bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of Hindered Molecular Rotation between Unequal Potential Wells upon Nuclear Magnetic Resonance Spin—Lattice Relaxation Times and Second Moments
Authors: Look D. C. , Lowe I. J.
Published: January 2005

The nuclear magnetic resonance spin—lattice relaxation times along the static and rotating magnetic fields, and the second moment of the nuclear magnetic resonance line shape are calculated for a system of two identical spins that undergo a hindered rotation about the perpendicular bisector of the line joining the spins. The hindered rotation treated in this model is the jumping of the spins among four equally spaced equilibrium positions, two of the positions differing in energy from the other two. For finite temperatures, this difference in energy is shown to lengthen the spin—lattice relaxation times and inhibit the narrowing of the resonance line shape.
Electrical Resistance of Dilute Solutions of Hydrogen in Palladium
Authors: Simons J. W. , Flanagan Ted B.
Published: January 2005

The temperature coefficient of the relative resistance of the α phase of hydrogen/palladium has been investigated as a function of hydrogen content (0°—90°C). The relative resistance R/R0 increases linearily with hydrogen content at each temperature. In contrast to earlier studies, a temperature coefficient has been found which can be expressed as d(R/R0)/dn=0.542+1.042×103/T (0∘ to 90∘C).
Energy Transfer Between Rare-Earth Ions
Authors: Van Uitert L. G. , Johnson L. F.
Published: January 2005

Fluorescence quenching effects caused by multipolar energy-transfer interactions between rare-earth ions are examined. Transfer between remote ions by way of transitions that are matched in energy appears to be essentially dependent upon concentration alone (e.g., Sm3+ self-quenching) while transfer by non-resonant mechanisms can show a strong dependence upon crystal structure (e.g., Eu3+ self-quenching). Examples of thermally dependent multipolar transfer interactions are also given.
Effect of Pressure on the Curie Temperature and Volume of GdN
Authors: McWhan D. B.
Published: January 2005

The change in the Curie temperature of GdN with pressure has been measured up to 100 kbar by a transformer method and is given by dTc/dP=+0.08°±0.04°K/kbar. The compressibility has been determined relative to NaCl by powder x-ray-diffraction methods and can be expressed by a Birch equation with B0=1920±350 kbar and ξ=0. The results are compared with EuO: (d logTc/d logV) = −2.2±1.5 for GdN and −5.5±1.5 for EuO. This difference results from the combined effect of the valence difference between the compounds and a difference in the exchange mechanism.
On Nachtrieb's Tm/T Law for Diffusion in Solids under Pressure and Its Relation to Simon's Melting Equation
Authors: Rapoport Eliezer
Published: January 2005

Nachtrieb's Tm/T law for diffusion in solids under pressure is derived under the assumption that the melting curve of the solid can be described by Simon's melting equation. A necessary condition for the Tm/T law to hold is that the activation energy ΔH and volume ΔV obey an equation ΔH = AcΔV, where A and c are the Simon constants. This equation is substantiated by a comparison with the available experimental data.
Calculation and Interpretation of the NMR Chemical Shift and Its Pressure Dependence in the Alkali Halide Lattices
Authors: Hafemeister D. W. , Flygare W. H.
Published: January 2005

The symmetrical orthogonalization technique of Löwdin has been applied to the alkali halide outer-shell s and p wavefunctions. The resulting distorted ionic wavefunctions have been used to calculate the pressure dependence of the NMR chemical shift within the framework of the Kondo—Yamashita overlapping-ion model. The computed values of the rate of change of the chemical shift with respect to pressure, the change of the chemical shift at the NaCl–CsCl phase transition, and the chemical shift of the ionic reference solutions are compared to Baron's experimental data. Reasonable agreement is found although discrepancies exist and are discussed. No adjustable parameters are used in these calculations.
Many-Electron Theory of Nonclosed-Shell Atoms and Molecules. II. Variational Theory
Authors: Silverstone Harris J. , Sinanoğlu Oktay
Published: January 2005

The theory developed in I is generalized into a variational many-electron theory by making the first-order correlation processes in the wavefunction ``exact to all orders'' and by including the effects of unlinked clusters. Equations for obtaining the significant correlation effects are derived by the variation—perturbation approach. Symmetry properties of the correlation functions and a semiempirical theory based on the form of the variational energy are also discussed. The examples of lithium, 1S-carbon, and C2H4+ illustrate some aspects of the theory.
On the Nature of Paired Vacancy in Molten Electrolytes
Authors: Majumdar Dilip Kumar
Published: January 2005

Paired-jump diffusion coefficients have been calculated using rate theory equations of Eyring in molten electrolytes for two pathways, namely, diffusion into an adjoining pair of holes, and a hole of twice the average size. It appears that the paired-hole diffusion model gives a better representation of behavior than other models.
Algebraical Analysis of Diffusion Coefficients in Ternary Systems
Authors: Yao Y. L.
Published: January 2005

Using de Groot and Mazur's approach, the thermodynamic restrictions of diffusion coefficients in ternary systems are stated. The relative advantages of using different frames of reference are discussed. Different definitions of mobilities or friction coefficients are compared. A simple model of diffusion coefficients, tentatively called ``small-interaction'' model, is proposed. It is shown that this model, the ``mobility'' model, and the Darken model are algebraically equivalent to different assumptions in simplifying the matrix of diffusion coefficients. Physically the assumption for the small-interaction model is weak, but the assumptions for the mobility model are too strong to approach reality. The Darken model is valid only if the density of one component is low.
Multicenter Integrals in Quantum Mechanics. II. Evaluation of Electron-Repulsion Integrals for Slater-Type Orbitals
Authors: Harris F. E. , Michels H. H.
Published: January 2005

A unified method for the evaluation of electron-repulsion integrals for Slater-type orbitals is described. This method applies to every electron-repulsion integral needed for polyatomic molecule calculations, and is sufficiently general to handle integrals over orbitals with higher quantum numbers, located at arbitrary spatial points with arbitrary relative orientations.

