bluuebunny/crossref_metadata_2025 huggingface dataset:
Infrared emission spectra from cryogenic proton-irradiated helium gas
Authors: Tokaryk D. W. , Wagner G. R. , Brooks R. L. , Hunt J. L.
Published: July 2002

Spectra from proton-beam irradiated helium gas, near 4.2 K, have been examined in the near infrared spectral region using a sensitive photomultiplier-grating spectrometer combination (750–1000 nm) and a Fourier transform infrared spectrometer with an InGaAs detector (800–1800 nm). Most of the observed features can be assigned to known He2 transitions, and two bands, not previously observed (d 3Σ+u(v=4)→c 3Σ+g(v=3,4)) have been identified and analyzed. Some of the unidentified spectra have qualitative similarity to a set of visible emission lines tentatively assigned to an excimer–dimer (i.e., He4, two He2 excimers weakly bound together). These features, unlike the normal He2 spectra, demonstrate pronounced dependence on both the pressure and temperature of the sample. A comparison of the intensities of the singlet and triplet D(d)→C(c) transitions to the singlet and triplet C(c)→A(a) transitions is made.
Surface energy anisotropy for dipolar lattices
Authors: Lobkovsky Alexander E. , Halsey Thomas C.
Published: July 2002

Lattices of parallel dipoles, which appear in, e.g., electrorheological fluids, have strongly anisotropic surface energies. We compute these energies for a number of lattices as a function of polar angle θ and azimuthal angle φ with respect to dipole direction. Logarithmic cusps appear at low order lattice planes, so that the surface energy near a lattice plane oriented at a polar angle θ̄ is given by σ(θ)=σ(θ̄)+K±‖θ−θ̄‖log‖θ−θ̄‖, where K− or K+ are chosen depending on whether θ is less or greater then θ̄. K± for any cusp are exactly calculable with a continuum method.
A general quantum chemical approach to study the locally perturbed periodic systems: A new development of the ab initio crystal elongation method
Authors: Mitani Masaki , Imamura Akira
Published: July 2002

We have recently proposed the elongation method which is a novel molecular orbital method at the Hartree–Fock level to calculate the electronic structures of large periodic or aperiodic polymers efficiently. This method has the idea of the successive connection of any fragments to obtain the electronic properties of large molecules with any units. In this approach, the stationary conditions of the electronic states against the size extension have been formulated. Studies for molecular systems have suggested that the elongation technique with the stationary conditions may be applicable to periodic systems described by the crystal orbital. A one-dimensional polymer, a two-dimensional surface, and a three-dimensional crystal with a local disordering part can be treated systematically by introducing the elongation technique into a large extended supercell model. In the present study, we develop a new quantum chemical approach for the study of locally perturbed periodic systems by the ab initio crystal orbital calculation. The description for the methodology of this approach is given in detail. Results of test applications to a perturbed two-dimensional surface are shown. A local adsorption of carbon monoxide on (001) surface composed with magnesium oxide is examined as a sample model to confirm the accuracy of ab initio crystal elongation method. The utility of our method is clarified by an application to the perturbed surface.
Effect of solvent on semiconductor surface electronic states: A first-principles study
Authors: Ursenbach C. P. , Voth Gregory A.
Published: July 2002

In this paper the first step is taken toward a first-principles molecular theory of the liquid–semiconductor interface. The focus is on the degree of rigor that must be applied to the solvent in order to obtain a realistic description of the interfacial electronic properties. To accomplish this, two different water solvent geometries on the H/Si(111)1×1 surface are assumed, and the electronic structure of the system is calculated using two different models for the water molecules, one which includes the electrons of water explicitly and one which does not. It is found that for a realistic description of the surface electronic structure it is necessary to use an ab initio description of the solvent molecules for at least the first layer due to the electronic state mixing. The issues of broken symmetry of the crystal surface and possible dissociation of the solvent molecules are also discussed.
The influence of applied external magnetic field on NO2 fluorescence intensity under excitation at λ=441.6 nm
Authors: Lavrik N. L.
Published: July 2002

The magnetoinduced quenching of the discrete and continuous regions of NO2 fluorescence spectrum excited by the light of a continuous He–Cd laser (λ=441.6 nm) was studied experimentally. There is a principle difference in the field dependences of these spectrum regions: for a resolved spectrum region these are nonmonotonous whereas for the continuous one these are monotonous. A similarity was established between the dependences of magnetic field effects on pressure for the discrete and continuous regions of the spectrum.
Analysis of static distributions in hydrogen hyperfine interactions in randomly oriented radicals in the solid state by using 2H electron spin echo envelope modulation spectroscopy: Conformational dispersion of β -2H coupling in the model tyrosyl radical
Authors: Warncke Kurt , McCracken John
Published: July 2002

