bluuebunny/crossref_metadata_2025_split huggingface dataset:
Thermodynamic Properties of a Simple Hard-Core System
Authors: Hoover William G. , Ree Francis H. 
Published: January 2005

A simple hard-core system is studied over the entire density range. The model embodies both the simplicity associated with a nearest-neighbor lattice-gas interaction and the realism of a continuous configuration space.
In two dimensions the model system shows no signs of a first-order phase transition. In three dimensions a transition is indicated.
Neutron-Scattering Study of the Intramolecular and Crystalline Modes of Polyethylene
Authors: Safford G. J. , Naumann A. W. , Simon F. T. 
Published: January 2005

The neutron spectra and the derived frequency distributions for polyethylene are compared with recent calculations. The skeletal-deformation and -torsion modes appear at 550±50 and 200±10 cm−1, respectively, in agreement with both theory and previous measurements. Five additional weak maxima, observed below 190 cm−1, correlate with predicted intermolecular frequencies and are observed to persist to 350°K (well above the glassy-transition interval) in both a high-density and a strongly branched polyethylene. In contrast, the skeletal torsion and deformation modes broaden and decrease in intensity with increasing temperature and with increasing branching disorder. With annealed samples, the skeletal torsion sharpens with increasing density (i.e., increased order), but the intermolecular modes do not. A spectrum taken normal to the direction of chain alignment in a specimen that had been cold drawn to the yield point showed a sharp, well-resolved, skeletal-torsion mode but no skeletal-deformation mode. This agrees with a previous assignment of the deformation as a longitudinal and the torsion as a transverse mode. The interchain modes were split and shifted in frequency in the drawn sample, presumably because of interchain disordering.
Eight additional weak maxima were observed between 800 and 200 cm−1 in high-density polyethylene. The nature of the molecular motions associated with these modes is uncertain, but it is suggested that they involve coupling with the skeletal modes.
Nonequilibrium Effects in Unimolecular Reaction Theory
Authors: Valance W. G. , Schlag E. W. 
Published: January 2005

The rate constant for a unimolecular reaction is just the lowest eigenvalue of a relaxation matrix which describes all microscopic processes which may occur in the system undergoing reaction. A general solution to this eigenvalue problem has been given in a previous paper, including explicit corrections for non-steady-state behavior, but with one restriction, that the reactant energy distribution of states with insufficient energy to react (below the critical energy for reaction) is the equilibrium distribution. This is the so-called equilibrium assumption for reactant states. States above this critical energy barrier will always have less than their equilibrium population, approaching their equilibrium value in the high-pressure limit.
In this paper the eigenvalue problem is solved without this restriction, i.e., for the case that any or all reactant levels have nonequilibrium effects due to depletion of levels by reaction. This then completes a direct connection between the master equation and the theoretical unimolecular rate constant. The input for the problem is a table of microscopic transition probabilities for the master equation. The method suggested here produces a theoretically and computationally useful expression for the rate constant as a direct consequence of this input. The solution explicitly includes such effects as (a) inelastic scattering in the reactive gas, including many-shot effects, (b) nonequilibrium effects, and (c) non-steady-state effects. The expected relative importance of these effects is discussed.
As a guide in writing a master equation a generalized Lindemann mechanism was assumed operative since it affords some comparisons with RRKM theory. However, other mechanisms with strong time ordering between heat bath relaxation and reaction can be employed in this formulation. (When this ordering breaks down one can, in general, no longer discern first-order kinetics, and a different observable must be defined.)
Chemistry of Positive Ions. VI. Positive-Ion Chemistry in Solid Methane
Authors: Davis Donald R. , Libby W. F. , Meinschein W. G. 
Published: January 2005

The polymerization of solid methane at 77°K by γ rays was studied at two dose rates in the dose range 4 to 150 Mrad, corresponding to 0.04% to 1.4% conversion. The polymer contains an average of 20 carbon atoms per molecule and its empirical formula is C20H40, independent of the dose. The yield is 0.32 methane molecules converted to polymer per 100 eV absorbed and any induction dose must be less than 1 Mrad (0.01% conversion). We report a detailed characterization of the polymer based on physical and spectroscopic studies. Products up to heptane are identified. Preliminary studies of solid ethane and propane afforded polymer with lower yields and fewer numbers of monomer units than the methane polymer.
From an analysis of the dose and dose-rate dependence of the polymerization of solid methane, we conclude that only two types of mechanism are possible. The most useful mechanism is one in which a carbonium ion is transformed directly into a heavy hydrocarbon ion. This process would require about 6 eV of excitation energy. The other possible mechanism requires an efficient energy-transfer process leading to repetitive polymer-building and polymer-degrading reactions.
Isotope Exchange Rates. III. The Homogeneous Four-Center Reaction H2+D2
Authors: Bauer S. H. , Ossa Eugenio 
Published: January 2005

