Volume 28, Issue 6, 01 June 1958

Electron Diffraction Study of Lead Tetramethyl
View Description Hide DescriptionA reinvestigation of Pb(CH_{3})_{4} has shown the diffraction pattern to be in agreement with a symmetrical molecular structure and the complex atomic scattering factors of Ibers and Hoerni. The measured interatomic distances are Pb–C = 2.203±0.010 A and Pb···H = 2.79±0.05 A.

Electron Diffraction Study of Trifluoromethyl Iodide
View Description Hide DescriptionA reinvestigation of CF_{3}I by electron diffraction for this molecule has confirmed the complex atomic scattering factors of Ibers and Hoerni. The structural parameters agree with those reported from earlier, inconclusive determinations.

Dispersion and Absorption of Sound in Ethyl Formate and Study of the Rotational Isomers
View Description Hide DescriptionIt was shown by Karpovich that there is the relaxational absorption of sound in liquid ethyl formate and that it results from the rotational isomerism about the C–O bond. Dispersion of sound velocity is expected from the absorption data, and its existence is confirmed by the present experiments. The sound velocities are measured over the frequency range from 300 kc to 9 mc/sec and at 5°C intervals from 10°C to 40°C by an ultrasonicinterferometer. From these experimental results, the energy differences, the equilibrium constant, the rate of isomerization, and the height of the potential barrier hindering the internal rotation are calculated. The value of the energy difference is 2.50 kcal/mole and the height of the potential barrier, referred to the higher energy state is 3.38 kcal/mole. The calculated values of the heat capacity owing to the chemical reaction, the sound velocity at infinite frequency, and the maximum values of the absorption coefficient of sound per wavelength are in good coincidence with the experimental data.

Electronic Structure of Some Diatomic Hydrides
View Description Hide DescriptionThe self‐consistent field molecular orbital (SCF‐MO) method is applied to the calculation of the ground states of the CH, CH^{+}, NH, OH, and FH molecules. Results are reported for the ionization potentials,excitation energies, binding energies, and dipole moments. The relative behavior of the bonding in these molecules is exhibited by an electron distribution analysis and the importance of s‐p promotion in a single configuration description of these molecules is emphasized.

Kinetics of the Triiodide Equilibrium
View Description Hide DescriptionThe I^{127}NMR absorption line of iodide ion in aqueous solution is broadened by the presence of iodine molecules. The broadening is consistent with the diminution of phase‐memory time caused by strong quadrupole coupling in triiodide ion, which occurs in dynamic equilibrium. On the basis of a simple bimolecular mechanism, the rate constants for the forward and reverse reactions of the triiodide equilibrium at ca 35°C are (4.1±0.4) × 10^{10} l/mole‐sec and (7.6±0.8) × 10^{7} sec^{—1} in the concentration ranges which have been investigated.

Raman Spectroscopy in Liquid Ammonia Solutions. Vibrational Frequencies and Force Constants for Isotopic Species of the Borohydride Ion Having Tetrahedral Symmetry
View Description Hide DescriptionAll fundamentals of the ions ^{11}BH_{4} ^{—}, ^{11}BD_{4} ^{—}, ^{10}BH_{4} ^{—}, and ^{10}BD_{4} ^{—} have been observed in the Raman spectra of some alkali metal salts dissolved in liquid NH_{3} or ND_{3} at —50°C. A complete assignment of all observed frequencies has been made and force constants have been calculated by the FG method of Wilson.

Molecular Association of Methanol Vapor
View Description Hide DescriptionThe absorption of infrared radiation by methanol vapor in the region 3200 to 3800 cm^{‐1} has been studied at 305° and 335°K as a function of vapor density. The results best correlate with the model that methanol vapor is composed of monomer, dimer, and tetramer, with little or no trimer present. The heat of dissociation of the dimer is found to be approximately 3 kcal/mole, and that of the tetramer approximately 17 kcal/mole. The greater stability of the tetramer compared to the dimer is consistent with a cyclic structure for the tetramer.

Viscous Flow of Liquids at Constant Volume and Constant Pressure
View Description Hide DescriptionThe energy of activation for viscous flow at constant volume, E_{v}, is dependent on the volume of a liquid. For normal liquids at elevated temperatures, E_{v} is only a small fraction of the value of the activation energy at constant pressure, E_{p}. However, for associated liquids, on account of the presence of hydrogen bonds,E_{v} is almost equivalent to E_{p}. It is suggested that in highly associated liquids like liquidsilica and some molten glasses where flow necessitates the rupture of bonds of even greater energy, the difference between E_{v} and E_{p} should be negligible.

