bluuebunny/crossref_metadata_2025 huggingface dataset:
Mixing and segregation in binary polar-molecule clusters
Authors: Molinero Valeria , Laria Daniel , Kapral Raymond
Published: July 2002

Structural and dynamical properties of liquid-state, binary, water-acetonitrile nanoclusters are studied. When acetonitrile is the solute species, it exhibits a propensity to reside on the cluster surface, although one may identify regions interior to the cluster that are favorable for solvation. The dynamics of the interchange of acetonitrile between surface and interior solvation regions and structural aspects of surface solvation states are studied. When water is the solute it tends to be solvated in the interior of the cluster and form aggregates. The nature and dynamics of contact and solvent separated water pairs in these clusters is investigated, and the recombination dynamics of larger water aggregates in concentrated water-acetonitrile clusters is examined.
Photoionization and photodissociation dynamics of H2 after (3+1) resonance-enhanced multiphoton ionization via the B 1Σu+ state
Authors: Scheper C. R. , Buma W. J. , de Lange C. A. , van der Zande W. J.
Published: July 2002

We present a study of the molecular photoionization and photodissociation processes in molecular hydrogen occurring after one-photon absorption from various rovibrational levels (v′=3–22, J′=0–3) of the B 1Σu+(1sσg)(2pσu) state using resonance-enhanced multiphoton ionization in combination with high-resolution photoelectron spectroscopy (REMPI-PES). For one-photon absorption from the v′=3–8 levels, molecular photoionization competes with photodissociation into a ground-state atom and an atom in an n=2 excited state. A detailed comparison of the photoelectron spectra obtained via different rotational branches and vibrational levels strongly indicates that singly excited bound Σg+1 and Πg1 Rydberg states at the four-photon level exert a significant influence on the final state distributions of H2+. In contrast, one-photon absorption from the v′=9 and higher levels leads almost exclusively to dissociation into a ground-state atom and an excited-state atom with n>2. Excited atomic fragments are ionized in a one-photon absorption step, and excited-atom distributions over the energetically allowed values of the principal quantum number n are obtained. Simulations of these distributions suggest that excitation of dissociative continua of bound Σg+1(1sσg)(nsσg), Σg+1(1sσg)(ndσg), and Πg1(1sσg)(ndπg) Rydberg states may dominate over excitation of dissociative doubly excited Σg+1(2pσu)(npσu) and Πg1(2pσu)(npπu) states when considering the dissociation dynamics after one-photon absorption from the v′⩾9 levels of the B-state.
Vibrational predissociation dynamics of ArHF (3000) and (3110): Lifetimes and HF product state distributions
Authors: Chuang Cheng-Chi , Tsang Susy N. , Klemperer William , Chang Huan-Cheng
Published: July 2002

The lifetimes and HF (v=2) product rotational distributions (j′=13 to 9) arising from the vibrational predissociation of ArHF (3000) and (3110) states have been determined by laser-induced dispersed fluorescence. The lifetimes of (3000) and (3110) are found to be 3.9(4)×10−6 and 7.3(8)×10−6 s, respectively, showing both intermolecular vibration and HF valence state dependence. The variation in the vibrational predissociation rate for these two states indicates a strong angular dependence of the change of interaction potential with the HF internuclear distance in the complex. The product state distribution of ArHF (v=3) reveals the excess energy, 3380 cm−1, resulting from vibrational predissociation (Δv=−1) is partitioned primarily into HF product rotation. The rotational distribution observed following excitation of the (3110) level is considerably broader than the relatively sharply peaked distribution from (3000). The vHF=3 ground state, (3000), vibrationally predissociates into j′=13 at a rate of τvp=1.2×105 s−1, which is one half of the rate of predissociation to all states.
Molecular polarizability and atomic properties: Density functional approach
Authors: Itskowitz Peter , Berkowitz Max L.
Published: July 2002

In our previous work [P. Itskowitz and M. L. Berkowitz, J. Phys. Chem. A 101, 5687 (1997)], we showed how in the framework of the density functional theory the energy of a molecule can be expressed as a functional of the perturbations on atomic densities. In the present work we discuss the forms of the atomic hardness kernels that enter the energy expression and apply our approach to the problem of finding the response of molecules to an applied electric field. We obtain a system of linear equations for the density perturbations on each atom in a molecule due to the applied electric field. The calculated values of polarization tensor components of several planar molecules are reported.
Microwave spectroscopy of the 2,4-pentadiynyl radical: H2C–C≡C–C≡CH
Authors: Chen Wei , Novick Stewart E. , McCarthy M. C. , Thaddeus P.
Published: July 2002

The 2,4-pentadiynyl radical, H2C–C≡C–C≡CH, was detected in a pulsed-discharge supersonic molecular beam by Fourier transform microwave spectroscopy. Four successive rotational transitions, N=2–1 through N=5–4 in the Ka=0 ladder were observed between 8 and 22 GHz. A total of 48 hyperfine components were measured and assigned with an uncertainty of 5 kHz. Seven spectroscopic constants, including rotational and centrifugal distortion constants, the spin-rotation constant, and hyperfine coupling constants for the methylenic and acetylenic protons were determined to high accuracy.
Atomic and molecular collisions in pulsed laser fields
Authors: Madsen Dane K. , Xiao Yiwen , Eno Larry
Published: July 2002

