bluuebunny/crossref_metadata_2025 huggingface dataset:
Extinction Coefficients of Triplet↔Triplet Transitions in Aromatic Compounds
Authors: Keller Richard A. , Hadley Steven G.
Published: January 2005

The extinction coefficients and oscillator strengths were measured for the first strong triplet↔triplet transition in naphthalene, phenanthrene, and triphenylene. The results are: naphthalene, εmax=14 000 liter/mole·cm, f=0.05; phenanthrene, εmax=27 000 liter/mole·cm, f=0.11; triphenylene, εmax=7000 liter/mole·cm, f=0.05.
Electron Spin Resonance Study of Irradiated Anhydrous Maltose
Authors: Herak J. N. , Adamič K. , Blinc R.
Published: January 2005

The electron spin resonance spectra of γ-irradiated powdered anhydrous α-maltose have been measured. By temperature annealing it has been found that:

(a) The room temperature ESR spectrum consists of at least three different types (A, B, C) of radiation-induced damage centers.

(b) Type A centers seem to result from a rupture of the C-1—H bond. They transform into Type B centers with a half-life of about 110 min (in air at room temperature). The most probable type of transformation seems to be a change in configuration of the pyranose ring.

(c) Whereas Type B centers recombine under the above conditions with a half-life of about 26 h, the half-life of Type C centers exceeds a month. There are some indications that Type C centers are connected with a breaking of the glycosidic bond between two pyranose rings.
Numerical Solutions of the Percus—Yevick Equation for the Hard-Sphere Potential
Authors: Throop Gerald J. , Bearman Richard J.
Published: January 2005

The exact solution of the Percus—Yevick equation for the radial distribution function of a classical fluid of hard spheres found by Wertheim is evaluated numerically for a range of number densities. Comparison is made with the radial distribution functions found using other integral equations and Monte Carlo calculations.
Chemical Substitution Treated by Partitioning in the Hückel Approximation
Authors: Harriman John E.
Published: January 2005

A scheme is presented for the discussion of the effects of chemical substitution on one-electron energies and LCAO molecular orbitals in the simple Hückel approximation. The method is based upon a partitioning of the Hamiltonian matrix corresponding to the physical division of the system into molecule and substituent interacting through one bond. Graphical methods are developed and a series of hypothetical substituted benzenes are considered as examples.
Study of 13C Chemical Shifts in Substituted Benzenes
Authors: Maciel Gary E. , Natterstad J. John
Published: January 2005

Carbon-13 chemical shifts of several monosubstituted benzenes and para-substituted fluorobenzenes have been determined. The shifts of the para carbons in monosubstituted benzenes are found to correlate with Taft's resonance parameters, especially when the shifts are corrected by the amount of the meta shifts. A much less precise correlation is found between the 19F chemical shifts in the para-substituted fluorobenzenes and the 13C–F chemical shifts in the same set of compounds than between these 19F shifts and the para 13C shifts in monosubstituted benzenes. The relationships between the 13C and 19F shifts are considered in terms of the available theories of Karplus and Pople, Karplus and Das, and Prosser and Goodman, and are discussed in terms of π-electron densities. It is concluded that both 13C shifts in monosubstituted benzenes and 19F shifts in para-substituted fluorobenzenes will be useful indices of the π-electron densities in para positions.
X-Ray Diffraction Studies of Stable and Supercooled Liquid Gallium
Authors: Rodriguez S. E. , Pings C. J.
Published: January 2005

X-ray diffraction studies of stable and supercooled liquid gallium (mp 29.8°C) between 0° and 50°C show no important dependence of internal structure on temperature. Atomic radial-distribution functions were determined for 0°, 10°, 20°, 29.5°, 30°, and 40° and 50°C, from diffraction data to S = 11 Å−1. Diffraction measurements were made in reflection geometry with silver Kα radiation, scintillation counter, and pulse-height analyzer. The density distribution indicates 11 near neighbors; there may be a Ga–Ga complex.
Photochemically Generated Free Radicals. I. The Perinaphthenone System
Authors: Rabold Gary P. , Bar-Eli Kedma H. , Reid Eric , Weiss Karl
Published: January 2005

The photolysis of perinaphthenones in 2-propanol generates the corresponding hydroxyperinaphthenyl radicals which are relatively stable at room temperature. The electron spin resonance spectra can in most cases be interpreted in terms of splitting by two distinct groups of protons. Under high-resolution conditions small differences within these groups become apparent, and splitting by hydrogen on oxygen is observed. Detailed assignments are made on the basis of McLachlan MO—SCF calculations. The data indicate that |QOH|≈7 G. The same value is obtained by a calculation based on the configuration-interaction model of McConnell.
Schumann—Runge Bands of O2 in Solid Phases: Spectroscopic Measurement of Intermolecular Potentials
Authors: Schnepp O. , Dressler K.
Published: January 2005

The absorption spectra of oxygen in solid solution in nitrogen and argon have been investigated in the spectral region of the Schumann—Runge bands. Eleven discrete bands have been measured in each case. The vibrational structure is very similar to that of the free molecule for low vibrational quantum numbers of the excited electronic state but the spacings deviate considerably from those of the gas for higher quantum numbers, inasmuch as their convergence is slower. Potential curves have been constructed from the data for the oxygen molecule perturbed by the solid solvent cage. The deviations of these curves from those of the free molecule have been analyzed in terms of oxygen—argon and oxygen—nitrogen pair repulsive potentials.
Dielectric Relaxation of Nearly Spherical Molecules in Pure Liquids
Authors: Krishnaji  , Mansingh Abhai
Published: January 2005

