bluuebunny/crossref_metadata_2025 huggingface dataset:
Phase diagrams calculated for quaternary polymer blends
Authors: Horst Roland , Wolf B. A.
Published: July 2002

A method is presented that allows the calculation of phase diagrams (spinodal, binodal, and tie lines) of quaternary mixtures on the basis of the Gibbs energy of mixing ΔG. No derivatives of ΔG with respect to the composition variables are required. This method is particularly useful in cases where the composition dependence of ΔG is very complex, and no analytical representation of ΔG can be found. Phase diagrams have been calculated on the basis of the Flory–Huggins theory for mixtures of four polymers. Blends that phase separate because of very favorable interactions (negative interaction parameters) were of particular interest. In this case miscibility gaps can be located inside the tetrahedron composed of the Gibbs phase triangles of the four completely miscible ternary subsystems. For symmetrical mixtures of K components (identical chain lengths and interaction parameters) equations are presented that allow the calculation of K-phase and (K−1)-phase regions for any value of K.
Integral equation theory for Lennard-Jones fluids: The bridge function and applications to pure fluids and mixtures
Authors: Duh Der-Ming , Henderson Douglas
Published: July 2002

The pure Lennard-Jones fluid and various binary mixtures of Lennard-Jones fluids are studied by both molecular dynamics simulation and with a new integral equation which is based on that proposed by Duh and Haymet recently [J. Chem. Phys. 103, 2625 (1995)]. The structural and thermodynamic properties calculated from this integral equation show excellent agreement with simulations for both pure fluids and mixtures under the conditions which we have studied. For mixtures, the effect of deviations from the Lorentz-Berthelot (LB) mixing rules for the interaction parameters between unlike species is studied. Positive deviations from the nonadditivity of the molecular cores leads to an entropy driven tendency for the species to separate. This tendency persists even in the presence of a deviation from the LB rule for the energy parameter which enhances the attraction of the unlike species. On the other hand, in the case of negative deviations from nonadditivity, the tendency for association may be either energy or entropy driven, depending on the size ratio.
A three-dimensional wave packet study of Ar⋅ ⋅ ⋅I2(B )→Ar + I + I electronic predissociation
Authors: Roncero Octavio , Halberstadt Nadine , Beswick J. Alberto
Published: July 2002

A three-dimensional wave packet study of Ar...I2(B)→ Ar + I(2P3/2)+ I(2P3/2) electronic predissociation, arising from the argon-induced electrostatic coupling between the B(3Π0+u) and the repulsive a(3Π1g) state of I2, is presented. A time-dependent golden rule approach is used. The initial wave packet corresponds to a bound vibrational wave function of the Ar...I2(B) complex (with zero total angular momentum) multiplied by the electronic coupling. A 3-D propagation in the final dissociative surface is then performed and the predissociation rates are obtained by Fourier transform of the wave packet autocorrelation function. The potential energy surfaces are described by sums of atom–atom interactions. For the B(3Π0+u) state potential, empirically determined van der Waals parameters available from the literature are used. For the final dissociative a(3Π1g) electronic state, the van der Waals parameters are adjusted to reproduce the experimentally observed oscillations of the electronic predissociation rate as a function of the initial vibrational quantum number v′ of I2. It is shown that good agreement between calculated and measured values can be obtained with a van der Waals well of 100 cm−1 and an interstate coupling of the order of 14 cm−1.
A time-dependent calculation of the alignment and orientation of the CN fragment of the photodissociation of ICN
Authors: Wei Hua , Carrington Tucker
Published: July 2002

We calculate the alignment and orientation of the CN fragment of the photodissociation of ICN using a time-dependent method and the diabatic potentials and transition moments of Morokuma and co-workers [J. Chem. Phys. 100, 4894 (1994)]. The time-dependent Schrödinger equation is solved using a Chebyshev method evaluating sums required to do the matrix-vector products sequentially. To prevent the wave packet from drifting off the edge of our grid we propagate in successive steps. The ground state wave function, from which the molecule is dissociated, is computed using a three-dimensional variational method. We calculate alignment and orientation for a Ji=0→Jf=1 transition. We use parity-adapted angular basis functions. Our calculated alignment and orientation are qualitatively close to experimental results.
Rotational excitations of a symmetric top in cubic orientational potentials. II. CHD3 matrix-isolated in argon
Authors: Asmussen B. , Prager M. , Müller M. , Balszunat D. , Press W. , Carlile C. J.
Published: July 2002

Inelastic neutron scattering has been performed on CHD3 molecules matrix isolated in solid argon. Four inelastic lines have been observed within the energy transfer range 0≤ℏω≤2.5 meV. The corresponding energy level scheme and the line intensities are explained in terms of a model based on a completely free rotation of the CHD3 molecule. An expression for the double differential neutron scattering cross section of the free CHD3 rotor is derived. In spite of a line shift of 44% with respect to the free rotor, no crystal field splitting was observed.
Cluster size determination from diffractive He atom scattering
Authors: Buck Udo , Krohne Reinhard
Published: July 2002

In a crossed molecular beam arrangement helium atoms are scattered from argon clusters which are produced in an averaged size range of n̄=6 to n̄=90 by adiabatic expansion through sonic and conical nozzles. The diffraction oscillations in the total differential cross section are used to derive information on the size distribution of the clusters by comparison with quantum mechanical calculations based on a model potential. In the size range covered by the measurements, the average cluster size is given by n̄=38.4(Γ*/1000)1.64, where Γ* is the scaling parameter of the source conditions introduced by Hagena [Z. Phys. D 4, 291 (1987)]. The results are in agreement with recent measurements of corrected mass spectra but disagree with the results obtained from electron diffraction. General relations are recommended which connect the scaling parameter with the averaged size.
Interaction between macroparticles in Lennard-Jones fluids or in hard-sphere mixtures
Authors: Kinoshita Masahiro , Iba Shin-ya , Kuwamoto Ken , Harada Makoto
Published: July 2002

We report results of numerical analyses on the surface (macroparticle) interactions in simple fluids. The singlet Ornstein–Zernike theories with hypernetted-chain closures are employed. With no (or very weak) attraction in the surface–fluid interaction uMS, both the interaction φMM and the force fMM between macroparticles in Lennard-Jones fluids are characterized by monotonically decreasing attraction. With increasing attraction in uMS, however, φMM and fMM become more oscillatory. The force between planar Lennard-Jones solids immersed in a Lennard-Jones fluid previously calculated by the grand canonical ensemble Monte Carlo simulation, which oscillates around zero with strong repulsion at small surface-to-surface separations, is well reproduced by our theoretical approach. The previously reported experimental observation that the addition of cyclohexane to octamethylcyclotetrasiloxane causes a reduction in the range of the solvation force is well demonstrated by our theoretical calculations on a simple binary mixture of hard spheres with different diameters. Similarly, the model reproduces the observation that at a sufficiently high mole fraction of cyclohexane there is less structure than in either of the pure liquids.
Kinetic phase diagram for CO oxidation on Pt(210): Pattern formation in the hysteresis and oscillation regions
Authors: Berdau Martin , Karpowicz Andrzej , Yelenin Georgii G. , Christmann Klaus , Block Jochen H.
Published: July 2002

The reactive behavior of catalytic CO oxidation on Pt(210) is studied by means of combined reaction rate measurements and photoelectron emission microscopy (PEEM). These methods allow an investigation of the phenomena at macroscopic and mesoscopic level, respectively. The external control parameters (flow rate, CO and oxygen partial pressures, surface temperature and scanning rates of pressure and temperature) are systematically varied to reveal various reactive regions in parameter space. The macroscopic measurements for a given temperature and flow rate (under isothermal conditions) show that lower pressures lead to a pronounced clockwise hysteresis in the production rate of CO2, while increasing pressures cause a systematic narrowing leading to a crossing of the two hysteresis branches into a region of counterclockwise hysteresis. A further pressure increase leads to macroscopic temporal oscillations. Mesoscopic spatiotemporal oscillations appear at the same conditions. The resulting macroscopic isothermal kinetic phase diagram exhibits a cross-shaped characteristic similar to that previously obtained for the Pd(110) surface. The mesoscopic lateral distribution of CO and oxygen adsorbed on the surface is monitored with the photoelectron emission microscope during the reaction at isothermal conditions and different constant oxygen pressures. The observed mesoscopic spatiotemporal patterns, such as islands, waves, target patterns and spirals, are correlated via the external control parameters with different regions in the macroscopic isothermal phase diagram. The results are compared with previous data of CO oxidation on other surfaces, like Pd(110) and Pt(110).
The doubly excited states of the H2 molecule
Authors: Sánchez I. , Martín F.
Published: July 2002

We report extensive calculations of energy positions and autoionization widths for the doubly excited states of H2 between the first and the second ionization thresholds. We present results for 60 doubly excited states of the Σg,u+1,3, Πg,u1,3, and Δg,u1,3 symmetries in the range of internuclear distances 0⩽R⩽5 a.u. The resonance parameters have been obtained using the Feshbach theory and an L2 representation of the non resonant continuum with B-spline functions. We discuss the main features of the energy correlation diagrams and the corresponding widths. We point out the existence of avoided crossings in the Franck–Condon region that significantly affect the autoionization properties of the doubly excited states. Our data are compared with previous results reported in the literature.
Dynamics of polybutadienes with different microstructures. 2. Dielectric response and comparisons with rheological behavior
Authors: Zorn Reiner , Mopsik Frederick I. , McKenna Gregory B. , Willner Lutz , Richter Dieter
Published: July 2002

A series of 1,2-1,4-polybutadienes with varying 1,2 (vinyl) content was investigated using time-domain dielectric spectroscopy. The time range was 10−5–300 s, which can be converted by Laplace transform to a frequency range of 10−3–6000 Hz. The samples were the same as those used in a previous rheological study from these laboratories. Therefore, a direct comparison of dielectric and mechanical responses was possible. Within experimental uncertainty, the α relaxation observed by both methods shows the same temperature dependence but there is an offset between the characteristic times of both methods, which increases with increasing vinyl content. This result can be qualitatively understood from the difference of the size of the dipolar groups, viz. cis and vinyl monomeric units, in the context of the DiMarzio–Bishop model. In addition, the question of time–temperature superposition was studied using the dielectric data. In the cases of a vinyl content ⩾0.53 no deviations from time–temperature superposition were detected. Only for the sample with the lowest vinyl content (0.07) does the attempt to construct a master curve from the dielectric loss data fail. In this case a fit with a combination of a Havriliak–Negami and a Cole–Cole functions suggests that this deviation from time–temperature superposition is an intrinsic feature of the α relaxation rather than an effect of its merging with the β relaxation. The absence of indications for such a deviation in the rheological study can be explained by the smaller frequency range of the latter. This stresses the necessity of a large dynamic range in experiments aimed at the examination of the time–temperature superposition principle.
Adiabatic electron affinities of small superhalogens: LiF2, LiCl2, NaF2, and NaCl2
Authors: Gutsev Gennady L. , Bartlett Rodney J. , Boldyrev Alexander I. , Simons Jack
Published: July 2002

Geometries and frequencies for the neutral MX2 and ionic MX2− species (M=Li, Na, and X=F, Cl) are studied by several theoretical methods: density functional theory (Becke-3-Lee-Yang-Parr) [DFT(B3LYP)], second-order many-body perturbation theory [MBPT(2)], and coupled-cluster with singles and doubles (CCSD). The geometries optimized at the CCSD/6-311+G(d) level are used in CCSD(T) calculations with a large atomic natural orbital basis to compute adiabatic electron affinities (EAad), which are found for LiF2, LiCl2, NaF2, and NaCl2 to be 5.45, 4.97, 5.12, and 4.69 eV, respectively. The highest EAs among all the atoms of the periodic table occur in the halogen atoms (fluorine, 3.40 eV; chlorine, 3.62 eV); therefore all four of these triatomic radicals are properly termed superhalogens. LiF2, LiCl2, NaF2, and NaCl2 are thermodynamically stable, and their dissociation energies computed at the CCSD with the noniterative inclusion of triples [CCSD(T)] level are 20.5, 24.9, 19.3, and 25.2 kcal/mol, respectively. LiF2−, LiCl2−, NaF2−, and NaCl2− are more stable than their neutral parents with CCSD(T) dissociation energies of 69.5, 58.7, 49.0, and 52.5 kcal/mol, respectively. The computed vertical electron detachment energies of LiF2−, LiCl2−, NaF2−, and NaCl2− are 6.51, 5.88, 6.18, and 5.77 eV, respectively, which are in nice agreement with the values calculated by Scheller and Cederbaum by the Green–Function method.
Ab initio geometry, quartic force field, and vibrational frequencies for P4
Authors: Joakim Persson B. , Taylor Peter R. , Lee Timothy J.
Published: July 2002

We have computed the equilibrium bond length and quartic force field for tetrahedral P4 using coupled-cluster methods and a variety of extended basis sets. Our calculations include the effect of correlating core, as well as valence, electrons. As with earlier P4 studies, our computed results show significant differences with current experimental estimates of both bond length and fundamental vibrational frequencies: possible reasons for this disagreement are discussed, with particular reference to the effects of the elevated temperatures used in gas-phase spectroscopic studies of P4. Our calculated endothermicity relative to two P2 molecules appears to be well converged with respect to correlation treatment and basis set, but again differs substantially from present experimental values, which we speculate are underestimates.
Spin-restricted Brueckner orbitals for coupled-cluster wavefunctions
Authors: Crawford T. Daniel , Lee Timothy J. , Handy Nicholas C. , Schaefer Henry F.
Published: July 2002

The theory of spin-restricted Brueckner orbitals for high-spin open-shell coupled-cluster wavefunctions is presented. The orbitals are based on single-excitation amplitudes constructed using the symmetric spin–orbital basis coupled-cluster method of Jayatilaka and Lee. It is shown how this approach may be easily implemented within existing open-shell coupled-cluster programs. The method’s performance is compared to conventional spin-unrestricted Brueckner orbitals for C̃ 2A2 NO2 and X̃ 2A2′ NO3, for which instabilities in the Hartree–Fock reference determinant cause serious difficulties for highly correlated wavefunctions.
Vector parametrization of the N-atom problem in quantum mechanics. II. Coupled-angular-momentum spectral representations for four-atom systems
Authors: Gatti Fabien , Iung Christophe , Menou Michel , Chapuisat Xavier
Published: July 2002

Within the framework of adapted coupled-angular-momentum spectral representations, the geometrical description of a four-atom molecular system by three Jacobi relative position vectors is shown to result in matrices representing the kinetic energy operator, prediagonalized to a very large extent. A fully diagonal representation is built for the angular (internal and rotational) part of the problem.
High level ab initio and density functional theory studies on methanol–water dimers and cyclic methanol(water)2 trimer
Authors: González Leticia , Mó Otilia , Yáñez Manuel
Published: July 2002

The methanol-water dimers and the potential energy surface of the cyclic methanol(water)2 trimer have been studied through the use of high-level ab initio calculations and density functional methods. The geometries have been optimized at the MP2/6-311+G(d,p) and B3LYP/6-311+G(d,p) levels of theory. The harmonic vibrational frequencies were obtained at the latter level. The final energies of the different local minima were calculated in the framework of the G2 and G2(MP2) theories. These values were compared with those obtained using the B3LYP/6-311+G(3df,2p) approach. At all the levels of theory considered the most stable conformer of methanol-water heterodimers corresponds to that in which water behaves as a hydrogen bond donor, in agreement with the most recent experimental evidences [P. A. Stockman et al., J. Chem. Phys. 107, 3782 (1997)]. The energy differences between the different conformers of the cyclic methanol(water)2 trimer are rather small, as well as the energy barriers connecting them. The global minimum corresponds to a conformer with the methyl group on one side of the O-O-O plane and the two free OH groups of the water molecules on the other side. Other stationary points associated with a systematic flipping of the methyl group and the free OH groups have been also located. These stationary points, which are transition states or saddle points of higher order, are very close in energy to the global minimum, indicating that the potential energy surface of the methanol(water)2 trimer is very flat and very similar to that reported before for water and methanol trimers. The calculated enthalpies of association are slightly smaller than those estimated for methanol trimers. The most stable trimer has three different O–H donor stretching frequencies, showing that the HB in which the methanol moiety behaves as a HB donor is the weakest one. Cooperative effects are significant. They are reflected in larger frequency shifts, greater lengthening of the O–H bonds, and shorter O⋯O distances than in (water)2 and methanol-water dimers. The estimated additive interaction energy is also significantly large.
Analytical derivatives for geometry optimization in solvation continuum models. I. Theory
Authors: Cancès E. , Mennucci B.
Published: July 2002

We report in this article a way to compute analytical derivatives for geometry optimization in solvation continuum models. This method can be applied to Molecular Mechanics as well as Quantum Chemistry calculations and is both simpler to implement and more powerful than other derivation methods used so far. Extensions to the cases when the solvent is either an ionic solution described by the linearized Poisson–Boltzmann equation or an anisotropic medium with a tensorial dielectric permittivity are discussed.
Ab initio investigation of the atmospheric molecule bromine nitrate: Equilibrium structure, vibrational spectrum, and heat of formation
Authors: Parthiban Srinivasan , Lee Timothy J.
Published: July 2002

The equilibrium structure, dipole moment, harmonic vibrational frequencies, and infrared intensities of BrONO2 are determined using the CCSD(T) (singles and doubles coupled-cluster theory plus a perturbational estimate of the effects of connected triple excitations) electronic structure method in conjunction with a TZ2P (triple ζ double polarized) basis set. The heat of formation of BrONO2 is predicted (10.1 kcal/mol at 298.15 K) using isodesmic reactions involving ClONO2, HONO2, HOBr, H2O, and HOCl. In addition, we draw attention to the importance of the reaction of O3 with BrONO2 in relation to ozone layer depletion and stratospheric reservoirs of bromine.
Structures and stability of hydrated clusters of hydrogen chloride, HCl(H2O)n, n=1–5
Authors: Re Suyong , Osamura Yoshihiro , Suzuki Youhei , Schaefer Henry F.
Published: July 2002

The molecular structures of the hydrated clusters of the HCl molecule, HCl(H2O)n, n=1–5, are examined by employing density functional molecular orbital methods. The most stable structures of the n=1–3 clusters are found to be of the proton nontransferred type. In the case of the n=4 cluster, the proton nontransferred and proton transferred structures have nearly similar energies. There are several stable isomers for the n=5 case and the structures of these isomers are found to be all proton transferred. The relative stabilities of the direct ion-pair H+Cl−(H2O)n and the indirect ion-pair H3O+(H2O)n−1Cl− are discussed in conjunction with their structures. The prediction of the IR spectra of the stable HCl(H2O)n clusters clearly indicate the large red-shifts of the H–Cl stretching and hydrogen-bonded O–H stretching frequencies.
An accurate ab initio HOCl potential energy surface, vibrational and rotational calculations, and comparison with experiment
Authors: Skokov Sergei , Peterson Kirk A. , Bowman Joel M.
Published: July 2002

Accurate ab initio multireference configuration interaction (CI) calculations with large correlation-consistent basis sets are performed for HOCl. After extrapolation to the complete basis set limit, the ab initio data are precisely fit to give a semiglobal three-dimensional potential energy surface to describe HOCl→Cl+OH from high overtone excitation of the OH-stretch. The average absolute deviation between the ab initio and fitted energies is 4.2 cm−1 for energies up to 60 kcal/mol relative to the HOCl minimum. Vibrational energies of HOCl including the six overtones of the OH-stretch are computed using a vibrational-Cl method on the fitted potential and also on a slightly adjusted potential. Near-spectroscopic accuracy is obtained using the adjusted potential; the average absolute deviation between theory and experiment for 19 experimentally reported states is 4.8 cm−1. Very good agreement with experiment is also obtained for numerous rotational energies for the ground vibrational state, the ClO-stretch fundamental, and the fifth overtone of the OH-stretch.
Why semilocal functionals work: Accuracy of the on-top pair density and importance of system averaging
Authors: Burke Kieron , Perdew John P. , Ernzerhof Matthias
Published: July 2002

Gradient-corrected density functionals provide a common tool for electronic structure calculations in quantum chemistry and condensed matter physics. This article explains why local and semilocal approximations work for the exchange-correlation energy. We demonstrate the high accuracy of the local spin-density (LSD) approximation for the on-top pair density, which provides the missing link between real atoms and molecules and the uniform electron gas. Special attention is devoted to the leading correction to exchange in the high-density (or weakly correlated) limit. We give an improved analytic expression for the on-top pair density in the uniform electron gas, calculating its spin-polarization dependence exactly in the high-density limit. We find the exact form of the gradient expansion for the on-top pair density, using Levy’s scaling of the interacting wave function. We also discuss the importance of system averaging, which unweights spatial regions where the density varies most rapidly. We show how the depth of the on-top hole correlates with the degree of locality of the exchange-correlation energy. Finally, we discuss how well fully nonlocal approximations (weighted-density, self-interaction correction, and hybrid-exchange) reproduce the on-top hole.
Theory of nonstationary activated rate processes: Nonexponential kinetics
Authors: Chaudhuri Jyotipratim Ray , Gangopadhyay Gautam , Ray Deb Shankar
Published: July 2002

