bluuebunny/crossref_metadata_2025 huggingface dataset:
Infrared multiphoton dissociation of 2-chloro-1,1,1,2-tetrafluoroethane in a molecular beam
Authors: Yokoyama Atsushi , Yokoyama Keiichi , Fujisawa Ginji
Published: July 2002

Mechanism and dynamics of the infrared multiphoton dissociation of 2-chloro-1, 1,1,2-tetrafluoroethane have been studied using a photofragmentation translational spectroscopy. The molecule dissociates competitively through three-centered elimination of HCl and C–Cl bond rupture. The HCl elimination reaction accounts for 74% of the total primary dissociation yields. The center-of-mass translational energy distribution for the HCl elimination indicates that an exit barrier of several kcal/mol exists along the reaction coordinate on the potential energy surface. The infrared multiphoton dissociation of CF3CF produced by the HCl elimination from CF3CHClF also occurs as a secondary process through its dissociation into two CF2 molecules. The average excitation energy of dissociating CF3CHClF has been determined to be about 20 kcal/mol above the C–Cl dissociation threshold of the molecule by comparing the observed center-of-mass translational energy distribution for the C–Cl bond rupture reaction with that calculated by Rice–Ramsperger–Kassel–Marcus (RRKM) theory.
Absolute cross sections for the electron impact ionization of the NF2 and NF free radicals
Authors: Tarnovsky V. , Levin A. , Becker K.
Published: July 2002

We report measurements of the absolute cross sections for the electron-impact ionization of the NF2 and NF free radicals from threshold to 200 eV. At 70 eV, the absolute parent NF2 and NF ionization cross section are 1.25±0.23 Å2 and 1.05±0.19 Å2, respectively. We found little evidence of the presence of vibrationally excited radicals in the incident target beam for both NF2 and NF. We also studied the dissociative ionization of NF2 and NF. The absolute cross section for the formation of the NF+ fragment ions from NF2 was found to be 0.75±0.15 Å2 at 70 eV. The NF+ fragment ions are formed with little excess kinetic energy. The only significant pathway leading to the observed NF+ fragment ions is the single positive ion formation. Our data show no indication of the presence of double positive ion formation channels or positive–negative ion pair formation processes. Cross sections for the formation of the atomic fragment ions F+ and N+ from both NF2 as well as NF were found to be small with maximum values of less than 0.1 Å2 at 70 eV in all cases.
Chain-length dependence of interlayer interaction in crystalline n-alkanes from Raman longitudinal acoustic mode measurements
Authors: Snyder R. G. , Strauss H. L. , Alamo R. , Mandelkern L.
Published: July 2002

An analysis of the observed frequencies of the Raman-active longitudinal acoustic mode (LAM) bands of room-temperature crystalline n-alkanes in the chain-length range C33 to C246 indicates that, as the chains become longer, there is a significant decrease in interlayer interaction, this interaction being measured by the vibrational coupling between the ends of chains in adjoining layers. This conclusion is based mainly on LAM measurements on n-alkanes longer than 100 carbons that have recently become available. In the present analysis, the n-alkane crystals are modeled as collinear monatomic chains having end-to-end interactions to simulate interlayer interaction. One intrachain force constant (F) and one interchain coupling force constant (f ) were evaluated from the observed frequencies of the LAM-1 and LAM-3 bands for each of seven n-alkanes: C48, C62, C70, C72, C94, C150, and C192. As expected, the values found for F are essentially independent of chain length. However, the values of f were found to decrease in going to longer chains, contradicting the generally held view that the interlayer force constant is independent of chain length. The value of f is proportional to 1/n within experimental error. If we assume this proportionality, the observed Raman frequencies of the LAM-m modes can be accounted for quantitatively. This is not the case if f is assumed independent of chain length. The decrease in the interchain interaction with increasing chain length is attributed to an increase in the layer separation resulting from an increase in chain-end amplitudes associated with the low-frequency longitudinal vibrations of longer chains.
Near-infrared spectrum and rotational predissociation dynamics of the He–HF complex from an ab initio symmetry-adapted perturbation theory potential
Authors: Moszynski Robert , Jeziorski Bogumil , van der Avoird Ad , Wormer Paul E. S.
Published: July 2002

Starting from an ab initio symmetry-adapted perturbation theory potential energy surface we have performed converged variational and close-coupling calculations of the bound rovibrational states and of the positions and widths of rotationally predissociating resonances of HeHF and HeDF van der Waals complexes. The energy levels were used to compute transition frequencies in the near-infrared spectra of these complexes corresponding to the simultaneous excitation of vibration and internal rotation in the HF(DF) subunit in the complex. The computed transition energies and other model independent characteristics of the near-infrared spectra are in excellent agreement with the results of high-resolution measurements of Lovejoy and Nesbitt [C. M. Lovejoy and D. J. Nesbitt, J. Chem. Phys. 93, 5387 (1990)]. In particular, the ab initio potential predicts dissociation energies of 7.38 and 7.50 cm−1 for HeHF and HeDF, respectively, in very good agreement with the Lovejoy and Nesbitt results of 7.35 and 7.52 cm−1. The agreement of the observed and calculated linewidths is less satisfactory. We have found, however, that the linewidths are very sensitive to the accuracy of the short-range contribution to the V1(r,R) term in the anisotropic expansion of the potential. By simple scaling of the latter component we have obtained linewidths in very good agreement with the experimental results. We have also found that this scaling introduces a very small (2%) change in the total potential around the van der Waals minimum.
A direct measurement of the polyion conformation in aqueous solutions at different temperatures. Small angle neutron scattering of PSSNa using zero average and full contrast
Authors: Boué F. , Cotton J. P. , Lapp A. , Jannink G.
Published: July 2002