The relaxation time of five nearly spherical molecules, nitrobromoform, nitrochloroform, tert-butyl cyanide, trimethylacetaldehyde, and tert-butyl amine, has been determined at different temperatures. The molar volumes of all these nearly spherical molecules are approximately equal, but the macroscopic as well as microscopic (molecular) relaxation times do not show linear dependence on viscosity. This suggests that although the molecules may have the same molar volume, they have different volume for dipole rotation. The Cole—Cole distribution parameter comes out to be zero for all these molecules. The free energy of activation for the dielectric relaxation process is much smaller than that for the viscosity process. The enthalpy of activation is less than free energy of activation, consequently the entropy of activation is negative for all the molecules.
Anisotropic Light Scattering by Inner-Field Fluctuations in a Dense Monatomic Gas
Authors: Theimer O. , Paul R.
Published: January 2005

The intensity of light scattering by monatomic gases is calculated by a new self-consistent-field method which is particularly suited to analyzing the effect of collective phenomena such as the fluctuations of the inner field. The polarizability density of atoms is introduced, and the properties of this space function are illustrated by an explicit quantum-mechanical calculation for the hydrogen atom. The Fourier coefficients of the polarizability density are combined with the Fourier coefficients of the particle density and are used as stochastic variables of the scattering system. The effective inner field for a specific particle configuration is obtained as an iterative solution of an integrodifferential equation, and is used for calculating the scattering intensity. This intensity is then averaged over all particle configurations. At ordinary pressures (αn≪1, α is atomic polarizability, n is the particle density) Rayleigh's scattering formula is confirmed, but at high pressures (αn→1) observable corrections to his formula are obtained. In particular, scattering is found depolarized contrary to Rayleigh's theory and to some recent work.
Viscosity and Thermal Conductivity of Gas Mixtures. Accuracy of Some Empirical Formulas
Authors: Burnett D.
Published: January 2005

Sutherland's empirical formula for the viscosity of a binary gas mixture is compared with the Chapman—Enskog theoretical first approximation. When the molecular weights of the constituents are nearly equal, Sutherland's formula is found to be a very good approximation to the theoretical result; when the molecular weights differ by a moderate amount, a formula of Sutherland's type may be theoretically impossible, but may nevertheless provide a satisfactory approximation; when the molecular weights are very different, a formula of Sutherland's type is theoretically possible, but the relative error is greater than when the molecular weights are nearly equal. Similar considerations apply to Wassiljewa's formula for the thermal conductivity. Generalizations of Sutherland's and Wassiljewa's formulas to mixtures of more than two components are not compatible with the theoretical first approximations.
Pi-Electron Valence Bond Theory; Application to Vinyl Chloride
Authors: Schug John C.
Published: January 2005

A valence bond theory is developed which accounts for differential overlap up to second order in both the Hamiltonian and overlap matrices. Electron-density operators are defined, and used to compute electronic charge distributions, also up to second-order in differential overlap. The theory is applied to the pi electrons of vinyl chloride, in order to investigate the effects of (a) omitting some of the polar electronic arrangements from the set of basis functions, and (b) neglecting differential overlap. Of these two common approximations, the latter is found to be much more serious. When it is used, the theoretically estimated integral parameters lead to very poor charge distributions; they must be changed by factors of 2 or 3 in order to obtain reasonable results. The neglect of polar structures which contain double ionizations has only small effects, but it appears that in general all structures involving single ionizations must be included. The most general calculations made on vinyl chloride, including differential overlap and all polar valence bond functions, gives results which compare very favorably with the available molecular orbital calculations, as well as with experiment.
Adsorption of Polymer Molecules at Low Surface Coverage
Authors: Hoeve C. A. J. , DiMarzio E. A. , Peyser P.
Published: January 2005

A statistical-mechanical treatment of a polymer molecule adsorbed on a solid surface is given. The surface coverage by adsorbed molecules is assumed to be sufficiently low that the interactions of the adsorbed polymer molecules with each other may be neglected. The partition function is derived for a polymer molecule with sequences of repeating units adsorbed at an interface and with other sequences (loops) held at the surface only at their ends. The assumption of Gaussian statistics for the loops leads to a formulation equivalent to that used for the helix-coil region in DNA molecules. A broad distribution of loop sizes is found, in contrast to Silberberg's theory in which a sharply peaked distribution is assumed. The latter theory predicts also small loops for all values of the adsorption free energy. In contrast, our theory predicts large loops and few units adsorbed for small adsorption free energies and small loops and more units adsorbed for larger adsorption free energies when the chains are sufficiently flexible. This result is also in partial disagreement with earlier theories that predict large loops, even for relatively large adsorption free energies. On the other hand, for stiff chains our theory predicts only a small number of loops with most units at the surface. A relationship between the initial slope of an adsorption isotherm and the molecular weight of the polymer is presented.
Two-State Theory of the Structure of Water
Authors: Davis C. M. , Litovitz T. A.
Published: January 2005

