bluuebunny/crossref_metadata_2025_split huggingface dataset:
Analysis of Carr—Purcell Spin-Echo NMR Experiments on Multiple-Spin Systems. I. The Effect of Homonuclear Coupling
Authors: Allerhand Adam 
Published: January 2005

The effect of homonuclear coupling in a molecule on Carr—Purcell spin-echo (CPSE) nuclear magnetic resonance experiments is investigated. A density-matrix approach is used to derive a general equation for the CPSE train of multiple-spin molecules in the liquid state. This general equation predicts a CPSE train modulated with one or more frequencies, whose relative amplitudes are, in general, not equal. Unmodulated contributions may also arise. It is shown that the modulation frequencies and their relative amplitudes are functions not only of the relative chemical shifts and coupling constants, but are strongly dependent on the separation between the 180° pulses, tcp. In addition, the modulation can always be eliminated by making the pulse repetition rate, 1/tcp, large with respect to all the relative chemical shifts and coupling constants. It is also shown that, for weak coupling, it is possible to derive simplified equations by neglecting the nonsecular part of the coupling Hamiltonian, but this approximation is valid only when 1/tcp is of the order of, or smaller than the relative chemical shifts.
Procedures for solving the general equation to obtain closed formulas for the CPSE train of a specific system are discussed. Solutions for spin-½ nuclei are treated in detail. Closed equations are derived for the AB, A2B, and A3B systems, and they are used in some numerical calculations. Simplified weak-coupling formulas are also derived for these systems, and their range of validity is discussed.
A very brief discussion of heteronuclear coupling is presented.
Simple Classical Model for the Scattering of Gas Atoms from a Solid Surface
Authors: Logan R. M. , Stickney R. E. 
Published: January 2005

A simple classical model for the scattering of gas atoms from a solid surface is proposed and its characteristics are determined. The principal assumptions are (1) the interaction of a gas atom with a surface atom is represented by an impulsive force of repulsion, (2) the gas—surface intermolecular potential is uniform in the plane of the surface (hence the interaction does not change the tangential velocity of the gas particle), (3) the surface atoms are represented by independent particles confined by square-well potentials, (4) the surface atoms have a Maxwellian velocity distribution. The model incorporates many of the same features as that proposed by Goodman. As a result of these assumptions, the model is simplified to the extent that it contains no adjustable constants.
Results are obtained for the angular distribution of the scattered particles. Significant quantitative results are not expected from such a simple model, but the qualitative behavior of the model does agree surprisingly well with the experimental results. The model correctly predicts the general appearance of the scattering pattern and its dependence on the angle of incidence of the beam and on the temperatures and masses of the gas and surface atoms. We conclude that, contrary to the assumption often made in theoretical studies of gas—surface interactions, the temperature of the surface is a significant parameter when considering the existing experimental data.
Pressure and Entropy for Hard Particles at High Density
Authors: Hoover William G. 
Published: January 2005

A hard-particle model is introduced. For this model it is possible to calculate the thermodynamic limit of both the pressure and the entropy at high (solid-phase) densities. The model embodies both the simplicity associated with a nearest-neighbor lattice-gas interaction, and the realism of a continuous configuration space.
The volume of the system is divided into V cells, with particles in nearest-neighbor cells interacting as hard parallel squares (cubes, in the three-dimensional case). When written in terms of the cell occupation numbers the configurational integral for this system bears a superficial resemblance to a lattice-gas partition function. It is possible to calculate the maximum term in the lattice-gas partition function explicitly. This makes it possible to establish tight bounds on the continuum configurational integral; these bounds show that the free-volume form of the pressure and the entropy is exact at high density.
At close packing, where the upper and lower bounds coincide, the absolute entropy is determined. It is shown that the communal entropy vanishes at close packing, and cannot be fully excited for densities greater than half the close-packed density.
Far-Infrared Spectra of HCl Trapped in the β-Quinol Clathrate
Authors: Allen S. James 
Published: January 2005

