bluuebunny/crossref_metadata_2025 huggingface dataset:
Theoretical study of the surface structures and vibrations of solid α-N2 a)
Authors: Kobashi K. , Etters R. D.
Published: July 2002

The structures and vibrations of the (001), (110), and (111) surfaces of solid α-N2 at zero temperature have been studied using the modified Thiéry–Chandrasekharan potential model. The equilibrium structures of the semi-infinite surfaces are determined using a static energy minimization scheme, and the Einstein frequencies for both translational vibrations and librations of the molecules near the surfaces, and their root mean square (rms) oscillation amplitudes, are calculated. Based upon the equilibrium surface structures, the lattice dynamical calculations for finite thickness slabs [20 layers for the (001) and (110) surfaces and 10 layers for the (111) surfaces] have also been made for wave vectors along certain paths in the two-dimensional Brillouin zones. It is found that the molecular centers of mass positions, orientations, and bond lengths for the molecules in the vicinity of the surfaces, as well as the Einstein frequencies and rms oscillation amplitudes, are significantly different from their bulk values. In the lattice dynamics of the slabs, various types of surface modes are observed. They are (1) surface phonon modes, (2) surface librational modes, (3) hybrid modes of surface phonons and librations, and (4) surface intramolecular vibrational modes. The first two types of the surface modes can also have bulk librational and bulk phonon components, respectively. Furthermore, mixed modes between surface and bulk modes are observed for each of these four types of modes. The influence of the zero-point vibrations on the surface structures and the surface modes is estimated.
Relaxation of HCl in liquid xenon solution by vibration–vibration energy transfer
Authors: Knudtson J. Thomas , Vlahoyannis Yannis P. , Krueger Herman , Weitz Eric
Published: July 2002

The deactivation of vibrationally excited HCl produced via excimer laser photolysis of chloroethylenes has been studied in both the gas phase and in liquid xenon solution. The rate of deactivation of HCl* by the chlorethylene parent is significantly faster in liquid xenon than in the gas phase. This occurs due to enhancement of the vibration–vibration energy transfer probability in solution. The nature of this enhancement is discussed.
On the effect of nuclear motion on chemical binding
Authors: Dohnert Anna
Published: July 2002

Extending the previous work of Berlin to the case where the atomic nuclei in molecules are moving, we obtain a generalized equation for the ‘‘surface of Berlin’’ starting from the hypervirial theorem. We apply our result to the hydrogen molecule in its ground state and show that the corresponding surface is different from the one obtained by Berlin for the case of fixed nuclei, because now it encloses a finite region of space, in which electrons contribute to the binding.
Nuclear spin relaxation in Li2BeF4
Authors: Matsuo T. , Ohno Hideo
Published: July 2002

Nuclear spin lattice relaxation time T1 of 19F and 7Li in the solid and liquid phases of lithium tetrafluoroberyllate (Li2BeF4) were measured in the temperature range between 250 and 630 °C by means of pulse NMR technique. Nuclear magnetic resonance absorption spectra were also measured by use of a spectrometer. T1 of 19F and 7Li in the solid phase decreases exponentially as the temperature is increased. The temperature dependence of T1 of 7Li can be attributed to the diffusion of Li nuclei with an activation energy of 1.03 eV while T1 of 19F can be explained by the dipole interaction modulated by the diffusion of F ions with an activation energy of 1.23 eV. In the liquid phase T1 of 19F goes through a minimum at about 550 °C and is expressed as T1−1∝τ/(1+ω2τ2) in terms of a correlation time, τ. We obtained τ=5×10−16 exp (1.08/kT) s. It is suggested that the relaxation mechanism of 19F in the liquid phase is due to the dissociation of F ions from BeF4 complex ions and the long distance diffusion of the F ions in the liquid.
Study of the relation between diffusion and sedimentation of charged silica sols by dynamic light scattering, ultracentrifugation, and turbidimetry
Authors: Finsy R. , Moreels E. , Bottger A. , Lekkerkerker H.
Published: July 2002

Measurements of the diffusion coefficient, the sedimentation constant and the turbidity by dynamic light scattering, ultracentrifugation and turbidimetry, respectively, of a charged silica sol as a function of the colloid concentration are reported. The strong repulsive interparticle interactions give rise to a spectacular increase of the inverse osmotic compressibility with concentration. It is found that the generalized Einstein relation between the sedimentation and diffusion coefficients and the osmotic compressibility is satisfied, within the experimental accuracies, at finite concentrations.
Fluctuations of the cell size in a model microemulsion
Authors: Borzi Carlos
Published: July 2002

Fluctuations and correlations involving the cell size in a model microemulsion are investigated. The results are plotted vs the curvature parameter, when approaching a critical end point, and their accord with theoretical expectations and available experimental data is discussed. The influence of the breakdown of the saturated interface assumption is analyzed.
Distribution functions for polyoxide chains, including poly(oxymethylene) helices containing known numbers of disruptionsa)
Authors: Curro J. G. , Mark J. E.
Published: July 2002

