Index of content:
Volume 39, Issue 5, 01 September 1963
39(1963); http://dx.doi.org/10.1063/1.1734404View Description Hide Description
The theory of the second moments of ESR spectra of π‐electron radicals in polycrystalline environments is discussed and extended. Expressions for the effects of finite linewidth, satellite lines, carbon‐13 in natural abundance, g‐value anisotropy, and σ‐electron spin density are developed. Second moments of a series of aromatic monopositive ion radicals of known spin density distribution are measured and compared with calculated values. The second moment theory is applied to a radical of unknown spin distribution, the benzyl radical. This radical is prepared photochemically and the experimental second moment, 127±6 G2, is compared with values calculated on the basis of a number of valence‐theoretical approximations. Closest agreement is obtained for an alternant MO wavefunction [J. Chem. Phys. 32, 176 (1960)] with b/a=0.1. In the course of this investigation the ESR component shape of radicals in single crystals is determined experimentally to be very closely Gaussian. The radical product of the ultraviolet illumination of an aromatic hydrocarbon in a rigid boric acid glass is unambiguously identified by ESR as the aromatic monopositive ion.
Radiative Transition Probabilities within 4fn Configurations: The Fluorescence Spectrum of Europium Ethylsulfate39(1963); http://dx.doi.org/10.1063/1.1734405View Description Hide Description
The relative intensities of the transitions occurring in the fluorescencespectrum of Eu(C2H5SO4)3·9H2O have been measured at 77° and 4°K. The strengths of both the ``forbidden'' electric dipole transitions and magnetic dipole transitions which were observed are compared to theoretical expressions and the results are discussed. The results are in reasonable agreement with recent phenomenological treatments of crystal‐field‐induced dipole radiation, and suggest that g‐orbital configurational mixing contributes significantly to the process.
39(1963); http://dx.doi.org/10.1063/1.1734406View Description Hide Description
The spectrum of the GaO molecule excited in the aureole of a dc arc has been investigated and previous assignments of the vibrational energy levels of the 2Σ+→X 2Σ+ transition have been re‐evaluated. The splitting of bandheads noted by earlier writers has been attributed to the isotope shift. A rotational analysis of the O–O vibrational band has been carried out. The B 0′ and B 0″ values are 0.3861 and 0.4108 cm—1, respectively, corresponding to internuclear distances r 0′=1.829 Å and r 0″=1.773 Å.
The effects of the nonrigidity of the rotator have been observed. The difference in the spin splitting of rotational levels has been determined to be γ′—γ″=0.01808 cm—1. The spectroscopic constants determined in this study were used to calculate the thermodynamic functions up to 3000°K.
39(1963); http://dx.doi.org/10.1063/1.1734407View Description Hide Description
At the interfaces of absorbing dielectrics reflected light undergoes an abrupt phase change other than zero or π. For a transparent interference film on an absorbing dielectric substrate this change has the effect of shifting the wavelength of maximum interference toward a higher value. From the spectrophotometrically determined rates of film formation an extrapolation to zero time (zero thickness) gives the fractional wavelength shifts directly. Values are obtained experimentally for the hydrofluorination reaction of uranium dioxide and are compared with values calculated from optical theory.
39(1963); http://dx.doi.org/10.1063/1.1734408View Description Hide Description
In the temperature interval 200°—300°C optically isotropic uniformly colored interference films are formed on the flat, polished surfaces of fused uranium dioxide by reaction with anhydrous gaseous hydrogen fluoride. The kinetics were determined spectrophotometrically at eight temperatures for film thicknesses up to ¼ micron. In the pressure interval 20–53 cm Hg, the rate of film growth is diffusion controlled with the thickness proportional to the square root of both pressure of HF(g) and time. The proportionality constant is in which β0=2.59×103 and Q=6930 cal/mole with a 90% confidence interval of ±140 cal/mole. From metallographic examinations it was concluded that the uranium tetrafluoride product is deposited at the film—UO2(s) interface.
