bluuebunny/crossref_metadata_2025 huggingface dataset:
Low-Temperature Thermal Properties of Calcium Tungstate
Authors: Lyon William G. , Westrum Edgar F.
Published: February 2004

The heat capacity of a single crystal of CaWO4 was determined by adiabatic calorimetry from 5° to 350°K and found to be without transitions or thermal anomalies. Deviation of the curve from normal sigmate shape is shown to be due to internal vibrations of the WO4= ions. Apparent Debye θ's for the lattice-only heat capacity and for that of the acoustical spectrum show “normal” deviation from simple Debye theory. Values of the heat capacity (Cp), entropy (S°), enthalpy function [(H°–H0°) / T], and Gibbs function [(G° / H0°) / T] at 298.15°K are 27.28, 30.21, 16.02, and −14.19, in calories per gram formula mass·degree Kelvin.
Perturbation Theory and the Equation of State of Mixtures of Hard Spheres
Authors: Henderson D. , Barker J. A.
Published: February 2004

The equation of state of a mixture of hard spheres of diameter σ11 and σ22 is evaluated by expanding the free energy of the mixture to second order in powers of the differences σαβ − σ, where σαβ = 12(σαα + σββ) and σ is the diameter of the unperturbed hard spheres. The parameter σ is chosen to make the sum of the first-order terms zero. The second-order terms are evaluated using the superposition approximation, and the equation of state is determined by analytic differentiation. The agreement of this perturbation calculation with quasiexperimental Monte Carlo and molecular dynamics results is excellent, even for quite large values of R = σ22 / σ11, but becomes less satisfactory as R becomes very large.
Double Perturbation Theory Applied to Frequency-Dependent Polarizabilities. I
Authors: Kestner Neil R. , Young Ralph H. , Deal Walter J.
Published: February 2004

The work of Tuan et al. is extended to the calculation of second-order frequency-dependent properties. Under certain circumstances, our previous method of calculating dispersion interactions can be simplified to the point where the two-electron equations may be solved by inspection. The simplified method is applied to a model system previously used to study the effect of intra-atomic correlation, and the results of the method are compared with those of an exact calculation and those of coupled time-dependent Hartree–Fock theory.
Collision-Induced Dissociation of Low-Kinetic-Energy Ions
Authors: Moran T. F. , Roberts J. R.
Published: February 2004

Dissociative reactions of H2+, O2+ and NO+ molecule–ions with various atoms and molecules have been measured for reactant ions having kinetic energies less than 50 eV. Initial velocity components of the reactant ions in these experiments are axial to the slit system of the analyzing mass spectrometer which serves to discriminate against charged reactants but allows for effective collection of the slow reaction products. The cross sections for the production of low-kinetic energy H+, N+, and O+ ions range from 0.01 to 1.0 Å2 near threshold. Reasonably sharp kinetic-energy thresholds for atomic ion formation measured in these experiments approximate known molecule–ion dissociation energies.
S = 1
Spin State of Divalent Iron. I. Magnetic Properties of Phthalocyanine Iron (II)
Authors: Dale B. W. , Williams R. J. P. , Johnson C. E. , Thorp T. L.
Published: February 2004

The magnetic properties of phthalocyanine iron (II) have been investigated in the temperature ranges 1.25°–20°K and 100°–300°K. Between 100° and 300°K the magnetic susceptibility obeys the Curie–Weiss law accurately, but the measured value of μeff at 25°C is 3.71 μB, which is intermediate between the theoretical spin-only values for S = 1 and S = 2 states. However, in the range 1.25°–20°K the susceptibility is virtually independent of temperature, and this result has been used to show that the ground state of the central iron atom is an orbital singlet, with a real spin triplet state. This is split by second-order spin–orbit coupling into a singlet ground state and a doublet state at 70 cm−1, to give g‖ = 1.9, g⊥ = 2.9.
S = 1
Spin State of Divalent Iron. II. A Mössbauer-Effect Study of Phthalocyanine Iron (II)
Authors: Dale B. W. , Williams R. J. P. , Edwards P. R. , Johnson C. E.
Published: February 2004

Measurements of the 57Fe Mössbauer spectrum of phthalocyanine iron(II) made at 4.2° and 100°K, with the sample in applied magnetic fields of 30 kOe, show that the electric field gradient at the iron nucleus is of positive sign and that there is no large asymmetry in the electric-field-gradient tensor. The major axis of the field-gradient tensor does not appear to be colinear with the magnetic symmetry axis, indicating that there is no orbital degeneracy in the molecule. The hyperfine field at the iron nucleus, measured in the plane of maximum magnetization, is estimated to be + 270 kOe.
14N Nuclear Quadrupole Resonance in Substituted Pyridines
Authors: Schempp Ellory , Bray P. J.
Published: February 2004

Nitrogen-14 nuclear quadrupole resonance has been observed in 15 substituted pyridines, including cyano, acetyl, and chloro derivatives and the methyl esters of the pyridine monocarboxylic acids. The substituents produce variations of 5%–10% in the field gradient at the nitrogen. The σ and π charge densities at the ring nitrogen are discussed in terms of the relative electrophilic properties of the substituents and excellent qualitative agreement with the classical ideas of chemistry is obtained. Correlation of the nitrogen NQR data with the Hammett σ values and with chlorine-35 NQR results is also considered. The charge distribution of the cyano group is analyzed and discussed in connection with the ring nitrogen. The ionic character of the cyano C–N bond is found to be approximately 17%.
Millimeter-Wave Rotational Spectrum of HSSH and DSSD. I. Q Branches
Authors: Winnewisser Gisbert , Winnewisser Manfred , Gordy Walter
Published: February 2004

