcal/mol barrier to dissociation. The SSIP well is about 2.0 kcal/mol deeper than the CIP suggesting that water molecules in the first hydration shell are strongly coordinated to the Ca++ ion. Our results do not agree with the pmf reported recently by Guardia, Robinson, Padro [J. Chem. Phys. 99, 4229 (1993)]. Possible reasons for the discrepancy are discussed.
Spectroscopic verification of the theoretical reassignment of H2O+ Ã-state vibrational levels
Authors: Dressler Rainer A. , Arnold Susan T.
Published: July 2002

A Franck–Condon analysis of a low-resolution (4 nm FWHM) H2O+ Ã 2A1–X̃ 2B1 spectrum is presented in order to support recent ab initio calculations of the H2O+ X̃/Ã Renner–Teller system that call for a reassignment of the Ã-state vibrational levels [Brommer et al., J. Chem. Phys. 98, 5222 (1993)]. Ã–X̃ luminescence with well-resolved vibronic bands is observed from near-resonant Kr+(2P1/2)+H2O charge–transfer collisions. It is demonstrated that the intensity of vibronic progressions is only truthfully reproduced if the previously calculated vibronic transition moments [Jungen, Hallin, and Merer, Mol. Phys. 40, 25 (1980)] are applied to the reassigned states, thus confirming the ab initio work.
Layers of semiflexible chain molecules endgrafted at interfaces: An off-lattice Monte Carlo simulation
Authors: Haas F. M. , Hilfer R. , Binder K.
Published: July 2002

A coarse-grained model for surfactant chain molecules at interfaces in the high density regime is studied using an off-lattice Monte Carlo technique. The surfactant molecules are modeled as chains consisting of a small number (e.g., seven) of effective monomers. For the modeling of lipid monolayers, each effective monomer is thought to represent several CH2 groups of the alkane chain, but applications of the model to other polymers end grafted at solid surfaces also should be possible. The head segments are restricted to move in the adsorption plane, but otherwise do not differ from the effective monomers, which all interact with Lennard-Jones potentials. Bond angle and bond length potentials take into account chain connectivity and chain stiffness. The advantage of this crude model is that its phase diagram can be studied in detail. Temperature scans show two phase transitions, a tilting transition at low temperatures between a tilted and an untilted phase, and a melting transition at high temperatures where the lattice of head groups loses its crystalline order.
Numerical Liouville approach: Three-type analysis of virtual excitation processes of third-order nonlinear optical spectra in third-harmonic generation
Authors: Nakano M. , Yamaguchi K. , Matsuzaki Y. , Tanaka K. , Yamabe T.
Published: July 2002

An analysis method based on virtual excitation processes is developed in numerical Liouville approach, which is a new nonperturbative method presented before. In this analysis method (NLA three-type analysis method), third-order nonlinear optical susceptibilities χ(3)g(ω) obtained by the NLA method can be divided into the three-type virtual excitation processes similar to the three-type analysis based on the time-dependent perturbation theory (TDPT three-type analysis). The NLA three-type analysis method is superior to the TDPT three-type analysis one in the points as follows: (1) the NLA three-type analysis can treat the phenomena caused by the intensity-dependent nonlinear susceptibilities including nonperturbative effects and (2) the NLA three-type analysis can divide the resonant nonlinear susceptibilities into three types of virtual excitation processes. In order to explain the procedure of the present analysis, we try to elucidate the characteristics of virtual excitation processes of nonlinear optical spectra in THG for a three-state model that mimics the electronic excited states of a system with noncentrosymmetric charge distribution.
Negative velocity correlation in hard sphere fluid
Authors: Burshtein A. I. , Krongauz M. V.
Published: July 2002

Two methods are used to calculate the velocity correlation function K(t) in a hard spheres fluid. The first is a conventional impact theory but with a non-Poissonian distribution of collision times found in simulation experiments. Although not large the negative loop in K(t) appears when the tagged molecule is surrounded by heavier neighbors and back reflection dominates. At liquid densities the heavy particles screen each other from grazing collisions and as a result there is a strong back reflection of the tagged molecule. An alternative mechanism which is appropriate for the particles of equal mass neglects the non-Poissonian corrections, but regards the velocity turns as a highly correlated Markovian process. This results in long-lived circulations inside the liquid cell as in an imperfect ring billiard. The extent of the negative velocity correlation and location of its minimum are described well in this model, which gives the most reliable explanation for quasiperiodic velocity relaxation exhibited in different spectroscopic manifestations.
Characterization of the theta state and transition curves of off-lattice three dimensional chains
Authors: Rubio Ana M. , Freire Juan J. , Clarke Julian H. R. , Yong C. W. , Bishop Marvin
Published: July 2002

