Index of content:
Volume 19, Issue 2, 01 February 1951
The Kinetics of the Forward and Reverse Reactions for the Vapor Phase Thermal Bromination of Chloroform19(1951); http://dx.doi.org/10.1063/1.1748148View Description Hide Description
The rates of the forward and reverse reactions for the homogeneous vapor phase thermal bromination of chloroform, CHCl3+Br2=CBrCl3+HBr(A), have been measured in the range 420–455°K and are given by ; where , log10 kr =(−32,930/4.575T)+12.70. The mechanism of the reaction is,where log10 k 1=(−9300/4.575T)+9.36, log10 k 4=(−10,200/4.575T)+10.91, (k 2/k 3)=0.040. The activation energies imply that the CH bond in CHCl3 is weaker than that in CH4 by 6+E 2 kcal and the Cl3C–Br bond is weaker than the H3C–Br bond by 11+E 2 kcal, where E 2, the activation energy of reactions (2) and (3), is estimated to be ≤7 kcal.
The directly determined equilibrium constant for (A) agrees well with the kinetic value, Ke =k 1 k 3/k 2 k 4; log10 Ke =(900/4.575T)−0.15. From these data, for CCl3Br, ΔH° f,298°=−9.4 kcal, S°298°=80 eu S° was calculated as 80 eu from electron diffraction and spectroscopic data.
The equation for k 4 is identical with that obtained in a previous study of the exchange of radioactive bromine between Br2 and CBrCl3, thus proving this free radical mechanism for the exchange reaction.
Solid Solutions of the Alkali Halides. II. The Theoretical Calculation of Lattice Constants, Heats of Mixing, and Distributions between Solid and Aqueous Phases19(1951); http://dx.doi.org/10.1063/1.1748149View Description Hide Description
In all previous attempts to calculate interionic distances and heats of mixing for alkali‐halide solid solutions, uniform anion‐cation distances throughout a given solution have been postulated. In the present paper, the more logical assumption is made that the ions are not at a constant nearest‐neighbor distance, but take up positions of minimum potential energy relative to one another. A method making use of the Born‐Mayer‐Huggins equation for the lattice energies of the alkali halides is developed for the determination of these positions in a simplified model and for the calculation of the potential energy associated with the resultant lattice. From these values, the average lattice constant, the heat of mixing, and the activity‐coefficient ratio in the solid solution are calculated; from the latter is obtained the theoretical distribution ratio between solid and aqueous solutions. Values of the listed properties have been calculated at various mole fractions for the solid solutions KBr–KCl, RbBr–RbCl, RbCl–KCl, RbBr–KBr at 25° and have been found to be in good agreement with the experimental data available.
19(1951); http://dx.doi.org/10.1063/1.1748150View Description Hide Description
The electronic energy levels of a molecular crystal are classified according to representations of the space group of the crystal. When the ground state of the free molecule is totally symmetric, the number of crystal levels active in the absorption of light can be determined by a simple rule. Applications to rare earth crystal spectra and to the crystalline benzene spectrum are discussed.
19(1951); http://dx.doi.org/10.1063/1.1748151View Description Hide Description
The vibration frequencies of a linear chain of the type ...XZYZXZYZ... are calculated and discussed. The chain is considered to be a model for a mix‐crystal of the components XZ and YZ. Only one vibration is strongly active enough to give a reflection band, its frequency being intermediate between those of the components, in agreement with observation. Another vibration, at lower frequencies, is very weak or inactive, depending on the exact nature of the mix‐crystal structure—whether the ions X and Y are distributed regularly or statistically.
19(1951); http://dx.doi.org/10.1063/1.1748152View Description Hide Description
The reactions of methyl radicals formed by the photolysis of acetone and deutero‐acetone with nine paraffins have been studied from 25°C to 340°C. If the combination of methyl radicals requires no activation energy, then the activation energies found for the abstraction of hydrogen atoms are as follows: ethane, 10.4; n‐butane, 8.3; 2‐methyl propane, 7.6; n‐pentane, 8.1; 2,2‐dimethyl propane, 10.0; n‐hexane, 8.1; 2,3‐dimethyl butane, 6.9 and 7.8; 2,3,4‐trimethyl pentane, 7.9; 2,2,3,3‐tetra‐methyl butane, 9.5 (all values in kcal). The collision theory steric factors of these reactions are all less than 10−3.
19(1951); http://dx.doi.org/10.1063/1.1748153View Description Hide Description
The abstraction of hydrogen atoms from eight olefins by deutero‐methyl radicals formed in the photolysis of deutero‐acetone has been investigated over a temperature range from 180°C to 340°C. The activation energies of those reactions have been determined relative to the activation energy of the combination of methyl radicals. The activation energies are 10.0 kcal for ethylene and 7.5±0.3 kcal for the other seven olefins. The collision theory``P'' factors are all of the order of 10−3 or less.