We examine collisions in pulsed laser fields within the framework of multimode Floquet theory; the approach as outlined in a recent paper allows us to deal with collisional problems in pulsed fields without having to explicitly follow the varying field amplitude. Our focus here is on pulses which are on a time scale of the order of the collision time or less; the interest follows from an analysis made by Lee and George suggesting that unusually strong radiative transitions are possible in such fields. We examine transitions in a model problem based on the Na–Ar collision system and conclude, as others have done for this system, that a short, intense pulse is indeed effective in promoting transitions. Our assessment is, however, only partly based upon the traditional measure of examining how transition probabilities vary as the pulse length is shortened/lengthened. This traditional measure, we argue, is not sufficient for investigation of the mechanism of Lee and George because the measure includes varying contributions to the pulse effect as the pulse frequency profile changes—and this is not accounted for in the analysis of Lee and George. We combine, then, the traditional measure with an examination of the effect of changes in pulse-frequency profile. More particularly, results are determined which embody the same frequency profile as the pulse train, but are generated from a series of fixed-amplitude sources and these results are, in turn, compared with ones generated from a single fixed-amplitude source operating at the central frequency of the train. While, as noted above, we conclude for the Na–Ar system that a short, intense pulse is indeed effective in promoting transitions, we find that combining the traditional measure with an investigation of the role of frequency-profile changes significantly alters assessment of the pulse effect.
A classical dynamics study of the anisotropic interactions in NNO–Ar and NNO–Kr systems: Comparison with transport and relaxation data
Authors: ter Horst Marc A. , Jameson Cynthia J.
Published: July 2002

Classical trajectory calculations were carried out on potential-energy surfaces (PES) that have been proposed for NNO–Ar and NNO–Kr. Two types of cross sections involving the molecular rotational angular momentum vector, σθ,2 and σJ, that are particularly sensitive to the anisotropy of the potential function are compared with new experimental data from our spin-relaxation measurements (see the preceding paper). There is very good agreement with one cross section, σθ,2, but less favorable agreement with σJ. Spectroscopic data for the van der Waals complexes are compared with the values calculated from the surfaces using quantum diffusion Monte Carlo simulations.
Comment on “A 250 GHz ESR study of o-terphenyl: Dynamic cage effects above Tc” [J. Chem. Phys 106, 9996 (1997)]
Authors: Giordano M. , Leporini D.
Published: July 2002

In a recent ESR study Earle, Moscicki, Polimeno and Freed investigated the rotational dynamics of tracers in supercooled o-terphenyl (OTP). Their claims were (1) the correct analysis of the ESR data must account for the possible nonexponential decay of the the rotational correlation function; (2) the average rotational correlation time, 〈τ〉 follows the Stokes–Einstein–Debye law (SED); (3) there are substantial voids in OTP. We show that (i) claims #1 and #2 become very questionable after having corrected inconsistencies between the data and the related plot; (ii) the claim #3 is not supported by additional experimental data.
A note on the green dyadic calculation of the decay rates for admolecules at multiple planar interfaces
Authors: Hartman R. L. , Cohen S. M. , Leung P. T.
Published: July 2002

The Green dyadic formulation for calculating classical decay rates of admolecules at multiple planar interfaces first published by Chance, Prock and Silbey is reexamined. It is pointed out that, for the case of fluorescing molecules sandwiched between a system of super- and substrate interfaces, the original formalism requires significant modifications in order to lead to results consistent with those obtained from the Sommerfeld radiation theory.
Probing the alignment of NO(X 2Π) by [2+1] resonance-enhanced multiphoton ionization via the C 2Π state: A test of semiclassical theory in 355 nm photodissociation of NO2
Authors: Mo Yuxiang , Katayanagi Hideki , Suzuki Toshinori
Published: July 2002

A theoretical method to analyze the two- and three-dimensional imaging data of photofragments with polarized angular momentum was tested by comparing with the experimental data on NO from 355 nm photodissociation of NO2. The alignment of NO(X 2Π) was detected by [2+1] resonance-enhanced multiphoton ionization via the C 2Π state. The data were analyzed by assuming the μ–ν–J triple vector correlation described by semiclassical multipole moments in the velocity-fixed frame. The geometrical factors for the two-photon absorption were derived rigorously for the intermediate coupling between Hund’s cases (a) and (b). It was also shown, however, that the geometrical factors for high J are independent of the coupling case and can be approximated by simple formulas. Excellent agreement between the simulation and the experimental data proved the validity of the semiclassical treatment.
Topology of electron-electron interactions in atoms and molecules. II. The correlation cage
Authors: Cioslowski Jerzy , Liu Guanghua
Published: July 2002

