bluuebunny/crossref_metadata_2025 huggingface dataset:
The computer simulation of correlated electron transfer across the electrode/electrolyte interface involving multiple redox species
Authors: Calhoun August , Voth Gregory A.
Published: July 2002

The basis for molecular dynamics simulations of the electron transfer between multiple redox species and a metal electrode is developed using the Anderson–Newns approach to model the effect of the electronic degrees of freedom. As an example, the free energy surface for 2 redox species coupled to the metal surface and with each other is computed via two dimensional umbrella sampling. The resulting free energy surface is found to be qualitatively in agreement with an analytic model for such a process [Y. Boroda, A. Calhoun, and G. A. Voth, J. Chem. Phys. 107, 8940 (1997)], with relatively minor quantitative disagreement.
Monte Carlo simulations of asymmetric polymer blends
Authors: Olaj Oskar Friedrich , Petrik Thomas , Zifferer Gerhard
Published: July 2002

Quasibinary asymmetric polymer blends (the average chain lengths of one component being 100 while the other is varied between N=1 and N=100) well below the critical temperature were simulated on a five-way cubic lattice using the Monte Carlo method. The bulk density was kept constant by adjusting the energy parameter ε according to a power law ε∼N−0.8. Quantities employed for the characterization of the interface region were its thickness (calculated from the shape of the density profile), the cavity structure (evaluated from the number of heterocontacts) and the roughness of the interface (described by the relative surface enlargement and some sort of fractal dimension). In general, the interface region becomes thinner, the roughness decreases, and the cavity structure becomes less pronounced with decreasing average chain length of one component, due to the reduced degree of entanglement in the interface.
Conformation of a polymer chain near the solvent critical region. I. The integral equation theory
Authors: Vasilevskaya Valentina V. , Khalatur Pavel G. , Khokhlov Alexei R.
Published: July 2002

Using the polymer reference interaction site model (PRISM) approximation and hybrid self-consistent MC/RISM method which combines the traditional Monte Carlo (MC) simulation with the numerical solution of the site–site Ornstein–Zernike-type (RISM) integral equation, we study solvent–mediated interactions and the conformational behavior of a single flexible-chain polymer immersed in a monoatomic solvent. The PRISM theory and the self-consistent MC/RISM method predict that in the vicinity of the solvent critical point there is an effective intrachain attraction between monomeric units of the chain. However, the strongly fluctuating solvent can induce significant conformational changes only if there is rather strong attraction between polymer segments and solvent particles. At such conditions, the collapse transition of long chains is possible near the solvent critical point. The equilibrium microstructure of the chain is modulated as a result of the competition between the intrachain short-range excluded volume repulsion and the nonlocal solvent–mediated attraction. For the dilute polymer solution without polymer–solvent attraction, the MC/RISM calculations show that the flexible polymer chain shrinks when approaching the critical point of the solvent. In this case, under the action of indirect intrachain attraction, long chain can take a specific winding conformation, with the fractal structure which is rather close to the globular structure.
Comment on “Local frequency analysis and the structure of classical phase space of the LiNC/LiCN molecular system” [J. Chem. Phys. 108, 63 (1998)]
Authors: Martens C. C. , Davis M. J. , Ezra G. S.
Published: July 2002

In this Comment, we correct a misunderstanding in the title paper concerning the accuracy of the fast Fourier transform method of local frequency analysis of Martens, Davis, and Ezra. We also discuss the application of the Martens–Davis–Ezra method to a two degree of freedom model of OSC.
Infrared photophysics in an ion trap
Authors: Stace A. J.
Published: July 2002

A Monte Carlo model has been developed which provides a very detailed picture of conditions during multiphoton infrared fragmentation experiments, as performed in an ion trap. Typically, two types of ion traps are used, an ion cyclotron resonance (ICR) instrument, and a quadrupole ion trap. Experiments fall into three separate categories: Low background gas pressure combined with either high or low intensity laser radiation, and moderate background pressures with low intensity laser radiation. Each set of experimental conditions brings to the simulation a dependency on a particular set of variables, and these can be refined to give a self-consistent picture of the complete photofragmentation process. At the low gas pressures (∼10−7 mbar) found in ICR traps, the simulation of experiments run at low laser intensities shows that radiative decay has an important influence on photofragment yield. In the same type of trap, but at high laser intensities, pulse shape and stimulated emission become important. Finally, at pressures found in a typical quadrupole ion trap (∼10−4 mbar), collisions with the helium background have a significant effect on the outcome of infrared excitation; however, the time scale of an experiment is such that radiative decay can also influence the results. The model has been applied to the infrared photofragmentation of the protonated diethyl ether dimer, [(C2H5)2O]2H+, where it successfully accounts for experimental results recorded under each of the three conditions identified above. Under circumstances where photofragmentation is in competition with either radiative or collisional relaxation, the calculations show that fragmentation requires the absorption of up to 20 photons (assumed to come from a CO2 laser, hν≈0.11 eV), as opposed to the 12 photons necessary to match the critical energy of reaction.
Stability study of icosahedral phases in diblock copolymer melt
Authors: Roan Jiunn-Ren , Shakhnovich E. I.
Published: July 2002