To investigate the rotational and translational states of HCl trapped in the β-quinol clathrate, the clathrate dielectric response has been measured in the spectral region between 10 and 100 cm−1, at liquid-helium temperatures. The attenuation coefficient of dilute mixtures of 40μ clathrate powder in paraffin was measured with a slowly modulated lamellar grating interferometer. By making suitable assumptions about the particle geometry, it is possible to extract, by an analysis based on the Hilbert transform, both real and imaginary parts of an effective dielectric constant for the suspended powder. Inherent in the technique are corrections for dielectric mismatch between the powder and paraffin. To help interpret the spectra, clathrates of SO2, HBr, N2, and a mixture of HCl, DCl have also been measured.
At 1.2°K, the HCl clathrate has three salient features: an absorption due to the β-quinol lattice at 66 cm−1, a translational vibration of the HCl dipole near 52 cm−1, and in the vicinity of 20 cm−1 a resonance absorption due to the rotational motion of the trapped dipole. The translational vibration is consistent with cage-wall interactions described by van der Waals forces. Both the intensity of this absorption and the shift of the rotational resonance 3 cm−1 below the gas-phase value of 21 cm−1, at the lowest concentration of HCl can be consistently explained by a coupling between the rotation and translational vibration. At high concentrations of HCl, the rotational resonance increases by nearly 50% and is commensurate with shifts predicted by dipole—dipole interactions between solute molecules.
Ultrasonic Shear and Longitudinal Measurements in Aqueous Glycerol
Authors: Slie W. M. , Donfor A. R. , Litovitz T. A. 
Published: January 2005

Glycerol—water mixtures containing up to 68 mole % water were subjected to ultrasonic shear and longitudinal measurements over the temperature range +50° to −120°C. The low-frequency absorption coefficient and the velocity of sound were measured at 25°C for various glycerol—water mixtures from pure glycerol to pure water. The results demonstrate the existence of low- and high-frequency limiting compressional moduli and a high-frequency limiting shear modulus. The relaxational moduli were found to be linear with temperature for each concentration, and also were linearly dependent on water content.
The width of the distribution of relaxation times was found to be independent of water content up to 68 mole %. This result fails to support Frohlich's model for the origin of the distribution of relaxation times.
Estimated values of the relaxation parameters of water were obtained from the data. Those which were obtained by extrapolation appear to be reasonable when compared with theoretically obtained values.
Species and Composition of Dilute Alkali-Metal—Ammonia Solutions
Authors: Golden Sidney , Guttman Charles , Tuttle Thomas R. 
Published: January 2005

Dilute solutions of the alkali metals in liquid ammonia are assumed to consist of the following constituents: M+, the solvated metal cation; S, the solvent; M−, the solvated metal anion; S−, the solvent anion (solvated electron); the ion pairs M+·M− and M+·S−. These species are assumed to be in equilibrium via M++2S−⇌M−+2Sand M++A−⇌M+·A−,where A− is either M− or S−. For simplicity, the activity coefficient of each species is assumed to be given by the extended theory of Debye and Hückel, with a fixed value of 5.5 Å for the interionic size parameter; the ion-pairing theory of Fuoss, with the same value for the interionic size parameter, is also assumed. The only adjustable parameter then remaining for a complete characterization of the composition of dilute alkali-metal—ammonia solutions is the redox equilibrium constant.
With generally good results being obtained, the foregoing model has been applied to account for the following properties of alkali-metal—ammonia solutions which are more dilute than about 0.10M: (1) the spin paramagnetic susceptibilities reported by Hutchison and Pastor; (2) the vapor-pressure-depression results of Kraus; (3) the electrical conductances measured by Kraus and the temperature coefficients of conductance, reported by Gibson and Phipps; (4) the Knight shifts for 14N and 23Na, reported by McConnell and Holm, Acrivos and Pitzer, and O'Reilly; (5) the optical-absorption results of Gold and Jolly, and those of Dye and Douthit. An a posteriori argument supports the neglect of ion triples for the solutions which have been considered, and reinforces a distinction to be made bewteen the metal anion Na− and the analogous ion triple S−·Na+·S−.
Ion—Molecule Reactions by a Photoionization Mass Spectrometer. I. Propylene and 1,3-Butadiene
Authors: Koyano Inosuke , Omura Itiro , Tanaka Ikuzo 
Published: January 2005

Ion—molecule reactions in propylene and 1,3-butadiene have been studied using a mass spectrometer which utilizes photoionization as an ion source instead of electron impact. The light source used was a low-pressure hydrogen-discharge lamp with a LiF window (substantially Lyman alpha, 1216 Å). It has been shown that this method has two major advantages in the study of ion—molecule reactions compared with the electron-impact method; one is the lack of unimolecular fragmentation of the parent ion and the other the absence of hot filament, which enables one to get high pressure without suffering pyrolysis.
Five reactions of C3H6+ with C3H6 and eight reactions of C4H6+ with C4H6 have been identified and the cross sections determined for these reactions. In propylene, the reaction leading to the formation of C3H7+ was found to be of the largest cross section and, although cross section was small, the reaction forming C3H3+ was found as a new reaction. It was shown that C3H6+ could not be the precursor of secondary C4H6+ in propylene, criticizing the Fuchs assignment. In 1,3-butadiene, three new reactions have been found; the reaction leading to the formation of C3H3+, the hydrogen-atom abstraction reaction just as in propylene, and the parent-minus-one reaction. The first one was of the largest cross section in all reactions in 1,3-butadiene. This reaction has been discussed in connection with the structure and the heat of formation of C3H3+.
Electron-Impact Ionization Cross Sections for Atoms
Authors: Cooper John L. , Pressley George A. , Stafford Fred E. 
Published: January 2005

The relative cross sections for electron-impact ionization of Fe, Co, Ni, and Ag have been measured using a crossed-beam technique with mass-spectrometric analysis of the ions. For 60-eV electrons, the relative cross sections for single ionization are 34, 39, 33, and 35, respectively.
The available absolute and relative cross-section data show singificant deviations from the correlations proposed by Otvos and Stevenson and by Lampe, Franklin, and Field.
Electron Spin Resonance of Parabenzosemiquinone-17O
Authors: Silver Brian L. , Luz Z. , Eden C. 
Published: January 2005

The electron spin resonance spectrum of 17O labeled parabenzosemiquinone in aqueous ethanol solutions has been examined. The hyperfine splitting constant due to 17O, a0, was 9.02±0.02 G. The splitting constant has the same sign as Δg, the anisotropy of the g tensor, and since Δg is negative, the sign of a0 is also negative. Taking into account the fact that 17O has a negative gyromagnetic ratio, the sign found for a0 implies that the net spin polarization at the 17O nucleus is the same as that in the radical.
Marked variations in linewidth occur between the different components of the 17O hyperfine structure due to modulation through molecular tumbling of the anisotropic dipolar and g-tensor interactions. These variations are quantitatively accounted for by the general theory of ESR linewidths due to Freed and Fraenkel. Analysis of the spectrum gives | Δg | = 0.0029, and a correlation time for the tumbling of the molecule of 9.4×10−11 sec.
Interpretation of NMR Spectra. I. A2BXx and A2BKk Spectra
Authors: Hirst Robert C. , Grant David M. , Paul Edward G. 
Published: January 2005

Difficulties encountered in analyzing NMR spectra are greatly multiplied as the number of interacting nuclei increases. The concept of effective internal chemical shifts or effective Larmor frequencies is developed here to simplify the study of complex NMR spectra in which a group of magnetically equivalent nuclei (Xx) is greatly shifted from a strongly coupled group. This concept permits decomposition of a complex NMR spectrum into two or more simpler groups of transitions called subspectra. These subspectra are then analyzed by previously developed methods. It is proved that this decomposition is independent of the nature of the strongly coupled group.
A discussion is given of the change in appearance of spectra as the shift of the Xx group from the strongly coupled group is decreased and small deviations from the ``X'' approximation appear. The Xx group is relabeled Kk (x=k) when such deviations appear. The form of this breakdown depends only on x (or k) and is given for general x (or k). Explicit expressions for the A2BXx system are tabulated. Detailed analyses are presented for the following series of A2BXx or A2BKk spectra: 2,6-dichlorofluorobenzene (A2BX); 1-chloro−2,3-butadiene (A2BX2 or A2BK2); methylallene (A2BX3 or A2BK3).
Hydrodynamic Properties of Flexible-Ring Macromolecules
Authors: Fukatsu Masaaki , Kurata Michio 
Published: January 2005

The sedimentation constant of a series of multiple-ring macromolecules has been calculated by the method of Kirkwood. The multiple-ring form consists of m subrings of equal size linearly connected by universal joints. In the limit of large intersegmental hydrodynamic interaction, the ratio of the sedimentation constant of a multiple-ring molecule to that of a linear chain of the same molecular weight is 1.18 for m=1, 1.32 for m=2, 1.40 for m=3, ··· and 2.00 for extremely large m. These values are found to be practically independent of the excluded volume effect.
The intrinsic viscosity of a single-ring macromolecule has also been calculated by the method of Zimm. The eigenvalue problem encountered in the Zimm theory can be solved exactly over the whole range of the hydrodynamic interaction parameter, h=0 to ∞. The ratio of intrinsic viscosities of ring and linear molecules is 0.500 for h=0 (free-draining case) and 0.645 for h= ∞ (non free-draining case). The latter value is slightly modified if the hydrodynamic interaction is treated by the Fixman method.
Dielectric and Viscous Behavior in the i-Amyl Bromide−2-Methylpentane System
Authors: Denney D. J. , Ring J. W. 
Published: January 2005

Equilibrium and dynamic dielectric and viscous-flow behavior in the i-amyl bromide−2-methylpentane system has been studied at five concentrations down to 7.7 mole % bromide (the equilibrium studies extend to more dilute solutions). Dielectric relaxation has been examined in the frequency range from 50 cps to 300 kc/sec and viscous flow in the range 102 to 107 P, the appropriate temperature ranges being practically coincident. Equilibrium dielectric constants have been measured from room temperature down to 122°K.
The equilibrium dielectric constants reveal no significant short-range correlations tending to preferentially align the dipoles. In the relaxation studies, only a single dispersion region appears at all concentrations. It is asymmetric and, except for the pure halide, not representable by any simple known dispersion function. It is qualitatively similar to the high-frequency dispersion in the 1-propanol−2-methylpentane system. The distribution width shows a maximum at some intermediate concentration which suggests that environmental heterogeneity is important but this hypothesis does not seem tenable in the light of other evidence.
The similarity in concentration and temperature dependences of dielectric relaxation and shear viscosity supports the idea that a large molecular region is involved in the relaxation process so that individual molecular interactions tend to be averaged.
Vibrational Levels of the 2D—E State of Ce3+ in CaF2
Authors: Struck Charles W. , Herzfeld Fred 
Published: January 2005

The equations for the Jahn—Teller problem of an E electronic state and an E vibrational mode are solved by a numerical technique analogous to the procedure for finding solutions for a lower-order eigenvalue equation.
The vibrational fine structure in the spectrum of CaF2:Ce3+ at 3100 Å is compared with the structure which a first-order Jahn—Teller effect would predict. A stabilization energy of about 330 cm−1 and a E mode vibrational frequency of about 210 cm−1 are indicated.
Theoretical Estimation of the Conductivity of Different Periodic DNA Models
Authors: Ladik J. , Biczó G. , Elek G. 
Published: January 2005

Using the deformation-potential approximation, we have estimated the conductivity of some periodic DNA models. The necessary band structures we had determined previously using the tight-binding approximation. We have obtained separately the contributions to the conductivity of the mobile electrons and the positive holes.
For the total conductivities (σ0) we have obtained values between 103−105Ω−1·cm−1. In the cases when the conduction bands are rather narrow we have obtained much larger values for the electronic contribution than for the contribution of the positive holes. This discrepancy can be interpreted within the framework of the deformation potential approximation, but it makes questionable the applicability of this method for narrow bands.
Double-Perturbation Treatment of the Ground State of the Hydrogen Molecule
Authors: Liu Bowen , Lyon William D. , Brown W. Byers 
Published: January 2005

The ground state of the hydrogen molecule is treated by a double-perturbation theory through second order. The unperturbed system is defined mathematically by the unperturbed spatial wavefunction, which has the form exp[—s(ra1+rb1+ra2+rb2)] where s is an energy-optimized scale parameter. The perturbations are (a) one-electron terms due to the simple form chosen for the zeroth-order wavefunction, and (b) the two-electron repulsion potential. The first-order wavefunction for (a) is known analytically and that for (b) is obtained approximately from the Hylleraas variational principle using a James—Coolidge type trial function.
The energy through second order gives an equilibrium distance within 1% of that obtained by Kol/os and Roothaan and a dissociation energy of 4.988 eV, 5% larger than their value of 4.747 eV. The wavefunction through first order gives expectation values for one-electron operators in good agreement with accurate calculations, but is poor when the electrons are close together.
Study of the Vapors of Liquid Lead and Bismuth
Authors: Kim Jang H. , Cosgarea Andrew 
Published: January 2005

