bluuebunny/crossref_metadata_2025 huggingface dataset:
Transport Properties of a Gas of Diatomic Molecules. VI. Classical Trajectory Calculations of the Rotational Relaxation Time of the Ar–N2 System
Authors: Russell J. D. , Bernstein R. B. , Curtiss C. F.
Published: February 2004

The rotational relaxation time of nitrogen in an excess of argon was computed by a fully classical, numerically exact procedure, starting from an estimated anisotropic potential for the Ar–N2 system. The method involves integration of the second moment of the rotational inelasticity probability density function P(Δ Erot) over the impact parameter of the collision and a Boltzmann distribution of translational and rotational energies. Confirmatory results were obtained from an integration of the first moment. Both moments of P(Δ Erot) were evaluated by the classical trajectory CT) method of a previous paper by Pattengill et al. The CT results, extrapolated to the limit of weak anisotropy, agree well with those of the classical generalized phase shift approximation discussed in the preceding paper (V). The present results for the relaxation time Pτ vary, over the temperature range 200–400°K, from 8 to 20× 10−10 atm· sec, corresponding to rotational relaxation Z numbers from 6 to 10.
Chemically Induced Dynamic Electron Polarization. I. A Simple Theory for H Atom
Authors: Hutchinson D. A. , Wong S. K. , Colpa J. P. , Wan J. K. S.
Published: February 2004

Anomalous ESR spectra have been observed for H and D atoms formed in aqueous solutions by γ irradiation. This paper proposes a model, based upon three assumptions, to describe this anomalous behavior. Some justification for the assumptions is provided. In the development of the model, fairly simple explicit expressions are given for the field dependent spin eigenvectors for an electron interacting with a nuclear spin I in an arbitrary magnetic field.
ESR Identification of Slater Configurations and of Exchange of the (AsO4)4− Radical in Irradiated Ferroelectric KH2AsO4 and KD2AsO4 and Antiferroelectric NH4H2AsO4 and ND4D2AsO4
Authors: Lamotte B. , Gaillard J. , Constantinescu O.
Published: February 2004

The ESR study of the g̃ spectroscopic tensor and the As hyperfine tensor T̃As of the (AsO4)4− radical in crystals of ferroelectric KDA and DKDA and antiferroelectric ADA and DADA shows that it is possible to identify selectively the Slater configurations of the radical in these crystals. In the paraelectric phase of these crystals we observe dynamic low frequency fluctuations of g̃ and T̃As which correspond to an exchange between the ``upper'' and ``lower'' Slater configurations of the radical. The temperature dependence of the correlation time of this process is close to Arrhenius laws: τc=τc0 exp(Ea/kT) with Ea=0.21 eV for KDA, 0.27 eV for DKDA, 0.33 eV for ADA, and 0.43 eV for DADA. For the ammonium ADA and DADA salts, the passage through the Curie point shows drastic changes of the spectra of this radical. The analysis of the lines shows that they correspond to several distinct new Slater configurations of the radical. Our general conclusion is that there exists a perturbed zone around the (AsO4)4− radical which exhibits several different features characteristic of local symmetry, local order, and, with regard to the paraelectric phase, local dynamics. The question of antiferroelectric order in ammonium salts is also discussed with reference to the Nagamiya model.
Reorientation and Angular Momentum Correlation Times in Fluorobenzene-d5 in the Liquid State
Authors: Assink R. A. , Jonas J.
Published: February 2004

The NMR spin-lattice relaxation times of deuterium and fluorine have been measured in fluorobenzene-d5 over the temperature range from 30 to 309°C and pressures from 1 to 2000 bar. Quadrupolar interactions represent the sole relaxation mechanism for deuterium. The spin-rotation interactions provide the dominant relaxation mechanism for fluorine but small corrections for intra- and interdipolar, and anisotropy of chemical shift contributions were carried out. Since the 2D quadrupole coupling constant is known and the 19F spin-rotation interaction tensor was determined by molecular beam measurements, the experimental 2D and 19F T1 data allowed us to calculate the angular position correlation time, τθ, and the angular momentum correlation time, τJ, respectively. The relationship of τθ and τJ over the range of temperatures and pressures provides useful information about the mechanism of molecular reorientation in liquid fluorobenzene. The experimental data indicate that at lower temperatures (≤ 100°C) the rotational diffusion theory describes well the reorientation and that the Hubbard's relation between τθ and τJ holds. At temperatures near to critical temperature the experimental results can be interpreted in terms of the extended J-diffusion model.
Force Constants and Dipole Moment Derivatives of Ammonia from Hartree-Fock Calculations
Authors: Pulay Péter , Meyer Wilfried
Published: February 2004

The complete force field of ammonia has been calculated from Hartree-Fock wavefunctions by the force method. Six Gaussian basis sets of increasing sizes were used. Contrary to previous calculations which were either inaccurate numerically or used very limited basis sets, our results reproduce the essential features of the experimental force field, even for the smallest basis set. In our best calculation we obtained the following results (experimental values in parentheses): symmetry force constants in mdyn/Å:  F11=7.376(7.075 ± 0.13),  F12=0.471(0.78 ± 0.2),  F22=0.579(0.543 ± 0.02),  F33=7.382(7.038 ± 0.13),  F34=−0.180(−0.174 ± 0.04),  F44=0.726(0.665 ± 0.01). Dipole moment derivatives in D/Å:μ1=−0.017(± 0.40), μ2=−1.819(± 1.52), μ3=+0.209(± 0.18), μ4=+0.439(± 0.34). From our results the probable value of F12 is estimated as 0.5 mdyn/Å. Dipole moment derivatives are more sensitive with respect to variations of the basis set, and only the sign of the two bending derivatives, μ2 and μ4 could be established with certainty.
Linear Spin-Wave Theory of Librational Motion in the α Phase of N2 and CO
Authors: Dunmore Paul V.
Published: February 2004