A new model of the structure of water is proposed. This model is based on the existence in water of puckered hexagonal rings similar to those in ice. In water, these rings are assumed to coexist in two structures: an open-packed icelike structure optimum for forming hydrogen bonds between rings, and a close-packed structure in which the molecules occupy a nearly complete body-centered cubic structure. Mole fractions and volumes consistant with both the specific volume and the observed radial distribution curve of water are obtained. The structure of water is estimated to be approximately 60% icelike at 0°C and 30% icelike at 100°C. The corresponding fractional amount of hydrogen bonding at 0°C is found to be 82% in good agreement with recent estimates from Raman spectral observations. A simple two-state expression for the expansivity is shown to agree with the observed thermal expansion of water over the temperature range 0° to 100°C. The lattice component (and therefore, at 4°C, the magnitude of the relaxational or configurational component) is 1.634×10−3/C°. Using these values, estimates of various other two-state thermodynamic parameters are obtained. Both the specific heat and compressibility of water are found to be approximately 50% structural over the temperature range.
Analysis of Relaxation Processes in a Multilevel System. A Many-Shot Expansion Technique
Authors: Serauskas R. V. , Schlag E. W.
Published: January 2005

The relaxation of a gas of polyatomic molecules, excited to a particular energy, is analyzed in terms of a random walk through all relevant energy states to yield the various stable products. The quantum yield is taken as the probability of forming a given one of these products. This relaxation problem is solved by means of an asymptotic, natural expansion in terms of the events in the process which are physically the most significant. The first term in this many-shot expansion is the strong collision result. For nearly strong collision processes and nonequilibrium systems this many-shot expansion yields a rapidly converging series for an arbitrary ordered set of transition probabilities, and hence constitutes a computationally useful solution to the Pauli master equation for such a nonequilibrium system. The relaxation problem is analyzed in terms of some assumed sets of collisional transition probabilities, including a model that assumes statistical equilibration of vibrational energy in every collision complex. It is shown in an application to an actual laboratory system that such a highly efficient model predicts many-shot behavior only slightly stronger than some recently observed experimental results.
Cluster Expansions and the Unitary Group
Authors: Yaris Robert
Published: January 2005

A further discussion of the possibility of product generators (see Paper II) is undertaken herein. In addition it is shown that the unitary group decomposes in a physically plausible way such that the cluster structure appears quite naturally. Through the use of this decomposition it is shown that there are three contributing effects which determine a cluster function: form, dynamics, and background. These three effects are then discussed in the context of the problem of the transferability of cluster functions.
Single-Crystal ESR Spectra and Covalent Bonding in [Cr(CN)5NO]3— Ion
Authors: Kuska H. A. , Rogers M. T.
Published: January 2005

ESR and infrared spectra of [Cr(CN)5NO]3— in dilute solid solution in various alkali halide lattices have been obtained, including ESR data as a function of crystal orientation. The anisotropic and isotropic hyperfine couplings to 14N, 13C, and 53Cr have been computed from the ESR data and values of g∥ and g⊥ are reported. The results have been interpreted, as far as possible, in terms of current molecular orbital theories of bonding in the complex ions of transition metal elements.
Spin-Echo NMR Studies of Chemical Exchange. III. Conformational Isomerization of Cyclohexane and d11-Cyclohexane
Authors: Allerhand Adam , Chen Fu-ming , Gutowsky H. S.
Published: January 2005

Equations derived previously for NMR chemical-exchange effects have been used in proton spin-echo studies at 26.85 Mc/sec of the chair—chair isomerization of cyclohexane and d11-cyclohexane in 1:1 v/v solutions in CS2. The d11-cyclohexane is particularly suited for such studies because of its very long natural relaxation time T20, and exchange rates of 0.53 to 3.8×104 sec−1 were determined for it in the temperature range between −98° and +25°C. The heteronuclear coupling in d11-cyclohexane does not affect the spin-echo results. However, in cyclohexane the homonuclear coupling and the shorter T20 restricted the temperature range of accurate rate determinations to −60° and −25°C, respectively. The exchange rates are the same within experimental error for both compounds, as are ΔH‡, ΔFcb‡, and ΔScb‡ which were found to be 9.1 and 10.3 kcal/mole and −5.8 eu. The latter are compared with the results of several previous studies, and it is demonstrated that, in most if not all of the studies, there are systematic errors in the exchange rates. These errors have a modest effect upon ΔH‡ but their effect upon ΔS‡ is so large that the values for it are unreliable. Several factors governing the use of spin echoes to study chemical exchange are discussed.
Cross Section for the Exchange of Charge Between Neutral and Positive Nitrogen
Authors: Breene R. G.
Published: January 2005

An ab initio calculation of the nitrogen charge-exchange cross section is described for (1) a single-interaction curve and unpolarized atomic functions of two types, (2) a single-interaction curve and a polarized neutral-atom function, and (3) multiple interaction curves and unpolarized functions.
Approximate Analytical Wavefunctions for Xe and Rn
Authors: Synek Miroslav , Stungis George E.
Published: January 2005

Approximate analytical wavefunctions were calculated for the atoms Xe and Rn. The minimum basis set expansion used makes the wavefunctions sufficiently simple for orientational calculations. The Hartree—Fock total energies are represented to three significant figures. The virial theorem and the SCF thresholds are quite satisfactory.
Zero-Field Splitting. V Aromatic Nitrogen Heterocycles
Authors: Boorstein Seth Alan , Gouterman Martin
Published: January 2005

Calculation of zero-field splittings in N-heterocyclic aromatic molecules involves consideration of unequal exponent zero-field splitting integrals and molecular symmetry. Five molecules are treated, and the variation of D and E values with the choice of configuration interaction basis set is discussed, using π-SCF molecular orbitals for the configuration interaction.
Least-Squares Determination of Structures of Gas Molecules Directly from Electron-Diffraction Intensities
Authors: Bartell L. S. , Kohl D. A. , Carroll B. L. , Gavin R. M.
Published: January 2005

A new least-squares procedure has been devised for interpreting electron-diffraction patterns by a comparison of experimental total intensities with calculated intensities. It differs from all procedures published previously in its direct use of observed quantities in place of the customary ``reduced intensities'' derived subjectively from manually smoothed data. The computer is programmed to select the atomic background function needed to establish the molecular intensity and to refine the background as it refines the desired structural parameters. Operating characteristics of the new procedure are discussed. Results of its application in the structure determination of gaseous B(CH3)3 are presented and compared with results determined by the radial distribution method. Although the new procedure requires somewhat more accurate trial values of molecular parameters for convergence than does the radial distribution analysis, it is intrinsically a faster, and, in some ways, a more powerful tool for deducing optimum molecular parameters.
Microwave Spectrum and Structure of Nitric Acid
Authors: Cox A. Peter , Riveros José M.
Published: January 2005

The microwave spectra of five further isotopic species of nitric acid, H18ONO2, cis-HON18OO, trans-HONO18O, cis-HO15N18OO, and D15NO3, have been investigated. The data have been used to refine the previous structural determination of Millen and Morton. In the calculation of structure, particular consideration has been given to the use of ground-state moments of inertia in Kraitchman's equations for near-oblate planar molecules. The structure was calculated using several different methods, including the double-substitution method due to Pierce, in order to substantiate the following structural parameters: O–H=0.964, (H)O–N=1.406, cis–N–O=1.211, trans-N–O=1.199 Å, ∠HON=102°9′, ∠cis-ONO=115°53′, ∠trans-ONO=113°51′, and O···O (NO2 group)=2.184 Å.

This determination clearly demonstrates a 2° tilt of the NO2 group away from the hydrogen atom. The NO2 parameters are close to those found in a number of related molecules, but there is some evidence that the cis-NO bond in HNO3 is slightly longer than the trans-NO bond. The NOH configuration is very similar to that found in formaldoxime.

Further evidence for the exact planarity of nitric acid is given. Stark-effect measurements limit the out-of-plane component of the dipole moment μC to a value less than 1.1×10−3 D. The other components have been redetermined to have the values μA=1.99 D and μB=0.88 D, giving a total dipole moment μ=2.17±0.02 D.
Photochemical Charge-Transfer Reactions in Frozen Aqueous Systems. II. Formation and Trapping of Holes
Authors: Moorthy P. N. , Weiss J. J.
Published: January 2005

Ultraviolet irradiation of frozen aqueous solutions of ceric ions was shown to lead to an electron transfer from the solvent molecules in the hydration shell to the ceric ion. The resulting ``hole'' in the matrix is trapped and exhibits a characteristic electron spin resonance (ESR) spectrum. The quantum yield for the trapping of the holes has been investigated as a function of the concentration of the solute and for different ice matrices. ESR spectra of trapped holes have also been observed on optical bleaching of the color centers which are produced by γ irradiation of frozen aqueous matrices containing H2SO4, H3PO4, or HClO4 at 77°K. Subsequent annealing of these colorless matrices at 120°K restores the original color centers, whereas the trapped holes disappear. These processes can be repeated through many cycles, practically without diminution of the color-center—hole concentrations.
Electrical Field Effect on the Critical Opalescence
Authors: Debye P. , Kleboth K.
Published: January 2005

The free energy involved in the concentration fluctuations of a liquid mixture is changed by an electric field. The effect becomes observable near the critical point. It increases with decreasing distance from the critical temperature and it is proportional to the square of the field intensity and to the second derivative of the dielectric constant with respect to the concentration. Temperature as well as field strength dependence are in agreement with a theory built along classical lines. The effect can be interpreted as a shift of the critical temperature and the sign of this shift is also predicted correctly. Experiments performed with a field intensity of 45 000 V/cm on the system nitrobenzene-2,2,4-trimethylpentane give a value of 0.015° for this shift (which is a depression), while the theory predicts the same.
Intramolecular Excimer Formation. I. Diphenyl and Triphenyl Alkanes
Authors: Hirayama Fumio
Published: January 2005

Fluorescence spectra have been measured for a variety of diphenyl and triphenyl alkanes in cyclohexane and in p-dioxane. A class of compounds which are so structured that the phenyl groups along a main alkane chain are separated by exactly three carbon atoms, e.g., 1,3-diphenylpropane, 1,3,5-triphenylpentane, has been found to possess unique fluorescence characteristics. These are the appearance of a long-wavelength band in the region of 330 mμ and a marked decrease in the fluorescence yield. The long-wavelength band is attributed to an emission from an excimer (a transient dimer) formed intramolecularly by the association of excited and unexcited phenyl groups. Formation of excimers in the specific class of compounds is discussed in relation to molecular configuration. Efficiency of excimer formation, solvent effects, and quenching by dissolved oxygen are some of the topics discussed through kinetic considerations.
Coherent X-Ray Scattering for the Hydrogen Atom in the Hydrogen Molecule
Authors: Stewart Robert F. , Davidson Ernest R. , Simpson William T.
Published: January 2005

The x-ray form factors for a bonded hydrogen in the hydrogen molecule have been calculated for a spherical approximation to the bonded atom. These factors may be better suited for the least-squares refinement of x-ray diffraction data from organic molecular crystals than those for the isolated hydrogen atom. It has been shown that within the spherical approximation for the bonded hydrogens in H2, a least-squares refinement of the atomic positions will result in a bond length (Re value) short of neutron diffraction or spectroscopic values. The spherical atoms are optimally positioned 0.07 Å off each proton into the bond. A nonspherical density for the bonded hydrogen atom in the hydrogen molecule has also been defined and the corresponding complex scattering factors have been calculated. The electronic density for the hydrogen molecule in these calculations was based on a modified form of the Kolos—Roothaan wavefunction for H2. Scattering calculations were made tractable by expansion of a plane wave in spheroidal wavefunctions.
Energy Levels of Trivalent Europium in Yttrium Gallium Garnet
Authors: Koningstein J. A.
Published: January 2005

Experimental and theoretical studies of the Eu3+ spectrum in yttrium gallium garnet are reported in the present article. From the crystal-field calculations on the split 7F manifolds it is concluded that the crystal field deviates from tetragonal symmetry. A reasonable agreement has been obtained between the calculated and observed splitting patterns of these manifolds. The best evaluation of the crystal-field parameters yields: A20=+105 cm−1, A22=±95 cm−1, A40=−170 cm−1, A42=∓100 cm−1, A44=850 cm−1, A60=50 cm−1, A62=±150 cm−1, and A64=+945 cm−1 where Anm=〈rn〉Bnm.
Effusion Studies, Mass Spectra, and Thermodynamics of Beryllium Fluoride Vapor
Authors: Hildenbrand D. L. , Theard L. P.
Published: January 2005

Torsion—effusion and mass-spectrometric techniques were used to obtain absolute pressures and composition data for vapors over condensed beryllium fluoride and beryllium fluoride-aluminum mixtures. Saturated beryllium fluoride vapor in the effusion pressure range was found to consist predominantly of monomeric BeF2, with less than 1 mole % of Be2F4. When aluminum powder was added to liquid beryllium fluoride, the vapor was found to contain a few mole percent of AlF in addition to the BeF2. The second-law entropy of vaporization of beryllium fluoride is consistent with a low bending frequency (320 cm−1) for BeF2(g). The heat of sublimation of BeF2 at 298°K was derived as 55.3±1 kcal/mole, leading to a value of −187.0±2.2 kcal/mole for the standard heat of formation of BeF2(g). Differential measurements of the AlF pressure over BeF2–Al mixtures yielded a value of −183.8±3 kcal/mole for the heat of formation of BeF2(g) at 298°K, and the corresponding value obtained from mass-spectrometric studies is −187.0±3 kcal/mole. Effusion measurements on Be–BeF2 mixtures indicate the dissociation energy of BeF at 298°K to be less than 6.8 eV, while data on the BeF2–Al system indicate D298<6.4 eV for BeF.
Neutralization of Intermediate Ions in the Radiolysis of Ethylene by Charge Exchange
Authors: Meisels G. G.
Published: January 2005

The presence of nitric oxide and other additives of low ionization potentials during the radiolysis of ethylene markedly enhances the formation of butene-1 and cis- and trans-butene-2. Yields of 1.4 to 1.6 molecule/100 eV absorbed in ethylene are reached at additive concentrations of 10% to 15%. The isotopic distribution of the butenes resulting from the radiolysis of ethylene—deuteroethylene—nitric oxide mixtures is typical of a molecular association mechanism, while the combined effects of electrostatic fields and nitric oxide demonstrate the absence of free radicals and excited species as precursors. The results suggest that intermediate butene ion produced by an ion—molecule association process of ethylene parent ion is relatively long lived and may be neutralized by charge exchange. This process apparently competes effectively with further ion—molecule reactions or unimolecular dissociation of [C4H8+]. The inference that G(C2H4+) ≦1.5 ion/100 eV is in good agreement with the estimate G(C2H4+) = 1.5 based on mass-spectrometric fragmentation patterns for 70-eV electrons.
Heat Conductivity of Polyatomic and Polar Gases and Gas Mixtures
Authors: Monchick L. , Pereira A. N. G. , Mason E. A.
Published: January 2005

A theory is presented which can be used for the practical calculation of the heat conductivity of polyatomic and polar gases and gas mixtures. For pure gases, the results are based on the Wang Chang—Uhlenbeck equations and involve no approximations, provided that a suitable definition of an internal diffusion coefficient is employed. This is compared with the known results for a gas of rough spheres, and found to hold to all orders of approximation. Approximations enter for real gases only in obtaining numerical estimates of internal diffusion coefficients and relaxation times. The result is essentially the same as that of Mason and Monchick. For mixtures, the results are based on the formal kinetic theory recently obtained by Monchick, Yun, and Mason. A brief digression on sound absorption and dispersion in mixtures is made in order to identify the cross relaxation times in the formulas. Two assumptions are required for mixtures to obtain usable formulas: neglect of ``complex collisions,'' and no correlation between internal energy states and relative velocities (or equal differential cross sections for all scattering channels). With these assumptions plus suitable definitions of internal diffusion coefficients and relaxation times, a usable formula for the heat conductivity is obtained. This formula is further simplified to include only first-order correction terms, and rearranged so that the heat conductivities of the pure components are automatically given correctly. Comparison with experimental results for a number of different types of mixtures showed that the calculated results were rather insensitive to inelastic collision corrections, provided they were forced to go through the correct end points. It was concluded that for most purposes a theory neglecting inelastic effects in the mixture would be adequate, but that inelastic effects must be included in calculations for the pure components.
Temperature Dependence of Chemical Shifts of Protons in Hydrogen Bonds
Authors: Muller Norbert , Reiter Richard C.
Published: January 2005

The observed variations with temperature of the NMR chemical shifts of protons involved in hydrogen bonds have hitherto been attributed to shifts in association equilibria favoring a larger fraction of broken hydrogen bonds with increasing temperature. We now suggest that another mechanism may account for a large part of these variations. The chemical shift for the associated species depends quite strongly on the degree of excitation of the hydrogen-bond-stretching vibrational mode. Because this is an unusually low-frequency motion, several excited states are significantly populated even at temperatures as low as 200°K. When an appropriate average over vibrational levels is used to evaluate the chemical shift, the results show a temperature dependence similar to those found experimentally even though it is assumed in the calculations that no dissociation of the hydrogen-bonded species occurs.
Temperature Dependence of the Rate of Conversion of He+ into He2+
Authors: Niles Franklin E. , Robertson W. W.
Published: January 2005

Measurement of the decay of atomic and molecular emissions in the early helium afterglow for temperatures from 77° to 449°K and pressures from 3.76 to 9.93 Torr, together with the known value for the ambipolar diffusion of He+, have shown that the temperature dependence of the conversion frequency for the reaction He++2He→He2++He goes as T−1. When the conversion frequency is expressed as βp2, the quantity βT3 is independent of temperature and pressure with an average value of 2.97×109°K3sec−1·Torr−2. When the conversion frequency is expressed as kR[He]2, the quantity kRT is independent of temperature and pressure with an average value of 3.19×10−29°K·cm6 atoms−2·sec−1. Theoretical considerations confirm that the temperature dependence of the conversion frequency goes as T−1.
Nuclear Magnetic Resonance Studies of Solvent Effects on the Hindered Internal Rotation of N,N-Disubstituted Amides. I. Dimethylformamide
Authors: Whittaker A. Greenville , Siegel Seymour
Published: January 2005

A number of chemical-shift measurements were made on dimethylformamide (DMF) as a pure liquid and in the solvents carbon tetrachloride, acetone-d6, fluorotrichloromethane, and hexamethyldisiloxane. These measurements included (1) the temperature dependence of the methyl-doublet separation, (2) the effect of solvent and concentration on the methyl-doublet separation, and (3) the effect of temperature, solvent, and concentration on the chemical shift of the formyl proton resonance and on the center of the methyl doublet. The results show large solvent effects that are interpreted in terms of dipolar association between DMF and solvent. No evidence for hydrogen-bonded species was found. Activation energies for the barrier to hindered rotation were calculated and were found to depend strongly on the nature of the solvent.
Electron Spin Resonance and Electronic Structure of the RĊHOR′ Ether Radicals
Authors: Griffith O. Hayes
Published: January 2005

Single crystals of inclusion compounds formed between urea and a series of aliphatic ethers were x irradiated and studied by electron spin resonance. The stable, x-ray-produced free radicals were all of the general type RĊHOR′. The approximate value for the spin density on the carbon atom is 0.70±0.10. The unpaired spin distribution is discussed in terms of the Hückel and approximate configuration interaction π-electron molecular orbital models and the valence bond method. The theoretical spin distributions are found to be in qualitative agreement with the experimental spin distribution.
Chemical Shifts and Electron Densities in Nitrogen Heterocyclic Molecules
Authors: Gawer A. H. , Dailey B. P.
Published: January 2005

The chemical shifts of a series of 11 nitrogen heterocyclic molecules have been measured and, after appropriate correction, compared to calculated π-electron densities. This study includes a comparison of the azines and diazines with their ``benzologs,'' analogous molecules in which a benzene ring has been fused to the original heteroaromatic ring. The chemical shifts and calculated π-electron densities were found to have only a rough correlation. The chemical shifts of protons bonded to carbon atoms adjacent to nitrogen atoms seem especially anomalous. The effect of irregular molecular geometry on the calculated electron densities, of the magnetic anisotropy due to lone-pair π-orbital promotion on the nitrogen, and of σ-π-electron interaction all appear to make significant contributions. The proton chemical shifts appear to be a somewhat unreliable measure of π-electron densities in the nitrogen heterocycles, especially in positions adjacent to a nitrogen atom.
Magnetic-Dipole and Electric-Quadrupole Transition Moments for the a 1Πg←X 1Σg+ Transition in N2
Authors: Vanderslice Joseph T. , Wilkinson P. G. , Tilford S. G.
Published: January 2005

The intensity distribution of the five branches in the a 1Πg←X 1Σg+ system of N2 has been analyzed in detail. The results are in agreement with theory (Chiu) and with line strengths calculated by Chiu give a ratio Q/D=0.33 between the relative transition probabilities of electric quadrupole (Q) to magnetic dipole (D). These results suggest a need for a theoretical study of these matrix elements in electric-quadrupole and magnetic-dipole transitions.
Chemisorption on Single-Crystal Planes: Nitrogen on Tungsten
Authors: Delchar T. A. , Ehrlich Gert
Published: January 2005

