bluuebunny/crossref_metadata_2025 huggingface dataset:
A direct comparison of vibrational deactivation of hexafluorobenzene excited by infrared multiple photon absorption and internal conversion
Authors: Gascooke Jason R. , Alwahabi Zeyad T. , King Keith D. , Lawrance Warren D.
Published: July 2002

We report the first direct comparison between energy transfer parameters measured using infrared multiphoton absorption (IRMPA) versus ultraviolet (UV) excitation followed by rapid internal conversion (IC). Highly excited hexafluorobenzene (HFB) molecules in the electronic ground state were prepared by (i) IRMPA by CO2 laser pumping to an average initial energy of 14 500–17 500 cm−1 and (ii) UV excitation to ∼40 300 cm−1 followed by IC. The vibrational deactivation of the highly excited HFB by the monatomic collider gas argon was monitored by time-resolved infrared fluorescence. The results for the two methods are identical within experimental error, demonstrating the viability of IRMPA as a method of state preparation for vibrational deactivation experiments involving large molecules.
Electrokinetic properties of colloids of variable charge. II. Electric birefringence versus dielectric properties
Authors: Mantegazza Francesco , Bellini Tommaso , Degiorgio Vittorio , Delgado Angel V. , Arroyo Francisco J.
Published: July 2002

We present a detailed comparison between the low frequency behavior of the electric birefringence (as measured by the Kerr constant, B) and of the real part of the dielectric increment Δε′, in aqueous colloidal dispersions of nearly monodisperse, elongated, polytetrafluoroethylene (PTFE) particles with adjustable surface charge. The electric charge and, consequently, the ζ-potential of the particles, is modified by adding a nonionic surfactant to the dispersion, as discussed in the first paper of this series [J. Chem. Phys. 103, 8228 (1995)]. We find that both B and Δε′ decrease upon decreasing the absolute value of the ζ-potential. Upon increasing the ionic strength, B and Δε′ display opposite behavior: B decreases while Δε′ increases. We propose an expression which connects B to the standard electrokinetic model and compare the experimental data to a calculation based on spheroids with thin double layers. If we calculate B and Δε′ using the particle ζ-potential obtained from electrophoretic mobility measurements, the theory qualitatively reproduces the experimental results, but systematically underestimates the values of both B and Δε′. If, instead, we take the ζ-potential as an adjustable parameter, we can reconcile both B and Δε′, although the values of ζ are larger than those derived from electrophoretic mobility measurements.
An efficient direct method for geometry optimization of large molecules in internal coordinates
Authors: Paizs Béla , Fogarasi Géza , Pulay Peter
Published: July 2002

A new efficient procedure has been developed for geometry optimization of large molecules using internal coordinates. The method stores only the nonzero elements of the large transformation matrices, in the spirit of direct methods in electronic structure theory. Matrix inversion has been replaced by iterative solution of linear systems of equations by the preconditioned conjugate gradient method. A new incomplete Cholesky preconditioner proved essential to accelerate the conjugate gradient procedure. The geometries of several alpha helical alanine polypeptides, up to 50 alanine units, have been optimized by the new method, using the SYBYL force field. For larger systems, the number of energy/gradient evaluations is reduced by a factor of 6–10, compared to Cartesian optimization, and the cost of the optimization is small compared to the energy calculation. We expect this method to be useful in molecular mechanics and in mixed quantum mechanics/molecular mechanics calculations.
Photodissociation spectroscopy of MgO2+
Authors: Chen J. , Wong T. H. , Kleiber P. D.
Published: July 2002

The chemically bound superoxide molecule MgO2+ has been studied by photodissociation spectroscopy in a reflectron time-of-flight mass spectrometer over the spectral range 247–540 nm. The experimental observations are consistent with ab initio calculations showing the ground state of MgO2+ to be of Mg2+O2− superoxide character [Chem. Phys. Lett. 203, 215 (1993)]. Through the visible and near UV spectral range 280 nm<λ<540 nm, we observe weak continuum absorption with evidence for two overlapping bound–free absorption bands, assigned as 1 2B1←1 2A2 and 2 2B1←1 2A2 in C2v symmetry. These bands correspond to radiative charge-transfer transitions of the form Mg2+O2−→Mg+O2. Both Mg+ and MgO+ fragments are observed, with a clear threshold for branching to MgO+ at a photolysis wavelength of 330 nm. This threshold yields limiting values for the MgO2+ bond dissociation energies of D0″(MgO+–O)⩽(3.75±0.04) eV and D0″(Mg+–O2)⩽(1.13±0.11) eV. For photolysis wavelengths λ<280 nm we observe a third structured absorption band showing a clear vibrational progression with an excited state vibrational mode spacing of ωe=520±15 cm−1. This electronic band is assigned as 3 2B1←1 2A2, with the upper state correlating to an excited electronic state of O2−. The vibrational progression is tentatively assigned to the ν2 (Mg–O) symmetric stretch mode of the complex (a1).
Structural properties of poly(propylene oxide) from diffraction experiments and reverse Monte Carlo simulation
Authors: Carlsson P. , Swenson J. , Börjesson L. , Torell L. M. , McGreevy R. L. , Howells W. S.
Published: July 2002

