Volume 20, Issue 3, 01 March 1952
Index of content:

Comparison of Absorption Spectrum of Uranium Tetrachloride with that of Praseodymium Trichloride and of Uranium Trichloride with that of Neodymium Trichloride
View Description Hide DescriptionThe absorption spectra of the following substances were taken at 77°K: crystals of anhydrous uranium tetrachloride, praseodymium trichloride, uranium trichloride, neodymium trichloride, uranium tetrafluoride, and a solution of uranium tetrachloride in a solvent consisting of propyl alcohol, propene, and propane.
The spectra of the ``iso‐electronic'' ions U^{+4} and Pr^{+3} exhibited a match in wave number of the centers of gravity of the separate absorption groups. The correspondence is especially evident between the spectrum of uranium tetrachloride and that of praseodymium chloride. The uranium tetrafluoride furnished a spectrum resembling the chloride but the separations of the absorption regions were not so clear.
These correspondences between U^{+4} and Pr^{+3} lead to term assignments for the basic and activated electronic states of U^{+4} since those of Pr^{+3} are known. Thus, the two least stable electrons of U^{+4} in the basic state and to a good approximation in the excited states lies within the 5f shell.
The interaction of the fields of the environment of the ions in the case of U^{+4} is greater than in the case of Pr^{+3} and is especially marked in their ^{1}I_{6} states. The greater interaction is in agreement with the greater effects of different ionic environments on the spectra of U^{+4}, the greater intensity of the ``forbidden'' transitions and other physical properties.
The correlation of the spectra of U^{+3} and Nd^{+3} is less distinct even though the same approach may well be valid in view of their similar color. The areas of absorption are not as distinctly separate and may even overlap.
A new method has been developed for obtaining the absorption spectra of very small anisotropic crystals.

Lifetimes of Active Molecules. I
View Description Hide DescriptionThe specific dissociation constants for various vibrationally excited molecules (methane, ethane, and propane) are inferred from an interpretation of experimental data on atomic crackingreactions and on the deuterization of free radicals. As was to be expected, these dissociation constants decrease with increasing number of degrees of freedom of the decomposing molecule.

Lifetimes of Active Molecules. II
View Description Hide DescriptionExperimental data on the atomic cracking of propyl radicals and on the deuterization of methyl radicals are compared with some theoretical calculations. With the aid of some assumptions concerning intramolecular energy transfer in the dissociating molecules involved in these and other reactions and concerning the corresponding activated complexes, data on a number of free radical reactions are correlated.

Unimolecular Dissociations and Free Radical Recombination Reactions
View Description Hide DescriptionThe steric and pressure effects associated with the recombination of free radicals both depend on the nature of the activated complex, and are therefore intimately related. From a consideration of the reverse process of unimolecular dissociation, some equations are derived for these properties using an extension of earlier transition state and quasi‐unimolecular theories. The present formalism differs from previous formulations of the latter in a number of ways, particularly in the expression used for the density of quantum states of the high energy molecules. Subsequent applications of the theory tentatively suggest that essentially all vibrational degrees of freedom of these molecules can contribute their energy to the vibrationally excited molecules. Consequently, vibrational anharmonicity would appear to be an important factor in intramolecular energy transfer. The present paper is an extension of a previously developed theory for the recombination of methyl radicals and iodine atoms.

Recombination of Methyl Radicals and Atomic Cracking of Ethyl Radicals
View Description Hide DescriptionThe characteristics of this atomic crackingreaction and of the pressure and steric effects associated with the recombination of methyl radicals are all intimately related. The available data on these reactions are correlated by means of a previously developed theory. Some experimental results on the steric factor and the data on the remaining subjects appear to be consistent with the assumption that the methyl radicals must be highly oriented with respect to each other in order that recombination occur. However, experimental steric factors of unity have also been reported in the literature. The corresponding assumption of no orientation leads to disagreement with the remaining data unless some of the rotational degrees of freedom of the ``active'' molecule, in addition to the vibrations, are assumed to be ``active.'' Even then, the difficulties are not completely removed. Further experimental work on these reactions is needed.

