bluuebunny/crossref_metadata_2025 huggingface dataset:
On the conformation of non-adsorbing polymers in colloidal suspensions
Authors: Haronska Peter , Vilgis Thomas A.
Published: July 2002

This paper investigates the interaction of dilute polymer solution with a colloidal suspensions. The object of consideration is the effective monomer–monomer potential between different polymer pieces. Unlike in the current polymer literature the effective potential is calculated beyond the pseudo potential approximation, i.e., detailed effects of the colloidal liquid is taken into account. It is shown that the effective potential can be used to discuss the stability of the three component system, polymer chains, good solvent, and colloidal particles. The latter are approximated as hard spheres. Two regimes are found: The polymer chains are collapsed (as a function of the packing fraction of the colloids), if the size of the polymer is much larger compared to the colloids. This corresponds to the unstable regime. If the polymer chains are smaller, it is shown that the depletion effect dominates the physical behavior and the solution is stable. Detailed criteria are worked out in the form of an eigenvalue equation for the interactions and conformations.
An embedding approach for the calculation of STM images: Method development and application to galena (PbS)
Authors: Becker Udo , Greatbanks Steve P. , Rosso Kevin M. , Hillier Ian H. , Vaughan David J.
Published: July 2002

A point charge cluster embedding technique is developed to model the electronic structure of flat PbS surfaces. This method removes edge effects of the wavefunction that are most significant at the edges of the cluster, and it was applied to mimic a greater cluster thickness without a significant increase in computational cost. Point charges are fitted to compensate for the difference potential between the PbS cluster and an infinitely wide slab with a greater thickness than the cluster. STM images calculated with the embedding technique hardly show any edge effects. The differences of calculated STS spectra for tip positions above corner, edge, and center atoms of the cluster are much reduced after applying the point charges. Surface relaxation calculated for the bare cluster results in differences in the vertical relaxation between corner and center atoms at the surface of up to 0.5 Å. These differences decrease to less than 0.1 Å with the embedding technique and the calculated relaxation agrees well with calculations using two-dimensional periodic slabs.
A quasioscillatory collisional model for two-dimensional soft disk fluids
Authors: Noro Massimo G. , Kivelson Daniel , Tarjus Gilles
Published: July 2002

Although Enskog kinetic theory describes intermolecular collisional dynamics in terms of hard core molecular cross sections with inclusion of environmental crowding (excluded volume), more detailed analyses must incorporate the effects due to softness of the intermolecular potential and correlations of successive collisions. We present a simple model for the probability, pr(t), for a collision (defined as a velocity reversal) to occur at a time t after the last collision, that includes all the effects listed. Although the model incorporates correlated collisions, it does so only in the sense that such correlations affect pr(t), an intrinsically Markovian property since it carries memory only back to the last collision. We also examine the sign velocity autocorrelation function in order to identify the strictly Markovian contributions to the collisional dynamics.
Ultrafast OH production in clusters containing N2O and HI
Authors: Ionov P. I. , Ionov S. I. , Wittig C.
Published: July 2002

The geometrical arrangement of reagents is an important factor influencing chemical reactions in condensed phases and molecular clusters. In the present study, OH buildup times have been recorded upon photolysis of (N2O)m(HI)n clusters in order to elucidate the role of the cluster environment on the reaction mechanism. The buildup times were measured for different molecular beam compositions (i.e., degrees of clustering). The buildup time changed from ⩽100 fs at the lowest backing pressures (119 and 132 kPa) to 430 fs at 188 kPa. It is argued that at the lower backing pressures the OH derives primarily from binary N2O-HI complexes. However, regardless of the cluster species involved, the fast OH buildup at the lowest backing pressures suggests a dominant direct oxygen abstraction mechanism rather than reaction via a vibrationally excited intermediate such as HNNO†.
Simulation study of polymer chain geometry in a one-dimensional periodic potential
Authors: Goulding D. J. , Teixeira P. I. C.
Published: July 2002

The behaviour of a freely-jointed chain in a one-dimensional periodic potential has been investigated by theory and simulation. Chain geometry is well described by perturbation theory in the limit of a weak potential. By contrast, predictions based on a tight-binding model for confinement and layer-hopping of chains, are found to have much more restricted validity than previously conjectured.
Dynamical catalysis
Authors: Deutch J. M.
Published: July 2002

If a biomolecular chemical reaction is mass transport limited, the reaction rate can be accelerated by embedding one of the reaction centers in a molecular framework. This framework acts as a lower dimensionality surface along which the other reaction partner can travel. The rate enhancement can be identified with an increase in entropy of activation. A model for this effect is constructed and applied to a reaction center embedded in three molecular framework geometries: a linear chain, a disk, and a polar reactive patch on a sphere. The new result for the reactive patch on a sphere is relevant for enzymatic systems where substrates must find their way to a small reactive region.
Kinetic description of diffusion-limited reactions in random catalytic media
Authors: Oshanin G. , Blumen A.
Published: July 2002

We study the kinetics of bimolecular, catalytically-activated reactions (CARs) in d-dimensions. The elementary reaction act between reactants takes place only when these meet in the vicinity of a catalytic site; such sites are assumed to be immobile and randomly distributed in space. For CARs we develop a kinetic formalism, based on Collins–Kimball-type ideas; within this formalism we obtain explicit expressions for the effective reaction rates and for the decay of the reactants’ concentrations.
Confocal micro-Raman spectroscopy of black soap films
Authors: Lecourt B. , Capelle F. , Adamietz F. , Malaplate A. , Blaudez D. , Kellay H. , Turlet J. M.
Published: July 2002

