bluuebunny/crossref_metadata_2025_split huggingface dataset:
Theory of Saturation and Double-Resonance Effects in ESR Spectra
Authors: Freed Jack H. 
Published: January 2005

A theory of saturation in the electron spin resonance spectra of dilute solutions of free radicals has been developed in terms of the general Boltzmann equation for the density matrix given by Bloch and Redfield. It is shown, in contrast to the earlier theories, that, in general, a composite line arising from a set of degenerate nuclear spin states must be described by a generalized (matrix) saturated Lorentzian rather than a single line of over-all saturated Lorentzian shape. The general properties of this solution are discussed in detail for the case when electron—nuclear dipolar interactions are the only important nuclear-spin-dependent relaxation mechanism. A particularly simple situation exists when similar nuclei are completely equivalent. Then each composite line consists of a superposition of saturated Lorentzians, and the linewidths and saturation parameters of each component depend only on the values of the total spin and of the z component of spin of the associated configuration of the completely equivalent nuclei. Thus it would be possible to saturate some components to a greater extent than others. These simple components may become coupled together in a complex fashion when the time-dependent fluctuations of the dipolar interactions for equivalent nuclei are no longer identical, which is often the case for aromatic-ring protons. Further coupling may be expected from the effects of nuclear-quadrupole interactions and from intermolecular-exchange phenomena and are only qualitatively discussed. In the absence of saturation the present theory reduces to the linewidth theory of Freed and Fraenkel.
Detailed expressions are obtained for steady-state electron—nuclear double-resonance (ENDOR) effects on radicals containing completely equivalent sets of nuclei. The saturating fields can lead to coherence and induced-transition effects. It is shown how the latter, under appropriate conditions, can result in enhancement of saturated ESR spectra. It appears necessary for enhancements that lattice-induced nuclear spin transitions should be comparable in magnitude to the lattice-induced electron spin transitions, or else cross transitions involving both nuclear and electron spins must be important in the relaxation process. Coherence effects involving both electron and nuclear spin transitions should be unimportant when the main contribution to the ESR linewidths results from secular processes and is nuclear spin independent. This is also the condition for NMR linewidths to be significantly narrower than ESR linewidths. Complications still arise from the coherence and linewidth coupling effects of the different nuclear transitions being excited. A brief discussion of enhancement by a transient ``heating'' effect is also given.
Dielectric Properties of Ices II, III, V, and VI
Authors: Wilson G. J. , Chan R. K. , Davidson D. W. , Whalley E. 
Published: January 2005

The dielectric properties of Ices II, III, V, and VI have been measured up to 300 kc/sec over a range of temperatures and pressures. All except Ice II exhibited well-defined dielectric dispersion and so are orientationally disordered under the experimental conditions. The dispersion loci were slightly broader than Debye curves, which may reflect the presence of nonequivalent crystal sites. As for Ice I, the static dielectric constants correspond to values of about 3 for the Kirkwood orientational correlation factor. This suggests that these forms of ice are four coordinated, in agreement with infrared and (for Ice III) x-ray evidence at low temperatures. The relaxation rates are considerably faster than for Ice I, and the activation energies and entropies somewhat lower. The volumes of activation are all about 4.6 cm3 mole−1. The relaxation mechanism appears to be similar to that in Ice I, i.e., relaxation occurs by diffusion of orientational defects.
X-ray and infrared studies have indicated that Ice II is rotationally ordered near liquid-nitrogen temperature. The absence of orientational polarization in Ice II at temperatures as high as −30° shows it to be ordered throughout its region of stability.
Electronic Specific Heats of Alloys of the MgCu2—xAlx System
Authors: Slick Paul I. , Massena Charles W. , Craig R. S. 
Published: January 2005