Cyanogen Flames and the Dissociation Energy of N_{2}
View Description Hide DescriptionThe behavior of cyanogen flames under a variety of conditions has been observed. Because of the relatively high stability of cyanogen, the behavior is considerably different from that of hydrocarbon flames. Because of the high stability of the combustion products, CO and N_{2}, the flame temperature can reach temperatures above 4800°K. Thus the cyanogen‐oxygen flame is the hottest natural flame known.
Spectroscopic observations have been made of both the reaction zone and the mantle of completely combusted gases. Rotational and vibrational temperature measurements using the CN violet system have indicated that the reaction zone is not in thermal equilibrium but that the mantle is in thermal equilibrium. Comparison of the calculated flame temperatures based on different heats of dissociation of N_{2} with the observed mantle temperatures has fixed DN_{2}=9.764±0.005 ev and D _{CN}=8.2±0.2 ev.

The Combination Band ν_{1}+ν_{2}+ν_{3} of Heavy Water Vapor
View Description Hide DescriptionThe vibration‐rotation band ν_{1}+ν_{2}+ν_{3} in the heavy water vapor (D_{2}O) spectrum has been remeasured. An analysis of the rotational structure has been carried out. From the interpretation of the data the value of the band center has been determined as well as the values of the reciprocals of inertia effective in this vibration state. The frequency ν_{1}+ν_{2}+ν_{3} is found to be 6533.4 cm^{−1}. The effective reciprocals of inertia are A(V)=15.70 cm^{−1}, B(V)=7.19 cm^{−1}, and C(V)=4.69 cm^{−1}.

The Infrared Spectrum of Nitrosyl Fluoride. Part I. Prism Spectrum
View Description Hide DescriptionThe prismspectrum of NOF is reported from 2–38μ. Twelve absorption bands were observed and identified in terms of the fundamental frequencies ν_{1}(N–O)=1844.03 cm^{−1}, ν_{2}(bending)=521 cm^{−1}, ν_{1}(N–F)=765.85 cm^{−1}. Comparison with the companion molecules NOCl and NOBr is made.

The Infrared Spectrum of Nitrosyl Fluoride. Part II. Rotational Analysis of ν_{1} and ν_{3}
View Description Hide DescriptionAnalysis of the rotational structure of ν_{1} and ν_{3} infrared bands of nitrosyl fluoride at 1844.03 and 765.85 cm^{−1} showed that this molecule is nearly a prolate symmetric top. The following rotational parameters based on symmetric‐top theory were found and are expressed in cm^{−1}: for ν_{1}, 2B″=0.7515, 2B′=0.7483, D_{J} ′+D_{J} ″=0.000027_{5}; for ν_{3}, 2B″=0.7521, 2B′=0.7503, D_{J} ′+D_{J} ″=0.000002_{36}. The anharmonic term x _{33} was found to be −1.06 cm^{−1} from ν_{3}. The anharmonic term x _{12} was found to be +1.03 cm^{−1} from ν_{1}.

Proton Relaxation in Paramagnetic Solutions
View Description Hide DescriptionThose paramagneticions, such as Fe^{+++}, Mn^{++}, and Gd^{+++}, which are in S spectroscopic states; and those ions, such as Cr^{+++} and Cu^{++}, which are effectively in S states through complete quenching of the orbital component, have been shown to produce reciprocal protonrelaxation times proportional to the square of the magnetic moment. Those ions such as Co^{++}, Nd^{+++}, and Dy^{+++}, with strong spin‐orbital coupling, have been shown to be, in equivalent concentration, about ten times less effective.

Kinetics of a Fixed Bed System for Solid Diffusion into Spherical Particles
View Description Hide DescriptionA solution is presented to the general problem of the transient behavior of a linear fixed bed system where the rate of adsorption is determined by the combined effect of a liquid film and soliddiffusion into spherical particles. The result obtained is an expression for the effluent following a sudden change in the influent concentration. It is given in two forms: (a) An exact solution to the problem in the form of an infinite integral suitable for numerical integration. (b) An approximation to the exact solution together with an expression for the leading error term in this approximation. The approximate solution and the error term are given as closed trigonometric expressions which can be easily evaluated.