39(1963); http://dx.doi.org/10.1063/1.1734409View Description Hide Description
The microwave spectrum of silyl acetylene has been investigated in the region 8–36 kMc. The rotational constants for the isotopic species H3Si28C12≡C12H, H3Si29C12≡C12H, H3Si30C12≡C12H, H3Si28C13≡C12H, H3Si28C12≡C13H have been determined to be 4828.66, 4775.65, 4725.65, 4803.98, 4662.34 Mc/sec, respectively. From these data the structural parameters r SiC=1.826 Å and r CC=1.208 are determined. If r CH is assumed to be 1.056 Å then r SiH=1.455 Å, <HSiH=110.3°. The length of the SiC bond is definitely shortened by its location adjacent to the CC triple bond and there may be some lengthening of the CC bond. Measurement of the Stark effect leads to a dipole moment of 0.316 D.
39(1963); http://dx.doi.org/10.1063/1.1734410View Description Hide Description
The configuration interaction method is applied to the nonrelativistic 23 S state of the helium atom. It is shown, by variationally adjusting the effective charges of the angular configurations, that no radial readjustment occurs for this state. The final set of 80 configurations then gives an energy of —2.1752259 a.u., recovering 27% of the energy difference between our best previous configuration‐interaction value and the extrapolated value of Pekeris. A discussion is then given concerning the relative merit of the configuration interaction method and suggestions given for its future application to the 23 S state of the two‐electron system.
39(1963); http://dx.doi.org/10.1063/1.1734411View Description Hide Description
Delayed fluorescence has been observed at 77°K from rigid‐glass solutions of some aromatic hydrocarbons and their halo derivatives. The intensity of the delayed fluorescence is proportional to the square of the phosphorescence intensity. The decay of delayed fluorescence in all cases may be represented as the sum of two first‐order processes; the lifetime of the longer‐lived component varies between 1/2↔1/5 that of the phosphorescence while the lifetime of the shorter‐lived component varies between 1/10↔1/50 that of phosphorescence. The intensity of delayed fluorescence observed from a solution of a given compound varies as the ratio φ f φ p 2/k 3 *2, where φ f and φ p are fluorescence and phosphorescence quantum yields, respectively, and where k 3 * is the emissive phosphorescence rate constant. Phosphorescence decay exhibits a slight nonexponentiality only in the case of those solutions where the delayed fluorescence is especially strong. Delayed fluorescence is spectrally identical, within the limits of spectrograph resolution available to us, with ordinary fluorescence. The above facts may only be interpreted by a kinetic mechanism which involves a triplet—triplet annihilation process, this mutual annihilation resulting in the eventual production of the singlet excited state of one of the partners. Some ancillary hypotheses are required to interpret the shorter‐lived component of the delayed fluorescence, and a kinetic flowsheet representative of the energy‐transfer processes involved in the production of delayed fluorescence is proposed.
39(1963); http://dx.doi.org/10.1063/1.1734412View Description Hide Description
The NMR spectra of several 2‐F, 6‐X‐benzotrifluorides yield values for J FF between the 2‐fluorine and the fluorines in the CF3 group of 13 to 34 cps, depending upon the 6‐substituent. Furthermore, the magnitude of this J FF is roughly proportional to the chemical shift of the CF3 group and to the size of the 6‐substituent. Two related factors appear to be involved: (a) the rotational configuration of the CF3 group with respect to the plane of the benzene ring, and (b) electrostatic deformation of the electron distribution about the fluorines in the CF3 group. Our results for the benzotrifluorides and values reported earlier for the cis‐F–CF3 coupling in CF3CX=CFY compounds are consistent with little or no hindrance to CF3‐group rotation in the latter.