The experimental analysis of static distributions in hydrogen hyperfine interactions in randomly-oriented organic radicals in the solid state by using 2H electron spin echo envelope modulation spectroscopic techniques has been examined systematically. The hyperfine interaction between the two β-methylene-2H nuclei and coupling π-spin density (ρπ) at ring carbon atom C1 in the tyrosine neutral radical trapped in a low temperature aqueous glass was addressed specifically. Stimulated echo envelope modulation generated by the microwave pulse-swapping sequence was collected for τ values of 176–1295 ns at external magnetic field strengths of 0.3258 and 0.3983 T. The spectra reveal weak (β-2Hw) and strong (β-2Hs) sets of hyperfine couplings. The envelope modulation depths and line shape responses to changes in τ and magnetic field strength could not be reproduced by simulations that incorporated discrete principal hyperfine tensors. Successful simulations were achieved by using two sets of distributed principal hyperfine tensors. The principal tensor distribution is caused by a variation in the isotropic coupling for β-2Hw and β-2Hs of 0–2.0 MHz and 2.0–6.8 MHz, respectively. The range of isotropic couplings corresponds to a distribution in the dihedral angles, θ, between the phenol ring normal and the Cβ–Hβ bonds of ‖θw‖=60°–90° and ‖θs‖=60°–30°. A common, θ-independent dipolar coupling constant for each rotamer yields a value for ρπ at C1 of 0.35±0.03. Relative rotamer populations in the distribution are given by the conformational weighting function required for exact reproduction of the line shapes. The quantitative relation between modulation amplitudes and the details of the electron–nuclear interactions that determine the hyperfine frequencies gives the 2H electron spin echo envelope modulation method unique merit for the detection and accurate description of static distributions of principal hyperfine tensors in randomly-oriented paramagnetic systems in the solid state.
Conformational equilibrium and orientational ordering: 1H-nuclear magnetic resonance of butane in a nematic liquid crystal
Authors: Polson James M. , Burnell E. Elliott
Published: July 2002

In this study we use multiple-quantum 1H-NMR spectroscopy to study butane, the simplest flexible alkane, dissolved in a nematic solvent. An analysis of the highly accurate 1H dipolar coupling constants gives important information about conformational and orientational behavior, including the trans–gauche energy difference, Etg, and the conformer probabilities and order parameters. An essential component of the analysis involves the use of mean-field models to describe the orientational ordering of solutes in a nematic solvent. Several models were found to adequately describe the molecular ordering, including the chord model of Photinos et al. [D. J. Photinos, E. T. Samulski, and H. Toriumi, J. Phys. Chem. 94, 4688 (1990)] and recent versions of a model proposed by Burnell and co-workers [D. S. Zimmerman and E. E. Burnell, Mol. Phys. 78, 687 (1993)]. It was found that Etg lies in the range 2.1–3.0 kJ/mol, which is significantly below most experimental estimates of the gas–phase value. An attempt to describe more realistically the conformational states by including torsional fluctuations about the rotational isomeric states did not significantly improve the quality of the fits or alter the results. Finally, the anisotropic component of the solute–solvent interaction was found to perturb only marginally the conformational probabilities from the isotropic values.
The vibrational spectrum of FC(O)O radical: A challenging case for single-reference electron correlation methods
Authors: Schneider William F. , Maricq M. Matti , Francisco Joseph S.
Published: July 2002

Calculation of the structure and vibrational spectrum of the fluoroformyloxyl radical, FC(O)O, using ab initio methods is complicated by orbital symmetry breaking in the Hartree–Fock wave function. While symmetry breaking is most directly corrected with multiconfiguration Hartree–Fock methods, the problem can also be attacked with single reference methods when combined with an electron correlation treatment that adequately mixes the Hartree–Fock determinants leading to the symmetry breaking. In this work the structure and vibrational spectrum of both the ground (X 2B2) and second excited (B 2A1) states of the FC(O)O radical are calculated using single-reference wave functions constructed in two different ways—unrestricted (UHF) and quasirestricted (QRHF) Hartree–Fock wave functions—in combination with a coupled cluster [CCSD and CCSD(T)] electron correlation treatment. We find that both methods must be employed with some caution, but in combination they provide reliable prediction of the vibrational spectrum of the FC(O)O radical.
Dipolar interactions in deuteron spin systems. II. Transverse relaxation
Authors: Diezemann Gregor , Sillescu Hans
Published: July 2002

The effect of the so-called local field terms of the dipolar interactions on the transverse relaxation of deuteron spin systems is investigated for a variety of physical situations. It is shown that the response of the spin systems to arbitrary pulse sequences can be calculated in an operator formalism. For illustration, calculations of the signals following a quadrupolar echo sequence are performed, and expressions for the echo amplitude are given including that for a system of a deuteron coupled to an arbitrary number of spins. A simple model is formulated and the transverse relaxation rate in supercooled liquids is calculated for the whole range of motional correlation times of experimental interest.
Electron mobility in liquid n-hexane/2,2-dimethylbutane mixtures under high pressure
Authors: Itoh Kengo , Nishikawa Masaru , Holroyd Richard A.
Published: July 2002

The effect of pressure on the electron mobility in mixtures of n-hexane and 2,2-dimethylbutane was studied over the whole concentration range. The variation of mobility with pressure is discussed in terms of a two-state model, and provides information on the volume changes occurring on localization. The observed volume changes are interpreted as the difference between the cavity volume and the electrostriction volume of the localized electron.
Preparation and characterization of long-lived molecular Rydberg states: Application to HD
Authors: Merkt F. , Xu H. , Zare R. N.
Published: July 2002

