bluuebunny/crossref_metadata_2025 huggingface dataset:
The topology of the charge distribution and the electric-field gradient at the N nucleus in imines and di-imides
Authors: Aray Yossien , Murgich Juan
Published: July 2002

A relationship between critical points that define the topology of the Laplacian of a molecular charge distribution ∇2ρ(r), and components of the electric-field gradient (EFG) tensor at the nucleus of the two-coordinated N, obtained from an ab initio molecular-orbital calculation, was found in di-imides (R–N=N–H with R=H, F, and CN) and imines (RHC=N–H with R=H, F, CN, and CH3). The qzz component of the EFG was found to be determined by the position and magnitude of the nonbonded charge concentration present in the N valence shell, as defined by a maximum in −∇2ρ(r). The orientation of the z axis of the tensor was determined to be associated with three local maxima in −∇2ρ(r) present in the N valence shell while the asymmetry parameter of the tensor was found to be related to saddle points of the N valence shell located above and below the molecular plane.
Effect of salt concentration on interfacial behavior in the surfactant system water+n-octadecane+diethylene glycol monohexyl ether
Authors: Chen L.-J. , Hsu M.-C.
Published: July 2002

We study the phase behavior and observe wetting and nonwetting behaviors, as well as the wetting transition which occurs in the four-component surfactant system water+n-octadecane+diethylene glycol monohexyl ether+NaCl. All experiments are performed at constant weight ratio of water:n-octadecane (1:1) and constant temperature 35.0±0.05 °C, with varying NaCl concentration. It is found that the system water+n-octadecane+C6E2 exhibits a wetting transition lying at 17.1 wt. % of NaCl in water. This observation is further confirmed by interfacial tension measurements. In addition to that of lyotropic salt (NaCl), the effect of hydrotropic salt on the interfacial wetting transition is also discussed.
Product kinetic energies, correlations, and scattering anisotropy in the bimolecular reaction O(1D)+H2O→2OH
Authors: King David S. , Sauder Deborah G. , Casassa Michael P.
Published: July 2002

Doppler spectroscopy of the 16OH and 18OH products of the 16O(1D)+H218O reaction reveals marked c.m. reactive scattering anisotropy: 16OH scatters in the hemisphere containing the 16O-atom velocity vector. Internal energies of geminate OH fragments are correlated: fragments of high internal energy form with cofragments of low internal energy.
Localization of a continuum shape resonance. Photoionization of CS2
Authors: Kakar Sandeep , Choi Heung-Cheun , Poliakoff E. D.
Published: July 2002

We report a vibrationally resolved investigation into the 5σu−1 shape-resonant ionization dynamics for CS2 in the range 18≤hν≤30 eV. The intensity of dispersed fluorescence from CS2+(B 2Σu+) photoions is measured to obtain partial photoionization cross-section curves for the v=(0,0,0) and (1,0,0) levels of CS2+(B 2Σu+), as well as the vibrational branching ratio. Our results indicate a shape resonance at hν≊21 eV which is insensitive to changes in the symmetric stretching coordinate. These data are consistent with recent theoretical efforts that predict a shape resonance in the 5σu→επg channel. All previous vibrationally resolved data on shape resonances have been obtained for systems whose shape resonances occur in the εσ continuum. The current results are in contrast to behavior observed for other shape resonances, highlighting both their diverse nature and possible extensions of the current measurements.
A partial averaging strategy for low temperature Fourier path integral Monte Carlo calculations
Authors: Lobaugh John , Voth Gregory A.
Published: July 2002

Partial averaging is a powerful technique for enhancing the convergence of Fourier path integral Monte Carlo with respect to the number of Fourier coefficients included in the calculation. In the original partial averaging method, a Fourier sine series expansion of the Feynman paths was implemented and higher-order Fourier modes were averaged over by virtue of a free particle reference system. In the present paper, it is shown that by making the alternative choice of a full Fourier series expansion of the paths and a locally harmonic reference system, the partial averaging technique can be improved for low temperature applications. This improvement is accomplished because the higher-order Fourier modes that partially average the potential are allowed to have locally harmonic fluctuations about the Feynman path centroid variable. The added statistical benefit from using path vs point estimators in the partial averaging scheme is also illustrated. Representative calculations are presented for a Morse oscillator at low temperature.
Gas-phase reactions of oxide and superoxide anions with CF4, CF3Cl, CF3Br, CF3I, and C2F4 at 298 and 500 K
Authors: Morris Robert A.
Published: July 2002

