bluuebunny/crossref_metadata_2025 huggingface dataset:
Studies of Chemical-Shift Anisotropy in Liquid Crystal Solvents. III. The NMR Spectra of Nematic Solutions of Ethane and 1,1,1-Trifluoroethane
Authors: Silverman David N. , Dailey Benjamin P.
Published: February 2004

The proton and fluorine NMR spectra of ethane and 1,1,1-trifluoroethane have been observed in the nematic liquid crystal solvent 4,4′-di-n-hexyloxyazoxybenzene. High pressure NMR techniques were used to achieve the necessary concentration of the gases in the nematic phase. Relative internuclear distances in these compounds have been determined from ratios of the dipolar coupling constants. A positive value for the absolute sign of the scalar coupling J(CH3,CH3) has been determined from the spectrum of oriented ethane. A positive J(CF3,CH3) in CH3CF3 gave internuclear distances in best agreement with microwave data. The proton chemical-shift anisotropies are found to be small; that of ethane is essentially indistinguishable from zero. Calculations of the fluorine shielding anisotropies of five compounds in terms of primary and secondary shielding indicates that double bond character in the C–F bond is of some importance in determining the anisotropy of the fluorine chemical shift.
Microwave Spectra of Nitrogen-Containing Molecules. IV. Conformation, Dipole Moment, and Quadrupole Coupling Constants of Cyclopropylamine
Authors: Hendricksen David K. , Harmony Marlin D.
Published: February 2004

The microwave spectra of three isotopic species of cyclopropylamine have been investigated. The A, B, and C rotational constants in megahertz for C3H5NH2 are, respectively, 16269.95, 6723.00, and 5795.33; for C3H5NHD they are 15957.12, 6382.01, and 5504.06; and for C3H5ND2 they are 15592.62, 6091.39, and 5246.56. Quadrupole coupling constants have been measured and have the values χaa = 2.288, χbb = 1.841, and χcc = −4.129 MHz for the normal isotopic species. The dipole-moment components are found to be μa = 0.43 D and μc = 1.11 D which lead to a total moment of 1.19 D. The molecular structure of cyclopropylamine has been discussed, and in addition to other structural parameters, we have found C–C = 1.520, C–N = 1.428, and amino H···H = 1.631 Å.
Presscure-Induced a″ 1Σg+ ← X 1Σg+ Absorption in the Vacuum Ultraviolet Spectrum of Molecular Nitrogen
Authors: Lutz Barry L.
Published: February 2004

The a″ 1Σg+–X 1Σg+ absorption band of N2, which has been reported recently in a preliminary communication, has been investigated at nitrogen pressures from 10 to 80 torr. A study of the absolute absorption coefficient and of its pressure dependence shows definitely that the observed band arises as a pressure-induced dipole transition at the quoted pressures. The integrated absorption represents an absolute oscillator strength of f / p = 3.2 × 10−8torr−1. The large bandwidth (400 cm−1) is attributed to the short duration of the dipole-inducing collisions: 1.3 × 10−14 sec on the average. Thus the observed intensity can be interpreted in terms of an average oscillator strength of f = 0.04 per molecule per collision. The apparent optical quadrupole absorption f value of less than 10−7 corresponds to a very small quadrupole matric element compared to the one which determines the electron excitation cross section. A search for the a″–X quadrupole emission band in the electron-beam-excited spectrum at very low pressures has resulted in strong evidence for the presence of the 0–0 band of this transition, and tentative assignments of O, Q, and S lines yield approximate values of υ0 = 99 005 cm−1 and B0′ = 1.85 ± 0.15 cm−1.
Hydrogen Motions in Liquid Hydrogen Fluoride
Authors: Ring J. W. , Egelstaff P. A.
Published: February 2004

The inelastic scattering of cold neutrons by liquid and solid HF has been used to study the low-frequency modes of motion. On melting a pronounced reduction in amplitude and increase in width is observed in the near-inelastic spectra. Analysis of the angular dependence of this effect suggests that it is due to rotational rather than translational modes of motion. In this event the rotational diffusion constant is ≈ 1 × 1012 sec−1. Additional evidence is given to support the idea of hydrogen-bonded polymers in the liquid state of HF.
Inelastic Cold Neutron Scattering from Different Forms of Silica
Authors: Leadbetter A. J.
Published: February 2004