Structural characteristics of an amorphous polymer melt, poly(propylene oxide) (PPO), have been studied by combining neutron and x-ray diffraction experiments and computer modeling using the reverse Monte Carlo (RMC) technique. The neutron diffraction experiments were performed on hydrogenous as well as deuterated samples. The experimentally determined nearest-neighbor distances were found to be in good agreement with literature data. The RMC modeling was applied for interpretation of the diffraction data to obtain more detailed structural information on bond angles, intermediate and long range correlations. For the intermediate range structure, the experimental structure factors demonstrate a first diffraction peak at about 1.45 Å−1, which from the RMC produced model can be related to the interchain distance of an almost random packing of the polymer chains. To investigate the chain conformation, partial atomic pair correlation functions have been calculated for atoms belonging to monomers close in sequence. The results show that the most probable conformation is a “stretched” trans conformation, where two consecutive methyl groups are pointing in almost opposite directions. Calculated bond and dihedral angle distributions support this finding and demonstrate the ability of the RMC method to produce polymer structures in good agreement with experimental results.
Irreversible adsorption/deposition kinetics: A generalized approach
Authors: Adamczyk Z. , Senger B. , Voegel J.-C. , Schaaf P.
Published: July 2002

A generalized random sequential adsorption (RSA) approach is developed by taking into account diffusion, particle/wall hydrodynamic interactions as well as external forces (e.g., gravity). In analogy to the previous concept of Pagonabarraga and Rubı́ [Physica A 188, 553 (1992)] the position dependent available surface function Φ(z,θ) is introduced. Using this definition, constitutive expressions for the adsorption flux are formulated which represent the generalization of previous models, including the widely used Langmuirian kinetic approach. It is shown that the overall available surface function Φ̄(Δ,θ) plays the crucial role in these expressions. It represents the net probability of transferring a particle from the arbitrary distance Δ to the interface for a given surface coverage. Explicit expressions in the form of definite integrals are formulated which enable one to calculate the Φ̄(Δ,θ) function in terms of the Φ(z,θ) function. In the case of hard spheres, Φ(z,θ) is calculated up to the second order of the surface coverage θ using geometrical arguments. The effect of an external force (gravity) is characterized by the dimensionless radius of particles R*, where R*→∞ corresponds to the purely ballistic case, R*=0 to the diffusion RSA, and R*→−∞ reflects the case of infinite gravity acting outwards from the surface. Using these expressions, the overall Φ̄(Δ,θ) function is also calculated. It is found that the RSA available surface function is not recovered for R*=0 as expected, but for R*→−∞. The transition from the R*=0 to the ballistic case (R*=∞) is analyzed. Unexpectedly, it is found that for R*=1 the second order term in the coverage expansion of Φ̄(Δ,θ) appears negative which seems an entirely new result. It is also deduced that in the case of an energy barrier, whose extension is much smaller than the particle dimension, the adsorption process can well be characterized for R*=0 in terms of the classical RSA model. This can be explained by the fact that for a high energy barrier the adsorbing particles could randomize over the deposition plane before crossing the barrier and adsorbing irreversibly.
Ab initio theoretical study of interactions in borazane molecule
Authors: Jagielska Anna , Moszyński Robert , Piela Lucjan
Published: July 2002

Some structural and energetical properties of the borazane molecule have been calculated by the Mo/ller–Plesset perturbation theory accurate to the second, third, and fourth orders (MP2, MP3, MP4), the coupled cluster [CCSD(T)] approach, and the symmetry-adapted perturbation theory (SAPT). The geometry has been fully optimized at the MP2 level. The theoretical values for bond distances, bond angles, rotational barrier, dipole moment, vibrational frequencies, and the nuclear quadrupole coupling constants agree well with the experimental data. The dissociation energy, the BN bond distance, and the vibrational frequencies obtained indicate that borazane is to a considerable extent a floppy system, which has to be classified as a strong van der Waals complex rather than a molecule similar (isoelectronic) to ethane. The best estimate for the complex formation enthalpy corrected for the basis set superposition error is equal to 25.7±2 kcal/mol. As revealed by the SAPT analysis the main binding contributions are the induction and electrostatic effects. The dipole moment of the complex increases very strongly [from 1.53 to 5.30 D at the CCSD(T) level] upon the interaction due mainly to the umbrella structural polarization of the BH3 molecule and to the polarization of the electron cloud.
Tracer particle diffusion in crystal-like ordered colloidal suspensions
Authors: Brands Andreas , Versmold Heiner , van Megen William
Published: July 2002

Dynamic light scattering experiments are reported on crystal-like ordered colloidal suspensions composed of two types of electrostatically interacting particles having similar size and effective particle charge. The particles differ only in their refractive indices. We obtain the incoherent (self) intermediate scattering function of these mixtures by using only a trace concentration of one of the species and matching the refractive index of the suspending liquid medium to that of the other. Localization of the tracer particles about randomly located lattice sites of the colloidal crystal is manifested in the nonergodicity of the incoherent intermediate scattering functions. The latter, and the particle mean-squared displacements obtained from them, plateau to finite values at long times. In analogy to neutron scattering these plateau values of the incoherent intermediate scattering functions may be called the elastic incoherent structure factor.
The influence of the random sequential adsorption of binary mixtures on the kinetics of hydrocarbon hydrogenation reactions
Authors: McLeod A. S. , Gladden L. F.
Published: July 2002

