Index of content:
Volume 25, Issue 5, 01 November 1956
25(1956); http://dx.doi.org/10.1063/1.1743126View Description Hide Description
An empirical method is suggested for the calculation of molecular screening constants. The method is based upon the values of the screening parameter in atomic orbitals. Comparisons are made with some variational calculations.
25(1956); http://dx.doi.org/10.1063/1.1743127View Description Hide Description
The rate of the dissociation reaction,has been measured by the shock tube method for argon, helium, nitrogen, oxygen, and carbon dioxide as inert gases, M, in the temperature range 1000°—1600°K. The shock wave results by themselves and the comparison of the shock wavemeasurements with the room temperature measurements of kR by flash photolysis both show that kR has a negative temperature coefficient. The absolute value of this negative temperature coefficient derived from the shock wavemeasurements is greater than the value derived from comparison of the average high temperature result with the room temperature result for any particular gas. This may be due to experimental error in the determination of dkR/dT at the high temperatures, but it is believed that the values of kR determined in the middle of the temperature range studied are reliable.
The experimental evidence indicates that, for the measurements with CO2, the rate of vibrational equilibration is so fast that the observations made here pertain entirely to vibrationally equilibrated CO2. Evidence from other experiments indicates that the rate of vibrational relaxation in oxygen is such that most of the dissociation reaction occurs in relaxed O2, but that nitrogen remains vibrationally unexcited under the conditions of the dissociation reactions studied here.
The ratio, kR, 12/kR,A, of the efficiencies of iodine and argon as third bodies is not greater than 30 at 1300°K whereas it is 250–600 at room temperature. The hypothesis is proposed that in general the ratio kR, gas/kR,A for complex gases will decrease with increasing temperature.
25(1956); http://dx.doi.org/10.1063/1.1743128View Description Hide Description
The rate of dissociation of molecular bromine in argon‐bromine mixtures has been measured in a shock tube and the rate constants for the reaction, in the temperature range 1400°—2700°K, computed. The data are not sufficiently accurate to reveal the temperature coefficient of kR. However, comparison of the average high temperature result, kR =3.4×108 liter2 mole—2 sec—1 at 1600°K, with the room temperature flash lamp result, kR =2.5×109, shows clearly that dkR/dT is negative. Experiments were performed with bromine: argon ratios of 0.01 to 0.10; the efficiency of Br2 as M, the third body for the recombination of bromine atoms, is not greater than 8 times that of argon. The agreement of the results of this investigation with those of an independent shock tube investigation by Palmer and Hornig is gratifyingly good.
The extinction coefficients of bromine at 436 mμ and 487 mμ were measured as a function of temperature.
25(1956); http://dx.doi.org/10.1063/1.1743129View Description Hide Description
The location and measurement of the nuclear quadrupole resonances of Cl35 in all multichlorobenzenes has been completed with the detection of resonance frequencies in five such compounds. A summary of the data on all the multichlorobenzenes is presented, including these reported for the first time: 1,2,3‐trichlorobenzene; 1,3,5‐trichlorobenzene; 1,2,3,4‐tetrachlorobenzene; 1,2,3,5‐tetrachlorobenzene; and pentachlorobenzene. These data indicate that the chlorine resonance frequency acquires frequency increments approximately proportional to the number of ortho chlorine neighbors. This effect may be interpreted primarily in terms of electron induction away from the resonant chlorine, although out of plane bending and double bond character also affect the resonance frequency. A correlation of the chlorine quadrupole frequencies with the Hammett sigma parameter is also given.
25(1956); http://dx.doi.org/10.1063/1.1743130View Description Hide Description
A discussion is given of adsorption systems where the energy of an atom in the second layer is appreciably larger than the energy of an atom in the liquid. It is shown that low temperature films of helium will correspond to this model, rather than the model leading to the B.E.T. equation. It follows that the ``anomalous'' adsorption properties of helium have been inferred from unreal parameters which are a result of the application of the B.E.T. theory. An alternate method of analysis based on a more realistic model is proposed. When a typical case is treated in this manner, reasonable values for the parameters in question result.
