bluuebunny/crossref_metadata_2025 huggingface dataset:
Molecular dynamics simulations of liquid binary mixtures: Partial properties of mixing and transport coefficients
Authors: Heyes D. M.
Published: July 2002

Equilibrium molecular dynamics (MD) computer simulations have been used to determine the transport coefficients of model Ar–Kr, Ar–CH4, and CH4–N2 mixtures at a large number of liquid and dense fluid state points for which experimental data are available. Both species in each mixture are represented by single-site Lennard-Jones pair potentials with Lorentz–Berthelot mixing rules for the unlike interactions. Green–Kubo formulas and mean-square displacements are used to calculate the self-diffusion coefficients for each species and mutual-diffusion coefficients. The shear and bulk moduli and viscosities, thermal conductivity and the thermal diffusion coefficient are determined by Green–Kubo in the [NVE] and [NVT] ensembles. The thermotransport coefficients employ a rigorous definition for the heat flux, which includes the partial enthalpy of the two species, used for the first time to compute these transport coefficients. The partial volumes and enthalpies, and chemical potentials for each species, were obtained from separate computations carried out at constant pressure in the [NPT] ensemble.

The simulated density at fixed pressure, shear viscosity, and thermal conductivity of the Ar–Kr mixtures are in excellent agreement with experiment. However, the bulk viscosity shows a significant qualitative difference in the composition and temperature dependence (the latter even in the single component fluids). Agreement with experiment deteriorates as the quasispherical molecules progressively depart from spherical shape. For Ar–CH4 the density (obtained using [NPT] MD) is in good agreement with experiment, whereas the shear viscosity is in progressively poorer agreement with increasing methane content. This is caused by an overestimation of the methane viscosity (∼50% higher than experiment for pure methane). For CH4–N2 there are substantial differences between the simulated quantities and experiment. The average simulated densities are ∼5% higher than experiment over a wide temperature and composition range. The simulated shear viscosity is typically ∼10%–50% higher than experiment. The enthalpy, chemical potential, and shear modulus of the model mixtures are reproduced well using a van der Waals one-fluid model. In this, there is little to distinguish between Enskog’s mixing rules for the model from the simpler mole fraction weighted (‘‘Kay’s’’) mixing rules. The shear viscosity and thermal conductivity are also predicted reasonably well with either model, recognizing the greater statistical uncertainty in the transport coefficients when compared with the thermodynamic averages, even for the pure Lennard-Jones fluid.
Dynamics of the two-step photodissociation of azomethane
Authors: Andrews B. Kim , Burton Katherine A. , Weisman R. Bruce
Published: July 2002

Time-resolved coherent anti-Stokes Raman spectroscopy (CARS) measurements have revealed aspects of energy disposal in the 355 nm photodissociation of gas phase azomethane. Interpretation of these results is aided by earlier experiments that kinetically resolved the two steps of dissociation leading to two methyl radicals and a nitrogen molecule. Methyl radicals were observed with zero to four quanta of vibrational excitation in the ν2 out-of-plane deformation mode. Kinetic analysis showed the first-step methyl radicals to carry far more ν2 excitation than the second-step methyl radicals. Through simulation of band contours, a rotational temperature was estimated for the vibrationally unexcited second-step methyls. In addition, nascent vibrational and rotational populations were determined for the nitrogen photoproduct, which is formed in the second dissociative step. These experimental findings are compared to the results of impulsive and statistical models of energy partitioning in this system. It appears that the first step may have significant impulsive character. In the second step, some experimental findings are predicted accurately by the separate statistical ensemble (SSE) model, while other findings fall outside the range spanned by the impulsive and SSE predictions. It is suggested that the second-step energy distributions may reflect the effects of specific exit channel interactions superimposed on statistical partitioning of available energy. Quantum chemical computations on the second step’s reaction path (leading from the methyldiazenyl radical intermediate to the methyl radical and nitrogen photoproducts) should permit deeper understanding of the dissociation dynamics.
Controlling bimolecular reactions: Mode and bond selected reaction of water with translationally excited chlorine atoms
Authors: Sinha Amitabha , Thoemke John D. , Crim F. Fleming
Published: July 2002

Vibrational overtone excitation prepares water molecules in the ‖13〉−, ‖04〉−, ‖02〉−‖2〉, and ‖03〉− local mode states for a study of the influence of reagent vibration on the endothermic bimolecular reaction Cl+H2O→OH+HCl. The reaction of water molecules excited to the ‖04〉− vibrational state predominantly produces OH(v=0) while reaction from the ‖13〉− state forms mostly OH(v=1). These results support a spectator model for reaction in which the vibrational excitation of the products directly reflects the nodal pattern of the vibrational wave function in the energized molecule. Comparison of relative OH product yield from Cl+H2O(‖04〉−) and the calibration reaction O(3P)+CH3OH shows that vibrational excitation of water to the ‖04〉− state leads to a near gas kinetic reaction rate (k(‖04〉−)=2×10−10 cm3 molecule−1 s−1). Relative rate measurements for the two vibrational states ‖03〉− and ‖02〉−‖2〉, which have similar total energies but correspond to very different distributions of vibrational excitation, demonstrate the control that initially selected vibrations exert on reaction rates. The local mode stretching state ‖03〉− promotes the Cl+H2O reaction much more efficiently than the state having part of its energy in bending excitation (‖02〉−‖2〉). The localized character of the vibrational overtone excitation in water permits the observation of bond selected bimolecular reaction using this approach. The reaction of chlorine atoms with HOD molecules excited in the region of the third overtone of the O–H stretching vibration, 4νOH, produces at least an eightfold excess of OD over OH, reflecting the preferential cleavage of the vibrationally excited bond.
A global optimization approach for Lennard-Jones microclusters
Authors: Maranas Costas D. , Floudas Christodoulos A.
Published: July 2002