Polymer NMR Spectroscopy. X. The Use of H1–H1 Spin Decoupling in the Elucidation of Polymer Structure39(1963); http://dx.doi.org/10.1063/1.1734413View Description Hide Description
High‐resolution NMR spectroscopy of vinyl polymers in solution is an effective means of determination of the stereochemical configuration of their chains. In polymers having only a single α substituent, however, spin coupling of α and β protons complicates the interpretation of the spectrum considerably and also makes it difficult to observe those stereoisomeric sequences which are present as minor components. Decoupling of the α‐ and β‐proton spins has been found to be helpful in the interpretation of the spectra of polyvinyl chloride, polyvinyl fluoride, and polyvinyl methyl ether. It is not helpful for polyvinyl acetate in carbon tetrachloride, since here the chemical shift of the α and β protons is not sensitive to stereochemical configuration.
39(1963); http://dx.doi.org/10.1063/1.1734414View Description Hide Description
The drift velocities of ions in nitrogen have been measured over very wide ranges of E/p 0, the ratio of electric field to pressure reduced to 0°C, under conditions permitting the pressure to be varied separately. The apparatus used consisted of a glow discharge as the ion source, an auxiliary drift space, an electronic shutter, and a final drift space. The readings were taken by measuring the time of flight of the ions across the final drift space. The ion current was amplified and displayed on an oscilloscope. Over a wide range of E/p 0 two ion species were present, and these ions are believed to be N2 + and N3 +. The N2 + drift velocities varied from 2.5×104 cm/sec at an E/p 0 of 20 V/(cm×mm Hg) to 3×105 cm/sec at an E/p 0 of 800. The N3 + drift velocity varied from 1.5×104 cm/sec at an E/p 0 of 6 to 5×105 cm/sec at an E/p 0 of 300.
39(1963); http://dx.doi.org/10.1063/1.1734415View Description Hide Description
Electron paramagnetic resonance studies of the fine structure of nitrogen atoms trapped in x‐ray‐irradiated sodium azide at low temperatures have revealed that the atoms are trapped interstitially. There are three similar sites with magnetic axes parallel to the three axes of the rhombohedral unit cell. The data are fitted to an appropriate spin Hamiltonian, and the temperature dependence of the fine‐structure parameters is discussed.
Isotropic NMR Shifts in Pyridine‐Type Bases Complexed with Paramagnetic NiII and CoII Acetylacetonates39(1963); http://dx.doi.org/10.1063/1.1734416View Description Hide Description
Isotropic contact interaction shifts in the proton NMRspectra of pyridine‐type molecules coordinated with paramagneticnickel (II) and cobalt (II) acetylacetonates have been observed. The role of π‐ and σ‐ bonding frameworks in the transfer of unpaired electron spin density from the metal ion to ligand protons is considered. Pseudocontact interactions appear to be significant in the cobalt systems but not in the nickel systems. When the pseudocontact contribution to isotropic shifts in cobalt systems is factored out there remain contact shifts which are in good agreement with the observed nickel shifts. It is concluded that the unpaired electrons in metaleg orbitals of both cobalt and nickel systems cause contact shifts in the pyridine‐type ligands whereas the t 2g electron in the cobalt system does not. The t 2g electron gives isotropic shifts by contributing to anisotropy in the electronic g factor.
39(1963); http://dx.doi.org/10.1063/1.1734417View Description Hide Description
Short‐lived colored species are formed when a water solution of 2,4‐dinitrotoluene is exposed to ultraviolet light. The colored species displayed an acid—base equilibrium and the absorption spectra of the acid form and the base form were measured. The half‐life time for the first‐order fading reaction was varied over a 105‐fold range (20 μsec to 1 sec) by changing the acidity and temperature. The fading reaction was found to be general acid‐catalyzed over the pH region 2–13. The experiment supports a mechanism having the following essential features. The nitro form is photochemically isomerized to an aci‐nitro form which is in equilibrium with the anion. The rate‐determining step in the fading reaction is the neutralization of this anion. The rate constants for the neutralization by various acids have been measured at 30°C to be 1.8×10—2 (H2O), 4.8×103 (HAc), 2.4×104 (+NH3CH2COOH), 7.4×104 (H+) liter sec—1 mole—1.