The decay dynamics by predissociation and rotational autoionization of high Rydberg states of HD close to the first few rotational levels of the ground vibronic state of the HD+ cation have been studied by delayed pulsed field ionization following resonant (1+1′) two-photon absorption via the B state. Although predissociation and autoionization both contribute to the rapid decay of Rydberg states with principal quantum number n≪100, the highest Rydberg states (n≳100) are stable for more than 20 μs. In contrast to H2, channels associated with an HD+ (v+=0, N+=even) ion core are coupled to channels associated with an HD+ (v+=0, N+=odd) ion core. We demonstrate that complex resonances that arise from rotational channel interactions between low (n∼25) Rydberg states characterized by a core with rotational angular momentum quantum number N++2 and the pseudocontinuum of very high Rydberg states characterized by an N+ core can be used with high efficiency to produce long-lived high Rydberg states. An investigation of the pulsed field ionization characteristics of these complex resonances enables us to measure the branching between diabatic and adiabatic field ionization and to determine the optimal conditions required to extend the method of H-photofragment Rydberg translational spectroscopy pioneered by Schnieder et al. [J. Chem. Phys. 92, 7027 (1990)] to molecular species.
Molecular dynamics studies of Langmuir monolayers of F(CF2)11COOH
Authors: Schmidt Mark E. , Shin Seokmin , Rice Stuart A.
Published: July 2002

We report the results of molecular dynamics simulations of close-packed Langmuir monolayers of F(CF2)11COOH using both a united atom model and a new anisotropic united atom model of the molecular force field. The stimulus for this investigation was the interpretation of the data from a recent grazing incidence x-ray diffraction study of this system [M. Goldmann et al., J. Phys. II France 4, 773 (1994)] to imply the existence of some degree of azimuthal ordering of the molecules in the close packed monolayer. With our new potential field, simulations at 275 K generate a monolayer in an azimuthally disordered rotator phase, while simulations at 150 K generate a monolayer phase with weak azimuthal order. We find that the relative intensities of peaks in the in-plane structure function can be used to distinguish between azimuthally ordered and disordered phases of the monolayer. Our simulation results are consistent with the available experimental data except for the temperature at which azimuthal ordering is detected, which difference is likely due to the inaccuracy of the intermolecular potential used in the simulations.
Taming the Ewald sum in molecular dynamics simulations of solvated proteins via a multiple time step algorithm
Authors: Procacci Piero , Marchi Massimo
Published: July 2002

Long range electrostatic forces are involved at a fundamental level in many biological phenomena. Their prohibitive computational costs often prevents their correct calculation in molecular dynamics (MD) simulations of biological molecules. In this paper we present a method to handle efficiently and exactly electrostatic interactions in MD simulations with periodic boundary conditions. Our scheme employs a multiple time step r-RESPA integration algorithm in combination with the Ewald summation technique, and is specifically targeted to simulation of large size complex molecular systems such as solvated proteins. In this approach, the force associated with each particle of the system is partitioned into four components which evolve in time with distinct and increasingly longer time scales. We found that a suitable time scale separation is achieved by subdividing the direct space nonbonded interactions, inclusive of Coulombic and van der Waals contributions, in a short, medium and long range shells.

The fastest bonded forces are associated to the component with the shortest timescale, while the reciprocal space nonbonded interaction is included with the medium range direct space forces. Our method is general. It can be straightforwardly implemented for any biomolecular force fields used in condensed phase simulations and can be applied to other complex molecular systems such as molecular liquids and heterogeneous solutions (e.g., micelles, membranes, etc.). We carried out tests on solvated proteins samples containing 7 040 and 20 627 atoms. Due to the nonlinear scaling of the Ewald computational cost with system size, the performance of our r-RESPA algorithm with respect to single time step algorithms with bond constraints grows with the number of particles. We reach a relative speed-up of 4.2 for a system composed of a solvated photosynthetic reaction center.
Electronic density of states of fluid mixtures in the single superchain/effective medium approximation
Authors: Lomba E. , López-Martín J. L.
Published: July 2002

We introduce a straightforward extension to binary mixtures of Logan and Winn’s formalism for the determination of the electronic density of states in disordered systems. Solutions obtained in the single superchain/effective medium approximation are also presented, and though reflecting the limitations intrinsic to the linear nature of this approximation, computer simulation results for the mixture of asymmetric hard spheres are reproduced quite satisfactorily. In particular the dependence of the band shape (and the width of the band gap) on the particle size asymmetry is correctly accounted for. The implementation of efficient integral equation algorithms to deal with complex Ornstein–Zernike equations also constitutes one of the key contributions of this work and is therefore treated in depth.
Segmental mobility in complexes of Pb(CF3SO3)2 and poly(ethylene oxide) studied by NMR spectroscopy
Authors: Johansson A. , Tegenfeldt J.
Published: July 2002