19(1951); http://dx.doi.org/10.1063/1.1748154View Description Hide Description
The mercury (3 P 1) sensitized decomposition of normal and iso‐butane at high temperatures is described. The reactions of both hydrocarbons result in the formation of decomposition products of butyl radicals at temperatures above 250°C. In the case of normal butane this reaction gives methane, ethane, ethylene, and propylene; and with iso‐butane, only methane and butane. Activation energies are assigned to the various radical decomposition steps.
19(1951); http://dx.doi.org/10.1063/1.1748155View Description Hide Description
The vapor phase photolysis of formaldehyde at wavelength 3130A has been investigated at high temperatures. The experimental results indicate that the rate of hydrogen formation is directly proportional to the first power of the intensity of absorbed light, and at constant intensity of absorbed light the rate is accelerated by the addition of formaldehyde or inert gas with about equal efficiency. Propylene and nitric oxide inhibit the rate. The mechanism of the photo‐decomposition is suggested to be:
19(1951); http://dx.doi.org/10.1063/1.1748156View Description Hide Description
A study has been made of the mercury photo‐sensitized decomposition of methyl chloride over a pressure range of 10 mm to 640 mm, a temperature range of 72°C to 328°C, and a threefold variation in light intensity. The stoichiochemical equation for the reaction at 250°C and 160‐mm pressure is . At high pressures, the following mechanism is shown to be consistent with the experimental results:The first of these reactions is shown to have a quantum efficiency of unity. From the effect of added CO2 on the reaction, an approximate value of σ2=5.7×10−16 cm2 has been calculated for the quenching cross section of methyl chloride.
19(1951); http://dx.doi.org/10.1063/1.1748157View Description Hide Description
The infrared and Raman spectra of liquid and solid nitric oxide have been studied in an attempt to prove the existence of the (NO)2 molecule. The liquid has four strong Raman lines at 1861, 262, 196, and 167 cm−1; two weaker Raman lines at 1760 and 487 cm−1; and two strong infrared bands at 1863 and 1770 cm−1. Several weaker infrared bands can be satisfactorily assigned as combinations by using the observed strong infrared and Raman frequencies. The spectra were found to be due exclusively to a dimer which probably exists as a bent ONNO molecule.
19(1951); http://dx.doi.org/10.1063/1.1748158View Description Hide Description
Considering the carbon‐hydrogen bond as an isolated unit, and including the effect of the interelectronic repulsions, a quantum‐mechanical calculation of the polarity of this bond is made. The bond dipole is found to be C+H−. On the basis of these calculations, the effect of change in hybridization on the C–H dipole, and the similar effect of replacement of hydrogen in a hydrocarbon by a highly electronegative substituent, is qualitatively discussed. It is predicted that, in both cases, there will be a tendency for the dipole moment to reverse itself.
19(1951); http://dx.doi.org/10.1063/1.1748159View Description Hide Description
In the behavior of the thermal hydrogen‐oxygen reaction there is an apparent discrepancy. The second explosion of the thermal reaction occurs over a wide range of low temperatures down to 350°C, even though the rate of steady thermal reaction is very nearly zero. The authors used a criterion stated by Lewis and von Elbe to show that the same volume chain which explains the reaction at higher temperatures applies to the low temperature range. The inhibition period within the low pressure explosion region and the branching of the chains are briefly discussed.
19(1951); http://dx.doi.org/10.1063/1.1748160View Description Hide Description
Previous experiments had left open the possibility that the melting phenomena of the methyl siloxanes might be an exception to the universal behavior of all other known substances, namely, that there is no experimental evidence for a critical point between solid and liquid or for a maximum or temperature asymptote on the melting curve. The special apparatus necessary for an examination of the siloxanes in this respect has been constructed, and it has been found that they do not constitute any exception. The abnormality of their melting behavior is sufficiently explained by abnormal subcooling effects connected with the abnormally large increase of viscosity of these substances under pressure.
19(1951); http://dx.doi.org/10.1063/1.1748161View Description Hide Description
The concept of intramolecular binding is given a precise definition in a way that relates binding to the forces acting on the nuclei in a diatomic molecule. A consequence of the definition is that the space around the nuclei may be separated into binding and antibinding regions. These regions are described and they depend on the Coulomb law of force and on the ratio of the nuclear charges: the internuclear distance is simply a scale parameter. The influence of a single electron on the binding due to other electrons is briefly discussed.