Black soap films from aqueous solutions of sodium dodecyl sulphate are studied by micro-Raman confocal spectroscopy. At the end of the draining process films of different thicknesses are obtained depending on the experimental conditions: Working in a closed humidified chamber leads to common black films while, under evaporation or in the presence of electrolyte, Newton black films are observed. From the Raman spectra of these films, quantitative information is deduced about the conformational and lateral order of the aliphatic surfactant chains, as well as the thickness of the residual water layer. More accurate measurements of the thickness of these ultimate films have been carried out by transmission ellipsometry and their effective refractive index measured by Brewster angle reflectivity. The thinner films present higher molecular organization and their aqueous core exhibits unusual spectral features.
Potential energy surfaces and dynamics for the reactions between C(3P) and H3+(1A1′)
Authors: Bettens Ryan P. A. , Collins Michael A.
Published: July 2002

Ab initio MCSCF/6−31G**  adiabatic potential energy surfaces have been determined for both the ground and first excited states of triplet CH3+. Classical trajectory studies of the collision of C(3P) with H3+(1A1′) on both surfaces yield an overall rate coefficient of 2.1×10−9 cm3 s−1 for the formation of CH+ (3Π)+H2 at 10 K, in good agreement with earlier work. A rate coefficient of 4.9×10−11 cm3 s−1 at 10 K has been determined for the previously unknown reaction which produces CH2+(2A1)+H. The properties of the reaction products are examined and the accuracy of the potential energy surfaces is investigated.
Path integral study of hydrogen and deuterium diffusion in crystalline silicon
Authors: Forsythe Kelsey M. , Makri Nancy
Published: July 2002

We use classical and quantum mechanical methods to calculate the site-to-site hopping rate of hydrogen impurities in crystalline silicon over a wide range of temperatures. The calculations employ a parameterized version of a potential surface calculated via density functional methods, expanded through quadratic terms about a Cartesian reaction path with a flexible reference. The hopping rate is obtained from the time integral of a flux correlation function which is evaluated using classical molecular dynamics and real-time path integral techniques. The latter are based on the quasiadiabatic propagator discretization and utilize a combination of discrete variable representations and Monte Carlo sampling for the evaluation of the resulting multidimensional integrals. Our results indicate that quantum mechanical tunneling plays a significant role in the diffusion process even above room temperature. In addition, the calculated diffusion rate exhibits a reverse isotope effect in the domain between activated and tunneling dynamics which arises from the zero point energy of the hydrogen atom in the direction perpendicular to the line connecting two stable minima.
An elementary description of nonlinear optical properties of octupolar molecules: Four-state model for guanidinium-type molecules
Authors: Cho Minhaeng , Kim Hyun-Soo , Jeon Seung-Joon
Published: July 2002

A theoretical description of the molecular polarizability (α) and first and second hyperpolarizabilities (β and γ) of a guanidinium-type octupolar molecule is presented. By using a valence bond and three charge-transfer states with nonzero transfer integrals, the electronic states are obtained as linear combinations of these basis states. It is found that a doubly degenerate excited state is only optically coupled to the ground state. Based on the analytic expressions for α, β, and γ obtained, it is shown that the resultant tensor elements of β satisfy the symmetry requirements of point group D3h. Unlike the linear push–pull polyenes, the magnitudes of α, β, and γ of the guanidinium-type octupolar molecules increase as the charge-transfer character of the electronic ground state increases. Also, their dependences on the distance (d) between the donor and acceptor are briefly discussed.
Tunneling chemical reactions in solid parahydrogen: A case of CD3+H2→CD3H+H at 5 K
Authors: Momose Takamasa , Hoshina Hiromichi , Sogoshi Norihito , Katsuki Hiroyuki , Wakabayashi Tomonari , Shida Tadamasa
Published: July 2002

Ultraviolet photolysis of CD3I in solid parahydrogen at 5 K gives CD3 radical, which decreases in a single exponential manner with a rate constant of (4.7±0.5)×10−6 s−1. Concomitantly, CD3H is formed, which is accounted for by the quantum tunneling reaction CD3+H2→CD3H+H. Under the same conditions, CH3I yields CH3 radical, but the corresponding reaction between CH3 and H2, expected to give CH4+H, does not proceed measurably at 5 K. The difference between the two systems is attributed to the difference in the zero point energy change.
Chiral discrimination in molecular excitation transfer
Authors: Craig D. P. , Thirunamachandran T.
Published: July 2002

A new aspect of chiral discrimination is described. The rate of resonance transfer between chiral molecules at all separation distances is calculated using molecular quantum electrodynamics. The leading correction to the familiar electric dipole transfer rate is shown to be discriminatory, and is proportional to the optical rotatory strengths. In the near zone the rate and the discrimination in the rates for transfer to the two chiral enantiomers are proportional to the inverse sixth power of the separation; in the far zone to the inverse square. Results are discussed for transfer between identical and nonidentical molecules. In the latter the discrimination between the two enantiomers of the molecule excited by transfer depends on the handedness of the initially excited molecule, a property that might be exploited in practical applications.
A three-dimensional potential energy surface for dissociative adsorption and associative desorption at metal electrodes
Authors: Koper Marc T. M. , Voth Gregory A.
Published: July 2002

