bluuebunny/crossref_metadata_2025 huggingface dataset:
Shapes of generalized random walks
Authors: Biswas Parbati , Paramekanti Anur , Cherayil Binny J.
Published: July 2002

Shape parameters for self-avoiding, random, and collapsed walks have been determined analytically and numerically using the results of recent work on the path integral description of generalized Brownian motion. The analytical calculations are both simple and exact, and the predicted values of the shape parameters in two and three dimensions are in close agreement with the simulation results and with the available literature data, which are generally obtained by approximate, non-trivial perturbation approaches. Typical realizations of actual walks in two dimensions suggest that despite its nonstationary character, the underlying generalized random walk process can serve as a useful minimal model of chain configurations.
Convergence study of multi-configuration time-dependent Hartree simulations: H2 scattering from LiF(001)
Authors: Capellini A. , Jansen A. P. J.
Published: July 2002

The multi-configurational time-dependent Hartree method is compared with close-coupling and close-coupling wave-packet calculations for a four-dimensional model of scattering of H2 from a LiF(001) surface. Accurate results are obtained for the rotation–diffraction scattering probabilities. An analysis of the accuracy of the simulations as a function of the number of configurations shows that more single-particle functions are needed for the scattering coordinate than for the others to obtain good results. Reasonable results are, however, already obtained with a small number of configurations.
The collapse transition of a single polymer chain in two and three dimensions: A Monte Carlo study
Authors: Wittkop M. , Kreitmeier S. , Göritz D.
Published: July 2002

The collapse transition of a single polymer chain in two and three dimensions was studied using the bond-fluctuation model. The obtained exponents ν of the scaling law 〈S2N〉∼N2ν agree with values proposed in the literature as well as above, at and below the Θ-temperature TΘ. Transition curves and scaling analysis plots are presented. The scaling function α3SτN1/2 vs τN1/2 has a pronounced maximum before leveling off in the fully collapsed regime in accordance with the theory [α2S=〈S2N〉/〈S2N〉Θ, τ=‖(T−TΘ)/TΘ‖]. An analyzing of the subchain distances leads to disagreements with the blob model. The subchains are locally swollen for T≳TΘ and shrunken for T<TΘ. The probability distribution function of internal distances for T≥TΘ can be described by scaling functions of the form fs(x)∼xκs exp(−Dsxδs) for large x, x being the scaled distance. In contrast for T<TΘ none of these functions describe the data. The dynamic properties above TΘ are in agreement with the Rouse model, but below TΘ differences occur; the center of mass diffusion becomes anomalous and the relaxation times rise with a power law in N of the form τi(N)∼N2+3/d (d being the dimension of space).
Dynamics of the photodissociation of triplet ketene
Authors: Gezelter J. Daniel , Miller William H.
Published: July 2002

Calculations of the microcanonical dissociation rate for vibrationally excited ketene on the first excited triplet surface (T1) are presented. The calculations utilize the quantum reactive scattering methodology of absorbing boundary conditions (ABC) with a discrete variable representation (DVR) to obtain the cumulative reaction probability for dissociation over the barrier. Model 1- and 2-degree of freedom potential energy surfaces for the T1 surface were obtained by fitting to the best available ab initio structures, energies, and frequencies. The dissociation rates in these reduced-dimensionality calculations give good overall agreement with the experimentally measured rates, although the steplike features seen in the experiments are washed out by the tunneling through the narrow barrier predicted in the ab initio calculations. Further model calculations reveal that a barrier frequency of approximately 50–100i cm−1 is required to recover the step structure seen experimentally, which suggests that there is either another transition state region on the T1 surface farther out towards the product channel, or that there is surface-hopping dynamics taking place between the T1 and S0 ketene potential energy surfaces, or that the ab initio barrier frequency is simply too large.
Vibrational nonequilibrium in a supersonic expansion with reaction: Application to O2–O
Authors: Shizgal Bernie D. , Lordet François
Published: July 2002

The hypersonic expansion of O2 through a nozzle is considered. The steady nonequilibrium vibrational distribution function of O2, and the nonequilibrium forward reaction rate coefficient for the dissociation of O2 are calculated theoretically. In the first instance, the vibrational relaxation of O2 in the absence of reaction is examined in the temperature range 500–2500 K. The master equation for the vibrational populations, coupled to the steady one-dimensional conservation equations is solved numerically. The vibrational population obtained in this way, is compared to the distribution calculated using either a Treanor model or a Boltzmann distribution characterized by a vibrational temperature. The transition probabilities between O2 vibrational levels employed take into account the vibrational anharmonicity, and the anisotropic intermolecular potential. For the temperature range 2500–5000 K, the vibrational relaxation including dissociation and recombination is studied. The reactive probabilities that are required for this calculation were obtained from the model of Kiefer and Hajduk [Chem. Phys. 38, 329 (1979)].
Simulation of the charge transfer absorption of the H2O/O2 van der Waals complex using high level ab initio calculations
Authors: Palmer Ian J. , Brown William Byers , Hillier Ian H.
Published: July 2002