The stability of three possible icosahedral phases (called here vertex, edge, and face models, respectively) in a diblock copolymer melt is examined, with higher harmonics included in the density-wave mean field theory developed by Leibler. Two cases are examined: (1) diblock copolymer melt with constant coupling vertices, and (2) diblock copolymer melt with general coupling vertices in which, unlike all the previous studies on the effect of higher order harmonics in diblock copolymer melt, the calculation is performed exactly. We find that icosahedral phases are always metastable and the mean-field phase diagram of the diblock copolymer is unchanged.
Droplet interaction in the spinodal decomposition of a fluid
Authors: Kumaran V.
Published: July 2002

The interaction between a pair of non-Brownian droplets in the spinodal decomposition of a binary fluid is examined. The interaction arises due to the convective term in the model H momentum equation, which is reciprocal to the convective term in the concentration equation. The dominant contribution to this convective term is due to the interface between the droplet and the matrix, where concentration gradients are large, and this contribution is determined in the limit where the distance between the droplets L is large compared to the radius of a droplet R. The force on the fluid due to the interfacial concentration gradient is first calculated, and it is found that there is a net force on the fluid only if there is a deviation of the interfacial concentration profile from the equilibrium profile. This deviation is related to the flux of solute at the interface, which is calculated correctly to (R/L)2 for the interacting droplets. The average velocity of the droplets is then calculated by solving the momentum equations for the system. It is found that the interaction between the droplets does cause a spontaneous motion of the droplets towards each other.
A generalized surface hopping method
Authors: Sholl David S. , Tully John C.
Published: July 2002

We present a method that allows the mixed quantum-classical dynamics of a system containing both bound and continuum quantum states to be simulated using a surface hopping method. In the limit where the quantum wave function is made up of only contributions from the continuum, this method reduces to mean-field (Ehrenfest) dynamics. We demonstrate the new technique by simulating a simple model of a quantum wave packet colliding with an adsorbed particle on a solid surface. By calculating the mixed quantum-classical evolution of this problem with both mean-field dynamics and our generalized surface hopping scheme and comparing these results to fully quantum solutions, we show that the surface hopping approach can avoid some of the inaccuracies that are common features of mean-field calculations.
Realistic master equation modeling of relaxation on complete potential energy surfaces: Kinetic results
Authors: Ball Keith D. , Berry R. Stephen
Published: July 2002

Using the potential surface information for (KCl)5 and Ar9 and partition function models introduced in the preceding paper [Ball and Berry, J. Chem. Phys. 109, 8541 (1998)] we construct a stochastic master equation for each system using Rice–Ramsperger–Kassel–Marcus (RRKM) theory for transition rates between adjacent minima. We test several model approximations to reactant and transition-state partition functions by comparing their master equation predictions of isothermal relaxation for (KCl)5 and Ar9 with the results of molecular dynamics simulations of relaxations performed in the canonical ensemble. Accurate modeling of the transition-state partition functions is more important for (KCl)5 than for Ar9 in reproducing the relaxation observed in simulation. For both systems, several models yield qualitative agreement with simulation over a large temperature range. This full treatment of small systems using realistic partition function models is a necessary first step in the application of the master equation method to larger systems, for which one can only expect to have statistical samples of the potential energy surfaces.
Dynamical study of the dissociation and elimination channels in the decomposition of methyl nitrite
Authors: Martı́nez-Núñez Emilio , Vázquez Saulo A.
Published: July 2002

The dynamics of the two unimolecular reactions that initiate the thermal decomposition of methyl nitrite were investigated by classical trajectories and statistical variational efficient microcanonical sampling-transition state theory. These two channels are (I) O–N bond dissociation to produce CH3O and NO and (II) concerted elimination through a four-center transition state to form CH2O and HNO. In order to perform both types of calculations, a potential energy function was developed, which reproduces reasonably well the energies, geometries, and frequencies selected from the literature. Microcanonical rate coefficients and branching ratios were obtained by each method at total energies ranging from 100 to 240 kcal/mol. The computed branching ratios indicated that reaction I is markedly faster than reaction II, which agrees with the experimental observations. It was found that for energies up to 160 kcal/mol the dynamics of reaction I is intrinsically Rice–Ramsperger–Kassel–Marcus (RRKM), but for the highest energies the behavior becomes intrinsically non-RRKM. The classical trajectories showed that the elimination process takes place via a regular dynamics during the last moments before reaction, which is clear evidence for nonstatistical behavior. Analysis of the trajectory rates computed for the deuterated species revealed that the dissociation process exhibits an inverse secondary isotope effect.
Protonated fluorine, HF2+: Ab initio potential energy surface and vibrational splittings for proton transfer
Authors: Chan Wai-To , Hamilton I. P.
Published: July 2002