25(1956); http://dx.doi.org/10.1063/1.1743131View Description Hide Description
An x‐ray absorption photometer capable of measuring gas densities with extreme rapidity and reasonable accuracy has been developed to study the reaction zone in detonation waves. At low initial gas pressures this zone is readily observable as a density peak at the wave front lasting a few μsec.
In general the results agree with the hydrodynamic theory of the detonation wave. The observed densities at the end of the reaction zone (the Chapman‐Jouguet state) compare well with the results of approximate equilibrium calculations and the observed shape of the density profile qualitatively confirms kinetic predictions. However, the observed peak densities are substantially lower than expected and the initial chemical reaction rates are faster. The most plausible explanation at present seems to be a lack of equilibration between translational and internal degrees of freedom in the shocked gas, lasting long enough for a substantial progress of the chemical reactions to take place.
The duration of the reaction zone in the mixtures studied is inversely proportional to the initial pressure of the detonating gas. This is true of the stoichiometric mixtures of hydrogen and oxygen which were studied most intensively. On varying the composition of these mixtures it is found that the duration is inversely proportional to the partial pressure of hydrogen and is independent of that of oxygen. An increase of the total pressure by the addition of a rare gas accelerates the reaction. Nitrogen and water vapor act as inert additives.
Replacement of some hydrogen by carbon monoxide does not alter the duration of the reaction zone. On the other hand, when small quantities of hydrogen are added to stoichiometric carbon monoxide‐oxygen mixtures, the reaction times are found to vary as P H2 —½. The addition of nitrogen has little effect on these mixtures but carbon dioxide proves to be a strong inhibitor.
Methane reacts with oxygen at a rate which is comparable with that observed for hydrogen, while acetylene oxidation is very much faster.
25(1956); http://dx.doi.org/10.1063/1.1743132View Description Hide Description
The Boltzmann equation for a spacially uniform situation is derived by the method of expansion of the molecular distribution functions in powers of the concentration. These expansions are shown to imply that after a time which depends on the initial phase space distribution and for configurations in which the molecules are not too widely separated, all the distribution functions are functionals of the single‐particle distribution. The single‐particle distribution obeys an equation which is a generalization of the Boltzmann equation which includes the effects of triple and higher collisions. The triple collision term is given explicitly.
25(1956); http://dx.doi.org/10.1063/1.1743133View Description Hide Description
The structure of the e‐type fundamentals of SiH3F, SiH3Cl, and SiH3Br has been resolved and analyzed. The values of the small amount of inertia thus obtained has been combined with microwave measurements to yield the dimensions of the SiH3 group in these molecules. It is shown that the differences in the structure of the SiH3 group are probably within experimental error. Several revisions have been made in the vibrational assignments for SiH3F and SiH3Cl.
25(1956); http://dx.doi.org/10.1063/1.1743134View Description Hide Description
An analysis of the velocity distribution of the molecules which escape through an ideal aperture from an isothermal enclosure has been made for ten alkali halides (CsCl, CsBr, RbCl, KCl, KI, NaF, NaCl, NaI, LiCl, and LiBr). It is found that there is a significant abundance of dimers in all cases except those of the cesium salts, and that an observable concentration of trimers occurs for NaF, LiCl and LiBr. Observation of the relative abundances of the polymers as the temperature of the effusing gas and the pressure within the source is varied permits the determination of the energy of dissociation of the dimer into two monomers and of the trimer into a monomer and dimer.
25(1956); http://dx.doi.org/10.1063/1.1743135View Description Hide Description
The intensity of the 15 μ band of CO2 has been redetermined by a curve‐of‐growth method, utilizing simple formulas derived for the absorption coefficient of the Q branch. The value obtained is 240 cm—1 per cm at S.T.P., some 25 to 30% higher than the results of the Wilson‐Wells method. Extrapolation methods in general can lead to considerable underestimation of intensity, especially when applied to bands containing narrow Q branches.