The concept of the correlation cage provides new insights into electron–electron interactions in atoms and molecules. The cage constitutes the domain in the space of interelectron distance vectors R within which correlation effects are substantial. Its shape and size are entirely determined by the topological properties of the electron intracule density I(R), thus avoiding any references to ill-defined “uncorrelated” quantities. Integration of observables related to I(R) over the correlation cage affords quantitative measures of electron correlation. The number of strongly correlated electron pairs Mcorr[I], their electron–electron repulsion energy Wcorr[I], and the cage volume Vcorr[I] that characterizes the spatial extent of electron correlation are functionals of I(R). The ratio κ[I] of I(0)Vcorr[I] and Mcorr[I], which measures the strength of short-range correlation effects, is small for systems such as H− and closer to one for those with weaker correlation effects.
Collisional relaxation of highly vibrationally excited CF2O prepared with different initial energies and distribution functions
Authors: Pino G. A. , Rinaldi C. A. , Coronado E. A. , Ferrero J. C.
Published: July 2002

The collisional relaxation of highly vibrationally excited CF2O* molecules prepared by infrared laser multiphoton absorption is compared with the results obtained when CF2O* is generated as a product of the reactions of CF3 and CF2Cl radicals with NO2. The three methods produce molecules with probably different energy distributions and also different average excitation energies 〈E〉. Thus, IR laser excitation results in a bimodal distribution, with average excitation energies in the range 3 000–20 000 cm−1, while the chemical reactions of CF3 and CF2Cl radicals produce CF2O* with a undetermined level of vibrational excitation that depends on the specific energy change of the process. Irrespective of the method of preparation, the same exponential decays are obtained for the each of various colliders studied (Ar, N2, NO2, and CF2O). It is shown that under these conditions, the observed bulk average energy transferred per collision, 〈〈ΔE〉〉, is equal to the microscopic value 〈ΔE〉. However, a single exponential energy decay is not sufficient condition to assure that equality.
A computational study of the enthalpies of formation of halomethylidynes
Authors: Marshall Paul , Misra Ashutosh , Schwartz Martin
Published: July 2002

The halomethylidynes CH, CF, CCl, CBr, and CI have been investigated using GAUSSIAN-2 theory and calculations up to QCISD(T)/6-311+G(3df,2p). At the latter level of theory bond lengths, harmonic frequencies, and anharmonicity corrections have been derived and compare well with experimental measurements where these are available. GAUSSIAN-2 and QCISD(T)/6-311+G(3df,2p) energies were employed in isodesmic reactions to obtain the following recommended enthalpies of formation (ΔfH298) in kJ mol−1: CF, 248.6; CCl, 436.9; CBr, 497.1; CI, 557.6. Estimated error limits are ±6 kJ mol−1.
Vibrational spectra of linear BC3 and linear B2C2 in argon at 10 K
Authors: Presilla-Márquez José Domingo , Carrick Patrick G. , Larson C. William
Published: July 2002

The vibrational spectra of linear BC3 and linear B2C2 in an argon matrix at 10 K have been observed for the first time by Fourier transform infrared spectroscopy. Mixtures of boron and carbon powders were vaporized in a resistively heated (∼3000 K) oven. BC3 was produced during codeposition of the vapors with argon onto a CsI substrate maintained at 10 K; B2C2 was observed only after annealing the matrices. Frequencies measured at 2002.1 and 1512.5 cm−1 have been assigned to the ν1(σ) and ν2(σ) B–C stretching fundamentals of linear BC3 and a frequency measured at 955.0 cm−1 has been assigned to the ν3(σu) stretching fundamental of linear symmetric BC2B. These assignments are based on FTIR measurements of the majority of the BC3 and B2C2 isotopomer frequencies and are consistent with the results from Rittby’s collaborative ab initio study.
Collision induced fragmentation of small ionic sodium clusters: Competition between electronic and impulsive mechanisms
Authors: Barat M. , Brenot J. C. , Dunet H. , Fayeton J. A. , Picard Y. J.
Published: July 2002

Collision induced fragmentation of small Nan+ (n=3–9) clusters with He atoms is investigated in the 100 eV center-of-mass collision energy range. The experiment is based on the determination of the velocity vectors of the fragments using a multicoincidence technique. The relative populations of the various fragmentation pathways are determined. Fragmentation mechanisms are discussed in detail. The most important pathways are primarily populated via momentum transfer in elastic binary collisions between the He atom and a Na+ core. Direct release of fast Na atoms is observed at variance with what is usually assumed at eV energies. However most of the fragmentation involves multistep dynamics with energy redistribution inside the cluster via Na–Na collisions. In contrast, production of Na+ fragments comes dominantly from electronic transitions towards repulsive potential energy surfaces of the cluster. The role of electron pairing is emphasized.
Variational mean spherical scaling approximation for nonspherical molecules: The case of dimers
Authors: Velázquez Esov S. , Blum Lesser
Published: July 2002

The variational mean spherical scaling approximation (VMSSA) is extended to nonspherical objects in ionic solutions. The mean spherical approximation (MSA) and the binding mean spherical approximation (BIMSA) are extensions of the linearized Poisson–Boltzmann (or Debye–Hückel) approximation that treat the excluded volume of all the ions in the system in a symmetric and consistent way. For systems with Coulomb and screened Coulomb interactions in a variety of mean spherical derived approximations, it has been recently shown that the solution of the Ornstein–Zernike (OZ) equation is given in terms of a screening parameter matrix Γ̳. This includes the “primitive” model of electrolytes, in which the solvent is a continuum dielectric, but also models in which the solvent is a dipolar hard sphere, and much more recently the YUKAGUA model of water that reproduces the known neutron diffraction experiments of water quite well. The MSA can be deduced from a variational principle in which the energy is obtained from simple electrostatic considerations and the entropy is a universal function. For the primitive model it is ΔS=−k(Γ3/3π). For other models this function is more complex, but can always be expressed as an integral of known functions. We propose now a natural extension of this principle to nonspherical objects, such as dumbbells, in which the equivalence to the OZ approach can be explicitly verified.
Comment on “Polymer localization in attractive random media” [J. Chem. Phys. 109, 10011 (1998)]
Authors: Grassberger Peter
Published: July 2002