We calculate 1542 ab initio points for the HF2+ ground state at the QCISD(T)/6-311++G(2df,2pd) level for a wide range of geometries. We fit the ab initio points to a multiparameter analytic function to obtain a multidimensional potential energy surface (PES) valid for large amplitude hydrogen motion. We then calculate and assign vibrational levels for this PES. There is intramolecular proton transfer when the hydrogen atom tunnels through a triangular transition state separating the two equivalent equilibrium geometries. The barrier to proton transfer is 9547 cm−1 (8340 cm−1 with zero-point correction). Below the barrier to proton transfer, the energy levels are split and measurable splittings are predicted for relatively low-lying vibrational levels that may be experimentally accessible. The first three levels with splitting greater than 0.01 cm−1 are, in order of increasing energy, (0,3,1), (0,4,0), and (0,3,2), while the first three levels with splitting greater than 0.1 cm−1 are, in order of increasing energy, (0,4,1), (0,5,0), and (0,4,2), where ν2 is the H–F–F bend quantum number and ν3 is the F–F stretch quantum number. We conclude that H–F–F bend excitation is essential for proton transfer, and that F–F stretch excitation facilitates proton transfer. In addition, there is a 3:1 Fermi resonance between the HF stretch (harmonic frequency 3334 cm−1) and the H–F–F bend (harmonic frequency 1141 cm−1), and levels with HF stretch excitation can have significant splittings. For example, the splitting is greater than 0.01 cm−1 for the (1,1,1) level, which is Fermi resonant with (0,4,1) and greater than 0.1 cm−1 for the (1,2,1) level, which is Fermi resonant with (0,5,1). This is relevant for the experimental observation of the vibrational splittings since the IR intensity of the HF stretch is four times that of the H–F–F bend.
Combination bands versus overtone stretch excitation and rotational effects in vibrationally mediated photodissociation of acetylene
Authors: Schmid R. P. , Ganot Y. , Bar I. , Rosenwaks S.
Published: July 2002

Rovibrational excitation combined with promotion of C2H2 molecules to the excited electronic trans-bent states Ã 1Au/B̃ 1Bu and photofragment ionization are used to generate action spectra, H Doppler profiles, and time-of-flight mass spectra. Rovibrational states of C2H2 in the 15 480–15 723 cm−1 region are photodissociated by 243.135 nm photons that subsequently tag the H fragments. The H photofragment yield is greatly enhanced upon rovibrational excitation. In the action spectra, the intensities of the combination bands that involve high stretch and low bend excitation, (1410020) and (2031100), are close to that of the fourth overtone of the C–H stretch, (2030000), while in the absorption spectra the intensities of the combination bands are much weaker. In addition, the effect of rotation on fragment yield and dissociation channels is demonstrated. Several pairs of rotational transitions (sharing similar J′) stand out from the rotational band contour and the R(13) line of (2030000) shows an anomalously high intensity. The origin of the intensity enhancement for the combination bands is the better Franck–Condon (FC) overlap of states containing bend excitation with the dissociative wave functions. The source of the high intensity for the pairs of rotational transitions is the interaction with zero-order dark states that also give rise to a better FC factor, whereas in R(13) excitation an additional bent state is prepared, followed by two photon ultraviolet absorption, leading to a different photodissociation pathway.
An interpretation for the thermodynamic features of ice Ih↔ice XI transformation
Authors: Johari G. P.
Published: July 2002

The unusual manner of entropy change at the ice Ih↔ice XI transformation has been interpreted in terms of an equilibrium involving the strain energy at the ice Ih/ice XI interface, the Gibbs energy and the grain-boundary energy, and a formalism given. The persistence of Ih below its first order phase transformation temperature of 72 K and the persistence of ice XI, the proton-ordered phase, at temperatures up to 110 K are attributed to the predominance of the strain energy which opposes the transformation favored by the Gibbs energy. Calculations using the measured entropy yield a strain energy of 3.5 J/mol at 72 K, which decreases on heating toward 120 K. Consequences of the theory are: (i) Thermodynamic quantities measured below 72 K would change on thermally cycling the sample through the transformation region, which has been observed; (ii) A procedure that reduces the strain at the ice Ih–ice XI boundary will increase the extent of transformation, i.e., mechanically deforming or grinding a partially transformed sample, or subjecting it to high energy irradiation to create more point defects that can bind with the ice Ih/ice XI interface, would transform ice Ih and ice XI more in either direction.
The effect of nonadiabatic coupling on the calculation of N(E,J) for the methane association reaction
Authors: Mardis Kristy L. , Sibert Edwin L.
Published: July 2002

A classic example of a barrierless reaction, CH3+H→CH4 is used as a model to test the extent of nonadiabatic coupling on the reaction rate constant. This coupling has two contributions. The first arises from the anisotropy of the CH3+H potential and the second is Coriolis coupling. A method is presented which calculates adiabats formally equivalent to those calculated by statistical adiabatic channel model (SACM) while still permitting the determination and inclusion of nonadiabatic coupling. Using the discrete variable representation (DVR) for the interfragment distance R, the ro-vibrational Hamiltonian is solved at particular R values. The eigenvalues and eigenfunctions are calculated at each of these R values to create the surface and the coupling elements used in a wave packet propagation. The dynamics of the reaction are investigated through a study of the cumulative reaction probability N(E,J) using energy resolved flux methods. We find that for J=0, 1, and 2, neglecting the coupling due to the changing anisotropy as a function of R results in a 20% error in N(E,J). Neglecting the Coriolis coupling results in average errors of 2% lending support to the helicity-conserving approximation. Finally, within the adiabatic approximation, the calculated adiabats provide a more realistic view of the barriers than the analytic functions of SACM, require no fitting parameter, and are obtained at reasonable computational cost.
A general finite-field scheme for the calculation of frequency-dependent electronic–vibrational hyperpolarizabilities
Authors: Bishop David M. , Kirtman Bernard
Published: July 2002

