Volume 46, Issue 3, 01 February 1967
Index of content:
46(1967); http://dx.doi.org/10.1063/1.1840812View Description Hide Description
The reaction rate of graphite and hydrogen has been measured between 2000° and 3400°K and pressures of 0.01 to 1.0 atm using a heated filament. At the lower temperatures and higher pressures, the reaction occurs on the surface and its rate is proportional to the hydrogen pressure and the KD where KD is the dissociation constant of hydrogen. At high temperature and low pressure, the reaction occurs in the vapor phase and is proportional to the sublimation rate of graphite. The sublimation temperature of graphite as a function of carbon to hydrogen ratio has been experimentally investigated at 0.1 atm and shown to agree with the estimated thermodynamic calculations.
46(1967); http://dx.doi.org/10.1063/1.1840813View Description Hide Description
The ortho—parahydrogen‐conversion reaction (p‐H2⇌o‐H2) and the hydrogen—deuterium exchange reaction (H2+D2⇌2 HD) were used to investigate the activated adsorption of hydrogen on aromatic—alkali‐metal charge‐transfer complexes. When deuterium gas was introduced onto these complexes, it was found that hydrogen exchange takes place between deuterium and the complexes (denoted as H2⇌D2). The kinetics of the parahydrogen conversion and of the H2—D2 exchange were studied as a function of the temperature and it was found that the rate of reactions was first order at a constant pressure. On the basis of the effect of the temperature on the rate constant, an apparent activation energyE for p‐H2⇌o‐H2 was 4.4 kcal/mole in the presence of phthalonitrile—Na (I), 3.7 kcal/mole with tetracyanopyrene—Cs (II), 10.0 kcal/mole with violanthrene B–Cs (III) and 0.6 kcal/mole with graphite—Cs (IV). Further, E for H2+D2⇌2 HD was 6.6 kcal/mole in the presence of (I) and ∼0.6 kcal/mole with (IV). These reaction suggests that the strong activated adsorption of hydrogen on the complexes takes place with the excess charge transferred from alkali metal to the organic molecule. Further, it was found that the hydrogen exchange reaction between gaseous hydrogen and phthalonitrile takes place reversibly.
46(1967); http://dx.doi.org/10.1063/1.1840814View Description Hide Description
Energy transfer from UO2 2+ to Eu3+ has been observed in several solutions. Quantitative data for the quenching of UO2 2+fluorescence at various [Eu3+] and the corresponding build up of Eu3+fluorescence has been obtained in D2O solutions. Rate constants for the transfer of energy from UO2 2+ to Eu3+ are derived, the value is determined to be about 107 M −1·sec−1. This value is less than the diffusion‐controlled reaction rate. The low value can be related to mutual ion—ion repulsion.
46(1967); http://dx.doi.org/10.1063/1.1840815View Description Hide Description
The effect of hindered internal rotation on the dielectric relaxation in a liquid whose molecules contain two polar groups of equal size with a common rotation axis is calculated for twofold symmetric cosine and parabolic potential barriers. The dielectric properties are calculated from the configuration space distribution function which is obtained by a perturbation expansion of a differential equation resulting from a rotational Brownian motion argument. The results are combined with a similar calculation involving an asymmetric potential to interpret the effects of symmetry, barrier height, and barrier shape on the relaxation process.
Symmetry Properties of One‐ and Two‐Electron Correlation Functions in the Many‐Electron Theory of Atoms and Molecules46(1967); http://dx.doi.org/10.1063/1.1840816View Description Hide Description
The symmetry properties of the many‐electron wavefunctions containing unlinked clusters are shown to follow directly from those of the wavefunctions with one‐ and two‐electron correlations occurring one at a time for both closed‐ and nonclosed‐shell states. The separate calculation and symmetry properties of distinct pair functions in general nonclosed‐ and closed‐shell states are discussed. The symmetry considerations carry over to the localized‐orbital+localized‐pair‐function descriptions.