Rate constants and product branching fractions have been measured for the gas-phase reactions of oxide (O−) and superoxide (O2−) anions with the halocarbons CF4, CF3Cl, CF3Br, CF3I, and C2F4 using a variable temperature–selected ion flow tube (VT–SIFT) instrument operated at 298 and 500 K. The reactions of O− with CF3X (X=Cl, Br, I) are fast and produce F−, XF−, and XO− for all X. For CF3Cl and CF3Br, X− is also formed. For CF3I, CF3− and IOF− are minor products. O− reacts rapidly with C2F4 producing F− as the major ionic product, along with contributions from reactive detachment and minor formation of FCO−, CF3−, and C2F3O−. The reaction of O2− with CF3Cl is slow, and both clustering and X− formation were observed. For CF3Br and CF3I, the reactions with O2− are fast, and nondissociative charge transfer was observed in addition to X− formation. O2− reacts rapidly with C2F4 by reactive detachment, in addition to producing F− as the major ionic product with smaller amounts of F2−, FCO−, FCO2−, CF3O−, and C2F4O−. O− and O2− were both found to be unreactive with CF4 at 298 and 500 K. The efficiencies of the reactions of both O− and O2− with CF3X are greater for the heavier halides at both 298 and 500 K. The rate constants for the reactions of O2− with CF3X appear to correlate both with the rates of thermal electron attachment to CF3X and with the electron affinities of CF3X, indicating that the O2−+CF3X reaction mechanism may involve initial electron transfer followed by dissociation. Thus the negative electron affinity of CF3Cl may explain the very slow rate for reaction with O2− despite the available exothermic pathways.
Physical origin of oscillations in the three-dimensional collision amplitudes of heavy–light–heavy systems. Semiclassical quantization of chaotic scattering
Authors: Grayce Beverly B. , Skodje Rex T. , Hutson Jeremy M.
Published: July 2002

The dynamics of three-dimensional heavy–light–heavy chemical reactions is studied using a new model which emphasizes the central importance of rotational motion in the reactive collisions. The single fastest vibrational motion is adiabatically eliminated. The reaction probability is then computed from a coherent sum of scattering amplitudes for two-atom–rigid-rotor scattering problems. The results for the reaction I+HI are shown to be accurate by comparison with available converged quantum results. Most of the analysis is devoted to a study of oscillations which appear in the reaction probability vs collision energy. The oscillations are found to result from extreme inelastic effects in the rotational scattering which are wholly unrelated to the light-atom exchange process and to the occurrence of rotational thresholds. In fact, similar oscillations are shown to exist in the nonreactive collision process, Ar+HBr. The primitive classical S-matrix semiclassical theory of Miller and Marcus is employed to relate the oscillations to interference between families of classical root orbits. These root orbits (which can number 50 or more per energy) generally exhibit extreme rotational–translational energy conversion, often including multiple scattering where the diatom rotates completely in the collision complex. The classical S matrix is shown to be useful even when the scattering dynamics is chaotic. The extreme sensitivity of the root orbits to initial conditions is suppressed since the boundary conditions are enforced at the beginning and end of the scattering process. This leads to a phenomenon of ‘‘phase coherence’’ where the semiclassical amplitudes add without the random phase cancellation one might expect in chaotic scattering.
Theoretical characterization of the reaction CH3+OH→CH3OH→products: The 1CH2+H2O, H2+HCOH, and H2+H2CO channels
Authors: Walch Stephen P.
Published: July 2002

The potential energy surface (PES) for the CH3OH system has been characterized for the 1CH2+H2O, H2+HCOH, and H2+H2CO product channels using complete-active-space self-consistent-field (CASSCF) gradient calculations to determine the stationary point geometries and frequencies followed by CASSCF/internally contracted configuration-interaction (CCI) calculations to refine the energetics. The 1CH2+H2O channel is found to have no barrier. The long range interaction is dominated by the dipole–dipole term, which orients the respective dipole moments parallel to each other but pointing in opposite directions. At shorter separations there is a dative bond structure in which a water lone pair donates into the empty a″ orbital of CH2. Subsequent insertion of CH2 into an OH bond of water involves a non-least-motion pathway. The H2+HCOH, and H2+H2CO pathways have barriers located at −5.2 and 1.7 kcal/mol, respectively, with respect to CH3+OH. From comparison of the computed energetics of the reactants and products to known thermochemical data it is estimated that the computed PES is accurate to ±2 kcal/mol.
Temperature dependence of the reaction of nickel clusters with deuterium
Authors: Zhu L. , Ho J. , Parks E. K. , Riley S. J.
Published: July 2002

The kinetics of reactions of D2 with Ni9 through Ni14 are studied in a flow-tube reactor over the temperature range 133–413 K. Rate constants for the dissociative chemisorption of the first D2 molecule have been determined. Reaction probabilities are near unity for Ni10–Ni14 and are essentially independent of temperature. For Ni9, the reaction probability is about 5% between 213 and 413 K, but increases below 213 K to ∼50% at 133 K. The possibility that these reactions are mediated by a physisorbed precursor state is discussed. The increase in Ni9 reactivity below 213 K is interpreted as due to an increase in the initial trapping of the D2 molecule on the cluster’s surface.
Solvated water molecules and hydrogen-bridged networks in liquid water
Authors: Corongiu Giorgina , Clementi Enrico
Published: July 2002

We have analyzed the molecular-dynamics (MD) trajectories for the oxygen and hydrogen atoms of liquid water, at six temperatures (from hot, T=361 K, to supercooled water, T=242 K); in the MD simulations the Nieser-Corongiu-Clementi ab initio potential has been used, since it yields reliable x-ray and neutron-diffraction data as well as infrared, Raman, and neutron-scattering spectra. Our analysis leads to two complementary models where we can consider each water as a solvated molecule (placed at the center of a solvation shell) or as a component of a cyclic polymer, a substructure of the hydrogen-bonded network. In the first solvation shell all water molecules are solvated with coordination values in the range 2–8. The most probable solvation number is four, at low temperature, and five at high temperature considering oxygen–oxygen pairs; however, the coordination number is four at all the temperatures if we consider oxygen–hydrogen pairs. The lifetime of the tetra coordinated complexes is the largest one and increases as temperature decreases. The computed population of cyclic polymers is highest for the pentameters in the studied temperature range, the second most probable cyclic structure is for hexamers. The average O–O distances in the liquid are temperature dependent and shorter than those in the gas phase, approaching ice values at low temperature (except for cyclic trimers, for which the O–O distance is nearly temperature independent). As a preliminary result, the lifetime of the polygons is estimated to be around 0.01 ps.
Prediction of the intensity and structure of spectra associated with the endohedral motions of metal-containing buckyballs
Authors: Van Cleef Garrett W. , Renkes Gordon D. , Coe James V.
Published: July 2002