A rigorous and transparent treatment for determining electronic–vibrational hyperpolarizabilities in the context of the finite-field method is developed. Our emphasis is on the vibrational contributions. The new formulation is more general than previous ones in that it yields properties over the entire frequency range.
Cavity size in reaction field theory
Authors: Zhan Chang-Guo , Chipman Daniel M.
Published: July 2002

The optimum size of the cavity accommodating a solute in the reaction field theory of solvation is considered by empirical calibration of the results of electronic structure calculations against experiment. To isolate the long range electrostatic free energy contributions treated by reaction field theory from the many other short range contributions not explicitly considered, computational results are compared to experimental determinations of conformational free energy differences in polar solutes having two or more stable or metastable isomers. When the cavity shape is defined by a solute electronic isodensity contour, it is found that the best overall agreement with experiment is obtained with a cavity size corresponding to the 0.001 a.u. contour.
Hot-atom mechanism and reaction sites in photoinduced carbon monoxide oxidation on a stepped platinum (113) surface
Authors: Yamanaka T. , Inoue Y. , Matsushima T.
Published: July 2002

Angular and velocity distributions of product CO2 were measured in CO oxidation photoinduced by 193 nm laser pulses of 16 ns duration on a stepped Pt(113) surface. At low oxygen and CO coverages, the CO2 desorption was collimated at ±23° from the surface normal in a plane along step edges. CO2 is formed by the collision between CO on an atop site and translationally hot oxygen atoms emitted along the axes of oxygen molecules that lie along the step edge, resulting in declined desorption due to momentum transfer from the hot atoms. With increasing oxygen coverage, new components collimating at the (111) terrace and (100) step normal became major, while an increase in the CO coverage enhanced the CO2 desorption collimated in the normal direction. It is likely that at high coverage the reaction sites extend to (111) and (100) microfacets and also to surface troughs, yielding the above three components. The translational temperature of product CO2 was often maximized at around angles where the CO2 desorption is collimated.
Solvation in high-temperature electrolyte solutions. I. Hydration shell behavior from molecular simulation
Authors: Chialvo A. A. , Cummings P. T. , Simonson J. M. , Mesmer R. E.
Published: July 2002

The behavior of the first hydration shell of species in solution and its relevant thermophysical properties are studied by molecular dynamics of infinitely dilute NaCl aqueous solutions at high temperature. The ion-induced effects on the water local properties are assessed in terms of the corresponding radial profiles for the local density, the local pressure, the local electric field, the local dielectric constant, and two alternative types of coordination numbers, along the near-critical reduced isotherm Tr=1.05 and the supercritical reduced isochore ρr=1.5. Simulation results are discussed in the context of their usefulness in enhancing the understanding and the modeling of supercritical aqueous electrolytes.
Measurements of Cl-atom photofragment angular momentum distributions in the photodissociation of Cl2 and ICl
Authors: Rakitzis T. Peter , Kandel S. Alex , Alexander Andrew J. , Kim Zee Hwan , Zare Richard N.
Published: July 2002

We have studied the complete Cl-atom molecular-frame photofragment angular momentum distributions from the photodissociation of Cl2 and ICl in the 320–560 nm region using time-of-flight mass spectroscopy with laser detection. The experimental signals were analyzed using the polarization-parameter formalism described in the preceding paper. These experiments study three distinct cases. The first case is the 470 nm dissociation of Cl2 through the B 3Π0+u state accessed via a parallel transition, yielding Cl-atom photofragments with polarizations described by the single parameter a0(2)(∥)=−0.7±0.2. The second case is the 320 nm dissociation of Cl2 through the C 1Π1u state accessed via a perpendicular transition, yielding Cl-atom photofragments with polarizations described by the two parameters a0(2)(⊥)=−0.50±0.10 and a2(2)(⊥)=−0.32±0.06. The third case is the dissociation of ICl in the 490–560 nm region in which dissociative states of both parallel and perpendicular character are accessed. In this wavelength region, the polarizations of the resulting Cl-atom photofragments are completely described by the approximately constant incoherent parameters, a0(2)(∥)≈+0.4, a0(2)(⊥)≈−0.2, and a2(2)(⊥)≈−0.2, whereas the interference contributions to the polarization, the Im[a1(1)(∥,⊥)] and Re[a1(2)(∥,⊥)], oscillate sinusoidally with excitation wavelength in a fashion that is sensitive to the shapes of the dissociative surfaces.
Response to “Comment on ‘Simple reversible molecular dynamics algorithms for Nosé–Hoover chain dynamics’ ” [J. Chem. Phys. 110, 3623 (1999)]
Authors: Jang Seogjoo , Voth Gregory A.
Published: July 2002

The purpose of Nosé–Hoover chain (NHC) dynamics at constant (N,V,T) is canonical distribution sampling, and this sampling is shown to be insensitive to fluctuations in the conserved quasi-Hamiltonian. For anharmonic potentials, it is also shown that the quasi-Hamiltonian conservation properties of the reversible algorithms developed by Martyna and Tuckerman degrade significantly.
Unusual conical intersections in the Jahn–Teller effect: The electronically excited states of Li3
Authors: Sadygov Rovshan G. , Yarkony David R.
Published: July 2002