46(1967); http://dx.doi.org/10.1063/1.1840817View Description Hide Description
The rate of recombination of electrons with ions behind shock waves in argon containing dilute lean mixtures of hydrocarbons and oxygen has been measured with a microwave system. The recombination reaction was found to be two body, as evidenced by insensitivity of the recombination coefficient α to changes in pressure and concentration; and the recombination coefficient for acetylene—oxygen and methane—oxygen mixtures was found to depend upon temperature as T −1.98 between 2500° and 5000°K. Extrapolating to room temperature gives a value of α=1.3×10−5 cm3/sec, consistent with the electron removal process being a dissociative recombination involving a complex ion. In the case of acetylene, the reaction is probably a previously proposed scheme: H3O++e −→neutral products. Measurements made at a temperature of about 3800°K, using ethane, ethylene, propane, and benzene, gave recombination coefficients similar in magnitude to those obtained for the acetylene and methane, consistent with the recombining ion being the same for all these hydrocarbon—oxygen reactions.
46(1967); http://dx.doi.org/10.1063/1.1840818View Description Hide Description
Oxygen chemisorption and the extent of oxidation on a tungsten ribbon at room temperature were studied by flash desorption using line‐of‐sight detection with a time‐of‐flight mass spectrometer. The only chemical species in the adlayer are oxygen atoms and the monotungstic oxides: WO, WO2, and WO3. These oxides are desorbed with an energy of ∼100 kcal. In agreement with earlier predictions, oxygen desorbs as atoms. Neither molecular oxygen nor polytungstic oxides were ever observed from a ribbon flashed after a room‐temperature exposure to oxygen. However, flowing oxygen on tungsten at surface temperatures > 1400°K does produce the polytungstic oxides. An order‐of‐magnitude estimate of the adlayer composition at room temperature suggests that at saturation there is approximately one monolayer of oxygen atoms in a random configuration with a few tenths of a per cent each of the monotungstic oxides. Consequently, oxidation is not a significant surface process on clean tungsten at room temperature.
46(1967); http://dx.doi.org/10.1063/1.1840820View Description Hide Description
The results of valence‐bond spin‐density calculations for a number of positive (+) and negative (—) aromatic ion radicals are given. For naphthalene, it is found that the α and β position spin densities in the positive ions do not equal those in the negative ion; that is (ρα(+)/ρα(−)) = 1.01 and (ρβ(+)/ρβ(−)) = 0.933. This result, which contrasts with that of molecular‐orbital calculations, may provide a partial explanation of the small hyperfine splitting differences found in (+/—) ion pairs. It is noted that for a number of ion radicals, predominantly positive ions, the symmetries of the ground state, as predicted by the valence‐bond and molecular‐orbital methods, are not the same. Although these symmetry disagreements introduce only small changes in the spin densities of most of the molecules studied, the azulene positive‐ion results may be sufficiently different to permit an ESR test of the theories. Other experiments and calculations which would also be important for evaluating the valence‐bond method for ion radicals are discussed.
Study of BeH3 −, BH3, CH3 +, NH3 ++, and OH3 3+ by One‐Center‐Expansion, Self‐Consistent‐Field Method46(1967); http://dx.doi.org/10.1063/1.1840821View Description Hide Description
One‐center‐expansion, self‐consistent‐field calculations are carried out for BeH3 −, BH3, CH3 +, NH3 ++, and OH3 3+ systems. Using a limited basis set, all of these molecules are found to be planar (θZAH=90°), except the OH3 3+ system which is found to be unstable. The extended‐basis‐set calculations are carried out only for BeH3 −, BH3, CH3 +, and NH3 ++. These calculations give the internuclear distances to be 2.500, 2.191, 2.032 and 2.074 bohrs, respectively, the total energies −16.144047, −26.235841, −39.117795, and −54.980821 hartrees, respectively, and the first ionization potential (—ε1e ′) 0.1402, 0.4802, 0.9293, and 1.4329 hartrees, respectively. Estimated values of the Hartree—Fock energy and the total nonrelativistic energy are also given for these four molecules.
46(1967); http://dx.doi.org/10.1063/1.1840822View Description Hide Description
We have examined the electron spin resonance spectrum of trivalent dysprosium in a single crystal of anhydrous lanthanum trichloride and have identified weak forbidden resonances as originating from the ground doublet in sites of distorted and essentially undistorted crystal symmetry. Also present in the spectrum is an allowed transition between the energy levels of the first excited doublet. The ESR spectrum due to the ground doublet in an undistorted site can be described by an axially symmetric spin Hamiltonian with S=½, g ∥=10.43±0.05 and g ⊥<1. The ESR line from the excited‐state transition fits an axially symmetric spin Hamiltonian with involving the operators HiSi , Si 2, and HiSi 3; the latter terms being necessary to account for the effects of another doublet 0.135±0.01 cm−1 away. This measurement of the zero‐field splitting is in good agreement with the optical value of 0.15 cm−1. The remaining parameters of the spin Hamiltonian are also in reasonable agreement with values computed from crystal‐field theory.
Electrical Properties and Electronic Configuration of the Monocarbide, Monophosphide, and Monosulfide of Plutonium46(1967); http://dx.doi.org/10.1063/1.1840823View Description Hide Description
The electrical properties of PuC, PuP, and PuS were measured over the temperature range of 300° to 1000°K. The monocarbide and monophosphide had resistivities of 257 and 752 μΩ·cm, respectively, at room temperature; these values did not change appreciably with temperature.Plutonium monocarbide is a p‐type conductor with a low absolute thermoelectric power which is only slightly temperature dependent. The monophosphide is also a p‐type conductor at room temperature, but it changes to an n‐type conductor above 740°K. The quasimetallic conduction of PuC and PuP is essentially proportional to the mobility as indicated by thermoelectric power measurements.
Plutonium monosulfide was found to be an intrinsic semiconductor from 425° to 700°K with an activation energy for conduction of 0.240 eV. Calculations show that above 700°K PuS becomes degenerate as the carrier concentration exceeds 1019 cm−3 and a gradual transition to metallic conductivity is indicated. A peak in the absolute thermoelectric power of 115 μV°K−1 was observed for this compound which is an n‐type conductor over the temperature range studied.
In these plutonium compounds and similar actinide compounds conduction appears to be governed by a systematic change of carrier concentration in the s+d valence band and scattering caused by s+d, f transitions. A split in the f states is proposed as the mechanism responsible for the semiconductor properties of PuS.
A comparison was made of the compound unit cell volumes and metal radii for the rare‐earth and actinide mononitrides, monophosphides, and monosulfides. The data indicate effective valences of four electrons per atom for the actinide mononitrides and between three and four electrons per atom for the actinide monophosphides and monosulfides.
These plutonium compounds do not have much promise as thermoelectric materials because their figures of merit are all too low for acceptable power generation.
46(1967); http://dx.doi.org/10.1063/1.1840824View Description Hide Description
Thermal‐diffusion column transport coefficients were measured for isotopically substituted methane in a 7.32‐m hot‐wire column. At wire temperatures of 320° and 480°C, measured values of the convective and diffusive remixing coefficients were in agreement with values obtained by numerical integration, specifically for methane, of the equations of the theory of the thermal‐diffusion column. Measured values of the initial transport coefficient were consistent with the thermal‐diffusion factor measurements of Nier.
Significant‐Structure Theory of Liquids. Heat Capacities, Compressibilities, and Thermal‐Expansion Coefficients of Some Molten Alkali Halides46(1967); http://dx.doi.org/10.1063/1.1840825View Description Hide Description
By means of the significant‐structure theory of liquids,heat capacities at constant volume and pressure were computed, as well as thermal‐expansion and compressibility coefficients of the ionic melts: KCl, KBr, NaCl, and NaBr at 1073°, 1173°, and 1273°K. Some aspects of significant‐structure theory are analyzed in the light of the computations.
46(1967); http://dx.doi.org/10.1063/1.1840826View Description Hide Description
Wavefunctions, Hamiltonian matrices, and population analyses are obtained for PH3, PO, PO−, and P2 from accurate self‐consistent‐field calculations in which multicentered basis sets of Slater orbitals have been employed. The P3d exponent was optimized at 1.40 in PO and 1.10 in PO−, assuming Slater exponents for other orbitals. These 3d orbitals were found to be important in the bonding in each of these molecules. Studies were also made of dipole moments,ionization potentials, and binding energies.
46(1967); http://dx.doi.org/10.1063/1.1840827View Description Hide Description
An LCAO SCF wavefunction for BH4 − has been optimized with respect to variation of orbital exponents in a minimum basis set. The resulting charge distribution, quite different from that given by Slater exponents, yields Mulliken charges of +0.044 for B and −0.261 for each H atom. These results, and electron density maps, indicate that the unit negative charge is distributed among the four H atoms, which have an optimized exponent of 1.04.
46(1967); http://dx.doi.org/10.1063/1.1840828View Description Hide Description
An exactly solvable one‐dimensional pseudopotential model is employed to calculate the field‐emission probability and current from a free‐electron metal through both metallic and neutral absorbates. The adsorbate potential is taken to be atomistic in nature consisting of an attractive square well plus a delta‐function (orthogonalization) core outside the surface of the metal. Metallic adsorbates lead to the wide (Δ∼1‐eV) resonances in the emission probability, an additional peak or shoulder in the energy distributions, large (102−104) enhancements in the emission current, and reductions in the slope of the Fowler—Nordheim plots of In[current/(field)2] versus (field)−1 at fields F≳5×107 eV/cm. Neutral adsorbate potentials without bound states lead to reductions in both the emission probability and current, and to a simple scaling of the Fowler—Nordheim energy distributions. Neutral adsorbate potentials with bound states below the metallic conduction band lead to enhancements of the current for loose binding, additional asymmetry in the energy distributions for all strengths of binding, and to both reductions in the current and strong field reductions of the slope of the Fowler—Nordheim plots for tight binding. The reduction in the emission current by neutral adsorbates is used to interpret the simultaneous reduction of the work function and emission current by the adsorption of nitrogen on the (100) and (411) faces of tungsten. The prediction for metallic adsorbates of an extra peak in the energy distributions whose location shifts linearly in F distinguishes the results of the atomistic pseudopotential model from those of a surface‐film model.
46(1967); http://dx.doi.org/10.1063/1.1840829View Description Hide Description
Four one‐dimensional, solvable pseudopotential models are employed to calculate the field‐ionization energy distributions outside of (a) step‐function clean metal surfaces (b) step‐function metal surfaces with a monolayer or less of adsorbed metallic or neutral impurities, and (c) clean metal surfaces with finite thickness and various shapes. All models lead to resonances for low‐energy ions. For the step‐function clean metal surface these resonances occur near the Bohr quantization energies for an inverted, infinite triangular potential. Both adsorbates and changes in the shape of the surface potential alter the intensity pattern of the resonances. A model in which the surface region is described by a linear potential leads to a two‐slit intensity pattern. Potentials with a finite discontinuity in the zeroth, first, or second derivatives all lead to slowly damped interference effects explicitly associated with the nonanalytic dependence of the potential on the position coordinate.
46(1967); http://dx.doi.org/10.1063/1.1840830View Description Hide Description
The nuclear magnetic shielding constants of two simple systems are calculated explicitly. These systems are (1) a nucleus μ at a distance R from a hydrogenic atom of charge ζe in the presence of an external electric field, and (2) a nucleus μ of a hydrogenic atom of charge ζe in the presence of a point charge at R with the result expanded in inverse powers of R. These calculations should prove useful in understanding the long‐range contributions of functional groups and neighboring molecules to observed chemical shifts of molecules.
46(1967); http://dx.doi.org/10.1063/1.1840831View Description Hide Description
The use of the perturbation—variation method to calculate spin—spin coupling constants is examined. It is shown that the use of trial functions with δ‐function singularities leads to ambiguous results (this ambiguity is explicitly exhibited for HD). With the ambiguity removed a calculation of J HD is performed using correlated ground‐state and trial wavefunctions. It is concluded that convergence is slow.
46(1967); http://dx.doi.org/10.1063/1.1840832View Description Hide Description
The extent of ozone formation in the 1470‐Å photolysis of oxygen and 1:1 helium—oxygen mixture has been studied in a flow system as a function of pressure and temperature. A determination of the ozone quantum yield at atmospheric pressure gave φ = 2.0±0.2 in good agreement with previous values. At lower pressures, the production of ozone was found to decrease with pressure, and this effect has been interpreted as arising from the reaction of O(1 D) with ozone. The quantitative evaluation of the ozone quantum yields in terms of O(1 D) reactions provides ratios of rate constants for the reactions of O(1 D) with ozone, oxygen, and helium. From the temperature dependence, the reaction with ozone is found to have an activation energy of 2.4±1.0 kcal/mole. Absolute rate constant estimates are obtained by the application of independent data.