bluuebunny/crossref_metadata_2025_split huggingface dataset:
Bond Character in XYM-Type Molecules: Chlorine—Oxygen Compounds
Authors: Wagner E. L. 
Published: January 2005

An internally consistent LCAO—MO method is presented for determining the bond characters in XYM-type molecules involving second-row elements. This treatment is based on Moffitt's ``atoms-in-molecules'' method, but utilizes valence-state electronegativities of the atoms involved rather than the generally unavailable valence-state ionization potentials and electron affinities. Results from the two methods compare favorably for C–O and S–O compounds.
Application of the method to the oxygen compounds of chlorine shows that the chlorine atom 3d orbitals may significantly participate in the π bonding. A successful correlation between the calculated total π-bond orders and the Cl–O bond stretching force constants is obtained when a reasonable set of β(dπ) values is used, and the observed properties of the compounds are compatible with the calculated bond characters.
Some Theoretical Aspects of Bonding in N–F Compounds
Authors: Kaufman Joyce J. 
Published: January 2005

A separation of the effects of substituent groups into distinct σ and π contributions and the influence of the σ and π contributions on Hamiltonians and wave functions of various cores has proved useful for describing several diverse phenomena in N–F compounds.
(1) The ``anomalous'' difference in bond strengths in NF3 and NF2 where the N–F bond in NF3 is weaker than that in NF2.
(2) The nonconstancy of the δK value for F in ionization potentials of NF3 and CF3, whereas other substituent group effects were constant.
(3) The similarity between the effect of F on ionization potentials of NF2 and of CF3.
Theory of Paramagnetic Excitons in Solid Free Radicals
Authors: Lynden-Bell Ruth M. , McConnell Harden M. 
Published: January 2005

A linear chain of S=½ molecules with alternating intermolecular exchange integrals is used as a model to investigate the properties of paramagnetic excitons in solid free radicals from a theoretical point of view. The spin exchange Hamiltonian for the problem is H=Σν(JS2ν·S2ν+1+J′S2ν·S2ν−1),when J&gt;J′&gt;0. In the limiting case that J≫J′, the exciton bandwidth is simply J′. For this case we have calculated (a) the fine-structure splitting of the paramagnetic resonance of isolated (noninteracting) excitons, (b) wave functions for a free exciton gas of arbitrary excitation density, (c) the magnetic susceptibility of the exciton gas, and (d) first-order exchange interactions between excitations.
In the case that thermal energies are much larger than the exciton bandwidth, exchange interactions between excitations are simply proportional to the excitation concentration, but at temperatures corresponding to thermal energies less than the bandwidth, there is a ``Fermi hole'' type repulsion between the excitons that reduces the exchange interaction. In fact, the excitons behave somewhat like Fermi particles, except their number is not constant (depending on the temperature), and only one exciton may occupy a given momentum state, irrespective of spin. Also, when the exciton bandwidth is comparable to or larger than kT, it is predicted that the exciton fine-structure splitting will be temperature dependent.
When J′ becomes comparable to J, then the elementary exciton takes on a complicated, distributed spin structure.
Relationship between Absorption Intensity and Fluorescence Lifetime of Molecules
Authors: Strickler S. J. , Berg Robert A. 
Published: January 2005

The equations usually given relating fluorescence lifetime to absorption intensity are strictly applicable only to atomic systems, whose transitions are sharp lines. This paper gives the derivation of a modified formula 1/τ0=2.880×10−9n2〈ν̃f−3〉Av−1(gl/gu) ∫ εdlnν̃,which should be valid for broad molecular bands when the transition is strongly allowed.
Lifetimes calculated by this formula have been compared with measured lifetimes for a number of organic molecules in solution. In most cases the values agree within experimental error, indicating that the formula is valid for such systems. The limitations of the formula and the results expected for weak or forbidden transitions are also discussed.
Effect of an Electric Field on Silicon Oxidation
Authors: Jorgensen Paul J. 
Published: January 2005

Modified-marker experiments have shown that silicon oxidation is controlled by the diffusion of oxygen through the silicon dioxide film.
The application of an electric field across the silicon dioxide layer enhances oxidation when the silicon is made positive with respect to the oxide—gas interface, and retards oxidation when the silicon is negative with respect to the oxide—gas interface. Oxidation under a retarding electric field ceases when the voltage developed across the growing oxide is equivalent to the change in free energy of the oxidation reaction.
The dependence of the oxidation rate on an electric field indicates that oxygen ions are the predominant species diffusing through the growing oxide film.
Exchange of Interacting Nuclear Spins in Nuclear Magnetic Resonance. II. Chemical Exchange
Authors: Alexander S. 
Published: January 2005

The study of the effect of exchange on high-resolution nuclear magnetic resonance spectra is extended to chemical exchange. A Boltzmann equation for the nuclear spin density matrix is derived. This equation is nonlinear because two uncorrelated molecules participate in each exchange process. For temperatures high compared to the nuclear Zeeman energy the equations can, however, be linearized. In particular the steady-state (slow-passage) solutions can be obtained from a set of linear algebraic equations for the elements of the density matrix.
Explicit approximation formulas for the slow- and fast-exchange limits are derived.
As an example the spectrum of a molecule with two nonequivalent spin ½ nuclei, one of which is exchanging, is calculated explicitly for arbitrary exchange rates and coupling strengths, and the behavior of the spectrum is discussed in detail.
Vacuum Ultraviolet Photochemistry. IV. Photolysis of Propane
Authors: Okabe H. , McNesby J. R. 
Published: January 2005

The direct photolysis of propane has been carried out in the vacuum ultraviolet region, at room temperature. Isotopic analyses lead to the conclusion that hydrogen and methane are formed almost entirely by molecular detachment processes. Among the three observed modes of hydrogen detachment CH3CH2CH3→CH3CCH3+H2,CH3CH2CH3→CHCH2CH3+H2,CH3CH2CH3→CH2CHCH3+H2.Process (1), the detachment from the central carbon atom, is most important when the exciting wavelengths are the Xe resonance lines, whereas process (2) increases in importance with the Kr lines. Two modes of methane detachment were observed, CH3CH2CH3→CH3CH+CH4,CH3CH2CH3→CH2CH2+CH4,in which process (5) is more important at both wavelengths. It has been shown that ethylene is formed almost equally from the molecular detachment process and from the disproportionation of of ethyl radicals. Ethane is formed from the disproportionation of ethyl radicals as well as from the association of methyl radicals, the former being more important.
The processes (1) to (6), along with two additional processes, CH3CH2CH3→H+C3H7,CH3CH2CH3→CH3+C2H5,followed by secondary reactions of radicals, can reasonably account for the formation of the photolytic products.
Solubility of Naphthalene in Compressed Gases
Authors: King A. D. , Robertson W. W. 
Published: January 2005

The influence of hydrostatic pressure upon the equilibrium between solid naphthalene and its vapor in various inert gases has been measured over pressure ranges extending from 1 to 1100 atm and temperature ranges of 20° to 75°C, using spectrophotometric techniques.
The variation of naphthalene vapor concentration with the diluent gas density has been used to evaluate second virial coefficients representing diluent—naphthalene interactions for the following diluent gases: He, H2, Ar, N2, CH4, and C2H4. Although the virial coefficients thus obtained vary from large negative values for the systems involving the hydrocarbon gases to small positive values for the helium—naphthalene system, attractive 6–12 intermolecular potentials may be fitted to all the data.
Raman Spectral Studies of Molten Selenious Acid
Authors: Walrafen G. E. 
Published: January 2005

Photoelectric Raman spectra of molten selenious acid, obtained in the temperature range of about 70° to 175°C, provide evidence for the existence of a new species H2Se2O5. Comparisons of Raman bands of molten selenious acid with those of very concentrated aqueous solutions of sodium biselenite indicate common frequencies. In concentrated aqueous solutions, the equilibrium 2HSeO3— = Se2O52—+H2O provides Se2O52— ions in considerable concentrations. Similarly, in molten selenious acid the reaction 2H2SeO3 = H2Se2O5+H2O produces H2Se2O5 molecules in quantity. Raman lines common to the two spectra arise from vibrations which are characteristic of the Se2O5 grouping. Such vibrations are those produced by the bending and stretching motions of the Se′–O–Se′ linkage.
When the stoichiometric composition of the melt is between 50 and 66.67 mole% SeO2 in H2O, (H2SeO3)2 and H2Se2O5 are important species. At the higher compositions, however, one weak band of the melt was observed which is close in frequency to an intense band of solid selenium dioxide. The complexity of the Raman spectrum of that solid suggests considerable polymerization. Accordingly, (SeO2)n may make an increasing contribution as the composition is increased above 66.67 mole% SeO2 in H2O, although the contribution of that species to the Raman spectra of the melt is considered to be small at the lower compositions.
Use of Pressure to Investigate Local Symmetry and Compressibility in Glass
Authors: Tischer R. E. , Drickamer H. G. 
Published: January 2005

The effect of pressure to 120 kbar has been studied in several silicate and phosphate glasses. Transition metal ions have been introduced as tracers, and from the effect of pressure on the peak locations and intensities, local symmetry, local compressibility, and local distortions are deduced.
Tetrahedral sites are less compressible than octahedral sites. Both are less compressible than the bulk glass and about as compressible as transition metal oxides. Fe2+ and Ni2+ apparently enter octahedral sites which show nearly the bulk compressibility, and considerable distortion.
With increasing pressure there is a tendency for Co2+ and Ni2+ ions to transfer from tetrahedral to octahedra sites, as is predicted from the direction of change of site preference energy with pressure.
There is an irreversible change in structure of the phosphate glass above 50 kbar.
Vapor-Phase Growth Kinetics of Potassium Whiskers by Field Emission
Authors: Parker R. L. , Hardy S. C. 
Published: January 2005

Using the method originated by Gomer, we have studied the growth kinetics of potassium whiskers, as they grow from the vapor phase in an electron field emission tube. To the best of our knowledge, this represents the first use of an alkali metal as a field emitter.
The results for potassium whiskers show some features similar to those previously found for mercury, and some new features. The former include positive exponential growth, and the attainment of terminal lengths. New features include the absence, usually, of symmetric emission patterns, a very low activation energy for surface self-diffusion, the fact that the whiskers cannot be pulled off but instead appear to shorten gradually if the field is raised, and finally, an interesting sensitivity of the growth processes to illumination of the whiskers with visible light.
Influence of Vibrations on Molecular Structure Determinations. I. General Formulation of Vibration—Rotation Interactions
Authors: Herschbach Dudley R. , Laurie Victor W. 
Published: January 2005

The structure of a polyatomic molecule derived from observed spectroscopic moments of inertia differs appreciably from both the equilibrium structure and the zero-point average structure. A perturbation expansion of the moments of inertia is derived here in a form which allows the vibrational displacements to be referred to any fixed configuration of the atoms as origin. A method for evaluating the expansion coefficients is given which proceeds ``atom by atom.'' Linear WXYZ, branched WXYZ, and symmetric WXY3 molecules are treated in detail.
Empirical data show that the anharmonicity of the vibrations gives rise to the most important of the corrections required to obtain the equilibrium structure. Except for a few molecules, the anharmonicity is not accurately known. It is shown, however, that to a practical approximation the anharmonic contributions are completely absorbed in displacing the average configuration from the equilibrium one. Therefore the moments of inertia for the zero-point average configuration of a molecule can be derived from the observed effective moments by applying corrections which depend only upon the harmonic part of the vibrational potential.
Energy-Transfer Phenomena and Dissociation Processes in Electronically Excited Molecules
Authors: Lorquet J. C. , ElKomoss S. G. , Magee John L. 
Published: January 2005

This paper is an extension of previous work by Magee and Funabashi on the influence of electronic coupling on dissociation mechanisms. The same model (linear chain of diatomic molecules) has again been considered. The use of a dynamical version of the Hellmann—Feynman theorem allows the development of a unified treatment valid for any coupling. An application to an infinite linear chain of hydrogen moleculeions is made, and the following results are found:
(1) In the weak-coupling case, immediate dissociation occurs.
(2) In the intermediate case, the electronic excitation energy is converted into vibrational energy, mainly localized on the molecule initially excited. Redistribution of the vibrational energy then occurs.
(3) In the strong-coupling case, dissociation does not occur. Many molecules vibrate with comparable amplitudes.
Short chains, containing two, three, four, and seven molecules, are also considered. For these it is found that two or more dissociations usually occur regardless of the type of coupling and initial position of the excitation.
Finally, an application to intramolecular energy transfer is attempted. The case of the radiation induced isomerization of cis-polybutadiene into its trans isomer, studied by Golub, is examined. With certain restrictions, our results can be considered as supporting Golub's views.
Attractive Forces between Long Saturated Chains at Short Distances
Authors: Salem Lionel 
Published: January 2005

The assumption of locally additive forces is used to describe the London—van der Waals dispersion attraction between long saturated chains. For two linear chains of length L, opposed and parallel, each built out of N identical units of length λ(Nλ=L), the total dispersion energy is given by W=A4λ2D4ρ(3tan−1ρ+ρ1+ρ2),  ρ=LD, where D is the mutual distance of the two chains; A is the coefficient of the dispersion interaction between two basic units. At distances D much smaller than the molecular length L, the energy becomes proportional to L (or N) and inversely proportional to the fifth power of the intermolecular distance.
The coefficient A is calculated for the interaction between two CH2 units in hydrocarbon chains and the result is used to estimate the sublimation energy at 0°K of paraffin crystals. Sound agreement with experiment is obtained. Finally, the particularities of the attractive forces between long saturated chains are examined in more detail and their great sensitivity to distance is shown by several examples. Various applications to systems of biological interest are also suggested.
On the Optimum Use of Ionization-Efficiency Data
Authors: Morrison J. D. 
Published: January 2005

The electron energy spread causes much fine detail in ionization efficiency curves to be obscured. The reduction of this spread by electron velocity analyzers is experimentally quite difficult. The present paper examines whether it is possible to reduce the effect of the energy spread by any analytical method.
It is found that the problem is very similar to that of the detection of radar signals in noise, and that similar methods can be applied. It is shown that typical ionization efficiency data contain a very much greater amount of information than is generally believed. Methods are developed for obtaining a marked reduction in the scatter in the experimental data and for reducing the effective electron energy spread by a factor of four or five times. The methods do not require an exact knowledge of the form of the electron energy spread, although the better this knowledge, the better the ultimate result.
Tests made on several artificial examples and on two actual cases show that the methods are of some promise. The results of this study also indicate a possible unconventional solution to the problem of achieving even higher resolution in energy in experimental ionization efficiency curves.
Unimolecular Decomposition of Chemically Activated Propyl Radicals. Normal Intermolecular Secondary Kinetic Isotope Effect
Authors: Falconer W. E. , Rabinovitch B. S. , Cvetanović R. J. 
Published: January 2005

The vibrationally excited radical pairs n-propyl-d0, n-propyl-d6 and isopropyl-d0, isopropyl-d6 were produced at 25°C by the addition of H atoms to propylene-d0 and propylene-d6. Decomposition of n-propyl is by C–C rupture, whereas C–H rupture appears to be the predominant path for isopropyl decomposition, possibly to the exclusion of C–C rupture. The average rates of decomposition ka were determined relative to collisional stabilization by complete product analysis: for n-propyl-d0 C–C rupture, kan≃108 sec—1; for isopropyl-d0 C–H rupture, kai(—1)≃10—2 kan. For vibrationally excited propane, formed by the addition of H atoms to isopropyl radicals, kapr≃6.6×105 sec—1. The secondary kinetic isotope effect for n-propyl unimolecular decomposition in this nonequilibrium system was found to be ∼5.0 at the low-pressure limit, and ∼4.5 at the high-pressure limit; that found for isopropyl decomposition is ∼6.
The Marcus—Rice specific rate expression was used with a quantum statistical harmonic oscillator model to calculate theoretical rate constants for the decomposition of n-propyl radicals which are in good agreement with the experimental values. A loosened activated complex was assumed, in which all vibrational modes and internal rotations were taken as active. The effect of different treatments of the internal rotations on the distribution function f(E)dE was demonstrated to be small.
Interaction Energies and Transport Coefficients of Li+H and O+H Gas Mixtures at High Temperatures
Authors: Krupenie Paul H. , Mason Edward A. , Vanderslice Joseph T. 
Published: January 2005

Accurate potential energy curves for the X 1Σg+, A 1Σu+, B 1Πu, and C 1Πu states of the Li2 molecule are calculated from observed spectroscopic data by the method of Rydberg—Klein—Rees (RKR), and compared with previous quantum-mechanical calculations, Long-range attractive potentials are estimated by extrapolation of functions fitted to the RKR ground-state curves of Li2, LiH, and OH. From these, the repulsive potentials derivable from interacting ground-state atoms are estimated semiempirically. Collision integrals are computed from the potentials, and transport coefficients of the gaseous systems Li+Li, Li+H, and O+H are calculated for temperatures of 1000° to 10 000°K.
A surprising result is the extraordinarily large values of the collision inregrals for Li+Li (and Li+H) interactions, which result in unexpectedly small values of diffusion coefficient, viscosity, and thermal conductivity. For traces of Li in Li+H mixtures at low temperature, the thermal diffusion factor is very large. Various approximate formulas for viscosity and thermal conductivity of mixtures are seen to give poor agreement with exact calculations for the systems considered.
Dielectric Constant and Interactions of Polar Molecules
Authors: Cole Robert H. 
Published: January 2005

The theory of the dielectric constant previously developed by Van Vleck and others for permanent dipoles is extended to third-order interactions with inclusion of induced polarization represented by harmonic oscillators. This is carried out by a high-temperature expansion of the partition function in powers of dipole coupling energy.
Third-order interaction terms are of the triangle and shuttle forms found by Rosenberg and Lax for permanent dipoles, but for induced moment contributions only the triangle interaction sum is involved. For neither type of term can agreement with Onsager's model be secured in any reasonable way.
Kirkwood's treatment of long-range interactions is shown to be consistent with the series expansion results if short-range correlations are properly evaluated, but the results of this evaluation depend on molecular distributions.
Vibration—Rotation Spectra of CH4 and CD4 Impurities in Xenon, Krypton, and Argon Crystals
Authors: Cabana A. , Savitsky G. B. , Hornig D. F. 
Published: January 2005

The infrared spectra of CH4 and CD4, present as substitutional impurities in crystals of argon, krypton, and xenon, were studied at temperatures ranging from 5° to 40°K.
Both v3 and v4 of CH4 in xenon showed a simple four line pattern which is consistent with that expected for a slightly hindered rotor. In krypton and argon a fifth line appeared on v3, as expected from King's theoretical calculations for a tetrahedral rotor in an octahedral field. In addition, argon showed absorption due to pairs or higher aggregates of CH4 molecules.
The rotational spectrum of CD4 is much more highly perturbed. Detailed assignments to hindered rotational levels could not be made but the general features are in accord with King's model.
The frequencies of the CH4 and CD4 absorption bands increase steadily in going from xenon to krypton to argon. This can be accounted for by the decrease in the sizes of the cavities occupied, with corresponding larger repulsive interactions between the CH4 or CD4 and the host atoms.
Numerical Calculations by the Hellmann—Feynman Procedure
Authors: Salem L. , Alexander M. 
Published: January 2005

Simple calculations of atomic forces and of force constants in homonuclear diatomics confirm the unreliability of the Hellmann—Feynman procedure for calculating molecular forces unless very accurate one-electron densities are used.
Electronic forces are extremely sensitive to polarization and to small density changes near atomic centers, so that in general they cannot be used alone to determine accurately a totally a priori one-electron density. They may, however, be useful in testing the features of the density obtained from a variational wavefunction.
Statistics of Irreversible Termination in Homogeneous Anionic Polymerization
Authors: Coleman Bernard D. , Gornick Fred , Weiss George 
Published: January 2005

The effect of irreversible termination on the molecular weight distribution of a polymer prepared by homogeneous anionic polymerization is examined for cases in which the initiator is monofunctional and initiation is instantaneous. A general procedure for the calculation of the chain length distribution, as well as its moments, is presented. The procedure is then illustrated by an exact treatment of an isothermal batch polymerization for which we obtain the ratio rσ(t) of the weight to number average molecular weights as a function of the fraction of monomer consumed, the initial ratio σ of the terminator concentration to that of the initiator, the ratio ρ of the rate constant for termination to the rate constant for propagation, and the average degree of polymerization at time t, 〈n1〉t.
When σ is much less than unity, and the limiting number average D.P. 〈n1〉∞ is very large, rσ(t) approaches the following value as the reaction nears completion: rσ(∞) = 1+σρ(2+ρ)—1.