A global optimization approach is proposed for finding the global minimum energy configuration of Lennard-Jones microclusters. First, the original nonconvex total potential energy function, composed by rational polynomials, is transformed to the difference of two convex functions (DC transformation) via a novel procedure performed for each pair potential that constitute the total potential energy function. Then, a decomposition strategy based on the global optimization (GOP) algorithm [C. A. Floudas and V. Visweswaran, Comput. Chem. Eng. 14, 1397 (1990); V. Visweswaran and C. A. Floudas, ibid. 14, 1419 (1990); Proc. Process Systems Eng. 1991, I.6.1; C. A. Floudas and V. Visweswaran, J. Opt. Theory Appl. (in press)] is designed to provide tight bounds on the global minimum through the solutions of a sequence of relaxed dual subproblems. A number of theoretical results are included which expedite the computational effort by exploiting the special mathematical structure of the problem. The proposed approach attains ε convergence to the global minimum in a finite number of iterations. Based on this procedure, global optimum solutions are generated for small microclusters n≤7. For larger clusters 8≤N≤24 tight lower and upper bounds on the global solution are provided serving as excellent initial points for local optimization approaches. Finally, improved lower bounds on the minimum interparticle distance at the global minimum are provided.
Experimental and theoretical study of the charge density in 2-methyl-4-nitroaniline
Authors: Howard S. T. , Hursthouse M. B. , Lehmann C. W. , Mallinson P. R. , Frampton C. S.
Published: July 2002

The in-crystal molecular dipole moment of the nonlinear optical material 2-methyl-4-nitroaniline has been determined from a charge density analysis of x-ray diffraction data. The results indicate a considerable enhancement of the free molecule dipole moment, due to the crystal field. The analysis suggests that aspherical pseudoatoms are essential for modeling the charge distribution in a noncentrosymmetric crystal. Careful consideration must also be given to the treatment of hydrogen atoms, in the absence of complementary neutron diffraction data. An analysis of the deformation density and Laplacian of the charge density proves useful for revealing weak hydrogen bonding effects. Ab initio calculations at the Hartree–Fock double-ζ level are reported for the molecule 2-methyl-4-nitro-aniline, with and without an applied electric field. In the former case, the magnitude and direction of the applied field were determined by a dipole lattice sum, to assess the magnitude of crystal field effects. The effect was to considerably enhance the molecular dipole moment, from 9 to 20 D, in agreement with the experimentally observed enhancement. Structure factors were generated from the ab initio wave functions and subjected to multipole refinement, to effectively project the theoretical ρ(r) into the same atom-centered multipole expansion form obtained from experiment. Monopole and dipole populations obtained in this way show convincing agreement with experiment.
Random sequential adsorption of anisotropic particles. II. Low coverage kinetics
Authors: Ricci S. M. , Talbot J. , Tarjus G. , Viot P.
Published: July 2002

We study the kinetics of random sequential adsorption (RSA) of anisotropic bodies (rectangles, ellipses, spherocylinders or, more precisely, discorectangles, and needles) at low-to-intermediate coverages. In this regime, the adsorption probability can be expressed as a power series in the coverage. We calculate numerically the second- and third-order coefficients of the series and compare the results to simulation data. The results for the low-coverage kinetics are then combined with the asymptotic results of Paper I [J. Chem. Phys. 97, xxxx (1992)] to construct approximate equations for the adsorption probability over the entire coverage range. While the equations provide a reasonably good description of the RSA kinetics, they produce unsatisfactory estimates of the saturation coverages. The effect of particle shape on the adsorption kinetics and surface structure is discussed. Finally, the available surface function is compared with that corresponding to equilibrium configurations of the adsorbed particles.
A chain of states method for investigating infrequent event processes occurring in multistate, multidimensional systems
Authors: Sevick E. M. , Bell A. T. , Theodorou D. N.
Published: July 2002

This paper describes novel numerical methods for constructing reaction paths and evaluating transition state theory (TST) rate constants for multidimensional, multistate systems. The reaction path is represented as a tethered, freely jointed chain of states with configuration specified by minimization of a function that is derived from the differential description of the path. The method is general and applicable to systems of arbitrary dimension and does not require a priori knowledge of the first-order saddle point, or the topology of the states. Also presented is a novel procedure for numerical determination of the TST rate constant. The procedure is based on Monte Carlo importance sampling using a tethered chain with links modeled as harmonic springs. The beads of the chain and the points at which links pierce the dividing surface separating states serve as biased sampling points for Monte Carlo numerical integration. The methods presented here are tested using the Muller potential surface. The application to problems involving transitions between clusters of states, i.e., macrostates, is discussed.
The reactions of N(2P) with O2 and O
Authors: Piper Lawrence G.
Published: July 2002