The homogeneous isotopic exchange reaction between hydrogen and deuterium in an excess of argon was studied in a 1-in.-diam single-pulse shock tube. Reaction times behind the reflected shock were close to 1 msec and the temperature range was 1060°—1420°K. The hydrogen and deuterium were present in concentrations from 1%—10%, at total reaction densities of 1.18 to 5.31×10−2 mole/liter. Conversions were kept low in order to minimize the reverse reaction. From the average rates the reaction was found to be second order (total) and empirically represented by Δ[HD]/Δt=kp[Ar]0.98[D2]0.66[H2]0.38,kp=1012.84T12exp(−42 260/RT),cm3mole·sec−1.
This power-rate law, with the surprising asymmetry in the H2—D2 dependence, can be rationalized by assuming that exchange occurs with high probability only during encounters between H2—D2 pairs, one of which is vibrationally excited to approximately 30 kcal/mole; that the probability for metathesis is low between molecules in their low-lying vibrational states even when their relative kinetic energy along the line of centers plus vibrational excitation exceeds the activation energy. Support for this mechanism is provided by an approximate calculation of the pre-exponential term of the exchange rate constant from vibrational relaxation data.
Vibrational—Rotational Study of the Electronic Ground State of the Hydrogen Molecule
Authors: Wolniewicz L. 
Published: January 2005

The Schrödinger equation for nuclear motion in H2, HD, and D2 has been integrated numerically with accurate internuclear potential for numerous vibrational and rotational states. The theoretical dissociation energies are larger than the experimental results by 3.9, 4.7, and 3.6 cm−1 for H2, HD, and D2, respectively. It is argued that this discrepancy exceeds the possible computational inaccuracies and is not due to the adiabatic approximation. The radiative corrections are also discussed and it is shown that the correction to the dissociation energy is definitely smaller in absolute value than 0.4 cm−1.
The computed vibrational quanta are larger than the observed ones by about 1 cm−1 while the theoretical rotational quanta are in a very good agreement with experiment.
The vibrational—rotational wavefunctions are used for averaging of some electronic expectation values. The computed quantities agree with experiment within the experimental errors.
Considerations on the Rotation—Vibration of Triatomic Molecules
Authors: Freed Karl F. , Lombardi John R. 
Published: January 2005

In order to incorporate the variations of the moments of inertia and the reduced masses for vibration generated by large-amplitude vibrations into a treatment of the vibration—rotation interaction of polyatomic molecules, we investigate the use of the instantaneous moments of inertia to define the rotating reference frame. This technique does not suffer from the disadvantage of tying the treatment to any equilibrium configuration and treats all vibrating molecules as asymmetric tops.
The case of large-amplitude bending in triatomic molecules is used to illustrate the general features of such a method. An adiabatic approximation is used to separate the stretching from the bending vibrations, exhibiting the important aspects of the coupling of these modes. The properties of angular momenta are utilized, and the resulting equations for the bending vibration are a set of coupled differential equations which are analogous to the algebraic equations encountered in the treatment of the rotational motion of asymmetric-top molecules.
As a simple test of the validity of the Hamiltonian, the l-type doubling parameter is calculated for a number of molecules in good agreement with experiment.
Non-Gaussian Behavior of the Polyethylene Chain of Finite Length
Authors: Nagai Kazuo 
Published: January 2005

The second and fourth moments 〈r2〉 and 〈r4〉 of the end-to-end distance r of the polyethylene chain of length n are calculated, based on expressions previously derived. The model employed is the familiar three-state [trans (θ(T)=0°), gauche (θ(G)=120°), and gauche prime (θ(G′)=−120°)] rotational—isomeric model. An approximate expression for the reduced distribution function Pn(ξ)=4π〈r2〉*32ξ2Wn(〈r2〉*12ξ), which is readily obtainable from 〈r2〉 and 〈r4〉, is calculated as a function of the reduced end-to-end distance ξ=〈r2〉*−½r. Here Wn(r) is the distribution function in the usual sense and 〈r2〉*=nlin lim n→∞n−1〈r2〉.(An asterisked quantity denotes the corresponding Gaussian analog.) Approximate expressions for Wn(0)/Wn*(0) and 〈r−1〉/〈r−1〉* are also calculated as functions of n.
For the GG′-eliminated trans-preferred chain with reasonable values for parameters, which approximates the polyethylene chain: (i) 〈r2〉 and 〈r4〉 are smaller than their respective Gaussian analogs, the deviation being more pronounced with the latter, (ii) the approximate distribution function becomes sharp and somewhat shifted toward ξ=0 compared with the Gaussian distribution, and (iii) as n decreases from infinity, both Wn(0)/Wn*(0) and 〈r−1〉/〈r−1〉* first decrease gradually from unity, then pass through minima, and subsequently increase rapidly, largely exceeding unity. The GG′-eliminated gauche-preferred chain behaves like the chain just mentioned above in all respects. For the gauche-preferred independent-rotation chain, the opposite trends result for Items (i) and (ii) above; for Item (iii), both quantities monotonously decrease as n decreases. It is shown that for the polyethylene chain, Kuhn's equivalent random chain does not correctly predict (considerably underestimates) the departure from the Gaussian distribution.
LCAO SCF Computations for Ammonia
Authors: Kaldor U. , Shavitt I. 
Published: January 2005

A series of LCAO SCF calculations with Slater-type atomic orbitals were performed for the NH3 molecule over a range of bond lengths and angles (keeping at least C3v symmetry). In addition to a minimal basis set (with two different values for the hydrogen 1s orbital exponent), two types of extended basis sets were examined for the planar and the experimental equilibrium configurations. One of these consisted of a ``double zeta'' set on N and one 1s function on each H (with exponent 1.20), while the other involved a minimal set augmented by four ``bond orbitals''−2s and 2p functions centered on N but having their maximum densities in the bond regions.
The results for the minimal basis set are typical for this type of calculation, giving good ionization energies and an inversion barrier height which is twice the experimental value. The extended basis sets, though giving fairly low total energies, resulted in an excessive concentration of charge on the nitrogen atom, with a high dipole moment and a zero, or even small negative, inversion ``barrier''. Direct calculations were also performed for the NH3+ ion in the pyramidal and planar configurations.
Electronically Excited NO by Photodissociation of NO2 and NOCl
Authors: Welge Karl H. 
Published: January 2005

The photodissociations of NO2 and NOCl yielding electronically excited NO are observed by the fluorescence of NO(γ) and NO(β) bands: NO2→ lim hνNO(A 2Σ+;v=0)+O(3P),NO2→ lim hνNO(B 2Π;v=0,1,2,3)+O(3P),NOCl→ lim hνNO(A 2Σ+;v=0,1,2)+Cl(2P0).
The dissociations of NO2 are observed at wavelengths 1165, 1236, and 1295 Å but not at 1470 Å and longer wavelengths, and the dissociation of NOCl is found at wavelengths from 1165 Å up to about 1600 Å. The NO(A 2Σ+) and NO(B 2Π) molecules from NO2 and NOCl are strongly rotationally excited.
Emission Polarization and Circular Dichroism of Hexahelicene
Authors: Weigang O. E. , Turner J. A. , Trouard P. A. 
Published: January 2005

Measurements of the relative polarization of electronic transitions and their circular dichroism are reported for hexahelicene (phenanthro[3,4-c]phenanthrene). A new assignment scheme for the electronic spectrum is proposed in the light of these measurements.
The longest wavelength 1Lb (375–425 mμ) transition shows evidence of mixed polarization in the fluorescence which would arise from vibronic coupling in the emission process. Its circular dichroism, which may contain ``forbidden'' character, is negative for the dextrorotatory molecule.
The 1La (325–375 mμ) transition has a strong positive circular dichroism which determines the optical activity at visible wavelengths. The success of previous chirality calculations for hexahelicene hinges on a satisfactory treatment of this transition and the rather complex detailed polarization of its excited state. Discrepancies between predicted optical activity by free-electron and by Hückel MO calculations are noted. The 1Bb (280–325 mμ) transition has a weak circular dichroism whose sign is difficult to determine from experiment; calculations predict that it should have the same sign as 1La. What appears to be the 1Ba (or 1Cb) (230–280 mμ) transition has a stronger negative circular dichroism.
Because of the out-of-plane distortion of the molecule, relative transition polarizations somewhat peculiar for polynuclear aromatics are expected. These account for certain data in the emission polarization, phosphorescence lifetime, and phosphorescence—fluorescence relative quantum yield.
Thermodynamic Properties of Condensed CO
Authors: Gill E. K. , Morrison J. A. 
Published: January 2005