Model calculations of the UV charge transfer (CT) absorption spectrum of a water–oxygen collision pair are reported. The motivation is an assessment of this process as the initial stage of a new nucleation mechanism for atmospheric water vapor. Ab initio MP2(full)/6-311++G(2d,p) geometries of ground state van der Waals dimers and (complete active space self-consistent field) CAS-SCF(10,7)/6-311++G(2d,p) geometries of the CT state are detailed. MR-ACPF/ 6-311++G(2d,p) potential surfaces have been calculated along the intermolecular coordinate. The bound and continuum nuclear wave functions have been calculated numerically on these potential curves and, with the MR-CISD/6-311++G(2d,p) electronic transition dipole moment integral, used to calculate the unary and binary absorption coefficients. The implications of the results for explaining water photonucleation in the presence of oxygen are discussed.
Homogeneous nucleation rates for n-pentanol in argon: Determination of the critical cluster size
Authors: Hrubý J. , Viisanen Y. , Strey R.
Published: July 2002

We have measured homogeneous nucleation rates of n-pentanol droplets in argon using the nucleation pulse chamber recently described. The nucleation rates J were measured as functions of activity a at constant temperatures in steps of about 2.5 K for 241.8<T/K<271.0. The experimental data were compared with the predictions of the so-called self-consistent classical nucleation theory containing an additional exp(Θ)/a factor. Smaller systematic deviations of the predictions of the theory from the experimental onset activities were observed than previously for other systems. Using a simple, alternative derivation of the so-called nucleation theorem we, furthermore, determined the average size of the critical cluster (n*) from the slopes of the isothermal J–a curves. We found that the self-consistent classical nucleation theory systematically overestimates the experimental n*’s.
Molecular quadrupole moment functions of HF and N2. II. Rovibrational effects
Authors: Špirko Vladimír , Piecuch Piotr , Kondo Anne E. , Paldus Josef
Published: July 2002

Relying on the molecular quadrupole moment functions of the HF and N2 molecules in their electronic ground state, as obtained by the ab initio calculations at various levels of approximation of the coupled cluster method, as well as on the corresponding theoretical and experimental potential energy functions, the effective rovibrational averages and transition moments were evaluated over a wide range of values for the vibrational and rotational quantum numbers. The reliability of the ab initio data was also checked by evaluating the theoretical vibrational energies and comparing them with the experimental ones. The predicted quadrupole moments were found to be in close agreement with the available (rather scarce) experimental data, indicating the reliability of the calculated effective quadrupole moments for the broad range of rovibrational levels where no experimental data are available.
A quantum-classical study of the reaction CO(v1,j1)+OH(v2,j2)→CO2+H
Authors: Balakrishnan N. , Billing G. D.
Published: July 2002

The dynamics of the complex-forming reaction OH+CO→CO2+H is investigated using a recently reported quantum-classical approach for diatom-diatom reactive scattering. In the present study, the OH and CO vibrations are treated quantum mechanically using the time-dependent wave packet approach and their relative translational and rotational motions are treated classically. Results of total reaction probabilities, total reaction cross sections and thermal rate constants obtained from our calculations are compared with those from quasiclassical trajectory and different reduced dimensional quantum mechanical calculations.
Stress relaxation in unentangled and entangled polymer liquids
Authors: Chatterjee Avik P. , Geissler Phillip L. , Loring Roger F.
Published: July 2002

We develop a stochastic model for the dynamics of a dense fluid of flexible linear macromolecules. A polymer is represented by a harmonic chain of beads whose mobilities fluctuate in time between two values. A bead in the low-mobility state does not execute local motions, but may move by a cooperative slithering process involving the entire chain. A bead in the high-mobility state may execute both local and slithering motions. The rate at which the mobilities fluctuate is determined self-consistently as a function of chain length through an ansatz that associates these fluctuations with the configurational relaxation of neighboring molecules. We calculate the viscoelastic shear modulus and the coefficients of shear viscosity and self-diffusion for this model. The coefficient of shear viscosity η shows three regimes of dependence on chain length N. For a fluid of short chains, η∼N, in agreement with the Rouse model and with the behavior of laboratory polymers. For a liquid of longer chains, η displays an N dependence that is intermediate between N3 and N4, in agreement with laboratory measurements. In the asymptotic limit of large N, η∼N3, in agreement with the prediction of the tube model.
Master equation simulations of the vibrational overtone activation of methylcyclopropene
Authors: Ouporov I. , Grinevich O. , Snavely D. L.
Published: July 2002

The complete kinetic data for the vibrational overtone activation of methylcyclopropene have been simulated using master equation calculations. The simulation included photoactivation, collisional energy transfer, and reaction into three unimolecular channels. A good fit to the Stern–Volmer plots for all the products at six different photolysis energies was obtained. The fit required an adjustment of the thermal activation barriers input into the RRKM calculation. The best fit barriers were 12 833 cm−1 for 2-butyne, 14 547 cm−1 for 1,3-butadiene, and 14 685 cm−1 for 1,2-butadiene. The collisional deactivation was fit with a single exponential energy transfer distribution function with an average amount of energy transferred down per collision of 1000 cm−1. This average value fit all of the Stern–Volmer plots. The product yield ratios were examined for local mode specific effects, but none were found. Previously obtained thermal data can be fit if log A is changed from 12.72 to 12.30. Stern–Volmer plots were constructed for methylcyclopropene diluted in helium, argon, and sulfur hexafluoride for the Δv=6 olefinic CH stretch transition. These plots were simulated using the same calculation parameters as mentioned earlier except for those having to do with the collider gas. For these simulations the average amounts of energy transferred down per collision were 150, 200, and 500 cm−1 for helium, argon, and sulfur hexafluoride, respectively.
Molecular motion through fluctuating bottlenecks
Authors: Eizenberg N. , Klafter J.
Published: July 2002

