Index of content:
Volume 16, Issue 4, 01 April 1948
16(1948); http://dx.doi.org/10.1063/1.1746864View Description Hide Description
The formulas previously published for the average (square) length and radius of long‐chain molecules with restricted internal rotation are applied to a somewhat idealized model of the normal paraffin hydrocarbons. The potential barrier for rotation about the carbon‐carbon single bond has been compounded of the usual threefold component plus a smaller onefold component which serves to stabilize the trans configuration of three successive bonds. Calculations are presented for a wide range of values of the parameters, and the most probable values of the parameters applicable to the normal paraffins, as determined from the thermodynamic and spectroscopic data, are discussed. The predicted ratios of the values of the average length and radius to the values calculated for free rotation are 1.76, 1.48, 1.36, and 1.29 at 0°, 100°, 200°, and 300°C, respectively. The effect of steric hindrance is discussed, and it is pointed out that the calculated values are probably appreciably low because of the neglect of steric hindrance.
16(1948); http://dx.doi.org/10.1063/1.1746866View Description Hide Description
A study has been made of the absorption spectra between 3μ and 15μ of methyl alcohol at varying concentrations in solution. Clear indications are obtained of at least two new bands outside the 3μ region which arise from the associated molecules. The data allow of a further discussion of the vibrational assignments in methyl alcohol and it is hoped that some of the uncertainties in that respect have now been removed.
16(1948); http://dx.doi.org/10.1063/1.1746867View Description Hide Description
A low temperature adiabatic calorimeter suitable for thermal measurements on adsorbed gases has been developed. Heat capacity values for two amounts of nitrogen adsorbed on titanium dioxide (rutile) are presented. While the measurements were complicated by the appearance of warm drifts in the calorimeter over part of the temperature range when nitrogen was present, it appears that at the concentrations used the adsorbate was present as a condensed phase.
16(1948); http://dx.doi.org/10.1063/1.1746869View Description Hide Description
The π‐electrons belonging to the double bonds of trans‐conjugated polyenes are assumed to be a one‐dimensional electron gas in a field of uniform potential energy whose length is approximately that of the conjugated system. The energy levels, the selection rule (Δn odd) and the transition intensities are deduced. The model predicts that the oscillator strength of the main absorption band of a trans‐conjugated polyene is given by f = 0.134(2N+1), where N is the number of double bonds. The calculated values are in good agreement with experimental data. The wave number of the main band is related to the length of the conjugated system and the number of double bonds with moderately good agreement with experiment. The second, weaker absorption band of trans‐carotene is discussed briefly.
16(1948); http://dx.doi.org/10.1063/1.1746871View Description Hide Description
The vapor pressure of benzene and the heat of dilution of benzene‐biphenyl mixtures have been measured at 25°C. The results are shown to be in fair agreement with Guggenheim's formulae based on the lattice model, assuming that the biphenyl molecule occupies twice the volume of the benzene molecule, and that the coordation number of the lattice is between 6 and 12. No agreement can be obtained if the effect of the different sizes is neglected.
The experimental results of other authors are critically examined and shown to conform to the above picture.
16(1948); http://dx.doi.org/10.1063/1.1746873View Description Hide Description
The gas heat capacity of 1,2‐dichloroethane was measured in the temperature interval from 356°K to 556°K. The gas heat capacity of 1,2‐dibromoethane was measured in the temperature interval from 383°K to 473°K. A potential barrier to internal rotation for 1,2‐dichloroethane was selected which is in agreement with all of the data on heat capacity,entropy, and dipole moment of the gaseous molecule. An approximate barrier to internal rotation was selected for 1,2‐dibromoethane.
16(1948); http://dx.doi.org/10.1063/1.1746875View Description Hide Description
A general method is developed for separating slow internal motions of molecules from their rapid vibrations, with the rotational motion. The zero approximation for the slow motion can be obtained by Schrödinger's rule from the classical motion in which the rapid vibrations are replaced by constraints.
The theory is applied to the symmetrical double‐minimum vibration of ammonia, the combined vibration‐rotation energy levels being evaluated as characteristic numbers of a Sturm‐Liouville equation in one variable. The microwave absorption frequencies of N14H3 and N15H3 are computed, also the pure rotation frequencies in the far infra‐red.
16(1948); http://dx.doi.org/10.1063/1.1746877View Description Hide Description
Classical electrostatics were employed to study the configuration and potential energy of the (hypothetical) (HF)2 molecule in the gaseous state. Of the planar models studied, a Z‐shaped structure of C2h symmetry with both F–F–H angles equal to 32.5 degrees had a minimum potential energy.
