bluuebunny/crossref_metadata_2025 huggingface dataset:
Mössbauer Effect of Exchangeable Ferrous Ions in Y Zeolite and Dowex 50 Resin
Authors: Delgass W. Nicholas , Garten Robert L. , Boudart M.
Published: February 2004

The area under the Mössbauer absorption curve of Fe2+ in organic (Dowex 50) or inorganic (Linde Y zeolite) ion exchange materials is greatly enhanced when the water-saturated solids are evacuated at room temperature or cooled to near liquid-nitrogen temperature. This result, considered in the light of findings by other techniques, is evidence of the solvation of ions in the water-filled resin or zeolite. Solvation of cations and its appearance upon hydration following treatment at high temperature is proposed as a criterion of reversible exchange in some materials.
Low-Temperature Magnetic Susceptibility of FeCl3
Authors: Jones Edwin R. , Morton O. B. , Cathey L. , Auel Theo , Amma E. L.
Published: February 2004

We have measured the magnetic susceptibility of carefully purified FeCl3 and FeCl3·6H2O over the temperature range from 4.2° to 30°K. An antiferromagnetic transition with a Néel temperature TN = 9.75° ± 0.2°K was observed in FeCl3. This is significantly lower than previously reported values determined by Mössbauer effect and neutron diffraction. Mössbauer resonance was found to be an excellent test for small amounts of water decomposition products of anhydrous FeCl3. Ferric chloride hexahydrate was found to be paramagnetic and obeyed the Curie law down to 4.2°K.
Study of Electronic Structure of Molecules. IX. Remarks on the Cyano Group
Authors: Clementi E. , Klint D.
Published: February 2004

We have attempted to characterize the cyano and the isocyano groups by stressing their analogy to the CN− radical. Some cyano compounds (like vinyl isocyanide, methyl cyanide, and methyl isocyanide) have been investigated by performing SCF computations. In addition R–CN and CN–R compounds have been simulated by perturbing the CN− with positive or negative point charges. The simulation reveals that classical electrostatic perturbation by point charges induces a two-way charge transfer, characteristic of R–CN or NC–R compound computed in the SCF approximation. In addition, the analysis of dipole moment for the point-charge perturbed CN− gives results which explain the high dipole moment present in many R–CN or CN–R compounds.
Third Virial Coefficients for Mixtures of Nonspherical Molecules
Authors: Stogryn Daniel E.
Published: February 2004

The influence of a large number of tensor interactions on the third virial coefficient, in some cases through the third order of perturbation theory, is evaluated for gas mixtures. The spherically symmetric components of the potential functions are assumed to be either of the Lennard-Jones 12–6 or 18–6 form. Molecular asymmetries of very general types are described in this work by permanent electric multipole moments, induced dipole moments, and anisotropic dispersion forces. Permanent moment interactions involving dipoles and quadrupoles are considered through the third order. Other asymmetric interactions involving permanent multipole moment tensors up to and including the hexadecapole moment are considered through the second order of perturbation. Pairwise potential nonadditivity due to three-body induced moment interactions occur in the second-order results and are studied for molecules with permanent dipole and quadrupole moments. Numerical examples of the effects of tensor interactions are given for CO2, N2, and C6H6.
Deactivation of Vibrationally Excited Carbon Dioxide (ν3) by Collisions with Carbon Dioxide or with Nitrogen
Authors: Rosser W. A. , Wood A. D. , Gerry E. T.
Published: February 2004

The rate constants associated with the deactivation of vibrationally excited CO2*(ν3) by collision with either CO2 or N2 have been experimentally determined from 300°–1000°K by a laser fluorescence method. The reactions investigated were CO2*(ν3)+N2⇆ lim k−5k5CO2+N2*, CO2*(ν3)+N2→ lim k5CO2*(ν1, ν2)+N2, CO2(ν3)+CO2→ lim k1CO2*(ν1, ν2)+CO2, N2*+CO2→ lim k7CO2*(ν1, ν2)+N2, where the asterisk denotes a vibrationally excited molecule, and the quantities in parentheses represent the specific excited modes of CO2. The rate constant k5 varies with temperature T (°K) as 8.60 × 107 / T3/2torr−1·sec−1. From 400°–1000°K the rate constant k1 (per torr−1 ·second−1) varies with temperature as log10k1 = A − BT−1 / 3 with A = 6.79 and B = 30.8. From 300°–400°K the measured values of k1 are greater than those corresponding to the above relation. The rate constant k6 increases from a value of 110 torr−1·sec−1 at room temperature to a value of 2700 torr−1·sec−1 at 1000°K, but the variation of k6 with temperature cannot be simply expressed. The rate constant k7 was found to be negligibly small compared to the other rate constants.
Magnetic Rotation Spectrum of the A 2Σ+ ← X 2Πr Transition in NO. II
Authors: Robinson Dean W.
Published: February 2004

The magnetic rotation spectrum of nitric oxide in the 44 000-cm−1 region, the 0, 0 “γ band,” has been photographed with about 0.07-cm−1 resolution yielding fairly clear line shapes and relative intensities. The spectrum has been synthesized line by line using classical formulas for the dispersion of the refractive index and the absorption coefficient through each Zeeman component as shifted in a field of 2500 G. For the ground state Π no perturbation of the field-free wavefunctions was incorporated; for the upper state Σ2 complete coupling of the spin to the field was assumed. This picture led to complete agreement with the details of the observed spectrum.
Natural Orbital Expansion of Interacting Geminals
Authors: Silver David M.
Published: February 2004