Hydrogen Fluoride Polymer Spectrum, Hexamer and Tetramer
View Description Hide DescriptionThe experimental techniques employed to investigate the polymer absorption are presented.
Experimental results at various pressures at temperatures from —70°C to +73°C are presented. The more important qualitative features noted are: (1) the absorption is strongly dependent upon both the temperature and pressure; (2) in some cases the shape of the bands is dependent upon the pressure; (3) at low temperature only very low pressures of vapor (hence of polymer) are necessary to observe some absorption. The application of Dalton's law, the law of mass action, Lambert's law, and Beer's law, as well as Van't Hoff's equation to the hydrogen fluoride monomer‐polymer system is discussed.
It is then shown that under some special circumstances the absorption varies simply as the fourth power of the pressure, under others as the sixth power of the pressure, but more generally where the absorption is strong as the sum of two terms, one containing the fourth power of the pressure the other the sixth power. The absorption thus corresponds to the overlapping spectra of a tetramer and of a hexamer, which can be separated on the basis of this pressure dependence. From the temperature variation of the absorption, the heats of polymerization are estimated as ΔH _{4} = —19 kcal/mole and ΔH _{6} = —40 kcal/mole. In general, the absorption outside the range of monomer absorption is as consistent with a tetramer‐hexamer interpretation for deuterium fluoride as for hydrogen fluoride.

Mean First Passage Time for a Random Walker and Its Application to Chemical Kinetics
View Description Hide DescriptionThe mean first passage time for a random walk with reflecting and absorbing barriers is computed by assuming Onsager's reciprocal relation for the transition probabilities. The result, which is valid for an arbitrary dimensional random walk, appears as a quotient of determinants whose elements are the transition probabilities and the initial distribution. For the one‐dimensional case, this result is written as a series which converges rapidly when nearest neighbor transitions predominate; in fact, the series reduces to a closed form for the case when only nearest neighbor transitions are allowed, and always converges for the case when only nearest and next‐nearest transitions are allowed. Montroll and Shuler's computation of the mean first passage time for the case of the truncated harmonic oscillator model of a diatomic molecule is simplified and extended to an arbitrary initial distribution. In addition, the mean first passage time for the case of an‐harmonic oscillator model is computed by using the transition probability obtained recently by Shuler, Bazley, and Montroll. In terms of the mean first passage time, the range of validity of the equilibrium theory in chemical kinetics is discussed.

Equation of State for Water at High Pressure
View Description Hide DescriptionAn equation of state for water at high pressures is derived by assuming water to be an amorphous mixture of Thomas‐Fermi‐Dirac oxygen atoms and Wigner‐Huntington hydrogen atoms, the mixture having the correct proportion of atomic constituents. Comparison is made with more intricate models in which interaction of the fields among neighbors is assumed.

Interactions of H^{—} Ions and H Atoms with Ne, A, and H_{2}
View Description Hide DescriptionPrevious work on the systems H^{—}–He and H–He serves as a model for the calculation of the interactions of H^{—} ions and H atoms with Ne, A, and H_{2} from an analysis of beamscattering measurements. It is shown that a mutually consistent interpretation can be developed on the assumption that the forces are repulsive in the region of interest. The H^{—} ion elasticscattering cross sections are used to calculate the interactions of H^{—} with Ne, A, and H_{2}, and the electron detachment cross sections are used to locate the points at which these potential energy curves cross the corresponding curves for H atoms. The crossing points are shown to be consistent with potential energy curves for the interactions of H atoms with Ne, A, and H_{2} obtained directly or indirectly from independent scattering measurements involving beams of H atoms. The crossing points are used to extend these potential energy curves over a wider range of validity than previously available. Such a mutually consistent picture does not seem to emerge when attractive forces are assumed for the H^{—} interactions.

Relative Detection Efficiency of LiBr and (LiBr)_{2}
View Description Hide DescriptionThe relative efficiency for the production of positive ions when LiBr and (LiBr)_{2} are incident on a hot oxidizedtungsten wire has been determined. It is found that the ratio of the number of ions produced for the molecule (LiBr)_{2} and the molecule (LiBr) is 2.0±0.1. The result is consistent with the heretofore unverified assumption that has been made in the study of the thermochemistry of the vapors of the alkali halides. Effects arising from the long mean free paths of molecules in ovens used for such studies have been observed and are discussed.

