bluuebunny/crossref_metadata_2025_split huggingface dataset:
Organic Liquid Scintillators. VI. Substituted Distyrylbenzenes: Scintillation Properties and Spectra of Absorption and Fluorescence
Authors: Heller Adam 
Published: January 2005

The preparation, spectra of absorption and fluorescence, and the scintillation properties of 28 derivatives of trans,trans-p-distyrylbenzene are described.
Most of the soluble derivatives are efficient primary scintillators in alkylbenzene solutions and are highly efficient secondary scintillators in alkylbenzene solution of p-terphenyl. Solutions with efficiencies exceeding those of the best-reported liquid scintillation systems are described.
Nuclear Magnetic Resonance in Molten Salts. I. Chemical Shift of Crystalline and Molten Thallium Salts
Authors: Hafner Stefan , Nachtrieb Norman H. 
Published: January 2005

Relatively large chemical shifts in the resonance frequency of the thallium nucleus are observed in both the crystalline and molten states of different thallium salts. Relative to TlNO3, which appears to be the most ionic of all thallium salts studied, the 205Tl resonance frequency at fixed field increases in the order: NO3—&lt;Cl—&lt;Br—&lt;I—. The results are interpreted in terms of a second-order paramagnetic contribution to the field at the Tl nucleus, arising from an admixture of its excited states with the ground state, and the chemical shift is approximately proportional to the square of the overlap integral. Although cations and anions in the melts are partly associated, the NMR shifts of the Tl nucleus show that the cause arises from incipient covalency rather than polarization of the cation by the halide ions.
In both the crystalline and molten states, a linear paramagnetic shift with temperature is observed, which is attributed to the enhanced overlap of excited states which are induced by thermal vibrations.
Kinetics of Pyrolysis of Isopropyl Iodide
Authors: Teranishi Hiroshi , Benson Sidney W. 
Published: January 2005

The kinetics of the pyrolysis of isopropyl iodide was investigated in the temperature range 270°∼300°C using a spectrophotometric technique for determination of I2. The reaction was found to proceed through an I atom and radical chain mechanism to yield C3H6, C3H8, and I2. The reaction velocity is determined by the slow unimolecular decomposition (5) i−C3H7I⇌ lim 65C3H6+HI.The rate data give the following equation for k5: logk5=12.96–43.5/θ (sec−1)with θ=2.303RT in kilocalories per mole. The activation energy E5 and Arrhenius factor, A5, show good agreement with the values previously reported by Jones and Ogg.
The contribution of a reported autocatalytic reaction to the velocity of pyrolysis is found to be negligibly small or less than 8% of that of the unimolecular decomposition process throughout the temperature range of the present experiment.
There is evidence for the beginnings of a falloff in rate constant with pressure at about 30 mm Hg. At 15 mm Hg the falloff does not exceed 25%.
Combination of the rate constants for the forward and reverse reactions leads to an equilibrium constant K5.6 lower by a factor of about 2 than that determined previously from direct equilibrium studies. It is proposed that the kinetic constant is probably the more reliable. Values of ΔS5.6 and ΔH5.6 computed from the kinetic data are in excellent agreement with independent estimates.
NMR Studies of Phase Transitions. II. NaCN
Authors: Coogan C. K. , Gutowsky H. S. 
Published: January 2005

The 23Na nuclear magnetic resonance absorption has been observed in the cubic phase of polycrystalline NaCN between about 15° and 170°C, but could not be detected in the orthorhombic phase which forms below 10.5°C. The observed lineshape and its second moment S are compatible with a model in which S=Sd+Sq0+Sq(T) , where Sd and Sq0 are temperature-independent terms, Sd≅0.39 G2 resulting from magnetic dipolar interactions of the ``rigid lattice'' of Na+ ions and Sq0≅0.16 G2 from first-order nuclear quadrupole interactions with field gradients produced by lattice defects. Sq(T) is a quadrupole broadening term which decreases exponentially from ∼13 G2 at 10.5°C to near zero at 170°C.
The electric-field gradients (EGF) involved in Sq(T) are attributed to the electric dipoles of the CN— ions, and the temperature dependence, to the averaging out of the gradients by thermally activated reorietations of the CN— ions. The reorientations are deemed responsible for the existence of a cubic phase. They are treated as a random-walk problem yielding algebraic expressions by means of which an activation energy of 6 kcal/mole is obtained from the experimental Sq(T), and 5×108 cps at 10°C and 1010 cps at 150°C are estimated for νi, the CN— reorientational jump frequency. A discussion is given of the relation between νi and the nucleation of the cubic to orthorhombic phase transition.
Molecular Structure of Dicyclopentadienylberyllium (C5H5) 2Be
Authors: Almenningen A. , Bastiansen O. , Haaland A. 
Published: January 2005

