bluuebunny/crossref_metadata_2025 huggingface dataset:
X-Ray Diffraction Studies of Aqueous Alkaline-Earth Chloride Solutions
Authors: Albright James N.
Published: February 2004

Electron radial distribution functions derived from x-ray diffraction measurements are presented for six concentrated aqueous alkaline-earth chloride solutions. Analysis of these functions indicates that magnesium, calcium, strontium, and barium ions are hydrated by 6.5–8.0 water molecules in concentrated chloride solutions. Magnesium ions significantly alter and order the water structure; calcium ions more markedly so as evidenced by significant second-nearest-neighbor coordinations. The hydration of magnesium and calcium ions appears to be independent of the concentrations of ions in solution from NMgCl2=0.071 and NCaCl2=0.055 to saturation.
Energy Disposal in the Photodissociation of Ethanethiol. II
Authors: White John M. , Johnson R. L.
Published: February 2004

Ethanethiol has been photolyzed at 189 and 214 nm in the presence of D2. The average partition of energy between translational and internal modes has been determined and compared with earlier results on methanethiol.
Molecular Structures of Perfluoromethyl Iodide, Perfluoroethyl Iodide, and Perfluoroisopropyl Iodide
Authors: Andreassen A. L. , Bauer S. H.
Published: February 2004

The molecular structures of three fully halogenated alkanes have been investigated by electron diffraction. The following rg parameters were determined for CF3I: C–I=2.101±0.009 Å; C–F=1.344±0.004 Å; and ∠FCI=111.3°±0.4°. For CF3CF2I: C–I=2.142±0.023 Å; C–F=1.338±0.004 Å; C–C=1.523±0.027 Å; with ∠CCI=113.4°±0.8°; ∠CCF=109.9°±0.8°; and ∠ICF=109.4°±1.0°. The rg parameters determined for (CF3)2CFI are: C–I=2.139±0.017 Å; C–F=1.338±0.003 Å; C–C=1.554±0.012 Å; ∠CCF=111.0°±0.4°; ∠ICF=111.8°±1.4°; ∠CCI=109.4°±0.6°; and ∠CCC=113.2°±1.2°. These are thermally averaged (nearly harmonic approximation) values.
Roles of Repulsive and Attractive Forces in Liquids: The Optimized Random Phase Approximation
Authors: Andersen Hans C. , Chandler David , Weeks John D.
Published: February 2004

The optimized random phase approximation (ORPA) is applied to the calculation of the thermodynamic properties and pair correlation function of simple liquids. General formulas are presented, together with results for the Lennard-Jones fluid, which are compared with Monte Carlo and molecular dynamic results. Excellent agreement is obtained for the entire single phase fluid region of the phase diagram, except for the critical region and the very low temperature vapor. The theory converges especially rapidly at high densities or at high temperatures. The importance of separating the intermolecular potential in the proper way to obtain most rapid convergence of perturbation theories is discussed. It is concluded that the structure of simple liquids is determined mostly by the rapidly varying parts of the potential. Further, the ORPA provides an accurate theory for calculating the contributions from different forces to the structure and thermodynamics of liquids.
Infrared Absorptions of Interstitial Hydrogen Atoms in Solid Argon and Krypton
Authors: Bondybey Vladimir E. , Pimentel George C.
Published: February 2004

Infrared absorptions are reported in the spectra of inert gas-hydrogen matrix samples which were deposited after the gas mixture passed through a glow discharge. For hydrogen in argon, absorption is recorded at 905 cm−1. This band shifts to 644 cm−1 for deuterium in argon and no new bands are observed with hydrogen-deuterium mixtures. With krypton matrix, similar absorptions are observed at 852 cm−1 (H2–Kr) and at 607 (D2–Kr). Prolonged infrared irradiation in the spectral range of the absorptions causes them to disappear, the hydrogen absorptions more rapidly than the deuterium features. No similar bands could be detected when xenon or neon matrixes were used. The very small Ar36–40 Ar isotopic shift (0.2±0.1 cm−1) shows that the argon absorptions cannot be attributed to HAr, to HAr+ or to HAr2+. The evidence is consistent with the interpretation that both in argon and in krypton, the absorptions are due to hydrogen atoms trapped in Oh interstitial sites in an undistorted inert gas lattice. This interpretation is supported by ab initio calculations for H atoms in a neon lattice. The loss on infrared irradiation is attributed to intramolecular energy transfer into local lattice vibrational motion which permits escape from the interstitial site and recombination.
Quantum Calculations of Planar Reactive H+H2. I. Theory
Authors: Saxon Roberta P. , Light John C.
Published: February 2004

Reaction coordinates for planar reactive collisions of an atom and diatomic molecule are described and a technique for the change of basis required by the coordinates is discussed. A quantum mechanical method of calculating the probabilities of reactive and nonreactive scattering and differential and total cross sections (of dimension length) is presented. The symmetry of the reaction is utilized to reduce the computation and in describing the change of basis.
Theory of Radiationless Relaxation of Rare-Earth Ions in Crystals
Authors: Fong Francis K. , Naberhuis Steven L. , Miller Marvin M.
Published: February 2004