The intrachain correlations in polystyrene sulfonate of sodium (PSSNa) have been measured as a function of the temperature, in semidilute solutions, using the zero average contrast technique. No temperature effect is observed. We also display and discuss data on interchain correlation, which are obtained from a scattering experiment at full contrast.
Semiclassical surface-hopping approximations for the calculation of solvent-induced vibrational-relaxation rate constants
Authors: Arce Julio C. , Herman Michael F.
Published: July 2002

Approximate schemes for the calculation of the rates of transitions between vibrational states of a molecule due to the interactions with a solvent are devised based on a rigorous, general semiclassical surface-hopping formalism developed earlier. The formal framework is based on an adiabatic separation of time scales between the fast molecular vibrations and the relatively slow bath motions. (The bath is composed of the solvent degrees of freedom plus all the molecular degrees of freedom other than vibrations.) As a result, the dynamics of the system are described in terms of bath motions occurring on adiabatic vibrational-energy surfaces, which are coupled by a nonadiabatic interaction. The time-dependent vibrational transition probability is evaluated by propagating the canonical density of the system, with the molecule in the initial adiabatic vibrational state, forward in time, and then projecting it onto the final adiabatic vibrational state of interest. The temporal evolution of the density is carried out with a semiclassical surface-hopping propagator, in which the motion of the bath on an adiabatic vibrational surface is described in terms of the familiar (adiabatic) semiclassical propagator, while transitions are accounted for in terms of instantaneous hops of the bath paths between the adiabatic vibrational surfaces involved, with an integration over all possible hopping points.

Energy is conserved in the hops, and the only component of momentum that changes is the one along the nonadiabatic coupling vector. When the nonadiabatic interaction is taken into account to first order, the transition probability is predicted to become linear in the long-time limit. Various methods for extracting the relaxation rate constant in this limit are presented, and a simple model system with a one-dimensional bath is employed to compare their practical efficiency for finite time. In addition, this system is used to numerically demonstrate that local approximations for the adiabatic vibrational surfaces and the nonadiabatic coupling yield accurate results, with great reduction of the amount of computation time. Since a local approximation for the vibrational surfaces makes an N-dimensional problem separable into N effectively one-dimensional ones, this treatment is seen to be more generally applicable to realistic systems.
An ab initio investigation on transition states and reactivity of chloroethane with OH radical
Authors: Sekušak Sanja , Güsten Hans , Sabljić Aleksandar
Published: July 2002

The reaction C2H5Cl+⋅OH→C2H5Cl⋅+H2O (α and β abstraction) has been investigated by ab initio molecular orbital theory with several basis sets and levels of correlation. Optimized geometries and harmonic vibrational frequencies have been calculated for all reactants, transition states, and products at the (U)HF/6-31G(d,p) and (U)MP2/6-31G(d,p) levels of theory. The correlation energy is found to play an important role in determining the barrier heights and reaction enthalpies as well as the geometry and the vibrational frequencies of the transition states. A pseudocyclic transition state is found to be favorable to the β-abstraction reaction since the participation of the chlorine substituent reduces the barrier height by 0.95 kcal/mol, through a relatively large inductive through-space effect. The best results for the barrier heights and reaction enthalpies have been obtained using the second-order Mo/ller–Plesset perturbation theory with spin projection employing the 6-311+G(2d,p) basis set. A satisfactory agreement is found with available experimental values.
Some consequences of the nucleation theorem for binary fluids
Authors: Oxtoby David W. , Laaksonen Ari
Published: July 2002

This paper describes some extensions and applications of the nucleation theorem, which relates the variation of nucleation rates with gas partial pressures to the size and composition of the critical nucleus. It is first pointed out that even when only critical supersaturations (as opposed to nucleation rates) are measured for binary systems, the composition of the critical nucleus can still be extracted from experimental data. The effect of carrier-gas pressure on nucleation rates is then examined and some rather general bounds are derived. Existing diffusion cloud chamber data appear to be inconsistent with these bounds, although more accurate measurements of binary gas second virial coefficients are necessary to draw a definitive conclusion.
The NaHg red bands revisited
Authors: Gruber D. , Domiaty U. , Li X. , Windholz L. , Gleichmann M. , Heß B. A.
Published: July 2002

Along with an ab initio recalculation of the NaHg(X1/2) ground state potential curve we present an identification of the chemiluminescence of the NaHg red bands. Based on quantum simulations of the relevant vibronic transitions following production of the NaHg molecule by the photochemical reaction via the sodium dimer, we propose the nascent population distribution in the upper NaHg(II1/2) state to peak at v′=2, described by a Gaussian distribution with α=0.023.
Multidensity integral equation theory for highly asymmetric electrolyte solutions
Authors: Kalyuzhnyi Yu. V. , Vlachy V. , Holovko M. F. , Stell G.
Published: July 2002