We have studied the kinetics of metastable N(2P) with atomic and molecular oxygen. The measurements were made in a discharge flow apparatus in which N(2P) was generated from the energy transfer reaction between N2(A) and ground state N(4S) and was monitored either by vacuum ultraviolet resonance fluorescence at 174 nm or else by observing the forbidden N(2P–4S) emission at 347 nm. The rate coefficient for N(2P) quenching by O2 is (2.2±0.4)×10−12 cm3 molecule−1 s−1 and that for quenching by O is (1.7±0.4)×10−11 cm3 molecule−1 s−1. One channel of the reaction between N(2P) and atomic oxygen appears to produce NO+ via a chemi-ionization mechanism.
van der Waals rovibration levels and the high resolution spectrum of the argon–benzene dimer
Authors: van der Avoird Ad
Published: July 2002

The van der Waals vibrations of Ar–benzene are calculated from two different intermolecular potentials, which are analytic fits to the same ab initio potential. The rovibrational Hamiltonian was derived earlier; the wave functions of the large amplitude vibrations are expanded in products of harmonic oscillator functions. The rotational structure of each van der Waals state is obtained from perturbation theory, as well as from variational calculations of the complete rovibrational states for J=0, 1, and 2. The degenerate bending modes and combinations have a large vibrational angular momentum; for their rotational structure it is important to include all first, second, and higher order rovibrational (Coriolis) coupling. The calculated vibrational frequencies, the information about rovibrational coupling, and the PI(C6v) selection rules for van der Waals transitions, in combination with the vibronic 601 transition on the benzene monomer, lead to a partially new assignment of the three van der Waals sidebands observed in high resolution UV spectra.
On the time scale and time course of protein conformational changes
Authors: Jackson Meyer B.
Published: July 2002

Proteins change their global conformation very slowly compared to the time scales of most localized internal motion. The relationship between these slow and fast processes was investigated with the aid of a stochastic model recently applied to a related problem of protein folding [Zwanzig et al., Proc. Natl. Acad. Sci. 89, 20 (1992)]. In the present study a change in conformation was treated as a bond rotational isomerization that may only occur when several other gating bonds simultaneously assume permissive orientations. The requirement of correct orientation of these gating bonds is analogous to the alignment of the tumblers in a lock. For a range of reasonable choices of parameters, this model generated realistic time scales for protein conformational changes. Although the transition time depended on the value of an energy bias against the permissive position of a gating bond, the model could still generate reasonable transition times in the absence of an energy bias. For a wide range of parameters chosen to give typical experimental rates for biological transitions, the model predicted conformational changes with simple exponential kinetics.
Ab initio search for the equilibrium structure of the ammonia dimer
Authors: Tao Fu-Ming , Klemperer W.
Published: July 2002

The equilibrium structure of the NH3 dimer is investigated using large, efficient basis sets, 6–311+G(3d,2p) and [7s5p3d,4s1p] extended with bond functions, at the second-order Mo/ller–Plesset perturbation approximation (MP2) and higher levels. Intermolecular energies and optimized dimer structures are obtained with the full counterpoise correction for the basis set superposition error. The stabilities of two possible equilibrium structures, one containing a nearly linear hydrogen bond with Cs symmetry and the other a cyclic configuration with C2h symmetry, are examined. In a basis without bond functions, the Cs structure is found more stable. As bond functions are added, however, the C2h structure becomes more stable. This establishes the importance of the dispersion energy which is disproportionally underestimated for the C2h structure in a purely nucleus-centered basis. The stability of the C2h structure relative to the Cs is retained at the higher levels up to the complete forth order (MP4SDTQ). The minimum energy path connecting the two equivalent Cs structures via the C2h structure is calculated. The resulting potential curves are extraordinarily flat in a broad region around the C2h structure but rise steeply upon approaching the Cs structure containing a nearly linear hydrogen bond, indicating that the donor–acceptor interchange barrier is absent in the NH3 dimer. The equilibrium structure for the NH3 dimer found in the present study probably has the cyclic form with C2h symmetry.
Optimal control of vibronic population inversion with inclusion of molecular rotation
Authors: Shen Liyang , Rabitz Herschel
Published: July 2002

This paper considers vibronic population inversion in the presence of molecular rotation. The objective is to invert the population of a vibronic level with populated rotational levels to a specific vibrational level in the excited electronic state regardless of the detailed population distribution in the final rotational levels. The control of the multilevel population inversion is achieved by design of a pump pulse through optimal control theory. The total energy fluence and the pulse peak intensity are imposed as physical constraints in the design cost functional. A model diatomic molecule is used as an example to investigate molecular rotational effects on the vibronic population inversion and the control properties at different target times. The numerical results indicate that the shape-optimized pulses can achieve nearly complete population inversion and largely overcome the difficulty that a rectangular pulse faces due to energy mismatches and multiple dipole transition moments.
The percolation-to-cluster transition during spinodal decomposition of an off-critical polymer mixture. Observation by light scattering and optical microscopy
Authors: Läuger J. , Lay R. , Gronski W.
Published: July 2002