Monte Carlo simulations based on rotational isomeric state models were used to generate distribution functions for the end-to-end separation of polyoxide chains having the repeat unit [–(CH2)mO–]. High temperatures minimize differences in the distributions arising from differences in conformational preferences, nonetheless even at 200 °C poly(oxymethylene) (m=1) has a narrower distribution than poly(oxyethylene) (m=2), poly(trimethylene oxide) (m=3), and polyethylene (m→∞), presumably because of its helicity. Decrease in temperature or increase in chain length bring out important differences in the distributions, including the appearance or disappearance of multiple maxima in the case of the helical poly(oxymethylene). The multimodal distributions are elucidated by construction of separate distributions for chains with specified numbers of helical disruptions.
Photoionization of perylene in polar solvents
Authors: Konuk Rengin , Cornelisse Jan , McGlynn S. P.
Published: July 2002

The photoionization of perylene in two common polar solvents (acetonitrile and dimethylformamide) is reported. The transient absorption spectra differ significantly from one solvent to the other: Positive and negative ion absorption intensities invert as also do ion decay times. These differences are attributed to solute–solvent interactions in the excited state. The mechanism for the photoionization process is posited to involve electron injection from perylene to the electrophilic solvent (i.e., acetonitrile) and hole injection from perylene to the nucleophilic solvent (i.e., dimethylformamide).
Ab initio MO study on equilibrium bond distance between Fe and pyridine in bis(pyridine)(porphinato) iron for various electronic states
Authors: Saito Minoru , Kashiwagi Hiroshi
Published: July 2002

For bis(pyridine)(porphinato) iron [FeP(py)2], a correlation between the iron electronic state and the equilibrium Fe–Npy distance was investigated. Potential energy curves as a function of the Fe–Npy distance were calculated for low-spin, intermediate-spin, and high-spin states of the ferric and ferrous ions by the ab initio SCF MO method. The equilibrium Fe–Npy distances were obtained from the potential curves. The values obtained for the ferric low-spin and high-spin states were in good agreement with the experimental values for Fe(III)(OEP)(3-Clpy)2 within the differences, ±0.05 Å. The following significant features were found. The equilibrium distance is elongated by the occupation of the 3dz2 orbital and is shortened by the oxidation of the iron atom. The distance and the force constant for the symmetrical py–Fe–py stretching are strongly correlated with the overlap population between the 3dz2 orbital and pyridine–nitrogen orbitals.
Application of inelastic epithermal neutron scattering to the vibrational spectroscopy of adsorbed molecules: Butane physisorbed on graphite (0001) surfaces
Authors: Wang R. , Taub H. , Lauter H. J. , Biberian J. P. , Suzanne J.
Published: July 2002

Inelastic epithermal neutron scattering (IENS) has been used to investigate the intramolecular vibrations of butane [CH3(CH2)2CH3] physisorbed on the (0001) surfaces of a graphite powder. The purpose of these studies was to assess IENS as a vibrational spectroscopy for adsorbed species by using a relatively well-characterized substrate (Carbopack B). The experiments were performed on the IN1 spectrometer located on a beam from the ‘‘hot’’ source at the Institut Laue-Langevin reactor in Grenoble. Film coverages of 1.0 and 3.6 layers were investigated at a temperature of 78 K. The IENS spectra are rich in structure, containing eight vibrational bands in the energy-transfer range from 280–3470 cm−1 (35–430 meV). The similarity in the spectra at the two coverages investigated indicates that in this energy-transfer range the butane intramolecular modes are not strongly perturbed by physisorption on graphite. A detailed comparison is made between the monolayer IENS spectrum and those calculated from models of a free and adsorbed molecule. Both models employ a previously derived intramolecular force field in the normal mode calculation. A third calculation assumes the displacement eigenvectors of the free molecule but replaces the mode eigenfrequencies by their measured Raman and IR values. It reproduces the observed IENS spectra very well. The large number of vibrational bands observed and the close agreement with the calculated spectra suggest butane as a desirable adsorbate for similar experiments on catalytic substrates.
ESR study of Cd(BrO3)2⋅2H2O single crystals γ irradiated at 300 and 77 K
Authors: Sastry D. L. , Rama Rao K. V. S.
Published: July 2002

In Cd(BrO3)2⋅2H2O single crystals γ irradiated at 300 K, BrO2 and O−3 centers are observed. A correlation between the ESR and optical absorption studies has revealed that the optical absorption band for BrO2 lies around 470 nm. After γ irradiation at 77 K, a complex defect center identified as H⋅⋅⋅BrO3⋅⋅⋅HO could be observed. The chemical inequivalence and the effect of thermal or UV light bleaching on the populations of these centers are discussed.
Study of the environment of Fe3+ in Na2Cd(SO4)2 2H2O by EPR
Authors: Gaite J. M. , Bulka G. R. , Hasanova N. M. , Nizamutdinov N. M. , Vinokurov V. M.
Published: July 2002

An EPR experiment on Fe3+ in a single crystal of Na2Cd(SO4)2 2H2O was carried out at Q band frequency and nitrogen temperature. The angular dependencies of the spectra were studied in the three crystallographic planes. In order to have a very precise orientation of the crystal, the experiment was done with a twinned crystal (superposition of two spectra in all three planes). The zero-field splitting of Fe3+ is of the same order of magnitude as the quantum hν, so that many ΔM=2 transitions were observed. All spin-Hamiltonian parameters (20) were calculated using observed ΔM=1 and ΔM=2 transitions for 30 different orientations of the magnetic field. The pseudosymmetries of the fourth-order term of the spin Hamiltonian of Fe3+ are compared to those we calculated using the superposition model. It is seen that Fe3+ is substituted for Cd2+ and that the orientation of the ocatahedral environment of Fe3+ does not differ much in orientation from that of Cd2+. Differences between the distortions of the Cd–O6 and Fe–O6 octahedra are discussed.
An ultrasonic absorption study of the complex formation of zinc(II) thiocyanate in aqueous solution
Authors: Tamura Kiyoshi
Published: July 2002