The rate of multiphonon relaxation of rare-earth ions in crystals has been calculated using the Kubo representation of the rate constant in terms of linear response time correlation functions. The nonadiabatic electron-phonon operator arising from the effect of the kinetic energy of the ions on the Born-Oppenheimer basis functions is employed in the first-order perturbation theory. The calculations yield the explicit dependence of the decay rate on the temperature, the transition energy gap, the phonon energy, and the displacements of the adiabatic potentials. Excellent agreement between theory and experiment has been obtained for the multiphonon relaxation phenomenon observed in LaF3:Er3+, LaCl3:Dy3+, LaCl3:Nd3+, and LaBr3:Dy3+. Although the comparison of theory and experiment is specialized to the nonradiative relaxation of rare earth ion excited states in crystals, the treatment given in the present paper is also generally valid for the relaxation of excited states by internal conversion in large molecules.
Primary Steps in the Photolysis of CD3CHCl2
Authors: Fujimoto T. , Wijnen M. H. J.
Published: February 2004

The photolysis of CH3CHCl2 and of CD3CHCl2 was studied at 25°C in the presence of iodine. Two series of experiments were carried out, one in which the initial pressure of 1, 1-dichloroethane was varied from 2.5 to 15.8 mm Hg and one in which nitrogen was used as an inert deactivator. The following reaction products were observed: hydrogen, acetylene, ethylene, vinyl chloride, 1, 1-dichloroethylene, 1, 1-chloroiodoethane, and iododichloroethane. The appearance of these products and their isotopic composition suggest that the following primary processes occur: CD3CHCl2* → CD3CHCl+ClCD2CHCl+DClCD2CCl2 + HDCD3CH+Cl2.Step (D) has been observed only below 2200 Å, Steps (A)-(C) occur also at wavelengths above 2200 Å. Primary steps (A)-(D) are discussed and compared with processes occurring during the photolysis of other ethanes.
ESR Study of Heisenberg Spin Exchange in a Binary Liquid Solution near the Critical Point
Authors: Lang John C. , Freed Jack H.
Published: February 2004

A careful study of the Heisenberg spin-exchange contribution ωHE to the ESR linewidths of the di-t-butyl nitroxide (DTBN) radical dissolved in mixtures of 2,2,4-trimethylpentane and n-perfluoroheptane was performed. The study includes samples of different radical concentration dissolved in the critical composition of the two solvents. This critical solvent system is known to exhibit an anomaly in the macroscopic kinematic viscosity ν near Tc. It is found that in the critical region, ωHE is not linear in T/ν. However, it was observed that ωHE is linear in T/ν′ both for noncritical compositions and critical compositions above Tc. Here ν′ is the macroscopically measured viscosity, but with the ``anomalous portion'' subtracted out. Deviations from ideal behavior of ωHE with respect to T/ν were observed and discussed. The experiments near the critical region required temperature stability and control to within ±0.01°C at the ESR sample, and a description is given of the experimental design.
Computer Generation of Molecular Symmetry Orbitals
Authors: Chung Alice L. H. , Goodman Gordon L.
Published: February 2004

We show how molecular symmetry adapted linear combinations of atomic orbitals can be produced by carrying out standard manipulations on the matrices that represent values for each of two rotationally invariant operators evaluated over the atomic orbitals of interest. Attention is given to obtaining final orbitals that have not only the desired transformation properties but that are also ``beautiful'' in the sense that they have simple atomic orbital coefficients like the symmetry orbitals that are traditionally discussed in the literature and generated by inspection. Our implementation of this method in Fortran on a CDC 3600 requires a time for symmetry adaptation that increases as the symmetry of the molecule becomes greater. In several calculations that we have done on small (5–12 atoms), reasonably symmetrical (D2h–Oh) molecules, the adaptation required times comparable to those required for calculating the overlap and kinetic energy matrix elements used in the symmetry adaptation.
Formation and Collisional Dissociation of Heteronuclear Rare-Gas Associative Ions
Authors: Kramer H. L. , Herce J. A. , Muschlitz E. E.
Published: February 2004

Ratios R of the cross sections for associative to the sum of associative and Penning ionization have been measured using molecular beams of thermal energy (330°K) metastable atoms, A*, incident on a gas B confined in a collision chamber at low pressures, where A* is either 21,3S helium or 3P2,0 neon and B is argon or krypton. By determining R as a function of the collision chamber pressure in the range 0.05–0.4 μ and by measuring the relative intensity of the secondary product ion, B2 +, cross sections σI and σII for destruction of the associative ions, AB+ + B → A + B+ + Band AB+ + B → A + B2 +,have been determined. Extrapolation to zero pressure gives the true value of R for the primary processes. The effect of a change in the ratio of 21S/23S helium in the metastable atom beam has also been investigated by changing the exciting electron beam energy. In collisions of He* with both argon and krypton, R is greater for 23S helium than for 21S helium by about 0.05. Within the experimental error, σI and σII for destruction of AB+ are independent of whether this ion is formed by collisions of 23S or 21S helium. The large size of the cross sections σI, which range from 210 Å2 for NeAr+–Ar to 470 Å2 for HeKr+–Kr, suggests that the associative ions are formed very close to the dissociation limit.
High-Resolution N2 Absorption Study from 730 to 980 Å
Authors: Carter Virginia L.
Published: February 2004