The hydrogenation of unsaturated hydrocarbons by transition metal catalysts demonstrates complex kinetic behavior that cannot be represented by mean-field kinetic models. In this paper, a Monte Carlo model of the Horiuti-Polanyi hydrogenation mechanism is introduced and is applied to the hydrogenation of a number of alkenes of varying chain length. We demonstrate that the origin of this anomalous kinetic behavior can be explained by assuming that the adsorption of the hydrocarbon on the catalyst surface approximates a random sequential adsorption (RSA) process. The model is shown to reproduce the kinetic discontinuities observed experimentally for the hydrogenation of ethene and ethyne. Monte Carlo simulations conducted on square lattices for a number of hydrocarbons indicate that the transition between the two kinetic regimes occurs at the point where the hydrocarbon surface coverage exceeds the dimer jamming limit.
Spectroscopic study of the mixed rare-gas–molecule van der Waals trimer NeArHCl
Authors: Xu Yunjie , Armstrong Geoffrey S. , Jäger Wolfgang
Published: July 2002

The van der Waals trimer NeArHCl was studied with a pulsed molecular beam Fourier transform microwave spectrometer. The trimer is of the type rare-gas–rare-gas′–linear molecule that is relatively unexplored and is of considerable importance for the understanding of new correction terms proposed in current investigations of nonadditive contributions in weakly bound molecular systems. Pure rotational spectra of four isotopomers of NeArHCl, i.e., 20NeArH 35Cl, 20NeArH 37Cl, 22NeArH 35Cl, and 20NeArD 35Cl were investigated. Nuclear quadrupole hyperfine structures due to the 35Cl, 37Cl, and D nuclei were observed and assigned. The resulting nuclear quadrupole coupling constants were used to provide information about the angular anisotropy of the NeArHCl potential energy surface. Structural parameters of the trimer were determined from the rotational constants obtained. A pseudotriatomic harmonic force field analysis was performed to provide qualitative frequency predictions of the heavy atom van der Waals vibrational motions. Information about the additive and nonadditive interactions from structural and force field analyses is discussed.
Freezing and melting of water in a single cylindrical pore: The pore-size dependence of freezing and melting behavior
Authors: Morishige Kunimitsu , Kawano Keiji
Published: July 2002

In order to clarify the origin of the hysteresis between freezing and melting of pore water, we performed x-ray diffraction measurements of water confined inside the cylindrical pores of seven kinds of siliceous MCM-41 (a member of ordered mesoporous materials denoted by Mobil Oil researchers) with different pore radii (1.2–2.9 nm) and the interconnected pores of Vycor glass as a function of temperature. The hysteresis effect depends markedly on the size of the cylindrical pores: the hysteresis is negligibly small in smaller pores and becomes remarkable in larger pores. This strongly suggests that the hysteresis is arisen from size-dependent supercooling of water confined to the mesopores. For the water confined to the mesopores with pore radius of 1.2 nm, a continuous transition between a liquid and a solid precedes the first-order freezing transition of the pore water which would occur by the same mechanism as in bulk water.
Gaussian-3 theory using reduced Mo/ller-Plesset order
Authors: Curtiss Larry A. , Redfern Paul C. , Raghavachari Krishnan , Rassolov Vitaly , Pople John A.
Published: July 2002

A variation of Gaussian-3 (G3) theory is presented in which the basis set extensions are obtained at the second-order Mo/ller–Plesset level. This method, referred to as G3(MP2) theory, is assessed on 299 energies from the G2/97 test set [J. Chem. Phys. 109, 42 (1998)]. The average absolute deviation from experiment of G3(MP2) theory for the 299 energies is 1.30 kcal/mol and for the subset of 148 neutral enthalpies it is 1.18 kcal/mol. This is a significant improvement over the related G2(MP2) theory [J. Chem. Phys. 98, 1293 (1993)], which has an average absolute deviation of 1.89 kcal/mol for all 299 energies and 2.03 kcal/mol for the 148 neutral enthalpies. The corresponding average absolute deviations for full G3 theory are 1.01 and 0.94 kcal/mol, respectively. The new method provides significant savings in computational time compared to G3 theory and, also, G2(MP2) theory.
Ab initioground potential energy surface, VTST and QCT study of the O(3P)+CH4(X 1A1)→OH(X 2Π)+CH3(X 2A2″) reaction
Authors: González Miguel , Hernando Jordi , Millán Judith , Sayós R.
Published: July 2002

An ab initio study of the ground potential energy surface (PES) of the O(3P)+CH4→OH+CH3 reaction has been performed using the second- and fourth-order Mo/ller–Plesset methods with a large basis set. A triatomic analytical ground PES with the methyl group treated as an atom of 15.0 a.m.u. has been derived. This PES has been employed to study the kinetics [variational transition state theory (VTST) and quasiclassical trajectory (QCT) rate constants] and dynamics (QCT method) of the reaction. The ab initio points have also been used directly to calculate the VTST rate constant considering all atoms of the system. The best VTST methods used lead to a good agreement with the experimental rate constant for 1000–2500 K, but QCT rate constant values are about one-third the experimental ones for 1500–2500 K. The cold QCT OH(v=0) rotational distribution arising from the simulation of the reaction with O(3P) atoms produced in the photodissociation of NO2 at 248 nm is in good agreement with experiment, while the very small QCT OH(v=1) population obtained is consistent with measurements. The triatomic PES model derived in this work may be used in studies of the kinetics and dynamics under conditions where the methyl group motions are not strongly coupled to the motions leading to reaction.
Gaussian-3 theory using density functional geometries and zero-point energies
Authors: Baboul Anwar G. , Curtiss Larry A. , Redfern Paul C. , Raghavachari Krishnan
Published: July 2002