The Q-branch rotational lines of H2S2 have been measured in the frequency range 80–200 Gc/sec, those of D2S2, in the range from 60 to 220 Gc/sec. For HSSH, measurements were made on the torsional vibrational state υt = 1, and the S–S bond-stretching vibrational state υs = 1 as well as on the ground state. The molecules are the most nearly accidentally symmetric tops of any so far reported. Wang's asymmetry parameter bp = (C − B) / (2A − B − C) for the ground vibrational state of H32S32SH is found to have the value bp = − 1.10 × 10−5; for H32S34SH, bp = − 1.07 × 10−5; for D32S32SD, bp = − 2.10 × 10−6. The asymmetry increases markedly with the torsional oscillation: for H32S32SH when υt = 1, bp = − 4.055 × 10−5; it decreases with the S–S stretching: for H32S32SH when υs = 1, bp = − 7.72 × 10−6. The molecule is found to have the nonplanar chain structure HSSH, with the structural parameters having the values: dSH = 1.327 Å, dSS = 2.055 Å, ∠HSS = 91.32°, and the dihedral angle η = 90°36′. The barrier to relative internal rotation of the two SH groups is evidently very high, much higher than in H2O2. The internal rotational splitting of the ground vibrational state is too small to be resolved. In the torsional vibrational state υt = 1 it was found to be 2Δν = 15.99 Mc/sec. The near orthogonality of the bond angles, the S–S length, and the high barrier to internal rotation indicate significant π bonding through the mechanism of hyperconjugation.
Atomic Binding of Transition Metals on Clean Single-Crystal Tungsten Surfaces
Authors: Plummer E. W. , Rhodin T. N.
Published: February 2004

The binding energy of single atoms of the Period-6 transition elements on atomically perfect single-crystal planes of tungsten was measured by utilizing low-temperature-pulse field desorption, in a ultrahighvacuum field-ion microscope. For any given adatom, the dependence of the binding energy on surface crystallography was observed to be inconsistent with a pairwise interaction model. A new model incorporating a surface-charge redistribution into a pairwise potential is shown to be consistent with the data. The variation in binding energy with electronic configuration of the adatom is interpreted in terms of an electron-energy band model. The binding energy depends upon the electron occupancy of two subbands assumed to exist in the d levels of the atom, when it is adsorbed on the surface. Finally, the abnormal field-desorption properties of adatoms on the (110) plane are discussed in terms of its anomalous work function.
Classical Probabilities of Reaction and Transmission Coefficients for Isotopic Reactions of the Type H+H2=H2+H
Authors: Mortensen Earl M.
Published: February 2004

Classical trajectories for the linear collision of an H atom and an H2 molecule and their isotopic substitutions have been calculated as a function of energy with the molecule having the zero-point vibrational energy. Probabilities of reaction are calculated by averaging over the vibrational phases during collision. These results are compared with recently reported quantum-mechanical calculations for the same systems.
Frequency Dependence of the Higher Susceptibilities
Authors: Sitz Paul , Yaris Robert
Published: February 2004

Expressions for the susceptibilities appropriate to third harmonic generation, second harmonic generation, and the induced Kerr effect are derived using time-ordered time-dependent pertubation theory. These expressions are used to calculate the susceptibilities for atomic hydrogen and helium. Using a six-pole trial function we obtain for He λ = 6943 Å,χzzzz(− 3ω; ω, ω, ω) = 47.7 a.u.;λ = 6943 Å,χzzzz(− 2ω; 0, ω, ω) = 45.2 a.u.;λ = 6328 Å,χzzzz(− ω, 0, 0, ω) = 43.6 a.u.; χzzzz(0; 0, 0, 0) = 42.6 a.u. The experimental value of χzzzz(− ω; 0, 0, ω) obtained from Kerr-effect measurements is 51.6 ± 8.6 a.u. The calculations are checked by the use of a number of sum rules with the largest disagreement being 0.05%.
Mean Amplitudes of Vibration in Molecules with Internal Rotation. IV. Theory with Application to Disulfur Dichloride
Authors: Cyvin S. J. , Elvebredd I. , Hagen G. , Brunvoll J.
Published: February 2004

A normal-coordinate analysis has been performed for disulfur dichloride, and used to calculate the mean amplitudes of vibration. A generalized definition of framework mean amplitudes is proposed, and is applicable to cases where the torsional mode (for internal rotation) is kinematically coupled with other normal modes of vibration. According to this definition the total mean-square amplitude is composed of a sum of the framework mean-square amplitude and the torsional mean-square amplitude; the latter quantity contains all contributions where the torsional coordinate is involved. Numerical results of framework and torsional mean amplitudes are reported for S2Cl2 at various temperatures as functions of the angle of rotation θ. The approximate formula for framework mean-square amplitudes, 〈Δρ2〉frm = (α + β cosθ + γ cos2θ) / ρ2, has been subjected to detailed studies. The α, β, and γ coefficients were evaluated in terms of certain mean-square amplitude quantities, and the following statement was deduced from the properties of their classical limits at high temperatures: α(θ, T), β(θ, T), and γ(θ, T) approach θ-independent values when T → ∞. Furthermore, it is explained why the above formula with constant coefficients may give a very good approximation even at low temperatures when the conditions of classical limits are not fulfilled. This feature was actually found for S2Cl2 even at absolute zero.
Dissociation Energies of GeCu, GeCo, GeFe, and GeCr
Authors: Kant Arthur , Strauss Bernard H.
Published: February 2004

