bluuebunny/crossref_metadata_2025 huggingface dataset:
Tuning of the Fermi resonance of CO2 and CS2 by temperature, pressure, and matrix material
Authors: Bier K. D. , Jodl H. J.
Published: July 2002

The influence of temperature, pressure, and matrix material on the Fermi resonance interaction of the molecular crystals CO2 and CS2 is investigated. The anharmonic constant κ122 and the unperturbed energy difference Δ are calculated from the experimental energy and intensity data. The sharp Fermi resonance of CO2 is mainly influenced by the change in unperturbed energy difference with a constant anharmonicity, while the tuning of the weak resonance of CS2 is also driven by changes in κ122. Moreover, the variation of the Fermi resonance quantity Δ is changed by the usual solid, temperature, pressure, and matrix shifts of the energy levels. Although the fundamental ν2 should be independent of the Fermi resonance interaction it shows an unusual behavior, which is influenced by its overtone 2ν2 and driven by the tuning of the Fermi resonance. A decrease of Fermi resonance is observed on the gas–solid transition, when increasing temperature and on increasing pressure. An increasing Fermi resonance, produced by an elevation of environmental symmetry and increasing lattice size, realized by matrix-isolated studies, is only significant for the smaller resonance of CS2. For matrix-isolated CO2 a decrease of Fermi resonance is noticeable only in the nitrogen matrix.
Thermodynamics of aqueous solvation: Solution properties of alcohols and alkanes
Authors: Fleischman Stephen H. , Brooks Charles L.
Published: July 2002

Molecular dynamics is used, with thermodynamic-cycle perturbation theory, to calculate relative Helmholtz free energies of hydration, ΔΔAsolv, for ethane, propane, methanol, and ethanol. The thermodynamic derivative properties ΔΔEsolv and ΔΔSsolv are also calculated from direct finite difference methods. The calculated values of ΔΔAsolv for hydrophobic→hydrophilic ‘‘mutations’’ (and the reverse) are in excellent agreement with the experimental values of ΔΔGsolv. This is believed to be the case because this transformation is dominated by the change in electrostatic interactions between solute and solvent. Similar agreement is not found for transformations between hydrophobic moieties where solute–solvent electrostatic interactions play no role. Changes to the existing model are suggested, namely the inclusion of explicit hydrogens containing partial atomic charges, and their evaluation through the use of ‘‘free energy’’ simulations is outlined.
Adiabatic invariance treatment of hitting collisions between ions and symmetrical top dipolar molecules
Authors: Bates David R. , Morgan Wm. Lowell
Published: July 2002

The adiabatic invariance method of calculating the rate coefficient kD(J,K) for hitting collisions between ions and symmetrical top dipolar molecules in level (J,K) is formulated. The rate coefficient for a particular quantum level does not depend on the rotational constant A of the symmetrical top molecule. This facilitates parametrization of the results. Representative results on NH3 and CH3F including the thermal average k̄D are presented. A formula that Su and Chesnavich obtained for linear molecules reproduces k̄D for NH3 and CH3F quite well. Measured rate coefficients kr for reactive collisions between several species of ion and NH3 yield collision efficiencies kr/k̄D that are rather less than unity and temperature independent in the low temperature region. Corresponding results involving H2O suggest that the linear molecule formula is useful even for an asymmetrical top molecule.
The molecular polarizability derivatives and their implications as interpreted from the surface enhanced Raman intensities: A case study of piperidine
Authors: Tian Bogang , Wu Guozhen , Liu Guohai
Published: July 2002

The distinct variation of the surface enhanced Raman spectral pattern of piperidine under different applied voltages is analyzed with the Wolkenstein’s approximation to reveal its very molecular implication. The calculated effective polarizability derivatives show that there are both chemical and electromagnetic mechanisms in the surface enhanced Raman scattering. The chemical mechanism results in: (1) The farther a bond is away from the metal surface, the larger its polarizability derivative. (2) The more negative the applied voltage (vs standard calomel electrode), the larger the polarizability derivatives are and the weaker the adsorption. (3) There is sharing of the electron charge between the ring skeleton of the piperidine molecule and the Ag electrode. The electromagnetic mechanism implies equatorial C–H bonds possess larger polarizability derivatives than those of the corresponding axial C–H bonds. The analysis confirms the head-on adsorption of the piperidine molecule on the Ag electrode through the nitrogen atom as interpreted independently from other spectroscopic analyses. The behavior of the polarizability derivatives as a function of applied voltages at the Ag electrode reveals certain structural information of the molecules adsorbed on the Ag electrode.
Thermodynamic perturbation theory of polymerization
Authors: Wertheim M. S.
Published: July 2002

