Index of content:
Volume 23, Issue 3, 01 March 1955
23(1955); http://dx.doi.org/10.1063/1.1742003View Description Hide Description
Total collision cross sections have been measured for neon atoms with energies between 500 and 2100 ev scattered in room temperature neon. The experimental results have been treated to obtain potential energy information for the interaction of two neon atoms. The analytical relation, which may be represented byfor r between 1.76 A and 2.13 A, is consistent with the potential, valid at larger separation distances, which has been derived from measurements of gaseous compressibility, viscosity, and crystal properties.
23(1955); http://dx.doi.org/10.1063/1.1742004View Description Hide Description
The radial distribution function of a fluid of rigid spheres has been calculated using IBM equipment by a Monte‐Carlo method which is valid only at relatively low densities. Up to the highest densities studied, 20 percent of closest packing, the radial distribution function agrees within the precision of the method with the one calculated by the use of the superposition approximation in triplet space. Another Monte‐Carlo approach was used at a higher density, 72.4 percent of closest packing, and the results agree with published computations.
23(1955); http://dx.doi.org/10.1063/1.1742005View Description Hide Description
The statistical mechanical theory of plane and spherical interfaces is developed on the basis of the virial theorem and the Gibbsian mechanical and thermodynamic definitions of surface tension. The expressions for the relevant thermodynamic functions in terms of molecular variables are employed in a discussion of the curvature dependence of surface tension which leads to an asymptotic expansion of the grand partition function in terms of the geometrical parameters that characterize the interface and thus provide a criterion for the range of validity of macroscopic concepts.
23(1955); http://dx.doi.org/10.1063/1.1742006View Description Hide Description
New equations for the excess sound absorption per wavelength and the relaxation frequency have been derived. These equations are derived on the assumption that a dissociation process of the type (AB)⇌(A)+(B) is irreversibly perturbed by the sound field due to assumed pressure dependence of the equilibrium. These equations are applied to the problem of excess sound absorption in dilute magnesium sulfate solutions.
By using mixtures of dioxane and water to vary the dielectric constant of the solvent the effect of varying the ion Coulombic interaction forces of the ions in solution has been studied experimentally. This work and the work of other investigators on the excess sound absorption in dilute magnesium sulfate solutions is explained quantitatively.
The specific rates of dissociation and ion association for magnesium sulfate in solutions of water and water‐dioxane have been determined. Good agreement is found between values of the mass action equilibrium constant determined by acoustic means and values determined by the usual conductivity method.
The experimental activation energy of molecular dissociation is determined to be 6.5 kcal/mole while the activation energy of ion association is estimated to be 8.5 kcal/mole.
23(1955); http://dx.doi.org/10.1063/1.1742007View Description Hide Description
The infrared spectra for the gaseous and liquid states, and the Raman spectra for the liquid state, including intensity and polarization data, are reported for ethyl cyanide (C2H5CN). A normal coordinate treatment has been made for four planar and five nonplanar skeletal vibrational modes. The observed spectra are in accord with a C 8 model for this molecule. A complete assignment of frequencies to the fundamental modes of vibration is made for ethyl cyanide. From the spectroscopic data, the height of the barrier hindering internal rotation is estimated to be 5200 cal. The thermodynamic functions for ethyl cyanide have been calculated to 1000°K.
Compressibilities of Ozone, Oxygen, and Ozone‐Oxygen Mixtures and the Thermodynamic Quantities for Pure Ozone23(1955); http://dx.doi.org/10.1063/1.1742008View Description Hide Description
The compressibilities of ozone and ozone‐oxygen mixtures were calculated from recently determined critical constants for pure ozone, and from well‐established data for oxygen. Data for pure ozone, pure oxygen, and mixtures containing 20, 40, 60, and 80 mole percent ozone are presented. The calculated compressibilities of pure ozone are used to correct thermodynamic quantities for the ideal gas, computed from spectroscopic data, to real gas conditions. Experimental determination of the compressibilities would be difficult because of the unstable nature of ozone.
23(1955); http://dx.doi.org/10.1063/1.1742009View Description Hide Description
A critical examination is given of a simplifying assumption made in an earlier calculation of the functional dependence of the integral dielectric constant of polar liquids on the applied field. It was assumed in this calculation that, for the cavity and reaction fields at all field strengths the usual expressions for low field strengths could be employed. It was pointed out that this simplification is plausible if the static dielectric constant is large compared with the optical dielectric constant. In the present note it is shown that, if it is postulated that the reaction field alone retains its conventional value, then the cavity field is appreciably increased by dielectricsaturation. In the case of water the effect is equivalent to an increase of the external field by about 10 percent; a modified relation between the field and the dielectric constant is suggested.