A linear spin-wave theory is used to calculate the frequencies of librational modes in the α phase of nitrogen and of carbon monoxide. Results are presented both for quadrupole-quadrupole interactions and for the potential of Kohin. From a comparison with the Raman scattering measurements of Anderson, Sun, and Donkersloot, it is concluded that the Kohin potential does not give a satisfactory representation of the anisotropic interactions in either crystal. The present spin-wave theory is compared with the recent excitonlike theories of Jacobi and Schnepp and of Raich. A numerical error in the classical harmonic calculations of Goodings and Henkelman is rectified.
Electron Impact Excitation Spectra in an Ion Cyclotron Resonance Mass Spectrometer
Authors: McAllister T.
Published: February 2004

Electron impact excitation spectra have been obtained from an ion cyclotron resonance mass spectrometer by monitoring the current of scattered electrons transmitted along the ICR cell. The CCl4 and SF6 scavenger techniques were also used to verify specific features of these spectra. The spectrum of N2 is strongly influenced by the depth of potential well employed and the SF6 technique gives an identical spectrum to that of the scattered current monitor at higher well depths, indicating that the narrow energy width of the SF6 resonance capture peak does not influence the spectrum obtained in the ICR. Spectra of C6H5NO2 and CH3NO2 showed peaks which could be attributed to the NO2 group. NO2− ions interfered with the C6H5NO2 spectrum but not the CH3NO2 spectrum.
Extension of Scaled Particle Theory for Rigid Disks
Authors: Cotter Martha A. , Stillinger Frank H.
Published: February 2004

Geometric information about rigid disk arrangements in the plane, not previously used, is assembled and incorporated into scaled particle theory. In particular the close packed density limit for the contact correlation function G(λ) has been obtained exactly. The revised theory provides substantially better agreement than its predecessor with molecular dynamics results for rigid disks in the fluid phase. Although high density G(λ) curves develop nonmonotonic behavior as expected of crystalline packings, the corresponding pressure isotherms manifest no freezing transition in the present version. Suggestions for rectifying this shortcoming are included.
Surface Crossing Model for Ion-Molecule Reactions
Authors: Gislason E. A.
Published: February 2004

The effect of surface crossings on ion-molecule reactions is investigated. A model is proposed to locate these crossings and determine their effect on reactive cross sections. The model successfully explains why some ion-molecule reactions have cross sections larger than the Langevin value. It also explains why the two spin-orbit states of Ar+ have different cross sections for reaction with H2.
Xenon Tetrafluoride: Heat Capacity and Thermodynamic Functions from 5 to 350°K. Reconciliation of the Entropies from Molecular and Thermal Data
Authors: Osborne Darrell W. , Schreiner Felix , Flotow Howard E. , Malm John G.
Published: February 2004

The heat capacity of a very pure sample of xenon tetrafluoride, XeF4, was determined by adiabatic calorimetry between 5 and 350°K. No irregular thermal behavior was observed, except in the region near 210°K, where slightly enhanced heat capacity values, ascribed to a small amount of impurity, were observed. The values of the thermodynamic quantities Cp°, S°, (H° − H °0)/T, and (G° − H °0)/T at 298.15°K are 118.39 ± 0.12, 167.00 ± 0.17, 77.24 ± 0.08, and −89.76± 0.09 J °K−1 · mole−1, respectively. The standard entropy of the gas at 298.15°K was calculated from the thermal data to be 323.2± 2.0 J °K−1· mole−1, in excellent agreement with the value 323.98 ± 0.4 J °K−1· mole−1 calculated from electron-diffraction data and the frequency assignment of Tsao, Cobb, and Claassen. The standard entropy of formation of solid XeF4 at 298.15°K was found to be −407.95 ± 0.17 J °K−1 · mole−1, and the standard Gibbs energy of formation at 298.15°K is −145.48 ± 0.88 kJ mole−1. The corresponding quantities for gaseous XeF4 at 298.15°K are −250.97 ± 0.40 J °K−1· mole−1 and −131.36 ± 0.91 kJ mole−1. The standard enthalpies of formation of the solid and of the gas at 0°K are −266.29 ± 0.88 kJ mole−1 and −201.39 ± 0.90 kJ mole−1, respectively. The equilibrium constant for the formation of gaseous XeF4 from the elements at various temperatures is tabulated and compared with the experimental measurements by Weinstock, Weaver, and Knop.
Classical Calculations of H2O Rotational Excitation in Energetic Atom-Molecule Collisions
Authors: Kolb C. E. , Baum H. R. , Tait K. S.
Published: February 2004

Classical rotational excitation probabilities for collisions of three atomic species with H2O have been calculated using a Monte Carlo procedure. The rigid rotor approximation for H2O is employed together with hard sphere scattering potentials permitting the excitation in a given collision to be calculated analytically. The results are applicable to high energy (≥ 1 eV) collisions. A procedure for constructing differential rotational excitation cross sections is presented.
Theory of Reactive Scattering. III. Exact Quantum-Mechanical Calculations for a Three-Dimensional Model for Three-Body Rearrangements
Authors: Baer Michael , Kouri Donald J.
Published: February 2004