The absorption spectrum of N2 obtained photoelectrically at a bandwidth of 0.04 Å from 730 to 980 Å is presented and compared to earlier, broader band measurements. At wavelengths longer than the ionization edge at 796 Å the peak absorption cross sections (and consequently band oscillator strengths) show a highly variable dependence on the number density of absorbing molecules in the light path, which is caused by saturation and bandwidth effects. The results of an attempt to obtain band oscillator strengths in the limit at low column numbers (1014 molecules/cm2) are given. At wavelengths shorter than the ionization edge, the data are only dependent on column number for values greater than ≈ 1016 molecules/cm2. Absolute values for a set of lower limits on the cross sections are given, the structure is identified, and results are compared to those of Cook and Metzger, Huffman et al., and Cook and Ogawa. Optical oscillator strengths are compared to those of Lawrence et al. for the four bands in the interval from 958 to 972 Å. These are the only bands for which optical oscillator strengths had been previously obtained. The results agree within experimental error with the earlier work except that the oscillator strength of the band at 972 Å is shown here to vary with absorbing column number. Oscillator strengths for a total of 102 intervals are given. They are grouped into the b 1Πu, b′ 1Σu+, c′ 1Σu+, o 1Πu, and c 1IIu vibrational band systems and are compared to normalized electron energy-loss band-head data of Geiger and Schröder. Marked perturbations in the vibration series are evident.
Gaussian Model Potentials for Molecular Interactions
Authors: Berne Bruce J. , Pechukas Philip
Published: February 2004

Simple analytical forms for the orientation dependence of the potential between two molecules are derived from a Gaussian overlap model. Orientation-dependent range and energy parameters are determined, which can be used with any two-parameter atomic potential to give simple and reasonable polyatomic potentials.
Monte Carlo Trajectory Calculations of the Dissociation of HCl in Ar
Authors: Shui Ven H. , Appleton John P. , Keck James C.
Published: February 2004

The modified phase-space theory of reaction rates is applied to the dissociation of HCl in a heat bath of argon atoms. Excellent agreement is obtained between the theoretical predictions and the shock-tube measurements of the dissociation rate coefficient over the temperature range 2500–5000°K. The recrossing correction factor and nonequilibrium correction factor are obtained from Monte Carlo trajectory calculations for states near the dissociation limit. The trajectories were sampled within the reaction zone, with a weight proportional to the equilibrium reaction rate, and numerically integrated in both timewise directions to determine the complete histories of the collisions. A simple separable function for the equilibrium transition rate R(εi, εf) from initial states εi to final states εf was obtained to fit the numerical data with sufficient accuracy and was used to solve the steady-state master equation. Important features of collisions of highly asymmetric diatomic molecules are discussed, and several typical reactive trajectories are shown to illustrate the importance of rotational motion in such collisions.
Phase Transitions Due to Softness of the Potential Core
Authors: Stell G. , Hemmer P. C.
Published: February 2004

This paper extends a previous demonstration [Hemmer and Stell, Phys. Rev. Letters 24, 1284 (1970)] showing that for a system in which the pair potential has a hard core plus a negative part, softening the hard core can produce a second transition if a first already exists. Detailed analytic and numerical results for one-dimensional continuum fluids are given, and our discussion of the lattice gas is further developed. In particular interactions that are repulsive over next-nearest neighbor cells as well as nearest-neighbor cells are considered, and it is rigorously shown that as many as four first-order phase transitions can occur for such potentials, even in one dimension. The relevance of our work to certain features found in real systems (e.g. the possible breakdown of the law of rectilinear diameters, and isostructural solid-solid transitions) is also discussed, as is the novel critical behavior to be expected of certain two- and three-dimensional lattice systems.
Conformational Equilibria in Isotactic Copolymers of Enantiomeric α-Olefins
Authors: Luisi P. L. , Bonsignori O.
Published: February 2004

The conformational equilibrium in random isotactic copolymers of enantiomeric α-olefins has been investigated. In particular, the conformational partition functions have been determined for copolymers of (R) and (S)-4-methyl-l-hexene (I), (R) and (S)-3,4-dimethyl-l-pentene (II), (R) and (S)-5-methyl-l-heptene (III). The molar ratio Wl of monomeric units left-handed spiraled as well as the molar optical rotation [φ]D of the copolymers, have been determined as a function of the monomer optical purity P. In the case of (I) and (II), the dependence of [φ]D on P (and of Wl on P) follows a hyperbolic behavior, which is more pronounced in (II), whereas in the case of (III) it follows a linear pattern. The reason for this difference in behavior is discussed. In the case of (I), for which a hyperbolic dependence of [φ]D on P had been experimentally found, the comparison between calculated and experimental data is discussed with reference to the stereoselectivity of the copolymerization process. The temperature coefficient of optical activity is also calculated as a function of P and in the case of (I) and (II) it is shown to be highly enhanced with respect to that homopolymer, with a maximum occurring at a P value, which is different in the two cases.
Point Charge-Point Dipole Model for Vibrating Triatomic Molecules
Authors: Simons Gary
Published: February 2004

A point charge-point dipole model for the electronic potential energy contribution to the Born-Oppenheimer potential of triatomic molecules is proposed. The polyatomic virial theorem, with appropriate approximations, is shown to allow one to proceed by implicit rather than explicit inclusion of the kinetic energy. Bending force constants and cubic stretching constants for XY2 molecules are predicted.
Microwave Spectrum of BF2OH
Authors: Takeo H. , Curl R. F.
Published: February 2004

The microwave spectrum of BF2OH has been observed and assigned. The rotational constants of the isotopic species 11BF216OH, 10BF216OH, 11BF216OD, 10BF216OD, 11BF218OH, and 10BF218OH have been determined. The inertial defects prove that the molecule is planar. Assuming the two BF bond lengths are the same, the bond distances and angles are found to be: rBF = 1.32 Å, rBO = 1.34 Å, rOH = 0.94 Å, ∠FBF=118°, ∠BOH=114.1°, ∠FBO=122.8° (this is the angle cis to the OH). The dipole moment is 1.86± 0.02 D. The dipole moment makes an angle of 50° with the BO bond with dipole moment line on the same side of the BO bond as the hydrogen.
Melting and Polymorphism of KHF2, RbHF2, and CsHF2 to High Pressures
Authors: White A. J. Campbell , Pistorius Carl W. F. T.
Published: February 2004