The temperature dependence of proton NMR spectra and spin–lattice relaxation in the rotating frame has been studied for Pb(CF3SO3)2 PEOn, where PEO is poly(ethylene oxide) (MW=4×106), and n=3–40. The dynamic properties of the crystalline PEO are not affected when salt is dissolved into the polymer. The chain mobility is, however, highly reduced in the amorphous regions and the mobility is in the same range as in crystalline PEO at room temperature. 19F NMR studies, spectra, and spin–lattice relaxation in the laboratory frame, suggest a correlation between the anion mobility and the polymer chain mobility, even though the interactions are weak.
Evaluating the accuracy of density functional methods for ClOO
Authors: Morris Vernon R. , Bhatia Subhash C. , Dibble T. S. , Francisco J. S.
Published: July 2002

We present results of density functional calculations on ClOO using the local density approximation (LDA) and the Becke–Lee–Yang–Parr functional (BLYP). The accuracy of computed geometries and vibrational frequencies appears comparable to that achieved with very high quality single-determinant methods [QCISD(T)].
Mean-field theory of solutions: Chemical potential and solution structure in the superposition approximation
Authors: Matubayasi Nobuyuki
Published: July 2002

A simple expression for the chemical potential of a solute in solution is presented under the superposition approximation. In this expression, only the knowledge of the solute–solvent pair correlation function within the solute–solvent interaction range is required to evaluate the chemical potential. The renormalization of the solute–solvent interaction is shown to be related to the indirect part of the solute–solvent potential of mean force. An inconsistency involved in the superposition approximation is then pointed out, and its origin is identified. The shift in a chemical equilibrium in solution induced by insertion of a solute is also investigated under the superposition approximation. It is shown that a correlation function characterizing the effect of a single solvent molecule on the chemical equilibrium plays an important role in determining the shift in the chemical equilibrium. Numerical results are presented for the chemical potential of hard sphere fluid.
Development and parametrization of continuum solvent models. I. Models based on the boundary element method
Authors: Horvath D. , van Belle D. , Lippens G. , Wodak S. J.
Published: July 2002

A series of different simplifications of the boundary element method (BEM) for solving the Poisson–Boltzmann equation is investigated in an effort to obtain an accurate and fast enough treatment of electrostatic effects to be incorporated in Monte-Carlo and molecular dynamics simulation methods. The tested simplifications include increasing the size of Boundary Elements, decreasing the surface dot density, and ignoring the interactions between the polarization charges. Combined with terms describing the nonelectrostatic solvation effects, the simplified BEM polarization terms were built into expressions for the solvation potential. The solvation potential is treated as empirical consistent force field equations. The intervening parameters, including atomic and probe radii, are derived by different fitting strategies of calculated vs experimental vacuum to water transfer energies of 173 charged, polar, and nonpolar small molecules. These fits are shown to yield very good correlations (rms ∼1.4 kcal/mol), even when the interactions between the polarization charges are neglected, proving that the most time-consuming step in BEM, which involves solving the linear system, can be successfully avoided. Finally, the computing efficiency of the method is tested on macromolecules and is found to be convenient for implementation in molecular dynamics or Monte Carlo simulations.
Temperature-dependent thermodynamic properties of refractory metals: Nonempirical study
Authors: Davidov Genady , Fuks David , Dorfman Simon
Published: July 2002

The second and fourth moments of the phonon spectrum are calculated for the refractory metals Mo and W, with the effective interaction potentials fitted to the results of the local density approximation calculations. The temperature dependence of the Debye temperature is discussed. It is shown that the obtained potentials give a good agreement of cohesive properties with the data calculated from the universal function of Rose et al. [Phys. Rev. B 29, 2963 (1984)]. The Gibbs–Bogoliubov inequality and the variational procedure of Ross [J. Chem. Phys. 71, 1567 (1979)] are used for the calculations of the temperature dependence of free energy in liquid tungsten. The calculated thermodynamic functions of solid and liquid phases are employed to determine the melting temperature.
Reaction dynamics in glass transition region: Propagating radicals in ultraviolet-irradiated poly(methyl methacrylate)
Authors: Plonka Andrzej , Bednarek Janusz , Pietrucha Krystyna
Published: July 2002

The decay kinetics of propagating radicals in UV-irradiated poly(methyl methacrylate) (PMMA) was followed at the irradiation temperatures in the range 300–400 K. At the highest temperatures, in the glass transition region of PMMA the radical decay patterns approached that of classical kinetics. Below the glass transition region, increase of reaction dispersivity with temperature decrease was found to be accompanied by a marked decrease, of about two orders of magnitude, of activation energy for the reaction. Both these phenomena are rationalized in terms of the stochastic model [A. Plonka and A. Paszkiewicz, J. Chem. Phys. 96, 1128 (1992)], which includes structural reorganization of the host matrix by imposing upon the static disorder model for radical reaction the additional assumption that matrix relaxation randomly reassigns the reactivity of guest species. Effective relaxation is shown to lead to the classical pattern of radical decay and the accompanying marked increase of activation energy is seen to be due to the contribution of activation energy for matrix structural relaxation.
The intramolecular vibrations of prototypical polythiophenes
Authors: Degli Esposti Alessandra , Moze O. , Taliani C. , Tomkinson J. T. , Zamboni R. , Zerbetto Francesco
Published: July 2002