Integral equation theory based on a recently developed multidensity formalism [Mol. Phys. 78, 1247 (1993)] is proposed to study highly asymmetric electrolyte (polyelectrolyte) solutions. The system studied consists of large and highly charged polyions and small counterions having one or two elementary charges. The potential energy of interaction between counterions and polyions is separated into two parts, a strongly attractive part responsible for the association and a nonassociative part. Due to the strong asymmetry in size we can treat each counterion as bondable to a limited number of polyions n, while each polyion can bond arbitrary number of counterions. In our cluster expansion appropriate to the problem the diagrams appearing in the activity expansion of the one-point counterion density are classified in terms of the number of associating bonds incident upon the labeled white counterion circle. The corresponding diagrams for the one-point polyion density are classified in the usual way. A generalized version of the Ornstein–Zernike equation, which involves n+1 counterion densities and one polyion density, together with hypernetted-chain-like (HNC) closure conditions are derived. The simplest two-density version of the theory yields very good agreement with new and existing computer simulations for both thermodynamical and structural properties of these systems. This good agreement extends into the region of parameter space where the ordinary HNC approximation does not have a convergent solution.
Use of recursively generated intermediates in state selective multireference coupled-cluster method: A numerical example
Authors: Ghose Keya B. , Adamowicz Ludwik
Published: July 2002

The present work represents the first attempt to utilize the idea of recursively generated intermediates (RGI) in the framework of the state-selective multi-reference coupled-cluster method truncated at triple excitations [SS CCSD(T)]. The expressions for stepwise generation of intermediates are so structured that the spin and point symmetry simplifications can be easily applied during computation. Suitable modifications in SS CCSD(T) equations are introduced to allow for optional quasilinearization of nonlinear terms in difficult convergence situations. The computational code is, as expected, much faster than the SS CCSD(T) code without RGI adaptation. This has been numerically demonstrated by potential energy surface (PES) calculation of the HF molecule using a double zeta basis.
Relaxation times in transient electric birefringence and electric-field light scattering of flexible polymer chains
Authors: Navarro S. , Lopez Martinez M. C. , Garcia de la Torre J.
Published: July 2002

We study in the present paper the response of a flexible macromolecular chain to the application or removal of an electric field. The polymer is mainly modeled as a Gaussian chain, and the case of freely jointed chains is also treated. We consider the dynamics of the chains, after the inception and subsequent cessation of an electric field. In particular, we calculate two properties. One of them is the time-dependent chain expansion, as measured by the components of the gyration tensor, that can be determined by transient electric-field light scattering. The other property is the transient electric birefringence, related to the reorientation of the chain segments. In this way, the dynamics of two different properties can be compared. The transient properties are analyzed in terms of a series of relaxation times. We propose the use of a mean relaxation time as a convenient representation of the rate of the dynamic process, and show that it can be deduced from simulation or experiments with more accuracy than the longest relaxation time. Our computational procedure is based on Brownian dynamics simulation. For Gaussian chains without hydrodynamic interaction, the results are compared with the predictions from the Rouse theory. We evaluate the influence of the strength of the force or field. Simulations are also carried out including hydrodynamic interactions, so that the importance of this effect can be assessed. We propose some combination of relaxation times with other macromolecular properties that take universal numerical values.
Wave-packet dynamics in the Li2 E(1Σ+g) shelf state: Simultaneous observation of vibrational and rotational recurrences with single rovibronic control of an intermediate state
Authors: Papanikolas John M. , Williams Richard M. , Kleiber Paul D. , Hart Jeffrey L. , Brink Carsten , Price Stephen D. , Leone Stephen R.
Published: July 2002

A three-step excitation sequence is used to study the wave-packet dynamics in the E(1Σ+g) ‘‘shelf’’ state of lithium dimer. In the first excitation step, a continuous wave (cw) dye laser prepares a single rovibrational level (v=14, J=22) in the intermediate 7Li2 A(1Σ+u) state. Ultrafast excitation of this single level with a 200 fs laser pulse centered at 803 nm creates a rovibrational wave packet (v=13–16; J=21 and 23) in the shelf region of the E(1Σ+g) state. The motion of this three-dimensional wave packet is probed via ionization by a second ultrafast laser pulse of the same color. The initial cw excitation step allows precise control of the states that compose the wave packet. Fourier analysis of the pump–probe transients shows 15 frequency components that correspond to energy differences between the levels that constitute the wave packet. Because of the large rotational energy splitting, the rotational beats occur in the same frequency range as the vibrational beats. Experiments performed with parallel and perpendicular pump-probe polarizations provide a ‘‘magic angle’’ transient in which only the pure vibrational beats are observed, thus aiding in the spectroscopic assignment. The observed beat frequencies agree well with conventional high resolution frequency-domain spectroscopy. Applications of the intermediate-state control of the initial wave packet are discussed.
On the nature of the coexistence surface and the critical double point in a polymer plus poor solvent system
Authors: Venkatachalam S. , Kumar A. , Gopal E. S. R.
Published: July 2002