Thermodynamic properties (heat capacities; vapor pressures; heats of transition, fusion, and vaporization) of condensed CO containing 2.6±0.4% N2 have been measured between 2.5° and 78°K. No anomalies nor unusual thermal behavior were observed at the lowest temperatures. The dielectric constant of the pure solid has been measured between 6°K and the melting point and found to be close to unity over the entire temperature range. The residual entropy of CO is discussed in the light of these results.
The N2 impurity displaces the melting and solid transition regions to lower temperatures but does not appear to alter their forms. Analyses of the vapor-pressure data show that the liquid solution of N2 in CO is nearly an ideal one. Deviations from ideality occur in the solid phases, particularly in the α phase that exists in the region T&lt;61°K.
A simple argument, based on the temperature dependence of the equivalent characteristic temperature for the entropy of the α phase, supports the view that CO molecules librate in the crystal and places an upper limit of 85 cm−1 on the librational frequency. This is to be compared with an average value of 70 cm−1 estimated from the optical spectrum of the crystal.
Deuteron Quadrupole Coupling Constant in CH3D
Authors: Caves T. , Karplus M. 
Published: January 2005

The deuteron electric-field gradient qD and quadrupole coupling constant (eqQ/h)D for CH3D have been calculated from extended basis-set LCAO-MO-SCF wavefunctions determined by Woznick for this system. Comparison of the results for different wavefunctions and different distances suggest that a reasonable theoretical estimate is (eqQ/h)D=210±30 kc/sec. Since this disagrees significantly with the present experimental estimate [(eqQ/h)D=100±50 kc/sec], a more refined measurement would be desirable.
Simplified schemes are examined for calculating (eqQ/h)D with neglect of three-center integrals or introduction of a two-center model; errors of 5% to 20% result.
Microwave Spectra of Some Isotopically Substituted Phenols
Authors: Forest Harvey , Dailey B. P. 
Published: January 2005

The rotational spectra of C6H518OH, C6D5OH, and C6D5OD were successfully analyzed to obtain their respective rotational constants in the region from 20 000 to 31 000 Mc/sec. The splitting of the rotational transitions due to the hindered internal rotation of the hydroxyl group was noted in C6H518OH and C6D5OH while the spectrum of the C6D5OD isotope was unsplit. As is the case in C6H516OH, the high barrier to internal rotation produced a doublet spectrum, where the splittings were effectively independent of the J transition and correspond to 115 and 103 Mc/sec for the C6D5OH and C6H518OH isotopes, respectively. The experimental rotational constants are: IsotopeA(Mc/sec)B(Mc/sec)C(Mc/sec)C6D516OH4682.677±0.0062422.815±0.0041596.930±0.004C6D516OD4654.378±0.0052342.102±0.0041558.383±0.004C6H518OH5650.076±0.0082487.321±0.0051727.232±0.005.
It was found that the structure of the C–O–H fragment could not be uniquely determined from the rotational constants. Two structures were found which were essentially the same except for C–O–H configuration relative to the internal rotation axis. Both structures fit the experimental spectra equally well and have standard deviations of 3.2 Mc/sec. The parameters for Structures (I) and (II) are, respectively: C–C, 1.3956±0.0002 and 1.3954±0.0002 Å; C–H, 1.081±0.001 and 1.082±0.001 Å; O–H, 0.985±0.01 and 0.944±0.009 Å; C–O, 1.370±0.001 and 1.379±0.001 Å; COH, 111.87±0.35 and 105.04±0.35 deg; α (off angle from rotation axis), 6.34±0.60 and −11.17±0.50 deg.
In Structure (I) the oxygen and hydrogen are on the same side of the internal rotation axis while in Structure (II) they straddle it. Structure (II) is believed to be the more reasonable.
Neither C–O–H structure could correlate the splitting data to one value of the barrier height, but good agreement was obtained if the moment of inertia of the hydroxyl group was based on CH3OH. The barrier height obtained was 1150 cm−1.
Comparison of Dielectric and Mechanical Relaxation Processes in Glycerol—n-Propanol Mixtures
Authors: Kono R. , Litovitz T. A. , McDuffie G. E. 
Published: January 2005