The molecules GeCu, GeCo, GeFe, and GeCr have been shown to exist in the vapor phase over liquid solutions of germanium and transition metal at temperatures above 1600°K. Using information derived from a combination of effusion and mass spectrometric techniques the equilibrium constants and energetics of the gaseous reactions GeX + Ge = Ge2 + X, where X = Co, Fe, and Cr and 2GeCu = Ge2 + Cu2, have been determined. The dissociation energies of the molecules GeCu, GeCo, GeFe, and GeCr as obtained by the third-law method are 47.8 ± 5, 56 ± 6, 49.4 ± 7, and 39.6 ± 7 kcal/mole, respectively.
Vibrational Spectra and Structure of Dimethyl Diselenide and Dimethyl Diselenide-d6
Authors: Green William H. , Harvey Albert B.
Published: February 2004

The vibrational behavior of the dimethyl diselenide and dimethyl diselenide-d6 molecules has been studied in the infrared region (4000–70 cm−1) and by Raman shifts (4000–50 cm−1). The molecule has been found to belong to point group C2. The fundamental vibrations, with the exception of the methyl torsions, have been assigned and supported by a normal-coordinate analysis.
Theoretical Study on the Proton Affinity of Small Molecules Using Gaussian Basis Sets in the LCAO–MO–SCF Framework
Authors: Hopkinson A. C. , Holbrook N. K. , Yates K. , Csizmadia I. G.
Published: February 2004

The proton affinities of certain small molecules have been calculated as an energy difference between the parent molecule and the protonated species. Various size Gaussian basis sets were used to see how the calculated proton affinities approximate the experimental values as the wavefunctions approached the Hartree–Fock limit. The correlation between experimental and calculated proton affinities was excellent with the extensive basis sets.
Exciton Interpretation of the Vacuum-Ultraviolet Absorption Spectra of Saturated Organic Polymers
Authors: Partridge Roger H.
Published: February 2004

A recent application of the molecular exciton theory to the σ bond absorptions of simple gaseous alkanes has prompted the extension of the theory here to saturated organic polymers in the solid phase. The theory explains the experimentally observed insensitivity of the absorption spectra to changes in polymer chain conformation and, in the case of simple alkanes, to changes from gas to solid phase. It also explains the observed similarity between the low-energy portions of the absorption spectra of such diverse materials as diamond and gaseous alkanes. The exciton energy levels of polyethylene, polypropylene, and diamond are calculated and are found to give very reasonable agreement with the previous experimental data on polyethylene and diamond, and with the absorption spectrum of polypropylene which is presented here between 2500 and 584 Å. The polarizations of the optically allowed transitions are also obtained but no experimental data is yet available on these. The possible implications of this exciton interpretation to the radiation chemistry of saturated hydrocarbon polymers are briefly discussed.
Studies in Molecular Dynamics. V. High-Density Equation of State and Entropy for Hard Disks and Spheres
Authors: Alder B. J. , Hoover W. G. , Young D. A.
Published: February 2004

The equations of state for periodic systems of hard disks and hard spheres in the solid phase have been accurately determined and used to evaluate the coefficients in the expansion of the pressure in powers of the relative free volume, α = (V − V0) / V0, where V0 is the close-packed volume. For disks pV / NkT = 2 / α + 1.90 + 0.67α + O(α2) and for spheres pV / NkT = 3 / α + 2.56 + 0.56α + O(α2). These coefficients are compared to cell models, and those models which include correlations between neighboring particles work best. An equivalent expansion of other thermodynamic properties requires the entropy constant to be evaluated in the close-packed limit. This constant is obtained here by integrating the equation of state over the entire density region. The Lennard-Jones–Devonshire cell-theory estimates of the entropy constant are nearly correct; that is, the cell-theory estimate is too small by 0.06Nk for disks and too large by 0.24Nk for spheres. The pressure difference and hence the entropy difference between the hexagonal and face-centered cubic packings of spheres could not be detected, and thus the relative stability of these two phases remains an open question.
Impurity States in a Linear Molecular Crystal
Authors: Julienne Paul S. , Choi Sang-il
Published: February 2004

The assumption of parallel impurity and host transition moments made in the existing theory of localized electronic states of impurities in molecular crystals is shown to be unnecessary for certain one- and two-dimensional model crystals. The dependence of localized impurity-state energy and radius on the angle between host and impurity moments is calculated for a linear crystal.
Structure and Vibrational Spectrum of Nitrogen Trichloride
Authors: Carter James C. , Bratton Raymond F. , Jackovitz John F.
Published: February 2004

The infrared and Raman spectra are given for nitrogen trichloride in carbon tetrachloride solution. All fundamental frequencies are identified, and all observed combination bands are assigned. The structure is established to be pyramidal, and the molecule obeys the selection rules for C3υ symmetry.
Effect of Pressure on the Mössbauer Spectrum of Fe2(CO)9 and Fe3(CO)12
Authors: Lewis G. K. , Drickamer H. G.
Published: February 2004

The effect of pressure to 160 kbars has been measured on the Mössbauer resonance in Fe2(CO)9 and Fe3(CO)12. Unenriched samples were used, but the major features are easily discernible. The di-iron carbonyl exhibits a very large shift to higher electron density with increasing pressure, and a moderate increase in quadrupole splitting. Evidently there are changes both in 3d–3s shielding and in orbital occupation with pressure. The quadrupole splitting indicates no significant distortion with pressure. The Fe3(CO)12 has three peaks of equal intensity. The outer pair shifts to higher electron density by an amount comparable to the Fe2(CO)9. The inner peak shows a more modest shift. There is a significant increase in the splitting of the outer peaks with pressure. The results are discussed in terms of the most probable structures.
Displacive Phase Transitions in Malononitrile
Authors: Zussman A. , Alexander S.
Published: February 2004