On the Frequency Dependence of the Viscosity of High Polymer Solutions
View Description Hide DescriptionThe ultrasonic shear waves technique introduced by Mason offers the possibility of measuring the viscosity of a liquid in laminar flow at ultrasonic frequencies. A dilute high polymer solution exhibits, besides the usual shear viscosity, a shear elasticity as well for rather low frequencies (of the order of 10^{5}). A theory is given for infinitely dilute solutions using as a model for the chain molecule an elastic sphere. This model, which the author has used previously for the theory of flow birefringence, represents the limiting case of complete impermeability. The theory is compared with experimental results of Baker, Mason, and Heiss and of Rouse on polystyrene. The theory gives a good picture of the phenomena as a whole and is particularly successful in describing the apparently anomalous temperature dependence of the elasticity coefficient of polystyrene solutions at high frequencies. The interest of combining results on flow birefringence and on high frequency viscosity measurements is emphasized, as well as the possibilities of the latter technique for determining the molecular elasticity and internal viscosity as a function of frequency. On the basis of preliminary results for a sample of polystyrene of molecular weight 130,000, the tentative conclusion is given that the elastic sphere model will not have a quantitative significance for chains of such a low molecular weight and that it might be necessary to take into account the effect of permeation.

The Pyrolysis of n‐Propyl‐Benzene and the Heat of Formation of Ethyl Radical
View Description Hide DescriptionThe investigation of the pyrolysis of n‐propyl‐benzene by the ``toluene carrier'' technique revealed that this compound decomposes initially into benzyl and ethyl radicals:Ethyl radicals decompose rapidly into ethylene and hydrogen atoms. The hydrogen atoms reacting with toluene produce H_{2} or CH_{4}. The rate of the initial dissociation process can be measured by the rate of formation of C_{2} hydrocarbons.
It was shown that the decomposition is a homogeneous, first‐order gas reaction. The activation energy was estimated at 57.5 kcal/mole, the frequency factor at 3.10^{12} sec^{−1}. It appears that the activation energy is equal to D(C_{6}H_{5}·CH_{2}–C_{2}H_{5}). Since the heat of formation of benzyl radicals is known, the estimation of D(C_{6}H_{5}·CH_{2}–C_{2}H_{5}) leads to the value for the heat of formation of ethyl radicals, i.e.,The latter value makes it possible to evaluate various relevant bond dissociation energies, i.e., D(C_{2}H_{5}–H) ≈94±4 kcal/mole.

The Pyrolysis of n‐Butyl‐Benzene and the Heat of Formation of n‐Propyl Radical
View Description Hide DescriptionIt has been shown that the pyrolysis of n‐butyl‐benzene results in its dissociating into benzyl and n‐propyl radicals,and the propyl radicals subsequently decompose to give ethylene and methyl radicals, . By the use of the toluene‐carrier technique, it has been possible to measure the rate of reaction (1) by measuring the rate of formation of ethylene. The activation energy of reaction (1) has been thus determined at 65 kcal/mole. The latter value has been identified with the C_{6}H_{5}·CH_{2}–C_{3}H_{7}bonddissociation energy, i.e., . Since and , one may compute the unknown value for ΔH_{ f }(n—C_{3}H_{7}), namely, ΔH_{ f }(CH_{3}·CH_{2}·CH_{2}·)≈24 kcal/mole. Having ΔH_{ f }(n—C_{3}H_{7}), we calculated the dissociation energy of the primary C–H bond in propane at . This value for D(CH_{3}·CH_{2}·CH_{2}–H) is compared with the findings of other workers. It agrees well with the recent findings of Stevenson, although the existing indirect chemical evidence seems to require a lower value.

Kinetics of Solidification of Supercooled Liquid Mercury Droplets
View Description Hide DescriptionThe solidification rate of supercooled liquidmercurydroplets is strongly dependent upon the nature of foreign substances on their surface or suspended in them. Droplets (2–8 microns diameter) coated with mercury laurate solidify at rates that are proportional to droplet volume and satisfactorily described by the theory of homogeneous nucleation of crystals. Droplets coated with mercury acetate solidify with frequencies that are proportional to droplet area and in good agreement with the theory of heterogeneous nucleation.
The results on Hg_{2}I_{2}‐coated droplets are interpreted on the hypothesis that nucleation is effected by suspended crystallites of one kind. For some droplet dispersions the kinetic results are apparently best described by a multiplicity of solidification frequencies/area or volume. Various hypotheses for this multiplicity are considered.

