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Volume 39, Issue 7, 01 October 1963
39(1963); http://dx.doi.org/10.1063/1.1734502View Description Hide Description
Phosphorescencepolarization measurements have been performed by the method of photoselection. Since this technique requires a knowledge of the polarization of at least two perpendicularly polarized absorption bands, the method can be fully exploited in the case of the N‐heterocyclics where the (n, π*) and (π, π*) absorptions are perpendicular to each other. The results on pyrazine (1,4‐diazabenzene) show that the π*→n emission is polarized along the N···N axis, in agreement with previous work. The polarization is uniform throughout the 600‐cm—1[ν6(ag )] progression and does not alternate as does the intensity and the vibrational frequency. This behavior might arise from a resonance‐type interaction which produces a progression with mixed Franck—Condon characteristics. Quinoxaline (1,4‐diazanaphthalene) is found to have a π*→π phosphorescence spectrum which is highly polarized out of the plane. Approximate calculations show that the (n, π*) singlet state in quinoxaline is at least as important as the (σ, π*) states in mixing with the emitting (π, π*) triplet state. The general conclusion is reached that in nitrogen heterocyclics when the phosphorescent state is of the (n, π*) type, the mixing singlet is of the (π, π*) type, or in‐plane Rydberg state, whereas if the emitting triplet state is of the (π, π*) type, the mixing singlet might be of the (n, π*), (σ, π*) and/or perpendicularly polarized Rydberg state.
39(1963); http://dx.doi.org/10.1063/1.1734503View Description Hide Description
The vibronic absorption spectra of praseodymium,neodymium, and erbium ethylsulfate were used to construct tentative vibrational energy‐level diagrams for the crystals. With this technique, 42 levels ranging from 18 to 360 cm—1 were identified. The energy‐level spectra were used to calculate the specific heat of the crystals from 0° to 15°K as an additional check on the origin of the observed absorption spectra and fair to good agreement with measured specific heats was achieved.
39(1963); http://dx.doi.org/10.1063/1.1734504View Description Hide Description
39(1963); http://dx.doi.org/10.1063/1.1734505View Description Hide Description
Electron spin resonance measurements have been made on dimethyl sulfoxide solutions of the anion radicals of s‐tetrazine, pyridazine, phthalazine, and phenazine, as well as the diprotopyrazine cation radical. The radicals were generated by constant potential electroreduction of the parent compound at a mercury pool cathode within the microwave cavity. Complete assignment of the hyperfine coupling constants to positions of phthalazine anion was made by selective deuteration and the help of a spectral synthesis program. Satellites due to the C13 hyperfine interaction of pyrazine anion have been observed, and the coupling constant measured. The mechanism of N14 hyperfine interactions in polyazine anion radicals is discussed in terms of earlier theories of C13 interaction. We find that π‐electron spin density on nearest‐neighbor N atoms affects a N strongly, whereas on nearest‐neighbor C atoms it does not. Values for several empirically determined spin‐polarization parameters are reported.
39(1963); http://dx.doi.org/10.1063/1.1734506View Description Hide Description
An asymptotic form for the low temperature limiting behavior of the gas imperfection due to gas—solid interaction is derived. Whenis plotted against 1/T it is shown that the plot must approach linearity at low T, and expressions for calculating the deviations of the plot from its limiting tangent at higher temperatures are presented. The slope of the limiting tangent depends only on the depth of the minimum in a plot of potential energy for gas—solid interaction against distance (and not on the shape of the potential energy curve). The intercept (at 1/T=0) of the limiting tangent depends principally on the surface area of the solid and the curvature of the potential energy plot at its minimum; neither of these quantities can be inferred from the intercept unless the other is known. The inference of potential energy forms from gas imperfection data is briefly discussed in the light of the asymptotic form developed.
Internal Conversion from Upper Electronic States to the First Excited Singlet State of Benzene, Toluene, p‐Xylene, and Mesitylene39(1963); http://dx.doi.org/10.1063/1.1734507View Description Hide Description
The variation with excitation wavelength of the fluorescence efficiencies of some alkyl benzenes has been studied from 2700 to 1600 Å. Measurements have been made in both liquid (pure and dilute solution) and vapor phases. For pure liquids the internal conversion efficiency βλ from upper electronic states to the first excited singlet state has been found to be significantly less than unity and to increase in the order benzene, toluene, p‐xylene, mesitylene when comparisons of βλ are made at comparable excitation wavelengths. Solvent effects on βλ have been noted. No emission is observed from any alkyl benzene studied in the vapor phase for excitations beginning on the long‐wavelength side of the second absorption band and extending to shorter wavelengths.
Dissociation,internal conversion to the ground state, and intersystem crossing to a triplet state are considered as mechanisms for the disappearance of the upper electronic singlet states. Measurements of the benzene‐sensitized phosphorescence of biacetyl indicate that if the disappearance mechanism involves upper‐level intersystem crossing to the triplet manifold, then the triplets so produced do not efficiently internally convert to the lowest triplet state.