The nitrogen‐broadened half‐width at one atmosphere and 298K is 0.064 cm—1.
25(1956); http://dx.doi.org/10.1063/1.1743136View Description Hide Description
From the two assumptions, that the pair distribution function of relative position is independent of the rate of strain or the temperature gradient and that the velocity distribution is locally Maxwellian, expressions are derived for the shear viscosity η, the bulk viscosity κ and the thermal conductivity λ of a hard‐sphere fluid at high densities. An expression is also derived for the coefficient of self‐diffusion D at any density by assuming that the autocorrelation function for the velocity of a given particle decays exponentially. The results arewhere m and a are the mass and radius of the spheres.
25(1956); http://dx.doi.org/10.1063/1.1743137View Description Hide Description
It is shown, by a procedure similar to one used by Abragam, Horowitz, and Pryce in their studies of hyperfine structure in paramagnetic ions and atoms, that configurational interaction may contribute to the isotropic hyperfine interaction in free radicals. For a configurational admixture of the type (σ B )2(π)+λ(σ B ) (σ A ) (π), the hyperfine interaction is 32π/3√6μημ e λσ B (r η)σ A (r η) where μ e and μη are magnetic moments of the electron and the nucleus η, and σ(r η) is the magnitude of the orbital σ at the position of the nucleus. Since the hyperfine coupling is linear in λ, only small admixtures of excited configurations are required to account for the protonhyperfine splittings observed in aromatic free radicals.
Microwave Spectra of Thiophene, 2‐ and 3‐Monodeutero, 3,3′‐Dideutero, and Tetradeuterothiophene. Structure of the Thiophene Molecule25(1956); http://dx.doi.org/10.1063/1.1743138View Description Hide Description
2‐ and 3‐monodeutero, 3,3′‐dideutero, and tetradeuterothiophene have been prepared and their microwave spectra recorded together with the microwave spectrum of ordinary thiophene (C4H4S). Lines originating from the 4% content of C4H4S34 and the 2% content of C3C13H4S in ordinary thiophene (both the 2‐ and the 3‐C13 species) were also identified. For all isotopic species rotational constants of high accuracy were obtained. The total material is sufficient for a complete determination of all the 8 geometrical parameters of thiophene.
25(1956); http://dx.doi.org/10.1063/1.1743139View Description Hide Description
Analytic expressions for tunneling frequencies through high periodic barriers are derived. The expression for wells of threefold symmetry is applied to the analysis of microwave data on CH3SiF3, CH3SiH3, and CH3SiD3 and values of the barrier heights of 1 kcal, 1.5 kcal, and 1.5 kcal per mole, respectively, are obtained, which are in reasonable agreement with previously obtained values.
25(1956); http://dx.doi.org/10.1063/1.1743140View Description Hide Description
Further experiments have been conducted to clarify the mechanism of the unique reaction whereby I128 freshly formed by the I127(n,γ)I128 process is able to replace hydrogen in methane (I128+CH4→CH3I+H). They show that the yield of CH3I128 is decreased by the presence of the inert gases He, A, and Xe but not to the extent which would be the case if the activation energy for the reaction were supplied solely by kinetic energy of the I128. I2, CH3I, C2H5I, C3H7I and NO, which have lower ionization potentials than that of the I atom are all more effective than the inert gases in reducing the yield. The organic yield of I128 is higher when alkyl iodides are used as the source of iodine than when I2 is used. The ability of the freshly formed I128 to react with C2H6 or C6H6 is much less than with CH4. Reactions of I2 with CH4 sensitized by argon and xenon occur when several hundred mm of the inert gas is present during exposures to 105 r or more of gamma radiation.