In this paper we study the motion of a molecule in a series of fluctuating bottlenecks. We solve explicitly the case of two bottlenecks and show that under steady state condition the problem can be formulated in terms of a two-dimensional reaction-diffusion equation. We present an analysis of the reaction-diffusion equation, with a sink term that depends on the coordinates of the two bottlenecks, and relate the solution to molecular motion through biomolecules.
Ab initio studies on the interstellar molecules C3H2 and C3H and the mechanism for the neutral–neutral reaction C(3P)+C2H2
Authors: Takahashi Junko , Yamashita Koichi
Published: July 2002

Ab initio molecular orbital calculations have been performed on the interstellar molecules C3H2 and C3H, and the reaction pathway including them for the neutral–neutral reactions C(3P)+C2H2 in interstellar clouds were examined. The optimized structures, rotational constants, dipole moments, vibrational frequencies, IR intensities, and relative energies for the various isomers of the C3H2 and C3H species were calculated and discussed. It was found that the cyclic and linear C3H species have, respectively, two forms, and that the cyclic-C3H is more stable than the linear-C3H. New pathways for the neutral–neutral reaction C(3P)+C2H2 along which the C3H2 species (cyclopropenylidene, propenediylidene, and propargylene) are involved were found, and, especially, the formation of the cyclic-C3H via the neutral-neutral reaction process C(3P)+C2H2→C3H+H passing through the cyclic-C3H2 was discovered. On the other hand, it was found that the formation of the linear-C3H cannot be explained by the mechanism via the neutral–neutral reaction.
Isotope selective overtone spectroscopy of CHCl3 by vibrationally assisted dissociation and photofragment ionization
Authors: Hippler Michael , Quack Martin
Published: July 2002

Overtone spectroscopy in the gas phase by vibrationally assisted dissociation and photofragment ionization (OSVADPI) has been coupled with mass spectrometric detection of fragments enabling us to separate congested spectra into components arising from several naturally occurring isotopomers. The new technique of isotope selective overtone spectroscopy (ISOS) has been applied to the 41 component of the CH chromophore absorption near 11 385 cm−1 in CHCl3 at room temperature and in supersonic jet expansions. These spectra allow us to assign a close local resonance with a CCl3 frame mode indicating vibrational energy redistribution within some ps. This is to be compared with the ultrafast (ca. 50 fs) redistribution between the CH stretching and bending modes established previously.
Resonances in the cumulative reaction probability for a model electronically nonadiabatic reaction
Authors: Qi Jianxin , Bowman Joel M.
Published: July 2002

The cumulative reaction probability, flux–flux correlation function, and rate constant are calculated for a model, two-state, electronically nonadiabatic reaction, given by Shin and Light [S. Shin and J. C. Light, J. Chem. Phys. 101, 2836 (1994)]. We apply straightforward generalizations of the flux matrix/absorbing boundary condition approach of Miller and co-workers to obtain these quantities. The upper adiabatic electronic potential supports bound states, and these manifest themselves as ‘‘recrossing’’ resonances in the cumulative reaction probability, at total energies above the barrier to reaction on the lower adiabatic potential. At energies below the barrier, the cumulative reaction probability for the coupled system is shifted to higher energies relative to the one obtained for the ground state potential. This is due to the effect of an additional effective barrier caused by the nuclear kinetic operator acting on the ground state, adiabatic electronic wave function, as discussed earlier by Shin and Light. Calculations are reported for five sets of electronically nonadiabatic coupling parameters.
Ultraviolet spectroscopy of HgAr in a supersonic jet: A new determination of the A 3+ state potential
Authors: Lawrence S. J. , Stacey D. N. , Bell I. M. , Burnett K.
Published: July 2002

We report high-resolution fluorescence studies of the rotational structure of the HgAr A 30+–X 10+ (1≤v′≤6, v″=0) vibrational bands which occur near to the Hg 6s2 1S0–6s6p 3P1 (254 nm) atomic resonance. The molecules were formed in a supersonic jet expansion and were excited by UV radiation produced by intracavity second-harmonic generation in a cw dye laser. The intensity profile of each band was reproduced by a theoretical model. Least-squares fits to the experimental spectra yielded accurate values for the band origins, rotational constants, and isotope shifts. This information was used in a Rydberg–Klein–Rees evaluation of the A 30+ potential, which was found to differ slightly, but significantly, from a Morse function in the region probed by this experiment.
Vibrational spectra of penta-atomic silicon–carbon clusters. III. Pentagonal Si3C2
Authors: Presilla-Márquez J. D. , Rittby C. M. L. , Graham W. R. M.
Published: July 2002