Dielectric and shear mechanical relaxation data are reported for mixtures of glycerol and n-propanol. Measurements of the dielectric permittivity were made at −60° over a frequency range from 50 cps to 600 kc/sec and indicate a distribution of dielectric relaxation times of the Davidson—Cole form. This distribution was observed to narrow with increasing n-propanol content. Shear mechanical measurements in the temperature range +10° to −150°C and frequency range 10 to 130 Mc/sec indicate a broad distribution of times which is essentially independent of the relative concentration of the two components of the mixture.
The width of the dielectric distribution is shown to be a function of the ratio of the average dielectric to shear relaxation times, τ̄D/τ̄S, becoming narrow as this ratio becomes large. These results are in accord with the concepts of McDuffie and Litovitz who proposed that the origin of the distribution of dielectric relaxation times is related to a cooperative breakup of the liquid structure and that the effects of this structural breakup on the dielectric process are lessened when the dielectric reorientation process takes a much longer time than the structural breakup.
In pure n-propanol it is found that τ̄D/τ̄S is temperature dependent in contrast to a temperature-independent ratio in glycerol. In order to explain these results, it is suggested that the local free volume in n-propanol needed for dielectric orientation is larger than that needed for the shear process. Using the free-volume theory of Macedo and Litovitz it is estimated that in pure n-propanol the critical local free volume for dielectric orientation is 40% larger than the critical local free volume for shear flow.
Transition Moment of the First Positive Band System of Nitrogen
Authors: Jeunehomme M. 
Published: January 2005

The light decay after interruption of a pulsed rf discharge was analyzed in the nitrogen first positive system as a function of nitrogen pressure and wavelength.
Values for the natural lifetime of a number of vibronic levels of the B 3IIg state were deduced. The variation of the electronic transition moment over internuclear separation was investigated. The results were found to be dependent on the potential model assumed in the calculations of the Franck—Condon factors. A value of about 2×10−3 is proposed for the oscillator strength f0,0.
Experimental and Theoretical Considerations for Ionization of Simple Hydrocarbons, H2, D2, O2, H2O, and the Rare Gases by 2.2-MeV α Particles and by Electrons in a Mass Spectrometer
Authors: Rudolph P. S. , Melton Charles E. 
Published: January 2005

The measurement of ionization cross sections and mass spectra produced by 2.2-MeV α particles is briefly discussed with emphasis on experimental details which lead to results which are more accurate than those of the previous study. Experimental techniques and apparatus have been developed and applied to these measurements for H2, D2, O2, H2O, CH4, C2H2, C2H4, C2H6, and the rare gases. The experimental results for H2 and He are compared with detailed calculations in the Börn—Bethe approximation and for Ar in the classical approximation. The ionization cross sections for the removal of one electron conform rather well to the predictions although the experimental results tend to be somewhat lower, than the theoretical values. Experimental values for He, Ar, H2, and N2 are in good agreement with extrapolated data of McDaniel and co-workers.
The mass spectra produced by 2.2-MeV α particles show the same fragments as those produced by electrons with the same velocity, but the abundance of some fragment ions differ by a factor of four. Mass spectra produced by 75-eV electrons (the usual energy employed in conventional mass spectrometry) also show significantly more dissociative ionization than those produced by 2.2-MeV α particles.
On the Eu3+ Fluorescence of Mixed Metal Oxides. IV. The Photoluminescent Efficiency of Eu3+-Activated Oxides
Authors: Blasse G. 
Published: January 2005

Four rules are proposed to explain the photoluminescent efficiency of Eu3+-activated oxides: (a) uv absorbing groups in the host lattice should be nearest neighbors in the crystal structure to have sufficient wavefunction overlap; (b) the emission spectrum of the absorbing group should overlap the absorption spectrum of the group; (c) the configuration consisting of the center of the absorbing group, the intermediary O2− ion, and the Eu3+ ion should be as collinear as possible; (d) O2− ions surrounding the Eu3+ ion must feel a high potential field.
If all these rules are satisfied, highly efficient Eu3+ phosphors can be expected; if one or more of the rules are violated, phosphors with low efficiency must be expected.
These ideas are applied to a large number of Eu3+-activated oxides. There is a very good agreement between experimental results and predictions from the rules.
As far as possible the physical background of the rules is discussed.