bluuebunny/crossref_metadata_2025_split huggingface dataset:
Gas-Phase Photolysis of Isopentane at Photon Energies below and above the Ionization Energy
Authors: Scala A. A. , Ausloos P. 
Published: January 2005

The photolysis of (CH3)2CDCH2CH3, (CH3)2CHCH2CD3, and of equimolar (CH3)2CHCH2CH3−(CD3)2CDCD2CD3 mixtures has been investigated in the presence of NO at 1470 Å (8.4 eV), 1236 Å (10 eV) and at 1048–1067 Å (11.54–11.72 eV). Furthermore, in order to obtain quantitative information concerning the free radicals produced in the fragmentation processes, (CD3)2CDCD2CD3 has been photolyzed at 1236 Å in the presence of various concentrations of H2S.
Fragmentation of the excited isopentane molecule produced at 8.4 or 10 eV, as well as of the superexcited molecule formed at 11.54–11.72 eV, yields mainly olefinic products and the following radicals, in order of decreasing yield: H&gt;CH3&gt;C2H5&gt;C3H7. In addition, on the basis of the various isotopically labeled products, it is concluded that methane, ethane, and propane are split off from the excited isopentane molecule by formation of (a) the corresponding carbene (1,2 elimination), or (b) the corresponding olefin (1,3 elimination). When the energy of the photon is increased from 8.4 to 10.0 eV, the 1,2-elimination processes become more important relative to the 1,3-elimination processes in all cases. At 11.54–11.72 eV, the parent ion decomposes to form methane and ethane mainly by a 1,3-elimination mechanism. It is derived from the results that, in addition to the C4H8+ and C3H6+ ions formed by the latter fragmentation processes, the ions C4H9+ and C3H7+ are produced with ion-pair yields (M/N) of 0.15 and 0.06, respectively. The ion-pair yield of the parent ion is estimated to be 0.35. In contrast to observations derived from an earlier investigation of photoionization of n-butane at 11.54–11.72 eV, an increase in pressure from 0.15 to 15 torr has only a minor effect on the fragmentation of the parent ion.
Microwave Spectrum, Molecular Structure, Barrier to Internal Rotation, and Dipole Moment of Methylketene
Authors: Bak Børge , Christiansen Jørn J. , Kunstmann Kaj , Nygaard Lise , Rastrup-Andersen John 
Published: January 2005

The microwave spectra of the CD3 species and of seven monosubstituted isotopic species of CH3CH=C=O have been examined, and a complete structure of methylketene has been obtained by the substitution method. The structural parameters found are: C=O=1.171, C=C=1.306, C–C=1.518, C–H (vinyl)=1.083, and C–H (methyl)=1.083 or 1.11 Å (see text). The ketene group is linear, and the methyl group is staggered, ∠CCC=122.6°, ∠C(1)C(2)H(2)=113.7°, and ∠HCH=109.9° or 108.8°.
The barrier splittings were used to determine the value of the barrier to hindered rotation and the angle between the methyl-axis and the principal-axis system for each of the monosubstituted species; V3=1177±20 cal/mole. The dipole moment of methylketene was measured on the 21,2→31,3transition, yielding μa=1.755, μb=0.35, μ=1.79 D.
On the Calculation of Autocorrelation Functions of Dynamical Variables
Authors: Berne Bruce J. , Boon Jean Pierre , Rice Stuart A. 
Published: January 2005

In this paper we develop a formalism for calculating the autocorrelation function of a dynamical variable in terms of a well-defined memory function. Guided by simple physical arguments, an ansatz is adopted for the functional form of the memory function. This ansatz asserts that the memory of dynamical coherence decays exponentially. It is found that:
(a) Despite the monotonic exponential decay of the memory function, the autocorrelation function deduced can display negative regions in some circumstances and decay monotonically in other circumstances.
(b) The form of the autocorrelation function deduced is identical with that obtained from two other very different analyses, suggesting that the major properties of the function are of general validity.
(c) The computed linear momentum autocorrelation function and power spectrum for liquid Ar are in good agreement with the computer experiments of Rahman.
(d) The computed dipolar autocorrelation function reproduces all the features of the experimentally determined autocorrelation function, though at present insufficient data are available to provide a quantitative test of the theory.
(e) The ansatz used, although obviously not exact, is consistent with the theory of linear regression (Appendix B).
Low-Pressure Study of Heterogeneous Hydrogen—Deuterium Exchange at Steady State
Authors: Bergsnov-Hansen Bjorn , Pasternak R. A. 
Published: January 2005

The hydrogen—deuterium exchange reaction on a clean molybdenum filament has been investigated at pressures of 10−8 to 10−5 torr and temperatures of 25° to 800°C; a steady-state approach was employed, and the exchange rates were measured with a mass spectrometer. The adsorption step is found to be rate determining and is characterized by pressure independent total sticking probabilities of about 0.4 at temperatures above 200°C, and declining to 0.2 at 25°C; the sticking probabilities are identical for the three hydrogen species. The net rates of adsorption and the saturation coverages at 25°C measured for hydrogen and for deuterium show also that the surface cannot distinguish between the hydrogen species.
Both the high value and the pressure independence of the sticking probability indicate that at either temperature, exchange occurs in adsorption states of low population. Surface coverage is immeasurably small at high temperatures. However, since at 25°C the total surface coverage is of the magnitude of a monolayer, exchange apparently involves adsorption states of low coverage which may be identified with low-energy states observed previously in flash-filament studies.
Atomic Distribution and Electrical Properties of Liquid Mercury—Thallium Alloys
Authors: Halder N. C. , Metzger R. J. , Wagner C. N. J. 
Published: January 2005

The x-ray-diffraction patterns of liquid binary Hg–Tl alloys have been measured using a theta—theta diffractometer and quartz crystal monochromatized Mo Kα radiation. Pure mercury and five alloys of 5, 8.5, 16, 28.6, and 40 at. % thallium were studied at room temperature (25°C), whereas pure thallium was studied at 350°C. Since mercury and thallium are next to each other in the periodic table, the contribution of the Laue monotonic scattering to the scattered intensity should be negligible.
The radial distribution functions were refined by an error-analysis program and then the refined interference functions were computed and used to calculate the electron transport properties of the alloys. The general equation for the scattering cross section of the electrons in liquid metals has been used and expressed in terms of the matrix elements of the pseudopotentials and the interference functions of the respective elements. Assuming that the atomic distribution is random, the resistivity and thermoelectric power of the alloys and the pure elements were calculated. There is some disagreement between the calculated or predicted and measured values of the resistivity. The predicted value of resistivity of mercury and thallium are 30 and 60 μΩ·cm, respectively.
The interatomic distances were found to vary linearly from 3.05 for pure Hg to 3.28 Å for Hg−40 Tl alloy. The value for pure Tl is 3.30 Å. This linear variation is not in agreement with previous results. The positive deviation from Vegard's law has been attributed to the electrochemical and valence difference of the two elements. The behavior of the coordination number with concentration, the change being from 10.0 to 11.5 for pure Hg to pure Tl, is similar to that of the interatomic distance with the concentration.
Calculation of the Lowest Energy Configurations of Nucleotide Base Pairs on the Basis of an Electrostatic Model
Authors: Nash H. A. , Bradley D. F. 
Published: January 2005

In this report we have taken as a point of departure the idea that the principal force acting between two nucleotide bases is the Coulomb law force between the localized formal charges on each atom of one base and each atom of the other base. The charges were computed by semiempirical quantum-mechanical methods for both σ and π electrons separately and added together. The energies of attraction between all possible pairs of the four common bases adenine, uracil, cytosine, and guanine in all possible coplanar orientations were computed automatically on a Honeywell 800 digital computer. Twenty-seven deep potential-energy minima were found. Although it was not possible to find gas-phase geometries and energies with which to compare these computed stable states, the energies in solution and known geometries in crystals, cocrystals, and polymer fibers were, with a single possible exception, completely consistent with the predicted structures and relative energies. Examination of the computed structures showed that, with few exceptions, the N–H···O and N–H···N short contacts are essentially linear, so that although the forces which led to these geometries were purely electrostatic in nature, the complexes may for convenience be thought of as being ``hydrogenbonded complexes.'' In several of the computed structures, nonlinear N–H···O and N–H···N close contacts were observed and these can be conveniently thought of as ``bifurcated hydrogenbond complexes'' which have been observed in nature although not, as yet, in purine and pyrimidine complexes.
The need for employing formal charges to reproduce the electrostatic forces was demonstrated by showing that dipole—dipole forces do not lead to potential minima and/or stable structures. Tests with a variety of computed charge densities showed that variation in the charges does not affect the equilibrium geometries or their relative stabilities although it does affect the absolute energies.
The method described therefore appears to be a satisfactory method for obtaining trial geometries for x-ray-diffraction analyses of crystals and fibers, and for estimating relative stabilities for use in predicting which pairings can occur where choice is possible, i.e., in codon—anticodon complexes, in hairpin-folded RNA complexes, as well as in cocrystals.
Infrared Spectrum of Crystalline CH2I2. Crystal Structure and Phase Transition
Authors: Marzocchi M. P. , Schettino V. , Califano S. 
Published: January 2005

The infrared spectra of two crystalline modifications, I and II of CH2I2, have been measured at various temperatures ranging from 3° to −196°C between 4000 and 400 cm−1. The transition between the two crystal forms I→II has been demonstrated to be monotropic.
The absence of the A2 fundamental in Form I allows a C2v site symmetry to be established, while for Form II the activity of the A2 fundamental and the observed factor-group splitting for A1, B1, and B2 fundamentals is consistent only with trivial site symmetry.
Among the multiplicity of factor groups consistent with the observed crystal spectra, a choice is possible using packing considerations. Orthorhombic, (D2h5) or (D2h13), and monoclinic (C2h5) structure are proposed for Forms I and II, respectively, which are consistent with the ir crystal spectra and densities of the two crystalline solids.
Some Considerations of Unimolecular Rate Theory. II. Aspects of the General Theory
Authors: Buff Frank P. , Wilson David J. 
Published: January 2005

The rate constant for a unimolecular reaction is obtained in alternative ways from the Liouville equation supplemented by the strong collision assumption. The final expression for the rate constant explicitly includes the details of the interactions between the collision process and intramolecular processes. The ``new-approach'' formula, obtained by Slater on intuitive grounds, is recovered when the collisional relaxation time is evaluated from the kinetic theory collision constant.
With this assumption, the rate constant is expressed in terms of the eigenvalue problem generated by the Liouville operator of the isolated molecule in its interesting states. This expression provides a basis for a perturbative evaluation of the rate constant.
Statistical arguments are used to develop consistency theorems for checking experimental data. Reversible isomerization reactions are discussed, and a transmission coefficient which monotonically increases with pressure is deduced.
Nuclear Magnetic Resonance and Relaxation of Hexafluoride Molecules in the Solid
Authors: Blinc R. , Pirkmajer E. , Slivnik J. , Zupančič I. 
Published: January 2005

The 19F magnetic resonance absorption spectra and spin—lattice relaxation times of polycrystalline UF6, PuF6, WF6, OsF6, PtF6, MoF6, and XeF6 have been studied as a function of temperature and magnetic field. In all investigated metal hexafluorides the chemical shifts were found to be quite large, exceeding the dipole—dipole broadening in the solid and allowing an unambiguous determination of the tetragonal distortion of the hexafluoride octahedra in the low-temperature orthorhombic phase. The fluorines can thus occupy either axial or equatorial sites. The chemical shifts of the axial and equatorial fluorines (σ0, axial=−5350 ppm, σ0, equatorial=−2800 ppm) with respect to HF, and the anisotropies of the 19F screening tensors in PtF6, |σ∥—σ⊥ |a=1300±100 ppm and |σ∥—σ⊥ |e=1380±100 ppm seem to be the largest of all known fluorine compounds. A slow, thermally activated motion where each fluorine atom is alternatively axial and equatroial, and which consists of a hindered rotation and subsequent distortion of the octahedron in the crystalline field of its neighbors, produces at higher temperatures a coalescence of the spectrum into a single, narrow line.
The same fluorine motion was found to influence the spin—lattice relaxation both by a direct modulation of the resonance fields due to anisotropic chemical shifts tensors as well as by a modulation of the dipole—dipole coupling. The solid—solid transitions to the cubic phase in PtF6, WF6, and MoF6, were found to be accompanied by a very sharp increase in T1 and a lowering of the activation energy for molecular motion.
Thermal Accommodation Coefficients of Helium on Tungsten and Hydrogen on Hydrogen-Covered Tungsten at 325°, 403°, and 473°K
Authors: Wachman Harold Y. 
Published: January 2005

Measurements have been made of accommodation coefficients of helium and hydrogen from 324° to 473°K on tungsten under conditions designed to minimize amounts of adsorbable impurities from the experimental system.
The helium accommodation coefficient values, lying between 0.0194 and 0.0228, compare favorably with measurements at lower temperatures, obtained by Thomas et al., which are probably the best available. The data are also correlated with results of theories on accommodation.
Values of hydrogen accommodation coefficients have been associated with the population of atomic hydrogen on the surface estimated from the adsorption studies of Hickmott. To a first approximation, hydrogen accommodation coefficients are found to be independent of coverage at the high coverages of these experiments and inversely proportional to hydrogen temperature.
On the Stereoregularity of Vinyl Polymer Chains
Authors: Frisch H. L. , Mallows C. L. , Bovey F. A. 
Published: January 2005

We explore quantitatively the kind and extent of information concerning the distribution of configurational sequences in vinyl (and related) polymer chains obtainable from data such as presently available high-resolution NMR spectroscopy measurements. After finding the number of observationally distinguishable types of sequences, containing n monomer units [n(ads)], we obtain the necessary consistency relations which must hold between the observed proportions of n(ads), explicitly for all n≤5. Geometrically, the possible regions accessible to the n(ad) proportions form certain simplexes in a space of D(n) dimensions. The extreme points always correspond to repeating polymers. The computed dimensionality number D(n) plays a central role in fixing the number of free parameters of the polymer propagation model determinable by an n(ad) analysis. We discuss in some detail certain Markovian and the non-Markovian Coleman—Fox propagation model in relation to n(ad) analysis. We introduce the concept of a completely reversible polymerization mechanism. We include in our discussion chains of alternating tacticity such as can be found on polymerizing α, β disubstituted monomers.
We present a number of actual examples based on available NMR spectra of certain polymer samples which illustrate how our formalism is useful in (1) checking the consistency of peak and (2) checking the consistency of peak proportion assignment, (3) predicting lower-order n(ad) frequencies (unobtainable because of spectral complexity) from higher-order n(ad) frequencies, (4) ruling out certain propagation mechanisms, etc.
Atom—Atom Ionization Mechanisms in Argon—Xenon Mixtures
Authors: Kelly Arnold J. 
Published: January 2005

The atom—atom ionization process occurring in high-purity argon—xenon mixtures has been investigated by means of a conventional shock tube employing a microwave probe to monitor the electron-generation rate. All tests were conducted at approximately atmospheric pressure and at temperatures in the range between 5000° and 9000°K, corresponding to a neutral-particle density of 7.0×1017 cm−3. The cross-sectional slope constant for xenon ionized by collision with an argon atom is 1.8×10−20 cm2/eV±20%, that is, equal to that for xenon ionized by collision with another xenon atom. The data for the reaction of argon ionizing xenon are consistent with an activation energy of 8.315 eV, that is, of the xenon—xenon, atom—atom ionization process. No data were obtained for xenon ionizing argon. Good correlation was obtained between the cross sections for electron elastic momentum exchange derived from the microwave experiment and those obtained from beam experiments.
The argon—xenon ionization cross section implies that, for atom—atom processes in the noble gases at pressures ∼1 atm and temperatures ∼⅔ eV, the ionization cross section is independent of the electronic structure of the projectile atom.
High-Pressure Decomposition of Synthetic Garnets
Authors: Marezio M. , Remeika J. P. , Jayaraman A. 
Published: January 2005

The stability of synthetic garnets {Y3}[Fe2](Fe3)O12, {Y3}[Al2](Al3)O12, and {Y3}[Ga2](Ga3)O12 has been investigated at high pressures and high temperatures. Decomposition to the YXO3 compound (X=Fe or Al) with the perovskitelike structure and to the sesquioxide with the corundum structure has been found to occur in the former two garnets: {Y3}[X2](X3)O12→3 YXO3+X2O3, while in the case of yttrium—gallium garnet no breakdown was detected at pressures up to 44 kbar and 1000°C. The three garnets seem to behave differently at high pressure and high temperature. This appears to be related to the differences in the preference of the three cations for the tetrahedral site.
Results of preliminary studies of the effect of high pressure and high temperature on Li0.5Fe2.5O4 are presented.
The geophysical implications of the decomposition under high pressure and high temperature of the garnet structure are discussed.
Growth of Potassium Chloride Crystals from Aqueous Solutions. I. The Effect of Lead Chloride
Authors: Botsaris G. D. , Mason E. A. , Reid R. C. 
Published: January 2005