A theoretical simulation is presented of the absorption spectrum of a particle trapped within a spherical potential with a barrier to motion through the center of the potential. The system is investigated as a simple model for ionic endohedral metallofullerenes, such as Na+@C60−. The radial Schrödinger equation was solved using an inverted Morse potential from Ballester and Dunlap, which they obtained by spherically averaging an icosahedral potential calculated with density functional theory for a Na+ in a neutral C60. The resulting energies and wave functions were used to generate a spectrum which lies in the microwave and far infrared regions. The energy levels have been characterized using relations generally applied to diatomic molecules with parameters analogous to ωe, ωexe, Be, De, and αe. There is a high degree of mixing of vibration and rotation in this system. Transition matrix elements were computed for transitions in both the microwave and far infrared regions which suggest that the transitions would be quite intense. It is hoped that this investigation will encourage an experimental search for these spectra and provide a useful first step in guiding the interpretation of the far infrared and microwave spectra of metal-containing fullerenes.
Rotational excitation with pointwise vibrational wave functions
Authors: Tennyson Jonathan
Published: July 2002

Finite element methods, particularly the discrete variable representation (DVR), have proved very successful for calculations on highly excited vibrational states. A procedure is given for calculating the corresponding rotationally excited states for triatomic molecules represented by either scattering (Jacobi) or Radau coordinates. By exploiting the quadrature approximation and the transformation between DVR and basis function representations, this procedure greatly simplifies the calculation of the Coriolis coupling matrix elements. The computational expense of the calculation is thus dominated by that of the underlying vibrational problem. Test results are presented for the benchmark molecules H2S, H2O and H3+.
A reactive phase diagram of CO oxidation on Pd(110): Steady and oscillatory states
Authors: Ehsasi M. , Berdau M. , Rebitzki T. , Charlé K.-P. , Christmann K. , Block J. H.
Published: July 2002

The steady and oscillatory regions of the CO oxidation reaction on the Pd(110) surface have been determined as a function of externally controlled parameters (flow rate, CO and oxygen partial pressures, temperature) over a wide range. At constant sample temperature and flow rate, the experiments yield a characteristic cross-shaped phase diagram separating regions of monostability, bistability, and oscillatory behavior. The existence of a cross-shaped phase diagram indicates the operation of a slow feedback process, which could be traced back to the (experimentally verified) formation and retarded removal of subsurface oxygen during the reaction. The diagram reflects one of the first well-defined oscillatory systems in heterogeneous catalysis and may provide a general basis for mechanistic studies and models of oscillatory surface reactions.
Solvation dynamics in a Brownian dipolar lattice. Comparison between computer simulation and various molecular theories of solvation dynamics
Authors: Komath Sneha Sudha , Bagchi Biman
Published: July 2002

Several recent theoretical and computer simulation studies have considered solvation dynamics in a Brownian dipolar lattice which provides a simple model solvent for which detailed calculations can be carried out. In this article a fully microscopic calculation of the solvation dynamics of an ion in a Brownian dipolar lattice is presented. The calculation is based on the non-Markovian molecular hydrodynamic theory developed recently. The main assumption of the present calculation is that the two-particle orientational correlation functions of the solid can be replaced by those of the liquid state. It is shown that such a calculation provides an excellent agreement with the computer simulation results. More importantly, the present calculations clearly demonstrate that the frequency-dependent dielectric friction plays an important role in the long time decay of the solvation time correlation function. We also find that the present calculation provides somewhat better agreement than either the dynamic mean spherical approximation (DMSA) or the Fried–Mukamel theory which use the simulated frequency-dependent dielectric function. It is found that the dissipative kernels used in the molecular hydrodynamic approach and in the Fried–Mukamel theory are vastly different, especially at short times. However, in spite of this disagreement, the two theories still lead to comparable results in good agreement with computer simulation, which suggests that even a semiquantitatively accurate dissipative kernel may be sufficient to obtain a reliable solvation time correlation function. A new wave vector and frequency-dependent dissipative kernel (or memory function) is proposed which correctly goes over to the appropriate expressions in both the single particle and the collective limits. This form is expected to lead to better results than all the existing descriptions.
Interfacial behavior of compressible polymer blends
Authors: Lifschitz Masha , Freed Karl F.
Published: July 2002