We study self-avoiding walks in an energetically disordered medium. We show that an effect interpreted as a localization phase transition in [A. Baumgärtner, J. Chem. Phys. 109, 10011 (1998)] actually is due to a sudden breakdown of effective ergodicity. While this problem occurs with any presently known Monte Carlo algorithm, there is a class of algorithms which at least warns the user when phase space sampling becomes insufficient.
Thermochemistry of ZnCl(g)
Authors: Hildenbrand D. L. , Lau K. H. , Roos J. W.
Published: July 2002

The gaseous species Zn, ZnCl, and ZnCl2 were generated in an effusion cell by reaction of Cl2(g) with ZnO(s) near 1400 K, and were identified and monitored by mass spectrometry. Equilibrium constants were evaluated for the gaseous reaction Zn+ZnCl2=2ZnCl from ion intensities measured over the range 1336–1436 K and used to derive the third law enthalpy change ΔH298°=176±8 kJ mol−1 and the dissociation energy D0°(ZnCl)=229±8 kJ mol−1. This result is in good agreement with an earlier value derived from equilibrium measurements made by monitoring the electronic spectrum of ZnCl(g) in equilibrium with gaseous Zn and ZnCl2. The results are useful in estimating the thermochemical properties of gaseous ZnOH and Zn(OH)2 for chemical modeling applications.
Nonequilibrium distribution function formalism for diffusion-influenced bimolecular reactions: Beyond the superposition approximation
Authors: Sung Jaeyoung , Lee Sangyoub
Published: July 2002

We introduce an improved and systematic procedure for solving the diffusion-influenced reaction kinetics problem based on a hierarchical set of kinetic equations for many-particle distribution functions. As a representative example, we apply the formalism to the reversible association kinetics problem. The new method gives an approximate analytic solution which is in almost perfect agreement with the Brownian dynamics simulation results for the whole time range. It is shown that results of previous nonequilibrium distribution function theories are reproduced as lower order solutions in the present formalism.
Tunneling manifold of two noninteracting NH4 torsional oscillators in polycrystalline ammonium iodide
Authors: Choi C. , Peternelj J. , Damyanovich A. , Pintar M. M.
Published: July 2002

Zeeman rotational tunneling level-crossing spectra have been interpreted on the assumption that only single noninteracting NH4 groups contribute to the spectrum. In this note, we report the results of an analysis which takes into account two noninteracting NH4 groups, in which case the inter NH4-group interactions become important. This analysis agrees with the accepted interpretation of the NH4 ground state manifold wherein the F level is triply degenerate.
Modeling of zeolite L crystallization using continuum time Monte Carlo simulations
Authors: Nikolakis Vladimiros , Vlachos Dionisios G. , Tsapatsis Michael
Published: July 2002

A model to investigate a gel-to-zeolite transformation as a possible growth mechanism through association and rearrangements of extended precursor structures is developed and solved via continuum time lattice Monte Carlo simulations. The model is used to study the morphology of zeolite L nanocrystals grown from an initial amorphous microporous precursor gel. The model considers gel dissolution to release growth precursor extended structures, precursor migration, precursor–precursor association, precursor incorporation into zeolite nanoparticles, and zeolite dissolution. It is shown that the gel-to-zeolite transformation can occur when dissolved precursor migration is slow compared to precursor incorporation and zeolite dissolution. Under these conditions, the initial gel microstructure (intraparticle porosity) has a significant effect on the zeolite morphology and on the crystallization kinetics. This transformation proceeds in two stages: A zeolitic framework forms initially without long-range order, followed by slow rearrangement of building units into nanocrystalline particles with possible defects. Finally different growth modes are identified as a function of microkinetic parameters and gel morphology.
Complexation to macromolecules with a large number of sites
Authors: Garcés Josep Lluı́s , Mas Francesc , Puy Jaume
Published: July 2002

This paper presents an approach based on the saddle-point approximation to study the equilibrium interactions between small molecules and macromolecules with a large number of sites. For this case, the application of the Darwin–Fowler method results in very simple expressions for the stoichiometric equilibrium constants and their corresponding free energies in terms of integrals of the binding curve plus a correction term which depends on the first derivatives of the binding curve in the points corresponding to an integer value of the mean occupation number. These expressions are simplified when the number of sites tends to infinity, providing an interpretation of the binding curve in terms of the stoichiometric stability constants. The formalism presented is applied to some simple complexation models, obtaining good values for the free energies involved. When heterogeneous complexation is assumed, simple expressions are obtained to relate the macroscopic description of the binding, given by the stoichiomeric constants, with the microscopic description in terms of the intrinsic stability constants or the affinity spectrum.
Vibrationally resolved cross sections for single-photon ionization of LiH
Authors: Lundsgaard Morten F. V. , Rudolph H.
Published: July 2002