Computation programs have been prepared to implement this analysis with emphasis on both the requirements of speed and accuracy in evaluating the large number of electron-repulsion integrals required for a polyatomic calculation. Comparisons with other methods are given which indicate that this unified approach can result in at least an order-of-magnitude saving in computation time for large molecular systems.
Change in the Thermionic Emission Current of Palladium Due to Chemisorption of Atomic and Molecular Hydrogen
Authors: Eisenstadt Melvin , Hoenig Stuart A.
Published: January 2005

The thermionic emission current from a palladium wire at 1000°C was measured while the wire was exposed to molecular- and atomic-hydrogen beams. The molecular-beam source was at 300°K; the atomic-beam source was at 2400°K. Ultrahigh-vacuum techniques were used, and background pressures were 10−9 torr.

The average sticking coefficient, for chemisorption, was found to be larger for molecular hydrogen than for atomic hydrogen. This may be understood in terms of the energy-exchange phenomena between the gas and the surface. The slow moving molecules (300°K) transfer enough kinetic energy to the lattice, during collision, to be adsorbed. The energetic atoms (2400°K) are not so successful in this energy transfer, and a considerable fraction of them are reflected from the surface.

Based upon heat of adsorption considerations, one would expect atomic hydrogen to have a higher sticking coefficient than molecular hydrogen. The data show the opposite, and it is concluded that the kinetic energy of the incoming particle affects the sticking coefficient more strongly than the heat of adsorption, for the conditions studied here.
Gaseous Reaction Rates at High Temperature. I. The Dissociation of Carbon Dioxide
Authors: Fishburne E. S. , Bilwakesh K. R. , Edse R.
Published: January 2005

The rate of dissociation of carbon dioxide has been measured in the temperature range 3000°—5000°K. The amount of carbon dioxide was varied from 1% to 10% in a diluent of argon or nitrogen. The concentration of the mixture was varied from 0.11×10−5 to 0.49×10−5 mole/cc. It is shown that the decrease in temperature and increase in density caused by the endothermic dissociation must be considered in the analysis of the data. A computer program was employed to account for the effect of the changes in temperature and density on the observed rate of dissociation. The rate constants for the reaction CO2+M→CO+O+Mwere determined to be k1,Ar=7.11×1011T12exp(−84 500/RT) cc/mole·secwith argon as the diluent and k1,N2=5.33×1011T12exp(−79 600/RT) cc/mole·secwith nitrogen as the diluent. The probable dissociation mechanism also is discussed.
Theoretical Rate Constant for Thermal Unimolecular Reactions in a Multilevel System
Authors: Valance W. G. , Schlag E. W.
Published: January 2005

A macroscopic unimolecular rate constant is given here in terms of the detailed microscopic rates of transport and relaxation in a many-quantum-level system. A rate equation for each quantum level is written considering all input and output processes. The resulting set of equations constitutes a master equation. The conventional unimolecular rate constant is then the lowest eigenvalue to the relaxation problem defined by the master equation. This is then a general prescription for the theoretical treatment of unimolecular behavior based only on the assumed mechanism and explicitly including the multilevel behavior of real systems undergoing chemical reaction.

General methods for solving this eigenvalue problem are discussed. These methods are illustrated with the master equation for a generalized Lindemann mechanism. The lowest eigenvalue for this mechanism is obtained by an iteration procedure. The zero-order result of the iteration is shown to be equivalent to the rate constant derived from a steady-state approximation. Both the zero-order and exact expressions for the lowest eigenvalue are given in terms of a computationally useful many-shot expansion. The lowest eigenvalue result then constitutes an exact and computationally simple version of the unimolecular rate constant which is an unambiguous consequence of the microscopic parameters inserted into the chosen relaxation mechanism.

Since the particular mechanism used here is also the point of departure in the derivation of the well-known RRKM theory of unimolecular reactions, the lowest eigenvalue solution is directly compared with the RRKM rate constant. The additional restrictive conditions for agreement between the two rate constants are enumerated.

Some generalized expressions for mean first passage times in unimolecular rate theory are also derived and expressed in terms of many-shot solutions. These latter expressions are computationally useful for extremely rapid reactions.
Effect of High Pressure on the Lattice Parameters of Cr2O3 and α-Fe2O3
Authors: Lewis G. K. , Drickamer H. G.
Published: January 2005

The effect of pressure to several hundred kilobars has been measured on Cr2O3 and α-Fe2O3. Both have the corundum structure. Results are compared with previously published data for Al2O3.

Cr2O3 exhibits a compressibility which decreases markedly with increasing pressure; Al2O3 has a compressibility nearly independent of pressure; the compressibility of α-Fe2O3 actually increases with pressure in the low-pressure region. This is probably associated with the unusual behavior observed in Mössbauer studies.

Based on hexagonal indexing, Al2O3 shows the smallest change in aH and the largest change in CH. Cr2O3 is at the opposite extreme with α-Fe2O3 in between. In rhombohedral indexing, Al2O3 exhibits the largest change in aR and Cr2O3 the smallest. Al2O3 shows an increase of ½ deg in α, Cr2O3 a decrease of about 1 deg, while for Fe2O3 α is nearly independent of volume.
Dissociation Energies of Group IIIA Monofluorides—The Possibility of Potential Maxima in Their Excited Π States
Authors: Murad Edmond , Hildenbrand Donald L. , Main Roger P.
Published: January 2005

The dissociation energies D0″ have been measured for GaF, InF, and TlF using high-temperature mass-spectrometric techniques. These results, along with those from previous studies, yield D0″=180±3, 159±3, 138±3.5, 121±3.5, and 101±3.5 kcal/mole for BF, AlF, GaF, InF, and TlF, respectively. Comparison of D0″ thus obtained with that derived from Birge—Sponer extrapolation of the A 1Π state indicates that for all these molecules the spectroscopically derived dissociation energies are higher than the thermochemical values. It is suggested that the difference may be due to a potential maximum in the A 1Π states BF, AlF, GaF, and InF. A maximum is also suggested for the a 3Π1 state of TlF. The appearance potentials of these monofluorides were measured and were found to be 11.5±0.4, 9.5±0.5, 10.6±0.4, 9.6±0.5, and 11.2±0.5 eV for BF+, AlF+, GaF+, InF+, and TlF+, respectively. The dissociation energies and the appearance potentials are briefly discussed in terms of the molecular states of the respective molecules.
NMR Relaxation Times of Adsorbed Ammonia
Authors: Reuben J. , Fiat D. , Folman M.
Published: January 2005

NMR relaxation times of NH3 adsorbed on modified silica surfaces have been studied using a progressive-saturation method. Measurements have been done for different surface coverages and frequencies. It has been found that the longitudinal and transverse relaxation rates of the adsorbate depend strongly on surface coverage. This has been explained by the existence of distribution of correlation times.

The relaxation rates differed in some respects from those known for liquids and solids, in other respects they showed some similarity. Possible relaxation mechanisms are discussed.
Theory of Correlated Wavefunctions. II. The Symmetry Properties of Atomic Correlated Wavefunctions
Authors: Steiner Erich
Published: January 2005

The conditions under which a correlated wavefunction for an atomic system can be an eigenfunction of angular momentum are investigated. It is shown how symmetry-adapted pair-correlated wavefunctions may be set up in which the pair functions satisfy the condition of strong orthogonality to the orbitals in the initial orbital approximation, and in terms of which the variational equivalent of the first-order perturbation equation reduces to a set of uncoupled two-body equations, one for each pair. The approach is illustrated by application to the lowest 3P, 1D, and 1S states of carbon and to the ground state of neon, the latter case being of particular interest as it shows how the separation of the orbitals into unique pairs is unsatisfactory when the orbital approximation contains a number of equivalent orbitals.
Self-Consistent-Field Calculation of the Energy of HeH+
Authors: Gallup G. A. , McKnight M. S.
Published: January 2005

The use of the exact solutions of the one-electron—two-nucleus problem as a basis set of functions for SCF calculations is investigated for the HeH+ ion. The calculations are made with both scaled and non-scaled functions. The best energy obtained here is −3.345 a.u. at an internuclear distance of 1.40 a.u. The conclusion is that these functions form an only moderately good basis set for such calculations. Their conditioning to the problem can only partially overcome their lack of completeness when continuum states are omitted.
Perturbation Theory of Constraints: Application to a Lithium Hydride Calculation
Authors: Chong D. P. , Brown W. Byers
Published: January 2005

The perturbation theory of the constrained variational method is applied to a calculation on the lithium hydride molecule by Browne and Matsen. The changes in various properties on constraining the expectation values of force and virial operators to vanish are calculated. It is shown that the perturbation series converge very rapidly, so that only the leading terms are required. However, in this particular case the imposition of the constraints has a negligible effect on the calculated properties.
Far-Infrared Spectra of Four-Membered-Ring Compounds. I. Spectra and Structure of Cyclobutanone, Cyclobutanone-d4, Trimethylene Sulfide, and Perfluorocyclobutanone
Authors: Durig J. R. , Lord R. C.
Published: January 2005

Two series of very sharp Q branches have been observed in the far-infrared spectra of cyclobutanone and cyclobutanone-d4 vapor. For cyclobutanone, these Q branches have wavenumbers of 35.3, 57.03, 64.99, 72.17, 77.77, 81.85, and 85.33 cm−1, and for cyclobutanone-d4 the wavenumbers are (∼30), 47.1, 54.2, 60.6, 65.6, 69.8, 73.2, and 78.9 cm−1. Relative frequency values were calculated with the energy levels of a pure quartic oscillator which fitted rather accurately all the transitions except the first observed one. This suggests that the four-membered ring of cyclobutanone is planar. The far-infrared spectrum of trimethylene sulfide revealed at least six Q branches of varying intensity and spacing. Their wavenumbers were 62.27, 84.50, 92.14, 100.18, 105.75, and 139.1 cm−1. It is impossible to fit the series to the energy levels of either a pure quartic oscillator or one perturbed by a quadratic term. Comparison of the statistical entropy with the measured calorimetric entropy indicated a double-minimum potential with a barrier above the first energy level. In the far-infrared spectrum of perfluorocyclobutanone no band was observed which could be confidently assigned to the ring-puckering vibration of this molecule.
Infrared and Raman Spectra of C6H5SiCl3, C6H5GeCl3, and C6H5SnCl3
Authors: Durig J. R. , Sink C. W. , Bush S. F.
Published: January 2005