A simple model is constructed to calculate the potential energy surface of dissociative adsorption and associative desorption reactions at the metal/solution interface. The model is based on an extension of the Anderson–Newns Hamiltonian and has three reaction coordinates; the bond length or the distance between the fragments, the distance from the surface, and the generalized solvent coordinate familiar from the classical theory of electron-transfer reactions. The properties of the three-dimensional potential energy surfaces are studied and the activation energy for dissociative adsorption is calculated as a function of the applied potential and the metal work function. In the observed trends, the absorption energy and hence the electrosorption valency of the fragments play an important role. For certain “extreme” values of the bonding or antibonding energy levels, molecular ions may become metastable and affect the reaction mechanism.
Anomalous heat capacity of nematic liquid crystals
Authors: Mukherjee Prabir K.
Published: July 2002

The anomalous parts of the specific heat capacity of nematic liquid crystals above the transition point is calculated on the basis of the continuum theory. The analysis of the temperature dependence of the specific heat provides a more detailed description of the pretransitional phenomena in liquid crystals.
The dynamic behavior of butanethiol and dodecanethiol adsorbates on Au(111) terraces
Authors: Terán Arce F. , Vela M. E. , Salvarezza R. C. , Arvia A. J.
Published: July 2002

The dynamics of butanethiol and dodecanethiol monolayers adsorbed on Au(111) studied by ex situ and in situ sequential scanning tunneling microscopy (STM) shows, at room temperature and in the range of seconds, √3×√3 R30°⇔c(4×2) transitions. High-resolution STM imaging also shows that these transitions can be explained by a displacement of adsorbed molecules from hollow to bridge sites and vice versa. Transitions from the p(n×1) superlattice to the √3×√3 R30° lattice were also imaged in real time. These processes appear to be coupled with fluctuations of the hole size of the Au(111) terrace.
Photoinduced reactions of methyl radical in solid parahydrogen
Authors: Fushitani Mizuho , Sogoshi Norihito , Wakabayashi Tomonari , Momose Takamasa , Shida Tadamasa
Published: July 2002

Photolysis of methyl iodide in solid parahydrogen (p-H2) at about 5 K is studied with ultraviolet light at 253.7 and 184.9 nm. It is found that the light at 253.7 nm produces only methyl radical, whereas the light at 184.9 nm yields both methyl radical and methane. The mechanism of the formation of the photoproducts is elucidated by analyzing the temporal behavior of the observed vibrational absorption. It is concluded that methyl radical in the ground state does not react with p-H2 molecules appreciably but that the radical in the electronic excited state of B̃(2A1′), accessible by reabsorption of 184.9 nm photons by the radical, decomposes to a singlet methylene CH2 ã(1A1) and a hydrogen atom (2S) and that the singlet methylene reacts with a p-H2 molecule to give methane.
Ab initio calculation of molecular energies including parity violating interactions
Authors: Bakasov Ayaz , Ha Tae-Kyu , Quack Martin
Published: July 2002

We present a new approach towards electroweak quantum chemistry including the parity violating weak nuclear force. After introducing the ground work of electroweak quantum chemical perturbation theory to calculate parity violating potentials, Epv, we present specifically a CIS-RHF method (configuration interaction singles—restricted Hartree–Fock). The method is compared to the previously established and widely used SDE-RHF method for calculations of Epv [single determinant excitations—restricted Hartree–Fock, R. A. Hegstrom, D. W. Rein, and P. G. H. Sandars, J. Chem. Phys. 73, 2329 (1980)]. It is demonstrated that the new CIS-RHF method can lead to values of Epv which are more than an order of magnitude larger than those obtained with SDE-RHF (for example in H2O2, where the new maximum value is Epv=3.7×10−19Eh). Furthermore, the importance of the tensor character of Epv is outlined by showing that the components of the trace of this tensor Epvxx+Epvyy+Epvzz=Epv evolve essentially independently from each other in magnitude and sign as functions of molecular structure and computational method. The total Epv results thus as a remainder after substantial mutual cancellation of these components. This finding explains the phenomenon of zero total Epv at chiral geometries, whereas the individual tensor components remain nonzero. We present systematic investigations of parity violating potentials as a function of structure for H2O2, H2S2, N2O4, C2H2, C2H4, C2H6, CH4, and alanine. The effect of nuclear charge Z is investigated for the pair H2O2 and H2S2 and a power law Z3+δ (δ≈1.5) for the enhancement of Epvii can be established with significance for the individual tensor components (i=x,y, or z), whereas just considering the total Epv would be misleading in analyzing the Z dependence. Contributions of hydrogen atoms to Epv are estimated and found to be orders of magnitude lower than those of the heavier atoms mentioned. The results are discussed in relation to a possible spectroscopic experiment to measure ΔEpv=2Epv in enantiomers of chiral molecules and in relation to various hypotheses for the origin of nature of homochirality in chemical evolution.
Orientational correlation of liquid–crystalline polymer chains in isotropic solutions. I. Anisotropic light scattering
Authors: Jinbo Yuji , Varichon Lionel , Sato Takahiro , Teramoto Akio
Published: July 2002