The problem of three-body reactive scattering in three dimensions is considered using coupled-channel operator equations. The discussion is in terms of two arrangement channels but the procedures employed may be used to treat the general case. Coupled integral equations are obtained which may be solved by a number of methods. In particular, simultaneous algebraic equations occur which are analogous to those encountered in previous studies of collinear reactive scattering. A model for a three-dimensional reactive process is considered for which one atom is infinitely massive and the interaction potential surface is isotropic. Exact calculations of the cross-sections and reaction probabilities are reported for a range of collision energies and differing degrees of internal excitation. The angular distribution of products is completely isotropic. The effects of varying the two finite masses are discussed.
Circular Dichroism of Oriented Helical Polypeptides: the Alpha-Helix
Authors: Mandel Richard , Holzwarth G.
Published: February 2004

The oriented circular dichroism spectrum of an alpha-helical polypeptide in solution is reported between 190 and 250 nm. The linear dichroism, absorption, and unoriented circular dichroism are obtained for the same solvent-polymer system. The four curves are resolved into a single set of bands consistent with the conclusions of exciton theory. It is shown that (1) the rotatory bands of the helix predicted by Moffitt are allowed only for light propagating perpendicular to the helix axis, (2) the additional rotatory strength predicted theoretically by Moffitt, Fitts, and Kirkwood is detectable and is allowed only for light propagating parallel to the helix axis. However, the Moffitt-Fitts-Kirkwood band has only 14 the intensity predicted by strong-coupling exciton theory.
All-Valence Electron CNDO Calculations on Transition Metal Complexes
Authors: Clack D. W. , Hush N. S. , Yandle J. R.
Published: February 2004

The CNDO-II approximate SCF molecular orbital method is extended to include molecules containing atoms of the first transition series. Valence orbitals explicitly considered in the transition atoms are 3d, 4s, and 4p. Calculations are reported for the monoxides of Sc, Ti, V, and Cu, using three sets of orbital exponents. The results are compared, where possible, with those obtained by ab initio methods, and appropriate values for off-diagonal core parameters are proposed. The method is used to evaluate the orbital levels for a number of hexafluorometallate complex ions. Fair agreement is found between the calculated and experimentally observed metal-fluorine equilibrium bond lengths. The eigenfunctions are used to obtain the transferred spin densities to the fluorine ligands and in general these compare favourably with the values derived from electron spin and nuclear magnetic resonance measurements.
Chemisorption of Hydrogen on the (111) Plane of Nickel
Authors: Lapujoulade J. , Neil K. S.
Published: February 2004

The adsorption of hydrogen on the (111) plane of nickel at room temperature has been investigated using the technique of flash desorption spectroscopy. The residual CO partial pressure has been reduced below 1 × 10−11 torr to minimize the effect of H2 and CO coadsorption. The experimental apparatus and crystal cleaning procedure are described. Hydrogen was observed to desorb in a single pressure burst for all the experimental conditions examined. Four techniques have been used to analyze the experimental results and a coherent picture emerges for low surface populations. The adsorption energy, Arrhenius preexponential factor and sticking coefficient have been determined for surface hydrogen populations less than 1013 atoms/cm2, the values obtained being 22.7 kcal/mole, 2×10−1 cm2 atoms−1 · sec−1, and 0.1, respectively. Furthermore it is shown that the adsorption energy is constant at low coverages. For surface populations greater than 1013 atoms/cm2, we have been able to reproduce the desorption spectra with a model in which either the adsorption energy decreased with adsorbed population or the pre-exponential factor increased with population. It is not possible to distinguish between the two possibilities. Such a model does not however describe very well the observed adsorption kinetics. We have proposed that this discrepancy may be due to diffusion of hydrogen into the nickel when the crystal is heated.
Nonlinear Behavior of Solutions Illuminated by a Ruby Laser
Authors: Zunger A. , Bar-Eli K.
Published: February 2004

The methods of calculating the deviations from Beer-Lambert's law and other related phenomena in solutions illuminated by a ruby laser are analyzed and criticized carefully. These methods include the steady-state and optically thin approximation as compared with the exact solution of a set of coupled differential equations which describe the temporal and spatial changes of the photons and populations of the various energy levels. Critical evaluation of the validity of the approximation is given. The computations are compared with the experimental data of Beer's law deviations, pulse shaping, recovery of the ground state after excitation, and blue and red fluorescence of solutions of cryptocyanine. A set of absorption cross sections and decay rates which fits best all these experiments is deduced.
Vibration-Vibration Energy Exchange in N2 with O2 and HCl Collision Partners
Authors: Berend George C. , Thommarson Ronald L. , Benson Sidney W.
Published: February 2004

A rigorous, classical solution of two colliding diatomic molecules is presented. Single quantum vibration-vibration energy transfer probabilities are calculated for the case of N2*+O2 and N2*+HCl. Results are in excellent agreement with experimental findings. Previous findings on N2*+N2 and N2*+CO reactions are included and it is demonstrated that the energy discrepancy between the respective vibrational states which exchange their energies is not necessarily the measure of the efficiency of the process. Geometry of the collision and the matching of interacting atomic masses seem to be of equal importance.
Role of Small Clusters in Nucleation about Ions
Authors: Castleman A. W. , Tang I. N.
Published: February 2004

