bluuebunny/crossref_metadata_2025 huggingface dataset:
Benzene–argon triplet intermolecular potential energy surface
Authors: López Cacheiro Javier , Fernández Berta , Koch Henrik , Makarewicz Jan , Hald Kasper , Jørgensen Poul
Published: August 2003

The benzene–Ar lowest energy triplet state T1 intermolecular potential energy surface is evaluated using coupled cluster methods and the aug-cc-pVDZ basis set extended with a set of 3s3p2d1f1g midbond functions. This surface is characterized by an absolute minimum of −392.5 cm−1, where the argon atom is located on the benzene C6 axis at a distance of 3.5153 Å, and has a general shape very close to the ground state S0 and the first singlet S1 excited state surfaces. Using the T1 potential, the intermolecular vibrational energy levels were evaluated and the results compared to those for the ground S0 and the excited S1 states. The calculated fundamental frequencies for the bending and the stretching modes are lower than those in the S1 state. The calculated data for the T1 state is expected to have the same accuracy as previously calculated data for the S1 state.
Unified analytical treatment of multicenter multielectron integrals of central and noncentral interaction potentials over Slater orbitals using Ψα-ETOs
Authors: Guseinov I. I.
Published: August 2003

Using expansion formulas for the translation of Slater-type orbitals (STOs) obtained with the help of complete orthonormal sets of Ψα-exponential type orbitals the addition theorems for STOs and interaction potentials are established. These addition theorems enable us to evaluate the multicenter t-electron integrals which arise in the solution of N-electron atomic and molecular problem (2⩽t⩽N) when a Hylleraas approximation is employed in Hartree–Fock–Roothaan theory. The relationships obtained are valid for the arbitrary location, quantum numbers, and screening constants of STOs.
Oxidation of the 3×3 6H-SiC (0001) adatom cluster: A periodic density functional theory and dynamic rocking beam analysis
Authors: Xie Xianning , Loh Kian Ping , Yakolev Nikolai , Yang S. W. , Wu Ping
Published: August 2003

The atomic reconstruction and the adsorption of oxygen on 6H-SiC (0001) surfaces have been investigated by reflection high energy electron diffraction (RHEED) dynamic rocking beam analysis. The various possible chemisorption states on the surface following the adsorption of one, two, and three oxygen molecules have been studied using periodic density functional theory. RHEED rocking beam analysis provided insights into the atomic structure of 6H-SiC (0001) 3×3 in terms of vertical and lateral displacements, as well as the initial chemisorption state of oxygen on the reconstructed surface.
Collision dynamics of large water clusters on graphite
Authors: Tomsic Anna , Andersson Patrik U. , Marković Nikola , Pettersson Jan B. C.
Published: August 2003

The emission of neutral cluster fragments during collisions of large water clusters with graphite surfaces has been investigated using molecular beam techniques. Water clusters with an average size of up to 1.4⋅104 molecules per cluster collide with the surface with a velocity of 1380 ms−1. Angular distributions for emitted large fragments are shifted towards the tangential direction and become increasingly narrow with increasing fragment size. The kinetic energy in the surface normal direction is efficiently transferred to internal degrees of freedom and to surface modes, while the momentum parallel to the surface plane is less affected by the surface interaction. Both a direct scattering channel and an emission channel mediated by cluster evaporation are concluded to be of importance for the collision outcome. The results for the evaporation-mediated emission channel agree well with previous experimental investigations and with recent molecular dynamics simulations, and the observations regarding the direct scattering channel qualitatively agree with the dynamics observed for macroscopic particles colliding with surfaces.
Reaction intermediates in the oxidation of methanol on a Pt(111)–(2×2)O surface
Authors: Liu Zhengxin , Sawada Takeshi , Takagi Noriaki , Watanabe Kazuya , Matsumoto Yoshiyasu
Published: August 2003

The oxidation of methanol on a Pt(111)–(2×2)O surface has been investigated by infrared reflection absorption spectroscopy and temperature-programed desorption. Methanol is dehydrogenated to produce methoxy species in the annealing temperature range from 130 to 170 K. Above 170 K, the reaction proceeds differently, depending on methanol coverage. At the saturation coverage, methanol adsorbates partly desorb molecularly and partly react with precovered oxygen atoms to produce CO, H2, and H2O. No detectable formaldehyde or formate is formed. In contrast, at submonolayer coverages, methoxy species is dehydrogenated to yield formaldehyde at ∼180 K and further oxidized to formate at ∼200 K. Formate is decomposed by 300 K. Defect sites such as steps are not relevant to the formation of the intermediates. When CO is coadsorbed on the surface, it destabilizes the reaction intermediates. The destabilization by coadsorbed CO makes the reaction intermediates short lived so as not to be detectable at high initial coverages of methanol.
Potential model for tetrathiafulvalene based on inelastic neutron scattering and Raman spectra
Authors: Burns Gary R. , Favier Frederic , Jones Deborah J. , Rozière Jacques , Kearley Gordon J.
Published: August 2003