The Structure and Internal Motion of 1,2‐Dichloroethane
View Description Hide DescriptionThe structure and internal motion of 1,2‐dichloroethane were investigated by means of electron diffraction. Values were obtained for several distances involving hydrogen atoms. In addition, the average amplitude of vibration was found for many of the distances. The predominant isomer was in the trans‐form and the less abundant one was in the gauche‐positions at 109±5° from the trans‐equilibrium position. The amount of gauche‐isomer was found to be 27±5 percent at 22°C.

Theory of Resonance Topology of Fully Aromatic Hydrocarbons. I
View Description Hide DescriptionThe computation of resonance forms of any specified number of unpaired electrons (``radicality'') of aromatic hydrocarbons is reduced to combinatorial problems, many of which can be solved on the basis of five simple lemmas. In this first of two papers, only unexcited (Kekulé) forms are computed. Algorithms are deduced for deriving the number N of such forms for classes covering practically all nonreticulate aromatic hydrocarbons and a number of singly, doubly, or triply infinite series of reticulate ones. One lemma leads to a simplification of the combinatorial problems from the hexagonal to the square lattice (``dot diagrams''). Points of purely mathematical interest arise and may merit further study.

Diffusion Coefficients of the Systems CO_{2}–CO_{2} and CO_{2}–N_{2}O
View Description Hide DescriptionThe radioactive tracer, C^{14}O_{2}, has been used to measurediffusion coefficients for the systems CO_{2}–CO_{2} and CO_{2}–N_{2}O in the temperature range −78° to 90°C. The apparatus, which is the Loschmidt type, uses saturation ionization currents as a measure of the extent of diffusion. The characteristics of this type of apparatus have been analyzed in detail to obtain working equations which permit use of experimental measurements at both small and large values of the time.
The present results, whose accuracy is estimated at 1.5 percent, agree with those obtained by other investigators using different experimental methods. Although the difference is small, 2.6±0.3 percent, the diffusion coefficient for the system CO_{2}–N_{2}O is consistently larger than that for CO_{2}–CO_{2} over the entire temperature range.

Polarized Infrared Spectrum of Sodium Nitrite
View Description Hide DescriptionThe infrared spectra of single crystals of NaNO_{2} were obtained in polarized light. The polarization behavior of the strong absorptions leads to the following assignment of the nitrite fundamentals ν_{1} = 1325 cm^{−1}, ν_{2} = 831 cm^{−1}, and ν_{3} = 1360 cm^{−1}. Other bands of lower intensity are observed. A combination band (ν_{1}+ν_{2}) at 2670 cm^{−1} shows a polarization behavior in accord with site group selection rules. An effect is discussed which results from the use of an infrared microscope in the investigation of polarizationspectra.

The Molecular Structure of MoF_{6}, WF_{6}, and UF_{6} from Infrared and Raman Spectra
View Description Hide DescriptionThis paper is a report of the complete investigation and interpretation of the Raman and infrared spectra of MoF_{6}, WF_{6}, and UF_{6}. The Raman photographs of MoF_{6} and WF_{6} exhibit three intense lines in accord with earlier measurements on UF_{6} in solution. The exploration of the infrared spectra of MoF_{6} and WF_{6} from 2–40μ yielded a great many bands for each molecule. Two of these were quite intense in each case and were identified as fundamentals while the remaining weaker ones were identified as overtones and combinations. The infrared spectrum of UF_{6} was repeated from 2–17μ with substantially the same results as in the earlier work. Extension of this spectrum from 17–40μ was made and three new bands were discovered.
Because of the great similarity of the spectra of these three molecules, an interpretation of the bands was made along lines suggested by Bigeleisen, Mayer, Stevenson, and Turkevich for UF_{6}, that the molecule belongs to the point group O_{ h } and has totally symmetric octahedral structure. It was possible to fit all but five very weak bands into an identification scheme based on this model. The success of this effort is taken to be conclusive evidence in favor of the O_{ h } type symmetry.