A consideration of the molecular aspects of ion induced nucleation shows that, in contrast to accepted theory, small clusters are a segment of the over-all nucleation size spectrum. No direct relationship exists between ion solvation energy and the free energy barrier for nucleation. Instead, it is established that the properties of the small clusters govern the free energy functions in the neighborhood of the peak cluster concentration, and that these in turn affect the over-all height of the free energy barrier for nucleation. The fact that ion-molecule reactions are taking place during the course of the usual ion induced nucleation experiment is clearly demonstrated. Assumptions that nucleation generally occurs directly about ions of the carrier gas are disproven.
Dynamics of Electrons in Nonpolar Liquids
Authors: Beck G. , Thomas J. K.
Published: February 2004

Electrons are produced in several organic liquids by the two-photon laser photolysis of dilute solutions of anthracene and pyrene in these liquids. The rate constants for reaction of the electron with biphenyl, carbon tetrachloride and oxygen in alkanes have been measured, and are found to be much larger than the values quoted in polar solvents. The rate constants for reaction of the electron with biphenyl increase with the square root of the electron mobility while the corresponding reaction with O2 is independent of the electron mobility. The mobility of the electron in hydrocarbon mixtures follows the exponential of the mole fraction of the hydrocarbon with the higher mobility. Complex behavior is observed in alcohol-hydrocarbon mixtures, which may be explained in terms of aggregates of alcohol in these mixtures. The data are discussed in terms of the present theories of the state of electrons in liquids.
Pulsed Vacuum-uv Photochemical Study of Reactions of OH with H2, D2, and CO Using a Resonance-Fluorescent Detection Method
Authors: Stuhl F. , Niki H.
Published: February 2004

Resonance fluorescence (A 2Σ+; v′=0→ X 2Π; v″=0) was used to monitor OH radicals in the pulsed vacuum-uv photolysis of H2O. From the exponential decays of OH radicals in the presence of a large excess of added reactants, absolute rate constants of the following reactions were determined at 298°K: OH+CO→ CO2+H;    k4=1.35 × 10−13cm3molecule−1· sec−1, OH+H2→ H2O+H;    k5=7.1 × 10−15cm3molecule−1· sec−1, OH+D2→ DHO+D;    k6=2.05 × 10−15cm3molecule−1· sec−1. The over-all uncertainty of these rate constants was estimated to be ±15%. The methods for production and detection of OH were discussed in detail.
Energy-Loss Cross Sections for 500 eV Electrons in the Continuum of O2 and CO
Authors: Tisone Gary C.
Published: February 2004

Energy-loss cross sections, differential in energy and angle, are presented for 500 eV electrons incident on O2 and CO in the angular range of 10°–30° for O2 and 6°–30° for CO. This region of the ionized continuum was studied with an energy resolution (Δ ε/ε) of 1.5% and an angular resolution of about 1.5°. The cross sections were placed on an absolute scale by normalizing to Bromberg's elastic scattering data. Integration of the differential energy-loss cross sections over angle yielded energy-loss cross sections, differential with respect to energy, for O2 of 0.078 a02/eV at 30 eV, 0.042 a02/eV at 50 eV, 0.013 a02/eV at 80 eV and 0.065 a02/eV at 100 eV. Corresponding cross sections for CO were 0.0077 a02/eV at 80 eV and 0.0043 a02/eV at 100 eV.
Microwave Spectrum and Barrier to Internal Rotation in CF3SiF3
Authors: Lide David R. , Johnson Donald R. , Sharp K. G. , Coyle T. D.
Published: February 2004

The microwave spectrum of CF3SiF3 has been analyzed. The ground-state constants are B0=1328.464± 0.001 MHz, and DJ=0.09± 0.05 kHz. An extensive series of torsional satellite lines has been studied in some detail. The barrier to internal rotation is found to be 489± 50 cm−1, corresponding to a torsional frequency of 37.0± 2.0 cm−1. The lowest degenerate rocking mode has been determined as 158± 12 cm−1. Other vibrational fundamentals are discussed.
Configuration Interaction Studies of NO and NO+ with Comparisons to Photoelectron Spectra
Authors: Thulstrup E. W. , Öhrn Y.
Published: February 2004

Full potential energy curves in the Born-Oppenheimer approximation are calculated for NO and NO+ with a configuration interaction method employing a minimal basis. Resulting energy differences and vibrational and rotational constants are compared to experimental results and assignments are suggested for transitions observed in the photoelectron spectrum of NO.
Pressure Dependence and Third Body Effects on the Rate Constants for H+O2, H+NO, and H+CO
Authors: Ahumada J. J. , Michael J. V. , Osborne D. T.
Published: February 2004

This paper presents measurements of the third order rate constants for the reactions H+O2+M→ HO2+M, H+NO+M→ HNO+M, H+CO+M→ HCO+M. The measurements were made by concentration analysis of hydrogen atoms by means of Lyman α photometry. All experiments were performed at room temperature. Particular emphasis was placed on the effect of changing the third body and, therefore, experiments were performed with M being H2, He, Ne, Ar, and Kr for all three reactions.
Generalized Mobility Theory of Electron Transfer Reactions in Solution
Authors: Schmidt P. P.
Published: February 2004