A variation of Gaussian-3 (G3) theory is presented in which the geometries and zero-point energies are obtained from B3LYP density functional theory [B3LYP/6-31G(d)] instead of geometries from second-order perturbation theory [MP2(FU)/6-31G(d)] and zero-point energies from Hartree–Fock theory [HF/6-31G(d)]. This variation, referred to as G3//B3LYP, is assessed on 299 energies (enthalpies of formation, ionization potentials, electron affinities, proton affinities) from the G2/97 test set [J. Chem. Phys. 109, 42 (1998)]. The G3//B3LYP average absolute deviation from experiment for the 299 energies is 0.99 kcal/mol compared to 1.01 kcal/mol for G3 theory. Generally, the results from the two methods are similar, with some exceptions. G3//B3LYP theory gives significantly improved results for several cases for which MP2 theory is deficient for optimized geometries, such as CN and O2+. However, G3//B3LYP does poorly for ionization potentials that involve a Jahn–Teller distortion in the cation (CH4+, BF3+, BCl3+) because of the B3LYP/6-31G(d) geometries. The G3(MP2) method is also modified to use B3LYP/6-31G(d) geometries and zero-point energies. This variation, referred to as G3(MP2)//B3LYP, has an average absolute deviation of 1.25 kcal/mol compared to 1.30 kcal/mol for G3(MP2) theory. Thus, use of density functional geometries and zero-point energies in G3 and G3(MP2) theories is a useful alternative to MP2 geometries and HF zero-point energies.
Formation of CF3O− in the gas phase
Authors: Morris Robert A. , Miller Thomas M. , Paulson John F. , Viggiano A. A. , Feldmann Michael T. , King Rollin A. , Schaefer Henry F.
Published: July 2002

We report experimental studies of the formation of CF3O− by ion-molecule and electron attachment reactions, and theoretical investigations of the structure and energetics of CF3O− and its neutral counterpart CF3O. The anion CF3O− is formed from the rapid attachment of free electrons to its neutral dimer, (CF3O)2. Potential sources of CF3O− through ion-molecule reactions of CF3− and F− were surveyed. CF3O− is formed in the bimolecular ion-molecule reaction of CF3− with SO2 and the third-order association reaction of F− with CF2O. In addition, rate constants for the reactions of CF3− with a variety of neutral compounds were measured. A number of cases were found in which formation of CF3O− was energetically allowed but was not observed. The potential energy surfaces of CF3O and CF3O− have been investigated using a variety of density functional theory (DFT) techniques. The ground-state minimum energy structure of CF3O was found to be a A′2 Jahn–Teller distorted Cs-symmetry structure, while for the anion the ground state is A11 with a C3v-symmetry minimum. A search for other low-energy minima for CF3O− was unsuccessful. The DFT methods support a value for the adiabatic electron affinity of CF3O near 4.1 eV.
Bound-free gas transport coefficients via the time correlation formulation based on an atomic picture
Authors: Snider R. F. , Alavi Saman
Published: July 2002

The values of gas transport coefficients, when both unbound particles and bound pairs are present, are standardly accounted for by treating the gas as a mixture of free atoms and bound pairs and using the well-known expressions for the transport coefficients for a mixture. But since the bound pairs consist of two atoms, it should also be possible to start from a purely atomic picture of the gas. This paper accomplishes that aim, starting with the time correlation function expression for the transport coefficient in terms of atomic variables.
Thermionic electron emission of small tungsten cluster anions on the milliseconds time scale
Authors: Weidele H. , Kreisle D. , Recknagel E. , Becker St. , Kluge H.-J. , Lindinger M. , Schweikhard L. , Walther C. , Ziegler J.
Published: July 2002

Small tungsten cluster anions Wn− (n=4–9 and 18–20) are stored in a Penning trap and electronically excited by photoabsorption (Ehν=1.815, 2.33, 3.5, and 4.66 eV). Delayed electron emission is observed on the milliseconds time scale by systematic variation of the storage duration between laser excitation and ion detection. Even if the photon energy exceeds the electron detachment energy, electrons are emitted several milliseconds after laser excitation. The electron emission time constant is determined as a function of the laser pulse energy. An Arrhenius analysis suggests that the observed delayed electron emission is a thermal process in analogy to thermionic emission of bulk materials. As shown by these experiments there is a simple rule for the dominating cooling channel of laser excited clusters: thermionic emission generally occurs as long as the electron binding energy is lower than the dissociation energy.
Hydrogen adsorption on GaAs (001) reconstructions
Authors: Hicks R. F. , Qi H. , Fu Q. , Han B.-K. , Li L.
Published: July 2002