Inelastic neutron scattering experiments are combined with infrared and Raman data to obtain a uniquely defined description of the intramolecular vibrations of three oligomers of polythiophene. Through refinement of ab initio force fields, the three vibrational spectra of each oligomer are simulated with remarkable accuracy. Two different basis sets of atomic orbitals are used: the first, is 6-31G* and is used to optimize the geometries and calculate the relevant force fields of α-2T and α-4T, the second is 3-21G* and is used for the same purpose for α-4T and α-6T. To improve agreement with the experiment, the force fields are scaled. In this way, one set of scaling parameters is generated for the 6-31G* basis and another for the 3-21G* basis. The parameters are common to both molecules calculated with either basis sets and are believed to be transferable to higher isomers. The fitting procedure is applied in steps: first, the calculated vibrational frequencies are assigned on the basis of the experimental infrared and Raman activity, then a fitting of the Inelastic Neutron Scattering profile is performed, finally, the infrared and Raman spectra are calculated with the new normal modes and the ab initio derivatives of the dipole moment and the polarizability. The procedure is iterated until the three spectra of each oligomer are satisfactorily reproduced. For α-4T, two scaled force fields are obtained—one for each basis set—and are shown to yield very similar normal modes. It is important to emphasize that not only the vibrational frequencies but also the spectral intensities are well reproduced by the simulations. Implicitly, this means that the dipole moment and the polarization tensor surfaces calculated ab initio at the potential energy surface minimum are of good quality. The procedure is absolutely general and can be applied to any molecular system. In the present case, it leads to well defined force fields that give us a stringent picture of the vibrations of these molecules.
Large odd–even effect in RbC−n cluster size distributions
Authors: Vandenbosch R. , Will D. I.
Published: July 2002

RbCn cluster anions have been produced by Rb sputtering of graphite. The intensity ratio of clusters with an even number of carbon atoms to those with an odd number of carbons is much larger for RbC−n clusters than for C−n clusters. Ab initio quantum mechanical calculations suggest that this arises from RbCn electron affinities that are close to zero or negative for odd n, rather than from an enhanced odd–even alternation in the affinities.
Compression of two polymer-coated surfaces in poor solvents
Authors: Singh Chandralekha , Balazs Anna C.
Published: July 2002

We use a two-dimensional self-consistent field theory to investigate two planar surfaces that are sparsely coated with end-grafted homopolymers, and immersed in a poor solvent. We determine the morphology of the layers and the forces between the surfaces as the plates are brought closer together. At large surface separations, each layer forms pinned micelles on the surface. When the surfaces are compressed, the force profiles show a net attraction between the layers. This attraction causes the micelles from the two surfaces to merge, forming a larger micelle or ‘‘bundle,’’ which extends from one surface to the other. This association happens at a distance greater than the vertical extent of the individual micelles.
Influence of the microstructure on the incoherent neutron scattering of glass-forming polybutadienes
Authors: Zorn Reiner , Kanaya Toshiji , Kawaguchi Tatsuya , Richter Dieter , Kaji Keisuke
Published: July 2002

This paper presents the first neutron scattering measurement of 1,2-1,4-polybutadienes with varying microstructure. Incoherent inelastic scattering experiments have been performed in the range of moderate (ΔE≊300 μeV) and high (ΔE≊20 μeV) energy resolution. For the moderate resolution experiments the vibration–relaxation model has been used as a guideline for interpretation. The most striking influence of the microstructure variation can be found in the position of the boson peak. Its position at low temperature as well as its temperature dependence is significantly different among the measured samples. The high resolution data have been Fourier transformed into the time domain yielding the intermediate incoherent scattering function in a time range of 5,...,150 ps. There, the basic properties found for 1,4-polybutadiene earlier could be reproduced (separation of α and βslow regimes of the slow relaxation, Kohlrausch–Williams–Watts function with β≊0.4). On the basis of the new data, no microstructure induced differences can be detected except for the changes of temperature dependence due to the strongly different glass-transition temperatures and different fragility.
Phase diagram of a semiflexible polymer chain in a θ solvent: Application to protein folding
Authors: Doniach S. , Garel T. , Orland H.
Published: July 2002