This paper reports a generalized theory of electron transfer reactions in solution; such reactions are typically termed oxidation-reduction reactions. The theory involves the use of the Kubo linear response analysis to examine the transport processes in an electrolyte system. In particular, we examine a system in which electron transfer between ions in the solution is possible. The substance of the theory is contained in an expression for the electron transfer rate constant. This rate constant is general, valid for all degrees of coupling between the electrons, ions, and surrounding dielectric medium. An expression of this type has not been reported previously. In deriving this general rate constant we find that there are new, hitherto unexpected processes which contribute to the electron transfer rate. Such processes are evident when, through the use of the Kubo theory, we examine the probability of electron transfer correlated with a simultaneous ionic diffusion step and a polarization displacement in the dielectric continuum. The degree to which these various transport processes contribute to the electron transfer rate is examined by means of a calculation on a model system. The system consists of two ionic species, one an electron donor and the other an acceptor, immersed in a dielectric continuum. The transfer electron is considered harmonically bound to the donor ion. The analysis is carried out without recourse to the Born-Oppenheimer separation approximation. We ignore the interaction between the transfer electron and the surrounding ions, but include a brief discussion of ways of handling this problem in the Appendix. This analysis of a model system points the way to future more detailed analyses of the problem.
Nitrogen-14 Nuclear Quadrupole Resonance in Compounds Containing N–N Bonds. III. Azines
Authors: Sauer Enrique G. , Bray P. J. , Marino R. A. , Oja Tõnis
Published: February 2004

Nitrogen-14 nuclear quadrupole resonance results have been obtained in benzophenone hydrazone, in azobenzenes, and in several azines. These data are analyzed by means of the semiempirical Townes and Dailey theory to yield the average sigma and the pi-electron charge densities as a function of the C–N–N angle.
Sound Velocity, Adiabatic Compressibility, and Specific Heat of Liquid and Dense Fluid Krypton
Authors: Streett W. B. , Ringermacher H. I. , Burch J. L.
Published: February 2004

The sound velocity, W, in liquid and dense fluid krypton has been measured with an estimated accuracy of 0.2%, using a double-transducer acoustic interferometer operating in the frequency range 1.05–1.44 MHz. Measurements have been made over the temperature range 145–240°K, and at pressures up to 680 atm. The results show good agreement with published data for pressures below 68 atm. Measurements at the critical temperature, 209.4°K, and at several nearby temperatures, provide information about the form of W–P–T and W–V–T surfaces near the critical point. Smoothed values of W have been used, together with published P–V–T data, to derive adiabatic compressibilities and constant-pressure and constant-volume specific heats for liquid krypton. The calculated specific heats, which have an estimated accuracy of 5%, show good agreement with the calorimetric measurements of Gladun and Menzel for the saturated liquid. However, our results suggest that Cv for liquid krypton is not independent of temperature in the region adjacent to the saturation line, as Gladun and Menzel have reported.
Excitation and Quenching of CO Fourth Positive Chemiluminescence Due to Reactions Involving C2O
Authors: Shackleford W. L. , Mastrup F. N. , Kreye W. C.
Published: February 2004

Vacuum ultraviolet chemiluminescence from photolyzed C3O2/O and C3O2/NO2 mixtures has been observed and identified as the CO fourth positive (A1Π → X1Σ) system. Photolysis of C3O2 is used as a method of producing C2O(X̃3Σ) molecules in concentrations which are viewed by adjusting the C3O2 flow rate or the photolytic flash energy. Microwave dissociation of O2 and photolysis of NO2 are used as alternative sources of O(3P) atoms, with equivalent results. The intensity of emission is found to be proportional to the product of the concentrations of O(3P) and photolytically produced C2O, indicating that the binary reaction C2O+O(3P)→ CO*(A1Π)+CO(X1Σ) produces the chemiluminescence. Absolute (300°K) rate constants for the reaction of C2O with O, and for quenching of the chemiluminescence by NO and O2, have been obtained by the method of kinetic emission spectroscopy. The data yield an over-all rate constant for the C2O+O→ 2 CO reaction of 9.5−3.6+5.7× 10−11 cm3 particle−1· sec−1, and quenching rate constants of 1.65± 0.35× 10−12 and 1.2± 0.2× 10−10 cm3 particle−1·sec−1 for O2 and NO, respectively. There is some question concerning whether the quenching effect of NO and O2 involves a chemical reaction with C2O or de-excitation of excited C2O*. On the basis of the over-all C2O rate constant obtained from this experiment and previously published estimates of relative rate of populating various CO* levels, the rate constant for the C2O+O(3P)→ CO*(A1Π)+CO(X1Σ) reaction is estimated to be between 10−11 and 10−12 cm3 particle−1·sec−1.
Self-Diffusion Coefficients and Rotational Correlation Times in Polar Liquids. V. Cyclohexane, Cyclohexanone, and Cyclohexanol
Authors: O'Reilly D. E. , Peterson E. M. , Hogenboom D. L.
Published: February 2004

Self-diffusion coefficients and proton and deuteron relaxation times have been measured in cyclohexane (C6H12), cyclohexanone (C6H10O), and cyclohexanol (C6H12O). The results are interpreted with the quasilattice random flight (QLRF) model and a dynamical rotational coherence (DRC) theory, a general formulation of which is given in the present paper. The experiments were carried out to ascertain the effect of adding (1) a large dipole moment and (2) a strong hydrogen bond to the cyclohexane molecule. Molecular reorientation or self-diffusion are not greatly affected by the addition of dipole moment but become more than an order of magnitude slower in cyclohexanol. The results of DRC and QLRF models are compared with experiment for a wide variety of liquids.
Three Atom Correlations in Liquid Neon
Authors: Raveché Harold J. , Mountain Raymond D.
Published: February 2004