Hydrogen adsorption on the c(4×4), (2×4), (2×6), and (4×2) reconstructions of GaAs (001) have been characterized by internal-reflection infrared spectroscopy. The infrared spectra contain up to 15 bands due to the stretching vibrations of arsenic hydrides (2150–1950 cm−1), terminal gallium hydrides (1950–1800 cm−1), and bridging gallium hydrides (1800–950 cm−1). These features arise from hydrogen adsorption on arsenic and gallium dimers, and second-layer arsenic and gallium atoms. The large number of peaks observed indicates that the surface atoms exist in a variety of different chemical environments.
Quantum mechanical study on energy dependence of probabilities of nonreactive vibrational transitions, atom exchange reaction, and dissociation in a collinear He+H2+ collision
Authors: Onda Kunizo , Sakimoto Kazuhiro
Published: July 2002

The accurate time-independent quantum mechanical method developed by the present authors [K. Sakimoto and K. Onda, J. Chem. Phys. 100, 1171 (1994)] is applied to investigate a nonreactive vibrational transition, atom exchange reaction, and dissociation processes in a collinear H2+(vi)+He collision. The algorithm based on the three-point finite difference formula is replaced with the Numerov algorithm to improve on numerical efficiency for directly solving the Schrödinger equation represented by the hyperspherical coordinates (ρ,ω). We have employed the interaction potential surface analytically fitted by Joseph and Sathyamurthy [J. Chem. Phys. 86, 704 (1987)] for this collision system. The energy dependence of the probabilities of the nonreactive vibrational transition, atom exchange reaction, and dissociation processes is investigated at the total energy from 4 to 10 eV, and the dependence of these probabilities on the initial vibrational state of the H2+(vi)(0⩽vi⩽17) ion is also studied to understand deeply this collision dynamics. These probabilities are undulatory as a function of the total energy, and show that the coupling among the channels defined by the reactant and product vibrational bound and continuum states is strong. The atom exchange reaction is the dominant process for vi⩽4, and the predominant process is dissociation of the H2+ for vi⩾14 at the total energy investigated here. In order to clarify the sensitivity of this collision dynamics to the interaction potentials, we have investigated an effect of an additive two-body and nonadditive many-body interaction potentials on the nonreactive vibrational transition, atom exchange reaction, and dissociation processes. It is found that the collision dynamics is extremely sensitive to the short-range part of the potential energy surface.
Ab initio molecular-dynamics simulation of K+ solvation in water
Authors: Ramaniah Lavanya M. , Bernasconi Marco , Parrinello Michele
Published: July 2002

Potassium ion in water plays a very important role in chemistry and biology. In this paper, we study the solvation of this important ion using ab initio Car–Parrinello molecular dynamics. We work within the pseudopotential, density-functional approach and use the BLYP (Becke–Lee–Yang– Parr) generalized gradient approximation to the exchange and correlation potential. An analysis of the structural properties of the solvation shell shows good agreement with existing experiments, as well as with previous simulations based on classical potentials.
On the short time motion of hydrogen-bonded molecules in supercooled water
Authors: De Santis Alberto , Ercoli Alessandro , Rocca Dario
Published: July 2002

The short time dynamics of tagged pairs of molecules that, at the initial time, are in the first coordination shell, is investigated in supercooled liquid water at 245 K by using the molecular dynamics technique with the four-points transferable intermolecular potential of Jorgensen et al. [J. Chem. Phys. 79, 926 (1983)]. The instantaneous normal mode approach and the results of the local structure investigations are exploited to build up a correlation function of the relative displacements that represents the projection of normal modes along the initial center of mass separation vector. By imposing simple constraints to the initial dynamical conditions, localized damped oscillations of the centers of mass are detected along the hydrogen bond directions. The corresponding density of states shows a maximum around the frequency of 230 cm−1 and its shape agrees with the frequency contributions expected from the translational phonon branches of ice. Total and radial correlation functions of the relative velocity are also computed. The radial component is dominated by oscillations at the frequency of 230 cm−1; it compares fairly well with that derived from the translational density of states, thus supporting the reliability of the employed method. A slower component of motion, absent along the hydrogen bond direction, is derived; in the density of states, it produces a contribution peaked around 50 cm−1.
Vector properties of O(3P) and O(1D) in OH photodissociation: Near-threshold resonance, neighboring resonance, and above-threshold behavior
Authors: Lee Sungyul
Published: July 2002

Theoretical analysis is presented for the vector properties of the oxygen atom produced from the photodissociation of OH at energies from near the threshold to O(3P) to above the threshold to O(1D). The vector properties of O(3Pj, j=0,1,2) (angular distributions and alignment parameters) change rapidly across multichannel resonances near the dissociation threshold to O(3P). The vector properties near the close-lying Lorentzian resonances depend highly on the geometrical factors for the electronic transitions, corresponding to different final total angular momentum quantum numbers, and change rapidly between the resonances. At energies above the dissociation threshold to O(1D), quantum interference between the two direct dissociation pathways gives rise to highly oscillatory pattern of the vector properties of O(1D).
Calculation of static vibrational polarizabilities of polymers: Poly(HF) in two configurations
Authors: Otto P. , Martinez A. , Ladik J.
Published: July 2002

A finite field approach for the calculation of static vibrational polarizabilities of molecules is extended to polymers using the previously developed method for the calculation of the electronic contribution of periodic quasi-one-dimensional systems. The method has been applied to study the effect of the vibrations on the static polarizability of an infinite stack of alternating HF molecules and of poly(HF) in the experimentally determined hydrogen-bonded zig–zag structure. The results are discussed and compared with a calculation for the isolated HF molecule and studies in the literature.
Basis-set convergence of the molecular electric dipole moment
Authors: Halkier Asger , Klopper Wim , Helgaker Trygve , Jo/rgensen Poul
Published: July 2002

The electric dipole moments (μ) of BH and HF are computed in conventional calculations employing different correlation-consistent basis sets at the levels of Hartree–Fock theory, second-order perturbation theory, and coupled cluster theory with single and double excitations, and single and double excitations with a perturbative triples correction. The basis-set convergence of μ is examined by comparison with results obtained with explicitly correlated wave function models. Inclusion of diffuse functions in the basis set is essential for accurate calculations of μ. They speed up the convergence at the Hartree–Fock level significantly and make the convergence at the correlated levels systematic. Once the outer valence regions important for μ are described accurately via the diffuse functions, the convergence at the correlated levels is governed by the interelectronic Coulomb singularity. For the aug-cc-pVXZ basis sets, the correlation contribution to μ follows μXcorr=μlimcorr+aX−3, which is similar to the form for the correlation energy, and extrapolated values based on this form represent a significant improvement on the ordinary basis-set results. Our estimates of the exact dipole moments μe(HF)=1.8037±0.0007 D and μ0(BH)=1.3586±0.0007 D are in excellent agreement with the experimental values μe(HF)=1.803±0.002 D and μ0(BH)=1.27±0.21 D.
Using stretching and bending vibrations to direct the reaction of Cl atoms with isocyanic acid (HNCO)
Authors: Woods Ephraim , Cheatum Christopher M. , Crim F. Fleming
Published: July 2002

Reaction of well-characterized vibrational states prepared in the region of three quanta of N–H stretching excitation explores how vibrations with different components along the reaction coordinate influence the bimolecular reaction of Cl atoms with isocyanic acid (HNCO) to form HCl and NCO. Near prolate symmetric top states corresponding to different amounts of a-axis rotation are well separated in energy, and perturbations by background states make each of the eigenstates a different mixture of zero-order states. Molecules in the essentially unperturbed K=1 and 4 states, which are nearly pure N–H stretching excitation, react efficiently, but those in the perturbed states, K=0, 2, and 3, which are a mixture of N–H stretching and lower frequency vibrations react only half as well. Detailed analysis of resolved, perturbed eigenstates for J=6 and 7 of K=3 reveals the relative reactivity of the two interacting zero-order states. The less reactive zero-order state, which most likely contains only two quanta of N–H stretch and several quanta of other vibrations, reacts only 10% as well as the pure N–H stretch zero-order state. Ab initio calculations suggest that bending excitation alters the interaction potential to reduce the fraction of reactive collisions.
Response to “Comment on ‘Positron and positronium chemistry by quantum Monte Carlo. IV. Can this method accurately compute observables beyond energy?’ ” [J. Chem. Phys. 111, 108 (1999)]
Authors: Mella Massimo , Morosi Gabriele , Bressanini Dario
Published: July 2002

We discuss the accuracy and applicability to molecular systems of the model proposed by G. G. Ryzhikh and J. Mitroy [J. Phys. B 31, 5013 (1998), ibid. 32, 2203 (1999)] to compute annihilation rate in positron-containing systems. Moreover, we propose small modifications to this model in order to improve its accuracy for molecular systems.
Rotational patches: Stark effect, dipole moment, and dynamics of water loosely bound to benzene
Authors: Emilsson T. , Gutowsky H. S. , de Oliveira G. , Dykstra C. E.
Published: July 2002

The geometry of the Fabry–Perot cavity makes it difficult to use for measuring the Stark effect. A “Stark cage” is described which generates an electric field suitable for this purpose. The cage is used to measure first and second order Stark splittings of several low-J transitions of the benzene-water dimer previously reported [Gutowsky, Emilsson, and Arunan, J. Chem. Phys. 99, 4883 (1993)]. The dipole moment is found to depend somewhat on rotational state, ranging from 1.65 to 2.00 Debye for both ground m=0 and first excited m=1 internal rotation states of the dimer. Additional m=1 transitions are reported, including the previously missing downshifted line of a k=0′ doublet. Its presence and various Stark effects require reassignment of the m=1 spectrum. The results demonstrate that each J→J+1 spectrum consists of three distinct components which arise from the H2O in an unusual way. In addition to the k-doublets, there are two progressions; a set of (J+1) negative k’s running from −J to 0, and a set of (J−1) positive k’s with somewhat smaller spacings, running from +2 to +(J−1). It is proposed that each of the three components is associated primarily with one of the three 101, 111, and 110 rotor states of the water. These “patches” are attributed to the potential energy surface (PES). Molecular mechanics for clusters (MMC) calculations indicate barriers of 300–600 cm−1 to internal rotation of the water about any of its inertial axes and nearly free rotation about an axis parallel to the benzene C6 axis. This novel PES leads to unusual internal motions; asymmetric top rotations of the water about the benzene C6 axis instead of the usual rotation about an inertial axis of the water.
Formalism, analytical model, and a priori Green’s-function-based calculations of the current–voltage characteristics of molecular wires
Authors: Hall Lachlan E. , Reimers Jeffrey R. , Hush Noel S. , Silverbrook Kia
Published: July 2002

Various Green’s-function-based formalisms which express the current I as a function of applied voltage V for an electrode–molecule–electrode assembly are compared and contrasted. The analytical solution for conduction through a Hückel (tight binding) chain molecule is examined and only one of these formalisms is shown to predict the known conductivity of a one-dimensional metallic wire. Also, from this solution we extract the counter-intuitive result that the imaginary component of the self-energy produces a shift in the voltage at which molecular resonances occur, and complete analytical descriptions are provided of the conductivity through one-atom and two-atom bridges. A method is presented by which a priori calculations could be performed, and this is examined using extended-Hückel calculations for two gold electrodes spanned by the dithioquinone dianion. A key feature of this is the use of known bulk-electrode properties to model the electrode surface rather than the variety of more approximate schemes which are in current use. These other schemes are shown to be qualitatively realistic but not sufficiently reliable for use in quantitative calculations. We show that in such calculations it is very important to obtain accurate estimates of both the molecule–electrode coupling strength and the location of the electrode’s Fermi energies with respect to the molecular state energies.
Dielectric properties of materials using whispering gallery dielectric resonators: Experiments and perspectives of ultra-wideband characterization
Authors: Annino G. , Bertolini D. , Cassettari M. , Fittipaldi M. , Longo I. , Martinelli M.
Published: July 2002

Measurements for the determination of the complex permittivity of liquid and solid materials based on the use of whispering gallery (WG) dielectric resonators are presented. The procedure implies the measurement of the electromagnetic parameters of the involved resonator interacting with the material under study. The field distribution in the different WG resonant modes was obtained by an analytical calculation under the mode matching method approximation. The results of this calculation, combined with the experimental data, well account for the values of the complex permittivity of materials of interest. Due to the peculiar properties of WG resonators, the method shares the advantages of the wideband systems with the sensitivity and accuracy of the resonator systems in dielectric measurements. Dielectric permittivity of alumina (Al2O3) and of cyclohexane is measured in the frequency range 18–26 GHz. The overall accuracy of the measurements is discussed and the different sources of errors analyzed. Specific attention is paid to the measurements of variations of dielectric properties to monitor some physico–chemical processes. Finally, preliminary measurements at hundreds of GHz show that the procedure is particularly useful for study of dielectric properties up to the THz frequency region.
Ab initio calculations of the spin–orbit splitting in the C̃ state of CF4+ including the dynamical Jahn-Teller effect
Authors: Sommerdijk Harry , van Hemert Marc C.
Published: July 2002

Detailed ab initio calculations on the lowest vibrational state of the electronic C̃ 2T2 manifold of CF4+ show a small Jahn-Teller effect in all asymmetric modes. This results in only small distortions from tetrahedral symmetry, but due to strong coupling between electronic and nuclear motions (dynamical Jahn-Teller effect), the spin–orbit splitting is reduced by an order of magnitude. This answers the outstanding question on the large differences between the results of electronic structure calculations and measurements on spin-orbit splittings in XY4 molecules or molecular ions [R. N. Dixon and R. P. Tuckett, Chem. Phys. Lett. 140, 553 (1987); J. F. M. Aarts and J. H. Callomon, Mol. Phys. 81, 1383 (1994)]. Complete agreement with experiment is however not reached because the dynamical Jahn-Teller effect is very sensitive to small errors in the potential energy surface.
The advantage of writing kinetic energy operators in polyspherical curvilinear coordinates in terms of zi=cos φi
Authors: Carrington Tucker
Published: July 2002

It is straightforward to use kinetic energy operators (KEOs) written in polyspherical coordinates without computing an extra potential-like term, frequently called, V′, or a vector of coefficients multiplying terms with first derivatives with respect to the coordinates. The general polyspherical KEO can be written so that it does not involve gφ, where g=Πn=1Nmn3grgφ.
An accurate semiclassical calculation of collision-induced dissociation
Authors: Sakimoto Kazuhiro
Published: July 2002

Collision-induced dissociation, He+H2→He+H+H, is studied by using a semiclassical method in which one degree of freedom (i.e., intermolecular radial motion) is described by classical mechanics and the others by quantum mechanics. In the present semiclassical treatment, the quantum motions, i.e., vibration, dissociation, molecular rotation, and intermolecular angular motion, are solved accurately without introducing any decoupled approximations. The energy dependence of the dissociation cross section and the translational and angular momentum distributions of dissociative fragments are calculated at the total energies E⩽10 eV. Also carried out is the semiclassical calculation using a centrifugal sudden approximation, and the reliability of this approximation for the dissociation process is examined.
π-dimers of oligothiophene cations
Authors: Brocks Geert
Published: July 2002

The structure and stability of dimers of quaterthiophene radical cations are studied using density functional calculations. Several dimer structures are identified that are metastable in vacuum, in which the cations are stacked coplanarly. Evidence for a weak chemical dimer bonding is found from the binding energy curves and the singlet–triplet splittings. Introducing solvent or solid-state effects by embedding the dimer in a dielectric medium results in stabilizing the dimer, as is observed experimentally. An infinite stack of quaterthiophene cations shows a weak spontaneous dimerization.
Dynamics of the spin-1 Ising Blume–Emery–Griffiths model by the path probability method
Authors: Keskin Mustafa , Solak Abdulkadir
Published: July 2002