Specific-heat measurements were made on a series of alloys of the pseudobinary system MgCu2—xAlx between 1.5° and 4.2°K. The compositions studied ranged from MgCu2 to MgCu1·15Al0·85, and included 11 alloys having the MgCu2 structure, two having the MgNi2 structure, and three in the two-phase region. From the measured values of γ, the density of states at the Fermi level N (EF) was calculated.
The variation of γ with composition shows a maximum at 1.44 electron/atom and a shoulder on the curve at 1.60 electron/atom. A rapid decrease in γ occurs just before the two-phase region is reached at 1.72 electron/atom. This is followed by a linear increase in the two-phase region as the phase with the MgNi2 structure is approached. The trend of γ and N (EF) with composition shows close correlation with published magnetic-susceptibility data. The variation of N (EF) with composition in the cubic (MgCu2) phase is discussed in terms of interactions between the Fermi surface and Brillouin zone faces, and a comparison is made between measured values of N (EF) and the results of theoretical calculations by Bode and Schnabl.
M Centers in Potassium Bromide. I
Authors: Neubert Theodore J. , Susman Sherman 
Published: January 2005

Oriented distributions of M centers have been produced in KBr and the resulting anisotropic spectra measured at −76° and −194°C. Dichroic absorption bands are observed corresponding to transitions from the ground state to a variety of higher excited states. Included in these are previously unreported transitions that lie 0.7–1.1 eV to higher energy than the F band. Some energy-level assignments are made using the F2 model of the M center.
A simplified treatment is presented for the interpretation of dichroic spectra arising from the optical dipole transitions of anisotropic centers in an otherwise isotropic lattice.
Hard-Sphere Lattice Gases. I. Plane-Square Lattice
Authors: Gaunt David S. , Fisher Michael E. 
Published: January 2005

A plane-square lattice gas of hard ``squares'' which exclude the occupation of nearest-neighbor sites is studied by deriving 13 terms of the activity and the virial series and nine terms of appropriate high-density expansions. Using the ratio and Padé approximant extrapolation techniques it is found that the gas undergoes a continuous (or ``second-order'') transition to an ordered state at an activity zt=3.80±2 and a density ρt=(0.740±0.008)ρmax. The ordered state is characterized by a difference of the sublattice occupation probabilities, R(z), which vanishes at the |transition as (z—zt)β with β≈⅛. The pressure at the transition is given by pa2/kBT=0.792±5. The compressibility exhibits a maximum at or near the transition point but probably remains finite and continuous through the transition. A suitably defined ``staggered compressibility,'' which measures the tendency towards sublattice ordering, diverges sharply as the transition is approached from either side.
A double expansion which converges at all densities is derived and examined numerically and the exact behavior of finite lattices of N=4, 16, 20, and 24 sites is discussed.
Theoretical Considerations Concerning the Intramolecular Heavy-Atom Effect on the Phosphorescence Process: C2v Symmetric Dihalonaphthalene
Authors: El-Sayed M. A. 
Published: January 2005

Interactions leading to the observed intramolecular heavy-atom effect on the phosphorescence process of C2v dihalonaphthalenes have been examined. Using one electron operator, the observed heavy-atom effects in these molecules are found to be due to:
(1) Mixing between the Pπ orbitals of the halogen and the π system of the aromatic hydrocarbon.
(2) First-order spin—orbit interaction between the lowest π, π* triplet state and σ, π* (or π, σ*) singlet states (this explains Subspectrum I).
(3) Second-order spin—orbit—vibronic interaction between the lowest π, π* triplet state and π, π* (or σ, σ*) singlet states (this accounts for the appearance of Subspectrum II). These mechanisms are found to give rise to one-center spin—orbit terms on the halogen atom (as well as halogen—carbon two- and higher-center terms). First-order spin—orbit interaction between singlet and triplet π, π* states is found to have vanishing one- and two-center terms involving the halogen orbitals. Direct spin—orbit interaction between π, π* states in haloaromatics does not give rise to intramolecular heavy-atom effect. Mechanisms involving the first-order spin—vibronic terms of the total Hamiltonian are found to give predictions inconsistent with the observed results and thus seem to be unimportant.
The S.O. matrix elements for the A31(σ,σ* or π,π*)↔3B1(σ,π* or π,σ*) is found to vanish for 1,4- but not for the 2,3-dihalonaphthalene. This result, together with the fact that the 1A1 (σ,σ*)←1A1 transition has zero moment for 1,4-dihalonaphthalene but not for the 2,3 derivative, leads to the conclusion that for the 1,4 compound, only the 1B2 (σ,σ* or σ,σ*) (short axis polarized) is an important perturbing singlet state while both the 1B2 and the 1A1 are perturbing the emitting triplet state in the 2,3 derivative. This is found to be in agreement with the observed fact in glasses at 77°K that in the 1,4 derivative, Subspectrum II is highly polarized and parallel to the 1La←1A1 transition (short axis polarized) whereas in the 2,3 derivative, Subspectrum II is almost depolarized with respect to 1La excitation. Using the proposed mechanism it is shown, in agreement with observation, that the relative importance of Mechanism I (giving rise to Subspectrum I) to that of II is practically independent of the atomic number of the halogen. The fact that Mechanisms I and II are comparably important even though Mechanism I involves a first-order interaction whereas II involves a second-order interaction is shown to result from the great difference in the values of the oscillator strength of the perturbing singlet states involved in the two mechanisms.
Nuclear Magnetic Relaxation of Polymer Solutions
Authors: Ullman Robert 
Published: January 2005

A theory of nuclear magnetic relaxation of solutions of chain macromolecules is presented. The relaxation takes place because of the relative motion of segments of the polymer chain containing magnetic nuclei. The analysis is based on the Brownian motion of isolated polymer molecules using a model which has been applied previously for dynamic viscoelasticity and dielectric relaxation of polymer solutions.
The relative motion of magnetic nuclei may be considered as arising from three sources: (a) the relative motion of pairs of atoms rigidly attached to a chain segment, (b) the relative motion of chain segments, and (c) the motion of atoms relative to a chain segment to which they are attached. Cases (a) and (b) are treated here.
Application of the Cell Method to the Statistical Thermodynamics of Solutions
Authors: Prigogine I. , Mathot V. 
Published: January 2005

The cell method for pure liquids in the form used by Lennard-Jones and Devonshire is extended to solutions. It is assumed that (a) the constituents are spherical in shape, with an isotropic field of force, (b) the distance of the maximum interaction for AA, BB, and AB pairs is about the same, and (c) there is random mixing.
For the mean field in the ``cage,'' the complete 6–12 law, the harmonic oscillator, and the smoothed potential model have been studied. The smoothed potential model (potential curve with vertical walls and flat bottom, both depth and width depending on concentration) fits the liquid state best. For this model, one obtains important corrections on both the heat of mixing and the excess entropy to the classical, strictly regular solutions. These corrections are related to the volume changes on mixing resulting from the changes of interactions.
According to the value of εAB*, the excess properties such as volume, entropy, heat of mixing, and free energy present a large variety of shapes, including dissymmetry and inversions which correspond rather nicely to experimental evidence.
This model permits discussion of the severe limitations of Longuet-Higgins's recent theory of conformal solutions.
The Dissociation of Ethyl Bromide and Iodide by Neutron Capture
Authors: Wexler Sol , Davies T. H. 
Published: January 2005

A study has been made of recoil halogen species ejected from gaseous ethyl bromide and iodide after neutron capture. The results demonstrate that a fraction of the radio-halide fragments carry a positive charge. The fraction charged is 12 percent for the 4.4-hr Br80m, 25 percent for the 36-hr Br82, and 50 percent for the 25-min I128. The corresponding estimate for the 18-min Br80 is very approximately 18 percent. The positive charges are attributed to internal conversion processes in the stabilization of the compound nucleus formed by neutron capture. The fraction of positive recoil ions is interpreted as the minimum fraction of neutron capture processes which are followed by an internal conversion process. A comment is made upon the possible significance of the ions in the study of chemical reactions induced by radiative neutron capture.
A study of the 18-min Rb88 from β-decay of the 2.8-hr Kr88 was made using the procedures and equipment employed in the halogen experiments. The results show that all Rb88 atoms are positively charged as expected.
The Franck-Condon Principle and Its Application to Crystals
Authors: Lax Melvin 
Published: January 2005