Coherent inelastic cold neutron scattering experiments have been carried out at room temperature on polycrystalline quartz, on three polycrystalline cristobalite specimens of different degrees of crystal perfection, and on vitreous silica. The results for quartz show marked coherence effects, and it was possible to derive the form of the directionally averaged lowest-energy acoustic branch of the spectrum. Coherence effects at higher energies are too complex to be interpreted in detail, but the results show the presence of a maximum in the vibrational frequency distribution [G(ω)] at ≈ 130 cm−1 and a broad maximum extending from ≈ 250–500 cm−1. These results are in excellent agreement with recent neutron scattering experiments on a single crystal and detailed calculations of the dispersion relations for a model of quartz. For cristobalite, the scattering was shown to be largely independent of crystal perfection so that the results obtained are characteristic of perfect cristobalite. The form of the directionally averaged lowest-energy acoustic branch of the spectrum was determined and found to be very flat: the top of the branch is centred around 40 cm−1 with a width of about 20 cm−1. The approximate form of a second average acoustic branch was determined and found to have a maximum frequency of about 85 cm−1 and a width of 20–30 cm−1. Coherence effects at higher energies are not amenable to simple interpretation, but maxima in G(ω) at around 150 cm−1 and between 200–500 cm−1 are indicated. The differential cross section for vitreous silica is almost identical with that for cristobalite for ω > 50 cm−1 and is significantly different from that for quartz. At lower energies, the pronounced peaks in the spectrum for cristobalite, arising from the low-energy acoustic branch, are smeared out into an ill-defined shoulder for vitreous silica, but there is a significant cross section in the glass extending down to < 20 cm−1. These results suggest that longitudinal phonons exist in the glass, but that the transverse phonon lifetimes are so short as to make a phononlike description of these excitations of somewhat doubtful utility. The over-all vibrational frequency distribution for vitreous silica appears to be very similar to that for cristobalite except that because of the broadening of the low-energy acoustic branch in the glass, the sharp peak in G(ω) for cristobalite at 40 cm−1 is much broadened in vitreous silica. These results provide an explanation for the unusual low temperature heat capacity of vitreous silica. They show that, except for the lowest temperatures (T<̃5°K), where localized defect modes are important, the unusually high heat capacity of vitreous silica is due to the presence of a highly disperse and much broadened (transverse) acoustic branch of the spectrum very similar to that observed in cristobalite.
Dynamics of Fluctuations in Nematic Liquid Crystals
Authors:   Groupe d'Etude des Cristaux Liquides (Orsay)
Published: February 2004

In a nematic system, the molecular axes tend to be aligned parallel to each other, but there are some fluctuations in the alignment. They give rise to a strong scattering of light. By a frequency analysis of the outgoing light, one can measure the power spectrum of these fluctuations, i.e., their dynamical behavior. The small motions of an incompressible nematic liquid crystal with coupled flows and molecular rotations are analyzed here in terms of the Leslie equations. The structure of the modes depends essentially on the parameter μ = Kρη−2, where K is an average elastic constant, ρ is the density, and η is an average viscosity. In the most usual case of small μ values the modes are purely dissipative. For a given wave vector q and polarization α there is one slow mode describing the relaxation of a twisted nematic configuration and a fast mode where the molecules exert no torque on the hydrodynamic motion. The light-scattering data should measure the relaxation frequency of the slow mode: from such data taken at different q values the six friction coefficients introduced by Leslie may in principle be determined.
Effects of Methyl and Hydroxyl Substitution on Barriers to Internal Rotation about Single Bonds
Authors: Lowe John P.
Published: February 2004

The electrostatic model for threefold barriers to internal rotation developed in earlier papers is updated. The barrier in methyl arsine is predicted to be 1.57 kcal/mole. It is also shown that the barrier recently measured for nitrosomethane is in fair agreement with the model result. The effect upon the barrier of methyl substitution in ethanelike molecules is shown to correlate well with the electronegativity of the central atom where substitution occurs. The very different barrier increases caused by methyl substitution in propene and methyl ketene are reproduced and shown to be simply related to structure. An explanation is proposed for the experimentally observed lowering of the barrier in going from acetaldehyde to acetone. The very low barrier in acetic acid is shown to be consistent with delocalization of hydroxyl pi electrons. A low barrier for ethanol is not supported by the model.
Magnetic Susceptibility of the H2O Molecule
Authors: Taft Harrison , Dailey Benjamin P.
Published: February 2004

High-resolution microwave spectroscopy has been used to determine the individual diagonal components of the magnetic-susceptibility tensor of H2O. The results are: χxx = − 12.1 ± 1.6, χyy = − 13.5 ± 2.0, χzz = − 13.7 ± 1.8, all in units of 10−6 emu. The large uncertainties in these numbers arise from the small anisotropy in the magnetic susceptibility. Only a rough value of the molecular quadrupole moment can be derived from these data.
Isotopic Distributions in Exothermic Ion–Molecule Reactions. A Simple Model
Authors: Light J. C. , Chan S.
Published: February 2004

A simple model of exothermic ion–molecule reaction is proposed in which the isotopic distribution of products is due to the separations of the center of charge from the center of mass of the diatomic reactant. Applications to the energy dependence of the isotope ratio of products of the reactions Ar + HD+ and Kr + HD+ indicate that this factor may be predominant in the energy range of 1–10 eV (center of mass).
Molecular Beam Study of the Hydrogen Iodide Reaction
Authors: Jaffe S. B. , Anderson J. B.
Published: February 2004

The results of experiments to determine reaction cross sections for the formation of HD in energetic collisions of HI with DI are reported. Molecular beams of HI, accelerated to energies of 40–215 kcal/mole by expansion from nozzles in mixtures with hydrogen or helium, were passed through a reaction chamber containing DI at low density. A mass-spectrometer monitoring the gases effusing from the chamber was used to detect the HD reaction product. In the range of collision energies investigated, at approximately one-half to two and one-half times the activation energy of 44 kcal/mole, no HD attributable to reaction was detected. With HD formation assumed to be that corresponding to the lower limit of detection, the upper limit to reaction cross section is determined to be 0.04 Å2. Comparison is made with several models for reaction with differing energy requirements. The results indicate that rotational or vibrational excitation of colliding partners is required for an appreciable probability of reaction upon collision.
Kerr Constant and Optical Anisotropy of Polymeric Chains
Authors: Nagai Kazuo
Published: February 2004