We have performed a Monte Carlo study of the dimensions (radius of gyration) and the intrinsic viscosity for three dimensional linear chains of different lengths. The chains are composed of Gaussian units which interact through a 6–12 Lennard-Jones potential. From the study of the radius of gyration, the theta state has been characterized for this model. Moreover, the results for different temperatures and chain lengths have been plotted in terms of transition curves in order to analyze the collapse and excluded volume limiting behaviors. Also, a lower bound for the viscosity parameter Φ has been obtained in the theta state, as well as in the collapse and excluded volume regimes.
Colloidal interactions mediated via polyelectrolytes
Authors: Podgornik Rudolf , Åkesson Torbjörn , Jönsson Bo
Published: July 2002

Using Monte Carlo simulation, scaling, variational and mean-field arguments we investigate forces between charged spherical aggregates conferred by oppositely charged polymeric chains. Two types of polymer mediated attraction are found in this system, both of a bridging type but differing markedly in terms of the range. The entropic bridging force is of a range comparable to the average monomer–monomer separation in the chain. It is present whenever many chains have to compensate the charge on two macroions. The energetic bridging force has a range of the order of the length of the polymer chain and pertains to situations when a single chain has to compensate the charge on more than one macroion. In what follows we shall give a detailed analysis of both bridging interactions with a special regard for polycounterion versus simple counterion effects. The two types of bridging are in a certain sense complementary and should be present in polymer– surfactant systems at different regimes of the polymer–macroion concentration ratios.
Symplectic integrators for the multichannel Schrödinger equation
Authors: Manolopoulos David E. , Gray Stephen K.
Published: July 2002

The multichannel radial Schrödinger equation that arises in time-independent inelastic scattering theory and certain bound state problems has a classical Hamiltonian structure in which the radial coordinate plays the role of time. One consequence of this Hamiltonian structure is that the Schrödinger equation has symplectic symmetries, which lead in the context of inelastic scattering to the unitarity and symmetry of the S matrix. Another consequence is that so-called symplectic integrators can be used to solve the radial Schrödinger equation, both for bound state and scattering problems. This idea is used here to derive a new family of symplectic integrator-based log derivative methods for solving the multichannel radial Schrödinger equation. In addition to being simpler to write down and program, these methods are shown to be highly competitive with Johnson’s original log derivative method for several inelastic scattering and bound state test problems. An equivalent solution following version of the symplectic integrator family is also introduced and shown to have similar advantages over the DeVogelaere method. A number of more formal consequences of the classical Hamiltonian structure of the radial Schrödinger equation are also noted.
The F+HD→DF(HF)+H(D) reaction revisited: Quasiclassical trajectory study on an ab initio potential energy surface and comparison with molecular beam experiments
Authors: Aoiz F. J. , Bañares L. , Herrero V. J. , Sáez Rábanos V. , Stark K. , Werner H.-J.
Published: July 2002

The dynamics of the F+HD reaction has been studied by means of quasiclassical trajectory calculations on an ab initio potential energy surface (PES) at several collision energies. At the collision energy of 85.9 meV and for the DF+H isotopic channel of the reaction, there is a remarkable agreement between calculated and experimental results, in both the center of mass (c.m.) differential cross sections (DCS) and in the simulation of the laboratory (LAB) time of flight (TOF) and angular distributions (AD). The good agreement also extends to the lower collision energy of 58.6 meV for this channel of the reaction. In contrast, the simulation of the LAB angular distributions for the HF+D channel shows strong discrepancies between theory and experiment at both collision energies, which can be traced back to the absence of a forward peak in the calculated c.m. DCS for HF(v′=3). Simulations made from QCT calculations on other PES with important HF(v′=3) forward scattering contributions also fail to reproduce the overall AD. The theoretical findings and especially the roles of translational energy and initial rotational momentum on the dynamics of this reaction are discussed in terms of the topology of the PES through the analysis of individual trajectories.
Intramolecular electronic coupling enhanced collisional deactivation of highly vibrationally excited molecules
Authors: Hartland Gregory V. , Qin Dong , Dai Hai-Lung
Published: July 2002