We consider the correlation of three atoms in liquid neon from the neutron diffraction measurement of the isothermal density derivative of the pair correlation function. Several closure approximations for the triplet correlation function are discussed. Three of the approximations are representations of the triplet correlation function as a functional of the pair correlation function, and the other is expressed as a simple function of the pair correlation function.
Self-consistent charge and configuration molecular orbital calculations on PF3
Authors: Wensky D. A.
Published: February 2004

We present the results of a self-consistent charge and configuration molecular orbital (SCCC-MO) calculation undertaken for the PF3 molecule using a basis set of 3s, 3p, and 3d phosphorus and the fluorine 2s and 2p atomic orbitals. The positions of the predicted occupied MO energies agree well with the observed vertical ionization potentials for PF3. The predicted charge distribution and dipole moment are also in fair agreement with experimental observations. It is shown that the predicted orbital energy scheme can lead to a plausible explanation of the observed absorption spectrum. In addition the predicted virtual orbitals agree well with other chemical and spectroscopic evidence available for PF3.
Bethe surface, elastic and inelastic differential cross sections, Compton profile, and binding effects for H2 obtained by electron scattering with 25 keV incident electrons
Authors: Ulsh R. C. , Wellenstein H. F. , Bonham R. A.
Published: February 2004

Electron impact spectra for H2 have been obtained at scattering angles of 1°, 1.5°, 2°, 3°, 4°, 5°, 7°, and 10° using 25 keV incident electrons. The measured intensities were converted to generalized oscillator strengths and placed on an absolute scale at each scattering angle by the use of the Bethe sum rule for the generalized oscillator strength distribution. The cross section differential with respect to both solid angle and energy loss of the scattered electron was corrected for relativistic and exchange effects and integrated over energy loss to obtain the inelastic differential cross sections. In addition the elastic cross section differential with respect to solid angle was measured. The results are all in excellent agreement with theoretical calculations. The total elastic cross section was determined using additional data from another source. The Compton profile was determined from the 7° scattering data and was found to agree well with the previous x-ray results. Consistent generalized optical sums (2,1, − 1, − 2) and optical sum inequalities for the generalized oscillator strength were also obtained. In addition inelastic scattering factors were computed from the (−1) optical sum and were found to agree well with available theory. Chemical binding effects were explored using electron density difference functions obtained from the differential cross sections. For the first time the intensity difference functions for the elastic and inelastic scattering were determined separately. It is argued that the experimental method used in this study to obtain the intensity difference functions represents the best approach so far developed for obtaining direct information about molecular real space charge densities.
Statistical mechanics of random-flight chains. V Excluded volume expansion and second virial coefficient for linear chains of varying shape
Authors: Gobush William , Šolc Karel , Stockmayer W. H.
Published: February 2004

The excluded-volume expansion factor of a flexible chain is calculated with a smooth-density model providing for a distribution of both sizes and shapes of the segment cloud. From the known average moments of the inertial tensor found previously by Monte Carlo methods, an approximate empirical function is constructed to describe the distribution of ellipsoidal dimensions. With a suitably related pseudopotential representing the excluded-volume forces, an expansion factor αs2 for the mean square radius is obtained which requires very little scale adjustment to match first-order perturbation theory and which also gives a good representation at higher values of the interaction parameter. The empirical function approach is supplemented with a Monte Carlo study to remove bias inherent in the choice of the functional form. Because of the small increase in asymmetry with increasing excluded volume, the direct effect of shape variation on the expansion factor is less important than that of size variation. A calculation of the interaction of two chains and the resulting second virial coefficient is carried out, and in this case both shape and size variations are important.
Hydrogen-deuterium isotope effect in the decay of radical pairs in x-ray irradiated hydroxyurea
Authors: Haven Ysbrand , Williams Rodney C. , Hamrick Phillip J. , Shields Howard
Published: February 2004

The rate constants for the transformation of radical pairs to single radicals in x-ray irradiated hydroxyurea have been determined from the decay of ESR spectra at temperatures from 253–268°K. The ratio of decay rates for a protonated to a deuterated sample changes from approximately 13 in the initial stage to 5 in the later stage. The decay curves are explained by assuming that the decay is associated with a diffusion of H (D) vacancies from one radical of a pair to a neighboring molecule, dissociating the radical pair. The decay rates in the initial stage of decay for a protonated sample, k11(H), and for a deuterated sample, k11(D), may be presented as k11(H) = 1 × 107 exp[(−0.50 eV)/k T], k11(D) = 6 × 1011 exp[(−0.82 eV)/k T].
Crystal structure refinements with generalized scattering factors. II. Melamine, 2, 4, 6-triamino-s-triazine
Authors: Larson Allen C. , Cromer Don T.
Published: February 2004

Melamine, C3N6H6, or 2,4,6-triamino-s-triazine, is monoclinic, space group P21/a, with a = 10.537(2), b = 7.477(1), c = 7.275(1) Å, β = 112°9′(1), and Z = 4. Data were collected with an automatic diffractometer using crystal monochromated MoKα radiation. In addition to the usual spherical free atom model with anisotropic thermal parameters, least-squares refinements were made with one-center and two-center generalized scattering factors. The aspherical distortions found are chemically reasonable and in most cases have the symmetry of the molecule although the symmetry is not crystallographically required. Addition of the generalized scattering factor parameters produced a significant improvement between observed and calculated intensities but made no significant change in the molecular geometry.
Vibrational spectra of β-cyclotetramethylene tetranitramine and some of its isotopic isomers
Authors: Iqbal Zafar , Bulusu Suryanarayana , Autera Joseph R.
Published: February 2004