Theories of the Kerr constant Km and the optical anisotropy 〈(Δγ)2〉 of polymer molecules are developed on the basis of the additivity principle of bond polarizabilities and the rotational-isomeric-state approximation for rotations about skeletal bonds. Expressions are derived for both (i) polymer chains of finite length but of an arbitrary sequence of bonds and (ii) stereo-regular (or multirepeat) polymers of infinite length. The close similarity between Km and 〈(Δγ)2〉, and of them with the optical anisotropy ΔΓ of Kuhn's random link, is pointed out.
Diffraction Pattern and Structure of the System Tetra-n-butylammonium Fluoride–Water at 25°C
Authors: Narten A. H. , Lindenbaum S.
Published: February 2004

The diffraction pattern of a concentrated solution of tetra-n-butylammonium fluoride in water (Bu4NF·41H2O) has been measured and analyzed at 25°C. The radial distribution function for the solution is surprisingly similar to that of pure water at the same temperature. Each water molecule has ∼ 3.8 neighbors at an average distance of 2.80 Å, compared to ∼ 4.4 neighbors at 2.85 Å in pure water. Intensity and radial distribution functions have been calculated for two different models. The respective models assume modified ice-I and gas hydrate structures, surrounded by a random distance distribution, as an adequate description for the average short-range order in the solution. In both models the F− ions and the N+ atoms of the cations are part of a hydrogen-bonded network, while the butyl chains of the cations are located inside the cavities which, although of different size and shape, are typical of both structures. The ice-I model describes the x-ray data of the solution and of pure water, and its parameters are related to bulk thermodynamic properties in a physically reasonable way. The gas hydrate model is incompatible with the experimental radial distribution function of liquid water, and the parameters of this model for the solution do not explain the large enthalpy and entropy of fusion of the solid hydrate.
Vibrational Assignments and Potential Constants for cis- and trans-1,2-Difluoroethylenes and Their Deuterated Modifications
Authors: Craig Norman C. , Overend John
Published: February 2004

Infrared and Raman spectra and a complete assignment of vibrational fundamentals are presented for cis- and trans-1,2-difluoroethylenes and their deuterated modifications. For cis-CFHCFH the fundamentals are: (a1) 3122, 1716, 1263, 1015, 237; (a2) 839, 495; (b1) 3136, 1374, 1130, 769; and (b2) 756 cm−1. For trans-CFHCFH the fundamentals are: (ag) 3111, 1694, 1286, 1123, 548; (au) 875, 329; (bg) 788; and (bu) 3114, 1274, 1159, 341 cm−1. The near degeneracy in ν7 and ν12 of the trans isomer is shown from low-temperature crystal and matrix studies and zero-order normal coordinate calculations. Analysis of the rotational structure in the region of the overlapped bands proves that these fundamentals are strongly perturbed by Coriolis coupling. From the vibrational fundamentals and a previous measurement of ΔH°650, the cis isomer is shown to have 1086 cal/mole less electronic energy than the trans isomer. For the in-plane vibrations refined Urey–Bradley potential constants are presented. These constants are obtained from an overlay calculation involving the full set of six molecules. For the out-of-plane vibrations, refined general valence constants, obtained for the separate isomers, are presented.
Electric Fields in Thin-Film Dielectrics on Metal Surfaces
Authors: Fromhold A. T.
Published: February 2004

Electrostatic fields in oxides and similar compounds formed on metals by thermal reaction with ambient gases are deduced for the case in which rate is determined by charged-particle diffusion through the compound. Specifically, the limiting case where the macroscopic electrostatic potential leads to a condition approximating local charge neutrality is examined. The position-dependent electric field in the compound leads to an electrostatic potential across the layer which is independent of thickness.
Nuclear Electric Field Gradient at the Iron Sites in Ca2Fe2O5 and Ca2FeAlO5
Authors: Grant R. W.
Published: February 2004

Mössbauer spectroscopy has been used to determine the nuclear electric-field-gradient (efg) tensor parameters at the Fe3+ sites in Ca2Fe2O5 and Ca2FeAlO5. In all cases to within experimental error, the efg is axially symmetric and the z principal axis of the efg is parallel to the crystallographic b axis. Analytic expressions are developed for the relative transition probability of M1 radiation connecting I = 32 to I = 12 nuclear states for the situation in which the z principal axis of an axially symmetric efg is perpendicular to an internal magnetic field.
Electronic Raman Spectra. V. Theory for the Electronic Raman Transitions of PrCl3
Authors: Koningstein J. A.
Published: February 2004

A calculation has been performed of the electronic Raman spectrum of PrCl3. It has been found that antisymmetry plays an important role in many of the H43 intermanifold transitions and the H43–H53 transitions. The frequency dependence of the antisymmetry has also been investigated and it turns out that a rather pronounced dependence exists. Transitions to the F23 levels are all related to a symmetric scattering tensor, but some of the elements of this tensor are zero. In order to predict the selection rules of the electronic Raman transitions, use has been made of the eigenvectors of the crystal-field states and a 3j symbol. The effect of the crystal field goes via these properties, and direct information on the value of the nonvanishing elements and also on the vanishing part of the tensor is obtained.
Use of an Analog Computer for the Study of Chemical Reaction Dynamics
Authors: Ottinger Ch.
Published: February 2004

A classical model calculation is performed of the dynamics of reactions of cesium atoms with alkyl iodides ranging from CH3I through C4H9I. The integration of the five coupled equations of motion is performed on an EAI 680 analog computer. This approach is attractive because of its simplicity and speed. A linear collision model was employed to study the energy partitioning between the final degrees of freedom. The results indicate that the C–C stretching vibrations do not significantly participate in the partitioning while it can be inferred that slow bending vibrations and rotations probably do. The linear model is therefore insufficient to explain recent experimental results on the energy disposal. Three-dimensional analog computer calculations for smaller systems seem quite feasible and promising.
Oxygen-17 Quadrupole Coupling Parameters for Water in Its Various Phases
Authors: Spiess H. W. , Garrett B. B. , Sheline R. K. , Rabideau S. W.
Published: February 2004