Tetrathiafulvalene (TTF) is a key molecule in the family of charge-transfer salts of interest for their novel transport properties. The correspondence between transport property and electron-molecular vibration coupling requires that we understand the phonon density of states for these materials. Here we report the results of a low temperature study of neutral TTF using both inelastic neutron scattering spectroscopy and Raman spectroscopy. The new data has been interpreted using the CLIMAX code to calculate the neutron spectral profile which has also been calculated based on a set of force constants derived using Density Functional Theory to calculate the ground state energy of neutral TTF. The two approaches lead to a concordant set of harmonic force constants for the in-plane and out-of-plane normal modes of TTF.
Micelle formation in a dilute solution of block copolymers with a polyelectrolyte block complexed with oppositely charged linear chains
Authors: Kramarenko E. Yu. , Khokhlov A. R. , Reineker P.
Published: August 2003

The theory of micelle formation in a dilute solution mixture of polyelectrolyte/neutral diblock copolymers and oppositely charged linear chains has been developed. The core of the micelles is formed by the hydrophobic monomer units of polyelectrolyte complexes between the oppositely charged polyions while the hydrophilic uncharged blocks of the block copolymers comprise the micellar corona. The process of the formation of polyelectrolyte complexes and micelles in the solution has been described as sets of association–dissociation reactions with certain chemical constants and the concentration distribution of micellar aggregates of a given composition has been calculated for various concentrations of linear chains and block copolymer macromolecules in the solution. It was shown that the micelles are formed by nearly fully neutralized block copolymers, so that the micellar charge is close to zero. The aggregation number of the micelles and their charge slightly depend on the concentration of the block copolymers as well as the linear chains in the solution. Micelles in the solution coexist with single block copolymer chain/linear polymer complexes of different composition.
Charge and bonding redistribution in octatetraene driven by a strong laser field: Time-dependent Hartree–Fock simulation
Authors: Suzuki Masato , Mukamel Shaul
Published: August 2003

The time evolution of the reduced single electron density matrix for the π electrons in conjugated molecules in an intense laser field is calculated by numerically integrating the time-dependent Hartree–Fock equations for the Pariser–Parr–Pople model Hamiltonian. We study the field frequency and intensity dependence of the π-electron response, the charge fluctuations with respect to the ground state, and the molecular polarization in unsubstituted and acceptor–donor substituted octatetraene, where we also examine the interaction between a permanent electric dipole moment and the laser field. Our results demonstrate the origin of large nonperturbative changes in the π-electronic charges and bond orders (diagonal and off-diagonal density matrix elements, respectively) induced by a strong laser field.
Distributed dispersion: A new approach
Authors: Williams Greg J. , Stone Anthony J.
Published: August 2003

Distributed polarizability and dispersion models can be constructed by analyzing the response, at each point in an array surrounding the molecule, to the field due to a point charge at each point of the array in turn, in an extension of the method described by Dehez et al. [Chem. Phys. Lett. 338, 180 (2001)]. Anisotropic distributed polarizabilities and anisotropic distributed C6 dispersion coefficients have been calculated for carbon dioxide, water, benzene and ethane.
Electric-field-pulse-assisted covalent immobilization of DNA in the nanosecond time scale
Authors: Fixe F. , Cabeça R. , Chu V. , Prazeres D. M. F. , Ferreira G. N. M. , Conde J. P.
Published: August 2003

A single square voltage pulse is used to achieve the selective covalent bonding of DNA probes to a functionalized thin-film surface (SiO2) on a plastic substrate. Significant covalent bonding occurs with only 100 ns of pulse duration, representing a decrease in immobilization time of more than ten orders of magnitude from the 2 h needed without electric field. A systematic study of the effect of the duration and rise/fall times of the voltage pulse on the immobilization of DNA probe molecules is presented. Successful hybridization of the DNA probes immobilized using this technique with a complementary target strand was achieved.
The performance of phase modulated heteronuclear dipolar decoupling schemes in fast magic-angle-spinning nuclear magnetic resonance experiments
Authors: De Paëpe Gaël , Lesage Anne , Emsley Lyndon
Published: August 2003

The experimental performance of the phase modulated decoupling schemes SDROOPY-1 and SPINAL64 is investigated for heteronuclear dipolar decoupling in solid-state magic-angle-spinning nuclear magnetic resonance (NMR), in comparison with existing decoupling methods. It is found that both sequences perform well over a range of spinning rates from 0 to 35 kHz, with linewidths that are virtually independent of the spinning rate. The signal intensity is found to diminish with increasing spinning rate, and this behavior is compared qualitatively with theory. It is also demonstrated that decoupling sequences strongly influence the refocused linewidths observed in solid-state NMR, and that SDROOPY-1 and SPINAL64 both yield particularly long dephasing times.
Isotope dependence of the O+O2 exchange reaction: Experiment and theory
Authors: Fleurat-Lessard Paul , Grebenshchikov S. Yu. , Schinke Reinhard , Janssen Christof , Krankowsky Dieter
Published: August 2003

The isotope dependence of the O+O2 exchange reaction is investigated by means of kinetic experiments and classical trajectory calculations on an accurate potential energy surface. The measurements confirm the previously reported negative temperature dependence and yield the rate coefficients for both the exothermic O18+16O2→18O16O+16O and the endothermic O16+18O2→16O18O+18O reaction between 233 and 353 K: k8=(3.4±0.6)×10−12 (300 K/T)1.1±0.5 cm3 s−1 and k6=(2.7±0.4)×10−12 (300 K/T)0.9±0.5 cm3 s−1. In addition, the ratio of these two rates, ℛ, has been measured with comparatively higher precision. It is 1.27±0.04 at 300 K and also shows a distinct negative temperature dependence. Four types of classical trajectory calculations are performed in order to interpret the experimental result. They differ by the way in which the quantum mechanical zero-point energy of the reactants and the differences of zero-point energies between reactants and products, ΔEZPE≈±22 cm−1, are phenomenologically incorporated. Only calculations which account for ΔEZPE are successful in reproducing the temperature dependence of ℛ. This emphasizes that ΔEZPE is an important factor in low-temperature ozone kinetics. At energies slightly above threshold a dynamical effect is found which additionally favors the exothermic over the endothermic reaction.
Measurement of the cross section for H+D2→HD(v′=3,j′=0)+D as a function of angle and energy
Authors: Ayers James D. , Pomerantz Andrew E. , Fernández-Alonso Félix , Ausfelder Florian , Bean Brian D. , Zare Richard N.
Published: August 2003

Scattering of the HD(v′=3,j′=0) product from the H+D2 reaction is measured as a function of angle and collision energy from 1.39 to 1.85 eV. The plot of the cross section vs angle and energy is believed to be the first fully experimental plot of its kind reported for this benchmark reaction. Changes in the differential cross section (DCS) are observed in this collision energy range, including a forward-scattering component that peaks at about 1.64 eV and is a strong function of collision energy. This feature has been assigned to result from a barrier resonance, but its full interpretation is presently unsettled. These changes in the DCS do not manifest themselves as variations in the integral cross section (ICS), which varies less than 25% over the energy range measured. Comparisons of the DCSs and the ICS with quantum mechanical calculations show quantitative agreement, although some aspects of the DCS near 1.54 eV are not fully satisfactory.
Formation of dipole-bound negative ions in Rydberg atom collisions: A signature
Authors: Parthasarathy R. , Suess L. , Liu Y. , Dunning F. B.
Published: August 2003

Collisional studies using velocity- and n-selected Rydberg atoms show that the properties of dipole-bound negative ions formed by electron transfer at low-to-intermediate n are not influenced by post-attachment interactions. The behavior contrasts that observed with targets that form valence-bound negative ions and provides a new signature pointing to the creation of dipole-bound negative ions.
Ultrafast relaxation dynamics of the one-dimensional molecular chain: The time-resolved spontaneous emission and exciton coherence
Authors: Tanaka Satoshi
Published: August 2003

Ultrafast relaxation dynamics of the photoexcited one-dimensional molecular chain coupled with the phonon field is theoretically investigated. The time evolution of the system is formulated by the time-dependent variational method with a coherent state trial function. The temporal changes of the exciton coherence size and the energy relaxation strongly depend on the exciton transfer, the exciton–phonon coupling strength, and the phonon bandwidth. The calculated time-resolved spontaneous emission spectrum well reflects the temporal change of the exciton coherence: The intensity and the peak position represent the coherence size and the energy dissipation. A cooperative spontaneous emission, super-radiance, periodically appears as long as the exciton coherence maintains over the entire system as a result of in-phase oscillation of phonon modes.
Ab initio simulation of H2S adsorption on the (100) surface of pyrite
Authors: Stirling András , Bernasconi Marco , Parrinello Michele
Published: August 2003

The adsorption properties of H2S on the (100) surface of pyrite have been studied by Car–Parrinello simulations. The predicted adsorption properties have been contrasted to those of water on the same surface. It was found that H2S prefers molecular adsorption on the surface iron sites while the dissociative adsorption is highly unfavorable. The binding of the H2S and HS− species to the Fe sites results from a coordinative covalent bond from the ligand sulfur atom. The adsorption energy at full coverage is much lower than at partial coverage due to steric repulsion among the adsorbates. As opposed to water adsorption, hydrogen bonding plays a marginal role in H2S adsorption.
Perfluorinated phthalocyanines for optical limiting: Evidence for the direct correlation between substituent electron withdrawing character and the nonlinear optical effect
Authors: Dini Danilo , Yang Guo Ying , Hanack Michael
Published: August 2003

The optical limiting performance exhibited by phthalocyanines substituted with electron withdrawing atoms such as F, results considerably enhanced with respect to the optical limiting effect produced by unsubstituted or differently substituted phthalocyanines. This enhancement is verified by comparing under analogous conditions the nonlinear optical transmission of hexadecafluorophthalocyaninato complexes containing TiIV, VIV, ZrIII, and InIII as coordination central atoms, and tetra- or octaalkylsubstituted phthalocyaninato complexes with the same central atoms by means of the Z-scan technique. The remarkable variations of the nonlinear transmittance of perfluorinated phthalocyanines in solution indicates the strong influence that electron-withdrawing groups exhibit upon the variations of the transition dipole moments involved in the electronic transition, which effectively limits the intense radiation.
Energetics, vibrational spectrum, and scanning tunneling microscopy images for the intermediate in water production reaction on Pt(111) from density functional calculations
Authors: Karlberg G. S. , Olsson F. E. , Persson M. , Wahnström G.
Published: August 2003

