Volume 36, Issue 7, 01 April 1962
Index of content:
36(1962); http://dx.doi.org/10.1063/1.1701253View Description Hide Description
The broad‐line protonmagnetic resonance spectra of polycrystalline samples of the compound formulated empirically as β—UO32H2O have been investigated in the temperature range 77°—350°K. The second moment of the absorptionspectrum at 77°K indicates that some of the protons in the crystal are not present in water of crystallization and an analysis of the line shape shows that the compound is the trihydrate of pyrouranic acid, U2O5(OH)2·3H2O. The chemical properties of the compound are accounted for on the basis of this formula.
36(1962); http://dx.doi.org/10.1063/1.1701254View Description Hide Description
The broad‐line protonmagnetic resonance spectra of polycrystalline samples of the compound formulated empirically as β‐UO3H2O have been examined in the temperature range 77°—370°K. Analysis of the spectrum at 77°K shows that the compound is uranyl hydroxide UO2(OH)2. The interrelationships between the different phases in the uranium‐trioxide—water system which involve the β‐modifications of the ``uranium trioxide hydrates'' are discussed.
36(1962); http://dx.doi.org/10.1063/1.1701255View Description Hide Description
The quantum statistical cluster model of the saturated vapor of mixtures of helium isotopes He3 and He4 has been subjected to a program on an electronic digital computer. The results permit a prediction of the saturated‐vapor density as a function of the mixture concentration for each of the isotherms given by experimental data on the saturated‐vapor pressure of the same mixtures.
Emission of Vacuum Ultraviolet Radiation from the Acetylene‐Oxygen and the Methane‐Oxygen Reactions in Shock Waves36(1962); http://dx.doi.org/10.1063/1.1701256View Description Hide Description
The C2H2–O2 and CH4–O2reactions were studied by observing the emission of vacuum ultraviolet radiation from shock waves in gas mixtures containing 85 to 99% Ar and with 2×10−6≤[O2]≤60×10−6 moles/l. In both reactions after an induction period of length ti, the emission intensity rises exponentially with a time constant τ and then decreases about as rapidly. For the C2H2–O2reaction, emission predominantly in the wavelength range 1500<λ<1700 A was observed. It was estimated that one photon was emitted in this range per 3×104 C2H2 molecules passing through the reaction zone at 1840°K. The integrated emission intensity depends on the temperature as if the process producing it has an activation energy of 15 kcal/mole. In the range of conditions 0.65≤[O2]/[C2H2]≤3.6 and 1300≤T≤2200°K it was found that . The data for τ from mixtures with 0.65≤[O2]/[C2H2]≤7.7 fit the expression with a standard deviation of a factor of 1.4 in τ. In the first approximation the values of [O2]ti and [O2]τ are functions of temperature only. In lean mixtures, however, [O2]τ and especially [O2]ti are larger than the average. These results are explained in terms of a branching chain mechanism in which the reaction H+O2=OH+O plays the dominant role in controlling the rate.
The CH4–O2reaction was studied in the range of conditions 0.5≤[O2]/[CH4]≤4.5 and 1800≤T≤2700°K. The emission intensities are an order of magnitude smaller and more strongly temperature dependent, τ is a factor of 2 or 3 larger, and ti is a factor of 20 larger at the low temperatures and a factor of 4 larger at the high temperatures than for the C2H2–O2reaction. The kinetic implications of these findings are discussed.
36(1962); http://dx.doi.org/10.1063/1.1701257View Description Hide Description
The rate of dissociation of Cl2 has been measured at high temperatures in a shock tube. The temperature dependence curves, regardless of Cl2 concentration, are fitted by invoking 4° of freedom of internal motion and an activation energy of 48 kcal.
From the temperature TH at which the decrease of Cl2 optical density through dissociation matches the increase due to the cooling, by reactionenthalpy, the dissociation energy of Cl2 has been calculated. The agreement with the well‐known spectroscopic value gives a confirmation of the shock‐tube temperature measurements and of many assumptions made in the other calculations.
36(1962); http://dx.doi.org/10.1063/1.1701258View Description Hide Description
Paramagnetic ions in water shorten the spin‐lattice relaxation timeT 1 of the water protons. The effectiveness with which the relaxation takes place depends not only on the ion concentration, but also on the environment of the paramagnetic ion. In some cases where the paramagnetic ion is bound to a large molecule such as DNA, it shortens the protonrelaxation time to a greater extent than when it is in solution. By measuring T 1 by a pulsed nuclear magnetic resonance method, we have studied the binding of transition metal ions to DNA. We have obtained estimates for the number of available binding sites as well as information on the type of site which the various ions are bound to.
36(1962); http://dx.doi.org/10.1063/1.1701259View Description Hide Description
The ultraviolet spectrum of benzene induced by oxygen was studied in the wavelength range 2200–3000 A. From the experimental results obtained, the electronic absorption of this system is interpreted as a charge‐transfer absorption induced during a chance contact between benzene and oxygen without formation of stable complexes. The general formula obtained by Tsubomura and Mulliken was used together with experimentally obtained energy of the charge‐transfer state (about 5.6 ev) to calculate the intensity of the charge‐transfer absorption borrowed from the 1 A 1g —1 B 1u and 1 A 1g —1 E 1u absorptions of benzene. The calculated intensity is in excellent agreement with experimental data, indicating that it is the interaction of the charge‐transfer state with the benzene 1 B 1u and 1 E 1u states which is responsible for the observed intensity of the charge‐transfer absorption.
Experimental evidence is also presented which indicates that the two previously observed weak absorptions in benzene at 4.2 and 4.9 ev, which several workers have conjectured might be the transitions to the higher triplet states, are not multiplicity‐forbidden transitions. The absorption at 4.2 ev has been interpreted as due to an impurity and the absorption at 4.9 ev has been tentatively ascribed as due to a solvent effect whose nature is not yet understood.
36(1962); http://dx.doi.org/10.1063/1.1701260View Description Hide Description
This paper develops a theory which interrelates several aspects of the behavior of rubberlike materials through the extension of statistical mechanical treatments which have been developed for fluids.
An additional term of the partition function of bulk polymers is found. This term decreases as the temperature increases.
It has been possible partially to evaluate this added term by replacing important portions of it with thermodynamic expressions appropriate to liquids. This eliminates the necessity of attempting to evaluate series which would be difficult at low temperatures. In this way it is shown with a minimum of approximation how it is that the well‐known and simple theory of rubber is valid at high temperatures and tends to show deviations as the temperature decreases. It further shows that the probability of finding a chain far removed from its most probable position decreases with decreasing temperature. It is hoped that these techniques can be used to explain some of the changes which occur in solid polymers as the temperature is lowered.
36(1962); http://dx.doi.org/10.1063/1.1701261View Description Hide Description
In this paper, we have assembled the molecular constants needed for the computation of the thermodynamic functions and the standard heats of formation for most of the low molecular‐weight C m H n species which may be present in equilibrium at very high temperatures under a variety of external conditions. In the literature, the essential data are recorded for only 34 of the 73 species considered, and for those which are listed thermodynamic functions are given for the lower third of the temperature range included in our computations.
In order to arrive at an estimate for each of the 39 molecular fragments the following steps were used: (a) assign to each fragment a formal electronic configuration (this establishes the framework for the analogies to be sought and fixes its ground‐state electronic degeneracy); (b) select several paths for dissociation which, when followed, generate the species under consideration in going from one molecule of known structure to another; and (c) estimate the bond dissociation energies from known simple dissociations, corrected for nonlocalized electron interactions. These corrections are relatively small and have been obtained by comparing similar reactions for known species. The success of this procedure is demonstrated by the good agreement found between a few predicted and experimental heats of formation. The self‐consistency of the method is proven by the close check between estimates made for the heats of formation for many species following independent paths.
36(1962); http://dx.doi.org/10.1063/1.1701262View Description Hide Description
Thermodynamic functions for a large number of C–H molecules were computed from the molecular parameters presented in the preceding paper. These functions and others taken from the literature were used to calculate the equilibrium composition of the C/H gas phase system over the composition range C/H=1/10, ¼, ½, 1, 2, 3. The temperature range was 500–5000°K, and the pressures used were 0.1, 1.0, and 10 atm. In additional calculations solid carbon was assumed to be present. Representative graphs of the results are presented. They show that the familiar, stable hydrocarbons are not important in characterizing the equilibrium composition above 2000°K. Acetylenic molecules and their radicals dominate the composition at high temperatures.
36(1962); http://dx.doi.org/10.1063/1.1701263View Description Hide Description
The decomposition of cyanogen has been studied using shock waveheating. Kinetic and equilibrium concentrations of CN were obtained spectrophotometrically on 1, 5, and 10% solutions in Ar, in the incident and reflected shock regions. These data giveThe dissociationreaction proceeds via a second‐order process with a rate constant fitted byThe three‐body recombination‐rate constant is given byPrevious data on this reaction have been summarized and critically analyzed.
36(1962); http://dx.doi.org/10.1063/1.1701264View Description Hide Description
The complex dielectric constants of supercooled, branched alkyl halides, if plotted on the complex plane, are described by the skewed‐arc expression of Davidson and Cole. Contrary to this, dielectric properties of a great majority of straight‐chain alkyl halides at room temperature are in fair accord with the circular‐arc rule of Cole and Cole. Glarum considered the former behavior as an anomaly and presented a defect diffusion model. In the present paper another new mechanism is proposed. According to this, the dielectric properties of all these nonrigid molecules at lower temperatures are explainable from the same principle which explains the dielectric properties of the same substances at higher temperatures. In this mechanism a distribution of relaxation times between two limits is assumed. Skewed arcs similar to the Davidson‐Cole type are to be observed at low temperatures, because the ratio of two limiting relaxation times becomes considerably larger with decrease in temperature.