The crystallographic phases and phase transitions of molecular crystals of malononitrile were investigated using 14N pure nuclear quadrupole resonance. The resonance frequencies, linewidths, and spin–lattice relaxation times were measured using pulse techniques. The relation between quadrupole resonance results and the order parameter of a phase transition are analyzed. Detailed measurements were carried out near one of the transitions (at 294.7°K) and the results are interpreted in terms of critical exponents. The resonance frequencies give β = 0.5 ± 0.02 and the linewidths γ = 0.3 ± 0.07 and γ′ < 0.1 ± 0.1. Our quadrupole resonance results are compared with specific-heat measurement of Girdhar, Westrum, and Wulff. As two of the phase transitions show very small transition entropies, it is suggested that these are displacive phase transitions associated with instabilities in the collective modes of the crystal. The meaning of such transitions is discussed both from a thermodynamic and a microscopic point of view.
Geometric Properties of Off-Lattice Self-Avoiding Random Walks
Authors: Loftus Elizabeth , Gans Paul J.
Published: February 2004

Simple geometric properties of self-avoiding random walks not restricted to lie on a lattice have been investigated. Both the mean-square end-to-end distance and the mean-square radius of gyration are found to deviate in their exponential dependence upon the number of steps in the walk from values established for lattice-restricted walks. As the allowed deviation of each step from its lattice-defined position increases, the exponent increases markedly from its lattice value and then attains a limiting value as free rotation is approached. For all degrees of deviation the exponent is significantly larger than the accepted value of 6/5. Although normal random walks have the same exponential dependence whether restricted to a lattice or not, self-avoiding random walks behave differently. Thus any generalization to off-lattice systems of properties calculated from self-avoiding walks constrained to a lattice may be difficult. Since all linear macromolecules have some rotational freedom, the lattice-constrained self-avoiding random walk may be an insufficient model of such systems, even though it is clearly better than the normal random-walk model.
Dispersion Curves and Frequency Distributions of Isotactic Polypropylene
Authors: Zerbi Giuseppe , Piseri Luigi
Published: February 2004

Dispersion curves and a frequency distribution were calculated for isotactic polypropylene on the basis of a single-chain model using a valence force field previously reported in the literature. The obtained results provide useful information for the interpretation of the inelastic neutron scattering spectrum published by other authors. It is shown that calculations on the single-chain model account for practically all peaks in the experimentally determined frequency distribution. Most of these peaks arise from phase values which do not correspond to spectroscopically active motions.
Vibronic Spin–Orbit Interactions in Heteroaromatic Molecules. II. Phosphorescence of Quinoxaline and Other Diazanaphthalenes
Authors: Lim E. C. , Yu Jack M. H.
Published: February 2004

Solvent effects on phosphorescence and its polarization characteristics indicate that the two major spin–orbit coupling mechanisms through which the 3B2(ππ*) state of quinoxaline acquires dipole-allowed character via nπ* states are B11(nπ*) ↔  lim soB23(ππ*) and A11(ππ*) ↔  lim b1vib1B1(nπ*) ↔  lim so3B2(ππ*). The deuterium isotope effect on the emission characteristic and the intensity of vibronic bands of the n→π* absorption suggest that the perturbing vibration(s) is an out-of-plane C–H bending mode(s).
Choice of Basis Set for Expansion of One-Dimensional Oscillator Eigenfunctions
Authors: Lesk Arthur M.
Published: February 2004

Endres has studied expansions of eigenfunctions of a Morse oscillator upon basis sets derived from different harmonic-oscillator Hamiltonians. We have shown how an optimal basis set for such a problem can be determined. For the particular case studies by Endres, our prediction compares favorably with his result.
Exciton-Migration Processes in Crystalline Lanthanide Chelates. I. Triplet Exciton Migration in Lanthanide Chelates of 1,10-Phenathroline
Authors: Kleinerman Marcos , Choi Sang-il
Published: February 2004

Efficient long-range energy transfer takes place from the ligand 1,10-phenanthroline to Tb3+ and Eu3+ in lightly doped crystalline chelates of gadolinium (1,10-phenanthroline) triacetate, GdPhenAc3, and from Tb3+ to Eu3+ in TbPhenAc3 doped with various concentrations of EuPhenAc3. A self-consistent interpretation of the observation is given in terms of a mechanism involving triplet exciton migration. In Eu3+-doped TbPhenAc3 the measured temperature dependence and the europium concentration dependence of the fluorescence decay time of the D45 level of Tb3+ and the buildup time of the Eu3+ fluorescence agree rather well with our theoretical prediction. The activation energy of the lifetime of the D45 level of Tb3+ can be identified with the difference of the ligand triplet level and the D45 level.
High-Resolution Determination of the Widths of Self-Broadened Lines of Carbon Monoxide
Authors: Hunt Robert H. , Toth Robert A. , Plyler Earle K.
Published: February 2004

The half-widths of self-broadened lines of carbon monoxide have been measured directly with a resolution of 0.045 cm−1 in the first overtone band at room temperature for | m | values up to 31 and for pressures between 1 and 2 atm. The results, which have an estimated accuracy of ±3.0%, vary from 0.088 cm−1·atm−1 at | m | = 1 to 0.044 cm−1·atm−1 at | m | = 31, and occupy an intermediate position relative to other measurements. The half-widths have also been determined from peak absorption measurements and previously measured line strengths and these widths agree with the directly measured values to better than 2%.
Fluorescence Lifetimes of Aromatic Hydrocarbons under Pressure
Authors: Offen Henry W. , Phillips David T.
Published: February 2004