We consider a lattice model of a semiflexible homopolymer chain in a bad solvent. Beside the temperature T, this model is described by (i) a curvature energy εh, representing the stiffness of the chain; (ii) a nearest-neighbor attractive energy εv, representing the solvent; and (iii) the monomer density ρ=N/Ω, where N and Ω denote, respectively, the number of monomers and the number of lattice sites. This model is a simplified view of the protein folding problem, which encompasses the geometrical competition between secondary structures (the curvature term modelling helix formation) and the global compactness (modeled here by the attractive energy), but contains no side chain information. By allowing the monomer density ρ to depart from unity one has made a first (albeit naive) step to include the role of the water. In previous analytical studies, we considered only the (fully compact) case ρ=1, and found a first order freezing transition towards a crystalline ground state (also called the native state in the protein literature). In this paper, we extend this calculation to the description of both compact and noncompact phases. The analysis is done first at a mean-field level. We then find that the transition from the high temperature swollen coil state to the crystalline ground state is a two-step process for which (i) there is first a θ collapse transition towards a compact ‘‘liquid’’ globule, and (ii) at low temperature, this ‘‘liquid’’ globule undergoes a discontinuous freezing transition. The mean-field value of the θ collapse temperature is found to be independent of the curvature energy εh. This mean-field analysis is improved by a variational bound, which confirms the independence of the θ collapse temperature with respect to εh. This result is confirmed by a Monte Carlo simulation, although with a much lower value of the θ temperature. This lowering of the collapse transition allows the possibility (for large εh) of a direct first order freezing transition, from a swollen coil to the crystalline ground state. For small values of εh, the mean-field two-step mechanism remains valid. In the protein folding problem, the ‘‘liquid’’ compact phase is likely to be related to the ‘‘molten globule’’ phase. The properties of this model system thus suggest that, even though side chain disordering is not taken into account, disordering of the backbone of a protein may still be a sufficient mechanism to drive the system from the native state into the molten globule state.
Solvation and melting in large benzene⋅(Ar)n clusters: Electronic spectral shifts and linewidths
Authors: Adams John E. , Stratt Richard M.
Published: July 2002

Although there has been considerable interest in solvation processes in small atomic and molecular clusters, uncertainties in the interpretation of spectral probes have made the experimental elucidation of the solvation, and in particular how it relates to bulk solvation, problematical. We demonstrate here that, through the application of a microscopic formalism which has the novel feature of accounting for the collective dielectric response of a cluster, the reported spectra of large benzene⋅(Ar)n clusters can be readily understood. Specifically, we show that the apparent lack of convergence of the benzene’s absorption spectrum to the corresponding bulk result derives from the dominance of nonwetting cluster structures for large n. Even observed peak multiplicities and individual linewidths may be understood within this formalism if the cluster structures upon which the calculations are based are generated in a nonequilibrium (rather than thermally equilibrated) simulation. Given this detailed understanding of the relationship between spectroscopy and structure, we also can clarify the experimental consequences of the so-called ‘‘melting’’ transition in benzene⋅(Ar)n clusters: The spectral signature of the melting is a change in the behavior of the linewidth of the absorption envelope which results from a subset, but not all, of the Ar atoms becoming fluid. This description of the melting behavior suggests an important refinement of the conventional picture of solid–fluid phase coexistence in clusters.
A reliable new potential energy surface for H2–Ar
Authors: Bissonnette Carey , Chuaqui Claudio E. , Crowell Kenneth G. , Le Roy Robert J. , Wheatley Richard J. , Meath William J.
Published: July 2002

A reliable new three-dimensional potential energy surface is obtained for the H2–Ar system using an exchange-coulomb potential model with five parameters determined empirically from a least-squares fit to experimental data. This surface fully accounts for new high resolution IR data, virial coefficients, and vibrational transition pressure-shifting coefficients used in the analysis, and yields excellent predictions of elastic and inelastic scattering cross sections and hyperfine transition intensities not included in the analysis. Quantitative comparisons with the best previous empirical potential and a high quality fully ab initio potential are also presented.
Ab initio calculation of hydrogen abstraction energetics from silicon hydrides
Authors: Srinivasan Easwar , Yang Hong , Parsons Gregory N.
Published: July 2002

In this article, we present calculated energies for the abstraction of hydrogen from silicon monohydride and silicon dihydride surface bonding units by atomic hydrogen obtained using ab initio configuration interaction theory. Three and four silicon atom clusters are used to model the dihydride and monohydride units, respectively. Heats of reaction and activation energy barriers are calculated, including the vibrational energies of the initial, final, and transition states. Hydrogen abstraction from a Si–H unit (H+Si4H10→Si4H9+H2) is found to be exothermic by 9.4 kcal/mol with a transition state energy barrier of 5.5 kcal/mol when H approaches along the surface normal. The dihydride abstraction reaction, H+Si3H8→Si3H7+H2, is exothermic by 7.7 kcal/mol and has an energy barrier of 7.3 kcal/mol when H is approaching along Si–H axis. The barrier is larger for hydrogen atom approaching along the surface normal. The larger barrier for abstraction from a dihydride unit is consistent with our experimental observation of a preferential reduction in monohydride bond concentrations when hydrogenated silicon films are exposed to atomic hydrogen during plasma deposition.
Electron stimulated desorption from PF3 adsorbed on Pt. I. Positive ions
Authors: Akbulut M. , Madey T. E. , Parenteau L. , Sanche L.
Published: July 2002