25(1956); http://dx.doi.org/10.1063/1.1743141View Description Hide Description
The color center at 460 mμ produced by x‐irradiation of naturally colorless or bleached smoky quartz is anisotropic. The absorption maximum of this color center for the extraordinary ray is 460 mμ and for the ordinary ray is 485 mμ. A second anisotropic peak with maximum in the vicinity of 625 mμ for the extraordinary ray is also present. The relative anisotropy of these color centers in a natural, a Bell synthetic and a Brush synthetic quartz are compared and found to be nearly identical in the case of the synthetic quartzes studied. The absorption coefficients of these color centers x‐rayed to saturation are compared with total aluminum content for five specimens.
25(1956); http://dx.doi.org/10.1063/1.1743142View Description Hide Description
The constants α and β appearing in assumed interatomic energies of the type α/rm —β/r 6, m>6, have been calculated for the rare gases for various m, using solid state data only. Using these results the equations of state of these gases in the solid phase have been calculated, and compared with recent experimental data. For neon, m = 14 gave excellent agreement, while for krypton and argon m = 12 gave reasonable agreement with experiment.
25(1956); http://dx.doi.org/10.1063/1.1743144View Description Hide Description
A sensitive method has been developed for measuring rapidly the latent heat of vaporization or sublimation of metals of low volatility. Atoms from the saturated vapor emerge from a small Knudsen‐type effusion cell and impinge upon a hot filament. A fraction of the incident atoms re‐evaporate from the filament as ions and are detected by a mass spectrometer. The Clausius‐Clapeyron equation is used to obtain the latent heat from the dependence of ion current upon cell temperature. Application has been made to aluminum,praseodymium, and neodymium. The work on aluminum yielded an enthalpy of sublimation at 298°K of 78.0±0.4 kcal/M. Some anomalous behavior was observed in the rare earths. Preliminary molar enthalpy values are: Pr, 79.3±2.0 kcal (vaporization at 1380°K); Nd, 70.6±2.0 kcal (vaporization at 1348°K) and 74.1±2.0 kcal (sublimation at 1236°K). An estimate of the heat of fusion of Nd yielded 3.5±1.0 kcal/M.
The method also provided a convenient measurement of first‐order phase transition temperatures. The melting points of Pr and Nd were found to be 1192±2°K and 1292±2°K respectively. In addition, a solid‐to‐solid phase transition was found in Nd at 1142±2°K.
25(1956); http://dx.doi.org/10.1063/1.1743145View Description Hide Description
The present work was undertaken with the purpose of considering why and how reaction takes place in an ultrasonic field, rather than studying the chemical kinetics of such reactions. Experimental evidence is presented which supports the conclusions that chemical processes brought about by ultrasonics require cavitation. Furthermore, the experiments were planned so as to show whether the reactions take place in the bubble or at the interface by studying the effect of the thermal conductivity and of the ratio of the heat capacities of various dissolved gases and their mixtures on the yields. From the results it was concluded that primary reactions seem to be gas phase reactions, probably of a thermal nature, taking place inside the gas bubbles which serve as ``hot spots'' in the liquid. The calculations show that, under the experimental conditions reported here, temperatures of several hundreds or thousands degrees can be easily reached inside the cavitating bubbles in resonance with the sound field; it is believed that at these temperatures the reaction which gives rise to H2O2 takes place. At this time it is not possible, however, to rule out completely the concomitant effects of gaseous discharge as suggested by others. It was also found that in conducting experiments like those in the present work, the cavitation threshold should be determined, effective intensities used, and systems maintained as simple as possible.
25(1956); http://dx.doi.org/10.1063/1.1743146View Description Hide Description
The absorptionspectrum of water vapor has been measured from 5.26μ to 7.14μ. The spectra were observed with a Pfund‐type vacuum spectrometer employing a 4500‐line‐per‐inch echelette grating as the dispersing element. Lines separated by intervals of the order of 0.1 cm‐1 were resolved. The rotational analysis leads to the following molecular constants for the H2O bending vibration: