bluuebunny/crossref_metadata_2025 huggingface dataset:
11B Quadrupole Coupling Constants in Solid Boron Halides
Authors: Casabella P. A. , Oja T.
Published: February 2004

The 11B nuclear quadrupole coupling constants have been measured in the solid boron halides and found to be 2.64 MHz in BF3, 2.54 MHz in BCl3, 2.46 MHz in BBr3, and 2.40 MHz in BI3. These values do not agree with calculated values based on descriptions of the boron–halogen bonds obtained from quadrupole coupling studies of the halogen nuclei. The discrepancy is attributed to the inadequacy of some assumptions used in the Townes and Dailey method for the case of boron. A fine structure was observed on the B11 nuclear magnetic resonance in BF3 and is completely explained in terms of a dipole–dipole interaction between the boron nucleus and the three neighboring fluorine nuclei.
Collision Integrals of a Gas of Rigid Convex Molecules
Authors: Hoffman D. K.
Published: February 2004

A review of the collision dynamics of rigid, smooth, and convex molecules with zero, two, or three collisionally active rotational degrees of freedom is given, together with a review of the kinetic theory of a general gas mixture of such molecules. A method for performing the momentumlike integrations of the collision integrals for gases of this type is discussed, and general formulas for the evaluated integrals are presented. Some properties of the linearized Boltzmann equation are demonstrated on the basis of these results.
Effect of Temperature on the Structure of Gaseous Molecules. II. An Electron-Diffraction Investigation of the Molecular Structures of B2Cl4 and SiCl4. The Potential Function for Internal Rotation in B2Cl4
Authors: Ryan Robert R. , Hedberg Kenneth
Published: February 2004

Electron-diffraction data for B2Cl4 with SiCl4 present as an impurity have been gathered at nozzle temperatures of 251°, 295°, 331°, 376°, and 423°K. Complete structural analyses of both molecules were carried out at the lowest and the highest temperatures by least squares based upon intensity curves, but converged results were obtained only at the lowest. Based upon molecular symmetry D2d for B2Cl4, the results (parenthesized values are 2σ) at 251°K are as follows: B2Cl4:r(B–Cl) = 1.750 Å (0.0106), r(B–B) = 1.702 Å (0.0692), r(B2···Cl2) = 3.000 Å (0.0494), r(Cl1···Cl2) = 3.011 Å (0.0081), r(Cl2···Cl4) = 4.087 Å (0.0400), ∠ Cl1BCl2 = 118.65° (0.66), l(B–Cl) = 0.0562 Å (0.0081), l(B–B) = 0.0500 Å (assumed), l(B2···Cl2) = 0.1223 Å (0.1022), l(Cl1···Cl2) = 0.0616 Å (0.0080);SiCl4:r(Si–Cl) = 2.019 Å (0.0088), r(Cl···Cl) = 3.299 Å (0.0241), l(Si–Cl) = 0.0539 Å (0.0122), l(Cl···Cl) = 0.0732 Å (0.0185. The ra and la are, respectively, inter-atomic distances and root-mean-square amplitudes of vibration; they differ slightly from the equilibrium values re and le. The form of the potential hindering internal rotation in B2Cl4 was investigated by comparing the experimental distribution of rotation-sensitive distances at each of the five temperatures with theoretical distributions calculated from the functions V = 12V0(1 − cos2θ) and V = V0θ2. In each case the cosine function gave better agreement. The barrier (V0) averaged for the five temperatures has the value 1.85 kcal/mole for the cosine function.
Molecular Freedom of the Ammonium Ion. Heat Capacity and Thermodynamic Properties of Ammonium Perchlorate from 5°–350°K
Authors: Westrum Edgar F. , Justice Bruce H.
Published: February 2004

The heat capacity of NH4ClO4 has been determined by adiabatic calorimetry from 5°–350°K and found to be of simple sigmate character without thermal anomalies. The heat capacity (Cp), entropy (S°), enthalpy function (H°−H°0) / T, and Gibbs energy function (G°−G0°) / T evaluated at 298.15°K from these data are 30.61, 44.02, 20.24, and −23.78 cal/(gfm °K). Combination of these values with aqueous NH4ClO4 thermochemical data suggests the absence of zero-point entropy. Comparison with the heat capacity of isostructural KClO4 permits resolution of the molecular dynamics of the ammonium ions and leads to the conclusion that these ions are restricted rotators, prevented from freely rotating by comparatively low-energy barriers.
Infrared Spectrum of the Difluoromethyl Radical in Solid Argon
Authors: Carver T. Granville , Andrews Lester
Published: February 2004

When HCBrF2 and DCBrF2 at high dilution in argon are codeposited with an atomic beam of lithium on a CsI window at 15°K, lithium bromide absorptions appear along with several new absorptions not present when the precursor was deposited without alkali metal. These new absorptions are assigned to the antisymmetric hydrogen bending and carbon–fluorine stretching modes and the symmetric C–F vibration of the HCF2 and DCF2 radicals. The antisymmetric vibrational assignments are supported by product rule and normal coordinate calculations which give the potential constants F55 = 5.27 ± 0.1 mdyn/Å, F56 = 0.44 ± 0.02 mdyn/rad, and F66 = 0.64 ± 0.05 mdyn·Å/rad2. An approximate force constant F22 = 7.19 ± 0.7 mdyn/Å is calculated from the symmetric C–F vibration. The carbon–fluorine valence force constant for HCF2 is in the range of those for typical fluorocarbons.
Dipole Davydov Splittings in Crystalline Anthracene, Tetracene, Naphthalene, and Phenanthrene
Authors: Philpott M. R.
Published: February 2004

The contribution of dipole–dipole interactions to the Davydov splitting (i.e., the dipole Davydov splitting) of the moderately intense Ag1 → 1La transitions of anthracene, tetracene, naphthalene, and phenanthrene has been investigated. Unlike most previous calculations, the Bose character of molecular excitons has been taken fully into account by using the classical theory of exciton states. In the dipole approximation this treatment is equivalent to the second quantized formulation of Agranovitch. Formulas are also derived for coupling of Born–Oppenheimer separable states through the full interaction potential. Dipole sums are calculated for infinite crystals by Ewald's method. High-energy π–π* transitions are shown to make sizeable contributions to the Davydov splittings and are responsible for increasing polarization ratios of the 0–0 transition. For anthracene these high-energy states push the splittings of the 3800-Å system beyond the accepted values, implying that the residual intermolecular interactions due to higher multipole and exchange effects act in the opposite direction, thereby reducing the total splitting. In tetracene the dipole splittings, after including higher states, are in excellent agreement with experiment, thereby relegating multipoles, etc., to a minor role. The La1 states of naphthalene and phenanthrene are subject to a pinching effect, due to the proximity of levels above and below, which inhibits the development of a large dipole splitting.
Natural Orbitals, Divergences, and Variational Principles
Authors: Kobe Donald H.
Published: February 2004

Bogoliubov's principle of compensation of dangerous diagrams (PCDD) is extended to finite fermion systems and is shown to give Löwdin's natural spin–orbitals (NSO) which diagonalize the single-particle reduced density matrix. Hartree–Fock theory corresponds to the compensation of the lowest-order dangerous diagrams, which cause divergences in the perturbation expansion of the ground-state energy. The PCDD is then derived from a variational principle that the number of particle–hole excitations in the true ground state is a minimum. The sense in which the NSO give the most rapid rate of convergence of the configuration-interaction expansion of the true wavefunction is also discussed.
Rydberg States of Benzene in Rare-Gas Matrices
Authors: Katz Benjamin , Brith Malka , Sharf Benjamin , Jortner Joshua
Published: February 2004

In this paper we present experimental evidence for the observation of Rydberg states of benzene in solid Ar, Kr, and Xe. On the basis of semiquantitative theoretical evidence we argue that molecular Rydberg-type states of a guest molecule are amenable to experimental observation in rare-gas solids. These host matrices are characterized by a free electronlike conduction band, leading to a manifold of Wannier-type states. The electron–atom interaction in rare-gas solids is relatively weak so that the line broadening of these Rydberg-type states will not be excessive. The identification of these “Rydberg-type” states in a rare-gas solid is based on matrix shifts, vibrational structure, isotope effects, linewidths, and site splittings. Two Rydberg-type transitions of the benzene molecule were identified. From our analysis we conclude that: (a) The lowest (n = 1) molecular Rydberg state in the rare-gas matrix can be correlated with the lowest Rydberg state in the gas phase, but is appreciably blue-shifted. The blue shifts are: 6000 cm−1 for Ar, 3150 cm−1 for Kr, and 1610 cm−1 for Xe. (b) The benzene Rydberg levels in the rare-gas matrix reveal some evidence for Jahn–Teller coupling effects, as is evident from the appearance of a nontotally symmetric vibrational component. (c) The linewidths observed for the Rydberg levels in the rare-gas matrix are qualitatively consistent with theory. (d) The second Rydberg transition in the rare-gas matrix is tentatively identified as corresponding to a n = 2 Wannier state, which has no relation to the molecular levels of the isolated molecule. This assignment makes possible an estimate of the ionization potential of the molecule in rare-gas solids. (e) Our results provide evidence that the lowest molecular Rydberg state and molecular impurity Wannier states are amenable to experimental observation in rare-gas solids.
ESR Relaxation Studies on Orbitally Degenerate Free Radicals. I. Benzene Anion and Tropenyl
Authors: Kooser Robert G. , Volland Walter V. , Freed Jack H.
Published: February 2004

Careful continuous saturation measurements, which include corrections for nonuniform microwave and modulating fields, have been used to examine, at different temperatures, the ESR linewidths and saturation behavior of the benzene anion in a tetrahydrofuran: dimethoxyethane solvent and the tropenyl radical in molten bitropenyl. It has been found that the ratios of the longitudinal to transverse electron-spin relaxation times (T1 / T2) for these free radicals in solution are about 1.1 ± 0.1 for the benzene anion below − 60°C and for tropenyl at 165°C. Analysis of the results has shown that at 9.1 GHz, g-tensor, anisotropic dipolar, and spin–rotational relaxation mechanisms do not contribute appreciably to the observed times. The anomalously small relaxation times associated with these radicals have been attributed to effects involving the degenerate ground states of these radicals. The spin-relaxation behavior of peroxylamine disulfonate anion in aqueous solution has also been studied.
Environmental Effects on Phosphorescence. VI. Matrix Site Effects for Triphenylene
Authors: Offen H. W. , Hein D. E.
Published: February 2004

The phosphorescence lifetime and spectra of triphenylene are studied in different matrices at 77°K and as a function of increasing pressure (0–30 kbar). The magnitude of the pressure shift of the spectra and the pressure coefficient of the lifetime depend upon the size of the molecules comprising the solvent cage. This difference in pressure response is attributed to the preponderance of repulsive interactions between highly symmetric triphenylene and smaller solvent molecules under pressure.
Production of Excited Atoms by Impact of Fast Electrons on Molecular Hydrogen and Deuterium
Authors: Vroom D. A. , de Heer F. J.
Published: February 2004

Absolute emission cross sections for production of atoms in the n = 2–6 excited states have been determined for dissociative excitation of molecular hydrogen and deuterium by electrons having energies in the range from 0.05 to 6 keV. The excited atoms have been observed by optical detection of their fluorescence, Lyman-α radiation in the case of n = 2 and Balmer-α, β, γ, and δ radiation in the case of the higher states. In general there is more than one possible molecular transition which can lead to dissociation into a particular atomic state. In the case of the n = 2 level, however, it has been possible with the aid of previous investigations to identify some transitions which may lead to both the 2s and 2p atomic states. For the formation of the higher excited states the systematics of the processes has been determined by means of analysis of the data in the Bethe–Born approximation. A strong isotope effect was found for the production of excited atoms from molecular hydrogen and deuterium. Approximately 20% fewer excited atoms are formed in the case of the heavier isotope. This difference arises due to a competition between dissociation and other processes, in particular autoionization.
Atom–Molecule Kinetics Using ESR Detection. V. Results for O+OCS, O+CS2, O+NO2, and H+C2H4
Authors: Westenberg A. A. , deHaas N.
Published: February 2004

The wide-temperature-range, fast-flow reactor with ESR detection used in previous papers of this series has been further improved so that rate coefficients of nearly 1013 cm3 mole−1·sec−1 can now be measured by the pseudo-first-order method. Results have been obtained over the temperature range 273°–808°K for the reaction O+OCS→CO+SO, over 227°–538°K for O+CS2→CS+SO, and over 297°–543°K for O+NO2→NO+O2 In Arrhenius form the rate coefficients are (cm3 mole−1·sec−1) k1 = 1.9 × 1013exp(− 4530 / RT); k2 = 1.2 × 1013exp(− 1050 / RT); k3 = 1.0 × 1013exp(− 580 / RT). The product SO in [1] and [2] was measured in its Σ3 ground state, and excited vibrational states could not be detected by ESR under our experimental conditions, although the SO must have been so excited initially. Stoichiometry and mechanisms are discussed. The H + C2H4 reaction was also measured in room-temperature helium and argon, and in helium at 525°K. The data were obtained over a pressure range of about 0.5–2.5 mm, and the pressure dependence of the net rate coefficient kt for H-atom decay was clearly measurable. Mass spectrometer analysis showed essentially all of the C2H4 to be converted to C2H6 and CH4. The mechanism is discussed. From the P−1 = 0 intercept of plots of kt−1 vs P−1, the data were assumed to yield values of the rate coefficient k4 for the initial step H + C2H4→C2H5*, i.e., the production of vibrationally excited C2H5* in the Rabinovitch manner. At room temperature the helium data give k4 = 2.2 × 1011 and the argon data 1.4 × 1011cm3mole−1·sec−1. From the slopes of the two plots the probability for transfer of vibrational energy from C2H5* to translation of argon is found to be about five times greater than for helium. The limited high-temperature data imply very little temperature dependence for either k4 or its reverse.
Effects of Details of Dynamics on dc Transport Coefficients in Solution
Authors: Harris S. , Friedman Harold L.
Published: February 2004

We consider, within the framework of Liouville's equation, the calculation of a simple higher-order transport coefficient, the Hall conductance. We show that this quantity can be resolved into two terms. One is the first moment of the time-relaxed momentum correlation function of the carrier in the field-free system while the other, an intrinsically non-Markoffian contribution, is not simply a functional of this correlation function. The latter term does not appear in earlier work. We show that it vanishes in limiting cases corresponding either to a dilute plasma or the converse extreme corresponding to ions in a dense, strongly interacting solvent.
Density Dependence of Experimental Transport Coefficients of Gases
Authors: Hanley H. J. M. , McCarty R. D. , Sengers J. V.
Published: February 2004

We analyze experimental transport data for several gases as a function of density and temperature. Necessary conditions are proposed to check if the data are consistent with a given density function at fixed temperature. It is found that the theoretically suggested density function, which involves terms logarithmic in the density, satisfies the conditions for the data examined in the temperature and pressure ranges considered. Based on the analyses, first density coefficients of thermal conductivity and viscosity are reported, together with an assessment of their precision, at reduced temperatures up to T* ≈ 30.
Experiments on Er3+ in SrF2. I. Fluorescence Quantum Efficiencies and Lifetimes
Authors: Brown M. R. , Thomas H. , Whiting J. S. S. , Shand W. A.
Published: February 2004

This paper is concerned with the measurement and interpretation of the variations in the quantum efficiences and lifetimes of four of the Er3+ fluorescing levels in the 1%–10% concentration range in SrF2 at the three fixed temperatures 300°, 70°, and 4.2°K. The interpretation has centered on the identification of the nature of ion–ion interactions present in the samples, and the establishment of their importance in the decay processes. The results show that of the four fluorescing levels studied, that is, the I13/24, I11/24, F9/24, and S3/24 levels, only decay from the F3/24 level is strongly influenced by pair decay processes, and then, only at 300°K. The F9/24 decay processes are slightly affected by pair decays, while the remaining levels are basically free from such decay processes.
Percus–Yevick Approximation for the Truncated Lennard-Jones (12, 6) Potential Applied to Argon
Authors: Watts R. O.
Published: February 2004

The Percus–Yevick approximation is used in conjunction with a truncated Lennard-Jones 12–6 potential to predict the radial distribution function and thermodynamic properties of fluid argon. It is shown that the calculated distribution functions are in good agreement with the experimental data of Mikolaj and Pings, and that the calculated thermodynamic data are close to the experimental values given by Levelt.
NMR Study of BaFPO3: 31P and 19F Chemical-Shift Anisotropies and the Absolute Sign of the F–P Coupling Constant
Authors: VanderHart D. L. , Gutowsky H. S. , Farrar T. C.
Published: February 2004