The total vapor pressure and mass loss per unit time of lead and bismuth vapor were determined by the torsion—Knudsen-effusion—recoil method.
The average molecular weight of bismuth was determined as a function of temperature assuming the presence of a monomer and dimer species. The bismuth vapor composition is approximately 65% Bi2 and 35% Bi in the temperature range of investigation, 850° to 950°K. The average molecular weight of bismuth vapor decreased with increasing temperature. Vapors of lead were found to be monatomic.
In addition, the heats of dissociation and vaporization have been determined.
Electronic-Relaxation Processes in Acetone
Authors: Borkman Raymond F. , Kearns David R. 
Published: January 2005

The emission properties of acetone and acetone-d6 have been studied in order to investigate the mechanisms for relaxation of electronic excitation. The results are summarized in Fig. 2 and Table I.
The phosphorescence lifetime of acetone at 77°K in ether—IPA glass was 4×10−4 sec and the quantum yield for phosphorescence was ΦP=0.03±0.01. Deuteration of the acetone increased the phosphorescence lifetime to 1×10−3 sec and the quantum yield to ΦP=0.08±0.02. The phosphorescence of acetone was found to be quenched by the addition of pentene−2 to the rigid solution, but the fluorescence was unaffected. From the quenching data it is concluded that the excited-singlet→excited-triplet intersystem-crossing quantum yield is ΦIC=1.0±0.1, independent of temperature between 298° and 77°K. The intersystem-crossing rate constant is kIC=4×107 sec−1 and is not affected by deuteration. The excited triplet state is populated with nearly unit efficiency upon excitation of acetone to its lowest singlet state, and therefore the low phosphorescence yield, even in acetone-d6, indicates that radiationless transitions from the triplet state to the ground state are primarily responsible for determining the lifetime of the lowest triplet state.
A medium effect on the acetone phosphorescence lifetime at 77°K was observed; it was found to vary from 3×10−4 in glycerine solutions to 1×10−3 sec in crystalline acetone. The triplet-state lifetime in crystalline acetone decreased only slightly upon increasing the temperature to just below the melting point of crystalline acetone, but in the case of glycerine solutions, the triplet-state lifetime decreased by at least a factor of 10 in raising the temperature from 77° to 200°K, even though the glycerine remained quite solid. The fluorescence quantum yield in liquid solutions was found to be independent of solvent at 25°C.
The lifetime of the lowest (n, π*) singlet state of acetone is primarily determined by intersystem crossing to the lower-lying excited triplet state and is therefore τS=2.5×10−8 sec. The quantum yield for acetone fluorescence at room temperature was found to be ΦF=0.01±0.003. No deuterium effect on ΦF was observed and therefore it may be concluded that either there is no deuterium effect on the rate of internal conversion from the excited singlet state to the ground state, or, what is more likely, that internal conversion is much slower than intersystem crossing which is not affected by deuteration.
The fluorescence spectra of acetone and acetone-d6 were found to be identical under all conditions, as were the phosphorescence spectra of acetone and acetone-d6. The phosphorescence was found to be polarized perpendicular to the lowest singlet—singlet absorption, consistent with theoretical predictions.
One-Center-Expansion—Configuration-Interaction Studies on CH4 and NH3
Authors: Rutledge Ronald M. , Saturno Antony F. 
Published: January 2005

One-center-expansion—configuration-interaction calculations have been extended to include higher-order spherical harmonics and extra radial factors. Calculations have also been made to study the prediction of nuclear geometry. Computations on CH4 using as a basis set the orthonormalized Slater atomic orbitals through l=8, yielded a ground-state energy of −39.94 a.u. Further, the inclusion of additional Schmidt orthogonalized radial factors gave much less improvement than the addition of higher spherical harmonics.
The calculations with basis orbitals through l=4 on NH3 failed to predict a nonplanar configuration of minimum energy except for the case where the basis set was truncated to include only orbitals through l=2. For the l=2 case NH3 was predicted to have a nonplanar geometry within one degree of the experimental value.