bluuebunny/crossref_metadata_2025_split huggingface dataset:
Spin-Echo Studies of Chemical Exchange. II. Closed Formulas for Two Sites
Authors: Allerhand Adam , Gutowsky H. S. 
Published: January 2005

Closed expressions are derived for the decay in amplitude of successive echoes in nuclear magnetic resonance experiments for systems with two exchanging sites having different resonance frequencies but equal transverse relaxation times ``in the absence of exchange.'' The decay for a Carr—Purcell train of echoes is found to be the sum of two exponential terms and two ``pseudoexponential'' terms; the absolute values of the latter decay exponentially but the sign alternates from one echo to the next. For the case of two equally populated sites the decay simplifies to the sum of just one exponential and one ``pseudoexponential'' term. In most cases of experimental importance, a single exponential term is much larger than the rest and adequately describes the decay in echo amplitude.
An iterative computer procedure is described which uses the expressions to extract information about the system from measurements of the apparent decay time constant (1/T2) as a function of 180° pulse separation (tcp). In favorable cases the method yields values not only of the lifetime between exchanges (1/2τ) but also of the chemical shift (δω) and the relaxation time in the absence of exchange (1/T20). Moreover, it is applicable to both slow and fast exchange, whereas use of the approximate equation derived by Luz and Meiboom leads to inaccurate values for relatively slow rates with large chemical shifts between the sites. Our experience shows that the spin-echo technique can be very useful for the study of fast exchange processes, but even when closed expressions of appropriate accuracy are used the nature of these expressions is such that a high-speed computer is required for fitting the experimental data, unless some ``outside'' information is available about δω and 1/T20.
Irreversible Thermodynamics of Vectorial Phenomena. I. The Second-Order Transport Equations of Thermal and Electrochemical Diffusion
Authors: Hurle D. T. J. , Mullin J. B. , Pike E. R. 
Published: January 2005

A phenomenological description of transport processes in concentrated multicomponent systems driven by generalized thermodynamic driving forces of vectorial character is derived by the methods of classical irreversible thermodynamics. The approach differs from that used in previous work in two major respects, first, in an emphasis in the first-order equations on the use of experimentally measurable quantities, and, second, on mathematical consistency in the derivation of the second-order equations. This second requirement is shown to lead to the introduction of a number of new transport coefficients which are analogous to the already known Thomson coefficient. A further difference lies in the fact that electroneutrality is not assumed and Poisson's equation is incorporated.
A discussion of the nature and use of phenomenological transport coefficients is included, and an extension of the theory to large systems is made.
Energy-Band Calculations for Periodic DNA Models in the Hückel Approximation
Authors: Ladik J. , Biczó G. 
Published: January 2005

The energy-band structures of different periodic models of DNA poly(A−T),  poly(G−C),  poly(A−TG−C),  poly(A−TC−G),  poly(A−TT−A),  and  poly(G−CC−G) have been calculated in the Hückel approximation. The characteristic features of the results are compared with those previously obtained for homopolynucleotides and for the most simple heteropolynucleotides.
The widths of the energy bands make possible a small conduction in DNA. For the forbidden bandwidth between the highest filled band and the lowest unfilled singlet band the value 3.46 eV has been obtained in the case of the most complicated model systems, which do not differ too much from real DNA.
Production of Free Radicals in Organic Solids by Hydrogen-Atom Bombardment
Authors: Cole T. , Heller H. C. 
Published: January 2005

A beam of hydrogen (or deuterium) atoms, produced by a microwave electrodeless discharge, has been found capable of producing free radicals upon striking polycrystalline samples of malonic acid, malonamide, methyl malonic acid, and methyl malonamide. The radicals produced in these compounds, identified by means of their electron magnetic resonance signals, arise from the removal of a methylene hydrogen of the parent molecule. Only one radical species is produced in each compound, in contrast to x irradiation which often produces several types of radicals. The present study strongly suggests that the impinging hydrogen atoms (a) abstract methylene hydrogens from the organic molecules to form radicals and H2 and (b) penetrate several molecular layers of the crystallites before they react.
Radicals were also formed by impinging hydrogen atoms in naphthalene, hexamethylene tetramine, calcium malonate, calcium formate, and aluminum acetyl acetonate. However, no positive identification of these radicals could be made on the basis of their powder EMR spectra. On the other hand, several other organic powders gave no radicals by the hydrogen-bombardment technique.
Quantum Effect in the Diffusion of Gases in Liquids at 25°C
Authors: Nakanishi Koichiro , Voigt Eva M. , Hildebrand Joel H. 
Published: January 2005