The infrared-absorption spectra of liquid phenyltrichlorosilane, phenyltrichlorogermane, and phenyltrichlorostannane have been recorded from 4000 to 33 cm−1. The Raman spectra of the liquids have also been recorded and depolarization values have been measured. The phenyl vibrations have been assigned assuming C2v symmetry, although the molecular symmetry is probably Cs. All the spectra are interpreted in detail, and the 39 fundamental vibrations have been assigned based on previous assignments for substituted benzenes as well as the expected band positions and depolarization values. Comparison of the spectra reported herein to that of the carbon analog suggests the reassignment of a few of the fundamental vibrations in phenyltrichloromethane. The assignment of the lowest observed Raman line in each spectrum to the torsional vibration is discussed, but because of the large values calculated for the sixfold barrier hindering internal rotation it is discarded.
Growth and Impurity Effects on Fluorescence and Reduction of CaF2:Sm3+, Y3+
Authors: Ewanizky T. F. , Theissing H. H.
Published: January 2005

Observations are made of the effects of annealing and Y, O, or Na impurities on the growth and reduction of various types of Sm centers in CaF2. Increase of Sm3+ fluorescence with increase of annealing time is believed to be indicative of axial center formation. Radiation-induced reduction of Sm3+ centers in cubic, tetragonal, or trigonal crystalline fields is studied. The absorption spectra after irradiation are shown to be dependent on the original symmetry of the field surrounding the trivalent ions.
Spin—Lattice Relaxation in Organo—Sulfur Radicals
Authors: Akasaka Kazuyuki
Published: January 2005

Spin—lattice relaxation times (T1) of various organo—sulfur radicals, each with a different spectroscopic splitting factor g, were measured mainly at 77°K by the method of microwave saturation in electron spin resonance. All these radicals showed T1's which are smaller than those of organic (carbon) radicals. A remarkable correlation was found to exist between T1 and Δgmax, the maximum deviation of the g factor of a free radical from the free spin g value. These results have led to a conclusion that the spin—lattice relaxation in organo—sulfur radicals takes place mainly through spin—orbit and orbit—lattice coupling.
Transport Properties of Polyatomic Fluids. IV. The Kinetic Theory of a Dense Gas of Perfectly Rough Spheres
Authors: McCoy Benjamin J. , Sandler Stanley I. , Dahler John S.
Published: January 2005

The Enskog theory of transport in a dense gas is applied to the rough-sphere model. A formal analog of the Chapman—Enskog method is developed in order to construct the normal solution of the corresponding kinetic equation. Accurate estimates are given for the coefficients of shear and bulk viscosity and of thermal conductivity. A procedure is provided for computing the coefficient which characterizes the antisymmetric portion of the stress tensor and also for estimating the coefficients of proportionality between the flux of spin angular momentum and the gradient of the local spin velocity. Numerical values are given for these and other transport coefficients and characteristic relaxation times. A study is provided of the hydrodynamical significance of antisymmetric stress and of spin flux. Finally, the theory of this paper is compared with other, rather more heuristic, methods.
g-Value Calculations of Paramagnetic Centers in Amorphous Selenium
Authors: Chen Inan
Published: January 2005

The g values of several possible models for paramagnetic centers in amorphous selenium are calculated by the method described in the preceding paper. The results of that paper show that the electron spin resonance lines of amorphous selenium reported to date cannot be attributed to free chain ends because of the disagreement in the calculated and observed g values. In this paper, we invoke interaction between the chain end and other atoms, including those in the same chain and in a neighboring chain, as well as impurities. The results show that the most probable source of the sharp line with g=2.0039±0.0006 is an oxygen impurity terminating the chain; the broad line with larger g values (2.3 to 2.8±0.01) can be interpreted as inhomogeneously broadened by pairs of interacting chain ends.
Dipole—Dipole Scattering in Molecular Beams. Variation of Total Cross Section with Velocity and Rotational Overlap
Authors: Cross R. J. , Gislason E. A. , Herschbach D. R.
Published: January 2005

Total cross sections were measured for scattering of velocity-selected beams of alkali halide molecules KCl, CsCl, and CsBr by HBr and CH3I and, for comparison, for Kr and Xe. The pressure in the scattering chamber was monitored by a second crossed beam of alkali atoms, which allowed direct measurement of the ratios of cross sections for different molecules interacting with the same scattering gas. As expected from the first-order selection rules, the results indicate that the contribution of rotationally inelastic transitions produced by the angle-dependent dipole—dipole force depends strongly on the extent to which the rotational spectra of the two molecules overlap. Inelastic scattering appears to be dominant for the CH3I systems, where the overlap is very good. The cross sections are ∼5000 Å2 (compared with ∼1400 Å2 for elastic van der Waals scattering; both corrected for angular resolution) and the velocity dependence is much weaker than for elastic scattering. In the Appendix simple approximate formulas are derived from which the first-order inelastic dipole—dipole cross section can be calculated without numerical integrations over impact parameters and rotational distributions.
Collisional Energy Transfer. Thermal Unimolecular Systems in the Low-Pressure Region
Authors: Tardy D. C. , Rabinovitch B. S.
Published: January 2005

Collisional activation—deactivation efficiencies, β, for thermal unimolecular reactions in the second-order region were computed on a stochastic model by use of an iterative procedure. Four assumed collisional transition probability models were used: stepladder, Gaussian, Poisson, and exponential; detailed balance and completeness were observed. The collisional efficiencies increase with increase of average energy removed per collision, 〈ΔE〉, and with decrease in the average excess energy of the molecules, 〈E+〉, above the critical energy for reaction. Efficiency is defined as a product, β=γNγP, and varies with inert gas dilution.

Calculations were made for the nitrous oxide, nitryl chloride, methyl isocyanide, cyclopropane, and 1,2-dimethylcyclopropane systems over a range of temperatures. This provides a large variation in the internal vibrational-energy densities and critical energies in question. For a particular transition probability model, β may be expressed as a quasiuniversal function of the reduced parameter, E′ = 〈ΔE〉/〈E+〉. Experimental data may thus be readily related to a corresponding value of 〈ΔE〉. The relation between the various probability models is discussed. Various deductions made are reminiscent of those previously encountered in work on chemical activation systems. The range of validity of the conventional strong-collision assumption for thermal systems is made explicit.
Nonbonded Potential Parameters Derived from Crystalline Aromatic Hydrocarbons
Authors: Williams Donald E.
Published: January 2005

Nonbonded (exp−6) potential parameters for C···C, C···H, and H···H interactions were derived from the crystal structures and properties of aromatic hydrocarbons. The exponent of the C···C repulsion term was taken from a calculation based on the interplanar spacing and compressibility of graphite. The exponent of the H···H repulsion was taken from a quantum-mechanical calculation of the repulsion between two hydrogen molecules. The coefficients of the attractive and repulsive terms were fitted by weighted least squares to 77 observational equations involving the geometrical crystal structures, elastic constants, and sublimation energies of nine aromatic hydrocarbons. It was found necessary to externally estimate the H···H repulsion coefficient to obtain an appropriate partition between the C···C, C···H, and H···H energy terms. The geometrical mean combining law was applied for the attractive and repulsive coefficients.

The resulting nonbonded potential parameters, when used to minimize the lattice energy by a steepest descent procedure, were found to reproduce the crystal structures of benzene, napthalene, and anthracene to within about 0.05 Å in the carbon-atom positions and within about 1% in the lattice constants. No assumption of anisotropic forces based on π-electron polarizabilities was necessary to reproduce herringbone-type packing. The position of the minimum in the best C···H nonbonded potential was less than expected, while the minimum in the H···H potential was larger than expected. The minima positions were found to be sensitive to the H···H repulsion coefficient. The correlation coefficients between the potential parameters found by this method were found to be large.
LCAO Description of Symmetric Molecules by Unified Theory of Finite Graphs
Authors: Schmidtke Hans-Herbert
Published: January 2005

LCAO MO descriptions of symmetric molecules have topological and energetic aspects. Topological effects include the point symmetry of the nuclear configuration and the various connections between these points (connectedness). An appropriate mathematical tool for a discussion of molecular topoligies is given by the general theory of graphs. This theory anticipates various properties connected with the topological matrix. Special graphs and their matrix representations which reflect all topological properties of symmetric molecules serve to determine the molecular-orbital-term scheme and the symmetry orbitals for various sorts of molecules. In particular, tetrahedral, square planar, trigonal prismatic, and octahedral molecules are investigated. Finally some remarks are made on the relative choice of parameters in the semiempirical theory as suggested by the consideration of the virial theorem.
Dielectric Properties of Ice VII. Ice VIII: A New Phase of Ice
Authors: Whalley E. , Davidson D. W. , Heath J. B. R.
Published: January 2005

The dielectric properties of ice have been studied up to 107 kbar using opposed circular anvils. Ice VII undergoes orientational polarization, and consequently its molecules are orientationally disordered. The volume of activation for the relaxation of the orientational polarization is ∼3 cm3 mole−1, which suggests that the reorientation occurs by the migration of Bjerrum defects, as it appears to do in ice I, III, V, and VI. The energy and entropy of activation at 23 kbar are about 11.6 kcal mole−1 and 9.2 cal deg−1·mole−1, which are similar to those for other phases of ice.

At about 0°C, ice VII undergoes a transformation to a phase, here designated ice VIII, whose molecules are orientationally immobile at audio frequencies. This phase had previously been suggested by an analysis of the boundary line between ice VI and VII. The transformation occurs with a hysteresis of about 5.5°C at constant pressure, and the transformation temperature is independent of pressure to within about 0.5° up to 50 kbar. Some measurements previously identified with ice VII must now be identified with ice VIII or perhaps with another phase occurring at much lower temperatures.

The results illustrate the usefulness of dielectric measurements in an opposed-anvil apparatus for the study of phase changes in nonconductors at high pressures.
Electron Spin Resonance Studies of a Reaction Intermediate in Vacuum-Sublimed NaCl and KCl
Authors: Harrison L. G. , Adams R. J. , Catton R. C.
Published: January 2005

In accordance with predictions from kinetic studies, electron spin resonance has shown that treatment of vacuum-sublimed NaCl with F2 at room temperature produces a defect containing an unpaired electron, and that the decay of this defect is retarded by Cl2. The ESR spectrum indicates an anisotropic g factor and unresolved hyperfine structure. In KCl, a spectrum is obtained which is closely similar except that the hyperfine structure is partly resolved. The spectra appear consistent with interaction of the unpaired electron with Cl atoms, rather than with F or such impurities as O or Br, and can be accounted for well by a model of the defect as linear Cl43−, i.e., an H center or something very similar. For NaCl, the spectra indicate that the vacuum-sublimed material is sometimes partly oriented.