The alteration of the reaction field as a result of saturation, and the effect of this modification on the dielectric constant, are also examined.
23(1955); http://dx.doi.org/10.1063/1.1742010View Description Hide Description
The general statistical mechanical formulation of partial molar energies, ``internal pressure,'' and heat capacity of multicomponent systems is developed with use of the grand ensemble. In the case of two component systems, detailed concentration expansions are presented for partial molar free energies and energies and for radial distribution functions about standard states at constant temperature and pressure as well as constant temperature and partial molar volume.
23(1955); http://dx.doi.org/10.1063/1.1742011View Description Hide Description
The ion yield in the alpha‐induced decomposition of gaseous ammonia is found to increase slightly with the irradiation intensity, both in the presence and absence of electric fields. This is true for ammonia pressures of both 20 cm and 50 cm, at room temperature. Even a weak electric field increases the ion yield at 50 cm pressure. There is no evidence at 20 or 50 cm pressure of initiation of the reaction by ion recombination. Possible mechanisms are suggested.
23(1955); http://dx.doi.org/10.1063/1.1742012View Description Hide Description
The infrared spectrum of CO2 in the region of 2.7μ has been remeasured. Five rotation‐vibration bands have been identified and analyzed in detail. One of these is due to C13O2. The rotational constants obtained wereIn two of the bands l‐type doubling occurs, and the values of the splitting constants q were determined.
23(1955); http://dx.doi.org/10.1063/1.1742013View Description Hide Description
The accurate measurement of nuclear magnetic moments requires a calculation of the magnetic field at the nucleus arising from the orbital motions of the electrons in the external magnetic field. In this note we outline a solution of the ``magnetic shieldingeffect,'' using the quantum‐mechanical variation principle. Since all results are in terms of ground‐state wave functions, such an approach may facilitate a purely theoretical evaluation of shielding constants. As a variation function we take a function of the type suggested by Hylleraas and Hassé and applied successfully to calculation of polarizabilities. Previously, Ramsey used a second‐order perturbation method to calculate the magnetic shielding. His results are difficult to apply since the wave functions of excited electronic states of molecules are seldom available.
23(1955); http://dx.doi.org/10.1063/1.1742014View Description Hide Description
The mass spectrum of lithium‐iodide vapor produced from salt heated in an oven in the ion source of an analytical mass spectrometer was investigated as a function of oven temperature. Evidence was obtained for the existence in the vapor of both monomer and dimer. Partial heats of sublimation to monomer and dimer at 0°K were found to be 44.9±0.7 and 45.5±2 kcal respectively. The heat of dissociation of (LiI)2 into gaseous monomers was found to be 44.4±2.2 kcal. The partial heat of sublimation of monomer was used with supplementary thermochemical data to obtain values for the lattice energy and the heat of dissociation of the diatomic molecule.
Appearance potentials for the major ions in the spectrum where determined. Upper limits for the energy of dissociation of the gaseous diatomic molecule were derived from the appearance potentials of Li+ and I+ which were slightly higher than the thermochemical value.
23(1955); http://dx.doi.org/10.1063/1.1742015View Description Hide Description
The relation between the intermolecular potential field and the stability of crystal structures of the rare gases is investigated for a face‐centered cubic lattice and a structure of hexagonal closest packing. Two models of the potential field are used: a Lennard‐Jones (s,6) potential and a ``modified'' Buckingham potential (α,6). The effect of zero‐point energy is taken into account on the basis of Corner's method. It is found that for an addititive Lennard Jones (s,6) potential (s between 7 and 16) the hexagonal structure is the stable one. For an additive Buckingham potential (α,6), with α varying from 10 to 16, the hexagonal lattice is again more stable than the cubic structure. For both forms of the potential field considered in the analysis, the differences in energy between the two structures is of the order of one ten‐thousandth of the cohesive energies of the crystals. These results agree with the conclusions reached by Kihara and Koba, who neglected the influence of zero‐point energy. It is known from experiments that neon, argon, krypton, and xenon crystallize in the face‐centered cubic lattice. Therefore, if additivity of intermolecular forces is assumed, the results indicate that neither the Lennard Jones nor a modified Buckingham potential can explain the crystal structures of the rare gases except helium. The possible importance of many‐body interactions is discussed.