A new method of analysis for the ultrasonic absorption of systems involving multiple coupled equilibria is described. The method consists of calculating the relaxation frequencies and amplitudes under a postulated reaction mechanism using trial values of the rate constants and volume changes and of comparing the computed absorption α/f2 (absorption coefficient over frequency squared) directly with the experimental one. Application of this method to the ultrasonic absorption study of aqueous zinc(II)-thiocyanate solutions reveals that the relaxation absorption is ascribed to the successive complex formation equilibria Zn(SCN)3−nn−1 +SCN−(kn⇄k−n)Zn(SCN)2−nn, n=1–4. The rate constants and volume changes of the above reactions are determined. The method proves to be especially effective when the absorption spectra are associated with multiple coupled equilibria and accordingly too broad to be separated to discrete relaxation processes by the usual method of analysis.
Chemiluminescent reaction of selenium with fluorine: Assignment of the SeF A 2Π3/2 state
Authors: Thorpe William G. , Carper W. Robert , Davis S. J.
Published: July 2002

Selenium reacts with fluorine with a weak visible chemiluminescence between 3800 and 8900 Å. Identical spectra are obtained regardless of whether the source of selenium is the elemental vapor or hydrogen selenide. By analogy with C10, TeF, and reevaluated SF absorption data, the SeF band system is assigned to transitions between inverted 2Π states. The spectroscopic constants for the A 2Π3/2 to X 2Π3/2 system are: Te=14 680.4 cm−1, ω″e =688.8 cm−1, ωex″e =1.84 cm−1, ω′e =444.1 cm−1, and ωexe =2.93 cm−1. The weak 2Π1/2 to 2Π1/2 subband is described by the constants: Te =15 025.4 cm−1, ω″e =683.0 cm−1, ωex″e =2.11 cm−1, ω′e =436.9 cm−1, and ωexe =1.20 cm−1. Laser excitation spectra were recorded for the SeF chemiluminescence flame. The SeF fluorescence between 4200 and 5200 Å is rapidly quenched with increasing total pressure. A Stern–Volmer analysis gave a zero pressure lifetime of 12.8±3.9 μs. The measured quenching rate of 2.5×106 s−1 Torr−1 is nearly gas kinetic.
Electronic aspects of the hydride transfer mechanism. A
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o analytical gradient studies of the cyclopropenyl-cation/lithium hydride model reactant system
Authors: Tapia O. , Andres J. , Aullo J. M. , Bränden C.-I.
Published: July 2002

The electronic mechanisms of a model hydride transfer reaction are theoretically studied with ab inito RHF and UHF SCF MO procedures at the 4-31G basis set level and analytical gradient methods. The model system describes the reduction of cyclopropenyl cation to cyclopropene by the oxidation of lithium hydride to lithium cation. The molecular fragments corresponding to the asymptotic reactive channels characterizing the stepwise mechanisms currently discussed in the literature have been characterized. The binding energy between the fragments is estimated within a simple electrostatic approximate scheme. The results show that a hydride-ion mechanism is a likely pathway for this particular system. The system is thereafter thoroughly studied from the supermolecule approach. Reaction paths for the ground and first triplet electronic states have been calculated. The hypersurface is explored from a geometrical disposition of the reactants that mimics the one found in several dehydrogenases (perpendicular configuration). A hydride ion is found to be the particle transferred on the unconstrained as well as the constrained reaction pathways in the ground electronic state. In the triplet state (perpendicular configuration) the mechanism is stepwise: electron transfer followed by a hydrogen atom transfer. It has been noticed that the perpendicular geometrical disposition of the reactants plays an important role by polarizing the susceptible cyclopropene C–H bond in the sense of increasing the electronic density at the hydrogen nucleus. This provides a clue to rationalize several dehydrogenase’s active site structure and mechanism. The reactant molecular complex found in the inverted potential energy curves, namely the LiH---Cp+ association has an electronic distribution which can be described as a hydride ion cementing two electron deficient centers corresponding to the cyclopropenyl and the lithium cations. Direct CI calculations confirm the overall picture obtained above.
Solvent effects on chemical reaction profiles. I. Monte Carlo simulation of hydration effects on quantum chemically calculated stationary structures
Authors: Tapia O. , Lluch J. M.
Published: July 2002

Metropolis Monte Carlo simulations of hydration effects on rigid solutes characterizing the reactant, transition state (TS), and product for the rate limiting step of an acid catalized rearrangement of an a-acetylenic alcohol to α, β-unsaturated carbonyl compounds are calculated. The model compound corresponds to the protonated 3 methyl-but-1-yne-3-o1. The electronic structure and geometry of the corresponding species are determined with ab initio analytical gradient techniques; a 4-31G basis set has been used. Electrostatic and solute shape effects on samples having 125 MCY–water molecules at 300 K have been examined. Gurney’s model for ion–molecule interactions has been adopted. Although the solute–water potential used is very simple, the results on hydration energetics appear to be fairly reasonable. Solute shapes are found to play a significant role in producing differential solvation effects. Electrostrictive effects have been made evident by running MC simulations with fully uncharged solutes. A solvent activation barrier is detected. From the study of the TS solvation structures a possible incidence of ionic strength, counterion presence, and structure-making or breaking solutes can be conjectured. The structural features found for the solvation sheaths of reactant, TS, and product are in excellent agreement with the postulated molecular mechanism.
Polarimetry as a type of quantum nondemolition measurement
Authors: Macomber James D.
Published: July 2002

It is shown the polarimetric studies of chiral molecules are members of a type of experiment recently called quantum nondemolition measurements, thus pushing back the date of the first experiments of this type to the 19th century or earlier.
Classical response theory in the Heisenberg picture
Authors: Holian B. L. , Evans D. J.
Published: July 2002

A new derivation of classical response theory is presented from the Heisenberg point of view, where observable properties are followed along a classical many-body trajectory. The exact equivalence of the Heisenberg (observable) and Schrödinger (distribution function) pictures is shown and the nature of their relationship is clarified. The formal expression is derived for the response to time-dependent external fields, to all orders.
The second virial coefficient of 3He gas below 1.3 K
Authors: Cameron J. A. , Seidel G. M.
Published: July 2002

The second virial coefficient of 3He gas has been determined between 1.3 and 0.6 K from measurements of pressure and density on several isotherms. The density was derived from the resonance of a superconducting microwave cavity whose frequency was dependent on the dielectric constant of the gas within it. The pressure was also determined using a resonant superconducting rf cavity, in this case the flexing of a diaphragm being the source of the dependence of frequency on pressure. The experimental techniques work well at low pressure and densities. The virial coefficient appears well behaved and in agreement with the experimental work of others at higher temperatures.
The pressure tensor in nonuniform fluids
Authors: Mistura L.
Published: July 2002

Standard techniques of classical field theory are introduced in order to arrive at a unique definition of the pressure tensor in the thermodynamics of nonuniform fluids. We show that the familiar thermodynamic definition of the elastic stress as the derivative of the free energy with respect to strain can be used to this purpose by replacing the strain components with the components of the metric tensor as independent thermodynamic variables. As an application we calculate the pressure tensor in the case that the free energy is known as a local functional of the one particle density and, in the square gradient approximation, confirm Lovett expression. Moreover, the mechanical expression for the surface tension is obtained from the elementary thermodynamic definition; a scalar pressure is defined and shown to have a weak singularity at the critical point as expected; finally a modified Euler hydrodynamic equation is introduced.
Reactive scattering using pulsed crossed supersonic molecular beams. Example of the C+NO→CN+O and C+N2O→CN+NO reactions
Authors: Dorthe G. , Costes M. , Naulin C. , Joussot-Dubien J. , Vaucamps C. , Nouchi G.
Published: July 2002

The dynamics of the C+NO→CN+O and C+N2O→CN+NO reactions are reinvestigated using a pulsed supersonic crossed molecular beam apparatus. Laser vaporization of graphite at the exit of a pulsed nozzle is used to produce the atomic carbon beam. Laser induced fluorescence spectra of the CN scattered product have been obtained for both reactions.
Optical emission spectra and crystal-field analysis of Tm3+ in the cubic Cs2NaYCl6 host
Authors: Foster David R. , Reid Michael F. , Richardson F. S.
Published: July 2002

Optical excitation and emission spectra are reported for the Cs2NaY0.3Tm0.7Cl6 and Cs2NaY0.99Tm0.01Cl6 systems. The spectra of these two systems are compared and their differences are rationalized in terms of concentration-dependent excited-state relaxation mechanisms. Energy level data derived from this study and from several previously reported studies on Cs2NaTmCl6 and Cs2NaYCl6:Tm3+ systems are summarized, and these data are analyzed in terms of a 16-parameter model Hamiltonian for the 4f 12 electronic configuration of Tm3+ in an octahedral (Oh) crystal field. Eight of the parameters in this Hamiltonian are used to ‘‘fit’’ the empirical energy level data. The parameter sets obtained from several data fits are presented and discussed. Magnetic dipole oscillator strengths are calculated for several sets of transitions, and the calculated results are compared to experimental observation.
The Hilbert theory of transport phenomena in multiple gas mixtures
Authors: Delale Can F.
Published: July 2002

The Hilbert solution of the Boltzmann equation for a multiple gas mixture is presented. It is shown that the first approximation of the Hilbert theory yields the Euler equations for ideal flow. The Hilbert field equations of the second approximation are derived and the transport processes that appear in this approximation are discussed in a manner similar to that given in Chapman and Cowling. The Hilbert formulas for the transport coefficients of a multiple gas mixture are obtained and are seen to be precisely the same formulas of Chapman and Cowling obtained by the Chapman–Enskog method.
Reactivity of metastable helium molecules in atmospheric pressure afterglows
Authors: Pouvesle J. M. , Stevefelt J. , Lee F. W. , Jahani H. R. , Gylys V. T. , Collins C. B.
Published: July 2002

This work focuses upon previously encountered problems that have impeded the understanding of the role of He2(3Σu) metastable molecules in the afterglows of electrical discharges into atmospheric pressures of helium containing admixtures of reactants. New data are reported that generally confirm the literature values for the reactivities of this species at high pressures and continue the disagreement with earlier measurements in low pressure plasmas. Results are presented for reactions with HCl and HBr for the first time. A study of systematic dependencies of rates upon molecular parameters is reported which shows that the ratio for bimolecular reactions of vibrationally excited and unexcited He2(3Σu) molecules is largely independent of the identity of the reacting partner. This provides a means for using data on the pressure dependent part of the effective rate of reaction of He(2 3S) atoms to estimate the rate coefficients for the bimolecular reactions of the more elusive He2(3Σu) molecule.
A solution of Kramers equation for the isomerization of n-butane in CCl4
Authors: Blackmore Robert , Shizgal Bernie
Published: July 2002