Conical intersections of the 2 2E′ and 1 2A1′ states of Li3 are considered as a first step in the description of the excited state involved in the absorption at 14 583 cm−1. Quite novel results are obtained. In addition to the standard symmetry-required D3h seam of conical intersection, C2v symmetry-allowed seams are found and their locus determined as a function the symmetric stretch coordinate. The C2v seams of conical intersection intersect the D3h conical intersection seam at two distinct points producing two heptafurcations of the D3h seam. The locus of the C2v conical intersection points is explained in terms of a quadratic Jahn–Teller model. The overall geometric phase effect associated with the D3h and C2v conical intersections is well described using a previously suggested phase factor deduced from molecular properties. In addition to these conical intersections an intersection of the 1 2A1′ and 2 2E′ is states is located.
Monte Carlo simulations of nitrogen and hydrogen physisorption at high pressures and room temperature. Comparison with experiments
Authors: Darkrim Farida , Vermesse Jean , Malbrunot Pierre , Levesque Dominique
Published: July 2002

The excess surface adsorptions of nitrogen and hydrogen gases on graphite basal planes at a temperature of 293 K have been computed by numerical simulations. The results of the simulations are compared to experimental data on the adsorption of these two gases measured on activated carbons. The adsorption isotherms have been calculated up to 600 MPa, and the experimental isotherms have been measured in the pressure range from 0 to 500 MPa for nitrogen gas and up to 60 MPa for hydrogen gas. Both theoretical and experimental nitrogen adsorption isotherms have similar shapes. They present a maximum at moderate pressures, then decrease toward a constant value or a shallow minimum followed by a slight increase at very high pressures. In addition, we compare the values of the hydrogen adsorption computed by simulation at a temperature of 77 K and low pressures with those recently measured. The qualitative agreement between experimental and simulations results is rather good for both gases at the temperatures considered.
Path integral approximation of state- and angle-resolved inelastic scattering
Authors: Whiteley T. W. J. , McCaffery A. J.
Published: July 2002

We construct a path integral based approximation to rotationally inelastic collisions from which differential scattering cross sections are obtained for a number of atom–diatom systems. These are found to be in good agreement with IOS calculations on the same systems. In this approximation, the classical and near-classical paths that control scattering from a quantized system are deduced and this process reveals the origins of interference effects seen in theoretical calculations and some experimental measurements of angular distributions. This formulation provides physical insight into the important trajectories in systems where one or more degrees of freedom are quantized and could be regarded as an extension of classical S matrix theory, which for simple systems do not require root finding methods.
Unraveling the mysteries of metastable O4*
Authors: Peterka Darcy S. , Ahmed Musahid , Suits Arthur G. , Wilson Kenneth J. , Korkin Anatoli , Nooijen Marcel , Bartlett Rodney J.
Published: July 2002

A recent report from our laboratory described 1+1 resonant photoionization spectra of an energetic, metastable O4 species produced in a dc discharge [Bevsek et al., Faraday Discuss. 108, 131 (1998)]. Although a definitive assignment of the spectra was lacking, the long-predicted covalent forms of O4, either cyclic (D2d) or “pinwheel” (D3h), were adduced as possible candidates. We here present rotationally resolved photoionization spectra, photoelectron spectra, and ab initio calculations providing strong evidence for the identity of this species as a novel complex between a ground state O2 molecule and one in the c(1Σu−) state, which is excited via an allowed transition to the 1(1Πg)valence state. The latter then couples to the d(1Πg) Rydberg state, shifted in energy owing to the presence of the adjacent O2 molecule, from which it then ionizes. The results underscore the potential importance of the fully allowed but overlooked 1(1Πg)←c(1Σu−) electronic transition in O2 in the near ultraviolet, and provide a simple interpretation of puzzling results in an earlier study of electron transfer to O4+ [H. Helm and C. W. Walter, J. Chem. Phys. 98, 5444 (1993)].
Analysis of electric-field assisted photogeneration in polyparacyclophane doped with 2,4,7-trinitrofluorenone
Authors: Jung J. , Głowacki I. , Ulański J.
Published: July 2002

A new photoconducting polymer with extended cooperative electronics effects, poly((E,E)-[6.2]paracyclophane-1,5-diene) (PDE), doped with a low molecular weight organic acceptor, 2,4,7-trinitrofluorenone (TNF), was investigated by means of steady-state photoconductivity and xerographic discharge methods. The photogeneration quantum yield in the PDE+TNF charge transfer (CT) complex for blue light is comparable with the photogeneration yield in poly(N-vinylcarbazole) (PVK) doped with TNF. It is demonstrated that the electric-field dependence of the photogeneration quantum yield in PDE+TNF systems can be described by the Braun kinetic model of dissociation of the CT complex, and not by the Onsager model of geminate recombination, which is appropriate for PVK+TNF systems.
Fluorobenzene⋯water and difluorobenzene⋯water systems: An ab initio investigation
Authors: Tarakeshwar P. , Kim Kwang S. , Brutschy B.
Published: July 2002