Ab initio studies of the single-photon ionization of LiH(X 1Σ+) leading to LiH+(X 2Σ+) are reported. The process is studied as function of the internuclear distance (2 a.u.–6 a.u.) and the kinetic energy of the electron (0.05 eV–10 eV). The calculations are based on the so-called iterative Schwinger approach which is implemented within the frozen-core single center approximation. By comparing vibrationally resolved cross sections calculated beyond and within the Franck–Condon principle we find, that the Franck–Condon principle applies relatively well to the present photoionization process. The vibrational wave functions needed for these calculations are obtained from very accurate CI-potentials, also reported in this paper. For LiH the first 10 vibrational spacings are found to agree with the experimental data within 0.2%. Photoelectron spectra (PES) calculated under simulated experimental conditions suggest that photoionization experiments on LiH may yield information on the vibrational structure of LiH+. The calculated permanent dipole moment of the LiH ground state shows strong influence of the ion-pair channel (Li+H−).
Interface effects on the growth of cobalt nanostructures on molybdenum-based substrates
Authors: Clark P. G. , Friend C. M.
Published: July 2002

Using scanning tunneling microscopy and low energy electron diffraction, we show that the structure and morphology of cobalt islands deposited on clean, sulfur- and oxygen-covered Mo(110) at 400 K strongly depends on the nature of the interface. Two-dimensional islands of cobalt grow on clean and S-covered Mo(110), whereas three-dimensional islands grow on O-covered Mo(110). The islands on the clean surface are hexagonal, reflecting the structure of the bulk Co lattice. A square lattice is formed on the S-covered surface that is the result of significant S–Co interactions. The difference between the growth mode of the cobalt on sulfur- and oxygen-covered Mo(110) and the difference in island shape on clean and S-covered Mo(110) are both explained in terms of microscopic bonding differences at the interface related to the degree of charge transfer to the O and S from the Mo surface.
Master equation analysis of intermolecular energy transfer in multiple-well, multiple-channel unimolecular reactions. II. Numerical methods and application to the mechanism of the C2H5+O2 reaction
Authors: Venkatesh Prasana K. , Dean Anthony M. , Cohen Morrel H. , Carr Robert W.
Published: July 2002

Having elucidated a full theoretical analysis of the master equation for intermolecular and intramolecular energy transfer in multiple-well, multiple-channel chemically or thermally activated reactions [J. Chem. Phys. 107, 8904 (1997)], we now present efficient methods of numerical analysis for the computational examination of the dynamics of the master equation. We suggest the use of a Krylov-subspace method to determine the uppermost portions of the internal spectrum of the master equation kernel. Such a computation is pivotal in determining whether there exists a state of secular equilibrium for the population of the moieties and whether there exists within the possible state of secular equilibrium, a state wherein the dynamics are represented by an isolated dominating mode; for only in the state of secular equilibrium can one write rate equations for the dissociating processes that are local in time. And, if such a state is possible, we suggest the use of a Hermite–Laguerre orthogonal collocation method for obtaining highly accurate solutions to the population of the moieties. The theory and numerical analysis is then applied to study the dynamics of the chemically-activated reaction C2H5+O2. Comparison of the master equation treatment with modified strong-collision theory is also given for this system of multiple-well, multiple-channel reactions.
Raman study of intramolecular frequency noncoincidence effect in dialkyl benzenedicarboxylates
Authors: Kim Yoo Joong , Chang Hai-Chou , Sullivan Vivian S. , Jonas Jiri
Published: July 2002

Raman noncoincidence effects (NCE) of the C=O stretching band of dialkyl esters of benzenedicarboxylic (phthalic, isophthalic, and terephthalic) acids were measured as a function of concentration in acetonitrile and dioxane solutions. By quenching the intermolecular vibrational interactions between the C=O groups of neighboring molecules by dilution, the NCE arising from the coupling between two C=O stretching vibrations in a single molecule of the diesters was observed for the first time. The intramolecular NCE values for these molecules were negative and dependent on the relative orientation and distance between two C=O groups in a molecule. Similarly as for most intermolecular NCE, the intramolecular NCE is explained by the transition dipole coupling mechanism. The presence of a single dipolar coupling pair allows one to express the intramolecular NCE values by a simple analytical equation with a few molecular parameters including those for the arrangement of two C=O groups in a molecule. The observed intramolecular NCE values are well predicted, with a limitation of large dipolar distance, by the equation with the geometric parameters obtained from the ab initio optimized structures. In fact, the NCEs provided information about detailed conformational structure of the benzenedicarboxylates in dilute solutions. Since the intramolecular NCE value indeed corresponds to a frequency difference between the in-phase and out-of-phase C=O stretching normal modes of a molecule, the present result suggests that the vibrational coupling between the two C=O internal coordinates in these molecules is mainly due to the transition dipolar interaction.
Reaction field cavity optimization: A born-again Born model for ionic hydration
Authors: Hsu Chao-Ping , Head-Gordon Martin , Head-Gordon Teresa
Published: July 2002