A solution to the Kramers equation for the kinetics of the trans–gauche isomerization of n-butane in CCl4 is obtained in terms of the expansion of the probability density function in the eigenfunctions of the Kramers operator. The eigenfunctions. and corresponding eigenvalues are determined with the expansion in a basis set composed of Hermite polynomials in speed and the eigenfunctions of the Smoluchowski operator in the spatial coordinate. The Smoluchowski equation describes the situation when the speed variable is distributed in accordance with the equilibrium Maxwell–Boltzmann distribution and is the high collision frequency limit of the Kramers equation. The range of validity of the first order kinetics is examined in terms of the nature of the eigenvalue spectrum of the Kramers operator. The rate coefficient is determined in terms of the smallest nonzero real eigenvalue and comparisons are made with the results of other researchers.
Comment: On the equilibrium of Hg2 molecule
Authors: Stefanov B.
Published: July 2002

Mass-spectroscopic measurements of Hilpert [J. Chem. Phys. 77, 1425 (1982)] of the Hg2 number density n2 are shown to be in contradiction with the viscosity, thermal conductivity, and second virial coefficient data in Hg vapors. Calculations using available potentials of interaction between two Hg atoms result in 5 to 10 times higher values of n2 than Hilpert’s values.
Direct measurement of the stop gap for the 31 475 cm−1 singlet a exciton of naphthalene
Authors: Stevenson Sylvia H. , Small Gerald J.
Published: July 2002

The one-photon fluorescence excitation profile of the a exciton of naphthalene is reported. (AIP).
Is percolation of relevance to the superheating of light and heavy water?
Authors: Chukanov V. N.
Published: July 2002

The behavior of superheated light and heavy water is discussed. The nucleation rate was measured using the bubble-chamber method. It is assumed that the observed great discrepancy between experiment and nucleation theory is due to structural peculiarities of water. The temperature of maximum superheating T* is identified with the temperature corresponding to the bond percolation threshold. It is also assumed that the phase transition of superheated water is due to fluctuations of the hydrogen bond density near the percolation threshold. An exponential law is obtained for the nucleation rate J∼(T*−T)−λ, where the value of λ within the error bars does not depend either on pressure nor on the substance. Some consequences associated with anomalous behavior of water are discussed.
Host-lattice systematics of EPR spectra of Mn2+-doped isomorphic metal hexakisantipyrine perchlorate and EPR of Cu2+ in copper pentakisantipyrine perchlorate
Authors: Misra Sushil K. , Kahrizi Mojtaba
Published: July 2002

X-band EPR measurements on Mn2+-doped single crystals of isomorphous metal hexakisantipyrine perchlorate (metal=Ca, Cd, Mg, Co, and Pb) have been performed. For the case of calcium and cadmium metal hosts, the measurements were made from room temperature down to liquid helium temperature, while for the remaining metal hosts the measurements were made only at room temperature. The spin Hamiltonian parameters are rigorously evaluated using a least-squares fitting computer technique, especially adapted to electron-nuclear spin coupled systems. As for the systematics of the spin Hamiltonian parameters, at room temperature the variation of the zero field splitting parameters b02 and b04 is found to be approximately linear in the host metal ion radius. In order to estimate the Cu2+–Mn2+ exchange interaction, 5%–50% of Ca2+ ions in Mn2+-doped calcium hexakisantipyrine perchlorate were substituted for by Cu2+, and the EPR spectra were recorded from room temperature down to liquid helium temperature. However, even at liquid helium temperature, the presence of the paramagnetic ion Cu2+ in this host caused no significant change of the g value for Mn2+ as compared with that in the pure diamagnetic host indicating negligible Cu2+–Mn2+ exhange constant. EPR measurements on Cu2+ in copper pentakisantipyrine perchlorate were also made at room, liquid nitrogen, and liquid helium temperatures and the principal values and direction cosines of the principal axes of the g and A tensors were estimated, using a rigorous least-squares fitting procedure.
Compressive axial forces in long-chain molecules due to excluded-volume effects
Authors: Berman D. H. , Weiner J. H.
Published: July 2002

The force length relation f(r) for a freely jointed chain with excluded volume is studied by computer simulation. For the case of the strain ensemble (r fixed and f fluctuates) it is found that excluded-volume effects cause f to be compressive (f<0) for small r, in contrast to the stress ensemble (f fixed and r fluctuates) which leads to f≥0 for all r. The computation procedure also provides the separate covalent and noncovalent contributions to f and the average molecular shape; these provide additional physical insight.
Reply to the comment on ‘‘A proposal for the proper use of pseudopotentials...’’ by Igel-Mann and Stoll
Authors: Bagus P. S. , Nelin C. J. , Bauschlicher C. W. , Laskowski B. C. , Seel M.
Published: July 2002

We have shown that there are serious limitations in the use of a one electron pseudopotential to describe the chemisorption of CO on a transition metal. Igel-Mann and Stoll argue that the errors which we identified are due to our use of limited basis sets and to our neglect of corrections for the core–core interaction. We show that these claims are incorrect; a proper analysis of their results provides further support for our conclusions. For the metal–CO interaction, a one electron pseudopotential cannot describe the metal dπ–CO 2π* covalent bonding, an important feature of the interaction, because the d electrons are included in the pseudopotential. Our original conclusions about the requirements for a proper use of these potentials for chemisorption are unchanged.
Theory of propagation and scattering of light in dilute polymer solutions: Effects of solvation on the optical response of worm-like chain molecules
Authors: Dung Mei Hsu , Ladanyi Branka M.
Published: July 2002

We present a continuum model for the optical response of dilute solutions of polymer molecules. The solvent is represented as a uniform dielectric continuum. Polymer molecules are modeled as flexible cylinders of constant cross section. The optical properties of a dissolved polymer molecule are represented by two different refractive indices in inner and outer concentric regions about the cylinder axis. The model is thus designed to take into account the effects of solvation on the solution optical response. The molecular conformation is represented by the worm-like chain model. Isotropic and depolarized light scattering intensities due to dissolved polymer molecules are evaluated approximately to quadratic order in the scattering wave vector. The refractive index increment of the solution is evaluated as well. The effects on these quantities of molecular shape, size, stiffness, and relative sizes of concentric regions and refractive indices within them are studied. We compare the results of this model to those of several other optical response models of polymer solutions. We discuss how the results of the present model differ from those obtained using the independent scatterer approximation and suggest experiments which would reveal these differences.
On the effect of slightly endothermic reactivity on ion–molecule association rates
Authors: Bates D. R.
Published: July 2002

It has been proposed recently that the existence of a slightly endothermic mode may facilitate ion–molecule association by providing a pass leading to the most stable configuration of the ion formed and also by lengthening the lifetime of the activated complex. Accepting the first of these effects the second is investigated and shown to be generally inappreciable.
The choice of orbitals and single particle energies for the evaluation of molecular energies by a variation-perturbation approach
Authors: Bendazzoli G. L. , Palmieri P. , Fano G. , Ortolani F.
Published: July 2002

A multireference perturbative-variational method, proposed by the authors to compute the molecular energies and wave functions is investigated to assess the importance of the choice of the orbital basis and of the ‘‘zeroth order’’ Hamiltonian on the computed energies. A new definition is presented of single particle energy suitable to perform perturbative-variational computations with any type of orbital sets. Test computations performed with various orbital sets for the Be+H2 system at various geometries indicate that the computed energies are not very sensitive to the choice of the single particle energies but they are sensitive to the choice of the orbitals. The MCSCF orbitals appear to provide the best starting set of orbitals to perform this type of computation. The computed energies are compared to those evaluated by single and multireference coupled cluster, MBPT, and CI methods.
Statistical thermodynamics of insulators
Authors: Ruffa A. R.
Published: July 2002

A thermodynamic description of insulators is developed by means of an approach which uses a statistical treatment of coupled Morse oscillators representing the interatomic protentials. An equation of state is obtained in which the thermodynamic variables are expressed in terms of the microscopic parameters of the interatomic potentials and connected with such important properties of the solid–liquid transition as the melting temperature and the heat of fusion. Calculated values of the bulk modulus and its temperature and pressure dependence are found to agree favorably with available experimental data. Of practical importance is that the relevant quantities can be quickly calculated by personal computer without the need for experimental data.
Equilibrium polymerization of a mixture of two different monomers
Authors: Jensen Peter J. , Bennemann K.-H.
Published: July 2002

We present a general theory of equilibrium polymerization in a binary mixture by applying the n-vector model for magnetism in a weak field. Results are given for the temperature dependence of the order parameters, polymer length, and phase diagrams in the concentration–temperature plane. In addition to phase separations between two monomer phases and between a monomer and a polymer phase, the phase diagrams show the possibility of coexistence between two polymer phases with a critical point. It is shown that our theory becomes identical to the earlier theory for equilibrium copolymerization of Tobolsky and Owen when the molecular field approximation and some additional approximations are used.
Sulfate ion motion in ammonium sulfate crystal as studied by low field proton magnetic relaxation of 17O enriched species
Authors: Chiba Takehiko , Miyajima Seiichi
Published: July 2002

Proton T1D and T1 ρ are measured on ammonium sulfate crystal with 0.8% and 7.3% 17O content, at 30 and 10 MHz. A marked reduction is observed in T1D and T1 ρ compared with normal specimens at temperatures above about 200 K. At 30 MHz, T1D showed a minimum at about 240 K while two minima were observed for T1 ρ in the similar temperature range. Both T1D and T1 ρ at 10 MHz showed different behaviors from those measured at 30 MHz. These experimental results are interpreted in terms of H–O dipole modulation by the SO2−4 reorientation and by the fluctuation of the z component of 17O spin due to strong quadrupole relaxation. From the temperature dependence of the relaxation times, an activation energy of 21 kJ mol−1 and a preexponential factor of 3×10−11 s are obtained for the correlation time of the SO2−4 ion reorientation in the paraelectric phase. The correlation time exhibited a discontinuous change at Tc. Anomalous temperature dependence of the correlation time is observed in the ferroelectric phase. This behavior closely resembles that found for ammonium ions.
EPR of Gd3+ in Na2Cd(SO4)22H2O, comparison with previous results obtained for Fe3+
Authors: Gaite J. M. , Bulka G. R. , Hasanova N. M. , Nizamutdinov N. M. , Vinokurov V. M.
Published: July 2002