Fourier transform infrared measurements on the spectra of the products of the vaporization of silicon/carbon mixtures trapped in argon at ∼10 K combined with the results from a published ab initio study and from new theoretical calculations carried out in the present work, have resulted in the first observation of a spectrum for the pentagonal Si3C2 cluster. Three vibrational fundamentals have been assigned: the ν2(a1) symmetric ‘‘breathing’’ vibration at 681.1 cm−1, the ν7(b2) Siα–C–C–Siα stretching deformation at 956.7 cm−1, and the ν8(b2) C–Siβ–C antisymmetric stretching vibration at 597.8 cm−1. The observed frequencies, relative intensities, and 13C, 29Si, and 30Si isotopic shifts are in excellent agreement with the theoretical predictions.
State-to-state dissociation of OClO(Ã 2A2ν1,0,0)→ClO(X 2ΠΩ,v, J)+O(3P)
Authors: Delmdahl Ralph Felix , Baumgärtel Stephan , Gericke Karl-Heinz
Published: July 2002

Applying the two-photon laser-induced fluorescence technique for nascent state resolved ClO(X 2ΠΩ,v,J) detection, the photofragmentation dynamics of OClO in the (Ã 2A211,0,0) and the (Ã 2A218,0,0) state is investigated at fixed photolysis wavelengths of 351 nm and 308 nm. In both experiments the product fragments are formed in their electronic ground states, namely ClO(2ΠΩ) and O(3P). A complete product state analysis proves the ClO radicals originating from the OClO(Ã 2A211,0,0) dissociation at 351 nm to be formed in v=0–4 vibrational states with a spin–orbit ratio of P(2Π3/2):P(2Π1/2)=3.8±0.5. The ClO fragment shows moderate rotational excitation. The obtained ClO product state distributions and the relatively high translational energy of the fragments can be explained by predissociation of the (Ã 2A2ν1,0,0)-excited parent molecule in the course of which the initial symmetric stretch motion (ν1) of OClO is transferred into the dissociative asymmetric stretching mode (ν3). ClO line profile measurements indicate a dissociation time of less than 0.5 ps. Resulting from the OClO(Ã 2A218,0,0) dissociation at 308 nm ClO is generated in very high vibrational states. The rotational excitation is comparable to that of the 351 nm photolysis study.
Analysis of the mass-resolved two-photon spectra of jet-cooled ArKr near Kr*(5p) and Ar*(4s)
Authors: Lipson R. H. , Dimov S. S. , Bascal H. A. , Hu X. K. , Mao D. M. , Cai J. Y.
Published: July 2002

New (2+1) resonantly enhanced multiphoton ionization (REMPI) spectra of ArKr in the region of Kr*(5p) and Ar*(4s) between ≊92378.8 and 94250.7 cm−1 are presented. A time-of-flight (TOF) mass spectrometer was used to obtain single isotopomer data. Four band systems, two previously observed by Dehmer and Pratt [J. Chem. Phys. 88, 4139 (1988)], and two new ones, have been vibrationally analyzed. Excited state bond lengths have been found from Franck–Condon factor calculations while electronic symmetries were assigned from REMPI spectra recorded with circularly polarized light. Our excited state symmetry assignments differ from those recently proposed by Heck et al. [J. Phys. Chem. 99, 17700 (1995)]. The unusual vibrational band intensity distributions observed for some of the electronic systems are rationalized qualitatively in terms of interstate avoided crossings.
The suitability of scanning angle reflectometry for colloidal particle sizing
Authors: van der Zeeuw E. A. , Sagis L. M. C. , Koper G. J. M. , Mann E. K. , Haarmans M. T. , Bedeaux D.
Published: July 2002

We investigated the suitability of reflectometry as a sizing method for polystyrene latex particles. This technique, where the particles are adsorbed at a quartz/water interface, involves the analysis of the reflection around the Brewster angle of light polarized in the plane of incidence. A computation of a full reflectivity curve containing about 20 datapoints, taking into account all electromagnetic interactions, is extremely complex and may take up to 100 h on an Indy R4600 computer (Silicon Graphics, Mountain View, CA). To use the complete electromagnetic theory for analysis of experiments therefore is impractical. We analyzed the experimental reflectivity curves using an approximate theory (free-sphere model), which neglects all electromagnetic interactions between different particles and their images in the substrate. To assess the errors introduced by neglecting these interactions, we have calculated reflectivity curves using the full electromagnetic theory for this system, and analyzed them with the free-sphere model. We found that the diameters determined with the free-sphere model deviate at most 2% from the actual values. The surface coverage determined with the free-sphere model is systematically smaller than the actual coverage. The difference can be as large as 15% at a coverage of 9%. We also show that size polydispersity of the sample plays a minor role in the analysis of the data. We illustrate the usefulness of the free-sphere model, by analyzing measurements in which polystyrene latex particles are swollen by an organic solvent that is miscible with water.
High resolution near-infrared electronic spectroscopy of HCBr
Authors: Chang Bor-Chen , Sears Trevor J.
Published: July 2002

The rotationally resolved spectrum of the HCBr Ã 1A″(0,2,0)←X̃ 1A′(0,0,0) Ka=0←1 transition between 12760 and 12850 cm−1 was obtained for the first time at Doppler-limited resolution using a transient frequency-modulation absorption technique. Rotational structure of HC 79Br and HC 81Br was identified and analyzed. The analysis shows R″(C–Br)=1.852 Å and R′(C–Br)=1.749 Å. The observed band indicates a linear–bent transition. This yields an upper limit of approximately 1600 cm−1 for the barrier to linearity above the zero-point energy for the Ã 1A″ state. Perturbations caused by singlet–triplet interactions were also found in the observed spectrum. The analysis of these perturbations indicates a very low-lying ã 3A″ state.
Reverse Monte Carlo analysis of neutron diffraction results: Water around its critical point
Authors: Jedlovszky Pál , Vallauri Renzo
Published: July 2002