An antisymmetrized product of “interacting” geminals is proposed for the form of a many-electron wavefunction. The interacting geminals are given a natural orbital expansion where the natural orbitals of the geminals are also natural orbitals of the total wavefunction. This ansatz is more general than the strongly and weakly orthogonal geminal and identical geminal models and includes them as particular cases. However, the use of the interacting geminal wavefunction is not significantly more complicated for the purpose of calculations. Different constraints on the geminal occupation coefficients define the various geminal models and more than one model can be incorporated into a single wavefunction. The case of two singlet pairs of electrons is examined in terms of the interacting geminal model—density matrices, energy expressions, and variational equations being given.
Nuclear Spin–Lattice Relaxation in Axially Symmetric Ellipsoids with Internal Motion
Authors: Woessner D. E. , Snowden B. S. , Meyer G. H.
Published: February 2004

The calculation of the nuclear-magnetic-resonance spin–lattice relaxation time has been extended to include molecules which approximate an axially symmetric ellipsoid and have internal motion about an axis at any constant angle to the symmetry axis of the ellipsoid. The calculation is carried out explicitly for two models of internal motion. These are, random reorientation among three equivalent positions 120° apart, and rotational diffusion.
Optical Potentials in Molecular Collisions
Authors: Micha David A.
Published: February 2004

Inelastic transitions in molecular collisions are discussed in terms of optical (complex) potentials. Adiabatic and sudden approximations for molecular collisions are considered, as well as conditions for their validity. Numerical results obtained in an adiabatic approximation are presented for rotational and vibrational transitions in the H + H2 system, which include a study of the real and the imaginary parts of its optical potential. The relevance of optical potentials in the description of direct molecular reactions is also briefly considered.
Electron Spin Resonance Spectrum of N2− Trapped in KCl, KBr, and KI
Authors: Brailsford J. R. , Morton J. R. , Vannotti L. E.
Published: February 2004

The EPR spectra attributable to N2− trapped in KCl, KBr, and KI have been observed between 2° and 20°K. The N2− impurity occupies a negative-ion vacancy and is oriented so that the equilibrium direction of the N–N bond is a 〈110〉 direction of the crystal. The principal g factors of N2− are characteristic of a 2Π1/2 ground state, and the nodal plane of the π orbital is a {100} plane of the crystal. In KI, N2− appears to be undergoing a libratory motion, which is, however, confined to this {100} plane. The 14N hyperfine structure, which is perfectly resolved in KCl:N2−, is used to determine the atomic parameters 〈r−3〉s and (8π / 3)ψ2(0).
Solvable Quantum-Mechanical Model of Three-Body Rearrangement Scattering
Authors: Tang K. T. , Kleinman B. , Karplus M.
Published: February 2004

An “exact” quantum-mechanical calculation of three-body, colinear rearrangement collisions with certain idealized potentials has been performed. The method used is valid for collisions between particles of arbitrary mass. New results are obtained even in the simplest case, that of a scattering center of infinite mass with no multiparticle interaction potential. Transition probability curves are also presented for an infinite scattering center with a finite potential barrier, for collisions of particles of equal mass with and without a potential barrier, for an exothermic reaction, and for an example of resonance scattering. Quantum-mechanical tunneling, the likelihood of rearrangements involving vibrational transitions, and the effect of the relative mass of the scattering center are discussed.
E-Species Force Constants for POF3, PSF3, VOF3, and ClO3F
Authors: Hoskins L. C.
Published: February 2004

Calculations of the E-species force constants of the general harmonic force field have been carried out for POF3, PSF3, VOF3, and ClO3F using the vibrational frequencies and Coriolis constants of vibration–rotation interaction as experimental data. The Coriolis constants were determined from the shapes of the gas-phase infrared band envelopes. The calculations show that for these molecules F44 > F66 > F55, and all of the off-diagonal force constants are negative, except for F45 in VOF3 which is positive. Normal-coordinate calculations also show that the normal modes are strong mixtures of the corresponding symmetry coordinates.
Triplet–Triplet Energy Transfer in Ordered and Random Media
Authors: Hunter Thomas F. , McAlpine Robert D. , Hochstrasser Robin M.
Published: February 2004

The efficiency of transfer of benzophenone triplet energy to a naphthalene impurity has been examined in crystalline and glassy-state benzophenone samples. The efficiency, measured using relative emission yields and lifetimes, was about two orders of magnitude lower in the glass. This has suggested to us the possibility that coherent and incoherent transport of excitons occur at significantly different rates.
Magnetic Shielding of 19F in XeF2
Authors: Hindermann D. K. , Falconer W. E.
Published: February 2004

The rigid-lattice second moment of the 19F nuclear magnetic resonance of XeF2 has been measured as a function of the external magnetic field. The experimental shielding anisotropy | Δσ | = 105 ± 10 × 10−6 does not agree closely with a value of σ⊥ − σ‖ = − 506 × 10−6 previously calculated using a localized orbital model. The field-independent rigid-lattice second moment is ∼ 0.4 G2 larger than that calculated from crystallographic data, probably because of large amplitude vibrations in XeF2. The temperature dependences of the second moment and of the spin–lattice relaxation time are also reported.
Hypernetted-Chain Approximation Applied to a Modified Lennard-Jones Fluid
Authors: Watts R. O.
Published: February 2004

The hypernetted-chain approximation is used in conjunction with Baxter's form of the Ornstein–Zernicke equation to predict the thermodynamic properties of a Lennard-Jones fluid. It is shown that the approximation predicts a phase change for the fluid. Outside the critical region the HNC equation is at least two valued for all densities in the range (0–0.6). The critical temperature obtained from the approximation is dependent upon the range of the intermolecular potential function, but the critical density is relatively insensitive. Unlike the behavior reported for the Percus–Yevick approximation, the compressibility of the fluid does not become infinite at the phase change.
Rotational Energy Transfer Rates in HCN by Microwave Double Resonance
Authors: Gordon R. G. , Larson P. E. , Thomas C. H. , Wilson E. B.
Published: February 2004

Modulated microwave double resonance has been used to obtain values of collisionally induced transition rates between rotational levels of HCN. Double resonance intensities and phase shifts were measured for a number of pairs of l doublets in HCN. A computer program was written to calculate the intensities and phase shifts in terms of a set of rate constants. The rate constants were varied so as to produce the best least-squares fit to our data. As a check, the rate constants obtained from the least-squares fitting were used to calculate the results of some steady-state double resonance experiments. The calculations and experimental data are found to be in excellent agreement.
Radio-Frequency Spectrum of KBr by the Molecular-Beam Electric-Resonance Method
Authors: de Leeuw F. H. , van Wachem R. , Dymanus A.
Published: February 2004

The molecular-beam electric-resonance method has been used to observe the radio-frequency spectra of 39K79Br and 39K81Br in the J = 1 rotational state and the υ = 0, 1, and 2 vibrational levels. Analysis of the spectra has given dipole moments and hyperfine interaction constants of these molecules.
Crystal Field of Ruby and Stressed MgO:Cr3+
Authors: Stedman G. E.
Published: February 2004

The crystal field of ruby and the stress-induced crystal field of MgO:Cr3+ are re-expressed in terms of a semiempirical model of the crystal field. In this model, which allows for overlap and covalency, the observed crystal field parameters are related to the crystal field parameters for the single ligand system, Cr–O. The latter parameters are obtained for each system; they are constant for the two lattices to within 15%. This is indirect evidence for the existence of static relaxation around the Cr3+ ion. The dynamic properties of the bond and the consistency of the analyses are discussed.
Optical Absorption by Charge-Transfer Excitons in Linear Molecular Crystals
Authors: Hernandez John P. , Choi Sang-il
Published: February 2004

The optical absorption of linear molecular crystals is investigated theoretically. A generalization of the theory of excitons in linear molecular crystals was carried out by Merrifield who constructed the excited states of the crystal by mixing monomolecular states (Frenkel excitons) and bimolecular ionized states in which an electron is removed from one molecule in the crystal and placed on a second (charge-transfer states). If the monomolecular optical transition is allowed, we find that the optical absorption consists of a series of bound states and a continuum absorption, which gives rise to photoconductivity; the strength of all the absorption is predominantly borrowed from the Frenkel exciton. The experimental possibilities may be classified into three cases: (1) the Frenkel exciton, the charge-transfer state with electron and hole at nearest neighbors, and the state with the electron and hole at large separation are well separated in energy. This case gives rise to a strong absorption line, small charge-transfer sidebands, and very small absorption into the continuum. (2) Near resonance between Frenkel and nearest-neighbor charge-transfer states. The strong absorption is now shared by two absorption lines with weak sidebands and very small continuum absorption. (3) Resonance between the Frenkel state and the states with the electron and hole at large separation. This case corresponds to autoionization in a solid, the bound states have very weak absorption, and a strong sharp line will be observed within the continuum absorption. The width of the resonance line is directly proportional to the background absorption.
Dependence of Friction at Wet Contacts upon Interfacial Potential
Authors: Bockris J. O'M. , Argade S. D.
Published: February 2004

An analysis is made of the forces contributing to the coefficient of friction between two solids, at least one of which is an electronic conductor, through a film of liquid electrolytes. It is shown that the friction coefficient should vary parabolically with potential. The derived expression, without calibrated constants, fits experimental measurements of the dependence of friction coefficients as a function of applied interfacial potential.
Magnetic Susceptibility Anisotropy, Molecular Quadrupole Moment, and the Sign of the Electric Dipole Moment in OCS
Authors: Flygare W. H. , Hüttner W. , Shoemaker R. L. , Foster P. D.
Published: February 2004

The molecular Zeeman effects of 16O12C32S and 16O12C34S have been observed under high resolution in magnetic fields up to 30 000 G. Conventional microwave spectroscopy is combined with a 6-ft long magnet to yield accurate first- and second-order magnetic field parameters. The magnetic susceptibility anisotropy of the OCS molecule is χ⊥–χ∥ = (9.27 ± 0.10) × 10−6erg/G2·mole. The molecular g values for the two isotopes obtained from the J = 0 → J =  transitions are g⊥(16O12C32S) = − 0.028711 ± 0.00004 and g⊥(16O12C34S) = − 0.028127 ± 0.00004. These data yield the sign of the molecular electric dipole moment which has −OCS+ polarity. Combining χ⊥–χ∥ and g⊥(16O12C32S) gives the molecular quadrupole moment of Q∥(16O12C32S) = − (0.88 ± 0.15) × 10−26esu·cm2.
Quantum-Mechanical Formulas for the Inelastic Scattering of Fast Electrons and their Compton Line Shape. Nonrelativistic Approximation
Authors: Tavard C. , Bonham R. A.
Published: February 2004