The semiclassical Franck-Condon principle is shown to be related to the more rigorous (``exact'') quantum-mechanical perturbation formula in the following ways: (1) the Franck-Condon formula can be derived from the ``exact'' formula by using a mean value approximation or by neglecting certain commutators; (2) if the electric dipole moments are treated as approximately independent of position, the Franck-Condon and the ``exact'' absorption (or emission) spectrum have the same zeroth, first, and second moments, i.e., the same integrated spectrum, mean absorption frequency, and breadth; (3) the errors in higher moments than the second become relatively unimportant at high temperatures. If the electron-nuclear interaction is sufficiently strong the errors are unimportant even at absolute zero.
The use of a quasi-molecular description in a many particle problem is found to be possible only if the masses or stiffnesses are allowed to be temperature dependent.
A detailed analysis is made of the case in which the energy difference between the two electronic states is a linear function of the vibrational coordinates—and the latter are describable by normal modes. ``Exact'' formulas for the absorption and emission spectrum are obtained.
Raman and Infrared Spectral Data and Assignments for Dimethyldiacetylene
Authors: Ferigle Salvador M. , Cleveland Forrest F. , Meister Arnold G. 
Published: January 2005

Raman displacements and quantitative depolarization factors, and wave numbers for the infrared bands in the KBr region have been obtained for dimethyldiacetylene in CCl4 and benzene solutions. Since the results of the present work are inconsistent with some of the previous assignments, these have been changed.
In particular, the line at 475 cm−1, which was previously considered an A1-type fundamental vibration, is now proved to be depolarized and is assigned as an Ē-type fundamental. Consistently with this change, the assignment of the 644 cm−1 line of diacetylene as a bending mode is considered. The previous assignments of both of these lines were the cause of the conflicting data existing in the literature between the bond length and the stretching potential constant of the central bond in the structure –C≡C–C≡C–.
The Frictional Coefficient for Flexible Chain Molecules in Dilute Solution
Authors: Mandelkern Leo , Flory Paul J. 
Published: January 2005

The frictional coefficient, f0, of a polymer molecule in dilute solution is assumed to vary directly as an average linear dimension of the chain. From this assumption equations are developed, as follows, which are analogous to those used successfully in the interpretation of intrinsic viscosity measurements; f0/η0 =KfM½α and Kf=P(〈r02〉/M)½ where α represents the factor by which the actual root-mean-square end-to-end distance exceeds the unperturbed distance (〈r02〉)½, M is the molecular weight, η0 the viscosity of the medium, and P should be a universal constant.
Data in the literature on the dependence on molecular weight of sedimentation constants and diffusion coefficients, extrapolated to infinite dilution confirm the conclusions drawn from these equations.
The Infrared Spectra of CF4 and GeF4
Authors: Woltz P. J. H. , Nielsen A. H. 
Published: January 2005

The infrared spectrum of CF4 has been remeasured from 2–19μ and extended to about 33μ, and the infrared spectrum of GeF4 has been observed from 2–38μ.
Through the use of purer samples and more gas than in previous investigations, a large number of new overtones and combination bands were observed in CF4, and several bands were eliminated as impurities. Identifications of these bands are given. The infrared-inactive ν2 was observed, probably because of interaction with ν4, at 435 cm−1 in agreement with the Raman line at 437 cm−1.
In GeF4 the two active fundamentals ν3 and ν4 were observed at 800 cm−1 and 260 cm−1, respectively. Eight overtone and combination bands were discovered. From these the values of ν1 and ν2 were calculated to be 740 cm−1 and 200 cm−1, respectively. Compared with the fundamentals of GeCl4 and GeBr4, these frequencies seem reasonable. The most striking feature of the spectrum of GeF4 is the scarcity of bands as compared with both CF4 and SiF4.
Diffusion Coefficients of the Systems CO2–CO2 and CO2–N2O
Authors: Amdur I. , Irvine J. W. , Mason E. A. , Ross J. 
Published: January 2005