The reverse Monte Carlo method in its molecular version, is used to analyze neutron diffraction data for water in two thermodynamic states around its critical point. At the lower density we find that the oxygen–oxygen and the oxygen–hydrogen partial radial distribution functions cannot be reproduced simultaneously in a satisfactory way. At the higher density the first peak of the oxygen–hydrogen partial radial distribution function, which gives a measure of the number of hydrogen bonded neighbors, turns out to be unreproducible. To understand the possible origin of these discrepancies the reverse Monte Carlo method is applied to the analysis of the radial distribution functions which are obtained directly by Fourier transforming the measured structure factors. This procedure reveals that at the lower density the total radial distribution function measured in light water can be responsible for the apparent deviations between simulated and experimental curves. At the higher density the experimental inaccuracy in the measurements of both light and half-deuterated water structure factors is found to affect the evaluation of the partial radial distribution functions. The results of the present work demonstrate that reverse Monte Carlo is a valuable method for revealing experimental inaccuracies, and therefore its use in the data reduction process is suggested.
Orientational behavior of solid nitrogen at high pressures investigated by vibrational Raman spectroscopy
Authors: Scheerboom M. I. M. , Schouten J. A.
Published: July 2002

In this paper the results of an experimental study of the vibrational Raman shift of solid δ-, ε-, and β-nitrogen will be presented. The measurements have been performed in a diamond anvil cell, from 0.5 to 14 GPa and from 120 to 400 K. The accuracy of the spectroscopic measurements has been optimized in order to determine the Raman shift as a function of temperature on an isobar. As will be shown, measurements along an isobar are appropriate for obtaining information about the orientational behavior of the molecules. In this study the results of β-nitrogen will be compared with those of δ- and ε-nitrogen. In β-nitrogen the Raman shift as a function of pressure shows a remarkable curvature, as compared to the results in δ- and ε-nitrogen. Also in contrast to the behavior in the other phases, in β-nitrogen the slope of the shift versus the temperature on an isobar changes from negative to positive when the pressure is increased. It will be argued that these results might be due to a short range orientational ordering of the molecules, as predicted by recent computer simulations. This ordering might cause a resonance coupling effect, resulting in a negative effect on the Raman shift in β-nitrogen.
Long-range asymptotic behavior of ground-state wave functions, one-matrices, and pair densities
Authors: Ernzerhof Matthias , Burke Kieron , Perdew John P.
Published: July 2002

The asymptotic behavior of an N-electron ground-state wave function is analyzed, as one electron wanders far from the system. Implications for the one-matrix and pair density are described. The asymptotic behavior currently discussed in the literature, in which the remaining (N−1) electrons relax to their ground state, is generalized to the case where the (N−1)-electron ground state is degenerate. Infinitely long-ranged correlations are reported, in which the selected (N−1)-electron ground state depends upon the direction along which one electron wandered off. We correct a standard limit for the one matrix. Numerical and analytic studies of accurate correlated wave functions illustrate and support the standard asymptotic behavior for the nondegenerate case and its generalization derived here. We extract the (N−1)-electron density from the correlated N-electron wave function. We also discuss the question how large the separation of one electron must be to realize the limiting behavior.
Structure and spectra of the thioketenyl (HCCS) radical in its ground and first excited states obtained by ab initio coupled-cluster methods
Authors: Szalay Péter G.
Published: July 2002

There is recent interest in finding the HCCS radical in interstellar sources and a detailed investigation of its UV spectrum has also been reported. By using state of the art coupled-cluster (CC) based ab initio methods, such as coupled-cluster singles and doubles (CCSD) with approximate triple excitation correction (CCSD(T)), the equation-of-motion CC method for excited states (EOMEE-CC) and for ionization potential (EOMIP-CC) together with large basis sets, we present the structure, harmonic vibrational frequencies and dipole moment of the ground and first excited 2Π states. We confirm the experimental rotational constant and most of the assignment made on the vibrational structure of the UV spectrum, but we also suggest reassignments in some cases. Special attention is paid to the Renner–Teller splitting present for such 2Π states.
An ab initio study of the O(1D)+HCl reaction
Authors: Hernandez Maria Luz , Redondo Carmen , Laganà Antonio , Ochoa de Aspuru Guillermo , Rosi Marzio , Sgamellotti Antonio
Published: July 2002

The potential energy surface of the O(1D)+HCl reaction has been calculated at an ab initio level. The calculated values have been fitted using a bond-order polynomial. On this potential energy surface quasiclassical trajectory calculations have been performed. These calculations reproduce most of the available experimental information better than the other existing potential energy surfaces.
Adsorption of water and methanol on zeolite Bro/nsted acid sites: An ab initio, embedded cluster study including electron correlation
Authors: Greatbanks Stephen P. , Hillier Ian H. , Burton Neil A. , Sherwood Paul
Published: July 2002