Analysis of 17O NMR data in D217O ice yields a quadrupole coupling constant, e2qQ / h = 6.66 ± 0.10MHz, and an asymmetry parameter, η = 0.935 ± 0.01. Comparison with the data for liquid and gas indicates a temperature dependence of the 17O quadrupole coupling constant in the liquid and explains the unusual linewidth behavior for liquid D217O. Charge distributions in the water molecule in both the solid and gaseous states have been obtained by a valence-shell atomic-orbital population analysis based on the experimental data for the 17O quadrupole coupling parameters and the molecular dipole moment. The results were used to calculate the quadrupole coupling constant for the deuterium nucleus. Good agreement with experimental data was obtained for both the solid and gaseous states. Comparison of the calculations for the two phases suggests that the intermolecular effects in ice are mostly electrostatic.
General Equation for the Surface Tension of the Lattice Gas
Authors: Woodbury George W.
Published: February 2004

A general expression for the surface tension of a lattice gas is derived. The equation is γA / kT = 〈η〉g − 〈η〉1, where γ is the surface tension, A is the surface area, η is related to the eigenvector corresponding to the gas phase, and 〈 〉g and 〈 〉1 are averages performed in the bulk gas and bulk liquid phase, respectively. The derivation, which incorporates rigorously defined local thermodynamic functions, is similar in some ways to the Cahn–Hilliard development. Numerical results are obtained by applying the Bragg–Williams approximation to the general equation.
Intensity Measurements in the B3Πu+ − X 1Σ+g System of I2
Authors: Chutjian Ara , James Thomas C.
Published: February 2004

Equivalent width measurements were made on rotational lines in six vibrational bands in the B–X system of I2. All measurements were made photoelectrically. Calculations of the Doppler, natural, and collisional widths show the Doppler width to be 340 times larger than the combined natural and collisional width at 22°C. Measurement of the equivalent width at several path lengths for each line confirmed a pure Doppler curve of growth for the lines. From known spectroscopic data the line strengths gave a value for the square of the electronic contribution to the transition moment, and for the value of the total Einstein spontaneous emission rate from each υ′ to all υ″ with nonzero Franck–Condon factors. A comparison of these purely radiative decay rates with total decay rates measured by Chutjian, Link, and Brewer and by Davis and Grimes shows the presence of a significant amount of spontaneous predissociation of the B state to ground-state P3/22 iodine atoms. In the absence of collisions approximately 66% of the molecules which enter the B state predissociate, while only 34% radiate to the X state. In the presence of collisions, at 0.238-torr I2 pressure, approximately 20% spontaneously predissociate, 10% radiate to the X state, and 70% leave the B state by a process or processes which are nonradiative and collision dependent. Measurements were also carried out on the 25–0 band in the presence of a 15-kG magnetic field. No change in the equivalent width was observed in the presence of the field, indicating that the coupling to the repulsive state causing magnetically induced predissociation is insufficient to change the f value of the rotational transition.
Light Scattering by Orientational Fluctuations in Liquids
Authors: Ben-Reuven A. , Gershon N. D.
Published: February 2004

Tensor-algebra techniques are used to derive expressions similar to those of Pecora and Steele for light scattering by liquids of anisotropic molecues, in a more compact form. The correlation functions are cast into a form in which their geometrical meaning is made obvious and is more immediately applicable to liquid models. A generalization of the Kirkwood g factor to second-rank tensors is defined. The theory is applied to the depolarized Rayleigh wings. It is argued on the basis of intensity and half-width measurements that the diffuse line, which appears in the spectrum of benzene and several benzene derivatives, originates from reorientation of single molecules. This conclusion fits into a broader scheme incorporating results of dielectric, NMR, and viscosity measurements. It is suggested to relate the background, together with the narrow hole in the middle of the depolarized spectrum, to couplings of orientational fluctuations with shear waves.
Effusion. V. Angular Number Distributions of Gaseous CsCl from a Conical Orifice into Vacuum
Authors: Adams J. Q. , Phipps T. E. , Wahlbeck P. G.
Published: February 2004

Angular number distributions of CsCl(g) effusing from a divergent right conical orifice, located in the wall of an effusion cell, have been measured as a function of the state of the gas. The orifice had a circular gas opening with a diameter (dS) of 0.3239 mm, a length of 9.524 mm, and a half-angle of 15.93°. The effusion cell was constructed from oxygen-free, high-conductivity copper. Fifteen angular number distributions were measured with a range of Knudsen numbers (ratio of mean free path to orifice diameter dS) between 19.8 to 0.457. The angular number distribution data at large Knudsen numbers (molecular flow region) did not agree with theoretical angular number distributions. The fluctuation phenomenon, observed in similar experiments at low pressures in the molecular flow region with a right-circular cylindrical orifice, was observed also with this conical orifice. As with long cylindrical orifices, a decrease in Knudsen number in the transition region decreased the probability of effusion in the forward direction.
Limiting Laws and Counterion Condensation in Polyelectrolyte Solutions II. Self-Diffusion of the Small Ions
Authors: Manning Gerald S.
Published: February 2004