The molecular structure of gaseous (C5H5) 2Be has been determined from electron-scattering data. The molecule consists of two planar, symmetrical C5H5 rings. C1–C2 = 1.424±0.002 Å and C1–H1 = 1.070±0.005 Å. The rings are parallel and staggered with a vertical ring—ring distance h = 3.37±0.03 Å. The CH skeleton thus has point group symmetry D5d. The beryllium atom may occupy two alternative positions on the fivefold rotation axis h = 1.485±0.005 Å from one ring and h2 = 1.980±0.010 Å from the other. The complete molecule thus has point group symmetry C5v.
It appears then that the potential energy curve of the beryllium atom has two minima. This is readily understood from an ionic binding model. The discrepancy between the values found for h and h1+h2 is believed to be due to intramolecular motion.
Combination Reactions of Imino Radicals in the Flash Photolysis of Isocyanic Acid
Authors: Back R. A. 
Published: January 2005

The flash photolysis of HNCO vapor yielded CO, N2, and H2 in a ratio of 1/0.42/0.27, independent of HNCO pressure (7–40 mm) and flash energy (17–1150 J). From the mass balance, production of ammonia was implied; the stoichiometry was described approximately by 12 HNCO=12CO+5N2+3H2+2NH3. It was concluded that the reaction 2NH→N2H2 was much more important than 2NH→N2+H2, with the observed stoichiometry largely determined by the subsequent decomposition of N2H2.
The addition of ethylene or propane reduced the yield of CO (by competitive absorption of light), and reduced the value of N2/CO, apparently for the most part by interfering in the decomposition of N2H2 rather than by reacting with NH radicals. The effect on the hydrogen yield was complicated by the production of hydrogen by photolysis of ethylene and propane; in the HNCO—propane system, however, extra hydrogen definitely was produced, probably by the reaction of NH with propane.
Direct Determination of Natural Orbitals and Natural Expansion Coefficients of Many-Electron Wavefunctions. I. Natural Orbitals in the Geminal Product Approximation
Authors: Kutzelnigg Werner 
Published: January 2005

The approximation of a many-particle wavefunction as an antisymmetrized product of strongly orthogonal geminals (APSG) (geminal means two-electron function) is analyzed in terms of its natural expansion. APSG wavefunctions have very simple and interesting properties. The ``generating spin geminals'' (GSG) are automatically natural spin geminals (NSG) of the system with Occupation Number 1 (in the Löwdin normalization); 1 is the upper bound for the eigenvalues of the two-particle density matrix for wavefunctions of this type. The natural spin orbitals (NSO) of the GSG are automatically NSO of the total wavefunction and of all the NSG.
A generalized ``paired orbital function'' is defined in terms of its natural expansion. It contains the APSG type as well as the Bardeen—Cooper—Schrieffer (BCS) functions as special cases.
Integrodifferential equations determining the NSO and their expansion coefficients for an APSG function are derived for the special case of a singlet state by minimizing the total electronic energy. An iterative resolution procedure is proposed. When the state under consideration can be regarded as a closed-shell state the first iteration step may already be a good approximation. In this case the equations become very simple and can be given a straightforward physical interpretation.
Finally possible refinements of the theory are discussed.
Viscosity of Neon—Helium and Neon—Argon Mixtures at 20° and 30°C
Authors: Kestin J. , Nagashima A. 
Published: January 2005