We observe a critical double point (CDP) in a polystyrene (molecular weight=13 000) plus acetone mixture when a small amount of water is added to it instead of varying the molecular weight or the pressure. A very close access to CDP has been achieved, as exemplified by a sample with a miscibility gap, ΔTE (the difference between the upper and lower extremum temperatures, TEU and TEL, respectively) as small as 194 mK. The critical exponent (β) for the phase diagram exhibits a twofold increase in its 3-D Ising value for a sample with ΔTE=194 mK, when the usual field variable (the reduced temperature, t) is used. The universal value of β(=0.325) is retrieved for any ΔTE with the aid of the field variable tUL[=‖(TEU−T)(T−TEL) /TEU/BTEL‖]. Our evidence shows that, at least for the order parameter, ΔTE is preferable to ΔTc (the difference between the upper and lower critical temperatures, TU and TL, respectively) as a measure of the distance from CDP in this system. The critical line and the locus of the extremum temperatures are similar and are nearly parabolic in shape. Most of our observations can be understood in terms of the geometric picture of phase transitions as well as the Landau–Ginzburg theory, as applied to reentrant phase transitions. A microscopic understanding of the role of water in reducing the miscibility gap (ΔTE) in these complex systems remains to be achieved.
Colloidal dispersion confined in a planar slit: A density functional approach
Authors: Choudhury Niharendu , Ghosh Swapan K.
Published: July 2002

We present a simple density functional approach for the prediction of the local density profile of a colloidal suspension confined in a charged planar slit. Both the interparticle and wall–particle interactions are modeled to be of screened Coulomb type. The short range part of the interparticle correlation is treated through a nonperturbative weighted density approach, while the long range contribution is treated perturbatively in terms of the density inhomogeneity. The input correlation functions for the bulk fluid are obtained through the rescaled mean spherical approximation. The calculated density profiles are shown on an average to compare well with results from computer simulation.
Tunneling dynamics in water tetramer and pentamer
Authors: Gregory Jonathon K. , Clary David C.
Published: July 2002

We present a theoretical study of the structures, energetics and tunneling dynamics of the cyclic water tetramer and pentamer, both of which have recently been observed experimentally. The very good agreement between vibrationally averaged rotational constants from diffusion quantum Monte Carlo calculations and experimental values confirms that the structures are cyclic. We calculate a splitting for a tunneling motion that is a likely cause for doublets observed experimentally in the far infrared spectrum of (D2O)4. Predictions of tunneling splittings yet to be observed in the water pentamer are made.
Second virial coefficients, critical temperatures, and the molecular shapes of long n-alkanes
Authors: Vega Carlos , López Rodríguez Antonio
Published: July 2002

The second virial coefficient for a molecular model of n-alkanes with up to 200 carbon atoms has been computed for temperatures in the range of 1000 K<T<2000 K. The model used in the calculations is based on one which reproduces the experimental values of the second virial coefficient for short n-alkanes. The Boyle temperature of n-alkanes is not much affected by the length of the chain for n-alkanes with more than 100 carbon atoms. According to the Flory theory, for infinitely long chains the Boyle temperature and the critical temperature, denoted as the θ temperature are the same. On this basis using the present model we find θ=1620 K as the critical temperature of polymethylene. Scaling laws for the square of the end-to-end distance and for the radius of gyration at several temperatures are analyzed. For high temperatures, the scaling laws correspond to good solvent conditions whereas at the θ temperature the scaling laws are those of an ideal chain. The shape of long n-alkanes at several temperatures is also analyzed. We find that the polymethylene chains present an anisotropy similar to that of a parallelepiped with a ratio of sizes of 1:1.7:3.6.
Conformationally induced rotation of a molecular electronic transition moment
Authors: Joireman Paul W. , Kroemer Romano T. , Pratt David W. , Simons John P.
Published: July 2002

Rovibronic band contour analysis of the two S1←S0 origin bands of 3-phenyl-1-propionic acid has revealed a strong, conformationally dependent rotation of the electronic transition moment in the molecular frame. High level ab initio calculations confirm experimental observations, which are attributed to symmetry breaking and ring–chain interactions.
Spectroscopic gas phase determination of ΔH° [axial/equatorial] for 3-methyl cyclohexanone
Authors: Potts Alan R. , Baer Tomas
Published: July 2002

The gas-phase enthalpy of interconversion (ΔH°) between the axial and equatorial conformations of 3-methylcyclohexanone (3-MCHO) is determined to be 1.55±0.12 kcal mol−1. The relative conformational concentrations are probed at various equilibrium temperatures from 303 to 453 K by 2+1 resonance-enhanced multiphoton ionization (REMPI) spectroscopy. We report that under our expansion conditions the measured relative peak areas of the two conformers in the REMPI spectrum are proportional to their relative pre-expansion concentrations. ΔH° [axial/equatorial] is obtained from a van’t Hoff plot of the measured relative peak areas of the two conformers as a function of temperature. Comparisons are made to molecular mechanics and various ab initio molecular orbital calculations including G2(MP2) theory.
On the zero point energy in classical trajectory computations
Authors: Ben-Nun M. , Levine R. D.
Published: July 2002