An EPR experiment on Gd3+ in a single crystal of Na2Cd(SO4)22H2O (CdK) was carried out at Q band frequency and nitrogen temperature. Two spectra related by symmetry were observed. All spin Hamiltonian constants have been calculated. The pseudosymmetries of the fourth-order term of the spin Hamiltonian are compared to those of Fe3+. It is observed that the substitution of Gd3+ for Cd2+ induced a very important local distortion of the host lattice, which is discussed.
Critical behavior of the ultrasonic attenuation for the binary mixture: Triethylamine and water
Authors: Fast Steven J. , Yun S. S.
Published: July 2002

The experimental value of the slope of α/f 2 vs f −1.06 of the ultrasonic wave attenuation for the critical mixture is compared to the calculated value based on the dynamic scaling theory at the critical temperature. The values of α/f 2 vs f−1.06 yield a straight line as predicted by theory. The experimental values of α/αc for triethylamine and water are compared to the scaling function F(ω*) and to the values of α/αc for 3He and Xe. The adiabatic coupling constant ġ is calculated and compared to the experimental value.
Heisenberg spin exchange effects in powder spectra of weakly perturbed triplet states
Authors: Chesnut D. B. , Meinholtz Dore C.
Published: July 2002

Heisenberg spin exchange effects on the powder spectra of weakly perturbed triplet states have been studied by simulation techniques. From very small exchange frequencies on, the derivative powder spectrum is dominated by a sharp, essentially constant width, central line whose amplitude exhibits very close to a square-root dependence on the exchange frequency. The derivative powder edge lines are shown to behave according to the single crystal limiting exchange width equation and it is shown that the frequency dependence of the central dominating line provides yet another measure of exchange parameters in physical systems. Application of these methods to the (φ3AsCH+3) (TCNQ)−2 ion radical salt yields results in good agreement with those obtained by other methods.
The theory of relaxation processes in metastable systems
Authors: Blackwell M. F.
Published: July 2002

This paper presents a theory of relaxation for systems of spins that are metastable as a result of unimolecular spin decay. A complex Hamiltonian is used to describe the spin decay. Redfield’s theory [Adv. Magn. Reson. 1, 1 (1965)] which describes relaxation in stable spin systems, is treated as the interaction representation of the theory for metastable systems. The resulting rate equations are valid for systems in which spin relaxation is much more efficient than spin decay. An examination of the equations for the case of triplet state photokinetics reveals that the spin relaxation rate matrix is time dependent. A simple solution is offered which yields a new mechanism of spin polarization in metastable systems.
On the energy values of the Cu4OX6L4 type copper complexes
Authors: Buluggiu E.
Published: July 2002

The special form in terms of J+ and J− used by Lines et al. [J. Chem. Phys. 57, 1 (1972)] for the exchange energy in the tetrameric copper complexes of formula Cu4OX6L4, where X=Cl−1, Br−1, and L is a neutral or ionic ligand, is diagonalized to give exact analytical expressions for eigenvalues and eigenvectors. The energy levels have simple form and fit the graphical variations reported in that reference deduced by numerical diagonalization by computer.
Deuterium-induced 19F isotope shifts in fluoroethenes
Authors: Osten H. Jörg , Jameson Cynthia J. , Craig Norman C.
Published: July 2002

Deuterium-induced 19F isotope shifts in the NMR spectra of 17 fluoroethenes are reported here together with other NMR parameters (1H and 19F chemical shifts and FF, HF, DF, and DH coupling constants). The two-bond (gem) and three-bond (trans) isotope shifts exhibit correlations with nuclear spin–spin coupling constants 2Jgem (HF) and 3Jtrans (HF). The isotope shifts are interpreted using derivatives of nuclear shielding with respect to bond extension derived from the 19F temperature dependence at the zero-pressure limit, and the changes in the mean bond lengths due to isotopic substitution. The latter are calculated using the previously reported Urey–Bradley force fields for these molecules. The analysis of the isotope shifts lead to estimates of the change of 19F nuclear shielding due to extension of a bond which is located at a gem, cis, or trans position relative to the resonant nucleus. These 19F nuclear shielding derivatives correlate with the nuclear spin–spin coupling constants 2Jgem(HF) , 3Jcis (HF), and 3Jtrans(HF) which share the same pathway of electronic transmission.
Nonexponential nuclear magnetic relaxation of 13C nuclei in liquid chloroform
Authors: Rabii M.
Published: July 2002

By treating the 13C nuclear magnetic relaxation in liquid chlorofom as a dominant 13C–1H dipolar interaction mechanism and considering the other interaction effects as ‘‘leakage’’ relaxation terms, a general nonexponential decay was deduced with two relaxation times T′1 and T1 in fair agreement with the experimental results. By use of this model, the 13C spin-rotation relaxation time in liquid chloroform was estimated to be 1420 s at 30 °C.
Solitons and intermolecular interaction in the crystal of the helical polymer: Molecular motion and crystal structure
Authors: Saruyama Y.
Published: July 2002

Intermolecular interaction between the polymer molecules of nonuniform helices is investigated by extending a previous study on uniform helices. It was found that the sine-Gordon equation provides a reasonable equation of motion of the molecules in the crystal. Soliton solution of the equation of motion expresses cooperative molecular motion, instead of motion of a single molecule, around the helix axis. The solition propagates along the helix axis. Based on the static solution of the equation of motion a new model for the crystal structure is proposed. Recently this new model is found to be realized in polyoxymethylene crystal.
A curious result concerning the Clausius virial
Authors: Powles J. G. , Rickayzen G. , Williams M. L.
Published: July 2002