The fluorescence lifetimes of 12 aromatic hydrocarbons embedded in polymethylmethyacrylate have been measured as a function of pressure to 30 kbar. The lifetime τf is shortened 15%–40% in the 0–30-kbar interval for this class of compounds. The observed magnitude and sign of the pressure dependence of τf are in agreement with rough estimates derived from the compressibility and refractive index of the polymer matrix. The conclusion is reached that radiative and nonradiative transition rates are similarly (within a factor of 2) affected by external pressures.
Separation of Rotational Coordinates from the N-Electron Diatomic Schrödinger Equation
Authors: Pack Russell T , Hirschfelder Joseph O.
Published: February 2004

A formalism is presented for the exact separation of the center-of-mass and rotational coordinates from the Schrödinger equation for an arbitrary diatomic system. The use of operator arguments instead of the explicit representations of the rotation group simplifies the procedure. The coupled equations governing the internal motion of the system are obtained and expressed in a new and particularly simple form using angular-momentum raising and lowering operators. The formalism is carried out for three types of electronic coordinates (separated atom, center of mass of nuclei, and geometric center of nuclei) which differ as to the origin of the electronic coordinates.
Hindered Rotation of Adsorbed Hydrogen Isotopes: A Hybrid Approximation to the Quantum-Statistical Partition Function
Authors: Freeman M. P. , Hagyard M. J.
Published: February 2004

A new hybrid partition function is used to calculate the quantum-statistical behavior of the various isotopes and nuclear-spin isomers of diatomic hydrogen interacting with a plane structureless surface. Problems of convergence usually encountered are thereby avoided, and the rather pronounced effects of hindered rotation become readily apparent and are seen to be separable into two components which we designate as “adrotation” and “libration”. System measureables calculated from the new partition function are seen to be in excellent agreement with available experimental data. It is shown that use of the commonly employed Kirkwood–Wigner formulation can lead to serious errors in fitted energy parameters, errors usually ascribed to the polarizability differences of the isotopes.
Exponential Virial Equation of State
Authors: Starling Kenneth E.
Published: February 2004

An equation of state is derived which has promise for the prediction of high-density fluid behavior. An approximate form of this new equation yields expressions for approximate virial coefficients of all orders as functions of lower-order exact virial coefficients. Use of only the exact second and third virial coefficients in the second-order approximate equation leads to accurate prediction of rigid-sphere fluid behavior at high densities where even the fourth-order virial equation fails. For real fluids, the second-order approximate equation is accurate to high densities for temperatures above the Boyle temperature. At moderately lower temperatures, the third-order approximate equation yields accurate predictions at high densities where the third-order virial equation is in error.
Correlation of Experimental Rate Constants of the Hydrogen Exchange Reactions with a Theoretical H3 Potential Surface, Using Transition-State Theory
Authors: Shavitt Isaiah
Published: February 2004

The recent ab initio potential surface for H3 obtained by Shavitt, Stevens, Minn, and Karplus is used with one empirical parameter, a uniform scale factor for the energy profile along the reaction path, to compute rate constants for various isotopic forms of the H+H2 exchange reaction. The scale factor is adjusted to give agreement with the high-temperature part of the experimental rates recently measured by Westenberg and de Haas, and the results are compared with all the available experimental data. Tunneling is estimated by fitting a one-dimensional Eckart barrier to the higher 30% of the computed reaction-path barrier. It is concluded that the ab initio barrier needs to be scaled down by only 11% in order to reproduce the best experimental data, and the “experimental” barrier height is accordingly obtained as 9.8 ± 0.2 kcal/mole.
Transient Electro-osmosis in Capillary Tubes
Authors: Hanna W. T. , Osterle J. F.
Published: February 2004

Transient electro-osmosis of water in glass capillary tubes is treated analytically. The usual selective ion adsorption mechanism is assumed. The analysis is restricted to tube radii greater than about 3 × 10−5 m for which the effects of fluid compressibility and electrical capacitance and inductance are negligible. The problem considered is the transient response of the system to a step change in either pressure or voltage under various external loading conditions. The resulting time histories of the induced force (voltage or pressure) and the fluid flow and electrical current are solved for. A Laplace transform technique is employed. Coupling coefficients are established in the Laplace domain relating the electrical current and fluid flow rate to the electric field and pressure gradient. The cross-coupling coefficients are shown to obey the reciprocal relations for nonsteady-state irreversible processes.
Magnetothermodynamics of α-NiSO4·6H2O. IV. Heat Capacity, Entropy, Magnetic Moment, and Internal Energy from 0.4° to 4.2°K with Fields 0–90 kG along the a Axis
Authors: Fisher R. A. , Brodale G. E. , Hornung E. W. , Giauque W. F.
Published: February 2004