We present a new quantum reaction field model, the electronic reaction field (ERF), which does not require the empirical specification of cavity shape and size. We demonstrate our approach on one of the simplest reaction field theories, the Born model for aqueous atomic ion solvation. The ERF model enforces complete solute–solvent separability so that the solute wave function is optimized under the boundary condition that it is fully contained within the cavity. This model imposes the exact condition necessary for treating the solute quantum mechanically and the other as a macroscopic classical dielectric, and allows the testing of the primary assumption in the dielectric continuum approximation that the molecular character of solvent is unimportant. We show that the Born theory for monovalent and divalent cations is consistent with ERF model enforcing full wave function containment, while strict solute–solvent separability gives a poor description for anions of any valency.
Relations among the modern theories of diffusion-influenced reactions. I. Reduced distribution function theory versus memory function theory of Yang, Lee, and Shin
Authors: Sung Jaeyoung , Lee Sangyoub
Published: July 2002

It is shown that exactly the same results of the memory function theory of diffusion-influenced reactions, presented by Yang, Lee, and Shin, can be derived from the nonequilibrium reduced distribution function (RDF) theory. Instead of the usual dynamic superposition approximation (SA), which has been widely used to truncate the hierarchy of RDF evolution equations, we introduce another type of truncation approximation. The new approximation provides simple analytic solutions that are in better agreement with the computer simulation and the known exact results than those obtained with the SA.
Role of structural relaxation in peculiar permanent densification of fluorozirconate glass
Authors: Tamai Yoshinori , Kawamoto Yoji
Published: July 2002

Molecular dynamics simulations for a fluorozirconate glass (55ZrF4⋅17BaF2⋅5EuF3⋅23NaF in mol %) have been carried out in order to investigate a mechanism of a peculiar high-pressure behavior in the permanent densification of fluorozirconate glasses, which was found by experiments recently. Simulation conditions mimic experimental ones; compression of the glass under constant pressures at a high temperature, quenching under constant pressures Pq, and then decompression (release of pressure) at constant temperatures Td. The dependence of density on the quenching pressure Pq exhibits a maximum, which was also manifested in the experimental study. A mechanism of this peculiar densification behavior is proposed in terms of structural relaxation in the F− coordination environments around the network modifying ions (Ba2+ and Na+) during the decompression process. The “fragile” nature of fluorozirconate glasses plays an important role in this mechanism.
Light scattering spectra of fast relaxation in B2O3 glass
Authors: Surovtsev N. V. , Wiedersich J. A. H. , Duval E. , Novikov V. N. , Rössler E. , Sokolov A. P.
Published: July 2002

Light scattering spectra of B2O3 glass were measured in the frequency range 20–7000 GHz at temperatures in the range of 15–300 K and are compared with those previously obtained for Ca0.4K0.6(NO3)1.4 (CKN). The low-frequency contribution of the fast relaxation susceptibility spectrum is found to show a power-law behavior with an exponent α=0.6, the latter being temperature independent in contrast to the finding in other inorganic glasses such as silica and CKN. Possible explanations of the spectral shape in B2O3 are considered.
Coarsely resolved topography along protein folding pathways
Authors: Fernández Ariel , Kostov Konstantin S. , Berry R. Stephen
Published: July 2002

The kinetic data from the coarse representation of polypeptide torsional dynamics described in the preceding paper [Fernandez and Berry, J. Chem. Phys. 112, 5212 (2000), preceding paper] is inverted by using detailed balance to obtain a topographic description of the potential-energy surface (PES) along the dominant folding pathway of the bovine pancreatic trypsin inhibitor (BPTI). The topography is represented as a sequence of minima and effective saddle points. The dominant folding pathway displays an overall monotonic decrease in energy with a large number of staircaselike steps, a clear signature of a good structure-seeker. The diversity and availability of alternative folding pathways is analyzed in terms of the Shannon entropy σ(t) associated with the time-dependent probability distribution over the kinetic ensemble of contact patterns. Several stages in the folding process are evident. Initially misfolded states form and dismantle revealing no definite pattern in the topography and exhibiting high Shannon entropy. Passage down a sequence of staircase steps then leads to the formation of a nativelike intermediate, for which σ(t) is much lower and fairly constant. Finally, the structure of the intermediate is refined to produce the native state of BPTI. We also examine how different levels of tolerance to mismatches of side chain contacts influence the folding kinetics, the topography of the dominant folding pathway, and the Shannon entropy. This analysis yields upper and lower bounds of the frustration tolerance required for the expeditious and robust folding of BPTI.
Density fluctuations and shear viscosity of molecular liquids: Carbon dioxide and nitrogen
Authors: Rah Kyunil , Eu Byung Chan
Published: July 2002

An expression for the shear viscosity of molecular liquids is derived from the statistical expression for the stress tensor by taking into consideration density fluctuations over the intermolecular force range. The viscosity formula consists of a low density term given in terms of the Chapman–Enskog viscosity and a density dependent term reminiscent of the Stokes–Einstein relation between the viscosity and the self-diffusion coefficient. According to this formula, the shear viscosity of molecular liquids can be calculated in terms of intermolecular site–site forces, the corresponding pair correlation functions, and the self-diffusion coefficient as well as the Chapman–Enskog viscosity at low density. By treating the viscosity expression as a semiempirical formula where the experimental and numerically simulated self-diffusion coefficients available in the literature are used, the shear viscosities of nitrogen and carbon dioxide, both of which are treated as a rigid linear rotator with two sites, are calculated and compared with experiment. Agreement between theory and experiment is found very good qualitatively and quantitatively.
Quantum Monte Carlo: Direct calculation of corrections to trial wave functions and their energies
Authors: Anderson James B.
Published: July 2002

We report an improved Monte Carlo method for calculating the difference δ between a true wave function Ψ and an analytic trial wave function Ψ0. The method also produces a correction to the expectation value of the energy for the trial function. The nodes of the trial function are not corrected and the energy is corrected to the fixed-node energy of the trial function. Applications to several sample problems as well as to the water molecule are described.
Excitation energy transfer of a bichromophoric cross-shaped molecule investigated by polarized fluorescence spectroscopy
Authors: Schartel B. , Krüger S. , Wachtendorf V. , Hennecke M.
Published: July 2002

The excitation energy transfer (EET) of a bichromophoric cross-shaped molecule was investigated by stationary polarized fluorescence spectroscopy in the solid state. For this purpose 2,2′,7,7′-tetrakis(biphenyl-4-yl)-9,9′-spirobifluorene was embedded in a polymeric bisphenol-A-polycarbonate (PC) matrix. The dependence of the fluorescence on concentration and wavelength was determined. The role of the intermolecular and intramolecular EET is dealt with separately and discussed by means of the degree of polarization. The intermolecular excitation energy transfer is described in terms of a Förster transfer mechanism. The intramolecular transfer is prevented for the zero-point vibrational levels by the molecular cross-shaped structure, but is found for a wide range of wavelength, presumably based on vibrationally excited states.
Structure formation in dipolar fluids driven by rotating fields
Authors: Murashov Vladimir V. , Patey G. N.
Published: July 2002

The influence of rotating fields on strongly interacting dipolar fluids is investigated using molecular dynamics and Brownian dynamics simulations. Both one-component and two-component systems are considered. It is shown that under certain conditions rotating fields can induce spatial layering in these systems. This behavior is similar to that recently observed in experimental studies of electrorheological and magnetorheological fluids. The binary mixture results are particularly interesting. It is shown that components that differ only in their moments of inertia can be separated by adjusting the frequency of the applied field.
Low-energy electron attachment to fused 1,4-cyclohexadiene rings by means of electron transmission spectroscopy and exponent stabilization calculations
Authors: Venuti Marco , Modelli Alberto
Published: July 2002

Electron transmission spectroscopy is used for determining the energies of vertical electron attachment to the empty π* orbitals of ethene (1), 1,4-cyclohexadiene (2), 1,4,5,8-tetrahydronaphthalene (3), and 1,4,5,8,9,10-hexahydroanthracene (4), where the number of ethene double bonds, which interact through space and through the CH2 bridges, increases along the series. In contrast with the expectations based on a simple perturbational model, the energy of the first anion state is nearly constant on going from 1 to 4. Moreover, the energy splitting between the lowest and the highest anion states in the larger molecular systems 3 and 4 is smaller than in 1,4-cyclohexadiene. The experimental data are compared with the empty orbital energies of the neutral states supplied by HF calculations using both a standard basis set and one augmented with diffuse functions, using the exponent stabilization method for distinguishing the virtual orbitals which give rise to temporary anion states. The graphs of virtual orbital eigenvalues versus the exponent scaling factor display avoided crossings and regions where the π* molecular orbital energies are relatively stable. The orbital energies determined in correspondence with the avoided crossings do not reproduce the trends of the resonances observed in the spectra. A better match with experiment (although not completely satisfactory) is obtained by determining the energies in the region of stability of the graphs. This set of results also predicts smaller through-space and through-bond interactions.
Ground state isoconfigurational mixing in the V2, VNb, and Nb2 molecules
Authors: O’Brien Ted A. , Albert Katrin , Zerner Michael C.
Published: July 2002

Ground state spin–orbit effects in the molecules V2, VNb, and Nb2 have been investigated using self-consistent field configuration interaction (SCF/CI) calculations based on the intermediate neglect of differential overlap Hamiltonian parameterized for optical spectroscopy (INDO/S). Accurate results for the spin–orbit splittings of the X 3Σ− ground state of each molecule were obtained using a CI treatment designed to isolate essential correlation of the ground state and the isoconfigurational Σ+1 state causing the splittings. Energies computed for the Σ+1 states are compared to energies obtained from simple two-state perturbative models. It is observed that the consideration of only two states is a large source of error. Previous assignment of a low-lying excited state observed for V2 as the Σg+1 state is not supported. Additional results on excited states of VNb have also been obtained. A Σ-3←X Σ-3 transition observed experimentally for VNb is assigned as a σ*←σ promotion, and the presence of a second excited Σ-3 state of VNb that has not been observed is suggested. An analysis of correlation effects in the INDO/S model is given based on a comparison of results obtained with the approximate essential correlation CI calculations and with larger restricted active space full CI calculations.
Asymptotic behavior of line shifts in the 0-0 and 0-1 bands of HF in a bath of argon: Influence of vibration-rotation coupling
Authors: Grigoriev I. M. , Filippov N. N. , Tonkov M. V. , Boulet C. , Boissoles J.
Published: July 2002