The concepts developed in Part I of this series are applied to the self-diffusion of counterions and co-ions. The condensed counterions are assumed to have no mobility, while the uncondensed counterions and all co-ions execute Brownian motion subject to the locally inhomogeneous electric field created by the polyelectrolyte molecules. The polyelectrolyte field is assumed to have small amplitude, in analogy to the Debye–Hückel treatment of uncondensed mobile ions in Part I. The self-diffusion coefficients of the counterion and the co-ion are both less than the coresponding values in the absence of polyelectrolyte, but the coefficient for the counterion is much less than that for the co-ion. The results for the self-diffusion coefficient of the counterion in a salt-free polyelectrolyte solution are in good agreement with experimental data.
Infrared Spectra of Carbon Monoxide in Zeolites
Authors: Fenelon P. J. , Rubalcava H. E.
Published: February 2004

The infrared spectra of 12C16O and of a mixture of 12C16O and 13C16O in the zeolite NaA show bands that have been assigned as R-, Q-, and P-branch maxima of a relatively free rotor. This conclusion depends, partly, on small but experimentally distinguishable differences in the isotope shift of the bands of a pure vibrator and those of a vibrotor. The spectrum of carbon monoxide in the zeolite NaX is similar to that in NaA. However, in CaA, and in CaX, where it is known that CO is more strongly adsorbed than in the sodium zeolites, the bands have a different structure, corresponding to a strongly hindered rotor.
Scattering of High-Velocity Neutral Particles. XVI. Ar–Ar, Ar–He, and Ar–H2
Authors: Colgate S. O. , Jordan J. E. , Amdur I. , Mason E. A.
Published: February 2004

Measurements are reported for the scattering of argon beams with energies between about 200–1500 eV by room-temperature argon, helium, and hydrogen. Two separate investigations in essentially the same apparatus are involved, and interaction potentials are derived for Ar–Ar and Ar–He at internuclear distances where scattering measurements have not previously been made. For Ar–H2 there are no previous direct scattering results. The Ar–Ar system was studied in both investigations, and the combined results are represented by V(r) = 3.25 × 103 / r9.97eV, 1.96 Å < r < 2.41 Å. There is good agreement between this result and potentials from other scattering measurements and shock compression of liquid argon. One series of new measurements was made for Ar–He. The derived potential is V(r) = 22.2 / r5.42eV, 1.40 Å < r < 1.81 Å, which agrees well with other scattering results and with measurements of mutual diffusion coefficients at elevated temperatures. A combination of He–He potentials obtained from scattering with the present Ar–Ar results leads to a derived He–Ar potential which is in good agreement with that obtained from the direct measurements. One series of measurements was made for Ar–H2, and the resulting potential is V(r) = 159 / r6.28eV, 1.81 Å < r < 2.36 Å, which is about a factor of 2 higher than that derived from previous measurements on He–He, He–H2, and Ar–Ar. It is also higher by about the same amount than the Ar–H2 potential derived from other scattering measurements on Ar–Ar and H2–H2. Mutual diffusion coefficients for Ar–H2 at elevated temperatures also suggest that this potential is high. The present potential for Ar–Ar agrees well with theoretical calculations based on a delta-function model and a statistical (Thomas–Fermi–Dirac) model. The Ar–He potential also agrees with calculated potentials based on these models and with that obtained from ab initio Hartree–Fock calculations.
EPR of Cr(III) in a Single Crystal of NaMgAl(C2O4)3·9H2O
Authors: Bernheim R. A. , Reichenbecher E. F.
Published: February 2004

The electron paramagnetic resonance of Cr(III) substituted for Al in a single crystal of NaMgAl(C2O4)3·9H2O was examined at two different microwave frequencies. The Cr(C2O4)33− ion was found to be nonaxially symmetric in this host crystal and to occupy six nonequivalent positions in the unit cell. The distortion from axial symmetry is caused by the presence of one molecule of water of hydration per Cr(C2O4)33−. When gently heated the crystal loses one water molecule per Cr(C2O4)33− ion, and the EPR spectrum becomes that of an axially symmetric species. Upon being exposed to water vapor the heated crystal slowly absorbs water, and the EPR spectrum of the original nonaxial species is regenerated. The spin-Hamiltonian parameters for the two cases were determined. For the nonaxially symmetric, unheated crystal gzz = 1.9780, gxx = 1.9822, gyy = 1.9812, D = 0.7786 cm−1 and E = 0.0306 cm−1. For the axially symmetric, heated crystal, g‖ = 1.978, g⊥ = 1.980, and D = 0.7783 cm−1. The 53Cr hyperfine interaction for the heated crystal displayed a possible anisotropy with | A | = (15.7 ± 0.2) × 10−4cm−1 and | B | = (16.1 ± 0.3) × 10−4cm−1. The EPR spectrum of a Mn2+ impurity, presumably present as Mn(H2O)62+, could be observed to occupy two different sites.
Shear-Rate Dependence of the Intrinsic Viscosity of Flexible Linear Macromolecules. II. Solvent Effect
Authors: Suzuki Hidematsu , Kotaka Tadao , Inagaki Hiroshi
Published: February 2004