We derive several extensions of a previously given first-order perturbation theory (TPT 1) for fluids in which chain and ring polymers can be formed, due to the presence of two singly bondable molecular attraction sites. We retain graphs which contain a single chain of attraction bonds, and evaluate second-order perturbation theory (TPT 2) within this framework. The previous formulation with sites fixed in the molecule is generalized to allow movable sites, thus permitting calculations for flexible bead polymers. The TPT 1 result for the equation of state of an equilibrium mixture of chain lengths is in good agreement with simulations of flexible bead polymers of fixed bead number N, when the mean number ν of beads is equated to the fixed N of the simulations. TPT 2 differs from TPT 1 by a rather small term, with improved agreement. If the distribution of movable sites includes configurations such that bonding of one site blocks bonding of the other, then graph resummation must be used. Resummed TPT yield physically reasonable results over the entire range of conditions. For total blockage, where only dimers can be formed, resummed TPT reduces to a successful version of TPT given earlier for dimerizing systems. The equation of state appears in the form of two parametric equations. The first one gives the excess pressure over the reference system in terms of ν and the number density ρ̄ of beads. In TPT 1 this equation is universal for hard spheres with or without dimerization or chain polymerization. The second equation is nonuniversal and contains the dependence of ν on ρ̄, the strength of the bonding attraction, the number of sites, and the amount of blockage in the case of two sites. An additional simulational test of TPT 2 is suggested.
A statistical theory for the effect of nonadiabatic transitions on activated processes
Authors: Straub John E. , Berne Bruce J.
Published: July 2002

A simple statistical theory for calculating the effects of nonadiabatic transitions on activated barrier crossing is presented. It is based on the model of Cline and Wolynes which combines the impulsive BGK collisional model with the Landau–Zener theory for curve crossing to calculate rate constants for barrier crossing. We derive a closed analytical expression for the rate constant for nonadiabatic transitions which requires as input only the rate constant for barrier crossing on the adiabatic surface and the parameters for the Landau–Zener theory. Our theory gives excellent agreement with the numerical results of Cline and Wolynes.
Simple models for heterogeneous catalysis: Phase transition-like behavior in nonequilibrium systems
Authors: Meakin Paul , Scalapino Douglas J.
Published: July 2002

A simple model for heterogeneous catalysis, recently introduced by Ziff, Gulari, and Barshad (ZGB), has been explored using computer simulation. This is a nonequilibrium model which exhibits phase transition-like behavior. The only parameter in this model (Y) sets the ratio with which two reactive species (A and B2) are fed to the surface (lattice). On the square lattice, ZGB found a range of values of this parameter for which steady states with nonzero A and B concentrations occur. Outside of this range, the surface becomes completely covered with A or B sites. The range of Y values corresponding to steady state with nonzero A and B concentrations is bounded by two ‘‘critical’’ values Y1 and Y2. For values of Y close to but above Y1 we have obtained approximate values for the exponents which describe the dependence of the steady-state densities of A and B sites on (Y−Y1). These exponents both seem to have the same value independent of lattice details. We have extended the work of ZGB to the hexagonal lattice, one-dimensional lattice, and narrow strips. The results obtained for the hexagonal lattice are qualitatively similar to those obtained for the square lattice. For the 1D lattice and strips with a width of 2 lattice units, steady states with nonzero A and B site concentrations are not found. For widths of 3 or more lattice units, such steady states are found and the range of the relative probability parameter over which this behavior occurs grows steadily with increasing strip width. We have also obtained new results for the more simple C+D product reaction. Here C and D occupy single lattice sites and react immediately if they are nearest neighbors.
An experimental investigation of periodic and chaotic electrochemical oscillations in the anodic dissolution of copper in phosphoric acid
Authors: Albahadily F. N. , Schell Mark
Published: July 2002

The response of the current to an applied potential was monitored in an open electrochemical system consisting of a rotating copper disk as the working electrode, a calomel reference electrode, and a platinum counter electrode, all of which were in contact with a solution of 85% phosphoric acid. In addition to stable stationary states, the applied potential induced oscillatory states which were either periodic or chaotic. Transitions from a stationary state to sustained oscillations were found to take place either through a Hopf bifurcation or by way of a mechanism that gives rise to states possessing complex combinations of small and large amplitude oscillations (mixed-mode oscillations). Within the parameter ranges for which sustained oscillations occurred, we discovered sequences of period doubling bifurcations. Aperiodic oscillations were observed just beyond the limit at which a sequence terminated. Phase trajectories were constructed from time-series data for these aperiodic states from which we produced one-humped, one-dimensional maps. A trajectory was also constructed that closely resembled a homoclinic orbit. Mixed-mode oscillations were found which have the same properties as those previously observed in the Belousov–Zhabotinskii reaction. The production of copper was observed in this electrochemical system which suggests that the reaction of disproportionality may be part of a feedback mechanism that is responsible for the complex dynamical behavior.
The inverse operator representation of thermal diffusion factors in dense gas mixtures of rigid spheres
Authors: Monchick Louis
Published: July 2002

It is shown that, expressed in terms of the inverse of the Enskog collision operator, the Stefan–Maxwell equations in a multicomponent dense gas of rigid spheres take a form quite similar to that obtained in gases at moderate pressure. Two thermal diffusion factors are defined. The first is a ‘‘kinetic’’ thermal diffusion factor formally identical with the factors found by kinetic theory at moderate pressures. The second is a ‘‘phenomenological’’ thermal diffusion factor which is rather more pressure dependent. A brief comparison with experiment shows that the Enskog dense gas theory works best in systems rich in helium at elevated temperature.
Potassium on Ni(111): Quantitative determination and electronic structure
Authors: Bertolini J. C. , Duvault J. L. , Jugnet Y. , Ruiz Ph. , Tardy B.
Published: July 2002

Thermal desorption spectroscopy (TDS), low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), electron energy-loss spectroscopy (EELS), and x-ray photoelectron spectroscopy (XPS) techniques have been used to investigate the K adsorption on Ni(111). A quantitative evaluation of the K deposit is established from the combined use of TDS, LEED, and AES. The K electronic structure has been studied using AES, EELS, and XPS. Up to the monolayer completion the observed energy shifts are interpreted in terms of a continuous lowering of the positive charge carried by the K additive (charge transfer of about 0.5 electron) as the K coverage is increased.
Theoretical spectroscopy of the NO radical. I. Potential curves and lifetimes of excited states
Authors: de Vivie Regina , Peyerimhoff Sigrid D.
Published: July 2002

Multireference configurations interaction calculations are employed to study the low-lying excited states of the NO radical up to the dissociation limits. The states of 2Π symmetry are characterized by a change from Rydberg to valence character with increased internuclear separation; the interaction takes place over a fairly narrow range of R and both, Rydberg and valence states, show potential wells. The 2Σ+ states exhibit only distinct potential minima in the Rydberg area and are otherwise characterized at larger bond lengths in the main by interacting valence states. The lifetimes of the Rydberg states A, C, and D are calculated in the order of 5×10−8 s, those of the valence states B and L lie around 300×10−8 s while τ for B′ 2Δ (v=0) is calculated to be 17×10−8 s. The results are interpreted in terms of the spatial extension of the orbitals, their deviation from the gerade and ungerade character relative to a homonuclear molecule and the Franck–Condon factors. The lowest 2Σ+ state is found to possess a potential barrier relative to dissociation, whereby the tunneling probability for its v=8 level is found to correspond to a line broadening of 20.8 cm−1. The results form the basis for a study of spin–orbit, Λ-doubling, and predissociation effects to be reported in a consecutive paper.
Femtosecond real-time probing of reactions. I. The technique
Authors: Rosker Mark J. , Dantus Marcos , Zewail Ahmed H.
Published: July 2002

When a chemical bond is broken in a direct dissociation reaction, the process is so rapid that it has generally been considered instantaneous and therefore unobservable. But the fragments formed interact with one another for times on the order of 10−13 s after the photon has been absorbed. On this time scale the system passes through intermediate transition configurations; the totality of such configurations have been, in the recent literature, designated as ‘‘transition states.’’ Femtosecond transition-state spectroscopy (FTS) is a real-time technique for probing chemical reactions. It allows the direct observation of a molecule in the process of falling apart or in the process of formation. In this paper, the first in a series on femtosecond real-time probing of reactions, we examine the technique in detail. The concept of FTS is explored, and the interrelationship between the dynamics of chemical reactions and molecular potential energy surfaces is considered. The experimental method, which requires the generation of spectrally tunable femtosecond optical pulses, is detailed. Illustrative results from FTS experiments for several elementary reactions are presented, and we describe methods for relating these results to the potential energy surface(s).
Electrostatic model for protein/reverse micelle complexation
Authors: Bratko D. , Luzar A. , Chen S. H.
Published: July 2002

Thermodynamics of protein solubilization in water-in-oil microemulsions is analyzed in terms of the shell-and-core model for reverse micelles. The electrostatic contribution to the free energy of transfer of the protein from the aqueous solution to the microemulsion is determined via the solution of the nonlinear Poisson–Boltzmann equation for the protein/reverse micelle complex, for the protein-free micelle, and for the cell model of aqueous protein solution in equilibrium with the microemulsion. The electrolyte effect on the protein solubility in the microemulsion is studied. A good agreement between the predictions of the model and the known salting out effect of cytochrome-c in sodium di-2-ethylhexylsulfosuccinate AOT-water-in-isooctane microemulsion is observed.
General recurrence formulas for molecular integrals over Cartesian Gaussian functions
Authors: Obara S. , Saika A.
Published: July 2002

General recurrence formulas for various types of one- and two-electron molecular integrals over Cartesian Gaussian functions are derived by introducing basic integrals. These formulas are capable of dealing with (1) molecular integrals with any spatial operators in the nonrelativistic forms of the relativistic wave equations, (2) those with the kernel of the Fourier transform, (3) those with arbitrarily defined spatial operators so far as the integrals can be expressed in terms of the basic integrals, and (4) any order of their derivatives with respect to the function centers in the above integrals. Thus, the present formulation can cover a large class of molecular integrals necessary for theoretical studies of molecular systems by ab initio calculations, and furthermore provides us with an efficient scheme of computing them by virtue of its recursive nature.
Femtosecond real-time probing of reactions. III. Inversion to the potential from femtosecond transition-state spectroscopy experiments
Authors: Bernstein Richard B. , Zewail Ahmed H.
Published: July 2002

Femtosecond transition-state spectroscopy (FTS) of elementary reactions [M. Dantus, M. J. Rosker, and A. H. Zewail, J. Chem. Phys. 87, 2395 (1987)] provides real-time observations of photofragments in the process of formation. A classical mechanical description of the time-dependent absorption of fragments during photodissociation [R. Bersohn and A. H. Zewail, Ber. Bunsenges. Phys. Chem. 92, 373 (1988)] forms the basis for the present scheme for relating observations to the potential energy surface. A direct inversion scheme is presented that allows the difference in the two relevant excited-state potential curves to be deduced from observed transients at different probe wavelength tunings. In addition, from the shape and dependence of the transients on pump wavelength, information on the lower of the two potential curves (i.e., that of the dissociating molecule) is obtained. The methodology is applied to the experimental FTS data (Dantus et al.) on the CN photofragment from the ICN photodissociation.
Treatment of rotational isomers in free energy evaluations. Analysis of the evaluation of free energy differences by molecular dynamics simulations of systems with rotational isomeric states
Authors: Straatsma T. P. , McCammon J. A.
Published: July 2002