The orientational fluctuation in isotropic toluene and dichloromethane solutions of a stiff-chain polymer, poly(n-hexyl isocyanate) (PHIC), has been studied by anisotropic light scattering up to the vicinity of the isotropic–nematic phase separation region. The depolarized component ΔRθ,Hv of the Rayleigh ratio divided by the polymer concentration increased with increasing the polymer concentration. The ΔRθ,Hv data for different molecular weights were fitted almost quantitatively to the scaled particle theory for wormlike hard-spherocylinders with the hard-core diameter d, developed by incorporating the higher virial terms in the free energy. However, the fitted d value was appreciably smaller than that chosen to explain the experimental osmotic compressibilities and/or isotropic–nematic phase boundaries of the same systems. Then it was found that the “spinodal concentration” c*, where the isotropic phase becomes thermodynamically unstable, lies within the nematic region, in contrast with the prediction of the scaled particle theory that c* should be in the isotropic–nematic biphasic region. This inconsistency may come from a mean-field approximation used in the scaled particle theory.
Interactions, dynamics, and elasticity in charge-stabilized colloidal crystals
Authors: Weiss Jessica A. , Larsen Amy E. , Grier David G.
Published: July 2002

We describe real-space imaging measurements of the interactions and elastic properties of charge-stabilized colloidal crystals. Interactions between charge-stabilized latex spheres self-assembled into colloidal crystals are extracted from single-particle dynamics using new techniques in digital video microscopy. The spheres’ cooperative behavior makes possible simultaneous measurements of their crystals’ bulk moduli. Together, these measurements offer new insights into the microscopic origins of charge-stabilized colloidal crystals’ bulk elastic properties. In particular, they demonstrate that the crystals’ elastic properties cannot be ascribed to the linear superposition of pairwise interactions. This result highlights the potential pitfalls of using the conventional Derjaguin, Verwey, Landau, and Overbeek (DLVO) theory to describe the bulk properties of charge-stabilized colloidal suspensions.
A complete basis set model chemistry. VI. Use of density functional geometries and frequencies
Authors: Montgomery J. A. , Frisch M. J. , Ochterski J. W. , Petersson G. A.
Published: July 2002

The recently introduced complete basis set, CBS-Q, model chemistry is modified to use B3LYP hybrid density functional geometries and frequencies, which give both improved reliability (maximum error for the G2 test set reduced from 3.9 to 2.8 kcal/mol) and increased accuracy (mean absolute error reduced from 0.98 to 0.87 kcal/mol), with little penalty in computational speed. The use of a common method for geometries and frequencies makes the modified model applicable to transition states for chemical reactions.
Photo-induced electron-transfer processes in (anthracene–) quinquethiophene–fullerene diads and triads
Authors: Knorr S. , Grupp A. , Mehring M. , Grube G. , Effenberger F.
Published: July 2002

We have investigated novel model systems for photo-induced intramolecular electron-transfer processes. The molecules presented here are of the structure T5–C60 or A–T5–C60, i.e., fullerene C60 is used as acceptor, and quinquethiophene (T5) or anthracene (A) can act as a donor. From theoretical considerations we learn that a charge transfer should occur after excitation of the quinquethiophene or anthracene part, whereby an energy transfer is still not excluded. Optical absorption spectra show the electronic independence of the fullerene from the substituent and hint at isolated molecules in solutions. After laser-flash excitation of frozen solutions of the molecules, we used pulsed electron spin resonance (ESR) to detect the radical signals of the charge-separated state. In both the diad and the triad, we observe at low temperatures a free induction decay (FID) and a broad spin-echo spectrum with the same lifetime of 1–2 μs. The signals can be assigned to the C60− anion (FID) and the T5+cation (spin echo) by the g factor and the linewidth.
A structural and infrared study of the charge states of tetrakis (dimethylamino)ethylene (TDAE) in TDAE-C60 and (TDAE)(Cl)2
Authors: Pokhodnia K. I. , Papavassiliou J. , Umek P. , Omerzu A. , Mihailovič D.
Published: July 2002

The molecular structure and vibrational spectra of the different oxidation states of tetrakis(dimethylamino)ethylene (TDAE) are calculated using structural optimization and normal-mode calculations. The predictions are compared with x-ray structural data and infrared spectra of TDAE, TDAE-C60 and (TDAE)(Cl)2. The C–N stretch vibration frequency and axial torsional angle are shown to be sensitive indicators of the charge state of the molecule. TDAE+, which occurs in TDAE-C60 is found to have a near-degenerate configurational ground state.
Collisions of noble gas dimers with monomers: A classical trajectory study
Authors: Kalus René
Published: July 2002

Both classical and quasiclassical trajectories are used to study the dissociation of noble gas dimers and the transitions between stable and metastable dimers induced by collisions with noble gas atoms. Thermal reaction rates are reported for these processes over a 0.3–3.5 range of reduced temperatures, T*=(kBT)/ε. Classical and quasiclassical data are compared to assess the importance of quantum effects in the reaction dynamics.
Characteristics of impurity effects in isomorphous crystals and anisomorphous mixtures observed by 14N nuclear quadrupole resonance
Authors: Jung J. K. , Seo Y. M. , Choh S. H.
Published: July 2002