A critical discussion is presented on the various ways to calculate the total inelastic electron scattering in the Born approximation. It is shown that the Morse approximation, the simplest and most frequently used, fails at small and large scattering angles. An improvement can be obtained by use of the “quasi-free-scattering” approximation first proposed by Bethe. An attempt is made to take into account the binding of the target electrons through their momentum distributions and in this way to correct Bethe's approximation for Doppler effects. This quantum-mechanical treatment shows the contribution of exchange terms to the Compton profile of the scattered electrons.
Pairwise Trapping of Radicals in Irradiated High Polymers as Studied by Electron Spin Resonance
Authors: Iwasaki Machio , Ichikawa Takahisa , Ohmori Toshio
Published: February 2004

We have observed the ΔMs = 2 spectra due to radical pairs in a number of typical polymers irradiated at 77°K. The formation of radical pairs is not a particular phenomenon in some particular crystalline solids. The crystallinity of the polymers does not affect the formation of radical pairs in an apreciable amount. The buildup curve of the radical pairs suggests that the pairwise trapping is not due to the occasional overlapping of the spurs and that there is an intrinsic value of formation. The hyperfine separation of the ΔMs = 2 spectra of polyethylene and polypropylene have the values which are half of those of the isolated radicals. This means that the paired radicals are probably two alkyl radicals trapped in pair. The urea addition compounds of some polymers did not give any signal due to the ΔMs = 2 spectra.
Field Ionization of Water
Authors: Anway Allen R.
Published: February 2004

The field ionization of water is observed utilizing a mass spectrometer. The existence of H4O+ is confirmed. The energy distributions of the ions are investigated. Under certain conditions the energy distributions form two peaks. The appearance potentials of the polymers H3O+·nH2O, n = 0, 1, 2, 3, are determined. A model of the condensed layer is proposed which agrees with the observed phenomena.
Heats of Formation of H3O+, H3S+, and NH4+ by Electron Impact
Authors: Haney Max A. , Franklin J. L.
Published: February 2004

Electron impact fragmentations of some simple organic molecules have been investigated, making use of excess energy measurements. The results give values for the heats of formation to be 143, 191, and 148 kcal/mole for H3O+, H3S+, and NH4+, respectively. These values are consistent with limitations imposed by ion–molecule reactions and with crystal-lattice calculations.
Ground-State Splitting of Trivalent Gd and Cm in ZrSiO4, HfSiO4, and ThSiO4, Determined by ESR
Authors: Abraham M. M. , Clark G. W. , Finch C. B. , Reynolds R. W. , Zeldes H.
Published: February 2004

The electron spin resonance spectrum of trivalent gadolinium impurity in the tetragonal host crystals of ZrSiO4, ThSiO4, and HfSiO4 has been observed at X band, and the spin-Hamiltonian parameters were obtained at various temperatures. The dominant second-order parameter B20 for Gd was observed to be negative in all three crystals. Hyperfine structure for the odd isotopes 155Gd and 157Gd was clearly resolved and measured. In ThSiO4 crystals doped with trivalent curium, no ESR signals were detected, supporting the assumption of a large negative B20 term for this ion in these hosts.
Fluorescence Quenching and Isotope Effect of Tryptophan
Authors: Eisinger J. , Navon G.
Published: February 2004

The fluorescence spectra and quantum yields of trytophan (TRP) and several tryptophan derivatives in water and a polar glass have been measured over wide temperature ranges. For TRP, the wavelength of maximum emission shifts from 310 nm at 80°K to 355 nm at room temperature with almost all of the red shift occurring between 170° and 230°K, which is the temperature range where the glass softens. The successively more red-shifted spectra have no isoemissive wavelength, which supports the view that reorientation of several solvent molecules in the solvent shell of the excited TRP molecule and not a 1:1 exciplex is responsible for the red shift. The quantum yield remains constant until a temperature is reached at which solvent reorientation is virtually complete. Above that temperature, the quenching of the 355-nm emission can be fitted with a nonradiative de-excitation having an activation energy of 7 kcal/mole. A model for these spectral changes and quenching mechanism will be offered. In deuterated solvents, a large isotope effect of fluorescence yield of TRP has been reported. This effect is clearly not caused by proton transfer in the excited state since it is found to be virtually the same for TRP and 1-Me-TRP. At temperatures below those at which solvent reorientation occurs, the isotope effect vanishes, and above them it approaches an asymptotic value, the quenching activation energy being independent of the isotopic constitution of the solvent. The fluorescence of most proteins and hormones originates in their tryptophan residues. The quantum yields, isotope effects, and emission spectra of such polypeptides are compared with the corresponding parameters for solvated TRP.
Crystallography of Some High-Pressure Forms of C6H6, CS2, Br2, CCl4, and KNO3
Authors: Weir C. E. , Piermarini G. J. , Block S.
Published: February 2004