Using 1,2-dimethoxy ethane in aqueous solution as an example, the applicability of the perturbation method and the thermodynamic integration technique to evaluate free energy differences is considered for systems with multiple rotational isomeric states. It is shown that the naive application of these methods to evaluate free energy differences for such systems, even in a simple case such as the free energy of hydration of 1,2-dimethoxy ethane, may lead to unreasonable results. This problem is due to the fact that, in a conventional simulation, it is unlikely that all isomeric states will be sampled with the appropriate equilibrium probabilities. A procedure is proposed to estimate the contributions of isomeric states in free energy difference calculations.
Steady and nonsteady rates of reaction in a heterogeneously catalyzed reaction: Oxidation of CO on platinum, experiments and simulations
Authors: Ehsasi M. , Matloch M. , Frank O. , Block J. H. , Christmann K. , Rys F. S. , Hirschwald W.
Published: July 2002

The rate of reaction for oxidation of CO over (210) and (111) single-crystal surfaces of platinum has been studied as a function of reactant pressures (PO2,PCO) and sample temperature (T), both experimentally and by computer simulation. Experimental results on both surfaces show regions with a steady high rate of reaction followed by a nonsteady transition region and, at high CO pressures, a region with low reactivity caused by CO poisoning of the surface. At constant sample temperature, the transition region can be narrow and depends critically on the ratio of the gas phase concentration of reactants (PCO/PO2). The temperature dependences of the experimental data indicate that the critical ratio and the details for the occurrence of CO poisoning are strongly affected by surface processes such as adsorption, desorption, and diffusion ordering and reconstruction phenomena. A computer simulation model of the Langmuir–Hinshelwood surface reaction as developed by Ziff et al. was used for the simulation of the reaction under flow conditions. The initial fair agreement between this model and the experiment can be significantly improved if processes such as adsorption, desorption, and diffusion are taken into account in an extended simulation model which in turn provides an insight into the kinetics of adsorbate poisoning and the effect of adsorbate-induced processes on the reaction.
Isoelectronic analogs of molecular nitrogen: Tightly bound multiply charged species
Authors: Wong Ming Wah , Nobes Ross H. , Bouma Willem J. , Radom Leo
Published: July 2002

The structures and stabilities of N2 and its 15 possible first-row isoelectronic analogs (CO, BF, BeNe, NO+, CF+, BNe+, O2+2, NF2+, CNe2+, OF3+, NNe3+, ONe4+, F4+2, FNe5+, and Ne6+2) have been examined using ab initio molecular orbital theory. Equilibrium structures have been obtained at a variety of levels of theory including MP3/6-311G(d) and ST4CCD/6-311+G(2df ) and dissociation energies determined at the MP4/6-311+G(3d2f ) level. Full potential energy curves for dissociation, including dissociation barriers, have been obtained at the CASSCF/6-311G(d) level. Spectroscopic constants have also been determined at this level. For the neutral and monocation analogs of N2, the calculated equilibrium geometries, dissociation energies, and spectroscopic constants are in good agreement with the experimental values. The dication analogs of N2, namely O2+2, NF2+, and CNe2+, are all found to be kinetically stable species lying in deep potential wells. In particular, the hitherto unobserved NF2+ dication is predicted to have a short equilibrium bond length (1.102 Å) and a large barrier (445 kJ mol−1) to dissociation to N++F+. Thus NF2+ should be experimentally accessible in the gas phase. The (experimentally known) O2+2 dication is predicted to contain the shortest bond between any two heavy atoms, our best estimate of the bond length being 1.052 Å. The first excited state (A 3Σ+u) of O2+2 is predicted to be unbound, and observed metastable decomposition processes are reinterpreted in terms of the ground-state (X 1Σ+g) potential surface. In agreement with previous theoretical studies, we find that CNe2+ is a kinetically stable species, albeit with a relatively long C–Ne bond length. The OF3+ trication is calculated to have a relatively short bond but lies in a well of depth only 23 kJ mol−1. The potential energy curves of the other highly charged species are found to be purely repulsive.
Classical mechanics of intramolecular vibrational energy flow in benzene. V. Effect of zero-point energy motion
Authors: Lu Da-hong , Hase William L.
Published: July 2002

Zero-point energy excitation has a profound effect on the relaxation of benzene CH and CD overtone states. Only adding a fraction of the zero-point energy for each normal mode in the initial conditions results in smaller overtone relaxation rates. If no zero-point energy is added to C6H6, the n=3 and 5 CH overtones do not relax within 1 ps. Adding zero-point energy to different types of normal modes has nonequivalent effects on overtone relaxation. Zero-point excitation of modes with HCC bend character is particularly effective in enhancing relaxation of the overtones.
Normal-mode analysis of liquid-state dynamics
Authors: Seeley G. , Keyes T.
Published: July 2002