Single crystals of potassium chloride were grown from aqueous solutions under carefully controlled conditions of supersaturation, agitation, and impurity lead chloride concentration. The growth rate of the (100) face was determined.
It was found that PbCl2 concentrations as low as 10−8 moles PbCl2/mole KCl decreased the rate of growth, and the retardation increased markedly with increasing PbCl2 concentration. The growth process changed from a layer type to a dendritic form over a small supersaturation range. Mass transfer rates of both KCl and PbCl2 from the solution to the crystal face were estimated and conditions delineated where each may be the rate-controlling step. A mechanism is proposed to account for the formation of crystal clusters on the surface of KCl at slow stirring rates.
New Investigation of the 1SeAutoionizing States of He and H−
Authors: Holøien Erling , Midtdal John 
Published: January 2005

The present investigation should provide some insight into the convergence properties of some particular upper roots of the full Hamiltonian. In this connection detailed calculations using the scaling-variation orthogonalization procedure with different subsets of the basis up to order 54 have been carried through both for He and H−.
Subsets including basic functions up to g—g are used, and a complete mapping of the ten lowest roots of the energy matrix by a continuous variation of the scale factor has been carried out within a definite range. The investigation clearly demonstrates the stabilizing ability of particular upper roots throughout the extension of the basis if the orthogonalization is properly achieved. These roots in the second-step solution may be associated with certain autoionizing states (quasi doubly excited, discrete states). The eigenvalues, obtained by optimizing the scale factor for these roots, are compared with those obtained for resonance positions in He+ and H elastic-scattering calculations by using other approximate methods, and by experiments.
Interaction of Triplet Excitons and the Line-Shape Function
Authors: Iguchi Kaoru 
Published: January 2005

The general scheme of the interaction of triplet excitons is formulated to remove the degeneracy of spin energy levels of the system and predict the spread of each spin absorption line in the zero-field magnetic resonance spectra.
An approximate function of quasi-Lorentz type is derived for the shape of spectrum which represents the feature for different numbers of excitons interacting effectively. The maximum slope width is computed and it is proportional to the square of exciton density. Discussion is made in comparison with experimental data.
Pyrolysis of Ethylene in Shock Waves
Authors: Gay I. D. , Kern R. D. , Kistiakowsky G. B. , Niki H. 
Published: January 2005

The pyrolysis of ethylene, highly diluted by neon, was studied in the temperature range 1710°—2170°K with reflected shock pressures of 225 to 1600 torr. The products are C2H2 and H2 with some diacetylene at higher temperatures. The rate law is −d[C2H4]/dt=k32[C2H4][Ne]12 over a threefold ethylene and sixfold neon pressure range and the rate constant is given by log10k32=(0.01±0.19)−(50 500±1700)/2.303RT (cc/molecule)12sec−1.
In experiments with equimolar C2H4+C2D4 mixtures the hydrogen formed initially contains as much HD as H2 although at the same temperatures the reaction H2+D2→2HD is found to be relatively slow. In experiments with C2H4+D2 mixtures as much HD as H2 is formed in pyrolysis. Some relatively slow isotopic exchange between ethylenes and between C2H4 and D2 occurs, forming mainly C2H3D in the latter case.
An unexpectedly high sensitivity of the mass spectrometer to hydrogen formed in the pyrolysis suggested that it may be formed in vibrationally excited states. A number of diagnostic experiments could neither confirm nor reject this possibility.
Both a simple unimolecular reaction C2H4+M→C2H4*→C2H2+ H2*+M (where H2* is a vibrationally excited molecule) and several free-radical chain mechanisms of the Rice—Herzfeld type can describe the observed kinetics, although in all cases the relatively low observed activation energy is very difficult to explain. Thus no clear-cut identification of the most probable reaction mechanism is possible.