From single crystal x-ray diffraction at high pressure and room temperature unit-cell and space-group data were obtained for the following materials: C6H6, I—orthorhombic, a = 7.17, b = 9.28, c = 6.65, Pbca, CS2—orthorhombic, a = 6.16, b = 5.38, c = 8.53, Cmca, Br2—orthorhombic, a = 8.54, b = 6.75, c = 8.63, Cmca, CCl4, I—rhombohedral, a = 14.27, α = 90°, CCl4, II—monoclinic, a = 22.10, b = 11.05, c = 25.0, β = 114°, Cc or C2 / c, CCl4, III—orthorhombic, a = 11.16, b = 14.32, c = 5.74, C2221, KNO3, III—rhombohedral, a = 4.31, α = 78°54′, KNO3, IV(?)—orthorhombic, a = 5.58, b = 7.52, c = 6.58, P2lnb or Pmnb. All unit-cell dimensions are given in angstroms with estimated uncertainties of ± 2 in the last decimal place given and uncertainties of ± 0.5 deg in angles.
Computed Probability Density Functions of the Radii of Gyration of Long Random-Flight Chains
Authors: Hoffman Robert F. , Forsman W. C.
Published: February 2004

Probability density functions for the orthogonal components of the radius of gyration, Si, of a random-flight chain were computed using an indirect numerical integration algorithm which was based upon a multiple convolution proposed by Forsman and Hughes. Computations were performed for relatively long chains of 100–1000 statistical segments t. By using a reduced radius of gyration ξi = (π / 23) Si〈Si2〉−1 / 2 the distributions converged to a limiting function at about t = 500, except for extremely small radii, ξi ≤ [10(2t)1/2]−1. Additional computations using the one-dimensional distributions yielded the corresponding probability densities for both the two- and three-dimensional (polar) radii. The computed functions were in excellent agreement with all their limiting properties previously determined by analytical methods.
Rydberg Progressions in cis- and trans-Butene
Authors: McDiarmid Ruth
Published: February 2004

The far- and vacuum-ultraviolet absorption spectra of cis- and trans-butene and trans-butene-d8 have been recorded at low resolution. The more prominent bands are shown to be either the members of Rydberg series, ̄νn = 73 550 − R / (n − 1.03)2 (cis-butene), ̄νn = 73 650 − R / (n − 0.88)2 and ̄νn = 73 750 − R / (n − 0.24)2 (trans-butene), ̄νn = 73 900 − R / (n − 0.86)2 and ̄νn = 74 000 − R / (n − 0.23)2 (trans-butene-d8), or their vibrational subbands. The band observed at 49 690 cm−1 in trans-butene-d8 is seen, at higher resolution, to be polarized along the Z molecular axis, in agreement with its 3sR(Au) ← N(Ag) assignment. Finally, from a partial vibrational analysis of the trans-butene spectrum, the C=C and C–C bonds of the excited states are deduced to be slightly longer than in the ground state.
Ring Puckering in Five-Membered Rings. I. General Theory
Authors: Harris David O. , Engerholm Gail G. , Tolman Chadwick A. , Luntz Alan C. , Keller Richard A. , Kim Hyunyong , Gwinn W. D.
Published: February 2004

As a general introduction to a subsequent series of papers on pseudorotation and five-membered rings, the two-ring puckering modes of a five-membered ring are discussed in general terms. Five cases are separated depending upon the nature of the potential function for each mode. Correlation diagrams connecting the various cases are given. Consistent with our basic policy, we have gone directly from fundamental theory to an arithmetical development using large digital computers. The classical section of this development is outlined. The quantum-mechanical section is developed in the following paper.
Ring Puckering in Five-Membered Rings. II. The Microwave Spectrum, Dipole Moment, and Barrier to Pseudorotation in Tetrahydrofuran
Authors: Engerholm Gail G. , Luntz Alan C. , Gwinn W. D. , Harris David O.
Published: February 2004

The microwave spectrum of tetrahydrofuran has been studied. Nine complete rotational spectra have been observed. These arise from the ground and eight excited states. All of these states are less than 200 cm−1 from the ground state. The rotational constants and dipole moments exhibit a strong nonlinear dependence on the quantum number of the excited state. Vibration–rotation interaction is strong and the spectra of the first four states deviate from that of rigid rotor spectra. These deviations permit the determination of two energy separations: Δ01 = 0.67 cm−1 and Δ23 = 1.5 cm−1. All of the results are interpreted in terms of a model of restricted pseudorotation with a potential function of [30(1-cos2φ) / 2] + [40(1-cos4φ) / 2] cm−1, where φ is the angle of pseudorotation. The dipole moment varies from 1.52 to 1.76 D depending upon the pseudorotation state. The details of this variation indicate that the twisted configuration is at lower energy than the bent configuration.
Reinvestigation of the Rate Coefficients for O + H2 and O + CH4
Authors: Westenberg A. A. , deHaas N.
Published: February 2004