The FPO3= ion in polycrystalline BaFPO3 is investigated from the point of view that it is a very slightly distorted, axially symmetric two-spin system. NMR line-shape analysis for the 31P spectra as a function of magnetic field yielded the following information: (1) the chemical-shift anisotropy, (σ‖ − σ⊥), is − 145 ± 20 ppm for phosphorus; (2) the absolute sign of the F–P indirect dipolar coupling constant is negative; (3) the internuclear F–P distance is 1.63 ± 0.035 Å. Investigation of the 19F spectra was carried out through a second-moment analysis as a function of magnetic field. It was found that the fluorine chemical-shift anisotropy is + 182 ± 22 ppm, where the sign is deduced from the sense of the line-shape asymmetry. Further-more, an estimate of the intermolecular dipolar contribution to the second moment was obtained by extrapolating the total second moment to low field and then subtracting the intramolecular dipolar contribution which is known from the 31P spectra. The intermolecular dipolar contribution is such that if BaFPO3 has a structure analogous to BaSO4, K2SO4, and K2FPO3, then there is a high degree of disorder in the orientation of the FPO3= ions in the crystal.
Crystal Structure of (NH4)2MnF5
Authors: Sears D. Richard , Hoard J. L.
Published: February 2004

(NH4)2MnF5 crystallizes from acid solution as orthorhombic crystals of space group Pnma with a = 6.20 ± 0.03, b = 7.94 ± 0.01, and c = 10.72 ± 0.01 Å, and four formula units per cell. The stoichiometry is achieved by the growth parallel to b of infinite kinked anionic chains of composition (MnF5)n−2n, formed through sharing of opposite vertices of MnF6 octahedra. The shared fluorine atoms lie on mirror planes (y = 14, 34), as do the hexagonal networks of ammonium ions that bind the anionic chains together. Mn–F bond distances, corrected for the markedly anisotropic thermal motions, are 1.85 and 2.10 Å. The observed elongation of the MnF6 octahedra in the chain direction is largely, but not wholly, interpretable as a Jahn–Teller distortion.
Pulse-Radiolysis Observation of Molecular Cations of Aromatic Compounds in Halogenated Liquids
Authors: Arai Shigeyoshi , Ueda Hisashi , Firestone Richard F. , Dorfman Leon M.
Published: February 2004

Electron irradiation of dilute solutions of aromatic compounds in liquid chlorinated hydrocarbon solvents results in the formation of the molecular cations of these aromatic compounds. The optical absorption spectra of the molecular cations of diphenyl, p-terphenyl, t-stilbene, and anthracene were observed by pulse-radiolysis methods, and the identity of the reactive species established from the spectra and from various scavenger effects. At room temperature the half-life of these cations in the particular solutions is on the order of a few microseconds. In solutions containing two different aromatic compounds as solutes, it was found that an electron transfer may take place between the neutral aromatic molecule of one compound and the molecular cation of the other.
Use of the CNDO Method in Spectroscopy. IV. Small Molecules: Spectra and Ground-State Properties
Authors: Del Bene Janet , Jaffé H. H.
Published: February 2004

The modified CNDO method has been applied to the study of the electronic-absroption spectra of several small molecules; namely, formaldehyde, formic acid, formamide, allene, ketene, and diazomethane, with quite satisfactory results. The lowest energy excited state of the last three compounds has been assigned as the state which arises from a π → π′* transition. A study has also been made of the calculation of ground-state properties from the spectroscopic parameterization. It has been concluded that although this method can give reasonable estimates of these properties, it is most reliable when used to evaluate ground-state properties which depend upon calculated eigenvalues.
Temperature Effects on the Phosphorescence of Aromatic Hydrocarbons in Poly(methylmethacrylate)
Authors: Jones Peter F. , Siegel Seymour
Published: February 2004

Temperature effects on the phosphorescence lifetimes and phosphorescence intensities of naphthalene, phenanthrene, pyrene, biphenyl, and the perdeuterated analogs of these four molecules were measured with the use of poly(methylmethacrylate) (PMM) as a rigid solvent matrix. The phosphorescence lifetimes of these molecules decreased by only 13%–37% upon warming from 77°K to room temperature. On the basis of direct measurements of the permeability of PMM to oxygen, we conclude that oxygen quenching is not responsible for the observed temperature effects on the phosphorescence lifetimes. Although we cannot exclude the possibility that the rate constants for both radiative and nonradiative decay vary with temperature, we show that our results are consistent with an analysis that assumes that only the non-radiative rate constant is temperature dependent. On the basis of this analysis, we estimate an activation energy of 500 ± 100 cm−1 for the thermally activated nonradiative decay mode.
Projected Hartree Product Wavefunctions. II. General Considerations of Young Operators
Authors: Gallup G. A.
Published: February 2004

A discussion is given of the forms of two specific and one general Young operator for the irreducible representations of Sn important for fermion space functions. Comparisons are made of the projected Hartree product version of Löwdon's projected Hartree–Fock method with CI calculations.
Ground States of σ-Bonded Molecules. IV. The MINDO Method and Its Application to Hydrocarbons
Authors: Baird N. Colin , Dewar Michael J. S.
Published: February 2004