The paper gives the results of the absolute determination of the viscosity of two binary systems of mixtures: neon—helium and neon—argon, together with those of the viscosity of the three pure components. The measurements were performed with the aid of our high-precision oscillating-disk viscometer at 20° and 30°C; they covered a range of pressures up to 35 atm (densities from 0 to 0.09 g/cm3).
The density effect can be expressed, essentially, in the form of a temperature-independent, linear correction in the case of the Ne–He mixtures, and in the form of a quadratic correction in the case of Ne–Ar mixtures, both applied to the excess viscosity Δμ = μ—μ0 in which μ0 is a function of temperature only.
Quantum Calculation of the Sensitivity of Diffusion, Viscosity, and Scattering Experiments to the Intermolecular Potential
Authors: Mueller C. R. , Brackett J. W. 
Published: January 2005

Correlations between the form of the intermolecular potential as a function of distance and the quantum mechanical phase shifts as functions of the angular momentum quantum number have served as the basis for estimates of the sensitivity of viscosity, diffusion, and low-energy scattering experiments to the attractive and repulsive portions of the intermolecular potential. The sensitivities of the experimental techniques are approximately as follows: repulsive portion of the potential, diffusion&gt;viscosity&gt;scattering; to the attractive region of the potential, scattering&gt;viscosity=diffusion.
Quantum diffusion and viscosity cross sections for the Lennard-Jones potential have been calculated as functions of the reduced kinetic energy, K=μv2/2ε, and for decreasing values of the quantum parameter λ=h/σ(mε)½. The cross sections for systems with λ corresponding to T2, Ne, and CH4 are equal to the classical cross sections for K&gt;1.2. At lower K's the deviations from classical behavior become larger as λ increases.
Electronic Absorption Spectrum of Ni II in Cubic Perovskite Fluorides. I
Authors: Ferguson J. , Guggenheim H. J. , Wood D. L. 
Published: January 2005

The fine structure of the spectrum of Ni2+ in KNiF3 and KMgF3:Ni has been analyzed in terms of the spin—orbit splitting and vibrational intervals using a four-parameter crystal-field approach. The parameters obtained from the analysis differ from the free-ion values by relatively small amounts, but the differences are not interpretable physically because configuration interaction has been neglected. We find that Dq=698 cm—1, λ=—(305–320) cm—1, F2=1520 cm—1, F4=114 cm—1 (F4/F2=0.675). For the free ion we find that λ=—334 cm—1, F2=1622 cm—1, and F4=116 cm—1 by fitting the published values for the 3d8 energy levels in our analysis with Dq=0.
We find that the effects of exchange interaction on the excited state energy levels is small, and that no Jahn—Teller splitting is observed or expected for this crystal. An explanation is offered for temperature changes in the 3A2g→1Eg transition involving a breakdown of Born—Oppenheimer separability.
Reaction of O(1D) with CO
Authors: Raper O. F. , DeMore W. B. 
Published: January 2005

The photolysis of O3 in liquid CO at 77°K has been studied with 2537-Å radiation. Carbon dioxide is formed with a quantum yield of 0.07 and in a 1:1 ratio with O3 disappearance. The CO2 quantum yield is reduced by addition of N2, O2, or Ar, but is increased by lowering the temperature of the solution.
Formation of CO2 is attributed to the reactionO(1D)+CO(+M)→CO2(+M),and the low quantum yield is believed to be caused by the reactionO(1D)+CO+M→CO+O(3P)+Mwhich leads to O3 reformation. The reaction of O(1D) with CO is an example of an association process in which the excited association complex may undergo unimolecular decomposition to products of lower electronic energy in the course of stepwise vibrational deactivation. Formation of the association product is of higher order than one with respect to M, the actual order depending on the number of vibrational states in which unimolecular dissociation competes with collisional deactivation. Because of the high-order dependence on M, the rate of product formation is very sensitive to the deactivation efficiency of the third body. The effect of added diluents is believed to arise partly from competition of the added diluents for O(1D) and partly from the resulting change in solvent deactivation efficiency. The effect of reduced temperature is attributed to an increased rate of deactivation resulting from an increased solvent density.
Fifth and Sixth Virial Coefficients for Hard Spheres and Hard Disks
Authors: Ree Francis H. , Hoover William G. 
Published: January 2005