39(1963); http://dx.doi.org/10.1063/1.1734508View Description Hide Description
The conductivity of a slightly ionized gas in a uniform magnetic field has been obtained from the solution of the Boltzmann equation for cases in which the collision frequency between electrons and neutral particles are dependent on the electron velocity. The line shapes of the cyclotron resonance absorption of energy from a high‐frequency electric field and the derivative with respect to the magnetic field of the resonance absorption have been computed. Also the relationship between the linewidth of the above curves and the collision frequency was obtained.
The absorption of 9 kMc/sec microwave energy by the weakly ionized thermalized afterglows of helium, argon, oxygen, and carbon dioxide near cyclotron resonance has been measured experimentally. From these measurements the collision frequencies for thermal electrons with He, Ar, O2, and CO2 have been obtained. These collision frequencies are 2.0+0.2×108, 1.6±0.5×108, 1.7±0.2×108, and 24+2×108, respectively, for the above gases at 300°K and 1 mm Hg pressure.
39(1963); http://dx.doi.org/10.1063/1.1734509View Description Hide Description
The thermal decomposition of perfluorocyclobutane was studied behind reflected shocks in a single‐pulse shock tube over the temperature range 1040°—1200°K. Mixtures of approximately 1% to 1.5% of perfluorocyclobutane in argon were used. The reaction products were analyzed by vapor‐phase chromatography. The first‐order rate constants obtained at high temperatures are slightly higher than the values deduced from an Arrhenius extrapolation of the low‐temperature data; however, the activation energy is the same. The rate constant is: k uni ∞=2.1×1016 exp[—74.3×103/RT].
39(1963); http://dx.doi.org/10.1063/1.1734510View Description Hide Description
A number of substances are known to evaporate simultaneously into monomers, dimers, and higher polymeric species in the gas phase. For many of these systems the evaporation is expected to be limited by a surface reaction which precedes desorption. A method is given for determining from experiments whether the various species are evaporated independently. The experiments which will define the evaporation mechanism measure the variation of the evaporation rates with the size of the orifice of the evaporation chamber.
39(1963); http://dx.doi.org/10.1063/1.1734511View Description Hide Description
The absorption and fluorescence of Yb3+ in yttriumgalliumgarnet and yttriumaluminumgarnet have been investigated to locate the levels of the ground manifold2 F 7/2. From the temperature dependence of absorption in the gallium compound, it is concluded that the levels lie at 0, 112, 308, and 550 cm—1. For the aluminumgarnet, the fluorescencespectrum can be interpreted in terms of a ground manifold with levels at 0, 140, 490, and 620 cm—1.
39(1963); http://dx.doi.org/10.1063/1.1734512View Description Hide Description
The pyrolysis of neopentyl chloride has been reported to decompose by an ion‐pair mechanism in a gas‐phase homogeneous reaction at about 450°C. It is shown that a Rice—Herzfeld mechanism involving the principal radical chain:[Complex chemical formula]accounts well for the products and rate. It is followed by the fast unimolecular isomerization of 1,1‐dimethyl cyclopropane to the isomeric methylbutenes. It is shown that the system is quite complex, involving autocatalysis by HCl (one of the major products) and very likely inhibition by the concomitantly produced olefins.
Hydrogen‐Chloride‐Catalyzed Pyrolysis of Dimethyl Ether and the Use of HCl Catalysis for Diagnosis of Complex Chains39(1963); http://dx.doi.org/10.1063/1.1734513View Description Hide Description
The pyrolysis of CH3OCH3, catalyzed by gaseous HCl (3 to 16 mole %) has been studied in the temperature range 452° to 483°C. The rate approaches independence of HCl at the higher concentrations and is 3/2 order in ether. The products are the same as in the uncatalyzed decomposition (CH4+CO+H2 and CH4+CH2O) and no HCl is consumed. From the observation that there is up to a tenfold acceleration in rate with HCl it is shown that the slow step in the uncatalyzed pyrolysis is the reactionThe hitherto‐conceived slow stepresulting from the analysis of S. W. Benson and D. V. S. Jain [J. Chem. Phys. 31, 1008 (1959)] is instead very rapid, and thus CH2OCH3 cannot be an important chain breaker.
Recent data on the stability of HCO indicate that CH3 and CHO are the actual chain‐breaking radicals, both being important in the uncatalyzed decomposition. The H2S catalyzed reaction is discussed briefly and shown to be similar to the HCl system. HCl catalysis as a diagnostic tool in analyzing complex chains is discussed.
On the Excess Electron and Hole Band Structures and Carrier Mobility in Naphthalene, Anthracene, and Several Polyphenyls39(1963); http://dx.doi.org/10.1063/1.1734514View Description Hide Description
The excess electron and hole band structures of naphthalene, anthracene, and several polyphenyls have been calculated in the tight binding approximation. In addition the anisotropy and the pressure dependence of the mobilitytensor has been calculated in the constant‐free‐time and the constant‐free‐path approximations. The molecular wavefunctions were represented in the LCAO approximation using Hückel coefficients with the carbon atomic orbitals represented by the best available Hartree—Fock SCF carbon 2p ground‐state function involving a linear combination of four Slater‐type functions. By this choice of atomic orbitals, we hope to account properly for the behavior of the wavefunction at the large internuclear distances relevant to this problem. All of the above compounds are characterized by a crystal structure containing two molecules per unit cell. There are, therefore, two bands for both the electron and the hole cases. The bandwidths are calculated to be of the order of 0.1 eV. In the a —1 and b —1 directions, the symmetric and antisymmetric bands are degenerate at the zone edge. The electron bands are appreciably split in the c —1 direction.