Using density functional theory, the nature of the overlayer formed by dissociation of water on an oxygen covered Pt(111) surface has been studied. This overlayer has been argued to be equivalent to the reaction intermediate in the low-temperature water production reaction on Pt(111). The overlayer is assumed to be formed through either of the two reactions, 2H2Oad+Oad→3OHad+Had, or 2H2Oad+Oad→2OHad+H2Oad. Based on experimental data the products of these two reactions are reduced to two adsorbate compositions labeled OH+H2O and 2OH+H. The two compositions are analyzed in terms of their energetics, vibrational spectra, work functions, and simulated scanning tunneling microscopy (STM) images. The OH+H2O composition is found to be 1.4 eV more stable per OH+H2O compared to the 2OH+H composition. Only the vibrational spectrum corresponding to the OH+H2O composition agrees well with experiments, which provides strong additional support for this composition of the overlayer. A large redshift of the water scissoring mode is revealed for this composition. We also find a significant difference between the work functions of the two compositions, which could be used to discriminate between the two compositions. In contrast, we find our simulated STM images of both compositions to be consistent with the observed STM images.
Transport properties of dimethyl sulfoxide aqueous solutions
Authors: Nieto-Draghi Carlos , Bonet Ávalos Josep , Rousseau Bernard
Published: August 2003

The nonideal behavior of the transport properties of water–dimethyl sulfoxide (DMSO) mixtures has been studied through equilibrium and nonequilibrium molecular dynamic simulations. The shear viscosity and thermal conductivity of the mixture has been analyzed and compared with available experimental data at ambient conditions. The enhancement of shear viscosity at molar fractions xW=0.65 of water has been quantitatively reproduced in our simulations. In agreement with this fact, we have found an increase in the rigidity of the system reflected by an increase in the decay time of the survival probability of the H bonds. In addition, we compute the tetrahedral order parameter of water molecules in the solution at different molar fractions. This parameter indicates a reduction in the local tetrahedral order of water when the solute concentration is increased, followed by a clear minimum at the equimolar concentration near the locus of the maximum density of the mixture, probably due to the formation of water–DMSO complexes. We have obtained the thermal conductivity of the mixture for the first time. This property also presents a peculiar minimum at xW=0.4, precisely in the region of the minimum of the order parameter. However, no experimental confirmation of our results is available.
Quantum dynamical correlations: Effective potential analytic continuation approach
Authors: Horikoshi Atsushi , Kinugawa Kenichi
Published: August 2003

We propose a new quantum dynamics method called the effective potential analytic continuation (EPAC) to calculate the real time quantum correlation functions at finite temperature. The method is based on the effective action formalism which includes the standard effective potential. The basic notions of the EPAC are presented for a one-dimensional double well system in comparison with the centroid molecular dynamics (CMD) and the exact real time quantum correlation function. It is shown that both the EPAC and the CMD well reproduce the exact short time behavior, while at longer time their results deviate from the exact one. The CMD correlation function damps rapidly with time because of ensemble dephasing. The EPAC correlation function, however, can reproduce the long time oscillation inherent in the quantum double well systems. It is also shown that the EPAC correlation function can be improved toward the exact correlation function by means of the higher order derivative expansion of the effective action.
Mode bifurcation by pouring water into a cup
Authors: Kitahata Hiroyuki , Yamada Akiko , Nakata Satoshi
Published: August 2003

Mode bifurcation was investigated when water was poured into a cup. Three modes, i.e., accumulation flow (mode I), scattering flow (mode II), and oscillatory flow (mode III), were regulated by the flow rate and the size of the tube through which the water was poured. When the tube was wide, the flow rate at bifurcation from mode I to mode II was different from that from mode II to mode I, i.e., hysteresis was observed. Mode III was observed only when the flow rate was increasing and the tube was narrow. The water hollow was an important factor in mode bifurcation. The behavior of the system was reproduced by a numerical simulation using differential equations that included the dynamics of two variables, the radius of the water hollow and the pressure exerted on the water hollow.
Numerical implementation of some reweighted path integral methods
Authors: Predescu Cristian , Sabo Dubravko , Doll J. D.
Published: August 2003

The reweighted random series techniques provide finite-dimensional approximations to the quantum density matrix of a physical system that have fast asymptotic convergence. We study two special reweighted techniques that are based upon the Lévy–Ciesielski and Wiener–Fourier series, respectively. In agreement with the theoretical predictions, we demonstrate by numerical examples that the asymptotic convergence of the two reweighted methods is cubic for smooth enough potentials. For each reweighted technique, we propose some minimalist quadrature techniques for the computation of the path averages. These quadrature techniques are designed to preserve the asymptotic convergence of the original methods.
Deficiency of the Coulomb-field approximation in the generalized Born model: An improved formula for Born radii evaluation
Authors: Grycuk Tomasz
Published: August 2003

The weaknesses of the widely used Coulomb-field (CF) approximation in the generalized Born model are discussed from the standpoint of the Kirkwood model of biopolymer electrostatics. It is shown that the effective Born radii are overestimated by up to a factor of 2 because of the CF approximation. The problem is made worse by the fact that these critical parameters are used not only to model the self-contributions to the free enthalpy of solvation but they also appear in the analytical function for a pairwise contribution. A new simple formula for the effective Born radii evaluation overcoming some limits of the CF approximation is proposed and then tested on a model of a biopolymer with nonspherical symmetry. Some conclusions regarding the pairwise contribution are also formulated.
Many-body interactions and the melting of colloidal crystals
Authors: Dobnikar J. , Chen Y. , Rzehak R. , von Grünberg H. H.
Published: August 2003