Direct observation of the percolation-to-cluster transition of an off-critical mixture of polybutadiene and polyisoprene was achieved by optical microscopy. According to optical evidence, the transition occurs in a slow and a fast step not revealed in parallel light scattering experiments.
Pulsed-discharge nozzle Fourier-transform microwave spectroscopy of the HC4O radical
Authors: Kohguchi Hiroshi , Ohshima Yasuhiro , Endo Yasuki
Published: July 2002

A new carbon-chain free radical, HC4O, has been identified in the gas phase through its rotational spectrum. The molecule has been generated in a supersonic free jet by electric discharge of an acetylene/oxygen mixture diluted in Ar, and detected by a Fabry–Perot type Fourier-transform microwave spectrometer. Four harmonically related rotational transitions have been observed in the 9–23 GHz region. They all showed fine structure splittings accompanied with well resolved hyperfine structures due to the hydrogen nucleus. Observation of other isotopomers, DC4O and HC418O, has yielded confirmation for the molecular species. The observed spectra have been well characterized as Ka=0 transitions of a slightly asymmetric top molecule in a doublet vibronic state. The effective centrifugal distortion constant has been found to be fairly large, implying that the molecule has a bent carbon-chain framework. The present observation indicates that the radical is subjected to a strong (i.e., static) Renner–Teller interaction, resulting in a complete quenching of the orbital angular momentum in a nonlinear molecular structure as is the case of shorter members of the HCnO series. The isotropic hyperfine coupling constant shows that almost half of the unpaired electron lies on the carbon atom adjacent to the hydrogen.
Cluster structure determination using Gaussian density distribution global minimization methods
Authors: Tsoo Chiachin , Brooks Charles L.
Published: July 2002

Classical particle density distributions, approximated by floating Gaussians, are used to efficiently sample different structures in order to locate the global minimum on the multidimensional potential surface of van der Waals and water clusters. The Gaussian density annealing (GDA) approach of Ma and Straub [J. Chem. Phys. 101, 533 (1994)] provides a set of equations of motion for Gaussian widths and centers. These equations are used to anneal the system from high temperature, with large Gaussian widths describing the particle density distribution, to low temperature. In order to ensure a quasiequilibrium throughout this process, certain constraints are imposed during the annealing. The results of structure optimization of van der Waals clusters using different variants of the GDA are compared. These applications demonstrate the advantage and efficiency of our method, a variation of the GDA algorithm, which anneals on consecutive levels of lower temperature. Extensions of the approach for atomic systems to include rigorous bond constraints in molecular clusters are also presented. Water cluster structures are investigated and compared with other theoretical calculations. Our findings suggest that the nature of the underlying free energy surface may diminish the efficacy of the GDA, and related methods, in locating global minima for clusters.
Analysis of nuclear magnetic resonance spin echoes using simple structure factors
Authors: Sen Pabitra N. , Hürlimann Martin D.
Published: July 2002

The measured amplitude M(q,t) in a pulsed field gradient spin echo experiment probes the pore structure using two length scales, the gradient length q−1, and the diffusion length [D(t)t]1/2. M(q,t) can be well represented by a convolution of the structure factor of the connected pore space with an appropriate Gaussian propagator. This ansatz provides a model-independent way of obtaining the structure factor. In this paper, we consider the forward problem of understanding the amplitude M(q,t) using the simplest possible models for the structure factors which involve only one length scale. The most prominent among the simple models is the Debye model, which provides us with an understanding of the various length scales in the problem. We study several other structure factors, all definable by a single parameter—collections of tubes, collections of sheets, and collections of impenetrable spheres. We discuss periodic geometries for which the ansatz becomes exact in the long time limit. Implications for a ball and stick model for the pore geometry are discussed.
Ab initio electronic structure of a small band gap polymer: Poly-aminosquaraine
Authors: Brocks G.
Published: July 2002

Poly-aminosquaraine is the prototype of a class of organic polymers which recently has been shown to provide a route towards small band gap materials. We predict that poly-aminosquaraine has a small band gap of ∼0.5 eV. Our prediction is based upon a detailed analysis of first-principles calculations of the geometrical and the electronic structure, using the Car–Parrinello technique of simultaneous optimization. We analyze the bands around the Fermi level in terms of a simple tight-binding model based upon the highest occupied and lowest occupied (HOMO/LUMO) states of the individual squaraine molecules. The small band gap of the polymer is shown to be the result of the small splitting between the occupied and the unoccupied states of the squaraine molecule combined with a favorable hybridization in the polymer. It should be possible to analyze the electronic structure of a wide class of squaraine based polymers in the same way.
High frequency (140 GHz) dynamic nuclear polarization: Polarization transfer to a solute in frozen aqueous solution
Authors: Gerfen G. J. , Becerra L. R. , Hall D. A. , Griffin R. G. , Temkin R. J. , Singel D. J.
Published: July 2002