The heat capacity and magnetic moment of a 4-cm-diam spherical single crystal of α-NiSO4·6H2O have been measured over the range 0.4°–4.2°K, with stabilized magnetic fields of 0, 5, 10, 15, 25, 40, 65, and 90 kG along the a crystallographic axis. Comparison with our previous data with the field along the bisector of the a, b axes confirms the observation of Bose and Schoenberg that the magnetic characteristics in the a, b plane of this tetragonal crystal are not isotropic. The heat capacity and entropy of the lattice and electronic system approach zero at the lower temperatures and all fields. These data and closely interlocked observations of temperature vs magnetic field on isentropes which crossed the heat-capacity curves enabled an accurate tabulation of the entropy as a function of field and temperature. The derived differential magnetic susceptibility, the internal energy, enthalpy, and magnetic work are tabulated. At 90 kG the proton spins were in good equilibrium with the lattice above 1.1°K and were essentially out of equilibrium below 0.9°K. Details of heat-capacity measurements, out of equilibrium, in partial equilibrium, and in rapid equilibrium with the nuclear spin system are presented. With minor deviations it was found possible to represent the heat-capacity observations at all fields by means of three-state Schottky functions. The separations of the three states combined with the change of enthalpy of the ground state W0 with field enabled evaluation of the Zeeman pattern for the W0, W1, and W2 states between 0 and 90 kG. The W1 and W2 states were found to be degenerate in zero field. All of the observations were found to be in excellent agreement with values calculated from the spin-Hamiltonian–molecular-field parameters derived by Fisher and Hornung from previous observations with the field along the c axis together with those obtained when the field was along the bisector of the a, b axes.
Steady-State Thermodynamics of a Paramagnetic System
Authors: Barker William A. , McGuire Michael
Published: February 2004

A two-level paramagnetic system acted on by a static magnetic field and a resonant time-varying field is treated according to the thermodynamics of irreversible processes. The rate equations usually used to describe such a system are shown to be consequences of setting the internal entropy production equal to the rate of increase of internal energy. Expressions for the steady-state entropy and heat capacity are derived. The heat capacity exhibits large negative values if the relaxation rate is temperature dependent.
Paramagnetic and Electric Quadrupole Hyperfine Interactions of Ferric Ions in Ice and FeCl3·6H2O
Authors: Nozik Arthur J. , Kaplan Morton
Published: February 2004

Mössbauer resonance in 57Fe has been used to study the species Fe(H2O)5Cl2+ and Fe(H2O)63+ in frozen aqueous solutions of ferric chloride and ferric nitrate. The spectral lines display pronounced broadening as the temperature is lowered, as a result of paramagnetic relaxation effects. In dilute samples at 4.2°K, resolved paramagnetic hyperfine structure was observed for both species and permitted identification of the lowest crystal-field states of the ferric ion. The effective magnetic fields originating from the | ± 52〉 Kramers doublets were determined as 553 kG in Fe(H2O)5Cl2+ and 588 kG in Fe(H2O)63+. The latter ion showed no evidence of an electric quadrupole interaction, whereas the former exhibited a significant quadrupole splitting due to the coordinated Cl− ligand. The apparent electronic relaxation in the pentaquo–chloro complex was found to be substantially faster than that in the hexaquo complex, which may be indicative of a smaller zero-field splitting in the former case or an unbalanced spin pairing in the Cl ion. Results are presented for crystalline FeCl3·6H2O, in which the ion Fe(H2O)4Cl2+ exists as a structural unit. These spectra exhibit unusual line shapes and temperature variations which can be qualitatively understood by considering the effects of paramagnetic relaxation on the hyperfine interactions. The study of FeCl3·6H2O and its frozen solutions also demonstrates that common features as well as differences between the ion complex in solution and in the salt crystal may be inferred from a comparison of the Mössbauer spectra.
Clusters in Isentropically Expanding Nitric Oxide and Their Effect on the Chemiluminous NO–O Reaction
Authors: Golomb D. , Good R. E.
Published: February 2004

Recently a hypothesis was forwarded that molecular clusters are responsible for the enhanced light-emission rate in the reaction of adiabatically expanded nitric oxide and atomic oxygen. Clusters with masses up to 600 amu have now been detected by mass-spectrometric sampling of expanding NO jets. The fraction of dimers formed is strongly dependent on p0, the reservoir pressure, and d, the orifice diameter. Less than 1% dimers were detected at p0d = 10 torr·cm; at p0d = 35, the dimer concentration was 5%. When nitrogen is added to dilute NO, larger values of p0d are required to detect dimers. Addition of SF6 reduces the dimer concentration substantially, because the lower specific heat ratio of the mixture results in higher free-jet temperatures. Photometric measurements of the NO–O headglow show a direct relationship between radiance and dimer concentration in the expanding NO jet, confirming the above hypothesis. Intense headglows are obtained only for p0d ≥ 35, and in 50% SF6–50% NO jets, the headglow radiance is decreased significantly. Between 1 to 10 dimer–atomic-oxygen collisions yield an electronically excited NO2*, which subsequently radiates. The spectral intensity distribution of the headglow is similar to the normal NO–O chemiluminescence, indicating that the same excited state is involved. The onset of headglow radiation is within 100–200 Å of the normal afterglow onset; therefore, the dimer bond energy is probably less than 3 kcal mole−1.
Nuclear Magnetic Relaxation Studies. Nuclear Overhauser Effect in the Methanol System
Authors: Fukumi Toshio , Arata Yoji , Fujiwara Shizuo
Published: February 2004

Nuclear Overhauser effects were investigated in liquid methanol containing 10−1–10−6M p-nitrobenzoic acid, and negative enhancements observed over a wide range of acid concentrations. Longitudinal relaxation times were measured by an adiabatic rapid-passage method, and were used along with Overhauser enhancement data to estimate proton exchange rates in the methanol system. Up to 10−2M acid, proton transfer is approximately first order with respect to acid concentration with a rate constant of 1.8 × 108 liters mole−1·sec−1. This result is similar to that obtained by Grunwald, Jumper, and Meiboom, who studied a buffered methanol system and obtained a rate constant by extrapolating to zero buffer concentration.
One-Center Molecular Orbitals for HeH++ and HeH+ with Variable Origin
Authors: Combs L. L. , Runnels L. K.
Published: February 2004