The infrared and Raman spectra of the β-polymorph of cyclotetramethylene tetranitramine (commonly known as ``HMX'' in the explosives field) and its isotopic isomers, HMX-15NO2, HMX-15N(NO2), HMX-13C(50%), and HMX-d8, have been recorded in the spectral region between 30 and 3500 cm−1. Single crystal polarized Raman measurements on β-HMX were also made in order to separate out modes of Ag symmetry from those of Bg symmetry. Sizable correlation field induced splitting of some of the (NO2) modes was observed. Probable assignments have been proposed for the complex internal vibrations of β-HMX based mainly on the observed isotopic shifts. The spectra of HMX and HMX-d8 in solutions of dimethyl sulfoxide and acetonitrile have also been recorded. The results indicate that the molecular conformation in solution is centrosymmetric and probably similar to that in the β-HMX crystal.
Classical trajectory study of bond energy effects
Authors: Chapman Sally , Suplinskas Raymond J.
Published: February 2004

Three body classical trajectory calculations are used to investigate bond energy effects in the hot atom reactions of tritium atom with a series of hydrocarbons. The surface for T + CH3–H is a modification of that used by Polanyi and co-workers. The hydrocarbon series is represented by changing the bond dissociation energy, bond length, and mass only. Other potential parameters are fixed. It is found that the experimental ratios of abstraction yields are reproduced quite well theoretically at the single tritium atom energy of 2.8 eV. Of the parameters varied, only the bond dissociation energy had a significant effect on the abstraction ratios. These results lead us to support the high energy model for these effects.
Radiationless transitions in solutions: Isotope and proximity effects on Dy3+ by C–H and C–N bonds
Authors: Haas Yehuda , Stein Gabriel , Würzberg Elhanan
Published: February 2004

The lifetime and quantum yield of fluorescence from the 4F9/2 level of Dy3+ have been measured in perprotonated and perdeuterated dimethylsulfoxide and methylcyanide under various excitation conditions. The results are compared to those obtained in light and heavy water and with other rare earth ions and discussed with reference to the modified energy gap law in the theory of radiationless transitions. The specific energy gap in Dy3+ is such that efficiency of quenching depends on matching the electronic energy gap of the ion by a single vibration of C–H, C–D, or C–N and on the distance of the bond from the ion.
Vibrational analysis and molecular structure of the hydrogen polyoxides, H2O3, H2O4, D2O3, and D2O4
Authors: Arnau José L. , Giguère Paul A.
Published: February 2004

The fundamental skeletal vibrations of the H2O3 and H2O4 molecules and their deuterated analogs have been identified in the infrared or Raman spectra of the trapped products from reactions in electrically dissociated water vapor and related systems. All the observed frequencies have been assigned on the basis of assumed molecular structures of C2 symmetry, consisting of skew chains of single-bonded oxygen atoms with an OH group at each end. The assignments are consistent with the temperature behavior, 18O isotope shifts, and depolarization ratios of the various bands. Some unusual isotope effects were found in the deuterated species. A preliminary normal coordinate analysis of the skeletal vibrations of H2O3 and H2O4, including their 18O isotopic species, shows satisfactory agreement of the calculated and observed frequencies, thereby supporting the assumed molecular model. It also confirms that the bonding in these very unstable molecules is similar to that in H2O2.
Surface self-diffusion on an fcc crystal: An atomic view
Authors: Ayrault Guy , Ehrlich Gert
Published: February 2004

Migration of individual atoms self-adsorbed on different low-index planes of a face-centered cubic metal has been studied for the first time. Diffusion coefficients and activation energies for the motion of rhodium on perfect planes of the rhodium lattice, in the absence of high fields, have been derived from direct observation of atomic positions, using a field ion microscope. Correlation effects arising from interactions with other rhodium atoms are observed; these may be important even at interatomic distances greater than 7.5 Å. All diffusion parameters have therefore been derived from experiments with only a single atom on a plane. On the close-packed (111), motion is apparent even at cryogenic temperatures, with an activation energy Vm of only 3.6 kcal/mole. On the (311), (110), and (331) planes, atom movement is strictly one dimensional along close-packed [110] rows, with Vm at 12.4, 13.9, and 14.8 kcal/mole, respectively. The highest barrier, 20.2 kcal/mole, is found on the (100). On all surfaces the pre-exponential factor Do is normal. These results are at variance with previous measurements on similarly structured planes of tungsten; however, further observations of single atoms are needed to establish the exact role of surface structure.
Semiclassical calculation of the harmonic oscillator transition probability for a collinear hard sphere collision
Authors: Eastes Walter , Doll Jimmie D.
Published: February 2004

Closed form expressions are obtained for the vibrational transition probabilities in a semiclassical formulation for the inelastic hard sphere collision of an atom with a harmonic oscillator. The uniform semiclassical expression is found to be in excellent agreement with the numerically computed quantum mechanical transition probabilities. Expressions are also obtained for the average energy transfer in this collision and for the scattering of atoms from a hard cube model surface.
Argon matrix infrared spectra and vibrational analysis of the hydroperoxyl and deuteroperoxyl free radicals
Authors: Smith David W. , Andrews Lester
Published: February 2004