Impurity effects on 14N nuclear quadrupole resonance (NQR) measured at room temperature for various mixed complexes, [NaNO2]–[ANO2] and [(CH2)6N4]–[BNO2] with A=Ag or K and B=Na or K, respectively, have been compared. A classification of these mixed systems into isomorphous mixed crystals and anisomorphous mixtures provides a mean for obtaining systematic trends of impurity effects in these systems. With regard to the 14N NQR at room temperature, the isomorphous system [NaNO2]–[ANO2] is characterized predominantly by the static impurity effect caused by the random static distribution of the impurity ions in the host crystal. The anisomorphous system [(CH2)6N4]–[BNO2] appears to be subject to the dynamic secondary impurity effects due to the mobile point defects inside the matrix of the segregated host crystal.
Adiabatic bias molecular dynamics: A method to navigate the conformational space of complex molecular systems
Authors: Marchi Massimo , Ballone Pietro
Published: July 2002

This study deals with a novel molecular simulation technique, named adiabatic bias molecular dynamics (MD), which provides a simple and reasonably inexpensive route to generate MD trajectories joining points in conformational space separated by activation barriers. Because of the judicious way the biasing potential is updated during the MD runs, the technique allows with some additional effort the computation of the free energy change experienced during the trajectory. The adiabatic bias method has been applied to a nontrivial problem: The unfolding of an atomistic model of lysozyme. Here, the radius of gyration (Rg) was used as a convenient reaction coordinate. For changes in Rg between 19.7 and 28 Å, we observe a net loss of the native tertiary structure of lysozyme. At the same time, secondary structure elements such as α-helices are retained although some of the original order is diminished. The calculated free energy profile for the unfolding transition shows a monotonous increase with Rg and depends crucially on the nonbonded cutoff used in the potential model.
Photodissociation dynamics of argon cluster ions
Authors: Ikegami Tsutomu , Iwata Suehiro
Published: July 2002

The photodissociation process of argon cluster ions is studied by using Tully’s molecular dynamics with quantum transition method. It is demonstrated that the nonadiabatic transition plays an important role in the process. A lifetime of an initial photoexcited state and recovery time to the electronic ground state are obtained for several cluster sizes and excitation energies. A decay curve of the photoexcited state is composed of a Gaussian-type decay followed by an exponential decay. The initial Gaussian-type delay of the decay is attributed to the acceleration time of the nuclei. The recovery to the ground state is slower for the higher excitation energy, which is due not only to the increased number of the nonadiabatic transitions, but to the lower rate of each transition. The low recovery rate at the higher excitation energy might be explained from the low density of states in the energy range.
Hydrothermodynamic consideration on the steady-state motion of a solid/liquid interface
Authors: Mori Atsushi
Published: July 2002

We study a solid and a liquid coexisting at a plane interface in the nonequilibrium steady-state (NESS) condition on a hydrothermodynamic level. Both solid and liquid are treated as continuums. From the equations of mass-, momentum-, and energy-conservation we obtain relations for the velocity of the interface, the net mass-flow velocity, and thermodynamic variables such as temperature, pressure, and density. The second law of thermodynamics gives a criterion for the possibility of the NESS processes. It is shown that the NESS interface motion without extraction or addition of latent heat of fusion is possible.
Viscoelastic behavior of 1,3,5 tri α-naphthyl benzene (will the real TαNB please stand up)
Authors: Plazek Donald J. , Magill Joseph H. , Echeverrı́a I. , Chay In-Chul
Published: July 2002

The mistaken identity of 1,3-bis (1-naphthyl)-5-(2-naphthyl) benzene (1,3α-5β TNB throughout the article) for 1,3,5-tris (1-naphthyl) benzene (TαNB throughout the article) is acknowledged and viscosities and recoverable creep compliances above Tg for the real TαNB are reported. In addition, semicontinuous creep measurements have been carried out below Tg while time dependent structural changes occurred as the material drifted towards an equilibrium state, following temperature decreases (“down jump”) and increases (“up jump”). With this new measurement scheme, the evolution of both viscosity and recoverable creep compliance can be followed during the isothermal and isobaric drifts toward equilibrium. Since the steady state creep compliance of TαNB is nearly independent of temperature in the temperature range from 61.8 to 72.4 °C, the recoverable compliance curves did not change shape with changes in temperature; i.e., thermorheologically simple behavior was observed.
Effect of residual solvent on carrier transport in polysilane
Authors: Pan Lujun , Zhang Mei , Nakayama Yoshikazu
Published: July 2002

The effect of residual solvent on the carrier transport in poly(methylphenylsilane) films which were formed from toluene and tetrahydrofuran solutions and dried in different conditions has been investigated by a time-of-flight technique. The dependence of hole drift mobility on the electric field and temperature has been analyzed in a framework of the superimposed disorder and polaron effect. It is found that no parameter but the polaron binding energy has a strong relation with an amount of the residual solvent. The polaron binding energies of 0∼126 meV in addition to the native polaron binding energy of 80 meV for polysilane are discussed as solvent-related values.
Infinite basis limits in electronic structure theory
Authors: Fast Patton L. , Sánchez Marı́a L. , Truhlar Donald G.
Published: July 2002