Measurements of the shear-rate dependence of the intrinsic viscosity [η] were made on polystyrenes in several solvents, each of which differs in solvent power and solvent viscosity. Low-shear capillary viscometers of the Maron–Belner type were used: shear stresses can be varied continuously from about 10 to a few tenths of a dyn/cm2, which is low enough to measure the zero-shear intrinsic viscosity [η]0. The effect of excluded volume on the behavior of [η] / [η]0-vs-β (the generalized shear-rate) curve was examined. The curves at and near the θ condition resemble that of Saito–Scheraga's prolate ellipsoid with axial ratio p = 2, and those in good solvents resemble that of p = 3 ellipsoid; the variation with solvent power is not so large as the theory of either Fixman or Chikahisa predicts. In the region of moderate to large β, the [η] / [η]0-vs-β curve decreases continuously with increasing β, as contrasted to the behavior of the ellipsoid, which levels off rather rapidly. The non-Newtonian behavior is practically unaffected by solvent viscosity. In addition, the effect of polydipersity was discussed briefly.
Kinetics of Nitrogen Dioxide Fluorescence
Authors: Schwartz Stephen E. , Johnston Harold S.
Published: February 2004

The fluorescence lifetime and intensity of gas-phase nitrogen dioxide (2B1) have been measured as a function of excitation wavelength, fluorescence wavelength, and pressure (0.5–50 mtorr). The phase-shift method was used; this technique allows lifetime measurements to be obtained with signal intensities of 100 counts per sec and lower. The excitation source, tunable throughout the visible region, had a half-width bandpass as low as 15 Å. Fluorescence wavelength separation was accomplished with 15 interference filters between 400 and 8000 Å. The radiative lifetimes range from 55 to 90 μsec for excitation from 3980 to 6000 Å and tend to increase with excitation wavelength; however, the lifetimes exhibit considerable variation within a narrow excitation region. The fluorescence sample was contained in a 33-cm-diam spherical bulb; apparent fluorescence lifetimes in smaller cells were reduced because of migration of excited molecules (under collision-free conditions) and wall quenching. In order that the measured lifetimes exhibit no more than 5% error, observation must be extended beyond the excitation region to a distance five times the product of the lifetime by the most probable velocity. The Stern–Volmer analysis of fluorescence kinetics has been generalized to a multilevel system under conditions of modulated excitation and phase-sensitive detection. Analysis of fluorescence data in terms of this mechanism yields the product of energy-transfer rate constants times efficiencies (amount of energy lost by the excited polyatomic molecule per effective collision). This analysis implies the loss of at least one vibrational quantum (average value 1230 cm−1) per gas-kinetic collision over the entire range from 12 500–25 000 cm−1 energy above the ground state. This result indicates that molecules in the B12 excited electronic state are rapidly interconverted to the high vibrational levels of the ground electronic state.
Raman Spectrum of α-N2
Authors: Brith M. , Ron A. , Schnepp O.
Published: February 2004

The Raman spectrum of solid α-N2 has been investigated between 12°–35.5°K. Two lines were observed at 31.5 and at 35.8 cm−1, with intensity ratio 3.6 in favor of the lower-frequency line. A calculation of the scattering intensities was carried out assuming additivity of molecular polarizabilities. Consideration of relative intensities led to the assignment of the three predicted Raman-active librational lattice vibrations, assuming a close coincidence of two of these modes. The assignment is supported by an independent frequency calculation.
PVT Properties of Methane and Propene to 10 kbar and 200°C
Authors: Robertson Stanley L. , Babb Stanley E.
Published: February 2004

Isotherms of CH4 and C3H6 have been determined between about 1 and 10 kbar at temperatures of 35°, 100°, and 200°C. The results are reported.
Self-Consistent Calculation of Dynamic Polarizability and Excited p-State Wavefunctions of Two-Electron Atom and Ions
Authors: Mukherjee P. K. , Sengupta S. , Mukherji A.
Published: February 2004

Dynamic polarizability of He and isoelectronic ions has been calculated by a self-consistent variation–perturbation method. An interpolation relation has been obtained which may be used to estimate the polarizability of ions in actual crystals under the influence of a radiation field. Representations of excited p states have been obtained. They compare favorably with more accurate calculations available. The advantage of the present method lies in its simplicity. It can be easily extended to atoms containing many electrons.
Frequency-Swept Electron–Electron Double Resonance: DPPH in Liquid and Frozen Solution
Authors: Hyde James S. , Sneed Robert C. , Rist Günther H.
Published: February 2004

Instrumentation for frequency-swept electron–electron double resonance (ELDOR) at X band is described. Reliable control of the sweeping (pump) microwave frequency to 1 part in 106 (10 kHz) has been achieved. The technique has been applied to liquid and frozen solutions of DPPH in toluene. The isotropic nitrogen coupling constants are Aα = 7.933 ± 0.007 and Aβ = 9.739 ± 0.014 G including second-order corrections. The ratio Aα / Aβ = 0.815. In the solid phase the largest hyperfine coupling to the α-nitrogen | A + 2B |α and the largest hyperfine coupling to the β-nitrogen | A + 2B |β are each 20.1 G. The literature on this molecule is reviewed and the present experiments discussed in this context. Much of the paper is devoted to the physical basis of ELDOR spectroscopy. An analysis of ELDOR line shapes is given in an appendix.
ESR Spectra of ZnSiF6·6H2O:Ni2+ in a Parallel and Perpendicular Configuration
Authors: Fritz I. J. , Yarmus Leonard
Published: February 2004

The ESR spectra of ZnSiF6·6H2O:Ni2+ obtained with rf and dc magnetic fields parallel as well as perpendicular are studied for several crystal orientations. Experimental line positions and relative intensities are compared with those calculated by a numerical diagonalization of the spin Hamiltonian. The parameters of the spin Hamiltonian are found to be D = 1838 ± 15 G and g = 2.23 ± 0.01, with g isotropic.
Location of Energy Barriers. II. Correlation with Barrier Height
Authors: Mok M. H. , Polanyi J. C.
Published: February 2004