16(1948); http://dx.doi.org/10.1063/1.1746878View Description Hide Description
Fine structure has been observed in the nuclear paramagnetic resonance absorption line for protons in crystalline hydrates. The magnetic field of 6820 gauss was provided by a permanent magnet, the inherent stability of which facilitated detailed study of line shape. Measurements on a single crystal of CaSO4·2H2O show a splitting into four component lines with maximum separation varying from zero to 22 gauss, depending upon the direction of the externally applied magnetic field in the crystal. Both the number of component lines and the dependence of their spacing on field direction are calculated by treating the magnetic dipole‐dipole interaction as a perturbation of the proton two‐spin system within the water molecule; the effect of the more distant protons, neglected in this calculation, gives a finite width to the component lines. Variation of the splitting with field direction determines the orientation of the line joining protons in the water molecule, which is found to be consistent with positions ascribed to hydrogen nuclei in the lattice through simple considerations of chemical bonding. The distance between protons in the water molecule is measured by the splitting to be 1.58A for CaSO4·2H2O; if one assumes an H–O–H bond angle of 108°, the O–H distance is 0.98A. Powdered hydrates show a characteristic fine structure arising from isotropic distribution in solid angle of single crystal granules. This type of fine structure determines the proton‐proton distance somewhat less accurately than does the single crystal experiment.
16(1948); http://dx.doi.org/10.1063/1.1746880View Description Hide Description
The conductance of NaCl and KCl in 50‐mole percent methanol‐water mixtures has been measured at concentrations from 0.0005 to 0.01 N. The data can be represented within experimental error by the Onsager‐Shedlovsky equation, the limiting conductances being 66.62 and 75.10, respectively. The limiting conductances of chloride ion, obtained from these values, and the limiting transference numbers reported in the accompanying paper, satisfy the rule of independent ionic mobilities, but at finite concentrations the chloride ion conductance is less for the potassium salt than for the sodium. The decrease in limiting ion conductances as compared with those in water is greater than can be accounted for by viscosity alone. The results are also in disagreement with the Bjerrum theory of ion pair formation if the mean ionic diameter, determined thermodynamically, is significant in transport processes.
The Transference Numbers of Sodium and Potassium Chlorides in 50‐Mole Percent Methanol‐Water Mixtures16(1948); http://dx.doi.org/10.1063/1.1746882View Description Hide Description
The transference numbers of NaCl and KCl in 50‐mole percent methanol‐water mixtures at 25°C have been measured for concentrations from 0.005 to 0.08 N, employing both autogenic cation and sheared anion boundaries. In the cation measurements, there was no progression in t + as the boundary moved up the tube, and t + was independent of current; the same was true of the anion measurements at 0.02 N over a considerable range of indicator ion concentration, and the sum t ++t − was unity within experimental error. At 0.05 N, however, the measuredt − varied with current and at moderate and high currents frequently showed progression with boundary movement. The Longsworth function is linear in concentration for the more dilute solutions, but the plot is definitely curved for the more concentrated. For both salts t +° is greater than in water, and the variation of t + with concentration is distinctly different from that obtaining for aqueous solutions. A possible explanation is given for the latter effect.
16(1948); http://dx.doi.org/10.1063/1.1746883View Description Hide Description
Full quantum corrections are applied to Mayer's recent extension of statistical thermodynamics to the multi‐component liquid phase. The resulting formulae are too general in nature to permit quantitative interpretation, but qualitatively, in the case of Bose‐Einstein systems, the theory predicts a typical lambda‐point condensation in momentum space. A Fermi‐Dirac isotopic component dissolved to small concentrations in a Bose‐Einstein liquid phase, like He3 in He4, is shown to behave to a first approximation independently of the Bose‐Einstein degeneracy of the solvent at all temperatures above the lambda‐point. The theory appears unable to handle the situation below the lambda‐point.
16(1948); http://dx.doi.org/10.1063/1.1746885View Description Hide Description
The reactions of atomic hydrogen, produced by a discharge, with methyl ether, acetaldehyde, and ethylene oxide have been investigated.
The main products of the reactions are:
The mechanisms of the reactions are discussed in detail and it is concluded that the primary reactions are:and that the main secondary reactions in all cases are of the atomic cracking type:
16(1948); http://dx.doi.org/10.1063/1.1746887View Description Hide Description
A simplified procedure is described for computing, either manually or by means of punched cards, the equilibrium composition of a gaseous system of any degree of complexity. The method presents a system of n equations in n unknowns in such a form that each of the major components is given as a linear expression of the minor components only, while the logarithm of each of the minor components is given as a linear expression in terms of the free energy change for the dissociation reaction which defines the minor component and the logarithms of the major components and the total pressure. An iteration process then gives an exact solution with a rapidity which decreases as the fuel‐oxidant mole ratio approaches the stoichiometric value.
16(1948); http://dx.doi.org/10.1063/1.1746889View Description Hide Description
The reactions of atomic hydrogen produced by a discharge with ethane, propane, and neo‐pentane have been investigated. All three reactions proceed at approximately the same rate, with activation energies of ca. 9 kcal., assuming a steric factor of 0.1. It is suggested, however, that the activation energies are in reality somewhat lower than this, with correspondingly smaller steric factors.
In the reaction with neo‐pentane considerable ethane is produced. When deuterium atoms are used, this ethane is found to be only slightly exchanged. It is concluded, therefore, that ethane cannot arise by recombination of radicals, and must be formed in one step, presumably from an excited neo‐pentane molecule.