An embedded cluster model, employing a point charge field to correctly account for the electrostatic effect of the surrounding lattice, is used to study the adsorption of water and methanol at zeolite Bro/nsted acid sites. The effects of both the point charge field and electron correlation (studied at the second order Mo/ller–Plesset (MP2) and density functional theory (DFT) levels) are reflected in the predicted substrate structures, binding energies and 1H NMR shifts. Both result in enhanced substrate binding to yield good agreement with experiment, and an increased degree of proton transfer to the substrate, an effect particularly evident in the case of methanol. There is a suggestion from a comparison with measured 1H NMR shifts that this effect is somewhat overestimated by the model used here.
Isotope effects in trapping-mediated chemisorption of ethane and propane on Ir(110)
Authors: Kelly D. , Weinberg W. H.
Published: July 2002

We compare here recent results of molecular beam investigations of the initial probability of trapping-mediated C–H and C–D bond cleavage of C2H6, C2D6, C3H8, and C3D8 on Ir(110) at low beam translational energy and surface temperatures, TS, from 85 to 800 K. Each of these systems is highly reactive at low TS and displays decreasing reactivity with increasing TS. Measurements of the initial probability of trapping-mediated chemisorption for both ethane and propane reveal an isotope effect, which we attribute to zero-point energy differences, with the perhydrido-species exhibiting greater reactivity at a given TS. A difference in activation energies for desorption vs reaction (C–D bond cleavage) for C2D6 has been found to be Ed–Er=1.8±0.3 kcal/mol, cf. Ed–Er=2.2 kcal/mol for C–H bond cleavage of C2H6. For the trapping-mediated dissociative chemisorption of propane on Ir(110), Ed–Er=4.2 kcal/mol for C–H bond cleavage of C3H8, and Ed–Er=3.2 kcal/mol for C–D bond cleavage of C3D8. A quantitative analysis of the initial probability of trapping-mediated dissociative chemisorption of ethane and propane on Ir(110), within the context of a classical kinetic model of barrier crossing from the physically adsorbed state to the dissociatively chemisorbed state, provides the most reasonable description of the observed adsorption behavior.
Wetting transitions in polymer blends: Comparison between simulation and theory
Authors: Pereira Gerald G. , Wang Jian-Sheng
Published: July 2002

We investigate the problem of wetting transitions in polymer blends near a hard surface or wall by using a Monte Carlo technique to study the wetting transition along a path of increasing surface chemical potential. We introduce a parameter εs which describes the monomer–monomer interactions in the layer adjacent to the wall. This parameter is shown to behave similarly to the parameter g, used in mean field theory to describe the change in monomer–monomer interactions due to the wall. We identify a wetting tricritical point which may be defined either with respect to εs or the bulk density. For bulk densities less than the tricritical bulk density we obtain first-order transitions while for bulk densities greater than the tricritical bulk density we obtain critical wetting transitions, in accordance with mean-field theory. We also show how the molecular weight of the polymer can be varied to obtain first-order or critical wetting, as desired.
Infrared picosecond transient hole-burning studies of the effect of hydrogen bonds on the vibrational line shape
Authors: Bonn Mischa , Brugmans Marco J. P. , Kleyn Aart W. , van Santen Rutger A. , Bakker Huib J.
Published: July 2002

With infrared transient hole-burning spectroscopy we have investigated the influence of OD⋅⋅⋅X hydrogen bonds on the vibrational line shape of O–D stretch vibrations in acid zeolites. The effect of hydrogen bonding on the line shape depends critically on the type of hydrogen bond. For hydrogen bonding in a rigid structure, the hydrogen bond determines the inhomogeneous linewidth, but the homogeneous linewidth is determined by coupling to a ∼200 cm−1 lattice mode as concluded from the temperature dependence of the dephasing rate. When the hydrogen bond is formed with an adsorbing molecule, the coupling between the high-frequency O–D stretch vibration and the low-frequency OD⋅⋅⋅X hydrogen-bond stretching mode does determine the homogeneous linewidth. The difference between the two systems can be explained by the different hydrogen-bond potentials. Variation of the adsorbate provides a means of obtaining conclusive information on the coupling mechanism between the high-frequency O–D stretching mode and the low-frequency OD⋅⋅⋅X hydrogen-bond stretching mode.
Electronic absorption spectroscopy of diatomics on a dynamic surface: IBr on MgO(001)
Authors: Jiang Xue-Pei , Shapiro Moshe , Brumer Paul
Published: July 2002

A model for computing the absorption spectrum of a diatomic adsorbed on a nonzero temperature surface is developed based on the exact time-dependent expression for the absorption spectrum averaged over the surface variables. The required time evolution of the dipole transition density operator is modeled by a quantum Langevin-type equation derived by the method of Caldeira and Leggett. The resultant equation is treated numerically and sample spectra are obtained for IBr on an MgO(001) surface at a temperature of 150 K. The results, parameterized by a diatom-surface coupling constant, characterize the broadening of the spectrum in the presence of the stochastic thermal substrate motion.
Pressure and solvent isotope effects on the mobility of monovalent cations in water
Authors: Ueno Masakatsu , Tsuchihashi Noriaki , Yoshida Koji , Ibuki Kazuyasu
Published: July 2002