New expressions for the fourth, fifth, and sixth virial coefficients are obtained as sums of modified star integrals. The modified stars contain both Mayer f functions and f̃ functions (f̃≡f+1). It is shown that the number of topologically distinguishable graphs occurring in the new expressions is about half the number required in previous expressions. This reduction in the number of integrals makes numerical calculation of virial coefficients simpler and more nearly accurate. For particles interacting with a hard-core potential, values of the modified star integrals are shown to depend strongly on dimension; for example, several modified star integrals are identically zero for hard disks (two dimensions), but give nonzero values for hard spheres (three dimensions). Of all the modified star integrals contributing to the fourth, fifth, and sixth virial coefficients, the complete star integrals are shown to be the largest. Mayer's expressions for these coefficients made the complete star integrals the smallest contributing integrals.
The fifth (B5) and sixth (B6) virial coefficients of hard-sphere and hard-disk systems are obtained by Monte Carlo integration of the modified star integrals. The resulting values are spheres:B5/b4=0.1103±0.0003;B6/b5=0.0386±0.0004disks:B5/b4=0.3338±0.0005;B6/b5=0.1992±0.0008 where b is the second virial coefficient.
Estimated values of B7 obtained from a Padé approximation to PV2/(N2kT) — V/N are B7/b6=0.0127 for hard spheres and 0.115 for hard disks. For hard spheres virial series calculations including terms through the sixth virial coefficient give values of PV/(NkT) which agree closely, for densities less than half of closest-packing, with the molecular dynamics data of Alder and Wainwright. Furthermore the approximate Padé expression agrees within 2% with the molecular dynamics data for all densities on the fluid side of the solid-fluid transition. This agreement indicates convergence of the virial series along the entire fluid branch of the hard-sphere equation of state.
Configurations of Triarylborons
Authors: Weismann T. J. , Schug John C. 
Published: January 2005

Potential energies were calculated for a series of methylated triarylborons considering interactions between all nonbonded atoms and pi-electron resonance energies. All nonbonded interactions were estimated using modified Buckingham (exp-six) potentials obtained from the literature; pi-electron energies were calculated using simple molecular-orbital theory. The resulting predictions of stable geometries were compared with those derived from proton magnetic-resonance chemical shift data. All calculations were performed using a rigid geometric framework permitting only rotation of aryl groups. Reasonable agreement was found for several of the molecules studied; however, a number of discrepancies indicate certain inadequacies in the present treatment. In particular, future refinements must include the possibility of bond bending.
For tri-1-(2-methylnaphthyl)boron, temperature dependence of the NMR spectra was indicative of rotational isomerism.
Exchange Rates by Nuclear Magnetic Multiple Resonance. III. Exchange Reactions in Systems with Several Nonequivalent Sites
Authors: Forsén Sture , Hoffman Ragnar A. 
Published: January 2005

The nuclear magnetic multiple resonance method for the study of chemical exchange rates has been applied to systems in which a nucleus X is reversibly exchanged between three nonequivalent sites. A procedure is outlined, which is applicable to systems with any number of sites and permits the determination of the lifetime τK at any one site K. In this procedure the effective ``relaxation time'' τ1K and the equilibrium z magnetization, M0K (τ1K/T1K), are determined in experiments (Type I) in which the X signals at all other sites are completely saturated. The rate constants λKL and λLK of the individual exchange reactions (K⇄L) may then be determined in Type II experiments in which the signals of all sites excluding K and L are saturated. In Type II experiments only the equilibrium signal intensities need be measured.
The method has been applied to the base-catalyzed proton exchange in the keto—enol system of acetylacetone and it was found that exchange between the olefinic =CH and hydroxylic —OH protons occurs merely as a consequence of the keto—enol transformation. Within the accuracy of the measurements no evidence was found for a proton exchange between the =CH and —OH protons which does not involve the CH2 group in the keto form as an intermediate.
Vibrational Energy Exchange of Highly Excited Anharmonic Oscillators
Authors: Benson Sidney W. , Berend George C. 
Published: January 2005