The study of the diffusion of gas solutes in CCl4 by Ross and Hildebrand is extended and refined to give the following figures for 105D in square centimeters per second at 25°C: in CCl4: He, ∼20; Ne, ∼6.3 (subject to determination of solubility) ; O2, 3.8; SF6, 1.7. In (C4F9)3N; He, ∼13; H2, 6.5; D2, 5.55; Ne, ∼5.3; N2, 2.35; Ar, 2.75. The method is shown to yield not merely relative but absolute values of diffusivity.
The product 1021Dσ2, where σ is the molecular diameter, rises in CCl4 from 45 for the heavier gases to 132 for He; in (C4F9)3N from 32 to 86 for He. There is a linear relation between these values and the square of the quantum parameters of de Boer. The quantum effect is much stronger in CCl4 than in (C4F9)3N in accord with their internal pressures, 3350 and 2170 atm, respectively.
Diffusion of Thallous Chloride in Single Crystals of KCl by a Novel Spectrophotometric Method
Authors: Reisfeld Renata , Glasner Abraham , Honigbaum Alexander 
Published: January 2005

A novel attachment to the Beckman DU spectrophotometer is described, by means of which the diffusion profile of some heavy-metal ions in alkali halides may be recorded.
The diffusion of Tl+ ions in KCl in the temperature range 270°-400°C was measured. It is concluded that the activation energy for the diffusion of Tl+ varies with temperature, parallel to that of the self-diffusion of K+ ions, calculated from conductivity measurements. The diffusion constants of the Tl+, at any temperature, are several orders higher, as the associated vacancies of the divalent impurities present are also involved in the diffusion process.
Irradiation Damage in Type I Diamond
Authors: Dyer H. B. , Preez L. du 
Published: January 2005

In addition to the GRI and uv bands induced in all diamond by 0.78-MeV electron irradiation, another optical absorption feature, which we have named the ND1 band, is found in all Type I diamonds. A single EPR line appears to be associated with the ND1 band.
It is suggested that the ND1 center arises from the combination of a defect product—probably an interstitial—and nitrogen in platelet form. Certain observations can only be explained by assuming that the ND1 centers are not distributed randomly through the crystal, but occur in clusters.
The ND1 center acts as an acceptor, the center (probably a vacancy) responsible for the GRI and uv band as a donor. In the ionized state, GRI is optically inactive; ND1 is optically active. Electron transfer by thermal excitation results in the bleaching of GRI and enhancement of ND1. Illumination with light in the ND1 band causes electron transfer in the reverse direction, restoring band strengths to their former condition. A model is proposed which defines the energies within the forbidden gap of the ground and excited states of GRI and ND1.
Deep Impurity States in Molecular Crystals: The Optical Excitation of a Substitutional Argon Atom in Crystalline Neon
Authors: Webber Stephen , Rice Stuart A. , Jortner Joshua 
Published: January 2005

The calculation of the first electronic transition of an argon-atom impurity in a neon lattice is carried out in the Heitler—London scheme. The energy of this transition is taken to be the energy of the argon atomic transition, plus the correction to the SCF 4s orbital energy due to the presence of the crystal. The modification to the SCF 4s orbital is accomplished by the addition of charge-transfer functions on neighboring neon atoms. It is shown that the choice of the SCF atomic function is a very bad starting point for these systems.
A general method for choosing basis functions for bound systems is presented. It is shown that for many cases a suitably chosen ``model Hamiltonian'' may be appropriate to a more complex system, and the case of a dielectrically screened hydrogenic ``model Hamiltonian'' is worked out for Ar–Ne.
Rotational Isomerism and Microwave Spectroscopy. III. The Microwave Spectrum of 3-Fluoropropene
Authors: Hirota Eizi 
Published: January 2005