Recently, Brutschy and co-workers have reported the spectra of (substituted benzene)⋯(H2O)n systems. To investigate the possibility of these systems exhibiting a π–H kind of bonding interaction as observed in benzene⋯(H2O)n systems, we have carried out extensive ab initio calculations on different conformations of the fluorobenzene⋯(H2O) and p-difluorobenzene⋯(H2O) systems using various basis sets. Our results indicate that unlike the π interaction observed in benzene⋯(H2O)n, the O–H of the water molecule is involved in the formation of a six-membered ring system with the F–C–C–H of the aromatic ring. This six-membered ring which results from the formation of two H-bonds (water hydrogen and fluorine, water oxygen and benzene hydrogen), is extensively stabilized by electrostatic interactions. The strength of this σ-bonding interaction of water to fluorobenzene in C6H5F⋯H2O is nearly equal to the corresponding π-bonding interaction of water to benzene in C6H6⋯H2O. However the σ interaction of water to difluorobenzene in p-C6H4F2⋯H2O is somewhat higher than the π interaction in C6H6⋯H2O and slightly higher than the corresponding interaction in C6H5F⋯H2O. The frequency shifts of the predicted OH stretching modes are in reasonable agreement with the experimental vibrational frequency shifts for both C6H5F⋯H2O and p-C6H4F2⋯H2O.
State-to-state rate coefficients for rotational relaxation of CO in Ar
Authors: Belikov A. E. , Smith Mark A.
Published: July 2002

The rotational level populations of CO molecules were measured in CO (<10%)+Ar free jets by electron beam fluorescence (in a stationary jet) and resonantly enhanced multiphoton ionization (in a pulsed jet). The measured evolution of the nonequilibrium rotational energy was used to derive the rotational relaxation cross sections in the temperature range from 7 to 150 K. Using the pressure-broadened linewidth data the state-to-state rotational relaxation rate coefficients were found using the most popular fitting law forms: The modified exponential gap, the statistical polynomial-exponential gap, and the energy corrected sudden approximation. The fitted rates were compared with the experimental and theoretical data presently available in the literature. They were checked also by consideration of the rotational kinetics in free jets and by comparison between the computed and experimental rotational level populations both in near and far regions of a flow.
Formation of segmental clusters during relaxation of a fully extended polyethylene chain at 300 K: A molecular dynamics simulation
Authors: Liao Qi , Jin Xigao
Published: July 2002

Molecular dynamics simulations were performed on the relaxation of fully extended polyethylene chains, varying in length from 600 to 4000 CH2 units. When a chain has more than 1200 CH2 units, the relaxation proceeds in three stages, subglobule formation, subglobule growth, and subglobule coalescence into one globule. In the stage of subglobule growth, the number of subglobules is almost a constant because of kinetic barrier, which is different from the results based on Monte Carlo simulations of polymer collapse in dilute solution. The difference of lamella thickness between MD simulations and experiments is discussed. The degree of adjacent reentry is calculated, and amounts between 0.4 and 0.5 for the formed lamellae.
Complete active space self-consistent field and multireference configuration interaction studies of the differences between the low-lying excited states of HO2 and HO2–H2O
Authors: Aloisio Simone , Li Yumin , Francisco Joseph S.
Published: July 2002

The vertical excitation energies for the three lowest excited states of HO2 and the HO2–H2O complex and the molecular orbitals of the HO2–H2O complex map to the excited states have been calculated with complete active space self-consistent field and multireference configuration interaction (CASSCF) and (MRCI) methods of the corresponding states of HO2. Significant differences between the vertical excited state energies of HO2 and the HO2–H2O complex are found.
Dipole-bound anion of water dimer: Theoretical ab-initio study
Authors: Chen Hsing-Yin , Sheu Wen-Shyan
Published: July 2002

The dipole-bound anion of the water dimer is studied via an ab-initio approach. Two nuclear stable configurations, trans-(H2O)2− and cis-(H2O)2−, are both identified on the potential energy surface of the anionic water dimer. In addition, the transition state connecting the stable trans-(H2O)2− and cis-(H2O)2− is also located. The transition barrier, relative stability and vertical detachment energies (VDEs) of these two configurations are evaluated at high electron-correlated levels of theory. Consequently, the trans-(H2O)2− is found to be unstable against electron autodetachment. The only electronic and nuclear stable species of the anionic water dimer is the cis-(H2O)2−. The experimental implications of these findings are also discussed.
Calculation of OH-stretching band intensities of the water dimer and trimer
Authors: Low Geoffrey R. , Kjaergaard Henrik G.
Published: July 2002