The effects of surface structure on the adsorption of nitrogen have been examined by contact-potential measurements on macroscopic planes cut from single-crystal tungsten. On the (100), room-temperature chemisorption lowers the work function by 0.4 eV, confirming previous field-emission results. Flash desorption reveals only one state, β, in which nitrogen is bound as adatoms. On the considerably less reactive (111), the work function φ rises by 0.15 eV. Two states contribute: α, which is probably molecular, and β; both raise φ. At higher concentrations, a second β state is isolated by flash desorption. Nitrogen does not chemisorb at all on the (110) at 300°K and p∼10−7 mm. Adsorption of γ nitrogen can, however, be observed by lowering the temperature to T∼130°K or raising the pressure above 10−3 mm. On the (110), this state is atomic and lowers φ. On other planes, γ nitrogen has different properties and forms because of the heterogeneity induced in the surface layer by the presence of adatoms. Macroscopic surfaces can therefore be prepared showing extreme structural specificity. Previous correlations between the work-function changes and electronegativities are examined and are found insufficient. The effects of structure on the surface dipole are, however, correlated by the empirical rule that nitrogen atoms on flat planes lower the work function, whereas on rough surfaces the work function is increased.
Polyhedral Clathrate Hydrates. IX. Structure of Ethylene Oxide Hydrate
Authors: McMullan Richard K. , Jeffrey G. A.
Published: January 2005

A detailed single-crystal structure analysis is reported for ethylene oxide hydrate. This confirms the polyhedral host lattice formed by 46 hydrogen-bonded water molecules in a cubic unit cell of 12.03 Å with space-group symmetry Pm3n. Disordered ethylene oxide molecules occupy the six equivalent tetrakaidecahedral cavities, and their electron-density distribution suggests that they are hindered axial rotors. Additional guest molecules, either ethylene oxide, oxygen, or nitrogen molecules occupy the two dodecahedral cavities with high disorder and low statistical weight. The compound is therefore nonstoichiometric having the limiting formula 2M·6C2H4O·46H2O, in which the percentage of M is variable. If M is assumed to be entirely ethylene oxide, as is likely from the method of preparation, the composition of the crystals obtained from an equilibrated aqueous solution of 8 mole % ethylene oxide at 9°C is 6.4C2H4O·46H2O.
Polyhedral Clathrate Hydrates. X. Structure of the Double Hydrate of Tetrahydrofuran and Hydrogen Sulfide
Authors: Mak Thomas C. W. , McMullan Richard K.
Published: January 2005

The structure of the tetrahydrofuran/hydrogen sulfide double hydrate has been determined from three-dimensional single-crystal data. The analysis confirmed the clathrate host lattice characteristic of 17-Å cubic (Type II) gas hydrates. The hexakaidecahedral voids enclose tetrahydrofuran molecules which appear to undergo free rotation. Statistically, 46% of the pentagonal dodecahedra are occupied by hydrogen sulfide molecules.
Carbon-13 Chemical Shifts of the Carbonyl Group. II. A Correlation with CO Pi-Bond Polarity
Authors: Maciel Gary E.
Published: January 2005

The 13C chemical shift of the carbonyl group in molecules of the type XCOY is considered in terms of the theory for the local paramagnetic term σpAA for 13C shifts presented by Karplus and Pople. On the assumption of (i) an invariant, nonpolar sigma network, (ii) a constant average excitation energy, and (iii) neglect of neighbor anisotropy and intramolecular dispersion effects, a formula is derived which relates the chemical shifts to the polarity of the carbonyl pi bond. When the limitations imposed by Assumptions (i) and (iii) are minimized by an adjustment based on the 13C shifts of symmetrical disubstituted ethylenes, the resultant semiempirical theory predicts that the ``corrected'' carbonyl shifts should correlate with the CO pi-bond polarity, or substituent parameters to which the latter is related. The experimental data are found to correlate with Taft's reactivity parameter σI, in accord with the interpretation of substituent effects in terms of π-bond polarity in the carbonyl group.
Damping of Waves by Surface-Active Materials
Authors: Van Den Tempel M. , Van De Riet R. P.
Published: January 2005

A theory is presented for the damping of waves on a water surface in the presence of dissolved surface-active material. Equations are given which are valid for any wave on any liquid surface, subject only to the condition that the liquid be incompressible and the liquid layer be sufficiently deep. The equations are solved by the method of successive approximations; the first approximation is fully discussed. The resulting first-order equations are used in computing damping coefficients for solutions of surface-active materials, the adsorption of which follows a Langmuir isotherm. The computed damping coefficients depend on surfactant concentration and on frequency, as had already been found in experiments. Extension of the theory to insoluble monolayers is discussed.
Normal Vibrations of the Polymer Molecules of Helical Conformation. VI. Polytetrahydrofuran and Deuterated Polytetrahydrofurans
Authors: Imada Kiyohisa , Tadokoro Hiroyuki , Umehara Akira , Murahashi Shunsuke
Published: January 2005

Polarized infrared spectra of polytetrahydrofuran, polytetrahydrofuran-3,3,4,4-d4, and polytetrahydrofuran-d8 have been measured in the region from 4000 to 400 cm−1. The selection rules for polytetrahydrofuran molecule are derived under the factor group isomorphous to D2h. The normal vibrations of these polymer molecules are calculated for the infrared active B1u, B2u, and B3u species, by the use of the GF matrix method with the Urey-Bradley force field. The results show fairly good agreement of the observed and calculated frequencies, and give support to the 21 helix conformation, i.e., the planar zigzag conformation, reported previously. The assignments of the bands are given and discussed.
Set-Theoretic Approach to Reaction Kinetics
Authors: Jachimowski C. J. , Russell M. E.
Published: January 2005

A set theoretic approach to the study of stochastic reaction models is presented. This approach is based on the premise that the possible compositions of a reaction system form a so-called ``composition space.'' From the concepts of set theory and a frequency interpretation of probability a probability function of the reaction composition is formulated. The set-theoretic approach is applied to the first-order processes A→B, A⇄B, A→B→C and to the second-order process 2A→B. The results are compared with the stochastic Markov approach.
On the Distribution of Small Radii of Gyration of Linear Flexible Polymer Molecules
Authors: Forsman W. C.
Published: January 2005

The Fourier transform of the distribution function characterizing the three orthogonal components of the radius of gyration of linear flexible polymer molecules can be obtained from the properties of random-flight chains. In this publication we present a detailed analysis of the problem of inverting the Fourier transform of the distribution of small radii of gyration and present approximate and asymptotic expressions for the distribution. We consider random-flight chains of t statistical links. If we measure a given orghogonal component of the radius of gyration Si by the dimensionless parameter ξ=π(12)−12Si/〈Si2〉12, we find:

(1) In the domain of ξ between about (1/√3) and (3/t½), the distribution of ξ is adequately approximated by Wξ(ξ)=(π32)2−1ξ−3 exp (−π2/16ξ2). The expression is independent of t and therefore, as for larger radii, independent of how we subdivide a real polymer molecule into statistical segments. We also conclude, then, that for all radii such that ξ≥(3/t12), Wξ(ξ) is independent of the detailed structural features of real polymer chains.

(2) In the domain of ξ⩽(1/102t12) the distribution of ξ can be approximated by Wξ(ξ)=[2t!/(12t−1)!]ξt−1. The configurational statistics of such highly compressed coils is dependent on the number of statistical chain elements and thus on how we would choose to subdivide a real polymer molecule into such elements. We can also interpret these results to imply that the configurational statistics of such highly compressed real polymer molecules would be dependent upon the detailed structural features of the chain.
Anisotropy of the 19F Chemical Shift of Trapped CHF3 and NF3
Authors: Harris A. Brooks , Hunt Earle , Meyer Horst
Published: January 2005

We have studied the 19F chemical-shift anisotropy in NF3 and CHF3 trapped in β-quinol clathrates. It is known that molecules trapped in these compounds become aligned at low temperatures so that studies of relatively isolated oriented molecules become possible. The nuclear magnetic resonance was observed at several frequencies with the magnetic field applied at various angles relative to the crystal axis. The orientation of the molecules in the cavity and the dipolar broadening due to the lattice protons could be estimated rather closely by a qualitative analysis of the line shapes. The principal components of the chemical-shift tensor of NF3 could be determined from the observed variation of the first and second moments of the line with the strength and orientation of the magnetic field. This determination was further substantiated by the good agreement between the calculated and observed line shapes for various values of the magnetic field. For CHF3, where the chemical-shift anisotropy is smaller, only a cruder analysis was practicable. The values of the anisotropy of the chemical-shift tensor obtained were σ∥−σ⊥ = (390±60) ppm for NF3 and σ∥−σ⊥ = (105±20) ppm for CHF3, where σ⊥ is the average of the two components of the chemical-shift tensor perpendicular to the fluorine bond direction. For NF3 a difference of (140±40) ppm between the two perpendicular components was found, indicating a deviation from uniaxial symmetry of the NF bond. The sign and order of magnitude of the chemical-shift anisotropy are discussed and can be understood on the basis of current ideas about the relationship between the bond characters and the chemical-shift anisotropy.
High-Temperature Heat Contents of TiC and ZrC
Authors: Levinson Leonard S.
Published: January 2005

Using a drop calorimeter, the heat content of TiC was measured between 1274° and 2799°K, and that of ZrC between 1275° and 2788°K.
Different Screening Constants for Different Physical Properties. I
Authors: Sanders William A. , Hirschfelder Joseph O.
Published: January 2005

Expectation values of properties other than energy are calculated for the ground state of the two-electron atom with arbitrary nuclear charge. The calculations are designed to test the use of simple wavefunctions with embedded screening constants, where different screening constants are used for different properties. The results are compared with the perturbation expansion of Scherr and Knight, who have determined a large number of properties correct through sixth order.

Dalgarno has suggested that a screening constant be chosen so as to make the first-order perturbation correction vanish for the property under consideration. Robinson has shown that this choice is equivalent to the requirement that the zeroth-order wavefunction satisfy a hypervirial relation, where the hypervirial generator is related to the property through a differential equation. This procedure gives excellent numerical results for properties having positive definite operators.

For the particular examples considered here (but not generally), Dalgarno's method gives expectation values which are too small. When applicable, slight improvements are obtained by maximizing the expectation value calculated with the zeroth and first-order wavefunctions.