The feasibility of using a variable expansion point in one-center calculations is investigated for the ground states of HeH++ and HeH+ with very limited basis sets of Slater-type atomic orbitals. The energy improvement resulting from variation of the expansion point appears to be economical only if the wavefunction is of the simplest possible form, 1s or (1s1s). For these functions the maximum effect of varying the origin occurs in the vicinity of 0.4 a.u., where the improvement in the electronic energy is somewhat less than 2%.
Theoretical Study of the LiH Molecule
Authors: Bender Charles F. , Davidson Ernest R.
Published: February 2004

The low-lying electronic states of the LiH molecule have been examined using limited configuration interaction. Potential curves, dipole moments, and oscillator strengths are reported for the first 19 states. The dipole moment of the ground state was examined in detail. It was found that the discrepancy between SCF and experiment was largely due to the effect of single excitations in second order of perturbation theory.
Evidence of Multiple Vibrational–Translational Relaxation in CCl4
Authors: Barocchi F. , Vallauri R.
Published: February 2004

A multiple relaxation behavior in the vibrational–translational exchange of energy has been observed in CCl4 by means of SBS shifts measurements in the temperature range between the melting and the boiling points.
Spectra and Structure of Crystalline Cyclopropane and Cyclopropane-D6
Authors: Bates John B. , Sands Donald E. , Smith William Hayden
Published: February 2004

Infrared and Raman spectra of polycrystalline C3H6 and C3D6 were measured between 10 and 3500 cm−1 at 85°K. Infrared spectra of mixed C3H6 and C3D6 crystals were also obtained. From these measurements, an assignment of the origin of the multiplet splittings is proposed. An x-ray diffraction study of crystalline C3H6 suggests an orthorhombic unit cell with a = 10.0 Å, b = 6.8 Å, and c = 5.1 Å. Density measurements, spectroscopic data, and diffraction data indicate that the site group is Cs and that the space group is one of eight primitive D2h groups. Far-infrared spectra of crystalline C3H6 showed five absorptions attributed to k = 0 external modes.
Electromechanical Model of Diatomic Hydrides
Authors: Phillips J. C. , Sandrock R.
Published: February 2004

An electrostatic distributed-charge model for molecules is suggested and applied to diatomic hydrides AH of first- and second-row elements A. The atoms are represented by pseudopotential form factors rather than point ions. The parameters of the model—magnitude and location of the bond charge, internal polariza-ability of the A atom—are determined from equilibrium data. Calculated bond-stretching force constants and dipole moments agree reasonably with experiment.
Fluorescence and Intersystem Crossing of Benzene-h6, -d6, and -1,4-d2
Authors: Anderson E. M. , Kistiakowsky G. B.
Published: February 2004

Fluorescence produced by absorption of the 2537-Å radiation in benzene--h6, benzene-d6, and benzene-1,4,-d2 was studied down to 10−2 torr. At pressures below about 0.4 torr the spectral intensity distribution and quantum yield of the resonance fluorescence of each benzene are independent of pressure. The quantum yields of the three benzenes, determined by comparison of the work of others with benzene-h6 at high pressures, were found to be 0.39, 0.53, and 0.38. At higher pressures the yields are lower and the spectral intensity distribution is different. Benzene-1,4-d2 at low pressures showed no spectroscopic evidence of isomerization (to -1,3-d2) even on very prolonged exposure to 2537-Å radiation. Isomerization of cis- into trans-butene by benzene-h6 was studied at lower pressures. In the range of 0.5–0.1 torr a mechanism involving first-order rates of formation and destruction of the triplet state of benzene, the latter process competing with the energy transfer to butene molecules, describes the data well. At still lower pressures a direct photochemical isomerization of butene by the 2735-Å radiation, proceeding with quantum yields well above unity, complicates the interpretation of the benzene-sensitized reaction data. It is concluded that the “singlet-triplet intersystem crossing” in benzene is an intramolecular process. A model involving three vibronic levels of the B2u1 state, each of which fluoresces and also suffers intersystem crossing and the two upper levels of which also lose vibrational energy on collisions, describes quantitatively the dependence of the fluorescence yield on pressure for benzene-h6. The phosphorescence intensity of the triplet state of pyrazine C4H4N2 upon excitation into the first singlet state by 3110-Å radiation was studied at pressures from 5 to 0.05 torr. The pressure dependence observed is such that a mechanism involving only first-order (i.e., intramolecular) processes could not be devised to describe it. It is suggested that because of a smaller number of vibrational degrees of freedom and lower excitation energy above the ground level of the triplet state, the singlet–triplet intersystem crossing in pyrazine may to some extent be a collision-induced process.
NQR Zeeman Study of Gallium Trichloride
Authors: Peterson G. E. , Bridenbaugh P. M.
Published: February 2004

An NQR Zeeman study of a single crystal of gallium trichloride is made. The electric-field-gradient asymmetry parameters and major field-gradient directions for all atoms in the dimer molecule are obtained. The asymmetry parameters for the gallium and the bridge chlorines are quite large, being 0.867 and 0.473, respectively. An analysis of the data, based on the theory of Casabella, suggests the possibility of bent bonds.
Bimolecular Chemical Reaction Rates in Imperfect Gases
Authors: Emptage Michael R. , Ross John
Published: February 2004

The initial density dependence of the rate of a bimolecular chemical reaction in a mixture of imperfect gases is derived by the use of a simple physical cluster model. This result is compared with two forms of the activated complex theory; in one [denoted ACT(c)], the rate is assumed proportional to the concentration of the activated complex, and in the other [ACT(a)], proportional to the activity of the activated complex. In certain plausible limits, agreement is obtained between the cluster model and ACT(c).
Gaussian Lobe Function Expansions of Hartree–Fock Solutions for the Second-Row Atoms
Authors: Petke J. D. , Whitten J. L. , Douglas A. W.
Published: February 2004

Gaussian expansions of ground-state Hartree–Fock wavefunctions for the atoms sodium through argon are reported. The functions presented are designed primarily for use in molecular computations and consist only of functions of the form exp(− ar2); angular properties of real p orbitals are achieved by displacement of basis functions along coordinate axes. Variable exponents and coefficients are determined by maximization of the overlap of Gaussian orbitals with exponential Hartree–Fock expansions followed by self-consistent-field minimization of atomic energies.
17O Magnetic Resonance of Aqueous Solutions of Vanadium(II) and Chromium(III)
Authors: Olson M. V. , Kanazawa Y. , Taube H.
Published: February 2004

Oxygen 17 nuclei in aqueous V2+ and Cr3+ solutions give rise to a single NMR line emanating from the bulk solvent oxygens. In the V2+ case, the linewidth increases with temperature above 30° C due to chemical exchange between bulk and bound waters: k298 = 90 ± 20 sec−1, ΔH‡ = 16.4 ± 0.6 kcal/mole, and ΔS‡ = 5.5 ± 1.5 e.u. The linewidths narrow with increasing temperature up to 30°C for a V2+ solution, and up to 120°C in the Cr3+ case. Relaxation in these regions is attributed to time-dependent interactions between the unpaired electrons of the paramagnetic ions and outer-sphere solvent molecules. A detailed treatment shows that it may be important to consider scalar as well as dipole–dipole interactions in interpreting the observed outer-sphere relaxation.
First- and Second-Order Density Matrices of Symmetry-Projected Single-Determinant Wavefunctions
Authors: Simons Jack , Harriman John E.
Published: February 2004

We derive the first- and second-order density matrices of symmetry-projected single-determinant wavefunctions, in the case of finite groups. We also give the first-order density matrices for functions projected in the axial-rotation group. A method is presented for extracting from a density matrix its totally symmetric component, and the eigenfunctions of this component are shown to be the same after projection as before.
Structure of Pressure-Induced Fibrous Sulfur
Authors: Lind M. D. , Geller S.
Published: February 2004

The arrangement of the sulfur helices in the pressure-induced fibrous form of sulfur has been determined from three-dimensional x-ray diffractometer data. The crystals show orthorhombic diffraction symmetry, but analysis of their structure has shown that the most probable space group is P2 and that the orthorhombic symmetry results from multiple twinning. The pseudo-orthorhombic unit cell contains 16 ten-atom helix segments, half of which are right-handed and half left-handed, in an essentially close-packed arrangement. The monoclinic unit cell, which contains 8 ten-atom segments, has lattice constants a = 17.6, b = 9.25, c = 13.8 Å and β = 113°. There is strong evidence that the pressure-induced fibrous form and the ψ form have the same structure.
Measurements of Deuterium Quadrupole Coupling in Formic Acid
Authors: Kukolich Stephen G.
Published: February 2004

The 101–000 rotational transition was observed in HCOOH, HCOOD, and DCOOH using a maser spectrometer. Quadrupole hyperfine structure was completely resolved in HCOOD and DCOOH. The measured quadrupole coupling strength along the OD-bond direction was eqQ = 391 kHz and HCOOD and eqQ = 249 kHz along the DC-bond direction in DCOOH. These coupling constants and the deuterium quadrupole coupling in NH2D are compared with the corresponding force constants and the results are in fairly good agreement with the theory of Salem.
Calculations on Rydberg Terms of the Beryllium Atom
Authors: Lin T. F. , Duncan A. B. F.
Published: February 2004

Terms of P1,3 and S1,3 Rydberg series of Be are calculated up to n = 10. An effective Hamiltonian which contains two-electron operators for the core electrons is used. Trial Rydberg functions are used in a linear variation method, and the functions are optimized with respect to orbital exponents. The agreement with experimental term values is very encouraging. Penetration and exchange energies are calculated independently from the final Rydberg functions and are discussed for the different series.
Orbitally Nondegenerate Iron(III) Dithiocarbamates. I. Mössbauer Relaxation Phenomena
Authors: Wickman H. H. , Wagner C. F.
Published: February 2004

Mössbauer investigations of (i) bis(N,N-diisopropyldithiocarbamato) iron (III) chloride, (ii) bis, (NN-diethyldithiocarbamato) iron (III) bromide, (iii) iron (III) trispyrrolidinedithiocarbamate, and (iv) bis(NN-diethyldithiocarbamato) iron (III) chloride are discussed. Compounds (i)–(iii) are paramagnetic. Their Mössbauer spectra show the presence of “nonadiabatic” relaxation effects which require for their interpretation the recent theories of Blume and Tjon, or Gabriel. For these compounds a single phenomenological relaxation rate constant C, representing electronic relaxation, was sufficient to characterize the temperature-dependent features in the spectra. The approximation of a single relaxation rate C is rigorously valid only in a polarized sample. The error involved in use of the method in paramagnetic samples is estimated from experiments in ferromagnetic compound (iv), where the value of C consistent with data for temperatures below TC (2.43°K) was found to be roughly one-third of the relaxation rate derived from data at higher temperatures.