Matrix reactions of H and D atoms from a microwave discharge source with oxygen molecules diluted in argon have produced significantly higher yields of HO2 and DO2 than the previous photolysis methods. Oxygen isotopic enrichment confirmed the previous vibrational assignments to HO2; significantly greater concentrations of DO2 allowed observation of the weak 0–0 mode at 1123 cm−1. Normal coordinate calculations using HO2 and DO2 oxygen isotopic frequencies gave excellent agreement with the observed data. The bonding in HO2 is suggested to be largely covalent, in contrast to the ionic species Li+O2−.
Ab initio study of the potential energy surface of the B̃ 1A1 excited state of H2O
Authors: Flouquet F. , Horsley J. A.
Published: February 2004

A large part of the potential energy surface of the B̃ 1A1 (1A′) second excited state of the H2O molecule for dissociation into OH+H has been calculated using the SCF orbitals of the corresponding triplet state as the basis for a limited CI calculation. The form of the calculated surface is dominated by a conical intersection with the ground state surface in the linear configuration. Cross sections of the surface also show either a small potential maximum or a point of inflexion. These features appear to be related to the rapid transformation of the highest occupied orbital from a Rydberg orbital to an antibonding orbital. A cross section of the surface leading to O+H2 has also been calculated. This shows a very high (5 eV) potential barrier owing to an avoided crossing. The dissociation of H2O in the B̃ 1A1 state to give the OH (2Σ+) fragment with a highly abnormal rotational distribution is discussed in light of the calculated surface.
Multiconfiguration self-consistent-field theory for localized orbitals. II. Overlap constraints, Lagrangian multipliers, and the screened interaction field
Authors: Gilbert T. L.
Published: February 2004

The general, standard, and localized forms of the multiconfiguration self-consistent-field orbital equations, with and without overlap constraints, are derived and compared. The equations are very similar in form to the corresponding single configuration (Hartree-Fock) equations. The constrained equations may be formulated in terms of a single Hermitian orbital operator with a Lagrangian multiplier matrix which is always Hermitian. The equations for orbitals localized on an embedded fragment (an atomic shell, bond, atom, molecular fragment, or other subsystem that is part of a larger system) differ from the orbital equations for an isolated fragment only by the addition of a screened interaction field. Dropping the orbital overlap constraints introduces an additional condition on the Lagrangian multipliers that must be satisfied in order for the energy to be stationary with respect to variations in the overlap matrix.
Ultraviolet and x-ray induced recombination luminescence of L-tryptophan in low temperature polar matrices
Authors: Moan Johan
Published: February 2004

The long-lived isothermal luminescence and thermoluminescence of L-tryptophan in 9M LiCl and in ethylene glycol/water (EG/H2O) glass irradiated with uv light and x rays have been investigated. The spectra of the luminescence as well as the effect of an electron scavenger indicate that it arises from electron-cation recombination. In EG/H2O the decay kinetics of the long-lived isothermal phosphorescence and the shape of the thermoluminescence glow curve as well as the isothermal luminescence/thermoluminescence ratio are identical for the two types of irradiation. Hence it is concluded that the distribution of electron-cation distances is the same in the two cases for the ion pairs that give rise to recombination luminescence. In LiCl/H2O glass the decay kinetics of the long-lived luminescence is the same after x-ray and uv irradiation, but the glow curves are different. This difference may be explained by the following observations: (a) x rays induce radicals and long-lived luminescence in the pure solvent; (b) in an x-irradiated LiCl/H2O glass the absorption spectra indicate that tryptophan molecules react with radicals of the solvent at about 140 K to produce tryptophan cations. Luminescence may be emitted in the reaction of these tryptophan cations with trapped electrons or anions. In EG/H2O the uv-induced recombination luminescence is pure phosphorescence of tryptophan while in LiCl/H2O fluorescence is also seen at temperatures below 130 K. This is interpreted in terms of the trapping energy of the recombining ion pairs. The fraction of the recombinations that give rise to molecules in the triplet state has been estimated from the luminescence spectra.
On the asymmetry parameter and the principal directions of the electric field gradient tensor in NQR spectroscopy
Authors: Vega Shimon
Published: February 2004

The asymmetry parameter η of the electric field gradient tensor having reflection symmetry is discussed for spn hybridized atoms. It is shown that in this case η is a function of a single variable, which depends on the degree of hybridization and the occupation numbers of the hybrids. The model provides a physical picture for the various possible values of η as well as closed expressions for the occupation numbers. Two specific cases, sp3 and sp2, are discussed in detail.
Light scattering of spheroids. I. Extension of the Rayleigh-Gans-Stevenson functions and their application for molecular weight and shape determinations at θ = 90°
Authors: Heller W. , Nakagaki M.
Published: February 2004

The Stevenson extension of the Rayleigh-Gans theory on the light scattering by spheroids at an angle of observation θ of 90° with respect to the incident beam was supplemented by the inclusion of θ as a further variable. The somewhat lengthy equations were evaluated for various values of the parameters α, p, θ, and m. Because of the inclusion in the Stevenson theory of the quadrupole contribution, the new data should allow one to determine the molecular weight M up to about 100 million (or the corresponding particle weight) and the axial ratio p of spheroidal bodies assuming the use of visible radiation. Detailed theoretical results are given for θ=90°, and a relatively simple practical procedure is outlined for the simultaneous determination of M and p at θ=90°.