Dynamic nuclear polarization (DNP) transfers the large polarization of unpaired electrons to nuclei and thus significantly enhances the signal strength in nuclear magnetic resonance (NMR) spectroscopy. High frequency/field (140 GHz/5 T) DNP has been implemented in solid state NMR experiments using a nitroxide radical as the paramagnetic polarizing agent in a water:glycerol frozen solution. The 1H and 13C NMR signal strengths of both the solvent and an amino acid solute have been enhanced by a factor of 185, which represents a reduction of ≳102 in sample size requirements or ≳104 in signal acquisition time.
Global potential energy surfaces for the lowest 1A′, 3A″, and 1A″ states of HNO
Authors: Guadagnini Renee , Schatz George C. , Walch Stephen P.
Published: July 2002

We present global ab initio potential energy surfaces for the three lowest energy 1A′, 3A″, and 1A″ surfaces of HNO. These surfaces are the lowest three states of the HNO and HON molecules, and they correlate to the ground electronic states of H+NO and O+NH. In addition, the 3A″ surface correlates to the ground state of N+OH. The surfaces are based on approximately 800 ab initio calculations that were done using an internally contracted multireference configuration interaction calculation with a large basis set. The ab initio points were fit to a combination of Morse and spline functions in each of the three possible Jacobi coordinates, and the resulting splines were smoothly switched together, and combined with other functions to yield globally defined potentials. Properties of the HNO and HON minima and dissociation energies on these potentials are in good agreement with previous high quality calculations. The N+OH and O+NH reactions are found to have no barriers to formation of HON or HNO, respectively. Isomerization of HON to HNO involves barriers that are higher than the HON dissociation barrier on the singlet surfaces but not on the triplet surface.
On the mobility of stiff polyelectrolytes
Authors: Völkel A. R. , Noolandi J.
Published: July 2002

We present analytic calculations of the electrophoretic mobility of stiff charged molecules in solution, and results obtained by using different approximations for the hydrodynamic interactions are discussed. For low fields, where the average molecular conformation is hardly distorted from its Gaussian shape, we find an increase of the mobility with molecular size which, for chains up to 1000 segment lengths, follows approximately a power law with an exponent B. For sufficiently high counterion concentration (λD≲segment length) B is independent of the Debye length λD. Under the simplified assumption of spherical distributed counterions we find B to be proportional to the Debye length for λD larger than the segment length, except for the limits of Gaussian and rigid rod-type molecules. A brief comparison with numerical simulations is included.
Cluster adsorption on metallic surfaces: Structure and diffusion in the Cu/Pd(110) and Pd/Pd(110) systems
Authors: Massobrio C. , Fernandez P.
Published: July 2002

We investigate structural and diffusion properties of CuN and PdN (with N going from 1 to 30) clusters adsorbed on the (110) surface of Pd via atomistic simulations performed by employing embedded atom method potentials. For both systems, one dimensional linear chains are lower in energy than two dimensional structures, although the linear chain stability is more enhanced in the case of Cu/Pd(110). Our results on cluster stability are analyzed in terms of effective interactions and adsorbate arrangement upon relaxation. In close connection with STM experiments performed recently on Cu/Pd(110) [Roeder et al., Nature 366, 141 (1993)] we evaluate the diffusion barrier for atomic movement along and across the [11̄0] direction. A cross exchange mechanism is found to lower significantly the diffusion barrier across the [11̄0] direction, consistent with the value of the diffusion anisotropy found experimentally and the phase separation observed at the uppermost layer level.
Calculation of elastic scattering cross sections of low-energy electrons by PbH4 and SnH4
Authors: Bettega M. H. F. , Natalense A. P. P. , Lima M. A. P. , Ferreira L. G.
Published: July 2002

We report elastic integral, differential, and momentum transfer cross sections from 10 to 30 eV for electron scattering by SnH4 and PbH4, obtained using the Schwinger Multichannel Method with Pseudopotentials [M. H. F. Bettega, L. G. Ferreira, and M. A. P. Lima, Phys. Rev. A 47, 1111 (1993)]. With these molecules we close the series of XH4 molecules, with X=C, Si, Ge, Sn, Pb. We compare the present results with those obtained previously for CH4, SiH4, and GeH4. We find similarities in the cross sections for SiH4, GeH4, SnH4, and PbH4 and a distinctive behavior of CH4. We discuss the role of the center atom size in the scattering process. To our knowledge this is the first ab initio calculation of the SnH4 and PbH4 electron scattering cross sections.
Magnetic resonance and spin dynamics in radical ion pairs: Pulsed time-resolved fluorescence detected magnetic resonance
Authors: Shkrob I. A. , Trifunac A. D.
Published: July 2002

Pulsed time-resolved fluorescence detected magnetic resonance (FDMR) was applied to examine spin dynamics in radical ion pairs. Two time-domain FDMR phenomena were examined; microwave-induced quantum beats and the formation of the ‘‘echo’’ signal. These experiments demonstrate the fast decay of the first-order coherence generated by a microwave field on the time scale of 0.05–1 μs. This decay originates through the phase relaxation in the triplet manifold of the pairs and is probably caused by electron dipole–dipole interaction of radical ions in spurs.
Dissociation energies of tellurium cluster ions from thermoevaporation experiments
Authors: Bréchignac C. , Cahuzac Ph. , de Frutos M. , Garnier P. , Kebaili N.
Published: July 2002