We have calculated fundamental and overtone OH-stretching vibrational band intensities of the water dimer and trimer. The intensities were determined with a simple harmonically coupled anharmonic oscillator (HCAO) local mode model and ab initio dipole moment functions. The dipole moment functions were calculated at the self-consistent-field Hartree–Fock and quadratic configuration interaction including single and double excitations levels of theory with the 6–31G(d), 6–311+G(d,p), and 6–311++G(2d,2p) basis sets. The overtone spectra of the dimer and trimer have not been observed and a method of obtaining local mode parameters from scaled ab initio calculations has been suggested. We find that the falloff in absolute intensity with increasing overtone of the dimer and trimer is similar to the falloff observed for the monomer. Our calculations show that the total overtone intensity of the dimer and trimer, although distributed differently, is close to two and three times the total intensity of the monomer for a given region. One significant difference between the monomer and the dimer and trimer is the appearance of the red shifted hydrogen bonded OH-stretching band in the dimer and trimer spectra. We suggest that these red shifted bands are ideal for attempts to observe the water dimer in the atmosphere. The method presented can provide an accurate estimate of the OH-stretching intensities for molecules for which vibrational spectra have not been observed. Such calculations are of importance in atmospheric solar energy absorption models.
Electron transfer in three-center chemical systems
Authors: Zusman Leonid D. , Beratan David N.
Published: July 2002

Electron transfer is examined in three redox center (donor-bridge-acceptor) systems. Here, electron transfer cannot be described as arising from one-dimensional diffusional motion along the reaction coordinate. Instead, two-dimensional diffusion over two statistically independent reaction coordinates emerges. We show that electron transfer in three-center systems can be reduced to electron transfer between the pairs of adjacent centers. The interplay between sequential and superexchange mechanisms is examined. New expressions for the electron transfer rate, including effects of the two-dimensional reaction coordinate and of solvent dynamics, are derived. Using this analysis, new driving force effects are predicted, and rich behavior is revealed.
Polarization around an ion in a dielectric continuum with truncated electrostatic interactions
Authors: Baker Nathan A. , Hünenberger Philippe H. , McCammon J. Andrew
Published: July 2002

In order to reduce computational effort and to allow for the use of periodic boundary conditions, electrostatic interactions in explicit solvent simulations of molecular systems do not obey Coulomb’s law. Instead, a number of “effective potentials” have been proposed, including truncated Coulomb, shifted, switched, reaction-field corrected, or Ewald potentials. The present study compares the performance of these schemes in the context of ionic solvation. To this purpose, a generalized form of the Born continuum model for ion solvation is developed, where ion–solvent and solvent–solvent interactions are determined by these effective potentials instead of Coulomb’s law. An integral equation is formulated for calculating the polarization around a spherical ion from which the solvation free energy can be extracted. Comparison of the polarizations and free energies calculated for specific effective potentials and the exact Born result permits an assessment of the accuracy of these different schemes. Additionally, the present formalism can be used to develop corrections to the ionic solvation free energies calculated by molecular simulations implementing such effective potentials. Finally, an arbitrary effective potential is optimized to reproduce the Born polarization.
The role of the molecular shape on the conformational transition from B-to Z-DNA
Authors: Abascal José L. F. , Gil Montoro Juan Carlos
Published: July 2002

We have calculated by computer simulation the electrostatic contribution to the B-Z DNA free-energy difference for several simple models which differ only in the definition of the repulsive forces. The results are compared against experimental measurements. The conclusion is that the accessibility of the mobile ions to the grooves, i.e., the ability to penetrate inside DNA, is an essential feature for the transition to occur. Other consequences concerning the importance of specific solvent effects are also suggested from our calculations.
Molecular coordinates for instantaneous normal mode calculations. I. Coordinate dependence
Authors: Murry Robert L. , Fourkas John T. , Li Wu-Xiong , Keyes Tom
Published: July 2002

We demonstrate that the results of instantaneous normal mode (INM) calculations depend strongly on the coordinate system used, particularly when flexible molecules are employed. Appropriate INM treatments of rotation, bending, and rotational kinetic energy are illustrated and discussed, and a set of criteria for the selection of coordinates for INM calculations on molecular liquids is presented. A general scheme for developing molecular coordinates is introduced. This method simplifies the derivation of the expressions required for INM calculations, particularly for the kinetic energy. A technique for correcting the INM frequencies, applicable in some situations, is also presented.
Microwave spectroscopic detection of transition metal hydroxides: CuOH and AgOH
Authors: Whitham Christopher J. , Ozeki Hiroyuki , Saito Shuji
Published: July 2002

Pure rotational spectra of transition metal hydroxides have been reported. Millimeter wave spectra of CuOH and AgOH were recorded in a direct current discharge absorption cell in the range of 200–390 GHz. The spectra of both molecules correspond to that of a near prolate asymmetric top and only a-type R branch transitions were observed. The rotational and centrifugal distortion constants of CuOH and AgOH were precisely determined. Centrifugal distortion terms up to N2Nz10 were required to fit the observed line frequencies of CuOH63. A preliminary analysis showed that AgOH is also strongly bent like CuOH with a bond angle smaller by about 2°.
Critical conditions for stable dipole-bound dianions
Authors: Sarasola Cecilia , Fowler Joseph E. , Ugalde Jesus M.
Published: July 2002

Full configuration interaction calculations for two electrons moving in the field of an electric dipole have been carried out in order to determine whether bound states with respect to detachment of one electron exist. Model dipoles are constructed by the placement of point charges q=1, 2, 3 a.u. at varying distances. It was found that for a dipole constructed of point charges q=3 separated by a distance of 0.8788 a.u., the energy of binding one and two electrons is equivalent. In all cases, there exist charge separation lengths for which the binding of two electrons gives a binding energy only slightly less than that of binding one electron. Escape channels were not calculated, but may provide barriers allowing the existence of dipole-bound dianions with lifetimes long enough to allow detection.
Velocity correlations in liquid hydrogen fluoride
Authors: Balucani U. , Bertolini D. , Sutmann G. , Tani A. , Vallauri R.
Published: July 2002