It is suggested that centers with a one-electron deficiency are observable at such high temperatures in vacuum-sublimed material because this consists of essentially perfect crystals devoid of sites which could accept a second electron from the centers.
Gaseous Reaction Rates at High Temperatures. II. The Dissociation of Hydrogen Chloride
Authors: Fishburne E. Stokes
Published: January 2005

A shock tube has been employed to study the dissociation of HCl in an argon diluent in the temperature range 3300°—5400°K and at total gas concentrations of 0.21, 0.42, and 0.84×10−5 moles/cc. The rate constant was determined to be k1=1.92×1011T12exp(−69 700/RT) cc/mole·sec.Experiments were conducted employing gas mixtures containing 1% HCl and 99% Ar; 2% HCl and 98% Ar; and 1% H2, 2% HCl, and 97% Ar. The inclusion of H2 did not noticeably affect the rate of dissociation of HCl.

The dissociation of HCl is compared to that of H2, which possesses a similar dissociation energy. It is shown that the rate constant for the dissociation of HCl has a much lower activation energy than that of H2. The possible reasons for this lower activation energy are presented and discussed.
Theory of the Dissociative Recombination and Associative Ionization of Hydrogen
Authors: Nielsen Svend E. , Dahler John S.
Published: January 2005

A brief survey is given of the experimental information which is presently available on dissociative recombination and the dynamically inverse process of associative ionization. The mechanisms by which the reactions can occur are described in terms of adiabatic potential-energy curves of the ion and of the quasi-molecule formed from the union of ion and electron. The paucity of detailed information about the relevant potential-energy curves suggests that one focus attention upon hydrogen, the system about which the most is known.

A mathematical theory is then constructed which takes as its basis ingredients a Coulomb—Born description of the scattering of ion and electron and an adiabatic, or perturbed-stationary-state, treatment of atom—atom collisions. A very general approach is abandoned in favor of one based upon an approximate wave-function and which includes coupling of only a few entrance and final-state channels. Equations are derived for the scattering amplitudes in the two sets of channels and formulas are given for the relevant cross sections.
Simplified Procedure for the Theoretical Calculation of Isotope Effects Involving Large Molecules
Authors: Stern Marvin J. , Wolfsberg Max
Published: January 2005

It is demonstrated that one can simplify statistical-mechanical calculations of isotope effects on rates and on equilibria, without introducing significant error, by omitting large portions of the molecules involved. One can usually ``cut off'' parts of the molecules more than two bonds removed from the position(s) of isotopic substitution at which force-constant changes occur between reactant and transition state in a rate process or between reactant and product in an equilibrium process. Sometimes more extreme cutoff is possible. The force-constant and geometry changes at the isotopic position(s) must be the same in the cutoff and complete calculations. The success of the cutoff procedure focuses attention on the fact that experiments which determine isotope effects in the room-temperature region and above can directly yield information regarding only changes occurring in the neighborhood of the isotopic position(s).
Reversal of Fluorescence Quenching at High Concentrations in Aqueous Solution
Authors: Stein Gabriel , Tomkiewicz Micha
Published: January 2005

The self-quenching of fluorescence in aqueous solutions of sodium ortho-hydroxybenzoate (salicylate) and of sodium meta-hydroxybenzoate was found to follow a Stern—Volmer relationship at low concentrations. At higher concentrations, from 1 to greater than 4M solutions meta was found to show saturation of quenching. ortho, however, showed a reversal of the effect and increase in fluorescence in concentrated solutions. The effect of using cesium instead of sodium was tested as was that of added sodium perchlorate. The general and specific ionic effects observed were very small. The results in salicylate solution fit well the expression 1/I=k0+k1C—k2 C2 up to about 4M concentration. Possible mechanisms to account for this are discussed, and a mechanism involving intermolecular hydrogen bonding favored.
Spin—Lattice Relaxation of Rh(II) in Single Crystals of ZnWO4
Authors: Townsend M. G. , Orton J. W.
Published: January 2005

The spin-lattice relaxation time T1 of Rh(II) in a single crystal of ZnWO4 has been measured in the temperature range 1.5°—110°K. In the region 56°—110°K relaxation occurs by an Orbach process. From the temperature dependence of this process, the separation of the first excited Kramers doublet E′ from the ground doublet E″ is found to be 320 cm−1, and the lifetime of the excited state 1.7×10−11 sec. At low temperatures, the relaxation times are relatively short and depend upon the direction of the applied magnetic field H. With H aligned along the a, b, or c crystalline axes an anomalously strong dependence of T1 on temperature is observed.
Measurement of Enthalpies of Evaporation of Bi and Bi2 by an Optical Absorption Technique
Authors: Rice Philip A. , Ragone David V.
Published: January 2005

The enthalpies of evaporation of Bi and Bi2 were determined by the second-law method using an optical absorption technique to measure the change of vapor density with temperature for each species. The 3067-Å line was used for Bi and the band head at 2731 Å was used for Bi2. The enthalpy of dissociation of Bi2 is the difference between the two enthalpies of evaporation. The evaporation values obtained were   Bi(l)=Bi(g)ΔH298∘=49 600±1100 cal,  Bi(l)=12Bi2(g)ΔH298∘=24 700±1000 cal,12Bi2(g)=Bi(g)ΔH298∘=24 900±2100 cal.