The dynamic behavior of the spin-1 Ising Blume–Emery–Griffiths model Hamiltonian with bilinear and biquadratic nearest-neighbor exchange interactions and a single-ion potential is studied by using the path probability method of Kikuchi. First the equilibrium behavior of the model is given briefly in order to understand the dynamic behavior. Then, the path probability method is applied to the model and the set of nonlinear differential equations, which is also called the dynamic or rate equations, is obtained. The dynamic equations are solved by using the Runge–Kutta method in order to study the relaxation of order parameters. The relaxation of the order parameters are investigated for the system which undergoes the first- and second-order phase transitions, especially near and far from the transition temperatures. From this investigation, the “flatness” property of metastable states and the “overshooting” phenomenon are seen explicitly. On the other hand, the solutions of the dynamic equations are also expressed by means of a flow diagram for temperatures near and far from the transition temperatures. The stable, metastable and unstable solutions are shown in the flow diagrams, explicitly and the role of the unstable points, as separators between the stable and metastable points, is described. The dynamic behavior of the model is also studied by using the kinetic equations based on the Zwanzig–Nakajima projection operator formalism and the variational principle. Finally, it is found that one can investigate the dynamic behavior of the system by the path probability method more comprehensively than via the kinetic equations based on the Zwanzig–Nakajima projection operator formalism and the variational principle.
The mechanical strength of a covalent bond calculated by density functional theory
Authors: Beyer Martin K.
Published: July 2002

The rupture forces of covalent bonds in a polymer as a function of bond lifetime are calculated with an Arrhenius kinetics model based on high-level density functional theory calculations. Relaxed potential energy surface scans of small model molecules yield potential functions that account for the deformations and hybridizations caused by the application of force. Morse potentials chosen to exhibit the same well depth and maximum force are used as an analytic representation of an individual bond in an infinitely long one-dimensional polymer. Application of force deforms the potential, and the activation energy for the bond rupture event together with the frequency of an optical phonon in the one-dimensional polymer are the two Arrhenius parameters. Rupture forces of the bonds C–C, C–N, C–O, Si–C, Si–N, Si–O, and Si–Si are reported as a function of the lifetime of the bond.
The bonding of simple carboxylic acids on Cu(110)
Authors: Karis O. , Hasselström J. , Wassdahl N. , Weinelt M. , Nilsson A. , Nyberg M. , Pettersson L. G. M. , Stöhr J. , Samant M. G.
Published: July 2002

We present a study of the bonding of formate (HCOO) and acetate (CH3COO) chemisorbed on Cu(110) using core level spectroscopies in combination with theoretical calculations. For the first time, we apply x-ray emission spectroscopy (XES) to these systems. When XES is used in conjunction with x-ray absorption spectroscopy (XAS) and ab initio calculations, new information about the electronic interaction in the adsorbate–substrate system is provided. In particular, we have used the azimuthal orientation of the COO–surface bond on the (110) surface, to make a complete partition into x, y, and z orbital contributions. The surface bond is found to be predominantly ionic. For the case of adsorbed formate, the covalent bonding is dominated by 6a1/7a1, (σ)-type, frontier orbitals, interacting with the Cu valence band. The resulting hybrid orbitals form a distribution of states that cross the Fermi level. The contribution from adsorbate π-type orbitals is small. The chemical bond formation of adsorbed acetate is very similar to that of formate. In addition, states with metal character have been identified for the outermost CH3-group of acetate. These are delocalized states of mainly local σ-character. The spectral features due to states of local π-character in the adsorbed acetate are well described within the framework of hyperconjugation.
Rotational isomeric state chains on a high coordination lattice: Dynamic Monte Carlo algorithm details
Authors: Clancy Thomas C. , Mattice Wayne L.
Published: July 2002

A high coordination lattice model for simulating coarse-grained rotational isomeric state (RIS) chains has been under development recently. Initially, the model was developed for chains with symmetric torsional potential energy functions, E(φ)=−E(φ). A single-bead move Monte Carlo algorithm was used and found to be effective in simulating polyethylene chains. A modification was subsequently developed to allow for the simulation of chains with an asymmetric torsional potential, E(φ)≠−E(φ). The single-bead move Monte Carlo (MC) algorithm employed previously was found to be ineffective following this modification. Similar kinetic effects have been seen previously with single-bead MC moves on the cubic lattice, which lead to the Hilhorst–Deutch modification (two-bead crankshaft MC move) of the Verdier–Stockmayer (single-bead move MC) algorithm. A reptation MC move applied to this model appears problematic. A multiple-bead MC move is developed using the pivot algorithm formalism in order to avoid the lattice model specific kinetic effect seen with only single-bead MC moves. This allows for the effective simulation of vinyl polymers with asymmetric torsional potentials such as polypropylene and polyvinyl chloride. Polypropylene (PP) and poly(vinyl chloride) (PVC) chains of varying stereochemical structure are simulated. The chains are found to relax with reasonable efficiency. Polypropylene and polyvinyl chloride chains are reverse mapped back to the fully atomistic description. The solubility parameters of the reverse-mapped atomistic structures are found to agree reasonably well with experimental values.