In Paper I of this series a hypothetical potential-energy surface was used in order to examine the effect on the dynamics of exchange reactions A + BC→AB + C of moving the energy barrier from an “early” to a “late” position along the reaction coordinate (i.e., from the entry valley to the exit valley of the energy surface). In the present work an attempt has been made to correlate barrier location with other properties of the energy surface, as a step toward the application of the generalizations of Paper I to real cases. Related families of reactions have been examined in the London–Eyring–Polanyi–Sato (LEPS) and bond-energy bond-order (BEBO) approximations. The principal generalizations may be summarized as follows: (1) For substantially exothermic reactions the barrier is in the entry valley, and for substantially endothermic reactions the barrier is in the exit valley. In the light of Paper I this implies that the cross sections for these exothermic reactions will rise most steeply with increasing translational energy in the reagents, whereas the cross sections for the endothermic reactions will rise most steeply with increasing vibrational energy in the bond under attack. (2) For decreasing barrier height in related exothermic reactions the barrier moves to successively earlier positions along the entry valley, with increasing percentage “attractive energy release.” (3) In general [i.e., if (4), below, is obeyed even qualitatively], for increasing barrier height in related endothermic reactions the barrier moves to successively later positions along the exit valley. (4) The correlation between the classical barrier height EC and reaction energy qc, written ΔEC = −α Δqc by Ogg and Polanyi (OP) was found to be a weaker correlation than (2), above. For families where barrier height did decrease with increasing reaction energy, a logarithmic relationship, Δ logEC = −αl Δqc, was preferable for the exothermic reactions. The same relationship encompassed members of the family with qc < 0 (endothermic reactions). With the proviso that (4) shall be applicable, (1)–(3) are summarized in the proposition that the barrier moves to successively later positions along the reaction coordinate with increasing barrier height. This progressive shift of the barrier is embodied in the following approximate relationships: Δ logEC = −β1lΔr1‡,0, Δ log r2‡,0 = −aΔr1‡,0 + cΔ[(r1‡,0)−1], where r1‡,0 and r2‡,0 are the extensions of AB and BC, from their equilibrium separation, at the crest of the barrier. In view of (4), Δqc = (β1l / αl)Δr1‡,0, a relationship which encompasses both exothermic and endothermic reactions. The fraction of attractive energy release alters from one exothermic reaction to the next by approximately Δ(α⊥) = γ1Δr1‡,0; hence, Δ logEC = −(β1l / γ1) Δ (α⊥).
Infrared Spectra of MnO4− Ion in Alkali Halide Matrices
Authors: Manzelli P. , Taddei G.
Published: February 2004

The 300–1000-cm−1 spectra of KMnO4 embedded in KCl, RbCl, KBr, and RbBr pellets have been recorded at about 40°C and at liquid-nitrogen temperature in the concentration range 1/100 to 1/6000 molar ratios. In the chloride pellets, the fundamental modes of permanganate show many components which obey the selection rules of the unit cell of crystalline salt. Very sharp single bands of isolated tetrahedral MnO4− ion occur in the spectra of bromide pellets, superimposed on the bands of crystalline permanganate. The formation of solid substitutional solutions of MnO4− in alkali halide pellets is discussed in relation to size and symmetry of the occupied site of the host lattice.
Collision Theory and Activated Complexes in Bimolecular Gas-Phase Reactions
Authors: Snider Neil S.
Published: February 2004

The quasiequilibrium hypothesis for bimolecular reactions, which, as Marcus has shown, provides a bridge between the transition-state theory and the scattering theory of these reactions, was investigated from the standpoint of scattering theory. A modified formulation of R-matrix theory was employed. Assumptions necessary in order to unambiguously define a state of the activated complex and the transmission coefficient for this state in scattering-theory terms were given and partially justified. Approximations to the transmission coefficient expression thus formulated were considered, and relations between these approximations and the Marcus assumptions of adiabaticity, statistical adiabaticity, and separable reaction coordinate motion were noted.
Impurity Effects on the Ionization States of F-Aggregate Color Centers in Sodium Fluoride
Authors: Chandra Ashoka
Published: February 2004

A study has been made of the F-aggregate color centers produced in NaF crystals by x irradiation at room temperature followed by various thermal and optical bleaching treatments. Crystals used were grown from Harshaw NaF. In addition to undoped samples, crystals were grown with NaOH and Na2O2 dopings. Important differences are observed in the doped samples in relation to the charge states of F-aggregate centers. Electron-deficient centers are stabilized, and the probability of their formation is enhanced in hydroxide-doped samples. In peroxide-doped samples, the “extra-electron centers” are favored. Model photochemical reactions are introduced which show the generation of “electron traps” in hydroxide-doped samples and provide excess electrons in peroxide-doped samples. The behavior of the doped samples can be equivalently explained on the band picture in terms of the shift of the Fermi level caused by the introduction of acceptor and donor levels by the doped impurities. The postulated oxidation–reduction effects are supported by observations of impurity effects on (1) relative concentrations of charged and neutral centers after x irradiation, (2) thermal and optical bleaching rates, and (3) reactions involving conversion among various charge states.
Modified WKB Method as Applied to the Three-Dimensional Bound-State Problem
Authors: Lu Pao
Published: February 2004

The modified WKB method applied to the bound-state problem as given by the author is extended to include several high-order terms. It is then applied to the single-electron-atom case. Here we take the three-dimensional harmonic-oscillator problem as the solved part and the single-electron-atom problem as the part to be solved. It is shown that the successive terms in this modified WKB approximation consist of the series expansion which gives the correct eigenvalue.
Consolidated Configuration-Interaction Perturbation Method: The Ground State of Beryllium
Authors: Barr Tery L. , Simpson William T.
Published: February 2004

A method is developed for consolidating the configuration-interaction method and Rayleigh–Schrödinger perturbation theory. The technique requires the existence of a more or less conventional unperturbed Hamiltonian but not its eigenfunctions. The method depends crucially upon a particular partitioning of terms among perturbed and unperturbed parts of the full Hamiltonian expressed in spectral representation, and leads to having some perturbation denominators significantly different from the Rayleigh–Schrödinger ones. The beryllium ground-state energy was calculated as a test. An hydrogeniclike basis was obtained by interacting the Shull–Löwdin discrete Laguerres over an appropriate one-electron Hamiltonian. A single screening constant was employed and a rather unique method was used to treat the (zero-order) degeneracy of the | 1s22s2〉 and | 1s22p2〉 functions. Some two-electron integrals were calculated by the method of Jones and Brooks and others by modifying the formula of Brown. The value − 14.659 a.u. was obtained for the ground-state energy. This compares well with values obtained by more involved techniques. An analysis of the results shows that the method can be considered as generating the SCF and correlation energies in the normal course.
Dipolar Coupling and Molecular Vibrations in Crystals. II. Rhombohedral Lattices
Authors: Frech Roger , Decius J. C.
Published: February 2004

The Kornfeld–Ewald method of dipole summation in a lattice is applied to rhombohedral crystals with both unimolecular and bimolecular unit cells. These sums are evaluated as a function of the rhombohedral unit cell angle α. It is shown that the local field (Coulomb field) breaks into two parts, the macroscopic ε field and an “inner field” which is due to a transverse polarization wave. With the addition of the macroscopic ε field to the inner field the local field is that appropriate to a longitudinal polarization wave. The specific macroscopic shape dependence of the specimen implied by the use of the Kornfeld–Ewald method is discussed.
Composition Dependence of the Thermal Diffusion Factor of Binary Gas Systems
Authors: Nain V. P. S. , Saxena S. C.
Published: February 2004

The thermal diffusion factor, αT, is measured as a function of composition for binary systems Ne–Ar, Ne–Xe, H2–He, H2–Xe, H2–O2, and H2–N2 in a glass two-bulb apparatus at an average temperature of about 318.6°K. The gas samples from the two bulbs are analyzed on a differential thermal conductivity analyzer within an accuracy of about ± 0.025%. The estimated uncertainty of our αT values is about ± 2%, and these are compared with the existing αT values as reported by other workers and with the predictions of Chapman–Enskog kinetic theory and modified Buckingham exp-6 potential. This simple theory is not able to explain the results on systems involving polyatomic gases, and a theory which should consider the details of inelastic collisions is necessary for reliable interpretation of such systems.
Fluorescence, Phosphorescence, and Triplet Formation in Biacetyl at Low Pressures
Authors: Parmenter C. S. , Poland H. M.
Published: February 2004

The S1 → T intersystem crossing in biacetyl vapor has been examined by direct observation of emission from both the initial and final states. When biacetyl is excited by absorption of 4358-Å radiation, both the singlet- and the triplet-emission yields are independent of pressure in the range 40–0.1 torr. Since isolated molecule conditions are established for singlet relaxation in the lower limit of this range, triplet formation must occur in biacetyl in the absence of collisional perturbations involving the excited singlet state. Triplet emission continues to be observable at pressures descending to 0.03 torr, but the emission yield decreases due to triplet deactivation at the walls. The results are discussed within the framework of recent theories of radiationless transitions.
Self-Consistent-Field Molecular Orbital Theory of the Chemical Bond. I. Formal Theory
Authors: Peters David
Published: February 2004

A practical method of computing the self-consistent-field (SCF) localized molecular orbital (LMO) of a simple chemical bond in a large molecule is given. The method does not require the previous determination of all the canonical delocalized MO's of the entire molecule. The work is a step towards the unification of chemical valence theory and rigorous molecular quantum mechanics, and it sets up rigorous definitions of the various concepts used in chemical valence theory. This involves the use of a new secular determinant and the use of Rayleigh–Schrödinger perturbation theory in conjunction with the SCF MO theory in a way which is consistent with chemical valence theory.
Nonorthogonal Natural Orbitals from Open-Shell Configuration-Interaction Calculations
Authors: Weltin E. , Heaton Marie
Published: February 2004

The natural orbitals can be defined as the set of one-particle functions in which the spin-free one-density matrix is diagonal with the occupation numbers as diagonal elements. For an open-shell approximate wavefunction based on nonorthogonal one-electron orbitals the one-particle overlap matrix S in general does not commute with the Hermitian matrix D in γ(1; 1′) = Σi,jφi(1)dijφj*(1′). In this case two consistent sets of natural orbitals, the conventional orthonormal and a nonorthogonal set, are obtained. If D and S commute, the two sets are shown to be identical and orthonormal. For an intermediate-state multiconfiguration wavefunction of CH+ it was found that the natural orbitals in both sets are clearly grouped into highly occupied orbitals and orbitals with low occupation numbers. Single configurations based only upon the highest occupied orbitals gave approximately the same energy (99.93% of the energy of the multiconfiguration wave function) in both sets. The total charge-density distribution, however, was approximated significantly closer by the single configuration in the nonorthogonal set.