Volume 15, Issue 7, 01 July 1947

The Size and Vibration Frequency of the Excited Benzene Molecule
View Description Hide DescriptionIt is shown that the change in size and vibration frequency of a polyatomic molecule upon electronic excitation can be calculated if the excitation energy is known as function of the distance. This is applied to benzene, for which this function is calculated both with the Heitler‐London‐Slater‐Pauling method and the molecular orbital method. Upon comparison with experiment, it is found that both methods give the right sign and order of magnitude for the change in size, but the wrong sign for the change in frequency.

Equilibrium Atom and Free Radical Concentrations in Carbon Monoxide Flames and Correlation with Burning Velocities
View Description Hide DescriptionCalculations have been made of equilibrium concentrations of hydroxyl radicals and of hydrogen and oxygen atoms in moist carbon monoxide flames. Comparison with burning velocities,measured by Jahn by the Bunsen burner method, reveals a close correlation between observed burning velocities and such calculated hydrogen atom concentrations. Only a slight correlation exists with hydroxyl radical concentrations and none at all with calculated oxygen atom concentrations. A theoretical basis for these observations will be presented in a subsequent paper.

Theory of Burning Velocity. I. Temperature and Free Radical Concentrations Near the Flame Front, Relative Importance of Heat Conduction and Diffusion
View Description Hide DescriptionA previous paper having suggested that hydrogen atoms play an important part in combustion, an investigation is made of the relative importance of heat conduction and diffusion in establishing concentrations of hydrogen atoms near the flame front. To do this, differential equations are set up for heat transfer and for material transport. These equations are solved for two typical mixtures, one containing moist carbon monoxide, the other containing hydrogen. It is shown that the temperature falls rapidly as the distance from the flame front increases, and that the local thermal equilibrium concentration of hydrogen atoms, being a negative exponential of that temperature, falls more rapidly still. On the other hand, the local non‐equilibrium concentration of hydrogen atoms, which is caused by diffusion from the flame front into unburnt gas, falls only slowly with distance. It is thus concluded that diffusion plays a more important role than heat transfer.

Separation of the Nitrogen Isotopes by the Exchange Reaction between Ammonia and Solutions of Ammonium Nitrate
View Description Hide DescriptionThe fractionation factor for the exchange reaction between ammonia and ammonium nitrate was determined as a function of the dissolved ammonia content. It was found that for a stock solution containing 59.3–59.6 percent ammonium nitratewhere α is the fractionation factor at 25°C and M the fraction (Moles NH_{3})/(Moles NH_{3}+Moles NH_{4}NO_{3}), in solution. From these data, the equilibrium constant for the exchange reactionwas calculated to be 1.034, while that for the reactionwas found to be 1.005. Vapor pressure and density data were determined for the ammonia‐ammonium nitrate solutions as a function of the ammonia concentration.

The Rate of Adsorption of Phosgene and Chloropicrin on Charcoal
View Description Hide DescriptionMaking use of a relationship between the adsorptionrate constant and the maximum slope of the effluent concentration‐time curves derived from the rate equation of Bohart and Adams, it has been possible to estimate an absolute rate of adsorption of phosgene and chloropicrin on charcoal from the experimental data of Dole and Klotz. It is shown that diffusion to the most accessible part of the charcoal surface will occur faster than the observed rate, but more slowly to the least accessible surface areas. Calculated rates of adsorption using the statistical equations of Glasstone, Laidler, and Eyring and of Temkin are also of the correct order of magnitude when plausible assumptions are made concerning the nature of the activated state. It is concluded that both diffusion and surface reaction mechanisms must be postulated to gain a complete understanding of the adsorption rate of the two gases, but with diffusion being less important in the case of phosgene.

The Study of Reaction Intermediates by Means of a Mass Spectrometer Part I. Apparatus and Method
View Description Hide DescriptionA method for coupling a reaction chamber to a Dempster‐type mass spectrometer in such a manner that short lived intermediates can reach the electron beam is described. The presence of radicals is denoted by an increase in the ion current of the corresponding mass. The sensitivity of the method depends on the difference between the appearance potential of the given ion produced by electron bombardment of either reactants or end products and the ionization potential of the free radical. It is shown experimentally that excited molecules are not present in sufficient quantity to invalidate the assumption that free radicals are responsible for the changes in ion current. The method has already extended more than tenfold the pressure range in which radicals are detectable, thus bridging the gap between mirror and spectroscopic methods so that low pressure combustion phenomena may be studied.

Low Temperature Transport Properties of Gases. I. Helium
View Description Hide DescriptionValues have been computed for the viscosity,thermal conductivity, and coefficient of self‐diffusion of gaseous helium for temperatures below 200°K by application of classical scattering theory within angular regions where diffraction effects are absent. Values of classical total collision cross sections are obtained which are used to evaluate classical cross sections for viscosity,thermal conductivity, and self‐diffusion appropriate to a Maxwellian gas. Numerical values of the transport properties are obtained by substitution of the appropriate classical cross sections into the exact transport property formulas of Chapman and Enskog.
In the region 200° — 14°K, the average absolute deviations between calculated and experimental values are 1.9 percent for viscosity and 4.6 percent for thermal conductivity. Extrapolation of calculated viscosity values to 1.64°K appears to be justified on the basis of agreement with experimental values. It cannot be stated that similar extrapolation in the case of thermal conductivity is valid because of possible uncertainties in the experimental values between 3.95° and 1.62°K. Values of the self‐diffusion coefficient have been calculated between 200° and 5°K, but experimental values are not available for comparison.