Harmonic normal-mode analysis is applied to Lennard-Jones (LJ) liquids. The configuration-averaged density of (vibrational) states is obtained via numerical eigenanalysis of the force-constant matrices appropriate to an ensemble of liquid configurations; the configurations are generated by computer simulation. The contribution of unstable modes is included and plays a crucial role in the analysis. It is argued that the density of states contains information which may be used to construct theories of liquid-state dynamics. The argument is pursued at two levels. First, it is demonstrated that a glance at the density of states conveys a powerful, intuitive understanding of several aspects of the dynamics. Second, a theory of the self-diffusion constant is constructed which may be regarded as a preliminary attempt to base systematic transport theory upon normal-mode quantities.
Spectroscopy and electronic structure of jet-cooled NiPd and PdPt
Authors: Taylor Scott , Spain Eileen M. , Morse Michael D.
Published: July 2002

Resonant two-photon ionization spectroscopy of jet-cooled NiPd and PdPt has revealed a dense vibronic spectrum for NiPd and a much more sparse spectrum for PdPt. Four vibrational progressions have been identified for NiPd, and three have been located for PdPt. High resolution investigations of NiPd have established a ground state bond length of r″0 =2.242±0.005 Å with Ω″=2. The observed spectra have been used to bracket the ionization potentials, giving IP(NiPd)=7.18±0.76 eV and IP(PdPt)=8.27±0.38 eV. In contrast to previous work on Ni2, NiPt, and Pt2, no abrupt onset of rapid predissociation is observed for either NiPd or PdPt. A discussion of this result in terms of the expected potential energy curves for the palladium-containing diatomics is presented, which when combined with the frequencies of the highest energy vibronic bands observed yields estimates of D0(NiPd)≊1.46 eV and D0(PdPt)≊1.98 eV. The lack of observable vibronic transitions in Pd2 above 11 375 cm−1 places D0(Pd2) below 1.41 eV, in agreement with Knudsen effusion mass spectrometry. Finally a comparison of the platinum group dimers and the coinage metal dimers is given, demonstrating the increasing importance of d-orbital contributions to the bonding in the platinum group dimers as one moves down the periodic table. The anomalous behavior of the palladium-containing diatomics is also discussed in terms of the highly stable 4d105s0, 1S0 ground state of atomic palladium.
Rotational dynamics of anthracene and 9,10-dimethylanthracene in polyisoprene
Authors: Hyde Patrick D. , Ediger M. D.
Published: July 2002

Singlet and triplet transient grating experiments have been used to measure rotational correlation functions for anthracene and 9,10-dimethylanthracene in cis-polyisoprene from Tg+120 °C to Tg+20 °C. Over this temperature range, the rotational correlation time varies by seven decades and follows the temperature dependence of the bulk viscosity. The observed correlation functions are not consistent with predictions of the anisotropic rotational diffusion model. The observed correlation functions for anthracene are consistent with a change in the predominant axis for rotational diffusion as a function of temperature. At high temperatures, in-plane rotation occurs most rapidly, while at low temperatures rotation about the long axis predominates.
Palladium clusters: H2, D2, N2, CH4, CD4, C2H4, and C2H6 reactivity and D2 saturation studies
Authors: Fayet P. , Kaldor A. , Cox D. M.
Published: July 2002

In this paper both deuterium saturation measurements and gas-phase kinetic measurements of chemisorption of H2, D2, N2, CD4, CH4, C2H4, and C2H6 on neutral palladium clusters are reported. Saturation studies with D2 show that small palladium clusters can bind up to three deuterium atoms per palladium atom in the cluster, in contast to H/M ratios near unity typically reported for metal surfaces. In addition, the small palladium clusters exhibit pronounced discontinuities in deuterium uptake which may be indicative of structural transformations or selective desorption of deuterium. From the kinetic studies we find that, in general, the rate constants for a given size cluster towards different reagents tend to order as D2, H2>N2>C2H4>CD4, CH4, C2H6. The shape of the reactivity pattern with the different reagents varies strongly with cluster size for clusters containing less than 25 atoms. Finally, an inverse hydrogen isotope effect is observed for both hydrogen and methane, i.e., the D2 and CD4 rate constants are significantly larger than those of H2 and CH4, respectively.
Monte Carlo simulation of phase equilibria for the two-dimensional Lennard-Jones fluid in the Gibbs ensemble
Authors: Singh Rajiv R. , Pitzer Kenneth S. , de Pablo Juan J. , Prausnitz John M.
Published: July 2002

The coexistence curve of the two-dimensional Lennard-Jones fluid has been obtained by Monte Carlo simulation in the Gibbs ensemble. The calculated vapor–liquid equilibria show that the apparent critical exponent βe has a value near that of an infinitely large system, rather than the classical value, even though the correlation length is constrained by the box size. These results are similar to those for the three-dimensional case.
Strongly bound metastable states of B2+2
Authors: Bruna Pablo J. , Wright James S.
Published: July 2002

The stabilities of about 25 electronic states of B2+2 have been investigated using a multireference CI (MRD-CI) method and an AO basis set composed of 6s4p2d contracted Gaussian species per atom, including semidiffuse functions relevant for an adequate description of charge transfer interactions. The ground state X1∑+g (σ2gσ2u) is repulsive, as expected by its electronic configuration with a zero bond order. In spite of this and the doubly-positive charge, many excited states are found to be metastable, four of them (11∏g, 11∏u, 13∑−g, and 11Δg ) having potential wells from 1.52 eV (11∏u) to 2.83 eV (13∑−g). Relative to the ground state configuration, the metastable states arise from the excitations σu→σg(3∑+u), σu→πu(3,1∏g), σ2u→σgπu(3,1∏u), and σ2u→π2u(3∑−g, 1Δg, 1∑+g); they are analogous to those states showing deep local minima in B2 and B+2.

Differences in stabilities among quasibound states can be explained on the basis of the asymptotic ΔE’s between repulsive channels B++B+ and appropriate higher-lying limits (states) B+B2+ of bonding character; another important factor governing stability is the actual bonding character of the electronic configurations assigned to each state. The vertical double-ionization potential B2→B2+2(π2u→∞) between both ground states is 27.97 eV, the repulsive X1∑+g dication state being created with 8.37 eV excess energy relative to B++B+. Doubly-ionized states with a chance of being detected because of their long lifetimes against predissociation are those showing a strongly quasibound character, such as (with the vertical double ionization potential in eV given in parentheses): 11∏g(σuπu→∞; 30.29); 11∏u(σ2uπu→σg∞; 31.30); 13∑−g(σ2u→∞; 31.33), and 11Δg(σ2u→∞; 31.95).
Nematic–lamellar tricritical behavior and structure of the lamellar phase in the ammonium pentadecafluorooctanoate (APFO)/water system
Authors: Boden N. , Clements J. , Jolley K. W. , Parker D. , Smith M. H.
Published: July 2002

2H NMR spectroscopy has been used to map a high-resolution (±0.04 K) phase diagram for the ammonium pentadecafluorooctanoate(APFO)/heavy water system. It is qualitatively similar to that for the CsPFO/heavy water system. In particular, it exhibits a discotic micellar nematic phase N+D intermediate to an isotropic micellar solution phase I and a lamellar phase for weight fractions of APFO between 0.395 (φ=0.278) and 0.589 (φ=0.455) and temperatures between 292.10 and 338.10 K. The N+D to lamellar transition crosses over from second to first order behavior at a tricritical point similar to the superfluid transition in 3He/4He mixtures. X-ray scattering experiments shows there to be no dramatic change in the structure of the micelle at the I to N+D and the N+D to lamellar transitions. Nematic order parameters (orientational order parameters of the discoidal micelles) have been calculated from electrical conductivity measurements. Their variation with temperature in the nematic phase and across the nematic to lamellar transition are qualitatively consistent with the behavior expected for thermotropic calamitic liquid crystals undergoing an isotropic to nematic to smectic-A sequence of transitions.

It is, therefore, concluded that the transition from the N+D to the lamellar phase solely involves the propagation of long range positional ordering of the discoidal micelles into planes along the nematic director, that is, the lamellar phase is a discotic lamellar phase LD. This result reinforces previous claims about the structure of the lamellar phase of the CsPFO system as opposed to the alternative scenario of a classical lamellar phase in which the bilayers are perforated by microscopic defects. This conclusion is supported by measurements of 2H quadrupole splittings of heavy water. Similar measurements for deuterated ammonium ions show a preference for counterion binding to sites of lowest surface curvature. The fraction of counterions bound to the surface of the micelle is shown to increase as the temperature is lowered due to a growth in diameter and associated changes in surface curvature.
Rotational and translational diffusion of short rodlike molecules in solution: Oligonucleotides
Authors: Eimer Wolfgang , Pecora R.
Published: July 2002

Rotational and translational diffusion coefficients in aqueous solution of a series of three B duplex oligonucleotides 8, 12, and 20 base pairs in length are measured by depolarized and polarized dynamic light scattering, respectively. Theoretical relations for the diffusion coefficients of short rods by Tirado and Garcia de la Torre are used in combination with the experimental data to obtain the dimensions of the molecules. The theoretical relations are self-consistent through the series and give a hydrodynamic diameter for DNA of 20±1.5 Å. The results, furthermore, show that the combination of depolarized and polarized dynamic light scattering provides a powerful method for obtaining hydrodynamic dimensions of short rodlike particles and may therefore be used in applications which follow molecular sizes and size changes.
Optimal control of unimolecular reactions in the collisional regime
Authors: Gross Peter , Neuhauser Daniel , Rabitz Herschel
Published: July 2002

The possibility of controlling unimolecular–dissociation processes with multiple laser fields in the collisional regime is examined. Employing the Bloch equations to describe optical excitation and decay processes, optimal control theory is used to design amplitude modulated fields which produce the desired excited-state products. The selectivity of the product distribution of a simple four-state photodissociation system is shown to have a square-root dependence on the relative value of the mean dephasing time T2 to the pulse length τ, i.e, (T2/τ)1/2. The equivalence between T2 decay and phase disruptions occurring in a random-walk fashion is also examined. In the Appendix it is shown that the essential effect of the system temperature is to introduce a Boltzmann population factor on the product selectivity without affecting the nature of the optimal field.
Molecular dynamics simulation of a Langmuir–Blodgett film
Authors: Moller M. A. , Tildesley D. J. , Kim K. S. , Quirke N.
Published: July 2002

The structure and dynamics of a Langmuir–Blodgett film of stearic acid, physisorbed on graphite, has been examined by a combination of energy minimization and molecular dynamics. The model predicts that the molecules of this system are normal to the surface at head group areas below 21 Å2, but tilted away from the normal at head group areas above 21 Å2, and that this change occurs over a very small range in head group area. The system exhibited cooperative reorientations involving the average tilt and precessional angles. Gauche bond defects tended to appear near the ends of the molecules, and did not open up any significant free volume within the layer.
The dynamics of open-shell Van der Waals complexes
Authors: Dubernet Marie-Lise , Flower David , Hutson Jeremy M.
Published: July 2002

The theory of Van der Waals complexes formed from atoms and open-shell (Σ and Π) diatomic molecules is developed, paying particular attention to the quantum numbers that are conserved in the complex and the angular momentum coupling cases that may be observed. Complexes formed from diatoms in multiplet Σ states may exhibit several different coupling schemes closely analogous to Hund’s coupling cases for diatomic molecules. Complexes formed from diatoms in Π states usually exhibit a coupling scheme in which the (signed) projection P of the diatom angular momentum j onto the intermolecular axis is nearly conserved. Correlation diagrams showing the bending energy levels as a function of potential anisotropy are given for complexes containing diatomic molecules in both Σ and Π states. The transition from free internal rotor quantum numbers to near-rigid bender quantum numbers with increasing anisotropy is investigated. The cases of Ar–OH and Ne–OH are considered as examples.
Theoretical assignment of the visible spectrum of singlet methylene
Authors: Green William H. , Handy Nicholas C. , Knowles Peter J. , Carter Stuart
Published: July 2002

The potential energy surfaces of the two lowest-lying singlet electronic states of methylene (CH2) are determined by internally contracted multireference configuration interaction calculations, using a full-valence reference space, with an extended Gaussian basis set. The rotation–vibration levels on these surfaces are calculated by diagonalizing the rovibrational Hamiltonian matrix in a contracted basis. The rovibronic mixing due to the strong Renner–Teller interaction in this system is treated through the Coriolis term in the kinetic energy operator, using geometry-dependent electronic angular momentum matrix elements calculated from ab initio wave functions. The agreement between experiment and this high-quality ab initio calculation is sufficiently good that the calculation can be used to assign the observed vibronic bands in this very complex spectrum, where 90% of the observed lines remain unassigned. Many of the previous vibronic band labels are found to be incorrect. Most of the K>0 bands previously labeled b̃ 1B1 are actually predominantly ã 1A1 in character, and the vibrational numbering of their b̃ 1B1 components are also incorrect. This work demonstrates the importance of supplementing experimental data with good quality ab initio calculations.
Determination of the translational order parameter in the liquid crystalline smectic A phase using the x-ray diffraction method
Authors: Fan Zhong Xiao , Haase Wolfgang
Published: July 2002

On the basis of our recently suggested fan-shaped scattering model [J. Chem. Phys. 92, 5099 (1990)], a method to calculate the translational order parameter τ for the uniaxially oriented liquid crystalline smectic phase is suggested. Using this method, τ is determined from the scattering profiles of the two-dimensional inner reflexes for two samples of the liquid crystalline side chain polymer in their smectic A phase. The relationship between τ and the orientational order parameter 〈P2〉 is discussed.
Stochastic molecular dynamics in systems with multiple time scales and memory friction
Authors: Tuckerman Mark E. , Berne Bruce J.
Published: July 2002

The generalized Langevin equation (GLE) has been used to model a wide variety of systems in which a subset of the degrees of freedom move on a potential of mean force surface subject to fluctuating forces and dynamic friction. When there is a wide separation in the time scales for motion on the potential surface and for relaxation of the friction kernel, direct integration of the GLE is very costly in CPU time. In this paper we introduce an integrator based on our previous work using numerical analytical propagator algorithm (NAPA) and reference system propagator algorithm (RESPA) that greatly accelerates such simulations. We also discuss sampling methods for the random force. Accuracy of this algorithm is assessed by comparisons with an analytically solvable example. Introducing dynamic friction kernels determined from full molecular dynamics (MD) simulations allows us to compare the accuracy of the GLE simulations with full scale molecular dynamics simulations.
Interatomic potentials for excited states of XeHe and XeAr
Authors: Hickman A. P. , Huestis D. L. , Saxon R. P.
Published: July 2002

Interatomic potentials for the interaction of low-lying excited states of Xe(5p5nl) (nl=6s,6p,5d) with He and Ar have been calculated. A novel method has been developed that refines and extends the results of ab initio electronic structure calculations by incorporating available spectroscopic information using a model Hamiltonian. The ab initio calculations treat the heavy-atom cores using relativistic effective potentials and include spin–orbit effects at the configuration-interaction level. The model Hamiltonian depends on a small number of physically sensible parameters, some of which are extracted from the ab initio calculations, and others of which are determined more accurately from spectroscopy. The model Hamiltonian is then used to recalculate the potential curves and coupling matrix elements. The results obtained have significant implications for two classes of recent experiments. Recent measurements of rate constants for state-to-state transitions of Xe* induced by collisions with He or Ar have revealed that the magnitude of the rate depends strongly on the specific transition and collision partner. The present adiabatic potential curves exhibit several regions of strong coupling, which can be associated with the crossings of ‘‘diabatic states,’’ and which can be correlated with those transition rates found to be large. Other experiments have measured multiphoton ionization spectra of XeAr, thereby providing information on the vibrational levels of the excited-state potentials that can be compared with the present calculations.
The role of the torsion of the phenyl moiety in the mechanism of stimulated ultraviolet light generation in 2-phenylbenzazoles
Authors: Catalán Javier , Mena Elena , Fabero Fernando , Amat-Guerri Francisco
Published: July 2002

The general difference observed between the photophysical behavior of 2-phenylbenzazoles and parent unsubstituted benzazole is explained through an exchange of the energy order of the two lower excited singlet states of both compounds. The energy variation takes place because S1 -excited 2-phenylbenzazoles suffer a subnanosecond conformational change towards planarity by torsion around the phenyl-heterocycle bond. This torsion is in accordance with theoretical calculations, with ultraviolet (UV)-visible and fluorescence data, and with results obtained on rotationally-constrained 2-phenylindoles. These changes, as well as the usually high fluorescence quantum yields and small Stokes shifts exhibited by unsubstituted benzazoles, make 2-phenylbenzazoles suitable dye lasers for the UV-zone of the spectrum.
Structure and dynamics of water and aqueous solutions: The role of flexibility
Authors: Smith David E. , Haymet A. D. J.
Published: July 2002

The role molecular flexibility plays in the chemistry of bulk water and ionic solutions is evaluated using molecular dynamics simulations. For simplicity the flexible water model used here is the revised central force (CF) model of Stillinger and Rahman [J. Chem. Phys. 68, 666 (1977)]. A companion, rigid central force (RCF) model is invented to provide the most accurate average description of CF water. Discussion of the role of flexibility is divided into effects due to Coulombic and non-Coulombic parts of the potential energy. The Coulombic part provides the connection between flexibility and nuclear polarizability. A variety of thermodynamic and dynamical properties of the two models are compared. These include the static dielectric constant, orientational correlation functions, self-diffusion coefficient, pressure, intermolecular energy, and pair correlation functions. The effect of flexibility is greatest for dielectric properties, but in general it is found to be small. The dielectric constant of CF water is measured to be approximately 77, in good—perhaps fortuitous—agreement with the experimental value. A variety of ionic solution properties, including the free energy and dynamics of ionic association, are reported for model sodium chloride solutions at two concentrations. For these quantities also the effect of water flexibility is found to be small.
Dynamics of moderately concentrated salt-free polyelectrolyte solutions: Molecular weight dependence
Authors: Sedlák Marián , Amis Eric J.
Published: July 2002

Detailed static and dynamic light-scattering experiments were performed on salt-free solutions of sodium poly(styrene sulfonate) with 13 different molecular weights ranging from Mw=5000 to 1 200 000 at moderate concentrations in the interval c=10 – 45.6 g/L. Two diffusion coefficients, Df (fast) and Ds (slow), were characterized as is typical for such a system. The fast-diffusive mode, corresponding to the coupled diffusion of polyions and counterions, is completely dominated by the counterion’s influence. Df is found to be independent of molecular weight over the broad range used in this study. On the other hand, Ds, which corresponds to the dynamics of large multichain domains in solution, is strongly dependent on molecular weight. The apparent dimensions of the domains are calculated from the angular dependencies of Ds and from the total scattered light intensity. The concentration dependence of Ds is in the form of a power law Ds∼c−ν, where the exponent ν is dependent on molecular weight in the form ν∼Mμw. This is in contrast to the isotropic model of polyelectrolyte solutions proposed by de Gennes and Odijk, which predicts that the behavior of a semidilute solution does not depend on molecular weight. It appears that interactions on a much larger scale than the correlation length of the entangled network must be taken into account.
Search for the ground-state structure of C84
Authors: Zhang B. L. , Wang C. Z. , Ho K. M.
Published: July 2002

Using a well-tested tight-binding potential, we have studied all twenty-four isolated-pentagon isomers of the C84 fullerene. Unlike C76, the helical D2 isomer of C84 is energetically very unfavorable. Two isomers which have D2 and D2d symmetries are found to be much more stable than the rest. The energies of these two isomers are so close that they may coexist in synthesis.
Infrared multiphoton dissociation of styrene ions by low-power continuous CO2 laser irradiation
Authors: Dunbar Robert C. , Zaniewski Rebecca C.
Published: July 2002

The kinetics of infrared multiphoton dissociation of styrene ions under collision-free conditions in the ion cyclotron resonance ion trap were studied as a function of the intensity of the cw CO2 laser at powers up to 6 W. Following the beginning of irradiation an induction time was observed, followed by dissociation according to a first-order rate constant. The kinetics could be fitted to a random-walk simulation of a master-equation model, in the same way as previous studies. A matrix-algebra solution of the master-equation model is described which gave a better fit with greater computational convenience. From the modeling the rate of radiation of infrared photons (assumed to be at 940 cm−1) from the ions was estimated as 350 s−1 at an ion internal energy of around 3 eV. When the dissociation threshold Et was treated as an unknown it was found that master-equation modeling of the kinetic results could give an estimate of Et, but with large uncertainty. Application of simple thermal kinetic theory via Tolman’s theorem gave good qualitative understanding of the results, and predicted the intensity dependence of the dissociation rate with a deviation of about 30%.