The dissociation of tellurium cluster ions containing up to 40 atoms has been studied using unimolecular decay of thermoexcited clusters. Clusters with less than 10 atoms dissociate mainly by Te2 loss. Larger clusters fragment by loss of Te5, Te6, or Te7 species. As the cluster size increases, Te5 loss becomes the predominent channel, showing a dissociation which smoothly evolves to the bulk behavior. The dissociation energies of mass selected Te+n, with n=5–35, are deduced from the relative branching ratios of the competitive fragmentation channels. The changes in the observed neutral products are correlated to changes in the dissociation energies which are minima for cluster ion parents containing 13–25 atoms.
Selective efficiency of vibrational excitations in ion–molecule collisions: A comparison of behavior for H+–H2 and H−–H2
Authors: Gianturco Franco A. , Kumar Sanjay
Published: July 2002

The vibrational excitation processes which occur in molecular beam experiments on H2 molecules, and using H+ or H− as projectiles, are discussed from the theoretical viewpoint of the microscopic quantum dynamics and in relation to the various features of the two potential energy surfaces. The present study employs the vibrational close-coupling–rotational infinite-order sudden (VCC–RIOS) decoupling scheme and analyzes in detail the differences of behavior of the various inelastic differential cross sections in the small-angle region. It is clearly found that two separate mechanisms can be invoked in the two systems to explain the differences in efficiency between the two excitation processes. Such mechanisms can be related in turn to specific features of the two potential energy surfaces and to their bearing on the final dynamical observables. Rather good agreement between calculated and observed cross sections is found for both systems.
The shear induced disordering transition in a colloidal crystal: Nonequilibrium Brownian dynamic simulations
Authors: Butler Scott , Harrowell Peter
Published: July 2002

The shear induced disordering transition as observed in a dilute suspension of charged colloidal particles is modeled using nonequilibrium Brownian dynamics simulations. We report both real space and reciprocal space representations of the structure and dynamics of the sequence of steady states found as the shear rate is increased. While reproducing the observed steady-state structures at low shear rates, the simulated system was found to follow a different path to disorder with increasing shear. We find that the disordering process involves the accumulation of interstitial-vacancy defects in the shearing crystal as the shear rate increases. The disordering transition is also shown to exhibit an anisotropic dependence on system size. These two observations are combined in a new picture of the shear induced disordering transition. In this model a nonequilibrium defect density, generated by the coupling of long-wavelength fluctuations with the shear flow, eventually results in a collective disordering similar to the process of defect-induced amorphization in atomic solids.
Localized structures of polymers with long-range interactions
Authors: Muthukumar M.
Published: July 2002

The configurational properties of a heteropolymer with prescribed sequences of known concentrations of dipolar segments are investigated. The theoretical analysis of a model chain capable of aggregation into domains shows that entropic frustrations lead to localized structures. In these structures, polymer size is determined dominantly by the formation of aggregates and not by the chain length. This localization effect occurs in addition to the familiar aspects of helix-coil and coil-globule transitions. The formation of localized structures as predicted by the theory is verified by Monte Carlo simulations of heteropolymers bearing segments with dipolar interactions. The transition of a rodlike configuration into a coil can be induced in a sharp manner also by dilution of the chain backbone. The simulations show that disordered homopolymers bearing segments with dipolar or other long-ranged inverse power law interactions do not undergo any phase transition to an ordered state at nonzero temperatures. The ability of a heteropolymer with dipolar and neutral segments to form localized structures is found to be nontrivially controlled by the specificity of the sequences.
A localized orbitals based embedded cluster procedure for modeling chemisorption on large finite clusters and infinitely extended surfaces
Authors: Head John D. , Silva Susil J.
Published: July 2002

A new embedded cluster procedure for modeling chemisorption on metal surfaces is developed. The procedure is similar in philosophy to the approach used by Whitten and co-workers in that energy calculations are performed in a cluster region basis consisting of localized occupied and virtual orbitals. However, we present a new localization procedure to generate the cluster region functions which is based on orbital occupation numbers determined from the density matrix obtained in a calculation on the extended substrate. Our localization procedure avoids having to perform separate unitary transformations on the canonical occupied and virtual orbitals and as a consequence has the attractive feature of enabling the embedded cluster calculations to be applied to both large finite clusters and infinitely extended systems in essentially the same manner. We illustrate the embedded cluster procedure by performing partial SCF calculations in the cluster region basis for H adsorption at an on-top site of a Li(100) monolayer.

When the extended surface is modeled by large finite clusters, the localized orbitals in the cluster region rapidly converge to being completely occupied or completely empty, and we find partial SCF calculations to readily reproduce the full SCF results of the large finite cluster. For the infinitely extended surface, the occupation numbers for the localized functions in cluster regions converge much more slowly than in the finite case, but even with less than perfect occupation numbers we still obtain good H adsorption properties in the partial SCF calculations. Unlike the finite cluster case where charges are automatically balanced, we found in order to achieve good results in the partial SCF calculations on the infinitely extended systems it was necessary to carefully balance the charges used in the long range electron and nuclear interactions. All of the calculations involving clusters are performed with the GAMESS program and the calculations on the infinite extended surface are performed with the periodic Hartree–Fock CRYSTAL program.
A light scattering investigation of the sodium dodecyl sulfate–lysozyme system
Authors: Gimel J. C. , Brown W.
Published: July 2002