19(1951); http://dx.doi.org/10.1063/1.1748162View Description Hide Description
The nuclear spinrelaxation times of the protons in dilute solutions of sodium in liquid ammonia have been measured at 30 Mc/sec over the room temperature‐dry ice range. The data can be interpreted on the basis of an associative equilibrium existing between paramagnetic and diamagnetic solute species. This is in agreement with the results of the conventional magnetic susceptibility measurements. The paramagnetic component of the solutions seems to be about one‐tenth as effective as a cupric ion in inducing nuclear relaxation.
19(1951); http://dx.doi.org/10.1063/1.1748163View Description Hide Description
Electron interference has been obtained by the transmission of 50‐kv electrons through thin liquid films of the silicones: DC703, DC500, and DC200; and the hydrocarbons: octoil, castor oil, and glycerine. A commercial electron microscope (RCA, type EMU) was used after modification of the specimen holder. The electron diffraction patterns obtained from the hydrocarbons were all practically the same. They were characterized by three complete diffraction halos. The Bragg spacings (d) for octoil were found to be 4.93A, 2.26A, and 1.23A. These distances compare closely with values obtained previously for electron diffraction in liquid phytol (C20H39OH), Nujol, and Cenco pump oil. The patterns obtained for the silicones were significantly different from those of the hydrocarbons. Four complete rings were observed for DC703, giving d values of 8.17A, 4.18A, 1.97A, and 1.27A. The DC500 and DC200 fluids showed only three rings with Bragg spacings of 7.40A, 3.96A, and 1.27A; the ring which occurred at d=1.97A for DC703 was absent. The accepted Si–O distance of 1.60A is in agreement with the above data; however, the Si–O–Si–O angle is found to be approximately 104°. Such conclusions were confirmed by density calculations.
19(1951); http://dx.doi.org/10.1063/1.1748164View Description Hide Description
It is shown that orthogonal atomic orbitals, as introduced by Wannier for solid‐state problems and by Löwdin for molecular problems, do not lead to a simple way of eliminating the non‐orthogonality problem in the Heitler‐London method. Thus, if a calculation of the H2 molecule is made using orthogonal atomic orbitals in place of ordinary atomic orbitals, proceeding by the simple Heitler‐London method, the resulting diagonal energy of the lowest state shows no energy minimum. The orthogonal atomic orbitals are suitable for use, however, if we make a complete perturbation calculation, and for this purpose they are as convenient as LCAO's, but not appreciably superior.
19(1951); http://dx.doi.org/10.1063/1.1748165View Description Hide Description
The Jones reductor can be made to assume a steady state of operation in the reduction of ferric sulfate. Its behavior under such conditions strongly indicates that the rate limiting process is the diffusion of ferric ion through a static film of solution around the solid particles. This is particularly evidenced by the exponential dependence of the extent of reduction on the square root of the flow rate. The behavior of the reductor as a function of zinc ion concentration, zinc amalgam particle size, and temperature, also agrees with this result.
19(1951); http://dx.doi.org/10.1063/1.1748166View Description Hide Description
An apparatus has been designed and built for the study of low fluorescence intensities. The ``blue'' fluorescence of acetone vapor has been investigated from pressures of about 1 mm to pressures of about 200 mm, at temperatures from 25°C to 300°C both with and without the addition of foreign gases, such as O2, NO, N2, and (CH3CO)2.
It is found at temperatures over about 100°C, and in the presence of O2 and NO, that there is no structure in the fluorescence of acetone which can be detected, and the wavelength of the intensity maximum is different from that found at 25°C. The shapes of the quenching curves are such as to demand at least two upper electronic states which participate directly or indirectly in the fluorescence phenomenon. This is in agreement with previous work. The photo‐chemical data necessitate a high probability of predissociation of the initially formed state together with a small lifetime probably determined by the rate of predissociation. At least one other upper state, formed from the first through collisions, must be relatively insensitive to collisional deactivation. A complete theory cannot be given, but some important aspects are clear.
19(1951); http://dx.doi.org/10.1063/1.1748167View Description Hide Description
The kinetic theory cross‐section integrals are calculated for the square‐well molecular potential model. Parameters are determined for light gases from viscosity measurements and are used to reproduce viscosity data for pure gases and binary mixtures.
Comparison is made between these parameters and those found from compressibility data and other transport properties. Different sets of parameters are apparently required for each transport property in order to account for the experimental results. The thermal diffusion constant for this model is a function of temperature and one other parameter. Excellent reproduction of the neon isotope data is obtained, and predictions of the qualitative behavior of the thermal diffusion ratio are made.