The 3-fluoropropene molecule has been confirmed by microwave spectroscopy to exist in two rotational isomers, cis and gauche. Rotational constants in the ground vibrational state are A = 17236.638, B = 6002.915, C = 4579.827 Mc/sec for the cis and A = 27720.349, B = 4263.628, C = 4131.984 Mc/sec for the gauche form. Rotational transitions are also assigned for isotopic species, eight of the cis and nine of the gauche form. By using Costain's method the molecular structure is determined separately for the two isomers. The dipole moment and its components along the principal axes of inertia are μa=0.742±0.008, μb=1.601±0.011, μtotal=1.765±0.014 D for the cis and μa=1.595±0.006, μb=0.908±0.012, μc=0.623±0.014, μtotal=1.939±0.015 D for the gauche form. Through the temperature dependence of the intensity ratio the cis form is found more stable than the gauche form by 166±67 cal/mole.
The energies of the first and the second excited states of the C–C torsion are determined by the relative intensity measurement to be 169.6±4.2 cm−1 and 322.2±9.9 cm−1 for the cis and 84.6±2.8 cm−1 and 185±25 cm−1 for the gauche form. Some of the spectra in the excited states of the gauche form are doublets due to the tunnel effect through the trans barrier. Using these data a potential function for the rotation around the C–C single bond is discussed. A correlation is pointed out between the potential function and the binding states of carbon atoms forming the rotation axis.
Causes of l-Type Doubling in the 3p(E″) Rydberg State of Ammonia
Authors: Child Mark S. , Strauss Herbert L. 
Published: January 2005

Expressions are developed for the rotational levels of an electronically degenerate planar XY3 molecule. It is shown that there can be strong l-type doubling only in levels with no vibrational angular momentum and that this doubling has two main causes, namely, a rotational—electronic interaction and a rotational—Jahn—Teller interaction. The l-type doubling of other levels is largely quenched by vibronic coupling.
Estimates of the observed l-type doubling in the 3p(E″) state of ammonia indicate that the two above contributions act in opposite senses. A mechanism which accounts for the variation of the observed l-type doubling with vibronic state is suggested.
Rates of Decomposition of Light and Deuterated Alkyl Radicals. 2-Hexyl and 2-Hexyl-d12, and 2-Octyl and 2-Octyl-d16
Authors: Pearson M. J. , Rabinovitch B. S. , Whitten G. Z. 
Published: January 2005

The rates of decomposition of chemically activated 2-hexyl-d12 and 2-octyl-d16 radicals have been measured relative to their corresponding light radicals by an internal comparison method. The intermolecular secondary isotope effects so determined for these nonequilibrium systems were the high-pressure values and ranged from 4.6 for 2-hexyl at 300°K to 12.1 for 2-octyl at 195°K.
Application of the quantum statistical Marcus—Rice theory to appropriate vibrational models leads to theoretical rate constants for the light and heavy species. The resulting calculated isotope effects are in reasonable agreement with those measured. The effect of energy upon the theoretical magnitudes of the isotope effect has been examined by making a comparative study of the alkyl radicals, ethyl through 2-octyl, by varying the assumed ambient temperature over the range 100° to 1000°K. The accuracy of enumeration of vibrational energy densities for these large molecules was explicitly tested.
Triplet—Triplet Absorption in Sensitized Solvent Decomposition: An EPR Study
Authors: Siegel Seymour , Eisenthal Kenneth 
Published: January 2005

The decay of the steady-state electron paramagnetic resonance (EPR) signal from naphthalene triplet-state molecules in ether and other glasses under continuous near-ultraviolet irradiation at 77°K has been examined in some detail. It has been shown that free radicals are produced by sensitized solvent decomposition at a rate proportional to the rate of the disappearance of the triplet-state EPR signal. Also, it has been demonstrated that the latter decay process has a first-order dependence upon naphthalene concentration and that it exhibits a dependence upon excitation intensity, which is consistent with a stepwise two-photon absorption mechanism.
The experimental results are discussed in terms of a model that involves the resonant transfer of energy from a highly excited triplet state of naphthalene to the solvent. Excitation of the excited triplet state takes place via the absorption of a photon by a naphthalene molecule in its lowest triplet state. The free radicals are produced by the decomposition of the sensitized solvent molecule. It is believed that the naphthalene molecule is not destroyed in this process but that the EPR signal decay arises from a radical—molecule interaction which effectively shortens the lifetime of the triplet state.
Crystal and Molecular Structure of Hydrogen Peroxide: A Neutron-Diffraction Study
Authors: Busing William R. , Levy Henri A. 
Published: January 2005