The improved version of the wide-temperature-range, fast-flow reactor with ESR detection employed at this laboratory for a series of atom–molecule rate measurements has been used to reinvestigate the reactions O+H2ΔOH+H [1] and O+CHΔOH+CH3 [3] which had been previously measured with the original apparatus. The new data are uniformly lower than before, implying a systematic error in the old results. The revised Arrhenius expressions are k1 = 3.2 × 1013exp(− 10 200 / RT) cm3mole−1·sec−1 valid in the range 500°–900°K, and k3 = 2.0 × 1013exp(− 9200 / RT) over 400°–900°K. The new results are in better agreement with other data in the literature. Our previous measurements on O+C2H6ΔOH+C2H5 and O+D2→OD+D have also been checked and do not require revision.
Molecular Dynamics Studies of the Microscopic Properties of Dense Fluids
Authors: Fehder Paul L.
Published: February 2004

Molecular dynamics calculations have been performed on a two-dimensional system of Lennard-Jones disks. Equilibrium thermodynamics data for 18 temperature–density states of the system are presented and compared with values obtained from a simple Lennard-Jones–Devonshire treatment. Three types of graphical displays of the data have been examined: “snapshots” of the system configuration at some instant of time, motion pictures of the system dynamics created from sequences of snapshots, and plots of the trajectories of particle centers over various time intervals. The snapshots reveal the presence of relatively large vacancies in the spatial distribution of particles in the system, and the trajectory plots and motion pictures show that these vacancies may persist in the same region for times in excess of 10−12. The trajectory plots also indicate that cooperative motion is very important in the self-diffusion process. The nature of the vacancies and the diffusive motion suggest a more than passive role of the attractive interparticle potential in determining the microstructure and microkinetics of the liquid state.
Elastic Moduli of a Fluid of Rigid Spheres
Authors: Miller Bruce N.
Published: February 2004

This article presents a calculation of the elastic moduli of a monatomic fluid of rigid spheres. Although calculations for monatomic systems with a continuous and differentiable interatomic potential indicate that the elastic moduli become infinite as the potential becomes infinitely repulsive, we find that the hard-sphere moduli are nonsingular, and well defined. This result leads one to suspect that a fluid with a highly repulsive, but continuous and differentiable, interatomic potential may not always be even qualitatively represented by a fluid consisting of perfectly rigid spheres. Our method of attack consists of finding the general form of the stress tensor for a hard-sphere system, asserting the assumptions of local equilibrium, and expanding the stress tensor in terms of the strains using the techniques developed by Green.
Proton, Deuterium, and Sodium-23 Magnetic Resonance of Ferroelectric Lecontite NaNH4SO4·2H2O
Authors: Genin D. J. , O'Reilly D. E.
Published: February 2004

Proton, deuterium, and sodium-23 nuclear magnetic resonances of deuterated and ordinary lecontite have been studied from 77° to 300°K and particularly in the vicinity of the ferroelectric phase transitions at 92° and 101°K. Proton–proton vector directions were determined for the two kinds of water molecules in the unit cell and are observed not to alter as the phase transitions are traversed. Above 230°K the deuterium magnetic resonance of one type of water molecule is observed in the 180° flip state of motion. Below 101°K axial and equatorial deuterons of ND4+ groups rotating about the threefold axis are observed with no detectable change at the transition temperatures. At 300°K one kind of 23Na site is observed; below 101°K, 23Na satellite line splitting occurs indicating that there are two kinds of 23Na site in the unit cell. This splitting is attributed to an abrupt lattice distortion which occurs as the crystal is cooled below 101°K. Proton spin–lattice relaxation time data show two minima, one at 170°K and one at 120°K, which are, respectively, assigned to rotation about the twofold and threefold axes of the NH4+ tetrahedron. Rotating frame relaxation times show minima at 80°, 110°, and 210°K, respectively, due to the threefold and twofold axis rotations of the NH4+ ion and a 180°, flip motion of one kind of H2O molecule. Near 101°K the activation energy for the threefold axis rotation of the NH4+ ion changes from 2.0 to 3.1 kcal mole−1. It is proposed on the basis of the magnetic resonance data that the transition at 101°K in lecontite is of the displacive type with a relatively large lattice distortion.
Direct Electron–Hole Recombination in Anthracene
Authors: Morris R. , Silver M.
Published: February 2004

Temperature studies of the recombination coefficient of holes and electrons in anthracene have been made. There was no significant change in the recombination coefficient in the temperature range room temperature to 130°C other than that predicted by the Thomson or the Langevin theories of recombination. Kinetic analysis shows that the recombination center (thought to be the charge-transfer state) must have a binding energy E1 − ECT and a lifetime β−1 such that E1 − ECT > 0.033 eV ln(10RmNc / β), where Rm is the measured recombination coefficient and Nc is the density of states in the lower conduction band.
Rotational Relaxation in Parahydrogen and Its Mixtures with Helium, Neon, and Argon at 300°K
Authors: Valley Leonard M. , Amme Robert C.
Published: February 2004

Ultrasonic-velocity dispersion measurements have been performed in parahydrogen and its mixtures with helium, neon, and argon, all at 300°K. In each case the experimental dispersion curves can be matched successfully to those calculated under the assumption that the 0–2 rotational transition relaxes separately from the 2–4 and higher-order terms. For pure pH2 we find a relaxation time τ20 of 1.30 × 10−8 sec for the 2 → 0 transition and a τ42 of 3.90 × 10−8 sec for the 4 → 2 transition. Comparison with the quantum-mechanical theories of Roberts and of Davison for H2–H2 collisions using Morse potentials shows good agreement for τ20 over the temperature range of 75°–300°K. The Morse-potential asymmetry parameter yielding the best fit is β = 0.113 for Roberts' calculation and 0.108 for Davison's. It is found that He–pH2 collisions are more effective than pH2–pH2 in producing the J = 0 to J = 2 transition, but less effective for higher-order transitions. Collisions of neon with pH2 are found to be more effective at room temperature for inducing the 0 → 2 and 2 → 4 transitions than either helium or argon.
Mass-Spectrometric Investigation of the Reaction of Hydrogen with Graphite at 1900°–2400°K
Authors: Steck Sara J. , Pressley George A. , Lin Sin-Shong , Stafford Fred E.
Published: February 2004