We study the melting behavior of charged colloidal crystals, using a simulation technique that combines a continuous mean-field Poisson–Boltzmann description for the microscopic electrolyte ions with a Brownian-dynamics simulation for the mesoscopic colloids. This technique ensures that many-body interactions among the colloids are fully taken into account, and thus allows us to investigate how many-body interactions affect the solid–liquid phase behavior of charged colloids. Using the Lindemann criterion, we determine the melting line in a phase-diagram spanned by the colloidal charge and the salt concentration. We compare our results to predictions based on the established description of colloidal suspensions in terms of pairwise additive Yukawa potentials, and find good agreement at high-salt, but not at low-salt concentration. Analyzing the effective pair-interaction between two colloids in a crystalline environment, we demonstrate that the difference in the melting behavior observed at low salt is due to many-body interactions. If the salt concentration is high, we find configuration-independent pair forces of perfect Yukawa form with effective charges and screening constants that are in good agreement with well-established theories. At low added salt, however, the pair forces are Yukawa-type only at short distances with effective parameters that depend on the analyzed colloidal configuration. At larger distances, the pair-forces decay to zero much faster than they would following a Yukawa force law. Based on these findings, we suggest a simple model potential for colloids in suspension which has the form of a Yukawa potential, truncated after the first coordination shell of a colloid in a crystal. Using this potential in a one-component simulation, we find a melting line that shows good agreement with the one derived from the full Poisson–Boltzmann–Brownian-dynamics simulation.
An analysis of molecular origin of vibrational energy transfer from solute to solvent based upon path integral influence functional theory
Authors: Mikami Taiji , Okazaki Susumu
Published: August 2003

Molecular process of vibrational energy relaxation of CN− ion in the aqueous solution has been investigated based upon path integral influence functional theory. In order to obtain a molecular-based picture, bath normal coordinates were retransformed to Cartesian coordinate, Eulerian coordinate, and intramolecular vibrational coordinate of the solvent molecule. Then, based upon this retransformation matrix, coupling of the solute with the normal modes may be assigned to the couplings with the solvent molecules. Further, with respect to two-phonon process which is dominant in the relaxation of the present system, the relaxation may be divided into single- and dual-molecular processes. We show that the single-molecular relaxation is dominant in the relaxation. Further, water molecules in the first hydration shell play an essential role in the relaxation, whereas the solvent molecules outside the first hydration shell make little contribution. The solvent molecules located in the direction of CN− bond axis were found to make great contribution to the relaxation.
Rotational dynamics of nondipolar probes in ethanols: How does the strength of the solute–solvent hydrogen bond impede molecular rotation?
Authors: Dutt G. B. , Ghanty T. K.
Published: August 2003

Rotational dynamics of two structurally similar nondipolar probes; 2,5-dimethyl-1,4-dioxo-3,6diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and 1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP) has been investigated in ethanol (EtOH) and 2,2,2-trifluoroethanol (TFE) in the temperature range 243–298 K in an attempt to understand how the strength of the solute–solvent hydrogen bond impedes molecular rotation. It has been observed that the reorientation times of DPP are slower compared to DMDPP by about a factor of 2 in EtOH and this factor is only 1.3–1.4 in TFE. Another interesting observation is that the viscosity normalized reorientation times of DPP at a given temperature are almost identical in EtOH and TFE, whereas those of DMDPP are slower by a factor of 1.5 in TFE compared to EtOH. These observations have been rationalized on the basis of hydrogen bond donating and hydrogen bond accepting abilities of the respective solute and the solvent. Further evidence for such a rationale has been provided with the aid of ab initio molecular orbital methods.
Calorimetric study of plastically crystalline o- and m-carboranes
Authors: Yamamuro Osamu , Hayashi Masayuki , Matsuo Takasuke , Lunkenheimer Peter
Published: August 2003

The heat capacities of o- and m-carboranes were measured with an adiabatic calorimeter in the temperature range 5–310 K. m-carborane underwent two phase transitions at 170 K and 284 K with transition entropies of 12.70 and 15.52 J K−1 mol−1. The total transition entropy agreed well with the theoretical value, indicating that m-carborane is ordered at 0 K. o-carborane also exhibited two phase transitions at 160 K and 275 K with transition entropies of 3.72 and 13.72 J K−1 mol−1, and additionally a glass transition at 120 K. This glass transition does occur not in the supercooled intermediate-temperature phase but in the low-temperature phase of o-carborane. The high- and intermediate-temperature phases of o-carborane could not be quenched at the maximum cooling rate (10 K min−1) of this experiment. The total transition entropy of o-carborane was smaller than that of m-carborane by 10.8 J K−1 mol−1, suggesting that o-carborane has residual orientational disorder at 0 K. The enthalpy relaxation was examined by the temperature jump method around the glass transition of o-carborane. The results were reproduced well by the KWW function. The calorimetric relaxation time and nonexponential parameter agreed with dielectric ones at the glass transition temperature. This indicates that the glass transition of o-carborane is due to the freezing of molecular reorientation.
Heat and mass transfer during the violent collapse of nonspherical bubbles
Authors: Szeri Andrew J. , Storey Brian D. , Pearson Antony , Blake John R.
Published: August 2003