We have studied electron stimulated desorption (ESD) of positive ions from PF3 molecules adsorbed on a Pt substrate over a wide electron energy range (0–175 eV). Electron bombardment of 1 ML PF3 adsorbed on the Pt surface gives rise mainly to an F+ signal, whereas ESD from 6 ML thick PF3 film (thick PF3 layer) leads to P+, PF+, and PF+2 signals, in addition to F+. We find that the onset for F+ desorption from the 1-ML PF3/Pt is at ∼26.5 eV, while the F+ threshold from the thick PF3 layer is ∼28.5 eV. The P+ appearance potential from the thick PF3 layer is ∼23 eV. The ESD F+ ion energy distribution has a peak energy of ∼4 eV for all electron impact energies and a full width at half maximum (FWHM) of ∼3 eV. The P+ ions desorb with a peak energy of ∼2 eV under 55 eV electron impact; the FWHM of the P+ energy distribution is ∼2 eV. We suggest that the near threshold P+ formation from PF3 corresponds to the excitations of the 6a1 level, while the F+ threshold for adsorbed PF3 on the Pt surface is due to the excitation of the F 2s level. Our results suggest that beyond near threshold (≳32 eV), the excitation of the F 2s level also contributes significantly to the formation of P+ and PF+ ions from adsorbed PF3.
Classical trajectory and adiabatic channel study of the transition from adiabatic to sudden capture dynamics. III. Dipole–dipole capture
Authors: Maergoiz A. I. , Nikitin E. E. , Troe J. , Ushakov V. G.
Published: July 2002

Classical trajectory (CT) calculations of dipole–dipole capture processes are performed over wide ranges of conditions. Besides the anisotropic dipole–dipole potential, an isotropic dispersion potential is accounted for. The Massey parameter of the system characterizes the transition between adiabatic and nonadiabatic (sudden) dynamics. In the adiabatic limit, under classical conditions, CT and statistical adiabatic channel model (SACM) give identical thermal capture rate constants which can be expressed in simple parametrized form. The transition from quantum to classical SACM rate constants is represented analytically as well. Furthermore, harmonic oscillator versions of SACM for dipole–dipole capture are compared with the corresponding models for ion–dipole and ion–quadrupole capture.
The diagonal bond method: A new lattice polymer model for simulation study of block copolymers
Authors: Dotera Tomonari , Hatano Akira
Published: July 2002

A new lattice model for Monte Carlo simulations of dense polymer melts, developed in the spirit of Verdier–Stockmayer algorithm on square and simple cubic lattices, is presented. By introducing diagonals of squares and cubes as bonds, the lattice model acquires a large number of configurations and wiggling local moves. While it maintains the excluded volume interactions of monomers, it allows bond crossings and phantom moves, which result in a high mobility of polymers. For an application, we carry out simulations of symmetric A–B block copolymer melts and observe a first-order transition. We also show the stretching of the chains, namely, the non-Gaussian character, as a function of temperature. A quicker evolution towards thermal equilibrium enables us to form an ordered tricontinuous double-diamond (OTDD) phase for linear A–B–C triblock copolymers and a new cylindrical phase for star A–B–C triblock copolymers.
Vibrational energy transfer from levels below 410 cm−1 in S1 p-difluorobenzene. III. Different propensity rules for polyatomic partners
Authors: Mudjijono  , Lawrance Warren D.
Published: July 2002

Collision-induced vibrational energy transfer has been studied from three levels [302 (Evib=240 cm−1), 82 (Evib=361 cm−1), and 61 (Evib=410 cm−1)] in S1 p-difluorobenzene (pDFB) in a supersonic free jet expansion using the polyatomic partners methane, ethane, cyclopropane, and i-butane. The data indicate that vibration-to-vibration transfer is not efficient. Nevertheless, significant differences are found to exist between the state-to-state branching ratios for the polyatomic partners and those observed previously for monatomic and diatomic partners, with the exception of nitrogen. For the polyatomic partners single quantum changes in low frequency modes are no longer dominant. The polyatomic partners generally display a preference for transfer via channels involving large pDFB vibrational energy loss. There are similarities in the preferred two quanta channels for polyatomic and diatomic partners.
A note on Magnus formula
Authors: Prato D. , Lamberti P. W.
Published: July 2002

In this paper we reanalyze the Magnus formula for the time evolution operator in quantum mechanics from an algorithmic point of view. We give some rules to obtain the general term in the expansion as a compact time-ordered integral. These rules are easily applied to higher orders; in particular, we give explicitly the fifth order.
Direct observation of abrupt shape transition in ferrogels induced by nonuniform magnetic field
Authors: Zrı́nyi M. , Barsi L. , Szabó D. , Kilian H.-G.
Published: July 2002

Unidirectional elongation of magnetic field sensitive polymer gels, called ferrogels, have been studied. In ferrogels, finely distributed colloidal particles having superparamagnetic behavior are incorporated into a swollen network. These particles couple the shape of the gel to the nonuniform magnetic field. Shape distortion occurs instantaneously and disappears when the external field is removed. A discontinuous elongation and contraction in response to infinite-small change in the external magnetic field has been observed and a theoretical interpretation based on coupled magnetic and rubber elastic properties is provided.
X-ray and neutron scattering studies of the temperature and pressure dependence of the structure of liquid formamide
Authors: Bellissent-Funel Marie-Claire , Nasr Salah , Bosio Louis
Published: July 2002