``Structure'' in Liquids and the Relation between the Parameters of the Arrhenius Equation for Reactions in the Condensed Phase
View Description Hide DescriptionOn the basis of the liquid state being quasi‐crystalline, a modification of the transition state theory leads to a simple equation for the temperature dependence of the fluidity, which involves a characteristic frequency of molecular vibration. The parameters of the fluidity equation are shown to have a linear correlation for certain liquids. The quasi‐crystalline picture is then extended to a simple collision theory of uni‐ and bimolecular reactions in solution, and it is shown that the known correlation of the parameters of the Arrhenius equation arises from the fluidity correlation.

Solubilization by Solutions of Long‐Chain Colloidal Electrolytes
View Description Hide DescriptionIncrease in the concentration of a soap or other detergent does not increase the solubility of an oil above that in water until the critical concentration for the formation of micelles (cmc) is attained. Above this the solubility, designated as solubilization, increases and, in general, more rapidly as the soap concentration increases; i.e., per mole of soap the solubilization is greater in a 25 percent than in a 5 percent soap solution. For a homologous series the volume of oil solubilized at a constant temperature is to a first approximation inversely proportional to the molar volume. The polarity and shape of the molecules solubilized also play a role. Salts increase the extent of the solubilization; at low concentrations to an extent which may be accounted for by the increase in micellar area resulting from the depression of the cmc by the salt. At higher soap concentrations the increase in solubilization is greater than can be accounted for in this way.

The Magnetic Susceptibilities of the Vapors of Benzene and Carbon Tetrachloride
View Description Hide DescriptionAn apparatus has been constructed for measuring at elevated temperatures and at independently controlled pressures the magnetic susceptibility of vapors of substances which are normally in the liquid or solid state. Besides greatly increasing the range of substances for which such measurements can be made, the measurement at elevated temperatures is believed to permit considerably greater accuracy than has been attained previously in the susceptibilitymeasurements on vapors. Determinations of the susceptibilities of the vapors of benzene and carbon tetrachloride have been carried out at 110°C. The extreme limits of uncertainty in the results are estimated to be 3 percent. The values found for the mass susceptibilities of the vapors are in very close agreement with values which have been obtained for the liquids.

Statistical Mechanics of Binary Mixtures
View Description Hide DescriptionThe possibility of specifying both the enumeration of the complexions of an assembly and the evaluation of its configurational energy in terms of the numbers of closest neighbor pairs of sites of various kinds, without introducing parameters explicitly to specify the occupation of individual sites, is considered. The formula obtained on such a basis by Alfrey and Mark is examined. Their work depends on an assumption which at first sight appears plausible. Furthermore, if this assumption could be justified, it would imply that the quasi‐chemical equation, which has been introduced in the theory of regular assemblies as an ad hoc assumption, could be derived from the Boltzmann equilibrium law and the elementary formulas of algebraic combinations. If a pair of closest neighbor sites of which one is occupied by a molecule of species i and the other by a molecule of species j is called an i—j pair, then it is shown that the assumption made by Alfrey and Mark is equivalent to neglecting the restrictions on the free allocation (amongst the ½zN pairs available in all) of pairs of different kinds which are inherent in the interconnections of an assembly of interacting particles. It is concluded, therefore, that the assumption in question is unjustified and leads to an incorrect result, and that consequently the quasi‐chemical equation is correctly introduced as an ad hoc assumption.

An Equation of State for Gases at Extremely High Pressures and Temperatures from the Hydrodynamic Theory of Detonation
View Description Hide DescriptionThe hydrodynamic theory of detonation is derived in a convenient form for practical utility by employing the general equation of statepv=nRT+α(T, v)p. Two methods of solution of the general equations based on measured detonation velocity are discussed. In method (a) the detailed form of α(T, v) is unspecified. It is therefore, in principle, at least, a general solution. However, in practice one finds that it is impossible due to the experimental error in detonation velocities to evaluate the heat capacity at constant volume and hence the detonation temperature without specifying a particular form of α(T, v). The postulate (used only in the calculation of temperature) is α=α(v). Method (b) employs the approximation α=α(v) throughout. Methods (a) and (b) lead to identical results which one will find in view of the comparative nature of the two methods, is good evidence (but not conclusive proof) for the validity of the above approximation. This is supported also by the discovery that the same α vs. v _{2} curve applies to all explosives yet considered. As a matter of fact, it has been found that the detonation velocities may themselves be computed within experimental error, evidently for explosives of all types (where sufficient heat data are available) by employing the α(v) function evaluated from a few selected explosives. Several additional arguments supporting the above approximation are discussed. Data on the detonation properties of several explosives are presented and correlated with similar data obtained by other investigators.
 LETTERS TO THE EDITOR


The Temperature Dependence of Viscosities of Liquids and Activation Energy
View Description Hide Description 
On the Possibility of a Chemical Synthesis of Diamond
View Description Hide Description 
The Nature of the Hydrogen Bond in KHF_{2}
View Description Hide Description 
The Pressure Coefficient of Viscosity
View Description Hide Description 
Method for Extending Study of Metal Ion‐Cation Exchanger Isotope Fractionation
View Description Hide Description 
An Approximate Product Rule for the E_{g} and E_{u} Frequencies of the X_{2}Y_{6} Molecule
View Description Hide Description