The very high speed of collapse of cavitation bubbles is responsible for a number of phenomena of interest in science and engineering: Luminescence, sonochemistry, cavitation damage, ultrasonic cleaning, etc. Strongly forced bubbles may collapse with such violence that the relatively slow processes of diffusion of the heat of compression and of excess vapor to the bubble wall are obviated. This leads to an approximately adiabatic system with nearly constant mass during the final stages of extreme collapses, accompanied by the evolution of sharp thermal and compositional boundary layers on either side of the interface. It is shown that the boundary layers, which are involved in the determination of the interfacial temperature through the balance of sensible and latent heats, may profitably be described mathematically through integral equations. This complements well the boundary integral solution of the fluid dynamics, which has been the basis of much progress in the field.
Hysteretic behavior of the flow under a vertical sluice gate
Authors: Defina Andrea , Susin Francesca Maria
Published: August 2003

In the paper, the phenomenon of hysteresis which can develop in a supercritical channel flow approaching an obstacle is analyzed, and a simple theory to predict the occurrence of hysteresis is described. The results of an in-depth theoretical and experimental study of the case of flow under a vertical sluice gate in a rectangular channel are then presented. Possible flow regimes in the vicinity of a gate are classified on the basis of the nondimensional gate opening and the Froude number of the undisturbed approaching flow. It is shown that within a wide range of flow parameters both undisturbed and free outflow conditions may exist for the same gate opening. Within this range, the actual regime depends on the previous history of the flow, thus implying the hysteretic character of the flow. It is worth noting that a subcritical approaching flow may also exhibit such a hysteretic behavior provided the Froude number is greater than approximately 0.8. This occurrence, which has not previously been reported in the literature, is probably a result of the contraction affecting the flow issuing from under the gate. The results of an extensive series of experiments, performed over a wide range of flow parameters, are detailed in the paper and confirm theoretical predictions.
Dynamics calculations for the LiH+H⇆Li+H2 reactions using interpolations of accurate ab initio potential energy surfaces
Authors: Kim Kyoung Hoon , Lee Yoon Sup , Ishida Toshimasa , Jeung Gwang-Hi
Published: August 2003

Complete active space self-consistent-field calculations followed by multireference configuration interaction calculations are used to generate potential energy surfaces for the reaction of LiH+H⇆Li+H2. For interpolation of the potential energy surfaces, a recently proposed interpolant moving least squares(IMLS)/Shepard scheme is used. Trajectory calculations for LiH+H→Li+H2 on the interpolated potential energy surface illustrate that the initial vibrational state of LiH plays little role probably due to the absence of the energy barrier for this reaction. The reactive cross section shows a maximum value corresponding to the initial collision energy as low as 0.03 eV. Trajectory calculations for Li+H2→LiH+H show that high vibrational excitation (ν⩾4) of H2 molecule is the most efficient way and the collision energy inefficiently induces the reaction.
Information entropy production in non-Markovian systems
Authors: Bag Bidhan Chandra
Published: August 2003

In this paper we have calculated the information entropy production along with the entropy flux in the nonequilibrium and equilibrium states for the non-Markovian systems using the Fokker–Planck and the entropy balance equations.
Characteristics of flow over two circular cylinders in a side-by-side arrangement at low Reynolds numbers
Authors: Kang Sangmo
Published: August 2003

Two-dimensional flow over two circular cylinders in a side-by-side arrangement at low Reynolds numbers has been numerically investigated in this study. For the study, numerical simulations are performed, using the immersed boundary method, in the ranges of 40⩽Re⩽160 and g*<5, where Re and g* are, respectively, the Reynolds number and the spacing between the two cylinder surfaces divided by the cylinder diameter. Results show that a total of six kinds of wake patterns are observed over the ranges: antiphase-synchronized, in-phase-synchronized, flip-flopping, deflected, single bluff-body, and steady wake patterns. It is found that the characteristics of the flow significantly depend both on the Reynolds number and gap spacing, with the latter much stronger than the former. Instantaneous flow fields, time traces, flow statistics and so on are presented to identify the wake pattern and then to understand the underlying mechanism. Moreover, the bifurcation phenomena where either of two wake patterns can occur are found at certain flow conditions.
Characteristics of square cylinder wake transition flows
Authors: Luo S. C. , Chew Y. T. , Ng Y. T.
Published: August 2003

The wake transition regime in square cylinder flow is investigated experimentally. Mode A and B transitions similar to those captured in circular cylinder flows are found to exist in this regime and by using dye- and laser-induced fluorescence visualization, the spanwise and streamwise vortex structures are captured. Upon comparison with their corresponding modes in circular cylinder flows, some differences are noted. Most notably, the critical Reynolds numbers at which Mode A and B occur for square cylinder flows were estimated to be approximately 160 and 200, respectively, and are lower than those found in circular cylinder flows, which are generally agreed to be approximately 188–190 and 230–260, respectively. Also, the spanwise wavelengths for the two modes in square cylinder flows (5.2 Ds and 1.2 Ds for Modes A and B, respectively) are longer than their counterparts in circular cylinder flows (3–4Do and 0.8–1Do, respectively). Furthermore, by using a hot wire to measure the shedding frequency, it was found that the Mode A transition here does not exhibit a hysteresis phenomenon, which is unlike the case in circular cylinder flows. The Mode A to B transition here is also not associated with a sudden increase in Strouhal number, which is again unlike the corresponding situation in circular cylinder flows. However, the vortical structures are noted to be similar between the corresponding modes for the two bluff body shapes, and hence the formation mechanism can be deemed to be similar.
A mode-coupling theory of diffusion in supercritical fluids
Authors: Egorov S. A.
Published: August 2003

A mode-coupling treatment of diffusion in supercritical fluids is presented. Both neat fluids and dilute attractive supercritical solutions are considered. The solute velocity time correlation function and diffusion coefficient are obtained from the mode-coupling theory (MCT) method and from molecular dynamics simulations. Theory is shown to be in good agreement with simulation. The effect of the solute–solvent interaction strength and solvent clustering on the solute diffusion coefficient is analyzed within the MCT framework. Theoretical results for the diffusion coefficient are compared to the experimental data on the self-diffusion in supercritical xenon and the diffusion of the Xe+ ion in Xe.
Effect of compressed CO2 on the properties of AOT reverse micelles studied by spectroscopy and phase behavior
Authors: Liu Dongxia , Zhang Jianling , Han Buxing , Fan Jiufeng , Mu Tiancheng , Liu Zhimin , Wu Weize , Chen Jing
Published: August 2003

Combination of reverse micellar solutions and supercritical or compressed carbon dioxide (CO2) is a new and interesting topic. This work conducted the first study on the effect of compressed CO2 on the micro-properties (e.g., micropolarity, ionic strength, pH) of the sodium bis(2-ethylhexyl) sulfosuccinate reverse micelles in isooctane by phase behavior measurement, Fourier transform infrared and UV–vis spectroscopic techniques. The results show that CO2 can dissolve in both the organic-continuous phase and the water cores of the reverse micelles. The properties of the reverse micelles can be tuned continuously by changing the pressure of CO2 because the solubility of CO2 in the solution and in the water cores depends on the pressure. CO2 in the water cores can be ionized to produce HCO3−1 and H+. The micropolarity and ionic strength of the water cores increase with the pressure of CO2. Accordingly, the pH is reduced as the pressure and water-to-surfactant molar ratio (w0) are increased.
New surfactants design for CO2 applications: Molecular dynamics simulations of fluorocarbon–hydrocarbon oligomers
Authors: Kirmizialtin Serdal , Menceloglu Yusuf Z. , Baysal Canan
Published: August 2003

Novel block co-oligomers are designed as candidate surfactants in near-supercritical CO2 environment, with the CO2–phobic block consisting of ethyl propionate and ten different types of ethylene monomers, flanked on either side by eight repeat unit fluorinated CO2–philic blocks. Single chain molecular dynamics simulations are performed to understand their conformational and dynamic properties. Depending on the side chain type, the CO2–phobic blocks are prone to shrinkage in the CO2 environment, while the CO2–philic blocks preserve their vacuum dimensions. The overall chains form U-shaped planar structures with flapping motion of the fluorinated arms; thus, we expect bilayer micelle formation under these conditions. The origin of the CO2–oligomer interactions is investigated and van der Waals interactions are found to dominate over electrostatic interactions in the CO2 environment. Calculations of the radial distribution function for the solvent molecules around the oligomer backbone show a solvation shell around 5–6 Å, irrespective of the oligomer type; density of the solvent around the oligomer, on the other hand, varies with type of side chain due to the interactions between the CO2 molecules and the oligomer, and the available volume around the side chain. The local chain dynamics is investigated by orientational autocorrelation functions, and the characteristic time of the relaxation of selected C–H and C–F bonds is found to depend on the local friction experienced by the fluctuating atoms and the energy barrier that needs to be surmounted during the relaxation process. The simple exponential decay of the correlation functions for the C–H bond is common for all oligomer types, whereas the stretched exponents take on smaller values depending on the side chain for the C–F bond vector, implying that the fluorinated blocks are exposed to more complicated dynamical processes.
Role of supersecondary structural elements in protein G folding
Authors: Derreumaux Philippe
Published: August 2003

Knowledge on the folding of supersecondary structures is very important for a coherent understanding of protein folding mechanisms. Here, the hairpin 1, hairpin 1-helix and helix-hairpin 2 of protein G have each been subject of 50 folding Monte Carlo simulations and 48 ns unfolding molecular dynamics simulations. We find that the helix-hairpin 2 structure can stabilize itself to some extent independent of the rest of protein G but hairpin 1 cannot. Interestingly, an intermediate characterized by a helix-hairpin 2 packed in a non-native arrangement is observed in 50% of all simulations, independently of the temperature, force field, and exact amino acid sequence. This is in line with the framework model which envisages a secondary-tertiary hierarchical process.
Conformational statistics of bent semiflexible polymers
Authors: Zhou Yu , Chirikjian Gregory S.
Published: August 2003