The probabilities of dissociation Pdis, excitation Pex, de-excitation Pde, and the associated average energy exchanges were calculated theoretically for a highly energized, anharmonic oscillator (I2 or Br2) colliding classically with an inert gas atom C through a Morse-type potential. The effect of varying mass and well depth was investigated.
It was found that the effect of anharmonicity is to favor excitation and dissociation. However, the average ΔEv for these processes is about the same for harmonic and anharmonic oscillators and not much bigger than 1 to 2 RT. Pdis increases markedly with increasing mass. The well depth (V0) does not seem to have much effect in the range 0.004&lt;V0/D0&lt;0.14, where D0 is the bond dissociation energy. Exchange reactions are observed, but for shallow well depths they are rare events. From the results, it is possible to calculate λ, the probability of stabilizing an atom pair by a third-body collision. This turns out to be independent of temperature but depends strongly on the mass of the third body C. It is always in the range 0.1&lt;λ&lt;1.0 for the systems considered, and increases with increasing mass of C.
For the dissociation process, it is found that the average dissociation event gives rise to a pair of atoms with very little excess energy (∼0.10 RT) above the dissociation barrier. This implies an activation energy for dissociation of the order of D0°—2RT in the Benson—Fueno theory and also a T—2 dependence for the rate of recombination of atoms.
Analysis of Charge Distributions: Hydrogen Fluoride
Authors: Kern C. W. , Karplus M. 
Published: January 2005

Electron density maps and their appropriately weighted analogs (e.g., dipole-weighted density map) are found to provide a consistent and useful tool for the analysis and the comparison of atomic and molecular charge distributions obtained with different wavefunctions. Specific applications are made to hydrogen fluoride, for which a series of single-determinant SCF—LCAO—MO wavefunctions permit a study of the changes that occur in the electron density as the Hartree—Fock solution is approached. An examination of alternative basis-set functions of essentially the same size and the same energy shows that significantly different charge distributions can result; this points out the need for a careful selection of basis functions in large scale molecular calculations. It is found also that the charge density fluctuations from one basis set to another can be as large as the effects of chemical binding. Comparisons between density-map and population-analysis results indicate that considerable caution must be used in employing the latter for the interpretation of complicated wavefunctions.
Various methods for summarizing wavefunctions in terms of characteristic parameters of the charge distribution are considered. The forces acting on the nuclei are analyzed in relation to the Hellmann-Feynman theorem and a necessary condition on the exact molecular Hartree—Fock solution is obtained. Although none of the presently available wavefunctions for HF and LiF satisfy the condition to a high degree of accuracy, refinements in the wavefunctions are shown to yield improved results.
Collisional Transition Probabilities for Deactivation of Highly Vibrationally Excited Cyclopropane and Dimethylcyclopropane
Authors: Setser D. W. , Rabinovitch B. S. , Simons J. W. 
Published: January 2005

Collisional deactivation of vibrationally excited cyclopropane and dimethylcyclopropane molecules, in their ground electronic states, has been studied with ethylene and cis-butene as the respective deactivators. Activation is to an energy region just above 100 kcal mole—1, which is reached by chemical activation. The activated molecules may isomerize, and the measurable cascade is to a reaction threshold at 60–65 kcal; below the threshold the excited molecules are stable with respect to reaction. The principle and characteristics of this steady-state method has been described in detail in Ref. 5.
The two substrate molecules offer an interesting difference in internal rotational degrees of freedom. Cyclopropane was investigated at 298°, 598°, and 723°K, and dimethylcyclopropane at 573° and 673°K. The observed behavior was compared with model calculations for various assumed functional forms of collisional transition probabilities pij which include stepladder, Gaussian, and skewed distributions. The data, unfortunately, are too inaccurate to distinguish between these models clearly; but they do show that ethylene removes energy from cyclopropane in average steps which are at least 7 kcal (and in one system, perhaps, as high as 30 kcal); butene removes energy from dimethylcyclopropane in steps of at least 10–14 kcal; there is an indication that a step or Gaussian distribution fits the data better than a skewed exponential model. There is some evidence that the average step size decreases with increase of ambient temperature.
Studies on the Residual Lines of Mica
Authors: Sato Kenzo 
Published: January 2005

By the measurement of the residual lines of cleaved mica (muscovite), using an incident beam of unpolarized infrared radiation of 9–28 μ, where anomalous dispersion is seen, some information about the characteristic properties of the residual lines are obtained.
(1) Intensity of the residual lines of the 10- and 20-μ groups change regularly with a period of 2π by the change of the incident direction.
(2) Intensity of the residual lines of a sheet of cleaved mica changes almost alternately by the change of sample planes.
(3) Extreme values of the reflectivity of the residual lines are observed when the incident angle is set in the range of 15°—19° for the two groups commonly.
(4) The oscillators as origins of the residual lines are assumed to be oriented not parallel to the surface planes but declined about 15°—20°.