Limiting molar conductivities of alkali metal chlorides (LiCl to CsCl) and tetraalkylammonium bromides (Me4NBr to Bu4NBr) in H2O and D2O were determined at 25 °C as a function of pressure up to 196.1 MPa. The limiting molar conductivities of the ions were obtained with the aid of transference numbers of KCl under high pressure, and transformed into the residual friction coefficients Δζobs to see what kinds of factors are important in the mechanism of ion migration. The pressure and solvent isotope effects on Δζobs of the Li+ ion agree qualitatively with the predictions of the Hubbard–Onsager (HO) dielectric friction theory, which indicates that dielectric friction plays an important role for smaller ions. However, the Cs+ ion shows opposite pressure and solvent isotope effects. For the R4N+ ions, Δζobs increases with an increase in the ionic radius and the pressure, opposite to the predictions of the HO theory. The increase in Δζobs for large R4N+ ions with increasing pressure suggests that the structure of the hydrophobic hydration shell is not weakened by pressure as much as that of bulk water. To make sure of this, the rotational correlation times of water molecules in pure D2O (τc°) and coordinated to the Bu4N+ ions (τc+) were estimated from 2H NMR spin-lattice relaxation times (T1) of D2O molecules at high pressure. The pressure dependence of τ+c/τ°c was in qualitative agreement with that of Δζobs(Bu4N+).
Heat capacity extrema on isotherms in one-dimension: Two particles interacting with the truncated Lennard-Jones potential in the canonical ensemble
Authors: Freasier Ben C. , Woodward Cliff E. , Bearman Richard J.
Published: July 2002

Exact expressions are obtained for the heat capacity of a two-particle one-dimensional system interacting with the truncated, unshifted Lennard-Jones potential subject to periodic boundary conditions and the minimum image convention in the Gibbs canonical ensemble. Numerical calculations show that along isotherms the heat capacity exhibits maxima and minima as a function of density comparable to those found for three-dimensional models and experimental systems. For the present system, at very low temperatures, the maximum in the heat capacity arises because of a competition between low energy, which drives the particles towards the potential minimum, and high entropy, which drives the particles past the truncation distance where the force of interaction vanishes. The minimum arises because the range of integration in the partition function no longer is effectively infinite at sufficiently high densities. As the temperature rises, the locus of the maxima and the locus of the minima in the temperature–density plane move towards each other and finally merge at a reduced temperature T≊1.3. Above that temperature, the maxima and minima disappear. The contributions of different parts of the potential energy space are calculated. It is shown that the disappearance of the maxima and minima is related to the increasing probability of penetration of the two particles into the core region where the potential energy takes on large positive values.
Ion solvation in model polar clusters
Authors: Lu Dongsheng , Singer Sherwin J.
Published: July 2002

We have studied solvation of an ion in model polar clusters of Stockmayer (Lennard-Jones + point dipole) particles to test previous theories of cluster ion solvation, and to elucidate trends that may serve as a point of departure for analysis of more complex systems. We find that the pure solvent cluster is not isotropic, but has a well-defined pattern of orientational order which is converted to another distinct pattern by the ion. The extent to which the cluster is reordered depends upon the relative magnitude of ionic strength and solvent polarity. In many instances the two forms of order coexist with solvent shells far from the ion behaving as they did before inclusion of the ion, which shells close to the ion are reordered in a manner that best solvates the ion. The location of the ion in the cluster depends on relative ionic strength as well. The location changes from the surface to the center of the cluster in a gradual manner as the relative ionic strength increases. Qualitative agreement with dielectric continuum theory is found for large clusters. Deviations from dielectric continuum theory are found for small clusters, as expected, and compared to a previous theoretical prediction of those deviations.
Instabilities of polyampholytes in external electrical fields
Authors: Schiessel H. , Blumen A.
Published: July 2002

We consider the behavior of polyampholytes (PAs; heteropolymers carrying quenched positive and negative charges along the backbone) in external electrical fields. Whereas our previous treatments were devoted to the regime of a weak coupling of the charges, we consider here the regime where the electrostatic interactions between the charged monomers play the dominant role. Starting from a fluid drop picture we estimate the critical value of the external field that induces a breakup of the structure. As we proceed to show by scaling arguments, in strong fields the PA stretches out to a highly extended form, for which only a small fraction of the material is still organized in mesoscopic condensates along the rodlike configuration.
Reference hypernetted chain theory for linear molecular fluids: A comprehensive study of the gas-liquid coexistence
Authors: Anta J. A. , Lomba E. , Lombardero M. , Martín C.
Published: July 2002

We have used the reference hypernetted chain theory (RHNC) with a hard-core reference fluid treated in the modified Verlet’s approximation (VM) to obtain the gas-liquid coexistence curve of centrosymmetric linear molecular fluids modeled via site-site Lennard-Jones interactions with and without electrostatic forces. Explicit results are presented for the gas-liquid equilibrium of molecular nitrogen and carbon dioxide. Thermodynamic consistency constraints are crucial to define the reference system. Comparison with Gibbs Ensemble Monte Carlo and density functional theory (DFT) calculations evidence the RHNC-VM theory is an excellent tool for predicting coexistence densities and vapor pressures of linear molecular fluids.
Reorientational dynamics of an electric dipole in fluctuating electric fields
Authors: Seki Kazuhiko
Published: July 2002