The coordinates and thermal parameters of the O and H atoms in solid H2O2 have been determined by a single-crystal neutron-diffraction study. Final values were obtained by least-squares refinement based on 91 observed intensities of the hk0, hhl, and h0l reflections. The molecular parameters (uncorrected for the effect of thermal motion) are as follows: O–O distance, 1.453±0.007 Å; O–H distance, 0.988±0.005 Å; O–O–H angle, 102.7±0.3°; dihedral angle between the two O–O–H planes, 90.2±0.6°. Hydrogen bonding occurs with an O–H···O distance of 2.799±0.008 Å. The O–H bond distance corrected for thermal motion is 1.008±0.005 Å.
Comparison of these results with those reported by others from studies of H2O2 vapor and of various crystals containing H2O2 indicates that the dihedral angle is quite sensitive to the environment of the molecule.
Surface Tension of the {100} Face of Alkali Halide Crystals
Authors: Benson G. C. , Yun K. S. 
Published: January 2005

In this paper, the distinction between the surface free energy (or surface energy at 0°K) and surface tension is re-examined. The change in the state of stress in a crystal associated with the presence of the surface is represented by the surface stress tensor. The mechanical definition which is adopted for this tensor, is similar to that used in the theory of the surface tension of liquids. The surface tension is a scalar quantity related to the trace of the surface stress tensor.
Results of calculations of surface tensions are reported for the {100} face of all the alkali halides with the NaCl structure. The model used is the same as that employed in previous calculations of the surface energies of these crystals, and includes a consideration of distortion effects (displacement and polarization of the ions) occurring in the surface region. Positive surface tensions were found for all of the alkali halides investigated.
Vacuum-Ultraviolet Photolysis of Acetylene
Authors: Stief L. J. , DeCarlo V. J. , Mataloni R. J. 
Published: January 2005

The vacuum-ultraviolet photolysis of acetylene has been investigated at the Kr (1236 Å, 1165 Å) and Xe (1470 Å, 1295 Å) resonance lines. Evidence is presented for production of a long-lived excited state of acetylene. Decomposition of the excited state at low pressure leads to the formation of molecular hydrogen and excited C2. At the Kr resonance lines and at the 1295-Å Xe line, an emission consistent with Swan bands (A 3πg→X 3πu) of C2 is observed at low pressures. No emission is observed in the photolysis of acetylene at 1470 Å. Intensity of the emission, the quantum yield for hydrogen production, and the quantum yield for acetylene disappearance are all greatly reduced by the addition of a few millimeters of a quenching gas. Decomposition to H2 and C2 also occurs in the solid phase.
The over-all photolysis is explicable in terms of an excited molecule mechanism. Formation of C2 in excited A 3πg state depends on the energy of the incident photon: C2H2+hv→C2H2*,C2H2*+M→C2H2+M,C2H2*+C2H2→C4H4*,C4H4*→C2H2‡+C2H2‡,C2H2*→C2(A 3πg)+H2,C2(A 3πg)→C2(X 3πu)+hv,C2H2*→C2+H2,C2H2*→C2H+H,H+C2H2→C2H3,C4H4*+C2H2→C6H6*,etc.,C4H4*+C2H2→C4H4(vinyl acetylene)+C2H2,C6H6*+C2H2→C6H6+C2H2,2C2H→C4H2(diacetylene)
On the Kinetic Theory of Dense Fluids. XIX. Comments on and a Rederivation of the Kinetic Equations
Authors: Hiroike Kazuo , Gray Peter , Rice Stuart A. 
Published: January 2005

In this paper we rederive the Rice—Allnatt kinetic equation and show that, for the soft-force part of the interaction, the constant friction-coefficient Fokker—Planck operator is the first approximation to a more complex operator. This more complex operator is of the general Fokker—Planck form, but has momentum-dependent friction coefficients. The operator derived herein is identical with the operator obtained from the Prigogine theory in the weak coupling limit.
The consistency between the Rice—Allnatt analysis of transport in liquids and the exact perturbation analysis of Prigogine is examined, as is the internal consistency of the Rice-Allnatt analysis. At present, there appear to be no inconsistencies between the two theories.
Molecular Energy States and the Thermodynamic Properties of Solid Isotopic Methanes
Authors: Colwell J. H. , Gill E. K. , Morrison J. A. 
Published: January 2005