Although there is no attempt to make absolute calculations of the mobility, the experimental data seem to be in adequate agreement with the scattering models considered. Inferences can be drawn which suggest that further understanding of the mobility will arise from a detailed investigation of the interaction of the charge carriers and the intermolecular and intramolecular vibrations.
39(1963); http://dx.doi.org/10.1063/1.1734515View Description Hide Description
The least‐squares local energy method is applied to the lithium atom ground state. For a correlated wavefunction of 51 terms the calculated energy is —7.4653 hartrees compared with the exact nonrelativistic value of —7.4781 hartrees. Important limitations of the method are discussed and possibilities for avoiding them are proposed.
39(1963); http://dx.doi.org/10.1063/1.1734516View Description Hide Description
Magnetic susceptibility measurements have been made on UO2·xH2O for x=1.78 to x=2.13, and from 77° to 375°K. As the value of x decreased the susceptibility increased. Both these data and structural arguments imply that the formula of this compound is U(OH)4 rather than the dihydrate form. Based on this concept the data have been corrected for diamagnetism and also small amounts of UO2 and H2O which were present. The molar susceptibility of U4+ in U(OH)4 is nearly an order of magnitude less than in other uranium compounds, and it is suggested that this is probably due to superexchange between adjacent uranium atoms through intervening oxygen atoms.
Diffraction Studies of Photochemical Decomposition of Azides. I. X‐Ray and uv‐Induced Lattice Constant Changes in TlN339(1963); http://dx.doi.org/10.1063/1.1734517View Description Hide Description
Diffraction methods were applied to study irradiation and annealing effects on TlN3. Crystallographic lattice constant and cell volume changes upon x‐ray irradiation were determined as function of irradiation time and radiation intensity. From the character of these curves some conclusions were drawn about the physical processes causing the effects. The reversibility of these processes was studied by annealing experiments at various temperatures and in both air and helium atmospheres.
Preliminary studies of radiation damage on uv‐irradiated samples were carried out in order to relate the diffraction results to the TlN3absorptionspectrum and photoconductivity data.
39(1963); http://dx.doi.org/10.1063/1.1734518View Description Hide Description
Onsager's prediction of a first‐order phase transition in a gas of long thin rods is verified by extensive calculations on a simple model. The molecules are rectangular parallelepipeds, of length l and square cross section d×d. The long molecular axes can point in only three mutually perpendicular directions. Virial coefficients, up to the seventh, are calculated exactly as functions of orientation, in the limit l→ ∞, d→0, and l 2 d=constant. The transition predicted by Onsager's theory, based on a second virial approximation to the potential of mean force in the space of relative orientations, is observed also when all virial coefficients up to the seventh are included. The virial expansion appears to converge well for the isotropic phase; but the properties of the anisotropic phase depend sensitively on the order of approximation.
39(1963); http://dx.doi.org/10.1063/1.1734519View Description Hide Description
The ring protonchemical shifts of an extensive series of polysubstituted benzenes may be represented, with an accuracy of about 0.1 part per million, as simple sums of substituent shielding constants, except in the case of ring protons next to substituents which are ortho to one another.
The small, uniform deviations from additivity of the 46 shifts measured in a series of para disubstituted benzenes are well represented by a linear variation, so that the chemical shift of proton 2 in a 1,4‐substituted benzene may be expressed asThe polarizability γ decreases as d 0 increases.
The values of the shielding parameters d are consistent with a mechanism dominated by the pi‐electron charge distribution, although other effects are present, especially at the meta position, which cannot be accounted for in a consistent manner. It can be shown that the molecular‐orbital theory of substituent interaction with the pi‐electron system can yield a variation of polarizability with charge density of the order of magnitude of that observed; however, the symmetry of this variation is inconsistent with a single‐parameter interaction mechanism.
39(1963); http://dx.doi.org/10.1063/1.1734520View Description Hide Description
It is shown that the single‐parameter molecular orbital models of substituent perturbations of the benzene ring require that the charge density changes be odd, and the bond orders and polarizabilities be even functions of the perturbation parameters. This behavior excludes the possibility of a cross term in the MO description of those properties of disubstituted benzenes which are proportional to the pi‐electron charge densities.
39(1963); http://dx.doi.org/10.1063/1.1734521View Description Hide Description
The microwave spectrum of (CH3)2C=C13H2, CH3C13H3C=CH2, and (CH3)2C=CHD has been studied in the region 8–40 kMc/sec. The effective rotational constants of these species combined with previous microwave data give the structural parameters: r CC(single)=1.507 Å, r CC(double)=1.330 Å, average r CH(methyl)=1.08 Å, r CH(ethylenic)=1.088 Å, average <HCH (methyl)=107°, <HCH (ethylenic)=118.5°, <CCC=115.3°.