The problem of zero point energy in classical trajectory computations is discussed and illustrated by an example of dissociation where the zero point energy is used to provide the required energy. This is not possible in quantal dynamics. A proposed route to the alleviation of the problem, based on using classical-like trajectories which mimic the solution of the (expectation values) of Heisenberg equations of motion, is discussed. In general, one cannot simultaneously correct for all possible expectation values, so the remedy is at best partial. The variable whose expectation value and variance is to be handled correctly is examined in detail for a one-dimensional anharmonic potential, and is identified with the logarithmic derivative of the wave function in the Wentzel–Kramers–Brillouin (WKB) approximation. The multidimensional case is also discussed and it is pointed out that the zero point energy problem can be particularly severe for systems which exhibit a locally unstable classical motion.
The shape, size, aggregation, hydration, correlation times, and thermodynamic studies on macromonomer micelles
Authors: Geetha Bhaskar , Mandal Asit Baran
Published: July 2002

The hydration of the macromonomer micelles has been directly determined from the measurements of conductance of micellar solutions of macromonomer in 5 mM NaCl using the principle of the obstruction of electrolyte migration by the surfactants. This coupled with the intrinsic viscosity has helped in evaluating asymmetry of the micellar particles and their average axial ratios. The hydration has also been determined by Einstein and Vand’s equations, and are in good agreement with the conductivity results. Hydration number and micellar sizes are variable with temperature. The shape of the macromonomer micelles has been observed to be more or less spherical at 15 °C. However, above 15 °C the shape of the micelles are ellipsoids. From the absolute values of the axes, the micellar volume, hydrodynamic radius, radius of gyration, frictional, and diffusional coefficients as well as translational (τD), rotational (τr), and effective correlation (τa) times have been calculated. τr values for Triton X-100, Tween-20, Tween-80, and macromonomer at 25 °C are 45, 28, 25, and 8 ns, respectively. The aggregation number N̄ of the macromonomer in an aqueous solution was determined by fluorescence spectroscopic technique. Blueshift of ANS emission was observed in the presence of macromonomer micelles. Both N̄ and the Stern–Volmer rate constant (KSV) are variable with temperature and found them to be maximum at 25 °C indicating the macromonomer micelles attained a state of fully extended conformation at 25 °C. Diffusion energy (ED) of the macromonomer micelles in water has been calculated and found to be 21.0 and 21.1 kJ mol−1 for oblate and prolate models, respectively. The partial molal volume of the macromonomer micelles has also been determined and its comparison with the molar volume of pure macromonomer suggested a volume contraction due to immobilization of the water phase by the hydrophilic head groups of macromonomer. Thermodynamic activation parameters for viscous flow are in favor of the formation of structured water by the presence of macromonomer micelles. Enthalpy (ΔH*) and entropy (ΔS*) values show a very good compensation between them.
Proton transfer in the ground and lowest excited states of malonaldehyde: A comparative density functional and post-Hartree–Fock study
Authors: Barone Vincenzo , Adamo Carlo
Published: July 2002

Intramolecular proton transfer in the ground and the lowest two excited electronic states of malonaldehyde has been investigated by using density functional and post-Hartree–Fock methods. Our best estimates of the energy barriers governing proton transfer in the ground and lowest triplet state are quite low (4.3 and 6.6 kcal/mol, respectively), whereas a significantly higher barrier (12.0 kcal/mol) is obtained for the second triplet state. The coupled cluster approach provides reliable results already with relatively compact basis sets, its only drawback being the very unfavorable scaling with the number of active electrons. Among the cheaper methods, those based on the many-body perturbative approach provide good results for the ground electronic state, but their performances strongly deteriorate for excited states. The overestimation of correlation energy by conventional density functional methods produces an excessive degree of conjugation in the backbone of malonaldehyde with the consequent underestimation of energy barriers governing proton transfer. A more coherent picture is offered by a hybrid density functional/Hartree–Fock approach, which couples good structural predictions with a reduced, although still not negligible, underestimation of energy barriers. Furthermore, different electronic states are described with comparable accuracy.
A global H2O potential energy surface for the reaction O(1D)+H2→OH+H
Authors: Ho Tak-San , Hollebeek Timothy , Rabitz Herschel , Harding Lawrence B. , Schatz George C.
Published: July 2002

A global, single-valued ground-state H2O potential surface for the reaction O(1D)+H2→OH+H has been constructed from a new set of accurate ab initio data using a general multidimensional interpolation method. The ab initio calculations are of the multireference, configuration interaction variety and were carried out using augmented polarized triple zeta basis sets. The multidimensional method is formulated within the framework of the reproducing kernel Hilbert space theory. The H2O potential is expressed as a many-body sum of a single one-body term, three two-body terms, and a single three-body term. The one-body term is the dissociation energy to the three-atom limit 2H(2S)+O(3P). The two-body terms are two O–H and one H–H adiabatic diatomic potentials of lowest energy. Each diatomic term is obtained by interpolating a discrete set of ab initio data using a one-dimensional, second-order, distancelike reproducing kernel. The three-body term is obtained by interpolating the difference of the H2O ab initio data and the one- and two-body sum by means of a direct product of three one-dimensional reproducing kernels on an optimized regular three-dimensional grid. The H2O potential energy surface is accurate, globally smooth, easy to evaluate, and asymptotically correct. Extensive quasiclassical trajectory calculations based on this new potential energy surface have been performed and compared with the results based on the potential energy surface of Murrell and Carter (MC) and that of Schinke and Lester (SL). Comparisons with recent experimental measurements on total cross sections, isotope effects, rate constants, vibrational, rotational, and angular distributions of the O(1D)+H2/HD reaction show that the new potential energy surface is a significant improvement over the MC and SL surfaces.
The zero-point energy problem in classical trajectory simulations at dissociation threshold
Authors: McCormack Drew A. , Lim Kieran F.
Published: July 2002