For a sample of liquid confined in a hard-walled spherical container it is shown by computer simulation and by statistical mechanics that the conventional virial pressure for a notional sphere of radius r is not the correct thermodynamic normal pressure on the surface of the sphere but is the local kinetic contribution only, ρ(r)kT. In other words, ρ̄(r)kT+[1/3V(r)]〈𝒥iri⋅Fiθ(r −ri)〉=ρ(r)kT, where ρ̄(r) is the mean density inside the sphere of radius r and volume V(r). This result is true for any intermolecular interaction derivable from a potential and is a good approximation for any reasonable wall potential for regions outside the range of the wall potential.
Numerical solution of chemical equilibria with simultaneous reactions
Authors: Myers Andrea K. , Myers Alan L.
Published: July 2002

Numerical methods for the iterative solution of multireaction chemical equilibrium problems often diverge. The task is to find an initial estimate of the solution which is close enough to the actual value to guarantee convergence. The proposed algorithm first selects a set of reactions with equilibrium constants less than unity and then recalculates the initial composition of the mixture so that the extent of reaction vector ξ will be small and positive. The second-order numerical procedure is fast and convergent because it starts with a composition which lies close to the solution.
Spectra and structure of organogermanes. XXIV. Microwave spectra, quadrupole coupling constants, and structure of methylgermyl bromide
Authors: Durig J. R. , Sullivan J. F. , Mohamad A. B. , Cradock Stephen , Li Y. S.
Published: July 2002

The microwave spectra from 26.5 to 39.0 GHz of eighteen isotopic species of methylgermyl bromide, 12CH3GeH2Br, 12CH3GeD2Br, and 13CH3GeH2Br for three of the naturally occurring isotopes of germane, 70Ge, 72Ge, and 74Ge, as well as the two of bromine, 79Br and 81Br, have been measured and assigned. From these data, substitution structural parameters were determined except for the methyl group. The determined parameters are: r(Ge–Br)=2.308±0.002 Å, r(Ge–C)=1.933±0.002 Å, r(Ge–H)=1.520±0.001 Å, r(C–Hs)=1.100±0.012 Å, r(C–Ha)=1.097±0.030 Å, CGeBr=107.0±0.2°, HGeH=111.5±0.1°, HGeC=112.2±0.1°, GeCHs=110.3±4.7°, and GeCHa=111.9±1.7°. From a diagnostic least-squares adjustment to fit 36 rotational constants, r0 structural parameters were also obtained. Quadrupolar coupling constants have been obtained. These results are compared to similar quantities in some related molecules.
Canonical formalism in equilibrium thermodynamics. II. Homogeneity, stability, and generalized Maxwell relations
Authors: Rey de Luna A. , Zamora M.
Published: July 2002

The classical theory of equilibrium thermodynamics is formulated in a canonical context which has been recently proposed by us. It is shown that the equations of state correspond to what in analytical mechanics is called an involution system. The classical conditions of thermodynamic stability are deduced from a variational principle for the entropy function.
Epitaxial orientations of O2 monolayers on graphite by superlattice formation
Authors: Flurchick K. , Etters R. D.
Published: July 2002

A search for registered superlattices containing as many as 841 molecules has been conducted which shows that they provide a mechanism for understanding the orientational epitaxies observed in low-energy electron diffraction (LEED) measurements of monolayer O2 deposited on graphite. Superlattices for the δ, δ′, ζ1, and ζ2 phases were found that give excellent agreement with experiment for the epitaxial orientations and the departure of the unit cell from centered rectangular. It is concluded that the lateral variations in the admolecule–substrate interaction are responsible for the epitaxy and the departure of the unit cell from centered rectangular. The structure and epitaxial orientations of O2 clusters of size 1≤N≤16, deposited on graphite, are also investigated.
Herzberg band fluorescence from UV excitation of O2 adsorbed on aerosol surfaces
Authors: Manzanares E. R. , Suto Masako , Lee L. C.
Published: July 2002

The fluorescence spectra produced from KrF laser (248.5 nm) excitation of O2 adsorbed on aerosol surfaces were observed. The aerosols were produced from the reaction of O3+C2H4 or O3+C2H4+SO2 in O2 or N2 in a gas-flow reactor. The fluorescence spectra were identified as the O2 (A 3∑+u, A′ 3Δu, and c 1∑−u→X 3∑−g) Herzberg systems. The molecular fluorescence intensity was greatly enhanced by the aerosol surface. The possible mechanisms for the enhancement of the observed fluorescence by surface are discussed.
A model for the specific heat of amorphous polymers
Authors: Allen James P.
Published: July 2002

A model is presented that characterizes the temperature dependence of the specific heat of amorphous polymers. The model utilizes a density of vibrational states given by ρ(ω)∝ωd̃−1, where d̃ is the fracton, or spectral, dimension. This density of vibrational states, when introduced into the standard Debye model, leads to a specific heat temperature dependence of the form C(T)∝(T/Θd̃)d̃, where Θd̃ is a characteristic temperature. This model fits the data well for the temperature region ∼4 to ∼40 K. A key feature of the model is the prediction of a crossover of the polymer scaling properties that causes a crossover from T2 to T5/3 of the specific heat temperature dependence. In addition, this model allows a simple estimate of the specific heat for a wide range of polymers using only the formula weight.