The high-pressure phase diagrams of KHF2, RbHF2, and CsHF2 were studied by means of differential thermal analysis and volume displacement. The tetragonal / Fm3m transition lines of KHF2 and RbHF2 rise steeply with pressure to meet the melting curves of the Fm3m phases near 2 kbar. The resulting melting curves of the tetragonal phases are almost immediately terminated by the appearance of new phases which appear to be similar to the high-temperature forms with space group Pm3m of CsHF2 and the corresponding azides. In the case of KHF2 the tetragonal / Pm3m transition line meets the melting curve of the Pm3m phase at a triple point at 29 kbar, 451°C. A new high-pressure phase CsHF2 IV appears above 2.7 kbar at 20°C. This phase may be similar to CsN3 III. The phase diagram of CsHF2 is closely similar to that of CsN3.
Configuration Interaction Wavefunctions and Computed Inversion Barriers for NH3 and CH3−
Authors: Kari R. E. , Csizmadia I. G.
Published: February 2004

Ground state LCAO-MO-SCF wavefunctions, using a large contracted Gaussian basis set, have been constructed for NH3 and CH3−. These wavefunctions were determined for four points along the inversion coordinates of the respective molecules, and were then improved by the method of configuration interaction (CI). In these CI calculations, the computed energies of NH3 and CH3− were obtained to within 0.18 hartree of their respective nonrelativistic limits. The inversion barrier, as computed with the CI wavefunctions, was 6.9 kcal/mole for NH3 and 2.8 kcal/mole for CH3−.
Molecular Structure of Trimethylene Oxide from Its NMR Spectrum in a Nematic Solvent
Authors: Cole K. C. , Gilson D. F. R.
Published: February 2004

The 220-MHz NMR spectrum of trimethylene oxide, oriented in the nematic liquid crystal N-(p-ethoxy-benzylidene)-p-n-butylaniline, has been analyzed. The direct coupling constants have been determined and interpreted in terms of the molecular geometry and vibrational motion.
Optical Spectra of an Isoelectronic Series; the π*←π Transition in Phenyl Isocyanide, Ethynylbenzene, and Benzonitrile
Authors: Muirhead Alan R. , Hartford Allen , Huang Kwo-Tsair , Lombardi John R.
Published: February 2004

In this work we study the lowest-lying singlet π*←π transition in the isoelectronic series phenyl isocyanide, ethynylbenzene, and benzonitrile. The spectrum of phenyl isocyanide is analyzed for the first time and is found to be very similar to that of ethynylbenzene. The 000 band is assigned at 2723 Å. Rotational analyses are reported on the 000 and 23501 bands in phenyl isocyanide. Changes found in rotational constants for the 000 band are Δ A = −0.00789 cm−1, Δ B = −0.00101 cm−1, and Δ C = −0.00101 cm−1. Origin of the electronic transition is set at 36 712.96 cm−1. Dipole moment changes on excitation are determined for all three molecules by the Stark effect. For benzonitrile Δμ = 0.31 D, more than twice the change in the other two molecules. The differences in the over-all spectrum and dipole moment change for benzonitrile compared to the other two molecules are explained in terms of the possible influence of intramolecular charge transfer.
Quantum Mechanical Calculation of Molecular Radii. I. Hydrides of Elements of Periodic Groups IV through VII
Authors: Kammeyer Carl W. , Whitman Donald R.
Published: February 2004

The fraction of the total electron density within a sphere having the empirical van der Waals radius is calculated for the atoms of eight elements, using Hartree-Fock atomic wavefunctions. The sphere is found to contain 97%–99% of the electron density, indicating that such calculations correlate well with atomic sizes. The procedure is extended to eight hydride molecules of the type AHn, with molecular size determined by the radius of the 98% contour as computed from published one-center SCF molecular wave-functions. The predicted molecular radii agree well with experimental values and support the usefulness of this method for describing effective molecular size.
Far-Infrared Spectra of Ring Compounds. IX. Far-Infrared Spectrum and Hindered Pseudorotation in Cyclopentanone
Authors: Ikeda Tatsuya , Lord R. C.
Published: February 2004

The high resolution far-infrared spectra of cyclopentanone-α-d4, and d8 have been observed. The energy levels of the low-frequency vibration have been calculated as a problem of a particle in a two-dimensional potential well with a variable reduced mass. The eigenstates of the hindered pseudorotation have been described in terms of a mixture of the eigenstates for the two orthogonal motions, pure bending and pure twisting around the twisted equilibrium conformation. The conformational energy of the molecule is estimated to be V(x, y) in cm−1 = 1302 x2 − 14531y2 + 366x4+ 14765x2y2 + 70352y4 where x and y are dimensionless bending and twisting coordinates. The interconversion between the two twisted conformations occurs through the planar form whose potential energy is about 750 cm−1 higher than the equilibrium form. To account for the observed deuterium substitution effects, some coupling of the α-methylene rocking motions with the hindered pseudorotation has been postulated on an empirical basis. The major features of the far-infrared spectra have been explained and some of the implications of the model calculation have been discussed.
Electron Paramagnetic Resonance and Polarized Absorption Spectra of Cu(II)-Doped Single Crystal of L-Histidine Hydrochloride Monohydrate
Authors: Hirasawa Ryo , Kon Hideo
Published: February 2004