The radioactive tracer, C14O2, has been used to measure diffusion coefficients for the systems CO2–CO2 and CO2–N2O in the temperature range −78° to 90°C. The apparatus, which is the Loschmidt type, uses saturation ionization currents as a measure of the extent of diffusion. The characteristics of this type of apparatus have been analyzed in detail to obtain working equations which permit use of experimental measurements at both small and large values of the time.
The present results, whose accuracy is estimated at 1.5 percent, agree with those obtained by other investigators using different experimental methods. Although the difference is small, 2.6±0.3 percent, the diffusion coefficient for the system CO2–N2O is consistently larger than that for CO2–CO2 over the entire temperature range.
The Reaction of Nitrous Acid with Hydroxylamine
Authors: Bothner-By A. , Friedman L. 
Published: January 2005

Approximately equal quantities of N14N15O16 and N15N14O16 are produced by reaction of hydroxylamine hydrochloride and N15-enriched sodium nitrite in solution in O18-enriched water. The same reaction when carried out in a solution brought to a pH of 1 with gaseous HCl produces an excess of N14N15O16. In both experiments the concentration of N15N15O16 was accounted for on the basis of the reaction of nitrous acid with hydroxylamine to yield nitrous oxide molecules containing one nitrogen atom from each source. O18 exchange occurred to a greater extent in acid medium and the double tagged N2O so produced was mainly N14N15O18 rather than a mixture of the isomers N14N15O18+N15N14O18.
These observations lead to the following conclusions concerning the reaction mechanisms. In neutral solution most of the nitrous oxide produced is formed by dehydration of a symmetrical intermediate, probably hyponitrous acid. In acid medium a competitive dehydration of an unsymmetrical intermediate plays an important role. Mechanisms involving nitroxyl as an intermediate are untenable. Oxygen exchange is acid catalyzed and occurs exclusively with nitrous acid or with an unsymmetrical intermediate formed in the course of this reaction. Oxygen atoms from either nitrous acid or hydroxylamine appear in the product nitrous oxide.
A New Type of Disorder Affecting the Dielectric Properties of Long-Chain Esters
Authors: Crowe Robert W. , Hoffman John D. , Smyth Charles P. 
Published: January 2005

The dielectric properties of a series of long-chain esters have been investigated in the solid state. Like other long-chain molecules containing 20 or more carbon atoms, most of the ester molecules showed evidence of hindered molecular rotation about their long axes in the crystal lattice. It was also found that higher dielectric constants resulted when the samples were rapidly frozen than when they were allowed to solidify gradually, the effect being more pronounced as the chain length was increased. In order to explain this behavior, it has been assumed that the esters may crystallize in an ordered lattice in which the dipoles are in a plane, or in a longitudinally disordered lattice in which some of the chains are reversed. In the latter case, the dipoles may or may not form two planes depending upon the position of the polar group in the chain. It has also been assumed that the degree of longitudinal disorder increases with increase in freezing rate. Once frozen in, however, it remains fixed until the sample is remelted.
The potential barrier hindering rotation has been assumed to be much higher in the ordered state than in the disordered state because separation of the dipoles should lower steric and dipole interaction in the latter. Since the rotational transition temperature is proportional to a potential energy difference V0 related to the barrier against rotation, the transition temperature should increase with increase in longitudinal order. In a completely ordered ester or a nearly symmetrical disordered ester, the barrier may be sufficiently high to raise the transition point to a temperature near or above that of fusion. The experiments agree with these assumptions.
The Chemical Effects of Ultrasonics
Authors: Griffing Virginia 
Published: January 2005

A study of the effects of ultrasonics on the inversion of sucrose was made and negative results were obtained. In the light of these results one can make the following conclusions.
Chemical reactions attributed to ultrasonics can be classified according to three mechanisms. (1) Reactions due to cavitation never occur without the formation of bubbles. All of these reactions take place in the gas phase, are primarily thermal and are due to the heat developed during adiabatic compression. Thus, only reactants with an appreciable vapor pressure are affected. (2) Many reactions in the liquid phase are of secondary origin due to reactions with dissolved products of the primary reaction in the gas phase. Luminescense is also a secondary effect. (3) Some reactions are due to the uniform temperature elevation caused by absorption of acoustic energy of very high intensity in a short path length.