The center-of-mass velocity autocorrelation function is analyzed by computer simulation in a model of liquid hydrogen fluoride at two state points. In comparison with water (another hydrogen-bonded liquid) new features arise. To understand the peculiarities of HF, we have investigated atomic velocity correlations in both the laboratory and a molecular frame. The comparison of the frequency spectra permits to ascertain the role of fluorine–hydrogen correlations (or of rototranslational couplings) in the center-of-mass velocity autocorrelation function. At low temperature, the appearance of a long time tail is discussed in terms of projections in the two references frames, and found to be mostly associated with orientational correlations. A discussion in terms of velocity transfer between nearest-neighbor molecules is also given.
Femtosecond spectroscopy of the dissociation and geminate recombination of aqueous CS2
Authors: Thomsen C. L. , Madsen D. , Tho/gersen J. , Byberg J. R. , Keiding S. R.
Published: July 2002

The photolysis of aqueous CS2 has been studied using subpicosecond transient absorption spectroscopy. CS2 was photolyzed at 6.2 eV and the fast formation and decay of the photoproducts were monitored from 6.2 to 3.96 eV. Upon excitation, aqueous CS2 dissociates into CS+S. However, 93%±2% of the fragments geminately recombine on the electronic ground state potential of CS2 within a few picosecond leaving only 7%±2% of the CS+S fragments separated 100 ps after the excitation. In the gas phase, most of the dissociation occurs on a triplet-state potential surface, and the high recombination yield observed in aqueous solution therefore indicates a strong, solvent-assisted coupling between this state and the singlet ground state of CS2. The vibrationally excited CS2 molecule formed by the recombination transfers its high excess energy to the surrounding water molecules in two processes with time constants 8.4±1 and 33±7 ps. The rotational reorientation time of ground state CS2 is 6±1 ps, suggesting a surprisingly strong interaction between the neutral, nonpolar CS2 and the surrounding water molecules.
Pressure-induced structural and chemical changes of solid HBr
Authors: Ikeda Takashi , Sprik Michiel , Terakura Kiyoyuki , Parrinello Michele
Published: July 2002

The structural, dynamical, and electronic properties of solid HBr at high pressure are investigated using the ab initio constant pressure molecular dynamics method. A detailed analysis of the orientational distribution, and the reorientational and vibrational dynamics of the disordered phase I at ambient temperature showed that this phase can be described as a rotator phase with fluctuating hydrogen bonds up to pressures well over 10 GPa. We predict that the disorder at higher densities leads to cooperative proton-transfer dynamics. The approach to hydrogen-bond symmetrization is studied in phase I and the high pressure ordered phase III. The simulation results for phase III also indicate that this phase develops dielectric instabilities at high density. At pressures over 40 GPa we observe spontaneous formation of H2 with rearrangement of the Br lattice from fcc to hcp. The chemical reactivity is rationalized in terms of the electronic structure under conditions of near symmetrical hydrogen bonding.
Entropic trapping of a flexible polymer in a fixed network of random obstacles
Authors: Chern Shyh-Shi , Coalson Rob D.
Published: July 2002

Using Langevin dynamics simulations, we model the dynamics of a polymer in a fixed network of random obstacles containing a spherical cavity. We define the partition coefficient K as the time-averaged ratio of the number of monomers inside/outside the cavity and calculate this quantity as a function of polymer length N. Our results show that ln K(N) increases with N until the polymer’s radius of gyration is approximately equal to the size of the cavity Lh. Further increase of N leads to a decrease in ln K. The linear regime of this curve can be understood by comparing the free energy of a polymer confined through the spherical cavity to the corresponding free energy of the polymer in the mesh of the random network (this is the origin of the phenomenon of entropic trapping). The decrease in ln K when N is large results from imperfect confinement of the polymer inside the cavity. The number of monomers confined inside the cavity is limited by the size of the cavity and thus ln K decreases with N, roughly logarithmically, when N is very large.
Oxygen adsorption on Ag(111): X-ray photoelectron spectroscopy (XPS), angular dependent x-ray photoelectron spectroscopy (ADXPS) and temperature-programmed desorption (TPD) studies
Authors: Bukhtiyarov V. I. , Kaichev V. V. , Prosvirin I. P.
Published: July 2002

The O2 adsorption on Ag(111) single crystal in the range of 300–500 K has been studied by XPS, ADXPS, and TPD. At room temperature, the O1s line observed at 530.0 eV goes down to 528.2 eV when the sample is heated to 420 K. The comparison of XPS and TPD data indicates that both lines can be assigned to chemisorbed atomic oxygen. The depth concentration profiles calculated on the basis of the angular-dependent XPS data show that the line at 530.0 eV is responsible for oxygen adsorbed at the surface, as opposed to that at 528.2 eV which is assigned to oxygen between the top and second silver layers. The formation of surface silver oxide is concluded in the latter case.