Index of content:
Volume 14, Issue 8, 01 August 1946
14(1946); http://dx.doi.org/10.1063/1.1724175View Description Hide Description
The mercury photo‐sensitized polymerization of butadiene and butadiene‐acetone mixtures in the gas phase, and the photochemical reactions in solutions of butadiene and acetone in hexane, were studied. Metal mirror experiments demonstrated the absence of hydrogen atoms and the presence of alkyl radicals at low pressures. The mechanism proposed for butadiene vapor postulates an activated butadiene molecule produced by collision with activated mercury and competition between deactivation by collision,predissociation into free radicals, and a collisional process to give hydrogen gas. The polymerization chain proceeds by a mechanism involving either free radicals or hydrogen atoms. In the presence of acetone vapor, the rate is increased and the increase is directly proportional to the number of free radicals coming from acetone, indicating that polymerization chains are not broken by bimolecular gas phase reaction between free radicals. The quantum yield for butadiene alone at 53‐mm pressure and 43°C was 0.24. The average number of molecules reacting per free radical formed was calculated to be between 3 and 8. The liquid solution results are explained on the basis of a modified Frank‐Rabinowitch hypothesis for the primary photochemical process. Activated acetone molecules are deactivated by collision with hexane, and react with acetone to yield unsaturated compounds and with butadiene to initiate polymerization. The following kinetic equation is developed for the rate of polymerization,, where n is the average number of molecules reacting per free radical. n is taken equal to 4.
14(1946); http://dx.doi.org/10.1063/1.1724176View Description Hide Description
An electron diffraction investigation of dimethylketene dimer confirms the 2,2,4,4‐tetramethylcyclobutadione‐1,3 structure. The following parameters were determined for the symmetrical model: C–C (ring)=1.56±0.05A, C–CH3=1.54±0.05A, C=O=1.22±0.04A, ∠C–CO–C=93°±6°, and ∠CH3–C–CH3=111°±6°. The limits of error assigned to the angles apply only if simultaneous variation is excluded; otherwise much larger limits must be assigned. A notable feature of the structure is the large temperature factor which must be ascribed to the interatomic distances greater than 3A. The relation of this temperature factor to the unusually large atom polarization is discussed in terms of the probable amplitudes of the pertinent modes of vibration of the molecule.
The Determination of Critical Concentrations for the Formation of Soap Micelles by the Spectral Behavior of Pinacyanol Chloride14(1946); http://dx.doi.org/10.1063/1.1724177View Description Hide Description
The absorptionspectrum of pinacyanol chloride in aqueous solutions of anionic soaps changes sharply to that characteristic of its solutions in organic solvents over a short range of soap concentration. This effect is attributed to the formation of micelles, in whose hydrocarbon‐like layers or cores the dye is solubilized. The concentration of soap at which this spectral change occurs is taken as ``the critical concentration for the formation of micelles.'' In water the dye exhibits an absorption band at 5500A which disappears rapidly as the concentration of molecular soap increases. Thus, even 0.00006 molar potassium laurate gives a large effect. At about 0.0126 M the γ‐band at about 4800A reaches a high intensity, which falls, with further increase of concentration, very rapidly close to the critical concentration. At 2.3 to 2.4×10−2 for potassium laurate and 6×10−3 for the myristate the α‐band (ca. 6150A) and the β‐band (ca. 5700A) begin to increase rapidly in intensity and these are considered as the critical concentrations, since these are the prominent bands in the solution of the dye in an oil. Alkali oleate and dilinoleate soaps exhibit an additional band (ω‐band) at about 5200A. Some other critical concentrations determined by this method are 3.6–5.2×10−4 molar for sodium cetyl sulfate, 2.5–3.2×10−2 molar for potassium dehydroabietate, 7–12×10−4 molar for potassium oleate, and 2.5×10−4 molar for potassium dilinoleate. The values for the last two soaps are uncertain, since their behavior differs considerably from that displayed by the salts of saturated acids. All measurements were made at 25.8°C with the exception of the sodium cetyl sulfate which was investigated at 35.8°C. An absorption band at approximately 4800A appears in all the soap solutions measured below the critical concentration with the exception of potassium dilinoleate. This band is absent when salts or bases are added to the dye solution. A titrametric method has been developed for the determination of critical concentrations of soaps in the presence of added salts and hydrocarbons.
14(1946); http://dx.doi.org/10.1063/1.1724178View Description Hide Description
Biacetyl vapors have been decomposed by radiation of wave‐length 2537A in a flow system. The total amount of decomposition was determined by collection and analysis of the gaseous products. The amount of methyl radical formed was determined by allowing the products to pass over and react with lead mirrors. The amount of lead transported establishes the maximum quantity of methyl formed assuming a ratio of CH3 to Pb of 4:1. The data show that approximately one methyl group is released for every two or three molecules of carbon monoxide in the products and consequently that about half of the total reaction proceeds without the primary formation of free radicals. A Spence and Wild type of reaction scheme will explain both the analytical data and observations on fluorescence.
14(1946); http://dx.doi.org/10.1063/1.1724179View Description Hide Description
Crystal structure computations have been made for synthetic boehmite, the α‐modification of aluminum oxide monohydrate, using the powder method. The experimental data were found to fit an orthorhombic lattice with constants a 0 = 2.859A, b 0 = 12.24A, and c 0 = 3.691A. The assignment of four AlO (OH) groups to this unit cell gives a calculated density of 3.063 g/cc. The observed density is 2.977 g/cc. The general space group extinctions were found to agree with those of D 2h 17. Generalized atomic coordinates were roughly determined by application of Pauling's coordination theory for ionic crystals and the determinations refined by the usual intensity computations. The parameter values selected were u Al = −0.334, uOI = 0.287, uOII = 0.067. It is suggested that the inconsistencies which exist between the experimental and computed intensity data may be explained by the occurrence of a small quantity of γ—Al2O3 in the synthetic samples used. The value of 2.47±0.07A obtained for the hydrogen bridge distance is discussed. This value is less than that observed for any previously determined structure.