bluuebunny/crossref_metadata_2025_split huggingface dataset:
NMR Study of Electron Transfer Rates and Spin Densities in p-Xylene and p-Diethylbenzene Anions
Authors: de Boer E. , MacLean C. 
Published: January 2005

Electron transfer reactions between a diamagnetic molecule and its paramagnetic anion modify the NMR spectrum in two different ways. First, the NMR lines are broadened and secondly, the lines are shifted. The line broadening may provide information on rates of electron exchange; the line shift is related to the spin density at the proton.
The magnitude and the sign of the spin densities are quantities of great interest from a quantum-chemical point of view. It is useful, therefore, to provide methods for measuring spin densities. Usually they are derived from hfs parameters in ESR psectra. In this paper it is shown how they may be detected by the NMR line shifts. A special advantage of the NMR method is that the sign of the spin density is also easily available, because it is directly related to the direction of the line shift.
Basic equations that describe both line broadening and line shifts are derived in this paper. The theory has been applied to p-xylene and p-diethylbenzene. Rates of electron transfer of p-xylene have been obtained from the broadening of the NMR lines. A combination of ESR and NMR has been used to obtain the values of the spin densities at the sites of the methyl and methylene protons; contrary to predictions in the literature, the sign is negative. An explanation for this is given on the basis of MO theory.
Studies of Hydroxyl Radical Kinetics by Quantitative ESR
Authors: Dixon-Lewis G. , Wilson W. E. , Westenberg A. A. 
Published: January 2005

Reactions of the OH radical with OH, H2, and CO have been studied at room temperature in a fast flow system at pressures of the order of 1 mm. The very fast reaction between H atoms and NO2 was used as the source of OH. Electron spin resonance techniques were employed to examine the decay of OH concentration along the flow tube, using NO gas as the calibration standard. Rate constants at 300°K were found to be, in cubic centimeters per mole·second units OH+OH=O+H2O,k2=1.55±0.12×1012,OH+H2=H2O+H,k4=3.9±0.2×109,OH+CO=CO2+H,k5=1.15±0.05×1011.
Combination of the last two of these with published results at higher temperatures leads to k4=3.9±0.2×109exp{[(5200±300)/R][(1/300)−(1/T)]},k5=1.15±0.05×1011exp{[(600±300)/R][(1/300)−(1/T)]},or in simple Arrhenius form k4=2.3×1013exp(−5200/RT),k5=3.1×1011exp(−600/RT).
Determination of Hydration Numbers of Cations in Aqueous Solution by Means of Proton NMR
Authors: Swift T. J. , Sayre W. G. 
Published: January 2005

A technique was developed for the determination of the primary hydration numbers of cations in aqueous solution. The method was devised to be applied in particular to the study of those ions for which the average lifetime of water molecules in the primary hydration sphere is less than ∼10−4 sec.
The new technique is based on the kinetics of exchange of water molecules with hydrated manganous ions in solution and makes use of the relationship between this exchange and the proton nuclear magnetic resonance relaxation time T2 for such solutions. This relaxation time was found to be dependent on the nature and concentrations of added diamagnetic salts in a manner which could be related to the primary hydration numbers of the added cations.
An empirical relationship was derived and tested as a function of concentration, temperature, anion and for cations of known primary hydration number, and the relationship was found to be satisfactory in all cases. The precision of the technique is estimated to be ±0.2 of a hydration unit.
The empirical relationship was derived and tested with the ions Al3+, Be2+, H+, and NH4+. Ions to which the technique was subsequently applied included Mg2+, Ca2+, Sr2+, Ba2+, Zn2+, Cd2+, Hg2+, and Pb2+. The rather low numbers found for these ions are interpreted as a reflection of the role of the solvent structure of water in the determination of the structures of hydrated ions.
Cluster Expansion for the Ising Model
Authors: Farrell R. A. , Morita T. , Meijer P. H. E. 
Published: January 2005