We have developed a database of 29 molecules for which we have estimated the complete-one-electron-basis-set limit of the zero-point-exclusive atomization energy for five levels of electronic structure theory: Hartree–Fock (HF) theory, Mo/ller–Plesset second- and fourth-order perturbation theory, coupled cluster theory based on single and double excitations (CCSD), and CCSD plus a quasiperturbative treatment of triple excitations [CCSD(T)], all at a single set of standard geometries. Convergence checks indicate that the estimates are within a few tenths of a kcal/mol of the n=infinity limit of the cc-pVnZ basis set sequence. This data is then used to obtain optimized power-law exponents for extrapolating to the basis-set-limit from correlation-consistent polarized valence double and triple zeta (cc-pVDZ and cc-pVTZ) basis sets. This allows one to get thermochemical accuracy comparable to polarized quadruple or quintuple zeta (cc-pVQZ or cc-pV5Z) basis sets with a cost very comparable to polarized triple zeta, which is one order of magnitude less expensive than polarized quadruple zeta and two orders of magnitude less expensive than polarized quintuple zeta.
Reversible diffusion-controlled reactions in a disordered system of static spherical catalysts
Authors: Felderhof B. U. , Jones R. B.
Published: July 2002

The effective frequency-dependent rate coefficient for conversion of particles diffusing in a disordered system of static spherical catalysts is calculated for low density of catalysts by the method of multiple scattering theory. The rate coefficient follows from a Foldy–Lax type formula. The catalysts are assumed uniform, but different energies, conversion rates, and diffusion coefficients are allowed inside and outside the catalysts. The frequency-dependent rate coefficient is characterized by its steady-state value and by a spectrum of relaxation rates. The coefficient displays a wide variety of behavior as a function of the parameters of the model.
Calculation of optical spectra in liquid methanol using molecular dynamics and the chemical potential equalization method
Authors: Chelli Riccardo , Ciabatti Sonia , Cardini Gianni , Righini Roberto , Procacci Piero
Published: July 2002

We apply the chemical potential equalization (CPE) method to the calculation of the optical spectra in liquid methanol at 298 K and normal pressure. The configurations of the liquid are obtained by conventional molecular dynamics (MD) using a completely flexible all-atoms model. The infrared and Raman spectra are computed a posteriori using a CPE parametrization of methanol calibrated to reproduce the electronic properties of the isolated molecule evaluated with accurate ab initio calculations. The MD/CPE method reproduces correctly the optical spectra in the region of the intermolecular motions. The spectra are discussed and interpreted on the basis of hydrogen bonding structure and dynamics.
Multiple-scattering coefficients and absorption controlled diffusive processes
Authors: Godoy Salvador , Garcı́a-Colı́n L. S. , Micenmacher Victor
Published: July 2002

Multiple-scattering transmission and reflection coefficients (T,R) are introduced in addition to the diffusion coefficient D for the description of ballistic diffusion in the presence of absorption. For 1D (one-dimensional) systems, the measurement of only one between T and D imposes restrictions on the possible values of the other. If D is measured, then T is bounded between the Landauer and Lambert–Beer equations. Measurements of both (T,D) imply the theoretical knowledge of the microscopic absorption Σa and scattering rΣs cross sections.
A new (and better) lower bound for the excess internal energy of the one-component plasma
Authors: Caillol J. M.
Published: July 2002

We derive a new exact lower bound for the excess internal energy per particle of the classical three-dimensional one-component plasma. This bound is higher (better) than those obtained previously by Mermin, Lieb–Narnhofer, and Totsuji.
Two-parameter stochastic resonance in the absence of external signal for the photosensitive Belousov–Zhabotinsky reaction
Authors: Zhong Shi , Jiang Yongjun , Xin Houwen
Published: July 2002

Stochastic resonance (SR) is studied numerically in a modified Oregonator-type model, which was proposed recently to account for the photosensitivity of the Belousov–Zhabotinsky (BZ) reaction in a flow system. When either of the two control parameters, light flux and a flow rate, is modulated by multiplicative external noise, noise induced coherent oscillations (NICO) in the absence of deterministic oscillations are observed near Hopf bifurcation point, where the external noise is added to one parameter or the other. The signal-to-noise ratio (SNR) goes through a maximum with the increment of noise intensity indicating occurrence of SR. The aspects of the two-parameter SR in this system are discussed.
Relativistic MCSCF by means of quasidegenerate direct perturbation theory. II. Preliminary applications
Authors: Liu Wenjian , Kutzelnigg Werner , van Wüllen Christoph
Published: July 2002

The relativistic multiconfiguration self-consistent field (MCSCF) theory by means of (quasidegenerate) direct perturbation theory (DPT) is formulated and implemented through the leading order. Systematic investigations of nondynamical correlation and relativistic effects in berylliumlike systems are carried out. For this sequence, it appears that the single-state first-order DPT is valid only for the nuclear charge up to 30 or so. Calculations for the spectroscopic constants of the molecules of homonuclear halogen dimers X2 (X=F, Cl, Br, I, At) and TlX (X=F, Cl, Br, I) are performed at all-electron SCF (self-consistent field) and complete active space SCF levels, with the aim to demonstrate the efficiency of DPT by comparing with other relativistic calculations. Spin–orbit coupling affects the energies of the open-shell atoms and is taken into account for the binding energies.
Structure and phase behavior of a model clay dispersion: A molecular-dynamics investigation
Authors: Kutter S. , Hansen J.-P. , Sprik M. , Boek E.
Published: July 2002