A simple semiempirical valence shell SCF MO treatment of hydrocarbons is described which enables their heats of formation to be calculated to within a few kilocalories per mole, and which can be applied to a wide variety of saturated, unsaturated, and aromatic hydrocarbons including strained microcyclic systems. Geometries have to be assumed since the method does not lead to correct predictions of bond lengths, and it also fails to predict correctly the relative energies of rotational isomers. Otherwise the method is both simpler and more accurate than the PNDO treatment described in Papers I–III.
Conformation of Adsorbed Polymeric Chain. II
Authors: Motomura Kinsi , Matuura Ryohei
Published: February 2004

By developing the theoretical treatment of an isolated polymeric chain at an interface, which is based on a three-dimensional random walk on a simple cubic lattice, the average number of segments in the tail of a polymeric chain k̄0 and the average number of trains per polymeric chain m̄ are calculated in the limit in which the chain length n is infinite. With the aid of the average number of adsorbed segments ν̄ evaluated in Part I of this series, the average number of segments in a train and a loop, k̄′ and k̄, are also calculated. The average properties are found to change in their dependence on the adsorption energy ε and the total number of segments n at a critical point except k̄′. For η < ηc, where η = exp(ε / kT), k̄0 has the value 12n; this indicates that the polymeric chain, in effect, consists of two long tails extending into the solution, while k̄ is still proportional to n1/2. It is seen that at η = ηc only one-third of the chain length participates in adsorption, and m̄ and k̄ obey the half-power law in the same way as ν̄. When η exceeds ηc, k̄0 and k decrease rapidly with an increase in the adsorption energy, and m̄ becomes proportional to n. It is noticed that the length of train is relatively small even if the majority of segments are adsorbed at the interface.
Optical-Rotatory Power and Linear Electro-optic Effect in Nematic Liquid Crystals of p-Azoxyanisole
Authors: Williams Richard
Published: February 2004

Observations have been made on nematic liquid p-azoxyanisole in a magnetic field. The liquid is found to have optical-rotatory power and to show a linear electro-optic effect. Both are dependent on the direction of observation. A detailed examination of the symmetry elements of the experiments leads to a model in which all effects are traced back to the properties of a single molecule. It is concluded that the nematic liquid lacks a center of symmetry and is, in this way, similar to the polar crystal classes in solids.
Atomic and Molecular Calculations with Pseudopotential Method. IV. Excited Atomic Valence-Electron Energy Values, Wavefunctions, and Oscillator Strengths
Authors: McGinn George
Published: February 2004

The pseudopotential equation of Phillips and Kleinman is solved numerically for the first six S2 states, the first six P02 states, and the first five or six D2 states for the valence electron of the atoms Li, Na, K, Rb, Be+, Mg+, Ca+, Al++, Cu, and Zn+. From the calculated energy values it can be seen that the polarization effect of the valence electron on systems with large cores is important even when the valence electron is in a high excited state. The integrals needed to compute the oscillator strengths for all the allowed transitions between the calculated valence states are computed.
Statistical Mechanics of Linear Molecules. III. Hard-Core Model
Authors: Chen Yi-Der , Steele W. A.
Published: February 2004

Computations of the pair correlation functions have been carried out for model nonspherical molecules. The molecules were represented by two hard spheres with centers separated by a distance R (“diatomic” model). The usual expansion of the correlation function in powers of the density was generalized to treat this problem and terms in the series up to and including those in (density)2 were evaluated with the aid of the Percus–Yevick approximation. Both the angle-dependent and the angle-independent parts of the pair correlation function were computed for several values of the molecular length. These results were then used to calculate second, third, and Percus–Yevick fourth virial coefficients. The changes in pair correlation function, virial coefficients and equation of state were obtained as a function of molecular shape. It is shown that these changes are not large when compared to the hard-sphere results using proper units; i.e., that the most important factor in these calculations is the alteration in molecular volume rather than in molecular shape.
Stability of the Gaseous Ammonium Chloride Molecule
Authors: Goldfinger P. , Verhaegen G.
Published: February 2004

The equilibrium vapor effusing from a Knudsen cell, in which solid ND4Cl was evaporated, was analyzed by a quadrupole mass spectrometer. In the temperature range considered (335°–485°K) the main vaporization process corresponds to the decomposition of the solid towards gaseous ND3 and DCl; the measured enthalpy change ΔH°407[decND4Cl(s)] = 42.6 ± 2.0 kcal mole−1 is very close to that measured for NH4Cl: ΔH°407[decNH4Cl(s)] = 42.5 kcal mole−1. Apart from the main gaseous species, small proportions (5 × 10−5–2 × 10−3) of ND4Cl molecules were also observed. The ND4Cl molecule fragments (> 99%) under electron impact and yields the ND4+(m / e = 22) ion. From the observed ion intensity ratios, as well as their dependence on temperature, D0°(NH4Cl ⇋ NH3 + HCl) = 10.0 ± 3.0 kcal mole−1 was obtained confirming Clementi's computed prediction < 14.0 kcal mole−1.
Ring-Puckering Vibration of 2,3-Dihydrofuran
Authors: Green William H.
Published: February 2004

The far-infrared spectrum of 2,3-dihydrofuran vapor has been examined from 33 to 500 cm−1. Twelve sharp Q branches have been observed between 70 and 350 cm−1. These bands are assigned to transitions of the anharmonic ring-puckering vibration which is described by the following relation in reduced coordinates: V(cm−1) = 28.3(x4 − 3.44 x2). Such a potential predicts a puckered molecule in the ground and first excited states, a barrier to planarity of 83 cm−1, and a 0–1 transition of 20.5 cm−1. There is considerable disagreement between the previously predicted far-infrared spectrum and that observed.
Vibronic Intensity Distribution in the Phosphorescence of Benzene and Its Deuterated Isomers
Authors: Nieman G. C.
Published: February 2004

The gound-state vibrational frequencies and the relative vibronic intensities of the phosphorescence are calculated for all and measured for one carbon-13 and ten of the possible twelve deuterium-substituted benzenes. The intensities are calculated within the framework of the Herzberg–Teller model. It is shown that the relative intensities for different vibronic transitions of isotopically related molecules can be expressed in terms of two factors; a normal coordinate eigenvector matrix element which is isotope dependent, and an electronic term which is isotope independent. The observed C6H6 intensities are used to evaluate the electronic parameters. Excellent agreement is found among the calculated and observed intensities and frequencies. The relative importance of the different coordinates for vibronic coupling in benzene is indicated.
Experimental Measurements of the Resonant Vibrational Energy Transfer between Mode v3 of CO2 and N2
Authors: Taylor Raymond L. , Bitterman Steven
Published: February 2004

The rate of the resonant vibration–vibration energy-transfer process between mode ν3 of CO2 and N2 has been measured from 450°-2600°K by monitoring the infrared emission from mode ν3 at 4.26 μ behind incident shocks in CO2–N2 gas mixtures. The data are shown to be in agreement with recent laser fluorescent measurements from 300°–1000°K. The combined data for the probability per collision are compared to theoretical calculations and indicate that below 1000°K the interaction is dominated by long-range forces and a probability inversely proportional to temperature is obtained, while above 1000°K the probability increases with temperature and the interaction is due to short-range forces.
Vibration–Rotation Interaction Effects in Calculated Franck–Condon Factors. II. Hydrogen Lyman and Fulcher Bands
Authors: Villarejo Don , Stockbauer Roger , Inghram Mark G.
Published: February 2004

Calculated Franck–Condon factors, explicitly including previously neglected vibration–rotation interaction effects, are reported for 450 Lyman and 24 Fulcher bands of the hydrogen molecule. Large concellation effects, depending on rotational quantum number, are found for a number of Lyman bands. Where comparison is possible, the results show good qualitative agreement with experimentally determined intensity distributions. Using alternative representations of the B 1Σu+ potential curve (upper state of the Lyman bands) it is shown that the calculated intensity distribution for some bands are highly sensitive to detailed features of the molecular potential curves.
Elastic Scattering, Diffusion, and Excitation Transfer of Metastable Helium in Helium
Authors: Kolker H. J. , Michels H. H.
Published: February 2004

The dynamics of the resonance collision of metastable and normal helium atoms has been examined and calculations have been performed for elastic scattering, diffusion, and excitation transfer. To facilitate this analysis, accurate ab initio calculations of the pertinent interaction potentials have been carried out. The use of these more accurate potential curves in the framework of the original dynamical formulation of the collision process by Buckingham and Dalgarno leads to serious discrepancies between calculations and experiment for the elastic scattering cross section and the coefficient of diffusion. The rate constant for the excitation transfer reaction, however, is in reasonable agreement with experiment, over a wide range of temperatures.
Adsorbate Effects in Electron Ejection by Rare-Gas Metastable Atoms
Authors: Delchar T. A. , MacLennan D. A. , Landers A. M.
Published: February 2004

The effect of adsorbed-gas layers on the ejection of electrons by rare-gas metastable atoms has been investigated for some tungsten surfaces. These surfaces were the (111) and (110) planes of a tungsten single crystal and an essentially (100) oriented polycrystalline tungsten ribbon. With nitrogen as the adsorbate the ejected-electron yields were reduced by 42% on the polycrystalline ribbon and increased 1% on the (111) plane. The yield did not change for the (110) plane. With carbon monoxide as the adsorbate, the yield decreases were 63%, 51%, and 56% for the polycrystalline ribbon, the (100), and (111) planes, respectively. With hydrogen as the adsorbate the yield was decreased by 1% on the (110) plane and 4% on the (111) plane. In those cases where the ejected-electron yield was sharply reduced following adsorption, the electron energy spectrum revealed a marked diminution of the number of high-energy electrons and in the case of carbon monoxide on the (111) and (110) planes, essentially a “cutoff” in the energy spectrum. No correlation was found between the yield changes and the known work-function changes accompanying adsorption. The effectiveness of each adsorbate in reducing the ejected-electron yield was in the order CO>N2>H2. For CO it was found that the decrease in yield of ejected electrons was proportional to the number of CO atoms which had impinged on the surface. By this means the sticking coefficient for CO on the (111) and (110) planes was estimated to be unity. The results are interpreted with a model in which bonding electrons from the adsorbed atom participate in the ejection process, the degree of participation depending on the surface density of the adsorbed atoms as well as their size and position on the surface. The theory, applied to the data for CO, predicts that the CO bonding electrons lie 12.6 eV below the vacuum level. It is inferred also that nitrogen atoms adsorbed on the (111) plane lie in the surface, while on the (100) plane they protrude from the surface.
Time Dependence of Ion-Pair Recombination Rates in Liquids of Low Dielectric Constant
Authors: Ludwig Peter K.
Published: February 2004

An analog computer method is employed to solve the differential equation of diffusion when applied to ensembles of isolated ion pairs. The method permits evaluation of the probability density of the ion pairs as well as relative rates of their neutralization as function of time for a variety of initial conditions. The results are discussed with emphasis on ion neutralization processes in radiation chemistry.
Rotational Isomerism in Chloroacetyl Halides
Authors: Khan A. Y. , Jonathan Neville
Published: February 2004

Infrared and Raman spectra are reported for the acid fluoride, chloride, and bromide derivatives of chloroacetic acid. Data from vapor, liquid, and solid spectra are given. The results show that these acid halides exist in the vapor and liquid states as a mixture of two isomers, only one of which remains in the solid state. The more stable isomer was found to have the trans conformation. Calculations of the normal vibrations using a Urey–Bradley potential function serve to indicate that the other isomer has a staggered conformation with an azimuthal angle close to 180°. The measured enthalpy differences (H°trans − H°staggered) between the isomers in the vapor state were − 0.76 ± 0.2 kcal/mole, − 0.97 ± 0.2 kcal/mole, and − 1.86 ± 0.27 kcal/mole for the acid fluoride, chloride, and bromide, respectively.
Comment on Kirkwood Instability
Authors: Kunkin W. , Frisch H. L.
Published: February 2004

Kirkwood's instability incorrectly predicts a phase transition for hard rods. A reinterpretation, considering a uniform fluid perturbed by a small external force, in the absence of a phase transition is attempted, but is unsuccessful. In some cases, the perturbation results in a mechanically more stable system.
Theoretical Comparison of Formic Acid and the Formate Ion
Authors: Peyerimhoff Sigrid D. , Buenker Robert J.
Published: February 2004

Ab initio SCF MO and CI calculations are reported for formic acid HCOOH and the formate ion HCOO−. A simplified theoretical treatment for the salts of formic acid is considered in which a bare positive charge without an accompanying AO basis function serves as a model for the associated metal cation. The calculations suggest that the formation of an OHO bridge occurs in certain symmetric nuclear arrangements, even for monomeric HCOOH; the stability of the system is subsequently shown to be increased by the migration of the acidic hydrogen toward one of the oxygens and away from the other, thereby producing the known nonsymmetric equilibrium geometry of the acid. The influence of the acidic hydrogen is also apparent in the comparison of the spectra of HCOOH and HCOO−, causing the displacement of transitions to a 3,1B pair of states to considerably shorter wavelengths than are observed for the corresponding transitions in the anion. Calculated molecular properties of formic acid for various SCF and CI treatments are also reported and comparison with pertinent experimental data indicates certain definite advantages in employing CI techniques to improve the SCF ground-state electronic distribution.
Study of Some of the Parameters Affecting Knudsen Effusion. VI. Monte Carlo Analyses of Channel Orifices
Authors: Ward John W. , Fraser Malcolm V.
Published: February 2004

Techniques for Monte Carlo simulation of real particle behavior have been extended to study the performance of channel orifices. Transmission coefficient calculations and wall-flux gradient behavior are considered. For the particular case L / R = 4.0, functions are derived to allow for variable orifice-to-collimator distances when the target collection method is used. A “state-of-the-art” discussion on orifice behavior is included to help delineate the purposes of this study. It is concluded that use of both knife-edged and channel orifices is sometimes necessary to establish the cell-orifice behavior of some systems; information derived from experimental studies, and from Monte Carlo analysis of flux relationships appropriate to the geometry of the system, seems to indicate that the channel orifice is a better choice for most effusion work.
Production of Excited Hydrogen Atoms by Impact of Fast Electrons on Water Vapor
Authors: Vroom D. A. , de Heer F. J.
Published: February 2004

Lyman-α and Balmer-α, β, γ, and δ radiation has been observed resulting from dissociative excitation of water vapor by fast electrons. Absolute cross sections for emission of this radiation are reported in the electron energy range from 50–6000 eV. The results are analyzed by means of the Bethe approximation. Of all the relevant dissociative states of the molecule formed during the collision process, those which are populated by optically allowed transitions predominate. The relatively large cross sections for dissociative excitation producing excited hydrogen atoms may indicate the importance of such species in the radiolysis of water.
Photoionization of Ethylene with Mass Analysis
Authors: Chupka W. A. , Berkowitz J. , Refaey K. M. A.
Published: February 2004

The photoionization-efficiency curve of ethylene has been shown to have vibrational structure at the threshold for production of the first excited state of the parent ion. Autoionization of superexcited states in this region does not occur significantly. Appearance potentials were measured for the fragment ions C2H3+, C2H2+, and CH2+. The thermochemical values obtained were D0°(C2H3–H) = 3.85 eV; D0°(CH2–CH2) ≤ 8.28 eV; ΔHf0°(CH2) ≤ 95.5 kcal/mole.
Hyperfine Structure of Thallium Chloride
Authors: Hammerle Robert H. , Dickinson J. Thomas , VanAusdal Ray G. , Stephenson David A. , Zorn Jens C.
Published: February 2004

The radio-frequency spectra of TlCl at very weak electric and magnetic fields have been measured with a molecular beam electric resonance spectrometer. From these spectra the hyperfine interaction constants for the four isotopic species of the molecule were calculated. The constants for 205Tl35Cl in the J = 2, υ = 0 state are: eqQ = − 15793.32(50) kHz, cCl = 1.38(10) kHz, cTl = 76.35(10) kHz, c3 = − 0.13(10) kHz, c4 = − 1.54(10) kHz. A test was made for the polarization of the chlorine nucleus in the electric field of the molecule by comparing the ratio of the quadrupole interaction constants for 205Tl35Cl and 205Tl37Cl to the ratio of the quadrupole interaction constants for the free chlorine atoms. The agreement of the two ratios is within their uncertainties, thus providing no evidence for a polarization effect. In addition, the dependence of the spin–rotation and spin–spin interaction constants on isotope was found to show good agreement with theory.
Microwave-Optical Double Resonance. Frequencies and Linewidths of Stimulated Emission Transitions in the A 2Π3/2 State of CN
Authors: Pratt D. W. , Broida H. P.
Published: February 2004