A binary alloy system with nearest-neighbor interaction which forms a rigid lattice is the (ideal) Ising system. The present paper is an application of the theory of classical fluids, as developed by Morita and Hiroike, to such systems. The partition function for the Ising system is made formally equivalent to that for a binary fluid as discussed by Morita. Then, using the methods of graph theory, a high-temperature expansion is obtained for fixed long-range order parameters. By performing a partial summation, one finds that the free energy is given in terms of connected diagrams which are composed of βJ bonds, and black circles Mn. The quantities Mn are evaluated by noting that the diagrammatical expressions for these quantities appear in the expansion of the grand potential of a soluble problem.
The method is illustrated in the case of an AB alloy or the ferromagnetic Ising system which form cubic lattices. Chang's result is easily obtained. The extension of this method to higher terms and its application to other alloy systems are retained for a forthcoming paper.
Atomization Energies of the Polymers of Germanium, Ge2 to Ge7
Authors: Kant Arthur , Strauss Bernard H. 
Published: January 2005

Using information derived from a combination of effusion and mass-spectrometric techniques at high temperature, the atomization energies and free energies for the reactions Gen(gas)=n(Ge gas) (n=2, 3, 4, 5, 6, 7) have been determined. The values of the energy as obtained from the variation of the free energy of atomization with temperature (second law of thermodynamics method) are in kilocalories per mole: they are 65, 149, 238, 321, 407, and 481 for n of 2, 3, 4, 5, 6, 7, respectively. A comparison of these values with the atomization energies as calculated from the third law of thermodynamics (absolute entropy method) assuming that the geometric configuration of the molecules (n&gt;2) is either a linear chain or a cyclic structure indicates that the former structure is to be preferred.
The bond-dissociation energies (process Gen=Gen−1+Ge) as obtained from the second law are 75, 86, 83, 89, 84, and 65 kcal/mole for n of 7, 6, 5, 4, 3, and 2, respectively.
Crystal Structure of Solid Hydrogen and Deuterium, and of Neon—Hydrogen and Neon—Deuterium Mixtures
Authors: Barrett Charles S. , Meyer Lothar , Wasserman Judith 
Published: January 2005

Solid hydrogen (normal and para), deuterium, and the systems neon—hydrogen and neon—deuterium were studied by x-ray diffraction at various temperatures with and without prior plastic deformation. Under most conditions D2 freezes from the liquid phase as hexagonal close packed (hcp) and is stable in this form against plastic deformation; similarly H2 usually freezes in the hcp form. When gold foil with a cube texture is present, D2, normal H2 and para-H2 freeze in the face-centered-cubic (fcc) form. Plastic deformation at about 3° and 4.2° causes the addition of fcc reflections to the hcp diffraction pattern with samples of normal H2 that previously had shown only hcp.
It is concluded that normal H2 is metastable in the hcp structure at these temperatures, and that the spontaneous transformation to cubic on cooling may often be incomplete.
It was found that, in concentrations between 0.25% and 99.5% Ne, Ne and the hydrogens are immiscible.
Monte Carlo Study of a One-Component Plasma. I
Authors: Brush S. G. , Sahlin H. L. , Teller E. 
Published: January 2005

A Monte Carlo study has been made of a plasma of heavy ions immersed in a uniform neutralizing background. Systems containing from 32 to 500 particles, with periodic boundary conditions, were used. The results of the study are presented in terms of a dimensionless parameter Γ= (4πn/3) ⅓[(Ze)2/kT], where n is the ion density (particles per cubic centimeter), T is the temperature (degrees Kelvin), k is the Boltzmann constant, e is the electronic charge, and Z is the atomic number. Thermodynamic properties and pair distribution functions were obtained for values of Γ ranging from 0.05 to 100.0 from the canonical ensemble by the Monte Carlo (MC) method.
Two different methods were used to determine the potential energy of a configuration. The first is the ``minimum-image convention'' employed in many previous MC calculations. Each particle is allowed to interact only with each other particle in the basic cell, or with the nearest periodic image of each other particle if the image is closer. In the second method, the interaction of a particle with all the images of the other particles, and with the uniform background is taken into account by a technique similar to the Ewald procedure used to calculate lattice sums. It is found that both methods of determining the potential energy yield essentially the same results for the pair distribution function g for Γ values of 10 or less. For larger values of Γ the results given by the two methods differ significantly, indicating that the minimum image convention is inadequate for plasma systems at high densities and low temperatures.
Energies and values of the pair distribution function are compared with predictions of various approximate theories for small Γ values. It is found that the nonlinear Debye—Hückel (DH) theory is in agreement with the MC results for values of Γ up to 0.1. At Γ=1.0, significant deviations from the DH theory are observed. For Γ=1.0, g is found to be in close agreement with Carley's calculations based on the Percus—Yevick equation. For values of Γ above 2, g is no longer a monotonic function of the interparticle distance r, but begins to show oscillations characteristic of latticelike structures. For large values of Γ these oscillations are quite pronounced. The system is observed to undergo a fluid—solid phase transition in the vicinity of Γ=125.
Raman and Infrared Spectra and Structures of the Aluminate Ions
Authors: Carreira L. A. , Maroni V. A. , Swaine J. W. , Plumb R. C. 
Published: January 2005

The Raman and infrared spectra of the aluminate ions in alkaline solutions have been measured. Two ionic species, one predominant from pH 8 to pH 12 and the other predominant above pH 13, may be identified from the spectra. At intermediate pH's both forms are present. The selection rules and the polarization of the lines of the low-pH species are consistent with either a square-planar Al(OH)4− structure or a polymeric structure in which each metal ion is surrounded by six OH− ions and the perturbations on the Oh symmetry are not great enough to be resolved. The ion does not appear to be tetrahedral Al(OH)4−, as has been previously thought. Pronounced aging effects observed with the low-pH anion support a polymeric structure rather than the square-planar Al(OH)4−. The spectra show that the high-pH species is AlO2− with D∞h symmetry. The observation of two distinct species and measurements of the pH ranges over which they are stable, confirm the results of earlier measurements by some of these authors using Leden's techniques of studying complex-ion formation.
The Raman spectra of deuterated aluminate solutions were measured. The low-pH species shows the expected frequency shifts and the frequency of vibration of the high-pH form is not affected by deuteration, in agreement with the supposition that the aluminate ion contains hydrogen at low pH but does not contain hydrogen at high pH.
Chemisorption of Hydrogen on Tungsten (100)
Authors: Estrup P. J. , Anderson J. 
Published: January 2005

The interaction of hydrogen with a single-crystal tungsten (100) surface has been studied, using LEED in conjunction with flash-desorption and surface-potential measurements. As hydrogen is adsorbed at room temperature a sequence of LEED patterns is observed, consisting of the formation of ``extra'' ½ ½ spots, splitting and eventual disappearance of these spots with simultaneous formation of ¼-order spots, and finally, at saturation, the disappearance of all extra diffraction features. The possibility that these LEED patterns are caused by diffraction in the adlayer rather than in a reconstructed tungsten lattice is discussed in some detail.
The initial adsorption is found to occur with a sticking probability of 0.65 and is accompanied by dissociation of the hydrogen molecules. At saturation there are two hydrogen atoms per surface tungsten atom. The surface potential decreases approximately linearly with coverage and at saturation the total change is about −0.9 V.
For coverages above 0.25 the adsorption proceeds with formation of a series of arrangements characterized by large superstructure units with ``out-of-step domains,'' the size of which depends on the density of adatoms. It appears that these structures cannot be explained by existing theories of adsorption and lattice statistics.
Internal-Energy-Transfer Efficiencies in Eu3+ and Tb3+ Chelates Using Excitation to Selected Ion Levels
Authors: Dawson William R. , Kropp John L. , Windsor Maurice W. 
Published: January 2005

Fluorescence yields of solutions of several chelates of europium and terbium have been measured: (a) upon excitation in the ligand absorption bands and (b) upon selective excitation to individual upper levels of the rare-earth ion. Yields with upper-ion-level excitation are lower than when the emitting level is excited directly and yields with ligand excitation are lower still. Efficiencies of energy transfer to the emitting level from upper ion levels and from the ligands, for the solutions studied, are calculated from the data.
The fluorescence yield observed when the emitting level is excited directly, together with the fluorescence lifetime, allows both radiative and radiationless rate constants for deactivation of the emitting level to be calculated. The enhanced fluorescence yields of chelates compared to unchelated rare-earth ions are found to be due primarily to the enhancement of the radiative transition by the chelate environment rather than to a protective influence against quenching provided by the ligands.
For terbium chelates the strong temperature dependence of the fluorescence intensity is explained in terms of quenching of the emitting level via thermal excitation to the lowest triplet state of the ligand. This is indicated by the agreement between the activation energy for quenching and the energy difference between the emitting level and the triplet state.
Phase Diagrams of Sodium Nitrite and Potassium Nitrite to 40 kbar
Authors: Rapoport Eliezer 
Published: January 2005

The phase diagrams of NaNO2 and KNO2 were determined to 40 kbar. The NaNO2 phase diagram consists of five solid phases and the liquid. The ferroelectric Curie point (the III—II phase transition) and the Néel point (the I—II transition) were detected by differential thermal analysis (DTA) and determined to 40 kbar. The results are in good agreement with the work of Gesi and his co-workers to 10 kbar. Points on the III—IV boundary were determined by the volume-discontinuity method. The transition manifests itself as a break in the slope of the compression curve of NaNO2 and seems to be a second-order transition. The transition pressures are lower than those estimated previously. The melting curve increases with pressure. A new high-pressure solid-phase, NaNO2 V, wad discovered. The I—V phase boundary branches off the melting curve at the liquid—I—V triple point at 9.8 kbar and 343°C.
The KNO2 phase diagram consists of six solid phases and the liquid. The I—II and IV—V phase boundaries, determined by DTA, rise very slowly with pressure. The II—IV, I—IV, and I—V boundaries were determined by the volume-discontinuity method with application of piston rotation. Two new high-pressure phases, KNO2 V and KNO2 VI, were discovered. The melting curve of KNO2 exhibits a maximum at 7.0 kbar and 466°C, meets the I—V boundary at a triple point at 15.2 kbar and 413°C, and then increases steeply with pressure.
Free Energy of a Nonuniform System. III. Nucleation in a Two-Component Incompressible Fluid
Authors: Cahn John W. , Hilliard John E. 
Published: January 2005

By finding the saddle point in the expression derived in Paper I (see reference 8) for the free energy of a nonuniform system, we have derived the properties of a critical nucleus in a two-component metastable fluid.
At very low supersaturations, we find that the properties of the nucleus approach those predicted by the classical theory that assumes the nucleus to be homogeneous with an interfacial energy that does not vary with curvature. However, with increasing supersaturation, the following changes occur in the properties of the critical nucleus. (a) The work required for its formation becomes progressively less than that given by the classical theory, and approaches continuously to zero at the spinodal. (b) The interface with the exterior phase becomes more diffuse until eventually no part of the nucleus is even approximately homogeneous. (c) The composition at the center of the nucleus approaches that of the exterior phase. (d) The radius and excess concentration in the nucleus at first decrease, then pass through a minimum and become infinite again at the spinodal.
These properties are deduced without resort to any specific solution model. In addition, they are evaluated for a regular solution to permit a quantitative comparison with the predictions of previous treatments.
Electron Impact Studies of Aromatic Hydrocarbons. I. Benzene, Naphthalene, Anthracene, and Phenanthrene
Authors: Wacks Morton E. , Dibeler Vernon H. 
Published: January 2005

A systematic survey of ionization-dissociation processes of fused-ring aromatic compounds is initiated. Mass spectra and appearance potentials of the singly and doubly charged molecule ions are reported.
The observed ionization potentials of benzene (9.38 ev), naphthalene (8.26 ev), anthracene (7.55 ev), and phenanthrene (8.03 ev) are compared with available spectroscopic data, with values obtained from molecular orbital calculations, and with other electron impact data. The empirical method of group equivalents is extended to the calculation of the ionization potentials of the fused-ring aromatic compounds.
NMR in Hydrate Crystals: Structural Information from Broadened Fine-Structure Lines
Authors: Pedersen B. , Holcomb D. F. 
Published: January 2005

The effects of interpair broadening upon the NMR spectra of proton pairs in hydrate crystals are explored experimentally. The shape of a fine-structure line is generally found to be asymmetric. The importance of using the position of the center of gravity of a fine-structure line rather than the position of maximum absorption in determining structural parameters is emphasized by several experimental illustrations. In a coupled pair framework the former quantity is independent of interpair broadening to first order, the latter only to zero order (isolated pairs), but the position of maximum absorption is the quantity most commonly used in the literature. Data for a crystal not previously investigated with NMR, K2C2O4·H2O, illustrates the importance of the use of centers of gravity for a case in which the interpair interactions are relatively weak. It also demonstrates how the nature of the line asymmetry may be interpreted to give information about relative positions of neighboring water molecules.
The proton spectra of CaSO4·2H2O and Li2SO4·H2O have been remeasured and are reinterpreted. For these crystals short interpair distances suggest large modifications of the zero-order treatment given in the literature. It is demonstrated that in CASO4·2H2O a first-order treatment gives consistent results, with minor changes in the previously derived structural parameters. Second-order effects on the position of the center of gravity of the NMR line are largely suppressed by the relatively large number of interacting pairs.
In Li2SO4·H2O it is concluded that strong interpair couplings along a chain of water molecules cast grave doubts on any interpretation of the spectra based on an analytical treatment using a coupled pair framework.
Dilute Solution Properties of Linear Polystyrene in Θ-Solvent Media
Authors: Orofino T. A. , Mickey J. W. 
Published: January 2005

Dilute solution parameters descriptive of polymer coil size and segment—solvent interaction have been evaluated for each of three polystyrene—solvent systems at various temperatures in the vicinity of the Θ value characteristic of a given pair. The pure solvents selected for these studies were diethyl malonate, cyclohexane, and 1—chloro—n—undecane, which with polystyrene display Θ temperatures of 35.9°, 34.8°, and 32.8°C, respectively, as ascertained from analyses of second virial coefficient—temperature relationships. A single polymer sample of narrow molecular weight distribution was employed in all investigations.
Parameters interpolated for each polystyrene—solvent pair at the Θ temperature of the system are examined from the point of view of the specific influence of the medium on coil characteristics. Differences are observed, for example, among the intrinsic viscosity values. Accurate, relative comparison of such solution properties is facilitated through use of the common polymer sample and intentional selection of pure solvents displaying Θ temperatures in proximity. The complicating effects of variable molecular weight, molecular weight distribution, and temperature per se are thus conveniently eliminated.
Intrinsic viscosity and second virial coefficient data obtained for each pair at various temperatures near Θ are examined in accordance with the formal functional interdependence predicted by certain theoretical treatments, applicable to weakly interacting systems of the kind here investigated.
Centrifugal Distortion Effects in Asymmetric Rotor Molecules. I. Quadratic Potential Constants and Average Structure of Oxygen Difluoride from the Ground-State Rotational Spectrum
Authors: Pierce Louis , Di Cianni Nicholas , Jackson Robert H. 
Published: January 2005

The microwave spectrum of oxygen difluoride has been thoroughly investigated in the frequency regions 8 to 40 and 49 to 60 kMc. A total of 83 absorption lines have been assigned as ground-state rotational transitions in which the total angular momentum J ranges from 1 to 39. These data provide the means for a critical test of the Kivelson—Wilson theory of centrifugal distortion. It is found that the K—W energy expression only approximately accounts for the measurements. The K—W formula obtains when (1) separation of rotation and vibration is carried out to second-order of approximation, and (2) when the quartic (P4) distortion terms in the resulting effective rotational Hamiltonian are treated by first-order perturbation theory. It is found that both of these approximations must be carried one order higher to obtain a satisfactory fit of the observed rotational frequencies of OF2.
Analysis of the rotational spectrum by the method of least squares yields the following ground-state rotational parameters (Mc): A=58782.630, B=10896.431, C=9167.412, τxxxx=—0.0949559, τzzzz=—6.615402, τxxzz=0.399611, τxzxz=—0.162486, HJK=—2.2184×10—6, and HKJ=3.1073×10—6, where HJK and HJK are the coefficients of semiempirical P6 distortion terms in the effective rotational Hamiltonian. The quartic distortion constants yield vibrational frequencies which agree with the observed values to within 4% or better. By combining microwave data with the observed vibrational frequencies the following quadratic potential constants are obtained: fr=3.9499 mdyn/Åfrr=0.8059 mdyn/Å,fα/r2=0.7237 mdyn/Å,frα/r=0.1373 mdyn/Å.Using this potential function with formulas derived by Laurie and Herschbach the average structure of OF2 in the ground vibrational state is found to be 〈r〉=1.4124 Å,  〈α〉=103∘10′,where r is the OF distance, and α the FOF angle.