We have explored a simple microscopic model to simulate a thermally activated rate process where the associated bath which comprises a set of relaxing modes is not in an equilibrium state. The model captures some of the essential features of non-Markovian Langevin dynamics with a fluctuating barrier. Making use of the Fokker-Planck description, we calculate the barrier dynamics in the steady-state and nonstationary regimes. The Kramers-Grote-Hynes reactive frequency has been computed in closed form in the steady state to illustrate the strong dependence of the dynamic coupling of the system with the relaxing modes. The influence of nonequilibrium excitation of the bath modes and its relaxation on the kinetics of activation of the system mode are demonstrated. We derive the dressed time-dependent Kramers rate in the nonstationary regime in closed analytical form which exhibits strong nonexponential kinetics of the reaction coordinate. The feature can be identified as a typical non-Markovian dynamical effect.
Positron and positronium chemistry by quantum Monte Carlo. III. Ground state of [OH,Ps], [CH,Ps], and [NH2,Ps] complexes
Authors: Bressanini Dario , Mella Massimo , Morosi Gabriele
Published: July 2002

The stability of the [OH,Ps], [CH,Ps] and [NH2,Ps] complexes, where Ps = (e+,e−), have been studied using Monte Carlo techniques and explicitly correlated trial wave functions. For these systems we have computed the ground state energy values using both variational Monte Carlo and fixed node diffusion Monte Carlo methods. Diffusion Monte Carlo results allow us to predict [OH,Ps] and [CH,Ps] systems to be stable against positron and Positronium (Ps) loss. As far as [NH2,Ps] is concerned, its small value of the Binding Energy (BE) does not allow us to conclude about its stability: this is discussed qualitatively including zero point motion of the atoms in the complex.
Core hole effect in NEXAFS spectroscopy of polycyclic aromatic hydrocarbons: Benzene, chrysene, perylene, and coronene
Authors: Oji Hiroshi , Mitsumoto Ryuichi , Ito Eisuke , Ishii Hisao , Ouchi Yukio , Seki Kazuhiko , Yokoyama Toshihiko , Ohta Toshiaki , Kosugi Nobuhiro
Published: July 2002

Carbon K-edge NEXAFS spectra of chrysene, perylene, and coronene were measured using synchrotron radiation, and the results were analyzed by ab initio molecular orbital (MO) calculations. The spectra do not agree well with the calculated density of unoccupied states (DOUS), indicating significant core-hole effect leading to deviation of the NEXAFS spectra from the DOUS. On the other hand, the observed spectra were well simulated by theoretical calculations taking this effect into account by the improved virtual orbital method. This allowed a detailed analysis of the core-hole effect, which affects both transition energy and intensity. During the course of this analysis, the core-hole effect in benzene was also analyzed. It was found that the magnitude of the core-hole effect is strongly dependent on the combination of the excited site and final vacant orbital. This dependence could be semiquantitatively explained in terms of the combination of the excitation site and the orbital patterns of the final state. The severe deviation of the NEXAFS spectra from the DOUS is ascribed mainly to such variation of the core-hole effect, with additional effect by the site-dependent core ionization energy. In some cases, even the transitions to the same vacant orbital contribute to different spectral features, or an apparently single spectral feature consists of transitions to different vacant orbitals. Thus direct information about DOUS of a molecule with such inequivalent sites is not obtainable from the NEXAFS spectrum, although smaller deviation is suggested for other systems with single site, such as the C atoms in C60 and graphite, and the N atoms in Zn tetraphenyl porphyrin.
Spin–orbit effects in quantum mechanical rate constant calculations for the F+H2→HF+H reaction
Authors: Aoiz F. J. , Bañares L. , Castillo J. F.
Published: July 2002

Exact and approximate quantum mechanical calculations of reaction probabilities and cumulative reaction probabilities have been carried out for the F+H2 reaction on the ab initio adiabatic potential energy surfaces by Stark and Werner (SW) and by Hartke, Stark, and Werner (HSW), the latter including spin–orbit corrections in the entrance channel. These data have been employed to obtain thermal rate constants for the title reaction in the temperature range 200–700 K. The exact and approximate results have been compared with experimental determinations and previous theoretical predictions. In particular, the reaction probabilities obtained on the HSW surface are found to be in very good agreement with recent calculations by Alexander et al. [J. Chem. Phys. 109, 5710 (1998)] based on the exact treatment of spin–orbit and Coriolis coupling for this system. However, the rate constants calculated on the HSW PES are systematically lower than the experimental values, which indicates that the height of the adiabatic potential energy surface is too high. Furthermore, an estimate of cross sections from the reaction probabilities calculated by Alexander et al. shows that the contribution to the low temperature rate constants from spin–orbit excited F(2P1/2) atoms through nonadiabatic channels is very small and, thus, nonadiabatic effects are not sufficient to bring the calculated rate constants to a better agreement with the experimental measurements.
A new mechanism for penetrant diffusion in amorphous polymers: Molecular dynamics simulations of phenol diffusion in bisphenol-A-polycarbonate
Authors: Hahn Oliver , Mooney Damian A. , Müller-Plathe Florian , Kremer Kurt
Published: July 2002

Atomistic molecular dynamics simulations are performed to analyze the diffusion of phenol molecules in a bisphenol-A-polycarbonate melt in the zero concentration limit for the temperature range from 500 K to 640 K. The transition from a hopping diffusion to a continuous diffusion is observed. Visualization of the diffusion process reveals a strong coupling between the polymer dynamics, i.e., size and shape fluctuations of the pore space and the hopping of the penetrant. Proper equilibration of the melt has been ensured by application of a novel multiscale simulation approach.
Chain-length dependence of melting of n-alcohol monolayers adsorbed on graphite: n-hexanol, n-heptanol, n-octanol, and n-nonanol
Authors: Morishige Kunimitsu , Kato Takako
Published: July 2002

The structures and melting of n-hexanol, n-heptanol, n-octanol, and n-nonanol monolayers adsorbed on graphite have been studied by x-ray diffraction and compared with our previous results on n-alcohols with shorter chain length. All the monolayers take similar herringbone or related structures consisting of a pair of rows of the molecules in a head-to-head arrangement with OH groups close to each other. These monolayers, except for methanol and n-octanol, show an odd-even effect of carbon number on the melting behavior. The complete monolayers of n-alcohols with odd number of carbon atoms melt via two-dimensional (2D) smectic mesophases, while those of the even-numbered series melt directly into liquids. The odd-even effect on melting is associated with the difference in packing of the terminal CH3. Calculations of a diffraction pattern based on the Debye formula strongly suggest that the 2D smectic mesophase consists of random stacking of a pair of the molecular rows formed by hydrogen bonding and thus is a 2D smectic glass rather than a 2D smectic liquid crystal. The melting mechanism changes for n-octanol and n-nonanol on graphite.
A new, fully ab initio investigation of the ArNO(X 2Π) system. II. Bound states of the Ar–NO complex
Authors: Alexander Millard H.
Published: July 2002

We report an investigation of bound states of the ArNO complex, based on new coupled-cluster [CCSD(T)] ab initio potential energy surfaces. The position and relative spacing of the lowest bend–stretch levels of the complex are found to be in noticeable better agreement with available experimental results than the predictions based on earlier ArNO PES’s.
Nucleation at high pressure. I. Theoretical considerations
Authors: Luijten C. C. M. , van Dongen M. E. H.
Published: July 2002

A theoretical approach is presented that accounts for the influence of high pressure background gases on the vapor-to-liquid nucleation process. The key idea is to treat the carrier gas pressure as a perturbation parameter that modifies the properties of the nucleating substance. Two important mechanisms are identified in this respect: With increasing carrier gas pressure, the saturated vapor density tends to increase (enhancement effect), whereas the surface tension generally decreases. Several routes to obtain data for these pressure effects are outlined, in particular for the vapor–gas mixtures that have been studied experimentally. (The results of these expansion wave tube experiments are presented in Paper II of this paper [J. Chem. Phys. 111, 8535 (1999), following paper.]) Using classical nucleation theory, a criterion is then derived for the “pressure perturbation” approach to be valid: xgeq≪(S−1)/S, where xgeq is the carrier gas solubility in the liquid phase, and S is the supersaturation ratio. For the semiphenomenological Kalikmanov–Van Dongen model, the implications of the enhancement effect and surface tension decrease are briefly discussed. We also illustrate how these two effects can be obtained from (binary) density functional theory. Results of the latter for a mixture of Lennard-Jones particles are presented, with potential parameters that are characteristic for n-hexane with several carrier gases.
Experimental and theoretical study of line mixing in methane spectra. III. The Q branch of the Raman ν1 band
Authors: Pieroni D. , Hartmann J.-M. , Chaussard F. , Michaut X. , Gabard T. , Saint-Loup R. , Berger H. , Champion J.-P.
Published: July 2002

The shape of the ν1 Raman Q branch of CH4 perturbed by Ar and He at room temperature has been studied. Stimulated Raman spectroscopy (SRS) experiments have been made in the 2915–2918 cm−1 spectral region for total pressures from 0.4 to 70 atm and mixtures of ≈5% CH4 with He and Ar. Analysis of the spectra demonstrates that the shape of the Q branch is significantly influenced by line mixing and much narrower than what is predicted by the addition of individual line profiles. For the first time, a model is proposed for the calculation and analysis of the effects of collisions on the considered spectra. In this approach, the rotational part of the relaxation matrix is constructed, with no adjustable parameter, starting from semiclassical state-to-state rates. Two empirical constants which account for the shift and broadening of the branch due to vibrational effects are introduced and their values are determined from fits of measured spectra. Comparisons between measurements and results computed with and without the inclusion of line mixing are made. Although not perfect, our model satisfactory accounts for most effects of pressure at low densities, where rotational transfers are dominant, as well as at high densities, where the profile is strongly influenced by vibrational contributions. It is shown that collisions with He and Ar lead to different behaviors at elevated pressure. The influence of the perturbation introduced by the Fermi coupling between the ν1 and ν2+ν4 levels is discussed and the rotational and vibrational contributions to the spectral shape are pointed out.
Rovibrational Hamiltonians for general polyatomic molecules in spherical polar parametrization. II. Nonorthogonal descriptions of internal molecular geometry
Authors: Mladenović Mirjana
Published: July 2002

A rovibrational kinetic energy operator for a two-vector embedded reference frame is derived for general polyatomic molecules of arbitrary structure. New compact expressions for T̂ in terms of angular momentum and linear momentum vector operators are explicitly given for tri-, tetra-, and pentatomic molecules. The necessary actions for the evaluation of kinetic energy matrix elements in a standard rotation-angular basis are summarized. The structure of kinetic energy operators in the body-fixed formulation is explained with the help of Cartesian components of angular momentum and linear momentum vector operators. Kinetic energy operators for tetratomic molecules in a bisector embedded body-fixed formulation and for sequentially bonded pentatomic molecules are derived to show how to use our general result for diverse coordinate systems.
A study of water clusters using the effective fragment potential and Monte Carlo simulated annealing
Authors: Day Paul N. , Pachter Ruth , Gordon Mark S. , Merrill Grant N.
Published: July 2002

Simulated annealing methods have been used with the effective fragment potential to locate the lowest energy structures for the water clusters (H2O)n with n=6, 8, 10, 12, 14, 16, 18, and 20. The most successful method uses a local minimization on each Monte Carlo step. The effective fragment potential method yielded interaction energies in excellent agreement with those calculated at the ab initio Hartree–Fock level and was quite successful at predicting the same energy ordering as the higher-level perturbation theory and coupled cluster methods. Analysis of the molecular interaction energies in terms of its electrostatic, polarization, and exchange-repulsion/charge-transfer components reveals that the electrostatic contribution is the dominant term in determining the energy ordering of the minima on the (H2O)n potential energy surfaces, but that differences in the polarization and repulsion components can be important in some cases.
Tuning surface reactivity by in situ surface nanostructuring
Authors: Costantini G. , Buatier de Mongeot F. , Rusponi S. , Boragno C. , Valbusa U. , Vattuone L. , Burghaus U. , Savio L. , Rocca M.
Published: July 2002

As recently demonstrated, the morphology of a surface can be modified on the mesoscopic scale by ion sputtering. Here we show by microscopy and spectroscopy that the chemical properties of the surface are strongly affected by nanostructuring and that surface reactivity can be tuned by changing surface morphology. For the otherwise inert Ag(001) surface significant O2 dissociation takes place on the nanostructured surface, thus allowing us to control the relative coverage of admolecules and adatoms. The dissociation probability is determined by the experimentally tunable density of kinks.
Photodissociation dynamics of propyne at 157 nm
Authors: Harich S. , Lin J. J. , Lee Y. T. , Yang X.
Published: July 2002

Photodissociation of propyne at 157 nm has been investigated using photofragment translational spectroscopy. Detailed investigation of various photofragments from the deuterated compounds CD3CCH and CH3CCD, as well as the unlabeled propyne provides a uniquely clear picture of an inherently complex process. Hydrogen atom elimination processes from both the CH3 group and the C≡C–H group have been clearly observed. H atom elimination from the methyl group appears to be a single dynamical process, while ethynyl H elimination shows two distinctive dynamical pathways with a ratio of 0.30 (fast): 0.43 (slow). The relative contribution of the atomic hydrogen elimination from the two different sites was determined to be 0.73 (ethynyl): 0.27 (methyl). Molecular hydrogen elimination processes have also been observed, but with a much smaller yield compared to the atomic hydrogen elimination (1:9.6). Comparison of the H2 HD and D2 photoproducts from various deuterated propyne molecules shows that the molecular hydrogen elimination process is not sensitive to the origin of the two hydrogen atoms. This implies that scrambling (or isomerization) of H atoms is important prior to dissociation at 157 nm excitation of propyne. Two different C–C bond breaking processes have also been observed; one process breaks the C–C single bond to form methyl and C2H radicals, while the other process forms CH2 and C2H2. The existence of the CH2 channel also indicates that isomerization of propyne is significant prior to dissociation. The relative branching ratio of these two channels is estimated to be 2.2:1 for CH3 and CH2 formation, respectively.
Mechanism and rate constant of the reaction of atomic hydrogen with propyne
Authors: Wang Baoshan , Hou Hua , Gu Yueshu
Published: July 2002

The potential energy surface for the reaction of atomic hydrogen with propyne has been studied at the G3//UB3LYP/6-31G(d) level of theory. Three reaction entrances were revealed, namely, terminal addition, nonterminal addition, and direct H-abstraction, leading to CH3CCH2, CH3CHCH, and H2+C3H3, respectively. The respective activation barriers are 1.7, 3.9, and 8.4 kcal/mol. The CH3-extrusion from CH3CHCH forms C2H2 via a barrier of about 32 kcal/mol. Several H-shift paths along the CCC skeleton were also examined for three C3H5 isomers. Multichannel RRKM and TST calculations have been carried out for the total and individual rate constants over a wide range of temperatures and pressures. The total rate constants possess both positive temperature dependence and typical “S” shaped fall-off behavior. At atmospheric pressure, the collisional stabilization of the initial adducts dominates the H+CH3CCH reaction at temperatures lower than 500 K, and at T>1000 K, CH3 and C2H2 are the major products. Moreover, the direct H-abstraction channel also contributes significantly to the overall reaction. The theoretical results are compared with those of previous studies.
Multireference coupled-cluster calculations on the energy of activation in the automerization of cyclobutadiene: Assessment of the state-specific multireference Brillouin–Wigner theory
Authors: Sancho-Garcı́a Juan Carlos , Pittner Jiřı́ , Čársky Petr , Hubač Ivan
Published: July 2002