The depolarization of rotating dipoles in a fluctuating electric field is discussed. When the random field is weak, a Fokker–Planck equation is derived from a stochastic model for dipole rotation. In particular, the coefficients in the Fokker–Planck equation showing coupling between random fields and an external field are derived for the first time for dielectric relaxation. This Fokker–Planck equation reduces to the simple rotational diffusion equation of Debye’s theory when the random electric field is white noise and isotropic but not otherwise. The early part of the dipole relaxation is found to be Gaussian in time although in the present paper the discussion is restricted to to the time regime after the initial inertial motion. The results also indicate that the relaxation time along the static external field, T1, gets longer as the static external field increases; this is in agreement with recent molecular dynamics simulation findings.
Lattice models for dipolar adsorption at metal/electrolyte interfaces using Bethe approximation
Authors: Saradha R. , Sangaranarayanan M. V.
Published: July 2002

A two-dimensional lattice model for adsorption of solvent molecules at a metal/electrolyte interface is proposed assuming two configurational states for dipoles. The generalized Ising Hamiltonian incorporating Coulombic effects and nonelectrostatic metal–solvent interaction energies is derived and solved for obtaining the order parameters using the Bethe approximation. The significance of the formalism as well as the extension to adsorption of neutral organic compounds is indicated.
Electron states of benzene–Br2 donor–acceptor complex: HeI photoelectron spectroscopy and ab initio molecular orbital study
Authors: Ananthavel S. P. , Hegde M. S.
Published: July 2002

HeI photoelectron spectrum of weakly bonded π-donor complex of benzene with Br2 is obtained. Benzene orbitals are shifted to higher binding energy by 0.25 eV and the bromine orbitals to lower binding energy by about 0.5 eV due to the complex formation. Ab initio molecular orbital calculations predicted a weak π→σ* complex between benzene and bromine having a C6v symmetry. The charge transfer is from the benzene π cloud to the bromine orbital. The magnitude of charge transfer computed from natural bond orbital analysis is 0.002.
Single molecules of dibenzanthanthrene in n-hexadecane
Authors: Boiron A.-M. , Lounis B. , Orrit M.
Published: July 2002

We report on a new, highly fluorescent aromatic compound for single molecule spectroscopy, dibenzanthanthrene, studied in an n-hexadecane matrix. Narrow homogeneous lines are compatible with the measured lifetime. From the two components in the fluorescence correlation function, we determined the intersystem crossing rates towards and from the triplet states. The very weak triplet yields indicate that the molecule behaves as a nearly perfect optical two-level system, with negligible buildup of triplet population.
A special singular perturbation method for kinetic model reduction: With application to an H2/O2 oxidation model
Authors: Li Genyuan , Rabitz Herschel
Published: July 2002

An evolution of a homogeneous chemical reaction system is described by a set of nonlinear ordinary differential equations. When the evolution occurs on different time scales, a special singular perturbation method may be employed to extract the equations for the slow evolution. The resultant equations have a quite simple form and retain high accuracy in the solutions. For illustration, an H2/O2 oxidation model is treated by this method. A general reduced model containing no radicals has been obtained, which can be applied to a wide range of initial conditions and has very good accuracy for the temperature and all the species.
Ab initio cluster model calculations on the boson peak frequencies of B2O3 glass
Authors: Uchino Takashi , Yoko Toshinobu
Published: July 2002

We have performed ab initio molecular orbital calculations on the clusters modeling the medium-range ordering (MRO) region of B2O3 glass at the Hartree–Fock (HF)/3-21G and HF/6-31G* levels. Their equilibrium geometries, harmonic vibrational frequencies, and Raman scattering intensities have been calculated. The calculations have reproduced the boson peak frequencies of vitreous B2O3 observed at ≊24 cm−1 and ≊137 cm−1. The normal coordinates have demonstrated that these low-frequency vibrational modes are due to the wavelike motions of atoms within the region composed of one (for the mode at ≊137 cm−1) or two (for the mode at ≊24 cm−1) boroxol ring(s). The results suggest that the boson peak originates from the collective vibrations localized in the MRO region of glasses. We also propose that in glasses there are localized potential surfaces that are defined by the atoms in the medium-range scale. The driving force of the fast β relaxation will be the cooperative motions of atoms at the unstable sites toward the stationary point on the localized potential surface, which occasionally leads to the formation of the MRO region. We then interpret the glass transition phenomena and the strong/fragile differentiation of glasses in terms of the restoring force on the localized potential surface.
Acceptor bound hole states in solid xenon
Authors: von Grünberg H. H. , Gabriel H.
Published: July 2002

The energy spectrum of a hole in the valence bands of a Xe crystal which is bound to a negative point charge, is calculated and analyzed in form of a correlation diagram relating the individual bands of the valence band structure to the level scheme of the bound hole states. It is found that this energy spectrum depends critically on the whole band structure, and in particular on the splitting of the j=3/2 bands. Our calculated binding energies are in good agreement with recent experimental results.
Nonadditive interactions in the H2 trimer
Authors: McDowell Sean A. C.
Published: July 2002

Explicit equations for the second order nonadditive induction energy for interactions involving three linear molecules have been derived using the MAPLE symbolic computational language. This nonadditive induction energy (involving the permanent quadrupole moments and dipole polarizabilities) is compared with the triple-dipole dispersion energy for a system consisting of three hydrogen molecules, in two different configurations. It is found that for most of the orientations studied the nonadditive induction energy is comparable in magnitude or larger than the triple-dipole dispersion energy.