An integral furnace mass spectrometer was used to directly observe the products of the reaction of hydrogen with graphite between 1880° and 2430°K at pressures up to 0.5 torr and with 80-msec contact times. Ions observed to m / e 140 indicate formation of an extensive high-molecular-weight hydrocarbon system consisting mainly of unsaturated, acyclic, nonaromatic compounds. Many of these compounds were formed in steady-state concentrations much greater than expected for equilibrium. A reaction mechanism involving addition and insertion reactions is discussed.
Theoretical Investigation of the Lowest Double-Minimum State E, F 1Σg+ of the Hydrogen Molecule
Authors: Kolos W. , Wolniewicz L.
Published: February 2004

Accurate potential-energy curve for the first excited ∑1g+ state of the hydrogen molecule has been calculated for 1 ≤ R ≤ 12 a.u. The curve is known to have a double minimum, and for the outer minimum a significantly lower energy has been obtained than all previously reported results. The diagonal corrections for nuclear motion have also been computed and found to have a very sharp peak in the vicinity of the potential maximum. Vibrational and rotational levels have been calculated for all isotopes of the hydrogen molecule, and some results for H2, D2and T2 are shown and compared with experimental data. For the outer minimum, two, three, and four vibrational levels for H2, D2, and T2, respectively, are reported which lie below the lowest levels observed experimentally. For the observed levels good agreement with experimental data is obtained; however, the agreement is significantly worse when only the clamped nuclei potential without the diagonal corrections is used in the calculation of the vibrational energies. The wavefunction is analyzed in terms of some simple functions and appears to drastically change its character in the vicinity of the potential maximum.
Matrix-Isolation Study of the Vacuum-Ultraviolet Photolysis of Methyl Fluoride. The Infrared Spectra of the Free Radicals CF, HCF, and H2CF
Authors: Jacox Marilyn E. , Milligan Dolphus E.
Published: February 2004

Upon vacuum-ultraviolet photolysis of methyl fluoride in an argon or a nitrogen matrix at 14°K, the free radicals CF, HCF, and H2CF are stabilized in sufficient concentration for observation of their infrared absorption spectra. Studies utilizing carbon-13 and deuterium substitution confirm the identification of these species. Visible-ultraviolet absorption spectra of photolyzed Ar:CF3F samples include bands which may be assigned to CH, to CF, and to HCF. The reaction of carbon atoms, produced by the photolysis of cyanogen azide isolated in an argon matrix, with HF trapped in the matrix has also been found to lead to the stabilization of sufficient HCF for infrared spectroscopic detection. Reaction of HCF with a second molecule of HF to form CH2F2 also occurs. The force constants and thermodynamic properties of HCF have been calculated, and a partial vibrational assignment is proposed for H2CF. Although these experiments do not determine the primary photodecomposition processes important for methyl fluoride, there is evidence that both H atom and F atom detachment may occur under the conditions of these experiments.
Optical Properties of Tin Iodide Vapor
Authors: Zollweg R. J. , Frost L. S.
Published: February 2004

The optical absorption of SnI2, SnI4, and HgI2 were measured photometrically in the visible and near ultraviolet. The spectral and temperature dependence of radiation emitted by thermally excited SnI2 vapor was also measured and a tentative model for the electronic states of the SnI2 molecule deduced.
Possibility of Extracting Anisotropy Parameters from the M Dependence of Total Cross-Section Measurements
Authors: Miller William H.
Published: February 2004

The total cross section for the collision of an atom and a diatomic molecule (rigid rotor) initially in rotational state (Ja, Ma) is computed to first order in the anisotropic part of the potential. The dependence of the cross section on the quantum number Ma is examined, and it is seen that the anisotropy of the long-range attractive part of the potential can be determined from the Ma dependence of the average (average over glory oscillations) cross section, whereas the anisotropy of the short-range repulsive part of the potential can be determined from the Ma dependence of the glory oscillations.
Deuteron Quadrupole Coupling in Some Solid Chlorinated Hydrocarbons
Authors: Ragle J. L. , Sherk K. L.
Published: February 2004

We report deuteron quadrupole coupling constants in solids CDCl3, CD2Cl2, CD2ClCD2Cl, CD3CCl3, and CD3CCl2CD3. At 77°K these are 167.6 ± 0.8, 169.6 ± 1.1, 171.7 ± 0.8, 174.3 ± 1.6, and 174.7 ± 1.9 kHz, respectively, assuming an axial quadrupole interaction. We also report here a coupling constant of 149.3 ± 0.5 kHz for D in DCl at 77°K, in agreement with the data of Genin, O'Reilly, Peterson, and Tsang. Deuteron pure quadrupole spectra were observed by a pulse double-resonance technique suggested by Hahn and co-workers, in which the halogen serves as indicator nucleus. Interpretation of the results in terms of the electronic structure is briefly discussed.
Isotope Effect on the Hydrogen-Bond Length
Authors: Singh T. R. , Wood J. L.
Published: February 2004