The collisional deactivation of highly vibrationally excited NO2 and CS2 by a variety of buffer gases has been examined by time-resolved Fourier transform IR emission spectroscopy. The results show that there is a dramatic increase in the average energy removed per collision for NO2 excited above ∼10 000 cm−1 and for CS2 above ∼26 000 cm−1. These energies correspond to the origins of the lowest excited Ã 2B2/B̃ 2B1 states of NO2 and the lowest excited R 3A2 state of CS2. Mixing between these excited electronic states with the ground electronic state enhances collisional relaxation by allowing the electronic transition dipole to contribute to collisional energy transfer.
Effects of solvent polarization relaxation on nonadiabatic outersphere electron transfer reactions in ultrafast dipolar solvents
Authors: Roy Srabani , Bagchi Biman
Published: July 2002

Since the important work of Efrima and Bixon [J. Chem. Phys. 70, 3531 (1979)], it is believed that solvent polarization relaxation is usually too slow (compared to the rate of electron transfer) or the amplitude of energy fluctuation too large to have any noticeable effect on the dynamics of the nonadiabatic (NA) electron transfer reactions. On the other hand, recent studies have demonstrated that solvent polarization relaxation in several common dipolar liquids can proceed at a rate much faster than that anticipated in the earlier studies. This calls for a re-examination of the role of solvent dynamics on NA electron transfer reactions in these ultrafast solvents. In this paper, the results of such studies are presented for NA reactions in water and acetonitrile. It is found that because of ultrafast solvation, many NA reactions may lie in the dynamic region where the solvent effects are just beginning to be important. The present study further reveals the following new results. (i) In the case of high barrier reactions in solvents such as acetonitrile, the polarization relaxation in the reactant well can contribute significantly to the total rate of the nonadiabatic electron transfer reaction. (ii) In water, on the other hand, the reactive friction is still sufficiently high to make energy diffusion in the reactant well efficient and so, the solvent effects are predicted to be negligible. This is in accord with the earlier theoretical suggestions and is demonstrated here clearly for real systems. (iii) We find an interesting limiting situation where the long time rate can be significantly larger than the rate given by the Marcus expression.
One-dimensional chemical master equations: Uniqueness and analytical form of certain solutions
Authors: Samoilov Michael , Ross John
Published: July 2002

The eikonal (WKB) approximation is applied to a stationary one-dimensional master equation describing an arbitrary reaction mechanism. The uniqueness of a nontrivial (fluctuational) eikonal solution is proven. Consistent eikonal and exact analytical solutions are obtained for systems with an arbitrary, but unique step size of stochastic transitions. An analytical eikonal solution for the stationary probability density for systems with mixed step sizes of 1 and 2 is also obtained and found to differ significantly from the systems with a uniform step size, particularly in the case of multiple stationary states.
Competitive tracking of molecular objectives described by quantum mechanics
Authors: Chen Yu , Gross Peter , Ramakrishna Viswanath , Rabitz Herschel , Mease Kenneth
Published: July 2002

The control of molecular events by optical fields is sought with the methods of asymptotic inverse tracking, local track generation (model matching), and competitive tracking which are extensions of exact inverse tracking. The methodology is applied to infrared dissociation of a diatomic molecule and selective dissociation of the stronger bond in a highly coupled linear triatomic system. The major appeal of these methods is that they do not require costly iterations unlike other control studies in which optimization techniques are used to design fields to achieve desired molecular objectives. It is found that in exact inverse tracking where a requisite external field is obtained to exactly track a prescribed objective expectation value as a function of time, a high degree of intuition is required to find an a priori objective track such that the required fields are reasonable in terms of intensity and bandwidth. Furthermore, exact inverse tracking does not allow for tracking of multiple observables. The extensions of the inverse tracking method presented in this work help to alleviate these drawbacks. In all of these extensions the requisite field is computed locally in time through minimization of a cost functional which contains terms designed to minimize the error between the objective and actual tracks and also minimize the field fluence. The objective tracks can be prescribed a priori as in exact inverse tracking or from the present evolving system state (local track generation). Competitive tracking allows for the following of multiple observables although none will be tracked exactly. Locally generated tracks (model matching) require less physical intuition because it is easier to specify an objective track with current knowledge of the state of the system. However, the tradeoff with this method is that prediction of the behavior of the tracked observables may be elusive.