23(1955); http://dx.doi.org/10.1063/1.1742016View Description Hide Description
The infrared band intensities of formaldehyde have been measured in the gas phase using the pressure broadening method. From the results of the normal coordinate analysis and the experimental intensities, the bond moments and their derivatives have been calculated for the three symmetry species. The relative signs of ∂μ/∂Q 5 and ∂μ/∂Q 6 have been determined from the investigation of the Coriolis interaction between ν5 and ν6. The results chosen from the calculations are: μCH = 0.5D (C——H+), (∂μ/∂r)CH = 1.3×10—10 esu, μCO = 1.9D (C+—O—), (∂μ/∂r)CO = —1.9×10—10 esu. The B 1 symmetry block gave an abnormally large μCH of about 1D.
23(1955); http://dx.doi.org/10.1063/1.1742017View Description Hide Description
Convenient methods of analyzingdielectric dispersion and loss data satisfying the Debye equations are described and illustrated. The similarities and differences of the Fuoss‐Kirkwood and circular arc representations of symmetrical, non‐Debye‐type dispersions are considered. Interpretations given by various writers to the asymmetric dispersion found by D. W. Davidson and the writer for glycols are discussed. Finally, the possibilities and limitations of reduced representations of dielectricmeasurements are examined
23(1955); http://dx.doi.org/10.1063/1.1742018View Description Hide Description
An ammonia‐oxygen diffusionflame has been studied spectroscopically in the infrared region using a flat diffusionflame of the kind described by Wolfhard and Parker. The emissions of OH, NH3, and H2O and the absorptions of NH3 and H2O have been studied as a function of position in the flame. Combined with the information obtained earlier by Wolfhard and Parker concerning the distributions of OH, NH, NH2, and O2, a complete description of the flame is possible. Evidence is presented that OH is in vibrational as well as rotational equilibrium in the flame.
No evidence of infrared emission by NH or NH2 could be found.
23(1955); http://dx.doi.org/10.1063/1.1742019View Description Hide Description
The luminescence produced by intense ultrasonic waves has been investigated in a number of water solutions and organic liquids in the frequency range 0.66—2.0 mc/sec. Together with direct visual tests, quantitative measurements have been performed for which a photometric method has been developed. The results show that luminescence, when it does occur, is always present with cavitation and starts at the same sound intensity as cavitation. A quantitative relationship is established between intensity of luminescence and chemical yield induced by ultrasonics. From this it is concluded that, at least in some of these cases, the phenomenon is chemiluminescence. None of the organic liquids tested showed any luminescence except for nitrobenzene in which the luminescence was very weak in the frequency range and under the physical conditions examined. It is not possible at present to conclude whether this weak luminescence is due to some chemical reaction occurring in it or to a different process. An apparent frequency dependence for the intensity of luminescence was observed; however, it is shown that the threshold of cavitation is the frequency‐dependent phenomenon.
Compressibility of Gases. I. The Burnett Method. An Improved Method of Treatment of the Data. Extension of the Method to Gas Mixtures23(1955); http://dx.doi.org/10.1063/1.1742020View Description Hide Description
The theory of the Burnett method of measuring the compressibility of gases is discussed, and an analytical method of treating the data is presented. Determinations of the compressibility factor at 30°C for helium, nitrogen, and carbon dioxide show the convenience of the analysis and the precision that may be obtained by the method. Extension of the Burnett measurement to gas mixtures is also reported for some binary combinations of the three gases.
23(1955); http://dx.doi.org/10.1063/1.1742021View Description Hide Description
The relative ground‐state electronic energies of singly and doubly‐charged negative ions of unsaturated hydrocarbons are discussed in terms of systematically‐approximated self‐consistent molecular orbital theory. It is concluded that in the gas phase and in solution,R — ions will be stable by several ev to disproportionation into R and R =, which accords well with the recent paramagnetic resonance measurements of Lipkin et al. Further implications relevant to the solution chemistry of these ions are briefly discussed.
23(1955); http://dx.doi.org/10.1063/1.1742022View Description Hide Description
The relation between self‐diffusion coefficient and viscosity is examined in terms of the early hydrodynamictheory and Eyring's kinetic theory. They differ by a factor of about 6. A detailed consideration of the kinetic theory shows that this factor should have been included in Eyring's derivation of viscosity if a proper meaning is given to the relative velocity involved.