Stimulated emission at microwave frequencies in the lambda doublet of the A 2Π3/2, υ = 10, J = 72 level of CN has been reinvestigated. Observation of the increase in CN violet (B 2Σ–X 2Σ) emission caused by microwave pumping has allowed precise measurement of all seven of the allowed (ΔF = 0, ± 1) transitions between the hyperfine components of the lambda doublet. Effects of O2 and SF6 on electrical breakdown in the microwave cavity have been determined. Pressure dependence of the linewidths has been studied; comparison of these results with those of earlier experiments on CN suggests that the cross section for collisional energy transfer from the A 2Π state to the B 2Σ state is larger than that for rotational relaxation. The contribution of the optical radiative lifetime to the pressure-independent width is demonstrated.
Transfer of Electronic Excitation Energy in Polyvinyl Carbazole
Authors: Klöpffer W.
Published: February 2004

The energy transfer in amorphous polyvinyl carbazole (PVCA) has been studied by fluorescence quenching experiments using perylene, trinitrofluorenone, and hexachloro-p-xylene as guest molecules. The PVCA shows excimer fluorescence. The results are discussed in terms of exciton diffusion and competition between excimer-forming sites and guest molecules for the trapping of excitons. Single-step transfer from the excimer state by dipole resonance is considered as an alternative mechanism of energy transfer; in all three cases, however, exciton migration is the more probable energy-transfer mechanism. The concentration of excimer-forming sites is estimated to be 10−3 mole/mole basic unit of the polymer. In the pure polymer the exciton covers about a thousand basic units during its lifetime.
Exponential Solution of the Schrödinger Equation: Potential Scattering
Authors: Chang Daniel , Light John C.
Published: February 2004

The exponential matrix method of Magnus is applied to the solution of the time-independent Schrödinger equation for potential scattering. The formal properties of this method as a numerical technique are analyzed and the close connections to the WKBJ approximation are also shown. The use of the Magnus series is shown to yield the correct behavior near the origin without the Langer modification of the WKBJ equation. Applications to the scattering from above a potential barrier and to the phase shifts for a Lennard-Jones (12–6) potential are given.
Intensity and Polarization of Light Scattered by Some Permanent Gases and Vapors
Authors: Gucker Frank T. , Basu Sadhan , Pulido Agustin A. , Chiu Grace
Published: February 2004

A light-scattering instrument has been developed and used to determine the polarization ratios and scattering powers of nitrogen, oxygen, carbon dioxide, air, a 79.6% nitrogen–20.4% oxygen mixture, propane, butane, and dimethyl ether. Measurements were made with three types of incident light—unpolarized, horizontally plane polarized, and vertically plane polarized. Three methods were used to measure the polarization ratio for unpolarized incident light, ρu; two to measure that with vertically plane-polarized light, ρυ; and one for horizontally polarized light, ρh. Good agreement was obtained between experimental results and predictions from Rayleigh's theory of anisotropic scattering by small particles.
Analyses of the Vibration–Rotation Bands of HCF3, PF5, and Cyclo-C3H6 by Computer Techniques
Authors: Wyatt R. , Roberts J. T. , Wentz R. E. , Wilt P. M.
Published: February 2004

We have studied the ν3 fundamental of fluoroform at 700 cm−1 with a resolution of 0.3 cm−1, obtaining a contour showing a series of well-resolved P and R branch “lines.” A rotational analysis is performed which determines the ground- and first excited vibrational state rotational and distortion constants. A computer program for the calculation of symmetric-top parallel band contours is critically tested by comparison of the experimental and calculated envelopes of the ν3 HCF3 band. The ν4 band of phosphorous pentafluoride at 575 cm−1 is studied with a resolution of 0.5 cm−1 giving an unresolved but well-defined contour which is analyzed by comparison with calculated contours. The results give P–F axial bond length = 1.581 Å, P–F equatorial bond length = 1.534 Å in the ground vibrational state. The ν6 fundamental of cyclopropane is examined under a resolution of 0.3 cm−1, giving a well-resolved series of P and R branch “lines.” The B0 value for cyclo-C3H6 found from this band is 0.6688 cm−1, while C0 = 0.420 cm−1.
Mössbauer-Effect Study of Nitrosyliron Bis(N, N-diethyldithiocarbamate)
Authors: Johnson C. E. , Rickards R. , Hill H. A. O.
Published: February 2004

The 57Fe Mössbauer spectrum of nitrosyliron bis(N ,N-diethyldithiocarbamate) has been measured at temperatures between 300° and 1.3°K and in external magnetic fields. In zero field, the spectrum at all temperatures consists of a quadrupole split pair of lines of unequal intensity. The intensity of the absorption increases markedly on cooling from 300° to 77°K. The isomer shift (0.34mm sec−1 at 77°K, relative to iron) is higher than expected for a low-spin Fe(I) complex and implies that the metal s electrons are screened from the nucleus by electron donation from the ligands. The quadrupole splitting shows little dependence on temperature and the application of an external magnetic field shows the sign of the major component of the efg tensor to be positive. Magnetic hyperfine interactions are observed at low temperatures in large external magnetic fields. The variation of the effective hyperfine field at the nucleus with H / T follows the Brillouin function for S = 12, and the value of the saturation hyperfine field was found to be 110 ± 5 kG. The orbital ground state of the iron atom cannot be deduced from the sign of the quadrupole splitting owing to the large contributions from the bonding electrons. From the magnetic hyperfine splitting, it is deduced to be dz2.