36(1962); http://dx.doi.org/10.1063/1.1701265View Description Hide Description
A general expression is derived for the vapor pressure of a charged drop situated at the center of a spherical container with perfectly reflecting wall. It is assumed that the liquid is conducting, the medium devoid of external fields of force and that the gaseous phase comprises both neutral and charged molecules. The vapor pressure in the case where the radius of the container is close to that of the drop was evaluated after solving the equation for the distribution of the charged molecules near the surface.
It was shown that, in general, the vapor pressure tends to infinity with increasing distance from the droplet, thus rendering impossible the attainment of a thermodynamic equilibrium in the atmosphere. The vapor pressure of a charged mercurydroplet is given as a numerical example.
36(1962); http://dx.doi.org/10.1063/1.1701266View Description Hide Description
An analysis of the electron spin resonance spectra of X‐irradiated single crystals of MgHPO3·6H2O has shown that the principal long‐lived paramagnetic species produced by irradiation is a PO3 fragment. This fragment retains the same orientation within the unit cell as the undamaged molecule. The parallel and perpendicular components of the diagonal (electron‐spin) — (nuclear‐spin) coupling dyadic for the phosphorus nucleus are found to be of the same sign and of magnitude 2210 and 1730 Mc, respectively. These results are compared with theoretical values of the distributed dipole and contact hyperfine interactions calculated on the assumption that the unpaired electron is located in an sp 3 hybrid orbital centered on phosphorus.
36(1962); http://dx.doi.org/10.1063/1.1701267View Description Hide Description
In the absence of a suitable absorption band for a rare‐earth ion, its fluorescence can be excited by ultraviolet radiation through interaction with the host lattice when the latter is excited near its absorption edge. Rare‐earth ion fluorescence can be enhanced by localized lattice processes at frequencies lower than the absorption edge. Longer range effects occur as the absorption region of the crystal is approached. The enhancement due to these processes can be quenched by effects resulting from an increase in rare‐earth ion content.
Effects of Rare‐Earth Ion Substitution upon the Fluorescence of Terbium Hexa‐Antipyrene Tri‐Iodide and Sodium Europium Tungstate36(1962); http://dx.doi.org/10.1063/1.1701268View Description Hide Description
The directions and relative ease of transfer of energy between different rare‐earth hexa‐antipyrene aggregates are compared by examining the influences of rare‐earth substitutions upon the fluorescence of terbium hexa‐antipyrene tri‐iodide. Data concerning much shorter range interactions between rare‐earth ions are obtained employing the sodium rare‐earth tungstates. Energy transfer and quenching effects show systematic relationships to the total orbital angular momentum of the 4f subshell electrons of the rare‐earth ions.
36(1962); http://dx.doi.org/10.1063/1.1701269View Description Hide Description
A complete solution to all orders of perturbation theory is obtained without recourse to numerical approximation and it is shown that all corrections to the energy of higher order than the second do in fact vanish although the corrections to the wave function do not.
Critical Opalescence of Polystyrene in Cyclohexane : Range of Molecular Forces and Radius of Gyration36(1962); http://dx.doi.org/10.1063/1.1701270View Description Hide Description
Visible lightscattered by binary mixtures of critical concentration of ordinary molecules at small temperature distances above the critical temperature shows an angular dissymmetry which can be related to the range of molecular forces. The theory of critical opalescence is extended to polymer solutions of high molecular weight for which an additional dissymmetry due to the extension of the polymer coil is observed. The range of molecular forces and the radius of gyration at the critical concentration can be determined simultaneously by measurements of the angular dependence of scattered intensity at different temperatures above the critical temperature. Results of experiments on a solution of polystyrene (Mn =2 820 000) in cyclohexane are reported.
Absorption Spectrum of Phenazine Single Crystals at 77° and 4.2°K in the Region of the n→π Transition36(1962); http://dx.doi.org/10.1063/1.1701271View Description Hide Description
The absorptionspectrum of the long‐wavelength edge of phenazine and 1,2‐ 3,4‐dibenzphenazine has been measured at 77° and 4°K. A sharp band system is evident in crystals of each of these molecules. These systems have been assigned as n—π* transitions. For phenazine the band system involves a progression of a 398 cm−1 vibration frequency which has been attributed to a symmetric ring angular distortion. The band at 22 881 cm−1 was assigned as the allowed electronic origin of the transition (nπ*) B 1u ↔A 1g . The spectrum contained a weak component which was attributed to a symmetry forbidden n→π* transition lying at less than 96 cm−1 below the allowed origin. The analysis of the spectrum suggests that the phenazine molecule is twisted in the nπ*excited state. The 0–0 band only appears in the b‐polarized spectra and this indicates that crystal induced mixing with higher ππ* levels is occurring.
36(1962); http://dx.doi.org/10.1063/1.1701272View Description Hide Description
Several LiH valence‐bond wave functions of nonorthogonal structure are analyzed in terms of the ``natural orbitals'' and the ``structure projections.'' These wave functions are shown to agree well among themselves and to be closely similar to molecular orbital wave functions which give similar total energies.