Using the Stepanov approximation, computer calculations have been made of the effect of deuterium substitution on the length of the hydrogen bond. Expansion on replacement with deuterium is a feature of both symmetric and asymmetric double-minimum potentials, contraction of single-minimum potentials. The origin of the effect and its relation to observations is discussed.
Low-Energy Electron-Diffraction Studies of the Interaction of Oxygen with a Molybdenum (100) Surface
Authors: Kan H. K. A. , Feuerstein S.
Published: February 2004

Oxygen adsorption on the (100) surface of molybdenum single crystals was examined by low-energy electron-diffraction (LEED) in a display-type apparatus. Continuous diffraction-pattern observations from an initially clean surface were made at reaction temperatures between room temperature and 850°C. Below 650°C, no new ordered structures were observed. At 750°C, four ordered structures were formed on the surface with increasing oxygen coverage. The thermal characteristics and kinetics of formation of these oxides are discussed. In addition, models of surface structure with less than a monolayer oxygen coverage are presented.
Multiple Solutions to the Hartree–Fock Problem. II. Molecular Wavefunctions in the Limit of Infinite Internuclear Separation
Authors: King Harry F. , Stanton Richard E.
Published: February 2004

The restricted Hartree–Fock equation for a closed-shell molecule is investigated in the limit of infinite internuclear separation. The conventional molecular equation reduces, in the limit, to a set of modified Hartree–Fock equations for the individual fragments. The modification consists of admitting fractional occupation numbers for occupied orbitals. The individual fragment equations are coupled by conditions on the orbital energies and on the fractional “charges.” The self-consistent field equation, being nonlinear, admits a variety of mathematically acceptable solutions. For lithium hydride a wavefunction corresponding to ionic fragments Li+ and H− is one such limiting solution. Contrary to popular notion, it is not the solution of lowest energy even within the restricted SCF formalism. Limiting solutions are obtained for H2, LiH, and CH4. The separated hydrogen molecule is shown to have an SCF energy which is exactly one-quarter that of the helium atom. The long-range interaction of two hydrogen atoms is described by an inverse first-power potential according to Hartree–Fock theory. The Z dependence of correlation energy at infinite separation is analyzed in terms of a charge-transfer process. It is argued that in passing from the equilibrium separation to infinity the ground-state SCF wavefunction for a molecule such as lithium hydride becomes at first more and then less ionic.
Ultrasonic Measurement of the Elastic Constants of Anthracene
Authors: Huntington H. B. , Gangoli S. G. , Mills J. L.
Published: February 2004

Measurements have been made of the ultrasonic velocities for 18 modes of propagation in single-crystal anthracene. The accuracy of the measurements was limited by high attenuation, small specimen size, and inhomogenities in the material. Approximate values are deduced for all 13 elastic constants. The results are compared with those obtained by other investigators for naphthalene. A revised treatment of the data reduction for the latter is presented.
Quantum Corrections to the Thermal Conductivity and Shear Viscosity of a Moderately Dense Gas
Authors: Herman P. T. , McLennan J. A.
Published: February 2004

Using the correlation function approach to transport phenomena, a general formalism is developed for the calculation of the first-order collisional transfer contributions to the shear viscosity and thermal conductivity of a nondegenerate moderately dense monatomic gas. Applying this formalism to the hardsphere model, quantum corrections to the classical Enskog collisional transfer corrections are obtained and discussed.
Exciton–Photon Coupling in Crystalline Anthracene
Authors: Philpott M. R.
Published: February 2004

The form of the exciton–photon dispersion curve in an infinite crystal of anthracene is predicted by calculating the positions of the k = 0 polariton and Coulombic exciton levels for each vibronic level of the 3800-Å free molecule transition. Intermolecular interactions are restricted to dipole–dipole interactions and the 2500-Å system treated as if it were a single level. An exact classical solution of the problem posed here is employed in the calculations thereby circumventing perturbation theory. A slightly improved calculation of the dipole Davydov splitting is also reported.
Photoionization Cross Sections for the Ground State Configuration of Atomic Nitrogen and Oxygen
Authors: Koppel J. U.
Published: February 2004

Photoionization cross sections for the ground state configuration of N and O were calculated for photoelectron energies up to 50 eV using the scaled Fermi–Thomas potential method of Stewart and Rotenberg. All outer- and inner-shell transitions contributing to the cross section in this energy range were included. Results are in good agreement with experiment and with results obtained by more elaborate but numerically much slower methods of calculation such as Hartree–Fock–Slater and related methods.
Interpretation of the 57Fe Isomer Shift by Means of Atomic Hartree–Fock Calculations on a Number of Ionic States
Authors: Blomquist Jan , Roos Björn , Sundbom Marianne
Published: February 2004

The Hartree–Fock method is used to calculate the values of | Ψ(0) |2 for a number of configurations of different Fe ions. An interpolation formula for | Ψ(0) |2 as a function of the occupation number of valence electron orbitals (3d, 4s, 4p) is given. Experimental Mössbauer isomer shifts are correlated to estimated configurations which are compared to available MO calculations for a few cases.