Quasiclassical trajectory calculations offer a cost-effective means of investigating the dynamics of chemical reactions. However, they suffer from the zero-point energy (ZPE) problem, whereby the (quantum) ZPE motion can contribute to an overestimation of the rate coefficient. This paper reports on some dynamics of the Hénon–Heiles system. Dynamics of the water molecule at energies just below the (quantum) dissociation threshold, are also reported. The TRAPZ method [Lim and McCormack, J. Chem. Phys. 102, 1705 (1995)] leads to a definite improvement over unconstrained classical mechanics.
Chebyshev expansion methods for electronic structure calculations on large molecular systems
Authors: Baer Roi , Head-Gordon Martin
Published: July 2002

The Chebyshev polynomial expansion of the one electron density matrix (DM) in electronic structure calculations is studied, extended in several ways, and benchmark demonstrations are applied to large saturated hydrocarbon systems, using a tight-binding method. We describe a flexible tree code for the sparse numerical algebra. We present an efficient method to locate the chemical potential. A reverse summation of the expansion is found to significantly improve numerical speed. We also discuss the use of Chebyshev expansions as analytical tools to estimate the range and sparsity of the DM and the overlap matrix. Using these analytical estimates, a comparison with other linear scaling algorithms and their applicability to various systems is considered.
Vibrational-entropy-driven order-order phase transition at monolayer coverage: CO on Co{101̄0}
Authors: Gu J. , Sim W. S. , King D. A.
Published: July 2002

A novel reversible order-order phase transition is revealed by reflection absorption infrared spectroscopy and low-energy electron diffraction studies of a chemisorbed monolayer of CO on Co{101̄0} between 100 and 150 K. The high-temperature phase consists of tilted two-fold bridging CO molecules in the well-established p(2×1)g structure, while the low-temperature phase involves the movement of one-third of the CO molecules into three-fold hollow sites to produce a new p(6×1)g structure. The temperature-induced site switching is attributed to a vibrational-entropy- driven second-order displacive phase transition.
Avoided crossings, scars, and transition to chaos
Authors: Arranz F. J. , Borondo F. , Benito R. M.
Published: July 2002

The correlation diagram of the LiNC/LiCN isomerizing system as a function of ℏ, taken as a parameter, is considered in conditions of widespread classical chaos. Two series of isolated avoided crossings of very different nature, involving states related by the same 1:8 quantum resonance condition, are characterized and the corresponding interaction matrix elements between adiabatic states analytically calculated and analyzed. One of these series has been found to separate two distinct regions: one for smaller values of ℏ (at any given energy) corresponding to regular states, i.e., with a definite nodal pattern, and the other of much more stochastic behavior. Moreover, the effect of the interaction involved in these avoided crossings is to mix the regular wave functions giving rise to wave functions strongly scarred by the periodic orbits of the associated 1:8 classical resonance. This result constitutes an interpretation of the scarring effect from a new perspective.
Upconversion dynamics of Er3+ doped RbGd2Br7
Authors: Riedener Toni , Güdel Hans U.
Published: July 2002

NIR-to-VIS upconversion mechanisms upon excitation into I11/24 around 980 nm in the Er3+ doped low energy phonon host material RbGd2Br7 are identified and analyzed. The small energy difference between the first excitation step I15/24→I11/24 and the second step I11/24→F7/24 leads to a competitive situation of excited state absorption and energy transfer upconversion processes. Their relative efficiency strongly depends on the excitation energy and temperature. Fingerprinting techniques based on the temporal evolution of the upconverted luminescence after a 10 ns laser pulse are used to distinguish the two mechanisms. Rate constants for the relevant processes are determined. With Wt=68 s−1 at 10 K the energy transfer step in the upconversion process is comparable to Er3+ doped CsCdBr3.
Modeling the concentration dependence of diffusion in zeolites. I. Analytical theory for benzene in Na-Y
Authors: Saravanan Chandra , Auerbach Scott M.
Published: July 2002

We have developed an analytical expression for the diffusion coefficient of benzene in Na-Y at finite loadings in terms of fundamental rate coefficients. Our theory assumes that benzene molecules jump among SII and W sites, located near Na+ ions in 6-rings and in 12-ring windows, respectively. We assume that instantaneous occupancies in different supercages are identical, a mean field approximation yielding Dθ=16kθaθ2 where aθ≅11 Å is the mean intercage jump length and 1/kθ is the mean supercage residence time. We show that kθ=κ⋅k1⋅P1, where P1 is the probability of occupying a W site, k1 is the total rate of leaving a W site, and κ is the transmission coefficient for cage-to-cage motion. We assume κ=12 for all loadings, and derive analytical formulas for the T and θ dependencies of k1 and P1, assuming that SII and W site occupancies are either 0 or 1 and that benzenes do not otherwise interact. Exact expressions for P1 in the canonical and grand canonical ensembles are related for finite systems with a new correspondence rule. For θ<2/3, the SII→W→SII process contributes no loading dependence to kθ, while the SII→W→W process gives an increasing loading dependence of 1/(2−3θ). For θ>23, kθ initially increases due to enhanced W population, then decreases due to blocking of target W sites. In the article that follows this one we show that our theory agrees quantitatively with simulation, and agrees qualitatively with experiment for low to moderate loadings.
Mobilities of carbon cluster ions: Critical importance of the molecular attractive potential
Authors: Shvartsburg Alexandre A. , Schatz George C. , Jarrold Martin F.
Published: July 2002

Mobilities in helium gas for isomers belonging to the major structural families of carbon clusters identified in drift tube studies (chains, monocyclic and bicyclic rings, graphite sheets, and fullerenes and their dimers) have been evaluated by trajectory calculations employing a realistic ion-He interaction potential. For all the species considered, the agreement between the measured and calculated mobilities at room temperature improves by at least a factor of 3 over that obtained with the widely used hard-sphere projection approximation. Furthermore, for a large representative sample of clusters belonging to all the above families, the results of trajectory calculations as a function of temperature over the range of 78–360 K are in a good agreement with the measured mobilities. This shows that the C–He pairwise potential is only weakly dependent on the structure and chemical bonding of a carbon cluster. Thus this study demonstrates the universal suitability of trajectory calculations for the accurate prediction of the gas phase mobilities for polyatomic ions with various shapes and sizes, and the uniform superiority of this method over the previously used approximations. In particular, the trajectory calculations for large (n=120–140) fullerenes show that these cages have near-spherical shapes found by theory, while the projection approximation would erroneously assign them as “buckytubes.” It also appears that the mobility may be substantially affected by the degree of charge localization on a specific atom in the cluster, especially at low temperatures.
A comparison of density functional and integral equation theories vs Monte Carlo simulations for hard sphere associating fluids near a hard wall
Authors: Segura Chad J. , Vakarin Eduard V. , Chapman Walter G. , Holovko M. F.
Published: July 2002

We make a comparison of a perturbation density functional (DF) theory and an integral equation (IE) theory with the results from Monte Carlo simulations for nonuniform fluids of hard spheres with one or two association sites. The DF used applies the weighting from Tarazona’s hard sphere density functional theory to Wertheim’s bulk first order perturbation theory. The IE theory is the associative form of the Henderson–Abraham–Barker (HAB) equation. We compare results from the theories with simulation results for density profiles and adsorption of one- and two-sited associating fluids against a hard, smooth wall over a range of temperatures and molecular densities. We also report fraction of monomers profiles for the DF theory and compare these against simulation results. For dimerizing fluids, the DF theory is more accurate very close to the wall, especially at higher densities, while the IE theory has more accurate peak heights and positions away from the wall, also especially at higher densities. Accuracy of the IE theory increases with an increasing degree of association. For two-sited hard spheres, the DF theory is more accurate than the IE theory at lower densities; at higher densities accuracies are similar to that of dimerizing hard spheres.
Structure and dynamics of ring polymers
Authors: Brown Scott , Szamel Grzegorz
Published: July 2002

We study the equilibrium structure and dynamics of unconcatenated unknotted polymer rings in the melt. In accordance with earlier studies we find that rings in the melt are more compact than linear chains. We show that rings interpenetrate less and, perhaps as a result, diffuse faster than linear chains. Ring diffusion is faster for all ring sizes studied, the largest ring having numbers of monomers approximately ten times greater than the entanglement crossover of linear chains.
Orientation of the quadrupole and dipole tensors of hydroxyl groups by 17O quadrupole separated local field NMR
Authors: van Eck E. R. H. , Smith M. E.
Published: July 2002

The static 17O NMR spectra of Mg(OH)2 and amorphous Mg(OH)x(OCH3)2−x were measured. Simulation of these spectra gave e2qQ/h=6.8 MHz, η=0, and δiso=20 ppm, and e2qQ/h=7.25 MHz, η=0, and δiso=−25 ppm for the hydroxyl oxygen in, respectively, Mg(OH)2 and Mg(OH)x(OCH3)2−x. An OH distance in Mg(OH)2 of 1.001 Å was obtained using Lee–Goldberg decoupling to obtain the OH dipolar coupling constant. Dipolar oscillations in the 1H–17O cross-polarization curve provided an OH distance of 0.995 Å in Mg(OH)x(OCH3)2−x; these oscillations were not observed in Mg(OH)2. Based on differences in the OH distance and in the 17O quadrupole coupling constant it was concluded that the OH bond in Mg(OH)x(OCH3)2−x was more covalent. 17O 2D quadrupole separated local field experiments were performed on both samples in order to obtain the relative orientation of the 17O quadrupole and OH dipole tensors. In both cases the interaction tensors were found to be collinear. Lee–Goldberg decoupling during the dipolar evolution time t1 improved the resolution in the dipolar dimension. The resolution in the dipolar dimension of the 2D spectra obtained without proton decoupling during t1 was better for the Mg(OH)x(OCH3)2−x sample than for the Mg(OH)2 sample. Since the effect of the homonuclear proton dipolar coupling was less prominent in the amorphous sample, as evidenced by difference in CP-dynamics and spectral resolution, it was concluded that the hydroxyl groups were more isolated in the amorphous sample. The simulation of 2D separated quadrupole local field spectra showed that the experiment is very sensitive to differences in the relative quadrupole and dipole tensor orientation.
Morphology of thin block copolymer films on chemically patterned substrates
Authors: Chen Hao , Chakrabarti Amitabha
Published: July 2002