Reversible molecular-dynamics (MD) simulations have been carried out on simple models for dispersions of circular Laponite clay platelets to investigate the local structure on a mesoscopic scale. The platelets carry discrete charged sites interacting via a screened Coulomb potential. In model A all surface sites have identical negative charge, while model B also includes rim charges of opposite sign. These two models were used in a series of simulations in the semidilute regime, and for three values of the Debye screening length. The structure of the dispersions is characterized by translational and orientational pair distribution functions, and by the corresponding structure factors. Qualitative differences in the pair structure arising from variations in concentration and screening length lead to a tentative identification of sol, gel, and crystal phases. The rim charges have a dramatic effect on the local structure in the strong screening regime, leading to T-shaped pair configuration and clustering of the platelets at low clay concentrations, and at higher concentrations to a space-filling “house-of-cards” structure.
Charge transfer and chemical reaction dynamics in hyperthermal O++NO collisions
Authors: Levandier Dale J. , Chiu Yu-hui , Dressler Rainer A.
Published: July 2002

The O+(4S)+NO(X 2Πr) collision system has been investigated in a guided-ion beam experiment. Absolute cross sections for production of NO+ and O2+ have been measured at collision energies from near thermal to ∼12 eV. Time-of-flight measurements of the NO+ product recoil velocities, at collision energies of 1.3, 3.3, 5.2, and 9.1 eV confirm two hyperthermal charge transfer channels with thresholds at 0.2 and 2.0 eV. A kinematic analysis indicates mechanisms that give rise to spin-forbidden NO+(X 1Σ+)+O(1D) and endothermic NO+(a 3Σ+)+O(3P) products, respectively. The O2++N channel is observed to have a threshold of 0.7±0.1 eV. Time-of-flight studies of the O2+ product at collision energies of 3.3 and 5.9 eV indicate a complex-mediated mechanism.
Spectroscopic characterization of pure and cation-stabilized sodium phosphate
Authors: Cole R. S. , Frech Roger
Published: July 2002

A systematic study was conducted of pure Na3PO4 and solid solutions of Na3PO4 doped with Mg2+ and Zn2+. Na3PO4 has two solid phases: a low-temperature tetragonal phase and a high-temperature cubic phase. A factor group analysis of the two phases yielded the expected number of modes and their symmetry-based assignments. A temperature-dependent Raman spectroscopic study was then performed for the pure compound. Analysis of the doped Na3PO4 has also provided another description of the local structural distortions present in the cubic structure.
Structure of polydisperse dipolar hard-sphere fluids
Authors: Costa Cabral B. J.
Published: July 2002

We report Monte Carlo simulations of dipolar hard-sphere fluids with a nonuniform distribution of the dipole strengths or different hard-sphere diameters. Dipolar polydispersity significantly reduces the spontaneous orientational order observed in the ferroelectric phase of monodisperse dipolar hard-sphere fluids at high densities and low temperatures. Equimolar binary mixtures of dipolar hard spheres of different diameters σa and σb also exhibit ferroelectric order. However, the order parameters are very dependent on the ratio σa/σb. Chains of dipolar hard spheres, typical of strongly interacting dipolar hard spheres at low densities and low temperatures, were observed in binary fluids of dipolar and neutral hard spheres of different sizes.
Diffusion in supersaturated solutions: Application to the case of supersaturated protein solutions
Authors: Izmailov Alexander F. , Myerson Allan S.
Published: July 2002

The derivation of solute diffusion coefficient from the quasiequilibrium probability density function of the underlying stochastic quantity-characteristic size of subcritical solute clusters, was carried out within the classical formalism of metastable state relaxation (nucleation). It has been demonstrated that (a) the solute diffusion coefficient is necessarily concentration dependent; (b) the concentration dependent solute diffusivity is the function of only two parameters, solute diffusivity at saturation and total surface energy of a solute molecule. This energy, in principle, can be also a function of solute concentration for supersaturated solutions such as protein supersaturated solutions. In addition to that it has been demonstrated for supersaturated solutions of the proteins such as lysozyme and Bovine Pancreatic Trypsin Inhibitor (BPTI) solutions that (a) the total surface energy of lysozyme and BPTI molecules in the metastable state is concentration dependent. (b) The solute diffusivity as a function of solute concentration, observed in supersaturated protein solutions such as lysozyme and BPTI, can be accurately reproduced (with error less than 1.2%) on the base of classical theory of the metastable state relaxation, given that the surface energy of the protein molecules is concentration dependent.
A theoretical study of polyimide flexibility
Authors: Walsh Tiffany R. , Harkins C. G. , Sutton Adrian P.
Published: July 2002

The flexibility of a model polyimide, pyromellitic-dianhydride 1,4-oxydianiline (PMDA-ODA) is investigated using fully ab initio methods. Hartree–Fock, second-order Møller–Plesset theory (MP2) and density-functional theory (DFT) methods are employed along with both numerical and Gaussian basis sets. A hybrid scheme which combines energies and first derivatives is also used and appraised for this system. Energies of monomer fragments of the polyimide as a function of torsional angle are calculated using geometry optimizations. Extensive comparisons are made with maleanil, a smaller fragment of PMDA-ODA.
Experimental evidence of laser cooling of room temperature trans-stilbene upon excitation to the S1 state
Authors: Warmuth Christian , Milota Franz , Kauffmann Harald F. , Wadi Hicham , Pollak Eli
Published: July 2002

The room temperature photoinduced fluorescence decay of isolated trans-stilbene and trans-stilbene in the presence of 1 atm of Ar gas was measured as a function of the excitation laser frequency. Lifetimes were measured both to the blue and the red of the ground vibrational state of the ground electronic state (S0) to the ground vibrational state of the S1 state transition frequency ω00. The lifetime was found to decrease on both sides of ω00. The dependence of the decay rate on laser frequency in the presence of Ar gas is much weaker than for the isolated molecule. Both observations corroborate previous theoretical predictions of laser cooling of thermal trans-stilbene upon excitation at the ω00 frequency. The experimental results are in good agreement with theoretical analysis.
Theoretical study of the Jahn–Teller effect in X̃ 2E CH3O
Authors: Höper U. , Botschwina P. , Köppel H.
Published: July 2002

The Jahn–Teller effect in X̃ 2E CH3O is theoretically investigated in considerable detail. Making use of ab initio calculations carried out at the MRC-level we present the global shape of the adiabatic potential surfaces. Both sheets of the Jahn–Teller split X̃ 2E electronic state are used to determine the linear, quadratic and bilinear coupling constants of the e and a1 modes, as well as selected third-order constants. Dynamical calculations for the vibronic structure have been performed with these parameters. The dynamical Jahn–Teller problem is solved by taking into account all the three e modes or, alternatively, two e modes and two a1 modes. The resulting vibronic spectra are compared with those of previous work. Furthermore, ab initio calculated transition moments for Ã 2A1↔X̃ 2E are used to give an explanation for the forbidden Δj=32 transitions which were found by different experimental groups.
An accurate H2–H2 interaction potential from first principles
Authors: Diep Phong , Johnson J. Karl
Published: July 2002

We have calculated the potential energy surface extrapolated to the complete basis set limit using coupled-cluster theory with singles, doubles, and perturbational triples excitations [CCSD(T)] for the rigid monomer model of (H2)2. There is significant anisotropy among the 37 unique angular configurations selected to represent the surface. A four term spherical harmonics expansion model was chosen to fit the surface. The calculated potential energy surface reproduces the quadrupole moment to within 0.58% and the experimental well depth to within 1%. The second virial coefficient has been computed from the fitted potential energy surface. The usual semiclassical treatment of quantum mechanical effects on the second virial coefficient was applied in the temperature range of 100–500 K. We have developed a new technique for computing the quantum second virial coefficient by combining Feynman’s path integral formalism and Monte Carlo integration. The calculated virial coefficient compares very well with published experimental measurements. Integral elastic cross sections were calculated for the scattering of para-H2/para-H2 by use of the close-coupling method. The interaction potential model from this work is able to reproduce the experimental cross sections in the relative kinetic velocity range of 900–2300 m/s.
Identifiability of a model for diffusion-mediated intramolecular excited-state quenching
Authors: Novikov Eugene , Molski Andrzej , Boens Noël
Published: July 2002

A model for diffusion-mediated intramolecular fluorescence quenching based on the analytical solution of the Smoluchowski differential equation is considered. It is assumed that quenching occurs with a finite intrinsic rate at the distance of closest contact between the reactants. The solution has been worked out in terms of Laplace transforms with the aim to perform a deterministic identifiability study of this model. It is shown that the rate constant of excited-state deactivation and the system dimensionality can be uniquely recovered without additional information from a single fluorescence decay trace. The other system parameters (the diffusion coefficient, the intrinsic reaction rate constant, the distance of closest approach, and the distance of maximal separation), influencing the diffusion-controlled quenching, can be determined up to a scaling factor. However, if a priori information about one of these parameters is available, the model becomes globally identifiable (i.e., all model parameters can be uniquely determined).
A dynamic model for exciton self-trapping in conjugated polymers. II. Implementation
Authors: Kobrak Mark N. , Bittner Eric R.
Published: July 2002

We examine the electronic and vibrational dynamics of a model conjugated polymer using a particle–hole treatment for electronic excitation described in Ref. . We observe the transition from a delocalized free exciton state to a self-trapped exciton, and compare the characteristics of the process of localization to those predicted by existing theories. We find that the reaction path to self-trapping involves a well-defined intermediate state, complicating the process of cooling for the self-trapped exciton. We also find that high-energy excitons do not couple strongly to the lattice, and therefore do not self-trap.
Polymer melts and polymer solutions near patterned surfaces
Authors: Seok Chaok , Freed Karl F. , Szleifer Igal
Published: July 2002

We present analytical solutions for density profiles of homopolymer melts and of the mathematically equivalent, incompressible polymer solutions near heterogenous, periodically patterned surfaces. The theory employs an analytic density functional-self-consistent field theory, and particular applications consider striped and checkerboard patterns. The computations illustrate the competing influences of the pattern size and the bulk correlation length ξ on the density profile both at the surface and orthogonal to the surface. The density profiles are determined by the bulk correlation length ξ if the thickness of the stripes L is larger than 2ξ and by L if L<2ξ.