The optical absorption technique was checked on lead, using the 2833-Å line. For lead, the ΔH°v,298 measured was 47 240±850 cal, as compared with an accepted value of 46 600±300 cal.
Vacuum-Ultraviolet Photolysis of Ethane Films
Authors: Jackson William M. , Faris James , Buccos Nancy J.
Published: January 2005

The vacuum-ultraviolet photolysis of films of solid ethane has been studied both at 1470 and 1236 Å. The product distributions have been measured and evidence is presented for the quenching of excited ethane and ethylene. The over-all mechanism, with the exception of quenching reactions, appears to be the same as in the gas-phase photolysis.
Barrier Heights and the Separation of Torsional Motion from Frame Vibrations in Electron Diffraction
Authors: Karle J.
Published: January 2005

A method is presented for determining the barrier height hindering internal rotation from electron-diffraction data. The method is based on simple assumptions regarding the variation of the contribution of the frame vibrations as a function of the equilibrium positions on the circle of rotation. These assumptions, combined with the occurrence of two or more different interatomic distances which are affected by the torsional motion, permit a separation of the contribution of the torsional motion from that of the frame vibrations.

The assumed form for the functional variation of the squared amplitude of the contribution from the frame vibrations is α{1+βexp[−(π−|φe|)2]}, where α and β are constants and φe is the equilibrium angle for an interatomic distance on the circle of rotation, where φe=0 corresponds to the trans distance. When the variation is assumed to be constant, β=0, some typical values obtained from electron-diffraction data are 12.8 kcal mole−1 for Cl3CCCl3, 4.3 kcal mole−1 for F3CCF3, and 1.1 kcal mole−1 for Cl3SiSiCl3. It was found that a value of β=3.76 and a barrier height of 3.4 kcal mole−1 afforded a very good fit to the diffraction data for Cl3CNO2. Errors in the barrier heights are estimated to extend to 30% above and 20% below the given values. Further studies on Cl3CNO2 indicated that the barrier height is relatively insensitive to the assumptions made concerning the variation of the contribution of the frame vibrations, but strongly dependent on the assumed form for the potential barrier. Comparisons are made with the results of other methods for computing the barrier heights, and quite satisfactory agreement is obtained.
Chemionization in N and O Mixtures. II
Authors: Young Robert A. , St. John Gilbert
Published: January 2005

The rate of chemionization in mixtures of atomic oxygen and nitrogen is measured, and chemical processes leading to ionization are discussed.
Electrical Conduction in Thoria and Thoria—Yttria as a Function of Oxygen Pressure
Authors: Bauerle J. E.
Published: January 2005

The electrical conductivity of ``pure'' ThO2 and some ThO2–YO1.5 solid solutions has been measured at 1000°C under partial O2 pressures ranging from 1 to 10−14.6 atm. For all specimens the conductivity is well described by σ=σ1+σ2pO2¼. The term σ1 is interpreted as an oxygen ion vacancy conductivity which is fixed by the cation composition of the sample. The term σ2pO2¼ is interpreted as a positive hole conductivity arising from the incorporation of excess oxygen into lattice vacancies via the reaction 12O2+vO2−⇌O2−+2h,where vO2− is an oxygen ion vacancy, O2− is an oxygen ion, and h is a hole.
Simple Method of Deriving the Elements of the Tensor-Force Matrix for Monatomic Cubic Crystals
Authors: Shukla Ramesh Chandra
Published: January 2005

A correlation has been given between the force constants of the nth-neighbor axially symmetric and tensor-force models. The equations giving the correlation, lead to a very simple method of writing down the elements of the tensor-force matrix for any atomic position in a monatomic fcc or bcc lattice. A table of the tensor-force matrix is given for certain atomic positions and compared with similar results obtained by other authors from symmetry considerations.
ESR Studies of Coordination to the Sixth Position of Vanadyl Acetylacetonate
Authors: Walker F. Ann , Carlin Richard L. , Rieger Philip H.
Published: January 2005

The electron spin resonance spectra of vanadyl acetylacetonate (VOacac2) and a number of its adducts have been examined in benzene solution at ambient temperatures. It is found that the magnitude of the vanadium nuclear hyperfine splitting constant decreases from 108.19 to 105.3 G as amine adducts are formed, while the g value increases from 1.9695 to 1.9699. The anisotropies of the axially symmetric g and hyperfine tensors, Δg and b, and the rotational correlation time τR have been calculated from the linewidths of VOacac2, VOacac2·pyridine, and VOacac2·piperidine in benzene solution. The correlation time increases from 2.4×10−11 to 3.0×10−11 sec, |b| increases from 2.1×109 to 2.3×109 rad/sec, and |Δg| decreases from 0.034 to 0.029 on coordination of pyridine. Although the g values calculated from the electronic spectra using the theories of Ballhausen and Gray or Kivelson and Lee decrease slightly on adduct formation rather than increase as observed experimentally, the calculated anisotropies Δg follow the experimental trend. Analysis of the line broadening observed for mixtures of VOacac2 and an adduct allows the estimation of the rate of exchange of amine between the adduct and bulk solvent. Line shapes for the −72 line have been calculated for the intermediate exchange-rate region and compared with experimental spectra. For the reaction VOacac2+pyridine⇌ lim krkfVOacac2·pyridine,kr=1.3×107 sec−1, kf=1.0×109M−1·sec−1. For the similar reaction with piperdine, kr≤2×106 sec−1, kf≤2.8×109M−1·sec−1; and for 2-picoline, kr=7×107 sec−1, kf=5×107M−1 sec−1.
On the Correlation Problem in Atomic and Molecular Systems. Calculation of Wavefunction Components in Ursell-Type Expansion Using Quantum-Field Theoretical Methods
Authors: Čížek Jiří
Published: January 2005