39(1963); http://dx.doi.org/10.1063/1.1734419View Description Hide Description
Spectrophotometric data in the ultraviolet range at 25°C indicate that carbon tetrachloride forms a weak charge‐transfer complex with benzene and with mesitylene. For 1:1 complexes the equilibrium constants are, respectively, 0.009±0.004 and 0.113±0.044 liter/g·mole.
39(1963); http://dx.doi.org/10.1063/1.1734420View Description Hide Description
A method is presented by which barrier heights, in potential functions hindering the internal rotation of methyl groups, or spectroscopically inactive vibrational frequencies, can be obtained solely from thermal data for the solid phase. The observed heat capacities are analyzed in terms of contributions from the lattice vibrations, the internal vibrations, the expansion of the lattice, and the internal rotation. At low temperatures, the hindered rotation can be treated as torsional oscillation with a frequency related to the barrier height. It is shown that barrier heights derived by this method are in accord with those estimated from comparison of gas‐phase entropies, but can be determined with greater precision than such values.
39(1963); http://dx.doi.org/10.1063/1.1734421View Description Hide Description
Saturation current methods have been used to measure ion‐production rates with and without the addition of a nonreactive quenching compound (N2O). These measurements strongly indicate that metastable molecules play a decisive role in chemi‐ionization processes occurring in atomic O and N systems. Mobilitymeasurements, derived from an analysis of transient currents resulting from pulsed operation of the ion‐collection system, identify the predominant ion as NO+. No change in this mobility occurs from 0.6 to 100 msec after a pulsed clearing field has removed ions previously created. This and other observations indicate that NO+ is the initial ion formed by chemi‐ionization processes. Only the reactions N+N+NO*→N2+NO++e and N2 *+NO*→N2+NO++e are consistent with these observations.
39(1963); http://dx.doi.org/10.1063/1.1734422View Description Hide Description
A kinetics of formation of polymers on linear templates is developed which is applicable to the synthesis of biological macromolecules. Two extreme situations are given theoretical treatment. In both, monomer diffuses to the template surface where polymerization may occur if an adjacent site is suitable occupied. In addition the possibility of monomerdesorption is included. In one model (``unrestricted'') the reaction is initiated on each template by the polymerization of two monomers and proceeds through the addition of monomer to growing ends or by the coupling of the growing chains. In the second (``restricted'') model the number of growing centers per template is zero or one and growth is achieved through monomer addition solely. In both cases the template is assumed to be of uniform size and either single stranded or, if double stranded, with an unwinding rate that is rapid in comparison with the polymerization processes. The number of template molecules is assumed constant during the synthesis, which disregards the function of new chains as sites for continued synthesis. All degradation mechanisms are omitted from consideration, which may restrict the present theories to low conversion processes.
The pronounced differences between the unrestricted and restricted formulations in respect to kinetics and product molecular weight are elaborated and discussed in relation to recent experimental results on DNA synthesis in vitro. Finally, from known replication times in vivo, estimates of the rate parameters in the two models are derived.
39(1963); http://dx.doi.org/10.1063/1.1734423View Description Hide Description
The quadrupolar splitting of the Na23nuclear magnetic resonance signal has been studied in single crystals of borax, Na2B4O7·10H2O, and tincalconite, Na2B4O7·5H2O, placed in magnetic fields of 7536 and 5645 G, respectively. The resonancespectra show that the sodium atoms in both minerals occupy special positions with two unique sites in borax and three in tincalconite. The electric quadrupole coupling constants eqQ/h and asymmetry parameters η at these sites were determined to beThe orientations of the principal axes of the electric field gradient tensors for the above sites and their symmetry‐related sites have also been determined.