Correlation of Molecular Constants. I. Deduction of Relations
View Description Hide DescriptionMaking certain assumptions regarding constants in Sutherland's potential energy function, a number of relations have been derived between the molecular constants; viz., k_{e}, r_{e}, D_{e}, ω_{ e }, etc. The relations will be examined in succeeding papers.

Correlation of Molecular Constants. II. Relation between Force Constant and Equilibrium Internuclear Distance
View Description Hide DescriptionA new relation between force constant and equilibrium internuclear distance has been suggested:where d and A are constants for each molecular group. The relation has been examined for a number of molecular groups and found to be satisfactory.

Kinetic Studies of Hydroxyl Radicals in Shock Waves. I. The Decomposition of Water between 2400° and 3200°K
View Description Hide DescriptionThe decomposition of water vapor has been studied at temperatures between 2400° and 3200°K generated in plane shock waves by following changes in the concentration of OH with time. Specific radiation absorbimetry permits these measurements to be made with microsecond resolution. The source of OH radiation used was a flash lamp containing water vapor. Measurements were made on the gas behind reflected shock waves in argon at initial pressures near 50 mm Hg and containing the reactant, H_{2}O vapor, in amounts of the order of 1 mole %. The shock tube and associated electronic and optical equipment are described. Problems of purity, chemical analysis, and reduction of data are discussed. Absorption by OH was calibrated with the equilibrium mixtures resulting from the decomposition of H_{2}O and from the reaction of H_{2} and O_{2}. The rate of formation of OH from H_{2}O is proportional to the H_{2}O concentration and nearly independent of the argon pressure. The effective activation energy is about 50 kcal/mole. Addition of 2% O_{2} to the mixture approximately doubles the rate of OH formation and preserves the 50 kcal activation energy. Addition of 2% H_{2} decreases the rate and increases the activation energy to about 80 kcal/mole. Possible reaction mechanisms are considered. The initial rate of appearance of OH cannot be interpreted in terms of the simple dissociation of H_{2}O into H and OH, and it seems necessary to consider a relatively low‐energy path involving H_{2}O_{2} or the HO_{2} radical as an intermediate.

Magnetic Properties of Certain Rare Earth Double Nitrates
View Description Hide DescriptionParamagneticresonance experiments have been performed with crystals of diluted erbiummagnesium nitrate and diluted holmiummagnesium nitrate. Resonance was observed in the first case but not in the second. These experimental results and in addition the Zeeman effects in neodymiummagnesium nitrate are interpeted in terms of a crystalline electric field of predominantly icosahedral symmetry.

Free Radical Statistics
View Description Hide DescriptionOn the basis of a simplified statistical model, average concentrations of trapped free radicals condensed from a gaseous mixture are calculated. The trapped free radical concentration is obtained as a function of the free radical concentration in the gas and of the number of nearest neighbors in the solid. The mathematical problem presented by the model is solved rigorously in the one‐dimensional case (two nearest neighbors) and approximately for arbitrary numbers of nearest neighbors. Typical calculated percentages of trapped free radicals range from 14% for a simple cubic lattice to 10% for a face‐centered cubic lattice, when the solid is condensed from a completely dissociated gaseous mixture.

Determination of the Electronic Conductivity in Silver Halides by Means of Polarization Measurements
View Description Hide DescriptionThe current passing through a cell, silver/silver halide/graphite, has been measured as a function of applied potential in the temperature range between 200 and 370°C, for potentials less than 0.5 volt. Under these conditions, the current for steady‐state conditions is mainly electronic. Definite results were obtained for AgBr and AgI, but not for AgCl because of sluggish approach to steady‐state conditions. From an analysis of the current‐potential curves it is concluded that excess electron conduction prevails in AgBr, but hole conduction in AgI.

Interpretation of Infrared Dichroism in Axially Oriented Polymers
View Description Hide DescriptionAlthough the actual distribution of chain directions in a partially oriented polymer cannot be determined from observations of infrared dichroism it is desirable to express the results in terms of a plausible model. A general relation between dichroic ratio and the parameter describing any model distribution is derived and applied to a number of suggested distributions. None are found to give plausible density functions for chain directions and a model based on the orientation of crystallites in a stretched polymer is suggested as a more suitable representation.