A method is suggested for the calculation of the matrix elements of the logarithm of an operator which gives the exact wavefunction when operating on the wavefunction in the one-electron approximation. The method is based on the use of the creation and annihilation operators, hole—particle formalism, Wick's theorem, and the technique of Feynman-like diagrams. The connection of this method with the configuration-interaction method as well as with the perturbation theory in the quantum-field theoretical form is discussed. The method is applied to the simple models of nitrogen and benzene molecules. The results are compared with those obtained with the configuration-interaction method considering all possible configurations within the chosen basis of one-electron functions.
NMR Studies of Aqueous Electrolyte Solutions. I. Hydration Number of NaCl Determined from Temperature Effects on Proton Shift
Authors: Malinowski Edmund R. , Knapp Paul S. , Feuer Bernice
Published: January 2005

Proton chemical shifts of purified water and of aqueous sodium chloride solutions are measured as a function of temperature and concentration. The shifts are referenced with respect to the resonance signal of gaseous ethane, which does not vary with temperature. An increase in temperature causes an upfield shift. Increasing sodium chloride concentration also causes an upfield shift.

The shift of the single resonance line observed is interpreted as being a time average of the proton in its different envorinments, namely, in the normal water structure and in a form hydrated to the ions. Although water in the secondary layer may be affected by the ions, such effects are neglected. It is argued that the shift of a proton in the hydrated form does not vary with temperature, whereas the shift of a proton in the normal water structure increases with increasing temperature due to the breaking of hydrogen bonds. From this simple interpretation, the total primary, or so-called ``effective'' hydration number of sodium chloride, is determined to be 4.
Equilibrium Composition of Selenium Vapor; the Thermodynamics of the Vaporization of HgSe, CdSe, and SrSe
Authors: Berkowitz J. , Chupka W. A.
Published: January 2005

The vapor compositions of SrSe, CdSe, HgSe, hexagonal selenium, α-monoclinic selenium, and some mixed sulfur—selenium crystals have been studied by mass-spectrometric analysis. Dissociation energies of Se2 (71.2±2 kcal/mole) and SrSe (59.1±3 kcal/mole) were obtained. The heats and free energies of formation of the various Sen molecular species detected in the vapor are also given. There is no significant difference between the vaporization behavior of the two Se allotropes. Mixed sulfur—selenium molecules form very slowly at room temperature, but they form more readily at higher temperature, and quenching to room temperature retains the mixed configurations.
B–F Bond Distance of Boron Trifluoride Determined by Gas Electron Diffraction
Authors: Kuchitsu Kozo , Konaka Shigehiro
Published: January 2005

The B–F bond distance rg of BF3 was determined by gas electron diffraction to be 1.3133±0.0010 Å. The corresponding rotational constant B0 was calculated to be 0.3449±0.0006 cm−1 from the rg distance by the use of the theory of rotation—vibration interaction. This value was consistent with the rotational fine structure of the v2 infrared band reported by Nielsen, when slight changes in his J assignments were made. The reanalysis of his spectral data led to B0=0.3449 cm−1 and DJ=(4.1±0.4)×10−7 cm−1. The latter constant was in good agreement with the theoretical estimate, 4.13×10−7 cm−1. The zero-point average distance, rz(B–F), of BF3 was thus determined from the diffraction and infrared experiments to be 1.3112±0.001 Å.
Crystal-Field Term Symbols for Nd3+ in CaWO4
Authors: Gualtieri J. G. , AuCoin T. R.
Published: January 2005

The crystal-field term symbols for the F324 state and the three lowest 4I states of Nd3+ in CaWO4 are identified with the irreducible representations of the double group of S4 symmetry. The assignments were made after an analysis of the polarized fluorescence of Nd3+ in CaWO4 compensated by Na+. The selection rules corresponding to electric-dipole transitions were applied after establishing the multipolarity through polarized absorption.
Carbon Trioxide: Its Production, Infrared Spectrum, and Structure Studied in a Matrix of Solid CO2
Authors: Moll Norman G. , Clutter Dale R. , Thompson Warren E.
Published: January 2005

Reactions of oxygen atoms with CO2 molecules to give a species identified as CO3 have been observed in three systems: (1) the photolysis of solid CO2 at 77°K with vacuum-ultraviolet light from a xenon resonance lamp; (2) the photolysis of O3 in a CO2 matrix at 50°—60°K with 2537-Å light from a mercury arc; and (3) the radio-frequency discharge of CO2 gas followed by trapping of products at 50°—70°K. The infrared spectrum of the species includes absorptions at 568, 593, 972, 1073, 1880, 2045, 3105, and 3922 cm−1. Isotopic studies using CO2 enriched with 18O or 13C show that the molecular formula is CO3 and provide a basis for determining the molecular structure. Frequency assignments and isotopic product-rule calculations favor a planar C2v molecule in which the carbon atom is bonded to the unique oxygen atom by a strong carbonyl bond and to two equivalent oxygen atoms by weaker bonds. There is presumably covalent bonding between the two equivalent oxygen atoms. In the proposed frequency assignment it is assumed that the absorption arising from the out-of-plane bending mode is unobserved. Results of photolyzing isotopically nonequilibrated 18O-enriched CO2 indicate that a D3h or C3v species with three equivalent oxygen atoms is involved in the reaction mechanism or is a readily accessible excited state. There is evidence that the observed photodecomposition of CO3 by visible and ultraviolet light yields oxygen atoms and CO2.