Hilbert-space state-universal multireference coupled-cluster (MR CC) data on cyclobutadiene [A. Balková and R. J. Bartlett, J. Chem. Phys. 101, 8972 (1994)] were used as a benchmark for testing our recently developed state-specific (single-root) multireference Brillouin-Wigner coupled-cluster (MR BWCC) theory. For the energy of activation in the automerization of cyclobutadiene (i.e., the energy difference between the square and rectangular structures) at the CCSD/[3s2p1d/2s] level of theory, our MR BWCCSD method gives the value of 6.2 kcal/mol, compared to 6.5 kcal/mol given by MR CCSD. With the cc-pVDZ and cc-pVTZ/cc-pVDZ basis sets, the MR BWCCSD activation barrier is 6.4 and 7.0 kcal/mol, respectively. The effect of the triple excitations [in MR CCSD(T)] and of the frozen core approximation were estimated previously to be below 0.1 kcal/mol and in the opposite direction. This shows the way of how to arrive at a more accurate automerization barrier in future calculations: extension of the basis set seems to be more important than going beyond the CCSD(T) or MB BWCCSD level of the theory.
Helium atom scattering from isolated CO molecules on a Pt(111) surface: Experiment versus close-coupling calculations for a realistic He–CO potential
Authors: Choi B. H. , Graham A. P. , Tang K. T. , Toennies J. P.
Published: July 2002

The angular dependence of the elastic scattering of helium atoms from a low coverage (ΘCO=0.03) of randomly distributed isolated CO molecules adsorbed on a Pt(111) surface has been measured over a wide range of incident helium atom beam energies between Ei=5.25 and 62.1 meV. The results are compared with a close-coupling theory for a realistic He–CO potential. From a best fit of the experimental results the distance of the molecule from the surface and a modified gas phase He–CO potential are obtained.
A remarkable property of spherical harmonics
Authors: Beig R.
Published: July 2002

It is shown that a certain bilinear expression on the space of spherical harmonics of equal angular momentum is constant on the two-sphere.
The Trotter–Lie product formula for Gibbs semigroups
Authors: Zagrebnov V. A.
Published: July 2002

The trace-norm (and Hilbert–Schmidt-norm) convergence of the Trotter–Lie formula is proved for some classes of Gibbs semigroups.
Bispinor geometry for even-dimensional space-time
Authors: Crawford J. P.
Published: July 2002

The geometric properties of Dirac spinor fields defined over even-dimensional space-time are explored with the aim of formulating the associated nonlinear sigma models. A spinor field Ψ may be uniquely reconstructed from the real bispinor densities ρi=Ψ̄ΓiΨ, apart from an overall phase, so that the ρi constitute an alternate representation of the physical information contained in Ψ. For space-time of dimension N=2n, the corresponding Dirac spinor has D=2n complex components, and the bispinor densities satisfy a system of (D−1)2 homogeneous quadratic algebraic equations. The basis elements of the Clifford algebra {Γi} span a D2=2N -dimensional space whose Cartan metric is flat pseudo-Riemannian; the bispinor densities reside in the (2D−1)-dimensional curved subspace induced as an embedding by the algebraic constraints. The explicit geometric structure of the bispinor spaces are examined and found to be generalizations of Robertson–Walker space. In particular, the line element may be written as dS2=D dσ 2+σ 2 dΩ2, where σ=Ψ̄Ψ is the scalar density and dΩ is the line element for the homogeneous space: SU(D/2,D/2)/S(U(1)⊗U(D/2−1,D/2)).
On a characterization of classical and nonclassical probabilities
Authors: Beltrametti Enrico G. , Maczynski Maciej J.
Published: July 2002

In this paper a comparison of various approaches to the problem of characterization of probabilities is presented. The notion of multidimensional probability or S-probability is introduced and it is shown that this notion can be profitably used for characterization of probability systems. The main results are given in Theorems 3 and 4. In Theorem 3 the conditions are considered under which a system of S-probabilities admits a model on a generalized event space, while in Theorem 4 the conditions under which the system admits a classical model are considered. An interpretation for S-probabilities is discussed and it is shown that Theorems 3 and 4 allow one to derive the logical structure of events from the observed probabilities.
Killing tensors in two-dimensional space-times with applications to cosmology
Authors: Rosquist Kjell , Uggla Claes
Published: July 2002

The existence of Killing vectors and Killing tensors in two-dimensional space-times is discussed. The corresponding mechanical problem having a potential with two exponential terms is analyzed in detail and a number of cases admitting Killing tensors are given. The Killing tensors give rise to new exact solutions in perfect fluid Bianchi and Kantowski–Sachs cosmologies as well as in inflationary models with a scalar field source.
Complex Wishart matrices and conductance in mesoscopic systems: Exact results
Authors: Forrester P. J. , Hughes T. D.
Published: July 2002

It is noted that the hypothesis of independent random complex elements for the off diagonal blocks (say, M2 and M3) of the transfer matrix describing conductance in a mesoscopic wire allows the eigenvalue distribution of the matrix product M3°M3 (or M2°M2) to be computed exactly. Exact expressions, in terms of double Wronskian and Toeplitz determinants, are derived for the distribution of the smallest and second smallest eigenvalue of this and similar random matrix products.
The Heisenberg dynamics of spin systems: A quasi*-algebras approach
Authors: Bagarello F. , Trapani C.
Published: July 2002

The problem of the existence of the thermodynamical limit of the algebraic dynamics for a class of spin systems is considered in the framework of a generalized algebraic approach in terms of a special class of quasi*-algebras, called CQ*-algebras. Physical applications to (almost) mean-field models and to bubble models are discussed.