A structural investigation of liquid formamide was performed by neutron scattering at pressures of up to 4 kbar. We have also deduced the molecular pair correlation functions from x-ray studies at ambient pressure and in the 263–326 K temperature range. The spectra obtained are consistent with the presence of hydrogen-bonded open chains of molecules in the liquid state. The effect of both pressure and temperature on the hydrogen-bond network is examined.
Realistic representation of the induced electric dipole moment of a polarizable ligand: The missing factor in the Rittner polarization model
Authors: Field Robert W. , Gittins Christopher M.
Published: July 2002

We present a simple modification of the Rittner polarization model1 to better represent the dipole moment of a negative ion induced by the electric field of its counterion. At high fields the traditional second-order perturbation theory treatment of ion polarization is not appropriate. The internuclear separation dependence of the induced dipole moment is better described by explicitly modeling the mixing of free ion basis states. The dipole moment function of the ion is evaluated using the ab initio value of the free ion polarizability and its experimentally determined electron affinity. We present a simple and quantitative resolution of the mysterious difference between ab initio polarizabilities and Rittner model-derived values, which previously had been optimized separately for every different counterion.
Differences between the exact sum-over-states and the canonical approximation for the calculation of static and dynamic hyperpolarizabilities
Authors: Bishop David M. , Kirtman Bernard , Champagne Benoı̂t
Published: July 2002

When molecules are subjected to static or dynamic uniform electric fields the effects are conveniently described in terms of dipole polarizabilities and hyperpolarizabilities. The fields act on both the electronic and vibrational motions simultaneously and, through perturbation theory, this leads to the exact sum-over-states (SOS) formulas for the (hyper) polarizabilities expressed in terms of vibronic states. As an approximation, the fields can be considered to act on the electronic motions first and subsequently, through the distorted electronic ground state potential surface, on the vibrational motions. This commonly used canonical or clamped nucleus approximation, leads to (hyper) polarizability expressions that are different from the SOS ones. In this paper we develop a compact first-order treatment of the differences and carry out a preliminary examination of the numerical repercussions. Our calculations also bear on a proposed theoretical relationship between the vibrational and electronic first hyperpolarizabilities in push–pull conjugated systems.
Correlation states of propene
Authors: Bawagan A. D. O. , Desjardins S. J. , Dailey R. , Davidson E. R.
Published: July 2002

High resolution synchrotron photoelectron spectra (PES) of propene (C3H6) have been obtained at several photon energies (40–70 eV). The full valence shell binding energy spectrum reveals a well resolved correlation (satellite) peak at 20.3 eV and the photon energy dependence study of its photoionization cross section indicates that this new correlation peak is a “dynamic” correlation peak. MRSDCI calculations of the theoretical spectrum show semi-quantitative agreement between experiment and theory in terms of binding energies and intensities. A comparison of the full valence shell photoelectron spectra of ethylene, propene, 1-butene and 1,2-propadiene (allene) is also presented to illustrate trends in the correlation peaks of model alkenes.
Quasiclassical trajectory simulations of collisional deactivation of vibrationally excited HgBr(B 2Σ). I. Dependence on vibrational energy
Authors: Bollati Ricardo A. , Ferrero Juan C.
Published: July 2002

The collisional deactivation of HgBr(B 2Σ) by He, Ne, Ar, Kr, and Xe has been studied using quasiclassical trajectories calculations, with initial vibrational energy EV in the range 3000–20 000 cm−1. The rotational and translational energies used in the calculations corresponded to the Boltzmann distribution at 415 K. The first and second moments for the transference of translational, rotational, and vibrational energy are linearly dependent on EV. The spread of the distribution depends quadratically on EV for vibrational and translational energy transfer and linearly for the rotational change. The results could also be satisfactorily adjusted to power laws on EV and vibrational quantum number, which is indicative of the difficulty in interpreting experimental results through the usually available information of the dependence of the total average energy loss. Analysis of the correlation coefficients and the relative changes of average translational, rotational and vibrational energy transferred per collision indicates a strong V-R coupling for the heavier gases (Ar, Kr, and Xe).
Optical-optical double-resonance spectroscopic study of four ion-pair states of ClF and identification of the ClF(A 3Π1) valence state
Authors: Alekseev V. A. , Setser D. W.
Published: July 2002

Four of the six ion-pair states of ClF that correlate to Cl+(3PJ) and F−(1S0), the E(0+,3P2), f(0+,3P0), β(1,3P2), and G(1,3P1) states, have been identified by sequential, two-photon excitation via the ClF(B 3Π0+) valence state. The Ω=1 states, β and G, were studied by selection of rotational levels of the B 3Π0+ state that are perturbed by ClF(A 3Π1). Spectroscopic data from laser excitation and fluorescence spectra permit the assignment of vibrational energies and rotational constants to 30 levels. These four ion-pair states exhibit extensive homogeneous and heterogeneous interactions, and neither the vibrational energy nor the rotational constants are regular with increasing vibrational quantum number. The vibrational and rotational constants of the A 3Π1 state were identified from the low resolution ClF[β(1)−A 3Π1] emission spectra, and the dissociation limits of the A 3Π1 and B 3Π0+ states are compared. The dissociation energy of ClF(X) is confirmed to be 21 110 cm−1. Some qualitative information also was obtained about the D′(2,3P2) and A′ 3Π2 states of ClF.