16(1948); http://dx.doi.org/10.1063/1.1746891View Description Hide Description
A method of measuring the dielectric properties of medium and high loss liquids at microwave frequencies is described. In this method the liquid sample is placed in an open circuit‐terminated section of wave‐guide and standing wave‐ratios are measured at intervals as the length of the dielectric liquid column in the wave guide is changed. The method of measuring the wave‐length of the radiation in the liquid‐filledwave guide is indicated. A simple procedure for calculating the dielectric properties from these measurements is presented. Some dielectric properties of liquidsmeasured with this method at 3‐cm wave‐length are given.
16(1948); http://dx.doi.org/10.1063/1.1746893View Description Hide Description
In the sequence oxygen, sulfur, selenium, tellurium, and polonium a systematic alteration takes place from diatomic molecules, through ring and chain molecules, to a simple cubic lattice structure formed by atoms. This transition is paralleled by a modification in the electrical behavior from insulator (O,S), to semiconductor (Se,Te), to metal (Po). This paper is concerned with this progressive change in structure and conductivity and with the interrelation of the two phenomena. It discusses first the stability of ring and chain molecules and how the chain lattices of Se and Te may be derived by a simple distortion of the Po structure. Next, it considers schematically the electronic conductivity of selenium from the standpoint of the band picture, and finds that metallic Se is probably an intrinsic P‐type conductor. Finally an alternative approach is suggested which connects structure and conductivity by extending qualitatively the concept of quantum‐mechanical resonance from molecular structures to lattice structures and makes use of Pauling's resonating bond. The actual structure and conductivity of Se and Te thus appear as the outcome of a resonance between an insulating chain structure held by van der Waals cohesion and a metallic lattice of simple cubic structure, in which for Se the chain structure and for Te the Po lattice makes the stronger contribution.
16(1948); http://dx.doi.org/10.1063/1.1746894View Description Hide Description
The effect of pressure on the initial stages of the mercury photosensitized reactions of ethane has been investigated at room temperature. The production of methane increases with decreasing ethane pressure and is inhibited by the presence of molybdenum oxide. It has been shown that hydrogen atoms are produced in the initial stages of the reaction. It is concluded that the initial step is a C–H split and that an active ethane molecule is not formed. The dependence of quantum yields on pressure indicates that the initial step is about 100 percent effective and that inefficiency is caused by recombination of H and C2H5 resulting in the formation of ethane at high pressures by deactivation, and in the formation of methyl radicals at lower pressures.
16(1948); http://dx.doi.org/10.1063/1.1746896View Description Hide Description
The author's approach is based on the conception of ``structure'' of liquids. From rather simple assumptions about the symmetry of configuration of the nearest neighbors the entropy change connected with bringing up N‐Avogadro's atoms from the bulk of the liquid into its free surface is estimated and the temperature coefficient of the surface tension is calculated therefrom.
Two entropy terms are taken into consideration. The first term ΔS osc is due to the difference in the frequency of oscillation of the atom inside and on the surface, and is calculated from the respective number of neighbors. This term allows the calculation of the lower limit of the temperature coefficient of the surface tension (because the ``unsharpness'' of the free surface is not accounted for). The second term allows us to estimate—again from the number of the nearest neighbors in the bulk and on the surface—the maximal value ΔS conf, to account for the ``unsharpness'' of the surface.
The experimental data lie well within the calculated limits and almost coincide with the values calculated from (ΔS osc+½ΔS conf)/A where A is the surface occupied by N‐Avogadro's atoms. The influence of the thermal expansion is expressed by the change of A with temperature.
16(1948); http://dx.doi.org/10.1063/1.1746898View Description Hide Description
The author is opposed to the generally established opinion that the surface tension of liquid metals is abnormally high (as compared with that of non‐metallic liquids) and that the metallic character (the presence of free electrons) is the main cause of the ``high'' surface tension of metals. He shows that this opinion is due to the fact that specific surface tensions (ergs per cm2) are compared and that the influence of temperature is not accounted for. Total surface energies σ M —T(dσ M /dT)—where σ M =σspec×A is the surface tension referred to the surfaceA occupied (in monomolecular layer) by N‐Avogadro's molecules—are of the same order of magnitude.
The author expresses the opinion that the main cause of the surface tension of all liquids, whether metallic or not, is the same, viz., the total surface energy is determined by the amount of energy required to bring the molecules or atoms (and electrons) from the bulk of the liquid to its free surface while enlarging the surface area. He shows that in organic liquids showing no dipoles this energy can be calculated directly from the heat of vaporization at absolute zero and from the configuration of nearest neighbors. In the case of metals, the change of kinetic energy of electrons must be considered in addition. The author shows that the latter must contribute a negative term to the total surface energy. In fact, liquid metals show lowersurface energies than might be expected from their heats of vaporization and from the configuration of their nearest neighbors (Fig. 1).
A short review of the recent electron theories of the surface tension of liquid metals is given.