The heat capacities of the partially deuterated methanes, CH3D, CH2D2, and CHD3, have been measured between 2.5° and 27°K. In addition to the known transitions (two in each methane in the region 16°&lt;T&lt;26°K), each substance displays a large heat-capacity anomaly at lower temperatures (T&lt;8°K). Time effects were only observed with CH3D and it is suggested that these were caused by CH4 impurity in this methane.
None of the methanes was isotopically pure; the low-temperature properties of the ``pure'' methanes were estimated by assuming the heat capacities of the different isotopic species to be additive. On this basis, and neglecting the anomalies at the lowest temperatures, the residual entropies (i.e., the differences between the practical and the calorimetric entropies) are found to be very close to R ln4 for CH3D and CHD3 and to R ln6 for CH2D2. These results are to be compared with the values ∼0 and −0.58 cal/mole·deg for CD4 and CH4, respectively.
Results of magnetic resonance and spectroscopic experiments and information derivable from the thermodynamic properties of CH4 and CD4 at higher temperatures all indicate that intermolecular coupling in the methane lattices is relatively weak. For this reason, the properties of the solid methanes at the lowest temperatures are interpreted as resulting from separate contributions of lattice vibrational and perturbed molecular (rotational) energy levels. It is concluded that the residual entropies and the heat-capacity anomalies are related and that their origin is in the molecular energy states of the systems. We make a quantitative analysis of the thermodynamic data for all of the isotopic methanes, taking explicit account of the degeneracies of the different nuclear-spin species. The results are shown to be consistent with the known molecular symmetries and two particularly simple crystal site symmetries.
Intermediate Coupling Energy Levels for Nd3+(4f3) in LaF3
Authors: Caspers H. H. , Rast H. E. , Buchanan R. A. 
Published: January 2005

The absorption spectrum of Nd3+(4f3) has been investigated in the spectral region 4100 to 60 000 Å. Fluorescence at 77° and 4.2°K has been observed corresponding to transitions from the excited 4F3/2 levels to all the Stark components of the 4I9/2 and 4I11/2 levels.
By means of a least-squares method, we have calculated the Slater and spin—orbit parameters from experimentally observed levels whose LSJ assignments are reliable. The values of these parameters giving a best fit between the predicted free-ion spectrum and the spectrum of Nd3+ in LaF3 are F2=338.439 cm−1, F4=47.153 cm−1, F6=5.579 cm−1, and ζ=882.109 cm−1. The average deviation of eight of these levels is 10 cm−1.
Higher-Order Stationary-Phase Approximations in Semiclassical Scattering
Authors: Smith Francis J. , Mason E. A. , Vanderslice J. T. 
Published: January 2005

Higher-order stationary-phase approximations are used to calculate corrections to the classical expression for the differential cross section for elastic scattering. An expansion for the cross section as a series in ℏ2 is obtained, whose first term is the classical result. An oscillating term is also present, whose ``wavelength'' is approximately Δθ≈2π/kb. Both the ``wavelength'' and the amplitude of this term vary as ℏ. It is shown that the classical differential cross section is valid for angles as small as the critical angle defined by Massey and Mohr.
A similar technique is used to obtain corrections to Ford and Wheeler's semiclassical expression for the differential cross section at a rainbow angle. An expansion as a series in ℏ⅔ is derived, whose first term varies as ℏ—⅓ and hence diverges in the classical limit. It is shown that the leading term agrees with Ford and Wheeler's result and that the first correction term is very small for a 12–6 potential.
Long-Range Interaction in Two-Electron Systems
Authors: Carr W. J. , Ashkin M. 
Published: January 2005

The methods of Gor'kov and Pitayevski and Herring and Flicker for calculating the exact exchange interaction in a well-separated two-electron system have been examined in more detail, and attention has been given to the wavefunction.
An attempt has been made to apply the method to a case approximating two electrons in the electron gas problem as well as to the hydrogen molecule.
In the case of hydrogenic systems, it is concluded that the usual Heitler-London method is satisfactory for giving the exchange energy since there is a natural ``cutoff'' separation beyond which the exchange is of little interest, and up to this separation no important difference exists between the exact and the Heitler-London results. The ``cutoff'' is reached when the purely magnetic energy becomes equal to the exchange. However, differences between the exact and the Heitler-London wavefunctions can be significant, and an approximate form is discussed which may be of value in variational calculations for more involved systems.