We develop a new density functional treatment for the liquid–liquid interface of phase separated binary polymer blends. In contrast to previous density functional theory studies of these interfaces, we incorporate the compressibility (i.e., ‘‘equation of state effects’’) of the system through use of a compressible version of Flory–Huggins theory. The introduction of compressibility is demonstrated to severely complicate the mathematical description. The coupled, nonlinear Euler–Lagrange equations for the interfacial profile and tension are shown to become numerically unstable and permit analytic solution in the asymptotic profile wings. Our computations are limited to symmetric binary blends because this restriction enables us to compute numerically the central portion of the interfacial profile and thereby test more general variational methods (applicable also to unsymmetric blends) that we develop for computing the full interfacial profile and the interfacial tension. The total density of the symmetric blend is found to be reduced in the profile center relative to the bulk density by 0.4%–1.5%. While this total density fluctuation costs free energy, it reduces unfavorable polymer–polymer contacts, thereby diminishing the total interfacial tension. The applicability of our approximate scheme is tested.
Density functional pseudopotential studies of molecular geometries, vibrations, and binding energies
Authors: Chen Han , Krasowski Matthew , Fitzgerald George
Published: July 2002

This paper presents density functional calculations of structural and electronic properties of molecules by the use of ab initio pseudopotentials and the linear combination of Gaussian-type orbitals optimized by simulated annealing. A comprehensive study was carried out for various molecules whose constituent atoms cover a major portion of the Periodic Table from H to Po except for atoms in the lanthanum group. Bond distances, vibrational frequencies, and binding energies are obtained and compared with available experimental results and all-electron calculations. We find that, in general, pseudopotential bond distances are within 0.1 Å of experimental and all-electron results, and relative uncertainties of vibrational frequencies are, on the average, less than 12%. For binding energies, pseudopotential results agree well with corresponding all-electron results. For most cases, nonlocal gradient corrections to the local density approximation significantly improve both pseudopotential and all-electron binding energies.
Rotational population distributions of O2(X 3Σg−, v″=9, 12, and 15) following ozone photolysis at 248 nm
Authors: Daniels Michael J. , Wiesenfeld John R.
Published: July 2002

Vibrationally excited O2(X 3Σg−, v″=9, 12, and 15) produced from the 248 nm photolysis of O3 was probed using laser-induced fluorescence of the (0,v″) Schumann–Runge bands of O2(B 3Σu−←X 3Σg−). The nascent rotational distributions are all sharply peaked at 15% of the available kinetic energy. Comparison to statistical and impulsive models of photodissociation suggests that the energy release is largely impulsive. However, while the impulsive model accurately predicts the rotational energy distributions of O2(X 3Σg−), it does not simultaneously match previously published photoproduct angular distributions.
Adsorption kinetics for ethylsilane, diethylsilane, and diethylgermane on Si(111) 7×7
Authors: Coon P. A. , Wise M. L. , George S. M.
Published: July 2002

The adsorption kinetics for ethylsilane (ES), diethylsilane (DES), and diethylgermane (DEG) on Si(111) 7×7 were studied using laser-induced thermal desorption (LITD) and temperature programmed desorption (TPD) techniques. The initial reactive sticking coefficients were determined as a function of surface temperature using LITD measurements. In these experiments, the ethyl coverage vs adsorption time was monitored using CH2=CH2 (ethylene) LITD signals that were produced by the β-hydride elimination of the surface ethyl groups, e.g. Si–CH2CH3(ad)→Si–H(ad)+CH2=CH2(g). The initial reactive sticking coefficients were S0≊2×10−3, 4×10−3, and 5×10−2 for DES, ES, and DEG, respectively, at 200 K. As expected from a precursor-mediated adsorption model, the initial reactive sticking coefficients were observed to decrease with increasing surface temperature. Experiments with preadsorbed hydrogen also demonstrated that the initial reactive sticking coefficients of DES and DEG were reduced as a function of hydrogen coverage. This behavior indicated that alkylsilane and alkylgermane adsorption on Si(111) 7×7 requires free dangling bond sites. LITD experiments revealed that the ethyl surface coverage saturated after large exposures. The saturation coverage corresponded to a deposited Si or Ge coverage of Θ=0.13 monolayer for DES and DEG and was independent of surface temperature between 200–400 K. DEG adsorption cycles were used to deposit increasing amounts of Ge on Si(111) 7×7. The deposited coverage was examined using H2 TPD studies which indicated that the germanium may be forming islands. LITD experiments were also used to monitor ethyl surface diffusion on Si(111) 7×7 after DES adsorption. No evidence of significant ethyl surface mobility (D≤1.0×10−10 cm2/s) was found for surface temperatures as high as 600 K.
Monte Carlo simulation of the adsorption of diblock copolymers from a nonselective solvent. I. Adsorption kinetics and adsorption isotherms
Authors: Zhan Yongjian , Mattice Wayne L. , Napper Donald H.
Published: July 2002

The adsorption of diblock copolymers from a nonselective solvent onto a surface that attracts one of the two blocks has been simulated on a cubic lattice. The simulation uses periodic boundary conditions in two directions and places the surface at the edges of the box in the third direction. The diblock copolymers are ANABNB with 6≤NA≤20 and 10≤NB≤30, where A is the anchoring block and B does not adsorb. This report describes the adsorption isotherms and the kinetics of adsorption in the simulation. Good approximations to a Langmuir adsorption isotherm are observed only at small values of NA and the adsorption energy per segment. The kinetics observed in the isotherm has an initial stage that depends on √I, followed by a second stage that depends on [1−exp(−I/constant)], where I is the number of iterations. These results are in agreement with expectations based on previous experimental and theoretical results in the situation. The internal structure of the adsorbed layer is addressed in the following paper.
Dynamical properties of randomly cross-linked polymer melts: A Monte Carlo study. I. Diffusion dynamics
Authors: Sommer Jens-Uwe , Schulz Michael , Trautenberg Hans L.
Published: July 2002

In the framework of the bond-fluctuation method, we have studied the diffusion properties of randomly cross-linked polymer melts. From the connectivity matrices of simulated network samples, their ideal diffusion behavior was calculated. The resulting functions were compared with data from Monte Carlo simulations. For the cross-link densities under consideration, no dramatic deviation from prediction of ideal behavior was observed. In addition to the test of the generalized Rouse diffusion, our simulations illustrate the good mobility of cross-linked melts within the bond-fluctuation method.
Pseudospectral double excitation configuration interaction
Authors: Martinez Todd J. , Carter Emily A.
Published: July 2002

We present a pseudospectral formulation of the single reference, closed shell double excitation configuration interaction method using a generator state self-consistent electron pair approach. The method scales as O(n2N3), compared to the conventional scaling of O(n2N4+n3N3). In no case tested does the pseudospectral energy differ by more than 0.35 mhartree from the conventional result.
Translationally and rotationally resolved excitation of CO2(002) by collisions with hot hydrogen atoms
Authors: Khan Farooq A. , Kreutz Thomas G. , Flynn George W. , Weston Ralph E.
Published: July 2002

Time domain tunable diode laser absorption spectroscopy has been used to measure rotationally resolved transient absorption line shapes and nascent rotational populations for CO2 molecules excited into the (0002) vibrational state by collisions with translationally hot hydrogen atoms. The 0002 rotational population distribution and rotationally resolved linewidths are remarkably similar to those previously obtained for 0001. Within the context of a simple physical model used to interpret the data, the similar rotational distributions and translational recoils for 0001 and 0002 suggest that these two states are excited by similar collision trajectories, wherein asymmetric stretching excitation is optimized when H strikes near the end of the O–C–O molecule. The magnitude of population scattered into 0002 is ∼21 times smaller than that scattered into 0001.
Dielectric relaxation of electrolyte solutions: Is there really a kinetic dielectric decrement?
Authors: Chandra A. , Wei Dongqing , Patey G. N.
Published: July 2002

The dielectric behavior of model electrolyte solutions is studied by molecular dynamics simulations. For the systems considered, it is found that the zero-frequency dielectric constant depends only on equilibrium properties and that there is no measurable ‘‘kinetic dielectric decrement.’’ A theoretical explanation of the simulation results is presented. It is argued that no kinetic dielectric decrement exists for spherical ions in solvents of arbitrary molecular symmetry.
Symmetry selection rules for vibrational excitation by resonant electron impact and a unified treatment of vibronic coupling between resonances and to the continuum: A complete symmetry analysis of vibrational excitation in benzene
Authors: Gallup G. A.
Published: July 2002

A symmetry analysis of the contributions to the transition operator from vibronic coupling provides symmetry selection rules for vibrational excitation by resonant electron impact in polyatomic molecules. In the context of electron scattering vibronic coupling operates in two modes. These are named internal and external vibronic coupling. Internal vibronic coupling operates within and among the quasibound states giving rise to the resonances. External vibronic coupling operates between the quasibound states and the continuum. We discuss the differences, particularly with respect to the angular distribution of electron scattering. A complete symmetry analysis of vibrational excitation is given using benzene as an example.
Free energy and transport properties of the Gaussian overlap model: Variational approach
Authors: Orozco Jorge , Castillo Rolando
Published: July 2002

The purpose of this paper is to find a method for calculating the free energy and the transport properties of a fluid made up of nonspherical nonpolar molecules in the dense regime. The model potential used was the Gaussian overlap model with constant ε. Our procedure relies on the assumption that at high densities the behavior of a molecular fluid is dominated by the harsh repulsive forces. Hence, the properties of the fluid can be given in terms of an effective hard core fluid. Thus, the free energy was obtained through a variational method with the aid of a nonspherical reference potential. The results were compared with molecular dynamics calculations and with calculations using a perturbation method. These results are in a close agreement with simulation data. In a further level of approximation the transport properties, thermal conductivity and shear viscosity, can be estimated with an extension of the effective-diameter hard-sphere theory. The results of our calculations for transport properties were compared with experimental data, and with calculations using the effective-diameter hard-sphere theory, but with effective diameters coming from perturbation theory. In particular, for the case of shear viscosity the results were excellent. For thermal conductivity the results are not that good, however, the sources of discrepancy are discussed.
Fokker–Planck equation for nonlinear stochastic dynamics in the presence of space and time dependent friction
Authors: Pollak Eli , Berezhkovskii Alexander M.
Published: July 2002

The one-dimensional stochastic equation of motion for a particle in the presence of space and time dependent friction involves multiplicative fluctuations and a nonlinear friction kernel. We show how this rather complicated equation may be significantly simplified. Introduction of an auxiliary mode leads to a set of two nonlinearly coupled equations with space and time independent damping. An exact Fokker–Planck equation emerges naturally from this formulation.
Coexistence curve of polystyrene in methylcyclohexane. VI. Two-phase behavior of ternary system near the tricritical point
Authors: Dobashi Toshiaki , Nakata Mitsuo
Published: July 2002

Tricritical two-phase behavior of bimodal polystyrene (PS) in methylcyclohexane (MCH) was studied by using the Flory–Huggins theory combined with an empirically determined interaction parameter. In the ternary system PSI+PSII+MCH with the molecular weight M1=1.73×104 for PSI, the tricritical system was obtained for PSII with M2=2.24×105, which yielded a coincidence of the upper and lower critical end points. For the tricritical mixture the two-phase coexistence curve was calculated in the temperature range 0.01<Tt−T<6 K with Tt being the tricritical temperature. In a diagram of temperature, total PS volume fraction, the dilute and concentrated branches of the coexistence curve showed very different behavior. The log–log plots to estimate the exponent β for the dilute branch, concentrated branch, and the difference between them were curved, and the slopes of the plots, which decreased with decreasing temperature, were much larger than the exponent expected for the nonsymmetric tricritical point. This observation suggested that the asymptotic range is limited very near the tricritical point (TCP). According to analytical arguments with the original Flory–Huggins theory, the critical line of the tricritical system was conjectured to be tangent to the dilute branch of the coexistence curve at the TCP.
Structural transitions and thermally averaged infrared spectra of small methanol clusters
Authors: Buck U. , Schmidt B. , Siebers J. G.
Published: July 2002

Classical Monte Carlo and molecular dynamics (MD) simulations were carried out to investigate the structures, the infrared spectra, and the rigid–nonrigid transitions of small methanol clusters (CH3OH)n for n=3–6. The study was motivated by experimental results for these clusters from size specific infrared (IR) dissociation spectroscopy. The MD simulations revealed the following transitions: The trimer passes from a rigid ring configuration into a series of nonrigid open chain structures starting at 197 K. For n=4 and 5 such transitions occur between rings and rapidly fluctuating ring structures at T=357 and 243 K, respectively. For n=6 first a pure isomeric transition between the two energetically lowest isomers of S6 and C2 symmetry is found at 35 K, and then a similar transition to a nonrigid behavior as is observed for n=4 and 5 is seen at 197 K. The measured spectra display in all cases the rigid lowest energy configurations.
Temperature dependence of electron attachment and detachment in SF6 and c-C4F6
Authors: Datskos P. G. , Christophorou L. G. , Carter J. G.
Published: July 2002

In the temperature, T, range of 300–600 K and the mean electron energy range 〈ε〉 of 0.19–1.0 eV, the total electron attachment rate constant for SF6 and c-C4F6 measured in dilute mixtures with N2, is virtually independent of T. Under the same experimental conditions the stabilized SF−6 anion does not undergo autodetachment but the stabilized c-C4F−6 anion undergoes a profound increase in autodetachment (by about four orders of magnitude). This difference between the SF−6 and the c-C4F−6 anions is attributed to the larger electron affinity of the SF6 molecule compared to the c-C4F6 molecule. The heat-activated autodetachment for c-C4F−6 is related to increases in the internal energy content of the c-C4F−6 anion and is found to have an activation energy of 0.237 eV. The ‘‘limiting electric field strength’’ of SF6 was found to increase by ∼11% as T was increased from 300 to 600 K.
Photoionization mass spectrometric studies of Sb2 and Bi2
Authors: Yoo R. K. , Ruscic B. , Berkowitz J.
Published: July 2002

The photoion yield curves of Sb+2(Sb2) and Bi+2(Bi2) are presented. In both cases, two autoionizing series (designated pσ and pπ) are observed, converging on the excited 2Σ+g state. From this information, the ionization energy of the 2Σ+g state in Sb2 is lowered to 9.247 eV. The difference in quantum defects, δpπ−δpσ, is shown to be related to the quadrupole moment of the molecular ion core of the A 2Σ+g state in Pn+2. The adiabatic ionization energies (AIP) are also decreased: AIP(Sb2)≤8.43 eV, AIP(Bi2)≲7.34 eV. Although the uppermost occupied orbital is nominally a bonding pπ orbital, an analysis leads to the surprising conclusion that D0(Pn+2)≳D0(Pn2), where Pn=P, As, Sb, and Bi.
Infrared diode laser spectroscopy of the LiO radical
Authors: Yamada Chikashi , Hirota Eizi
Published: July 2002

The fundamental vibrational band of the 7LiO radical in the ground electronic state X 2Πi was observed in a region from 720 to 850 cm−1 using a source frequency modulation infrared diode laser spectrometer. Radicals were generated in a high-temperature cell by the reaction of lithium metal vapor with nitrous oxide. The observed spectrum was analyzed together with the radio-frequency and microwave spectra already reported. It was found that the vibration-rotation Hamiltonian employed in a previous paper was insufficient to fit all of the observed spectra simultaneously. The Hamiltonian was thus extended to include higher-order corrections for the centrifugal distortion and Λ-type doubling terms, and was used to derive molecular parameters.
Monte Carlo simulation study of adsorption from a liquid mixture at states near liquid–liquid coexistence
Authors: Fan Y. , Finn J. E. , Monson P. A.
Published: July 2002

We present a Monte Carlo simulation study of adsorption on a plane solid surface from a model binary liquid mixture with a miscibility gap terminating at an upper critical solution temperature. The model liquid mixture studied is a binary Lennard-Jones 12-6 mixture with identical pure components but with a weak unlike interaction. The components interact with the solid surface by means of 9-3 potentials. Adsorption isotherms in the approach to liquid–liquid coexistence from the phase dilute in the strongly adsorbed component have been found which are consistent with the presence of partial wetting and complete wetting by the more strongly adsorbed phase at coexistence. In addition prewetting transitions have been found. Resolution of the prewetting transition was made possible by using an isobaric semigrand ensemble, which for the system under consideration allows a precise tuning of the bulk state relative to saturation.
A high resolution infrared study of HCCCN–HCN and HCCCN–HF
Authors: Yang X. , Kerstel E. R. Th. , Scoles G.
Published: July 2002

The HCCCN–HCN and HCCCN–HF hydrogen bonded dimers have been investigated using optothermal techniques. Accurate molecular constants for both complexes are reported here and comparisons with similar complexes are made.
On the radiative lifetimes of the a 3Σ+ state of NO+
Authors: Palmieri Paolo , Tarroni Riccardo , Chambaud Gilberte , Rosmus Pavel
Published: July 2002

The radiative lifetimes of the lowest excited triplet state of NO+ have been evaluated using new integral and CI programs for the spinless and the spin dependent components of the molecular Hamiltonian. The Rayleigh–Schrödinger perturbation theory, compared to direct CI techniques, provides a numerically more stable and rapidly convergent expansion for the spin–orbit terms of the molecular wave functions. Based on this theory, we have performed the analysis of the Rayleigh–Schrödinger components of the singlet–triplet transition moment and found that the most important contribution to the radiative lifetimes is given by the second excited B 1Π state and not by the lowest A 1Π state as previously assumed on energy grounds. The radiative lifetimes computed for the lowest five vibrational states of a 3Σ+, resulting from the difference of two large contributions from the B 1Π and the A 1Π states, are found extremely sensitive to small variations in the computed 〈1Σ+‖μ‖B 1Πβ〉 transition dipole function and to the 〈a 3Σ+,1,1‖HLαSα‖A 1Πβ,0, 0〉 spin–orbit interaction. Using matrix elements evaluated at two levels of CI, the radiative lifetimes are estimated 330∼270 and 270∼250 ms, respectively.
On the cluster compositions in the classical binary nucleation theory
Authors: Laaksonen Ari , Kulmala Markku , Wagner Paul E.
Published: July 2002

The classical binary nucleation theory is considered. It is shown that as a consequence of the Gibbs–Duhem equation and the Gibbs adsorption isotherm, an equation emerges which can be used together with the interior (bulk) mole fraction and radius of the critical cluster given by the theory to calculate the total numbers of molecules of both species in the cluster. Accordingly the overall (average) composition of the critical cluster is determined. Furthermore, it is noted that it is possible to use this equation to determine the interior (bulk) mole fraction of any sufficiently large spherical cluster with known total numbers of molecules, which allows the construction of a free energy surface within the framework of the classical nucleation theory. However, numerical calculations for the water–ethanol system result in strange predictions at some typical experimental conditions. Possible reasons for the unsuccessful predictions by the classical theory are discussed.
Quadratic steepest descent on potential energy surfaces. III. Minima seeking along steepest descent lines
Authors: Sun Jun-Qiang , Ruedenberg Klaus , Atchity Gregory J.
Published: July 2002

A simplified quadratic steepest descent method, based on the availability of energies and gradients, is formulated for use in minimum searching. It requires only a fraction of the computational effort needed for the previously developed accurate steepest descent procedures and, typically, involves less work than standard quasi-Newton minimum searches. At the same time, it follows the true steepest descent curves reasonably closely and reaches the closest minima. This is in contrast to quasi-Newton procedures which cannot be relied upon to do so. The performance is documented by applications to a variety of searches on the Müller–Brown surface.
Monte Carlo simulation of the liquid–vapor interface of water using an ab initio potential
Authors: Lie George C. , Grigoras Stelian , Dang Liem X. , Yang Dah-Yen , McLean A. Douglas
Published: July 2002

Monte Carlo calculations have been carried out to study the interfacial properties of liquid water, using the Matsuoka–Clementi–Yoshimine (MCY) potential for the water–water interaction. The surface tension calculated at 298 K is 23.7±3.4 dyn/cm, to be compared with the experimental value of 72 dyn/cm. The interfacial 10–90 thickness is 4.70 Å, with the dipoles of the water molecules near the liquid phase pointing slightly towards the liquid phase and those near the gas phase pointing towards the gas phase. The interfacial water molecules are found to be more restricted in their rotation, as evidenced by the smaller root-mean-squared fluctuations of the dipole directions. The Volta potential difference across the interface arising from the permanent dipoles is estimated to be 0.024 V. A new and efficient method is proposed to calculated the surface excess energy. The excess energy calculated for the MCY water is 119 erg/cm2, to be compared with the experimental value of 120 erg/cm2. From the calculated surface excess energy, the temperature variation of the surface tension, or the surface entropy, for the MCY water is estimated to be −0.32 dyn/(cm2 K). This temperature variation is confirmed by another Monte Carlo study at 310 K to within the calculated uncertainty.
Local geometry and resonant vibrations of Cu+:NaF. Results of ab initio perturbed ion, cluster-in-the-lattice calculations involving clusters of 179 ions
Authors: Luaña Víctor , Flórez M. , Pueyo L.
Published: July 2002

The ground state electronic structure and energy of a CuF92Na5−86 cluster (Cu+ plus 12 shells of neighbors) embedded into a quantum lattice representing the NaF crystal are determined by using the ab initio perturbed ion (aiPI) method, with unrelaxed Coulomb–Hartree–Fock (uCHF) correlation energy corrections. Parallel calculations are performed on the NaF92Na5−86 cluster of the pure crystal in order to identify the changes induced by the impurity and to estimate the systematic errors in our calculations. The geometry of the first four shells (32 ions) is allowed to relax by following symmetric breathing modes. An inwards relaxation of −0.12 Å is predicted for the nearest neighbors (nn) shell, but negligible relaxations are found for the outer shells. The substitution of the Na+ ion by the Cu+ impurity is favored by −1.03 eV. The Cu+ ion is found to occupy an on-center octahedral position. The 138 independent Oh force constants corresponding to the vibration of the Cu+ and its first four shells of neighbors are then numerically computed from the aiPI energy using a Richardson iterated, finite-difference limit formula. These force constants give the vibrational modes of the impurity center. Strong couplings are found among vibrational modes of adjacent shells. Vibration frequencies characteristic of the doped system are obtained at 206 cm−1 (1a1g), 108 cm−1 (1t1u), and 173 cm−1 (1eg) determined mainly by the motions of the CuF6 octahedron.
Cl′+HCl(v=1, j=3)→Cl′H(v′, j′)+Cl reaction dynamics over an extended collision energy range
Authors: Aker Pamela M. , Fulmer John P.
Published: July 2002

Quasiclassical trajectory calculations have been used to model the Cl′+HCl(v=1, j=3) →Cl′H(v′, j′)+Cl exchange reaction dynamics over the 0–1.5 eV collision energy range. A kinematic constraint, which results from the H+LH mass combination, forces the reaction to strongly conserve initial vibrational excitation, independent of collision energy. This kinematic constraint is weaker than forces induced by the reaction potential anisotropy however, for the reaction does not strongly conserve initial rotational excitation. Reaction proceeds via two mechanisms which are classified as direct (single H atom exchange) and indirect (multiple H atom exchanges). Approximately 30% of the reaction proceeds via indirect exchange, independent of collision energy.
Analysis of the statistical errors in conditioned real time path integral methods
Authors: Amini Abolfazl M. , Herman Michael F.
Published: July 2002

An analysis is provided of the statistical errors in the Monte Carlo evaluation of the conditioned real time discretized path integral propagator. The analysis considers the case of a harmonic potential. For this case, all the required integrals can be performed analytically. This analysis is also relevant to a semiclassical evaluation of the integrals in more general problems. It is found (in the simplest case) that the optimal relative statistical error per independent sampling is proportional to DD/2, where D is the dimensionality of the integrand. Therefore, the number of Monte Carlo samplings must scale as DD in order to achieve a desired level of accuracy. Since D is proportional to the number of time steps in the discretized path integral, this analysis demonstrates that the length of the calculations required increases very rapidly as the number of time steps is increased.
Competing C–Br and C–C bond fission following 1[n(O),π*(C=O)] excitation in bromoacetone: Conformation dependence of nonadiabaticity at a conical intersection
Authors: Kash P. W. , Waschewsky G. C. G. , Morss R. E. , Butler L. J. , Francl M. M.
Published: July 2002

These experiments investigate the competition between C–C and C–Br bond fission in bromoacetone excited in the 1[n(O),π*(C=O)] absorption, elucidating the role of molecular conformation in influencing the probability of adiabatically traversing the conical intersection along the C–C fission reaction coordinate. In the first part of the paper, measurement of the photofragment velocity and angular distributions with a crossed laser-molecular beam time-of-flight technique identifies the primary photofragmentation channels at 308 nm. The time-of-flight spectra evidence two dissociation channels, C–Br fission and fission of one of the two C–C bonds, BrH2C–COCH3. The distribution of relative kinetic energies imparted to the C–Br fission and C–C fission fragments show dissociation is not occurring via internal conversion to the ground electronic state and allow us to identify these channels in the closely related systems of bromoacetyl- and bromopropionyl chloride.

In the second part of the work we focus on the marked conformation dependence to the branching between C–C fission and C–Br fission. Photofragment angular distribution measurements show that C–Br fission occurs primarily from the minor, anti, conformer, giving a β of 0.8, so C–C fission must dominate the competition in the gauche conformer. Noting that the dynamics of these two bond fission pathways are expected to be strongly influenced by nonadiabatic recrossing of the reaction barriers, we investigate the possible mechanisms for the conformation dependence of the nonadiabatic recrossing with low-level ab initio electronic structure calculations on the C–Br reaction coordinate and qualitative consideration of the conical intersection along the C–C reaction coordinate. The resulting model proposes that C–C bond fission cannot compete with C–Br fission in the anti conformer because the dissociation samples regions of the phase space near the conical intersection along the C–C fission reaction coordinate, where nonadiabaticity inhibits C–C fission, while from the gauche conformer C–C fission can proceed more adiabatically and dominate C–Br fission. A final experiment confirms that the branching ratio changes with the relative conformer populations in accord with this model.