This study seeks to determine the origin of several features observed in the behavior at high j values of shift cross sections in the 0-0 and 0-1 bands of HF in a bath of argon. These features were not explained in earlier quantum mechanical calculations. A semi-classical (classical path) simplified formalism is used to demonstrate that shift cross sections depend sensitively on vibration-rotation coupling in the high j limit.
Usual and unusual summation rules over j angular momentum
Authors: Elbaz E.
Published: July 2002

After recalling the usual j-summation rule over an angular momentum we describe two unusual summation rules. The first one concerns sum of products of ‘‘3nj’’ and ‘‘3j0’’ coefficients. The second involves sum of products of ‘‘3jm’’ coefficients but with the (2j+1) weighting factor missing.
Nonlinear classical scalar field theory in curved space-time
Authors: Okolowski J. A.
Published: July 2002

A generic Lagrangian based classical field theory is formulated for any space-time manifold in which certain postulated conditions remain valid. The choice of a specific field Lagrangian leads to a nonlinear model theory that admits a rigorous closed-form particlelike solution in isotropic homogeneous space-time of positive spatial curvature. This metastable solution, of finite positive energy, is discussed in relation to its counterpart in isotropic homogeneous space-time of negative spatial curvature.
Axially symmetric gravitational radiation from isolated sources and the deviation from spherical symmetry
Authors: Herrera L. , Jimenez J.
Published: July 2002

Axially (and reflection) symmetric space-times (Bondi metric) are studied in terms of scalar functions that measure the deviation of the system from spherical symmetry. The case of small departures from sphericity is considered, and the corresponding solution is completely specified up to the first order. Finally in discussing some aspects of the initial (characteristic) value problem, a very concise proof of a theorem by Papapetrou [A. Papapetrou, J. Math. Phys. 6, 1405 (1965)] is given.
‘‘Twist’’ for simply laced groups
Authors: Kim Jae Kwan , Kim Jeong Yong , Koh I. G.
Published: July 2002

‘‘Twist,’’ which is necessary to remove awkward signs in the commutator of vertex operators, is explicitly constructed with string momenta for simply laced groups. For different embeddings of roots into Euclidean spaces as well as for roots in nonorthogonal space, existence and construction of ‘‘twist’’ are shown in detail.
Multigroup transport equations with nondiagonal cross-section matrices
Authors: Willis B. L. , van der Mee C. V. M.
Published: July 2002

Multigroup transport equations with nondiagonal cross-section matrices are studied using the Wiener–Hopf method. Formulas for the solution and the exit distribution are given in terms of the factorization of the symbol of the Wiener–Hopf equation. Unlike the formulas for a diagonal cross-section matrix, these formulas involve derivatives of the H-functions. For the case of two groups, the H-functions are computed explicitly.
Reflection and refraction of a spherical acoustic wave from a thin layer
Authors: Banik N. C. , Lerche I.
Published: July 2002

The behavior of a spherical acoustic wave impacting on a thin, parallel slab of material of thickness L is investigated. It is found that the reflected wave may have a so-called head-wave contribution when the slab has a higher acoustic velocity than the surrounding medium. However, the effects of the finite slab thickness are to delay the head-wave arrival time relative to that from a single interface, to diminish its amplitude, and modify the frequency response of the amplitude, and to cause multiples from the base of the slab to produce far-field interference fringes (the analog of Newton’s rings). In addition, in the case where the slab has a lower acoustic velocity than the surrounding medium, no head wave results, but the far-field interference pattern persists. As the slab thickness is increased relative to the acoustic wavelength both the interference effects and the head-wave modifications increase with increasing thickness, for thicknesses small compared to the acoustic wavelength.
Euclidean invariant integral representations for Schwinger functionals
Authors: Yngvason Jakob
Published: July 2002

It is shown that if the Schwinger functions of a Wightman theory with a mass gap can be represented as moments of a complex measure, there is also such a representation by a measure that is invariant under the Euclidean group. A necessary and sufficient condition is also given for the exponential of a quadratic form on the test function space to be the characteristic functional of a complex measure.
Relation between the connected diagram and smoothing methods for rough surface scattering
Authors: DeSanto John A.
Published: July 2002

In previous work by the author on connected diagram expansion methods for the problem of scattering from a random rough surface a stochastic Lippmann–Schwinger integral equation in Fourier transform space for the scattered part of the Green’s function was derived. Averaging techniques using homogeneous statistics and a statistical cluster decomposition on the surface interaction function yielded a connected diagram expansion for the coherent and incoherent Green’s functions. Here it is demonstrated that the smoothing method applied to this stochastic integral equation yields a result that agrees with the connected diagram expansion only to second order in the surface interaction. For third- and higher-order interactions, the smoothing method does not yield connected terms.
Occurrence of secular terms in the Carleman embedding
Authors: Esperidião A. S. C. , Andrade R. F. S.
Published: July 2002

Secular terms occur in many perturbative solutions of nonlinear equation systems. In this work, an investigation is made of which cases they may occur in as the result of the application of the linear Carleman embedding to a system of nonlinear equations. The solution for the embedded system is written in a form that makes it convenient to see how these terms originate. Their occurrence for the general case is discussed and the results are exemplified by working out the Hénon–Heiles system.