This paper extends previous methods for obtaining the probability distribution function of end-to-end distance for semiflexible polymers, and presents a general formalism that can generate conformational statistics of any continuum filament model of semiflexible chains with internal bends and twists. In particular, our focus is distribution functions for chains composed of straight or helical segments connected with discrete bends or twists. Prior polymer theories are not able to fully account for the effects of these internal shape discontinuities. We use the operational properties of the noncommutative Fourier transform for the group of rigid-body motions in three-dimensional space. This general method applies to various stiffness models of semiflexible chainlike macromolecules. Examples are given which apply the stiffness parameters defined in the Kratky–Porod model, Yamakawa helical wormlike chain model, and revised Marko–Siggia double-helix model to chains with intrinsic bends or twists in their undeformed (minimal energy) state. We demonstrate how the location and magnitude of internal bends in the chain affect the distribution of end-to-end distances for each of these models. This capability allows one to study the entropic effects of intrinsic shape changes (e.g., bend angle) in various models, and may lead to coarse-grained continuum mechanical models of processes that occur during transcription regulation.
Ion pairing in model electrolytes: A study via three-particle correlation functions
Authors: Jiménez-Ángeles Felipe , Messina René , Holm Christian , Lozada-Cassou Marcelo
Published: August 2003

A novel integral equations approach is applied for studying ion pairing in the restricted primitive model electrolyte, i.e., the three-point extension (TPE) to the Ornstein–Zernike integral equations. In the TPE approach, the three-particle correlation functions g[3](r1,r2,r3) are obtained. The TPE results are compared to molecular dynamics (MD) simulations and other theories. Good agreement between TPE and MD is observed for a wide range of parameters, particularly where standard integral equations theories fail, i.e., low salt concentration and high ionic valence. Our results support the formation of ion pairs and aligned ion complexes.
Capillary driven flow in circular cylindrical tubes
Authors: Stange Michael , Dreyer Michael E. , Rath Hans J.
Published: August 2003

Capillary-driven flow of a perfectly wetting liquid into circular cylindrical tubes is studied. Based on an analysis of previous approaches, a comprehensive theoretical model is presented which is not limited to certain special cases. This model considers the meniscus reorientation, the dynamic contact angle as well as inertia, convective, and viscous losses inside the tube and the reservoir. The capillary-driven flow is divided into three successive phases where first inertia then convective losses and finally viscous forces counteract the driving capillary force. This leads to an initial meniscus height increase proportional to the square of time followed by a linear dependence and finally the Lucas–Washburn behavior where the meniscus height is proportional to the square root of time. The three phases are separated by two characteristic transition times which are determined by the Ohnesorge number and the inertia of the liquid. Experiments were carried out under microgravity condition in a carefully chosen range of Ohnesorge numbers and initial liquid heights to cover the complete process from the initial meniscus development to the final Lucas–Washburn behavior. Good agreement of experimental and theoretical data is found throughout the complete range of experiment parameters. The existence of all three flow regimes predicted by the theory is verified by the experiments.
Reversible and irreversible dispersion in a porous medium
Authors: Khrapitchev A. A. , Callaghan P. T.
Published: August 2003

Pulsed gradient spin echo nuclear magnetic resonance (PGSE NMR) has been used to examine the approach to asymptotic dispersion for flow of water in a randomly packed array of 500 μm latex spheres in a cylindrical tube of inner diameter 10 mm. Measurements have been performed using single pulse pair and double pulse pair PGSE NMR in directions both transverse to and co-linear with the flow direction, Péclet numbers (Pe) covering a range from 2.8×103 to 8.4×103. In the case of longitudinal flow we find evidence for both reversible and irreversible dispersion over observation times of up to 10τv where τv is the correlation time associated with flow across one bead diameter. The fluctuating component of the velocity field appears to obey a simple Ornstein–Uhlenbeck correlation function with a single correlation time in close agreement with τv. In transverse dispersion, no stationary velocity distribution is evident and the correlation time for fluctuation is much smaller, over the entire range of Pe values.
Mixing rates and symmetry breaking in two-dimensional chaotic flow
Authors: Voth Greg A. , Saint T. C. , Dobler Greg , Gollub J. P.
Published: August 2003

We experimentally determine the mixing rate for a magnetically forced two-dimensional time-periodic flow exhibiting chaotic mixing. The mixing rate, defined as the rate of decay of the root-mean square concentration inhomogeneity, grows with Reynolds number, but does not increase at the onset of nonperiodic (weakly turbulent) flow. The mixing rate increases linearly with a second non-dimensional parameter, the typical path length of a fluid element in one forcing period. The breaking of time-reversal symmetry and spatial reflection symmetry substantially increases the mixing rates. A theory by Antonsen et al. that predicts mixing rates in terms of the measured Lyapunov exponents of the flow is tested and found to predict mixing rates that are too large by approximately a factor of 10; the discrepancy is traced to the fact that large scale transport rather than stretching of fluid elements is the dominant rate limiting step when the system is sufficiently large compared to the velocity correlation length. An effective diffusion model gives a good account of the measured mixing rates. Finally, the formation of persistent recurrent patterns (also called strange eigenmodes) is shown to arise from a combination of stretching and effective diffusion.