The electron paramagnetic resonance of copper(II) doped in a single crystal of L-histidine· HCl monohydrate indicates that the Cu2+ ion is incorporated in a different environment from that in the ordinary bis(L-histidinyl)copper(II) complex. The coefficients of the ground state Kramer's doublet were determined from the observed g tensor (gz = 2.013, gx = 2.262, gv = 2.153) in such a way that the hyperfine parameters calculated from the coefficients closely reproduce the observed values. The results show that the half-filled d orbital is of the dz2 type, suggesting an axially compressed structure. The polarized single crystal absorption spectra taken in the three perpendicular crystal faces exhibit three d-d transitions at 14 000, 12 800 and 11 100 cm−1, which are polarized mostly along the c, b, and a crystal axes, respectively. By combining the optical and the paramagnetic resonance results, the one-electron orbital sequence was found to be dz2>dx2-y2>dyz>dxy>dxz. For a possible site of copper incorporation an interstitial position was chosen which has approximately D2 symmetry.
Theory of Zeeman Effect for Rare-Earth Ions in Crystal Field with C3h Symmetry. IV
Authors: Kambara T. , Haas W. J. , Spedding F. H. , Good R. H.
Published: February 2004

In earlier papers a quantum number ξ=±1 was assigned to states of the system (the ion in the crystal field and with magnetic field in the basal plane) according to their behavior under the antiunitary symmetry operation, time reversal combined with space reflection in the basal plane. The states were found to be of two types, Type A states for which the assignment of ξ number does not depend on choice of coordinate axes and Type B states for which it does. The types influence the dependence of absorption intensity on magnetic field direction: for A→ A and B→ B transitions it is qualitatively different than for A→ B and B→ A transitions. In this paper the type is shown to be related to the representations of C2h that the states make at the special magnetic field directions for which C2h is a symmetry of the system. It is shown also that the type of state may change when the energy levels, plotted as a function of magnetic field applied in one of those special directions, cross. This means that, wherever there is a crossing, there is a drastic change in the way the intensities of the lines vary with direction of the field. At high magnetic fields, past all crossings, all states are Type A. Application is made to some energy levels of erbium ethyl sulfate.
Pulse Radiolysis of Liquids at High Pressures. II. Diffusion-Controlled Reactions of the Hydrated Electron
Authors: Hentz Robert R. , Farhataziz  , Hansen Earl M.
Published: February 2004

For each of 11 presumably diffusion-controlled reactions of eaq−, absolute specific rates were determined by pulse radiolysis at four to seven pressures from atmospheric to 6.4 kbar. The ratio of specific rate at 6.4 kbar to that at 1 atm, followed by the corresponding activation volume in milliliters mole−1, is given in parentheses for each of the reactants: O2 (1.05, −0.2), H2O2 (0.88, 0.5), acrylamide (0.90, 0.4), benzamide (0.95, 0.2), p-nitrophenol (0.84, 0.7), NO3− (1.00, 0.0), S2O82− (1.07, −0.3), Cr(C2O4)33− (1.32, −1.1), Tl+ (0.74, 1.2), Cd2+ (0.75, 1.1), Sm3+ (0.86, 0.6). The Debye equation for specific rate of a diffusion-controlled reaction is used for analysis of the results. Such an analysis suggests that the diffusion coefficient of eaq− changes negligibly (compared to normal ions and molecules) and possibly increases with increase in pressure to 6.4 kbar. With the result for reaction of eaq− with NO3−, certain activation volumes obtained in previous γ-radiolysis studies (by assumption of an activation volume for the NO3− reaction) are corrected, and corrected values of 0 and 3 ml mole−1 are obtained for the partial molal volume and cavity volume, respectively, of an electron in water.
On the Calculation of the Molecular Cylindrical Distribution Function and the Order Parameter from X-Ray Diffraction Data of Liquid Crystals
Authors: de Vries Adriaan
Published: February 2004

A general formula is derived for the x-ray diffraction from an assembly of elongated molecules, and it is shown that this formula is too complex to be solved. To enable the calculation of the molecular cylindrical distribution function (mcdf) and the order parameter from the x-ray data, the approach taken in the literature has been to assume that only molecules approximately perpendicular to the scattering vector contribute significantly to the scattered radiation. Calculations on a model indicate, however, that this ``literature approximation'' is totally unjustified in the derivation of the mcdf, and, consequently, that the mcdf's obtained from x-ray data must be regarded as of no value. For the order parameter, the model calculations show that the errors made in using the ``literature approximation'' are not as large as in the case of the mcdf, and it appears that under favorable conditions the x-ray data may yield approximate values for the order parameter.
Vapor Pressures of TiO(g) in Equilibrium with Ti2O3(s) and Ti3O5(s, β); Dissociation Energy of TiO(g)
Authors: Wu Hung Yuan , Wahlbeck P. G.
Published: February 2004

The vapor in equilibrium with Ti2O3(s) and Ti3O5(s, β) at 1720°C was examined with a mass spectrometer equipped with a Knudsen cell high temperature inlet system. With 50-eV ionizing electrons, the ions TiO+, Ti+, and TiO2+ were observed. Ionization efficiency curves indicated that Ti+ resulted from dissociative ionization of TiO(g). The ions TiO+ and TiO2+ resulted from the parents TiO(g) and TiO2(g), respectively. The intensities of TiO+ and TiO2+ over the temperature range from 1675 to 1770°C indicated that PTiO/PTiO2=4.066. The principal vaporization reaction was 2 Ti2O3(s) ⇄ Ti3O5(s, β) + TiO(g).The vapor pressures of TiO(g) were determined as a function of T by using the Knudsen effusion method. A value of D0o[TiO(g)] of 160.82±4.56 kcal/mole was determined through a ``third-law'' analysis of the data. This value is compared with values determined recently through studies of isomolecular reactions. A value for D0o[TiO2(g)] of 310.37±2.96 kcal/mole was derived.
Reinvestigation of the Structure of Dinitrogen Tetroxide, N2O4, by Gaseous Electron Diffraction
Authors: McClelland Bruce W. , Gundersen Grete , Hedberg Kenneth
Published: February 2004