We present results from a numerical study of a coarse-grained model of di-block copolymer (BCP) thin films confined between two hard walls. One of these walls is neutral to the components of the BCP melt and the other one contains chemical inhomogeneities with a repeat spacing length scale comparable to the linear size of the BCP molecules. We find that the morphology of the BCP thin film is strongly influenced by the commensurability between the bulk unconstrained lamellar size λ*, and the linear size of the surface inhomogeneities w. When w≈λ*/2, the ordered morphology of the diblock copolymers has a strong overlap with the pre-assigned substrate chemical pattern throughout the film. However, for w≈λ*, the overlap strongly depends on the distance from the substrate surface. Close to the substrate surface, the overlap of the morphology with the pre-assigned chemical pattern is large but the pattern becomes out-of-phase at a distance of approximately λ*/2 from the substrate. For w≈3λ*/4, the morphology of the diblock copolymers is either similar to that for w≈λ*/2 or for w≈λ*, depending on the film thickness. For some film thickness, the morphology shows a mixed characteristics of the morphologies for w≈λ*/2 and w≈λ*.
Comparison of density matrix renormalization group calculations with electron-hole models of exciton binding in conjugated polymers
Authors: Yaron David , Moore Eric E. , Shuai Z. , Brédas J. L.
Published: July 2002

By analogy with inorganic semiconductors such as GaAs or Si, electron-hole models may be expected to provide a useful description of the excited states of conjugated polymers. Here, these models are tested against density matrix renormalization group (DMRG) calculations. The DMRG method is used to generate nearly-exact descriptions of the ground state, 11Bu optical gap state, and the band gap of the Pariser-Parr-Pople (PPP) Hamiltonian of polyenes with between 2 and 40 carbon atoms. These are compared with both bare electron-hole (singles configuration interaction theory and the random phase approximation) and dressed electron-hole (second and third order Green’s function) methods. For the optical gap, only second-order Green’s function results were obtained. When an unscreened (Ohno) electron-electron interaction potential is used, the dressed electron-hole methods work well for the band gap. The difference between the band gap predicted by bare and dressed electron-hole methods increases with chain length, suggesting the formation of a polarization cloud around the electron and hole on long chains. Dressed electron-hole theory does not work as well for the optical gap; however, the chain-length dependence of the error is weak and thus may be partially compensated by the parameterization of a semi-empirical Hamiltonian to experimental data. These results therefore support the use of dressed electron-hole theory to parameterize a semiempirical Hamiltonian to molecular data, and then make predictions for long polymer chains. When screened electron-electron interaction potentials are used, neither the bare nor dressed electron-hole models give predictions in agreement with the DMRG results. The effects of electron correlation on the ground state are shown to be larger with screened than unscreened potentials, and this may account for the breakdown in electron-hole theory for screened potentials.
Enthalpies, excess volumes, and specific heats of critical and supercritical binary mixtures
Authors: Kiselev S. B. , Rainwater J. C.
Published: July 2002

A scaling-law crossover thermodynamic model is applied to caloric properties of five binary mixtures, carbon dioxide + ethane, ethane + propane, n-pentane + benzene, n-pentane + acetone, and ammonia + water. Enthalpy increments, excess enthalpies, excess volumes, and specific heats are calculated on the phase boundaries and in the near-critical and supercritical one-phase regions, and calculated values agree well with experimental data. The method requires as input the mixture excess critical compressibility factor, as obtained from critical loci, which in this approach are determined from modified Leung–Griffiths correlations. The only additional variables needed are the acentric factors of the pure components and a few fitting parameters which describe the unknown thermodynamic energies along the critical locus and which are not needed for the phase boundary and supercritical properties in pressure–temperature–density space.
Capillary condensation of Lennard-Jones fluid in a slitlike pore filled with quenched disordered matrix
Authors: Pérez Luis , Sokołowski Stefan , Pizio Orest
Published: July 2002

We have studied a capillary condensation of a Lennard-Jones fluid in a slitlike pore filled with a quenched disordered hard-sphere matrix using the Born–Green–Yvon (BGY) equation with the Fisher–Methfessel (FM) approximation. The solution of the replica Ornstein–Zernike (ROZ) equation in the Percus–Yevick (PY) approximation for a fluid in a homogeneous matrix is used as an input. The adsorption isotherms exhibit hysteresis loops for matrix-free and highly microporous matrix-filled slitlike pores.