Dynamic and static light scattering measurements have been made on lysozyme solutions at 25 °C in 0.1 M phosphate buffer, pH 6.88, in the presence of sodium dodecyl sulfate (SDS). The concentration of SDS has been varied over a wide range (0–0.2 g/ml). It is found that under these conditions both SDS micelles and lysozyme(Lys)–SDS complexes coexist. Addition of lysozyme at constant SDS concentration leads to formation of more lysozyme–SDS complex. The translational diffusion coefficient of the complex increases with SDS concentration. It is also found that two lysozyme molecules are associated in the protein–surfactant complex.
Fourier transform emission spectroscopy of the B 1Π–X 1Σ+, C 1Σ+–X 1Σ+, and G 1Π–X 1Σ+ systems of ScH and ScD
Authors: Ram R. S. , Bernath P. F.
Published: July 2002

The emission spectra of ScH and ScD have been observed in the 380 nm–2.5 μm spectral region using a Fourier transform spectrometer. The molecules were excited in a scandium hollow cathode lamp operated with neon gas and a trace of hydrogen or deuterium. Three transitions with a common lower state, assigned as the ground X 1Σ+ state, have been observed in the near infrared and visible regions. The ScH bands with 0–0 band origins at 5404, 13 574, and 20 547 cm−1 have been assigned as the B 1Π–X 1Σ+, C 1Σ+–X 1Σ+, and G 1Π–X 1Σ+ transitions, respectively. A rotational analysis of the 0–0, 1–1, 1–0, and 2–1 bands of the B 1Π–X 1Σ+ system, the 0–0 and 1–1 bands of the C 1Σ+–X 1Σ+ system and the 0–0 band of the G 1Π–X 1Σ+ system has been obtained. The principal molecular constants for the X 1Σ+ state of ScH are ΔG(1/2)=1546.9730(14) cm−1, Be=5.425 432(48) cm−1, αe=0.124 802(84) cm−1 and re=1.775 427(8) Å. The corresponding band systems of ScD have also been analyzed. A rotational analysis of the 0–0, 1–1, and 1–0 bands of the B 1Π–X 1Σ+ system, the 0–0, 1–1, 0–1, and 1–2 bands of the C 1Σ+–X 1Σ+ system and the 0–0 band of the G 1Π–X 1Σ+ system has been obtained. The equilibrium molecular constants determined for the ground state of ScD are ωe=1141.2650(31) cm−1, ωexe=12.3799(15) cm−1, Be=2.787 432(41) cm−1, αe=0.045 321(73) cm−1, and re=1.771 219(13) Å. The ScH assignments are supported by recent theoretical predictions made by Anglada et al. [Mol. Phys. 66, 541 (1989)] as well as the experimental results available for ScF and the isovalent YH and LaH molecules. Although some unassigned bands have been attributed to ScH and ScD by previous workers, there have been no previous analyses of ScH or ScD spectra.
A new implementation of the second-order polarization propagator approximation (SOPPA): The excitation spectra of benzene and naphthalene
Authors: Packer Martin J. , Dalskov Erik K. , Enevoldsen Thomas , Jensen Hans Jo/rgen Aa. , Oddershede Jens
Published: July 2002

We present a new implementation of the second-order polarization propagator approximation (SOPPA) using a direct linear transformation approach, in which the SOPPA equations are solved iteratively. This approach has two important advantages over its predecessors. First, the direct linear transformation allows for more efficient calculations for large two particle–two hole excitation manifolds. Second, the operation count for SOPPA is lowered by one order, to N5. As an application of the new implementation, we calculate the excitation energies and oscillator strengths of the lowest singlet and triplet transitions for benzene and naphthalene. The results compare well with experiment and CASPT2 values, calculated with identical basis sets and molecular geometries. This indicates that SOPPA can provide reliable values for excitation energies and response properties for relatively large molecular systems.
Analytical Hartree–Fock calculation of the dynamical polarizabilities α, β, and γ of molecules in solution
Authors: Cammi Roberto , Cossi Maurizio , Mennucci Benedetta , Tomasi Jacopo
Published: July 2002

We present a theory for the calculation of the self-consistent-field dynamical polarizability and hyperpolarizabilities of molecular solutes described within the framework of the polarization continuum model. The formulation is characterized by the use of a cavity accurately modeled on the molecular shape of the solvated compound, and of a frequency dependent dielectric permittivity for the solvent. Calculations of the components of polarizability α, first and second hyperpolarizabilities β and γ tensors of H2CO in water and in benzene are reported and discussed.
Vibrational and geometric structures of Nb3C2 and Nb3C+2 from pulsed field ionization-zero electron kinetic energy photoelectron spectra and density functional calculations
Authors: Yang Dong-Sheng , Zgierski Marek Z. , Bérces Attila , Hackett Peter A. , Roy Pierre-Nicholas , Martinez Ana , Carrington Tucker , Salahub Dennis R. , Fournier René , Pang Tao , Chen Changfeng
Published: July 2002

Vibrational frequencies of three niobium normal modes of triniobium dicarbide neutral and cation have been determined from pulsed field ionization-zero electron kinetic energy photoelectron spectra. The niobium stretching mode has a frequency of 326 cm−1 in the neutral and 339 cm−1 in the ion. The two deformation modes have frequencies of 238 and 82 cm−1 in the neutral and a degenerate frequency of 258 cm−1 in the ion. The geometry of the triniobium dicarbide has been established by comparing the experimental spectra with theoretical calculations. The cluster has a trigonal bipyramid geometry with carbon atoms capping on each face of the metal frame. The cation cluster has D3h symmetry whereas the neutral cluster has lower symmetry resulting from a Jahn–Teller distortion. A second low-lying structure with doubly bridging carbon atoms has been identified by the calculations but has not yet been observed.
Structure and intermolecular motions of the water dimer anion
Authors: Bouteiller Y. , Desfrançois C. , Abdoul-Carime H. , Schermann J. P.
Published: July 2002

The electronic and geometrical structures of the water dimer anion are investigated by density functional calculations. Theoretical values of the very weak electron affinities and vertical detachment energies show good agreement with available experimental data. It is predicted that a significant modification of the neutral parent geometry is introduced by the electron attachment process and leads to a large excitation of anion intermolecular modes. The interpretation of field-detachment experiments of (H2O)−2 and (D2O)−2, produced in collisions between laser-excited Rydberg atoms and cold neutral water clusters, confirms the existence of large amplitude low-frequency motions in the anions.
The cumulative reaction probability for the H2 + OH reaction
Authors: Zhang Dong H. , Light John C.
Published: July 2002

The cumulative reaction probability [CRP or N(E)] for the four-atom reaction, H2+OH→H+H2O is calculated using one of the formulations of Miller, Schwartz, and Tromp [J. Chem. Phys. 79, 4889 (1983)] and the transition state wave packet (TSWP) approach of Zhang and Light [J. Chem. Phys. 104, 6184 (1996)]. It is shown that locating the dividing surface of the flux operator in the transition state region significantly reduces the number of wave packets which must be followed in order to converge the CRP as compared to the use of initial state selected wave packets (ISSWP). In addition we examine the use of transition state normal coordinates (versus Jacobi coordinates) and show that the use of transition state wave packets defined in normal coordinates yields more rapid convergence of the CRP and individual contributions of the TSWP to the CRP can closely approximate the probabilities of reaction for each transition state as a function of energy. Problems with large amplitude motions using the normal coordinates of the loose non-linear transition state are shown to be absent if normal coordinates of a linear transition state are used. Applications to the 3-D H + H2 (J = 0) reaction and to the 6D H2 + OH (J = 0) reaction demonstrate that both N(E) and the initial state reaction probabilities at many energies can be evaluated accurately and efficiently by propagation of each TSWP only once.
The rovibrational spectrum of hydroxylamine: A combined high resolution experimental and theoretical study
Authors: Luckhaus D.
Published: July 2002

This paper reports the rovibrational spectrum of hydroxylamine (NH2OH) recorded by interferometric Fourier transform spectroscopy with a resolution of up to 0.004 cm−1 close to the Doppler limit at room temperature, from 800 cm−1 up to the visible range of the spectrum. Detailed rotational analyses for 32 bands include all nine fundamentals and numerous overtones up to 10 500 cm−1. Approximate absolute band strengths, band centers and vibrational assignments are presented for a total of 72 bands up to the 4ν1 OH stretching overtone. The spectra are interpreted in terms of multidimensional vibrational calculations with potential and dipole surfaces constructed by multidimensional spline interpolation from more than 2×105 ab initioMP2/6-31G** points. The full three dimensional treatment of the torsion-inversion problem reveals well separated time scales for the two processes with a cis potential well supporting localized wave functions for the zero point and fundamental levels. Up to five dimensional normal coordinate models are employed for the analysis and dynamic interpretation of the complete vibrational spectrum up to 21 000 cm−1. Good quantitative agreement between observed spectra and results from ab initio calculations is obtained with a simple harmonic scaling procedure without any further empirical refinement. The comparison of various coupling schemes reveals an efficient path for the coupling between the OH and NH2 manifolds mediated through the OH bending mode. The implications for the effective homogeneous broadening at high energies are discussed.
A combined method for determining reaction paths, minima, and transition state geometries
Authors: Ayala Philippe Y. , Schlegel H. Bernhard
Published: July 2002

Mapping out a reaction mechanism involves optimizing the reactants and products, finding the transition state and following the reaction path connecting them. Transition states can be difficult to locate and reaction paths can be expensive to follow. We describe an efficient algorithm for determining the transition state, minima and reaction path in a single procedure. Starting with an approximate path represented by N points, the path is iteratively relaxed until one of the N points reached the transition state, the end points optimize to minima and the remaining points converged to a second order approximation of the steepest descent path. The method appears to be more reliable than conventional transition state optimization algorithms, and requires only energies and gradients, but not second derivative calculations. The procedure is illustrated by application to a number of model reactions. In most cases, the reaction mechanism can be described well using 5 to 7 points to represent the transition state, the minima and the path. The computational cost of relaxing the path is less than or comparable to the cost of standard techniques for finding the transition state and the minima, determining the transition vector and following the reaction path on both sides of the transition state.