A reinvestigation of the structure of N2O4 in the gas phase at −21oC has given results in good agreement with an earlier study so far as the molecular shape is concerned, but the size of the molecule appears to be about 0.9% larger than originally thought. The results for the coplanar (D2h symmetry) model are: rNN = 1.782 Å (0.0083), rNO = 1.190 Å (0.0018), ∠ ONO = 135.4o (0.58),  lNN = 0.0816  Å  (0.0178), lNO = 0.0381 Å (0.0019), lo1o2 = 0.0493 Å (0.0040), lN···O = 0.0729 Å (0.0061), lo1o1′ = 0.0970 Å (0.0167), and lo1o2′ = 0.0730 Å (0.0114). These distances and root-mean-square amplitudes are ra and la values; the parenthesized values are 2σ and include estimates of systematic error. The −NO2 groups have a structure very similar to that of NO2 itself. The very long N–N bond and its large amplitude of vibration imply a weak link and are in accord with the low dissociation energy.
Families of Thermodynamic Equations. II. The Case of Eight Characteristic Functions
Authors: Koenig F. O.
Published: February 2004

The geometrical methods of Koenig and of Prins for resolving into families a certain class of thermodynamic equations involving four characteristic functions, are extended to the next higher case, of eight characteristic functions. The extension consists of operations upon a regular octahedron analogous to those upon a square (``the thermodynamic square'') in the former case. In the present case, the number of distinct equations in a family may be 48, 24, 12, 8, 6, 3, 1. Some examples of physical interest are described.
Some Consequences of the Nonlinearity of the Hartree-Fock Approach, Demonstrated on the Example of the PPP Model for Closed Shell Alternant Hydrocarbons
Authors: Bonačić Vlasta , Koutecký Jaroslav
Published: February 2004

It is shown that the nonuniqueness of the H–F solutions of the PPP model for alternant hydrocarbons is generally a consequence of the fact that the part E2 of the energy expectation value E = E1 + E2, connected with two electron operators in the Hamiltonian, can exhibit various minima. Existence conditions for minima of E2 are given. These minima have bond orders corresponding to distribution of pure single and pure double bonds as in classical chemical structural formulas (Kekulé and Dewar formulas). There are cases for which the superposition of the part E1 does not erase the minima corresponding to Kekulé and Dewar formulas. By the direct minimization method, two distinct electronic H–F distributions are obtained for realistic or nearly realistic parametrizations and geometries in the case of annulenes and long polyenes. Cases of different H–F solutions with the same symmetry as the symmetry of the molecule itself are found. A conjecture is made that there may be two stable geometric configurations for very long polyenes. The possible connection is discussed between the strange features of the Hartree-Fock approach for long linear polyenes and their instability.
Quasielastic Thermal Neutron Scattering by Hydrogen in α-Vanadium Hydride
Authors: de Graaf L. A. , Rush J. J. , Flotow H. E. , Rowe J. M.
Published: February 2004

The diffusion process of hydrogen in vanadium has been studied by quasielastic thermal neutron scattering. Neutron linewidths associated with diffusion broadening have been determined at 485°K for VH0.198 and VH0.570. These measurements cover a range of momentum transfers (Q) for elastic scattering up to 4.1 Å−1. This is much larger than the momentum transfers obtained in earlier cold-neutron experiments, where no conclusive comparison was possible between data and proposed models involving diffusive jumps between octahedral and tetrahedral sites. The linewidth results for α-VH0.57 are compared with these models and with an extension developed in the present paper. Although no complete agreement was obtained, this comparison leads to the conclusion that jumps between tetrahedral sites are predominant in the diffusion process. Differences in the linewidth behavior for VH0.198 and VH0.570, and the fact that the measured widths at large Q increase rapidly with Q rather than approaching an asymptotic value as predicted by the theory, are discussed. It is suggested that these effects and the large Debye-Waller factors found in the present and previous neutron experiments are connected with the relative magnitudes of diffusion relaxation time and the jump time between sites.
Photodetachment of Electrons from Amide and Arsenide Ions: The Electron Affinities of NH2· and AsH2 ·
Authors: Smyth Kermit C. , Brauman John I.
Published: February 2004

The relative cross section for the gas-phase photodetachment of electrons has been determined for NH2 − in the wavelength region 1195–1695 nm (1.04–0.73 eV) and for AsH2 − in the wavelength region 620–1010 nm (2.00–1.23 eV). An ion cyclotron resonance spectrometer was used to generate, trap, and detect the negative ions. A 1000-W xenon arc lamp with a grating monochromator was used as the light source, except for one series of experiments in which a tunable laser was employed. Single sharp thresholds were observed in both cross sections, with the result that the following electron affinity values have been determined: E.A. (NH2 ·) = 0.744 ± 0.022 eV and E.A. (AsH2 ·) = 1.27 ± 0.03 eV.
Study of Hindered Rotation in Crystals by Neutron Transmission Measurements: Comparison of Calculated and Measured Scattering Cross Sections
Authors: Leung P. S. , Rush J. J. , Taylor T. I.
Published: February 2004

Total neutron cross sections have been measured as a function of neutron wavelength (λn = 5–11 Å) and sample temperature for (NH4)2SiF6, (NH4)2S2O8, durene and hexamethylbenzene. The experimental results are compared to previous cross-section data for other ammonium salts and methyl-substituted compounds and are correlated with the rotational freedom of the NH4+ ions and methyl groups. A relatively simple theoretical procedure is described for calculating total scattering cross sections of hydrogenous crystals using torsional and translational lattice-mode frequencies obtained from neutron or optical spectra. Cross-section variations vs wavelength (Δ σs/Δ λ) and temperature (Δ σs/Δ T) are calculated for a large group of ammonium salts and methyl-substituted organics (including those for which experimental cross sections are presented here) and are directly compared with measured cross section results. In every case the experimental and theoretical results agree within about 10% or less. Thus the previously observed empirical relationship between Δ σs/Δ λ and Δ σs/Δ T and the rotational and vibrational behavior of ions and molecules in solids is placed on a more quantitative basis. These comparisons reinforce the usefulness of the neutron cross-section method as a supplement to spectroscopic techniques.
Electron Paramagnetic Resonance of Gd3+ in Single Crystals of La(C2H5SO4)3· 9H2O, Bi2Mg3(NO3)12· 24H2O, and Sm(C2H5SO4)3· 9H2O Below 40 G
Authors: Gerkin Roger E. , Thorsell David L.
Published: February 2004

Electron paramagnetic resonance studies of Gd3+ ion in three types of magnetically dilute single crystals possessing C3 site symmetry have been carried out at magnetic fields below 40 G and at 77°K. The zero-field splittings and g factors of Gd3+ obtained from these studies are compared with those obtained from: (a) a previous low-field EPR study of Gd3+ in La(C2H5SO4)3· 9H2O, (b) conventional (high-field) EPR studies in this laboratory of Gd3+ in the same three host-crystal systems; (c) previous conventional (high-field) EPR studies of Gd3+ in La(C2H5SO4)3· 9H2O and in Bi2Mg3(NO3)12· 24H2O. While all possible such comparisons involving ±7/2, ±5/2 and ±5/2, ±3/2 transitions showed agreement within the stated uncertainties of the various data sets, those involving ±3/2, ±1/2 transitions did not. Moreover, anomalous absorptions of lower intensity consistently were observed to the low-field side of the +3/2, +1/2 and the −3/2, −1/2 transitions only. These anomalous signals are shown to be interpretable as arising from the ``forbidden'' +3/2, −1/2 and −3/2, +1/2 transitions. Detailed study of the normal and forbidden transitions permitted the discrepancy between the energies of the +3/2, +1/2 and the −3/2, −1/2 transitions (and between the energies of the +3/2, −1/2 and the −3/2, +1/2 transitions) to be rationalized in terms of a postulated nonlinearity in the field-dependence of the energies of these spin states at magnetic fields below ∼10 G, the lower limit of the present observations.
On the Variational Properties of Scott and van der Waals ``One-Fluid'' Theories of Mixtures
Authors: Mansoori G. Ali
Published: February 2004

In light of the Gibbs-Bogoliubov inequality for the Helmholtz free energy of the classical systems, the variational properties of Scott and van der Waals ``one-fluid'' theories of mixtures with Lennard-Jones potential functions are studied. It is shown that, while the Scott's theory gives an upper bound for the Helmholtz free energy of mixtures, it is not certain whether this upper bound is a least upper bound. It is also shown that the van der Waals theory is an approximation in order to get the upper bound on the entropy of mixtures.
Ionic Collision Processes in Gaseous N2O
Authors: Ryan K. R.
Published: February 2004

Ionic collision processes have been studied in N2O by mass spectrometric techniques with particular emphasis on the state of excitation of the N2O+ ion. Evidence is presented for the contention that electrons of energies ≥17 eV on collision with N2O molecules produce long-lived excited N2O+ ions which are capable of the reactions N2O++N2O→ NO++N+N2O,N2O++N2O→ O++N2+N2Owithout significant conversion of translational to internal energy. For electrons with energy ≥17 eV at least 1% of the N2O+ ions appear to have internal energy ≥2.4 eV. It is well known that excited N2O+ ions are involved in the unimolecular predissociation N2O+→ NO++N. However the abundance of these ions which survive for times appropriate to the collision experiments described here is only about 0.01%. In collision of Ar+ or N2+ with N2O simple charge transfer predominates over dissociative reactions even though sufficient energy is available for dissociation to occur.
Self-Consistent Polarization Propagator Calculations in the Pariser-Parr-Pople Model. A Modified Random Phase Method
Authors: Jo/rgensen P. , Oddershede J.
Published: February 2004

By decoupling the equation of motion for the two-particle Green's function, a self-consistent scheme for describing particle-hole excitations is developed. The self-consistent propagator is brought into a form which easily yields excitation energies and oscillator strengths. The method is applied to the π-electron systems of ethylene, butadiene, and benzene. Comparison is made with results obtained in the time-dependent Hartree-Fock approximation and related schemes. The problems concerning particle number conservation in these methods are discussed.
Information on Intermolecular Potentials Obtained from the Temperature Dependence of the ir Absorption Spectrum of Rare Gas Mixtures
Authors: Bar-Ziv (Bazini) Ezra , Weiss Shmuel
Published: February 2004

The infrared collision-induced absorption spectra of He–Kr and He–Xe mixtures were measured at 300° and at 620°K. The results were analyzed in terms of models previously suggested by Levine and Birnbaum and by Okada et al. The integrated intensities are found to be strongly temperature dependent. This finding indicates the molecules are actually soft. It is definitely concluded that studies of the temperature dependence of collision-induced absorption spectra can be used to obtain information on intermolecular potentials (particularly their repulsive part). For the case of He–Kr and He–Xe mixtures it is found that the results support the modified Buckingham exp-6 potential in preference to the Lennard-Jones potential.
Neutron Diffraction in Dilute and Semidilute Polymer Solutions
Authors: Cotton J. P. , Farnoux B. , Jannink G.
Published: February 2004

The small angle scattering of neutrons by dilute polystyrene molecules has been measured for concentrations c between 10−3 and 10−1 g/cm3. With these very low c values we can explore two different regimes: (1) A ``semidilute'' regime where each coil interacts with many others; (2) a dilute regime, where the coils are separated. There is a striking difference between the angular patterns observed in case (1) and in case (2). They can be interpreted in terms of calculations by Flory for the dilute end, and by Edwards in the semidilute end.
Kinetic Theory of the Thermomagnetic Torque
Authors: Park Won-Hoon , Dahler J. S. , Cooper Eugene , Hoffman D. K.
Published: February 2004

Kinetic theory based upon a classical Boltzmann equation is applied to the analysis of recent thermomagnetic torque experiments on nitrogen. To evaluate the collision integrals which arise it is assumed that the molecules behave as rigid ellipsoids. This model already is known to account quite well for the Senftleben-Beenakker effects upon thermal conduction but to fail in dealing with certain aspects of the analogous field effects upon shear viscosity. The torque dependence upon model parameters is examined and comparisons are made with experimental data and with estimates obtained from the theory of Levi, McCourt, and Beenakker. Predictions of the torque based upon this model fail in the same way—and for the same reasons—as they do for viscosity. The magnitude of the torque can be accounted for satisfactorily but the model underestimates the magnetic field strength at which the torque is maximal. To successfully deal with the latter it appears that one must introduce more realistic intermolecular forces. Calculations based upon the Levi-McCourt-Beenakker theory indicate that in the case of nitrogen thermal creep makes a very substantial contribution to the thermomagnetic torque.
Nonequilibrium Thermodynamics: Current-Affinity Relations and Thermokinetic Potentials
Authors: Tykodi R. J.
Published: February 2004

Two examples, heat conduction in wires and the Poiseuille flow of a gas through a capillary tube, are used to show that the presence or absence of higher order symmetry relations among the phenomenological coefficients can depend on the choice of boundary conditions. A family of thermokinetic potentials is developed for each example, and the over-all utility of the thermokinetic potential concept is discussed.
Reactions of Modulated Molecular Beams with Pyrolytic Graphite. I. Oxidation of the Basal Plane
Authors: Olander D. R. , Siekhaus W. , Jones R. , Schwarz J. A.
Published: February 2004

Reaction of molecular oxygen as a modulated molecular beam with the basal plane of pyrolytic graphite is investigated. The graphite surface participates in the reaction in an unusual fashion. Oxidation tends to create a highly reactive surface while thermal annealing tends to deactivate the surface. The imbalance of these two competing processes results in slow changes in surface reactivity during the course of an experiment, which is manifest as hysteresis in the rate of CO production. The surface chemical reaction occurs on a much smaller time scale than the processes responsible for hysteresis. The data indicate that a two-branch, two-site mechanism explains both the hysteresis and the surface chemistry. Surface migration of adsorbed oxygen is shown to be a step in the production of CO. Carbon dioxide is barely detectible; its production rate is two orders of magnitude smaller than that of carbon monoxide at all temperatures.
Reactions of Modulated Molecular Beams with Pyrolytic Graphite. II Oxidation of the Prism Plane
Authors: Olander D. R. , Jones R. H. , Schwarz J. A. , Siekhaus W. J.
Published: February 2004

The reaction of a modulated beam of molecular oxygen with the prism plane of pyrolytic graphite was investigated. Diffusional processes in the bulk dominated the response of the emission rates of CO and CO2. The phase lags of these products relative to the impinging reactant beam indicated that the surface reactions were strongly affected by diffusion of oxygen in the grain boundaries and then into the grains of the pyrolytic graphite structure. This double diffusion process so strongly demodulated the product signals that the apparent reactivity of the prism plane was less than that of the basal plane. This reactivity inversion is peculiar to the ac modulated beam method and would not occur in dc (steady state) experiments. The reactivity of graphite which had been annealed to 3000°C was found to be an order of magnitude larger than that of the as-received material. This increase in reactivity was due to reduction of the demodulation effect which resulted from closing off diffusional paths in the bulk by annealing. Hysteresis of the type found in the basal plane reaction was observed in the prism plane reaction as well. Approximately one-tenth as much CO2 was produced by the reaction as CO. Reaction with a beam enriched in oxygen-18 did not show complete statistical mixing of the isotropes of oxygen in the CO2 product, suggesting that a significant portion of the CO2 product was formed by a direct reaction of surface carbon with molecular oxygen.
Optical Absorption Spectra of Er3+:LaF3 and ErCl3 · 6H2O
Authors: Carnall W. T. , Fields P. R. , Sarup R.
Published: February 2004

The absorption spectra of Er3+:LaF3 and ErCl3 · 6H2O were investigated in the visible and ultraviolet regions at 4°K. Transitions to practically all of the J levels occurring at <50 000 cm−1 have now been observed. Theoretical analysis of the results including the effects of configuration interaction as expressed in both two- and three-body operators made possible a good correlation between calculated and observed free-ion level energies. Comparison of the levels observed in the crystal matrices with reported gaseous free-ion levels suggested some inconsistencies in the latter, but indicated that there is little change in the J-level structure in the 4f11 configuration when Er3+ is incorporated in the crystal lattices studied.