Volume 37, Issue 12, 15 December 1962
Index of content:
37(1962); http://dx.doi.org/10.1063/1.1733097View Description Hide Description
The electron paramagnetic resonance spectrum of Fe3+ substituted for Al3+ in guanidinium aluminum sulfate hexahydrate, C(NH2)3Al(SO4)2·6H2O, was measured at X‐band and K‐band frequencies at room temperature and 77°K. The spectra were observed with the magnetic field parallel to the c axis, which is also the axis of distortion of the octahedra formed by the water molecules. Relative values only of the spin Hamiltonian parameters for an S=5/2 ion were determined; the fitting procedure gave D negative and a and F positive. For room temperature and type‐II sites, the parameters in the spin Hamiltonian were calculated as; g=2.003, D=—0.17601 cm—1, a=0.020595 cm—1, and F=0.10047 cm—1. For type‐I sites at room temperature; g=2.003, D=—0.18508 cm—1, a=0.021966 cm—1, and F=0.10530 cm—1. At 77°K for type‐II sites; g=2.003, D=—0.18902 cm—1, a=0.053963 cm—1, and F=0.15203 cm—1. Type‐I sites at 77°K gave g=2.003, D=—0.19506 cm—1, a=0.059302 cm—1, and F=0.16389 cm—1.
Studies on the Alternant Molecular Orbital Method. IV. Generalization of the Method to States with Different Multiplicities37(1962); http://dx.doi.org/10.1063/1.1733098View Description Hide Description
A general formulation of the alternant molecular orbital method, applicable to states of different multiplicities in systems with an even number of atoms, is presented. The treatment is formulated in such a way that the same integral sums appear in the energy expression for the states belonging to different multiplicities. The resultant energy expressions show a simple dependence on the mixing parameter λ.
Application to cyclic systems shows that the best λ values decrease slowly with increasing multiplicity. Comparison with the configuration interaction treatment reveals that the AMO method corresponds to a restricted CI treatment, but it is capable of yielding a considerable part of the energy improvement obtained by the latter method.
37(1962); http://dx.doi.org/10.1063/1.1733099View Description Hide Description
The magnitude and the direction of the dipole moment of formic acid were determined through the Stark effect of rotational transitions. The dipole moments of HCOOH and HCOOD are 1.415±0.01 D and 1.39±0.01 D, respectively. The direction of the dipole moment lies between C–H and C=O bonds making an angle of 42.4°±2° with the C=O bond.
37(1962); http://dx.doi.org/10.1063/1.1733100View Description Hide Description
The electrical conductivity of nonstoichiometric α‐Nb2O5—x was measured as a function of both the composition over the range 0.001≤x≤0.137 and the temperature from 77° to 1270°K. The temperature dependence of the conductivity of a specimen of fixed oxygen content may be rationalized on the basis of excitation of electrons from donor centers at low temperatures and an exhaustion region at high temperatures. The conductivity data are interpreted in terms of an oxygen ion vacancy defect capable of trapping two electrons which can be thermally excited into a narrow 4dconduction band. Assuming this type of defect, an estimate of the mobility of the electrons in the conduction band is made from the conductivity data in the exhaustion region.
37(1962); http://dx.doi.org/10.1063/1.1733101View Description Hide Description
The electric field gradient (∂2 V/∂Z 2)atom has been evaluated for BI, CII, NIII, OIV, FV, NeVI with an open‐configuration wavefunction for the 2 P state. The fine structure splitting separation δ is calculated using the computed values of (∂2 V/∂Z 2)atom and the energy optimized orbital exponents for the 2p electron. The electric field gradient of BI agrees satisfactorily with the previous value obtained from the observed fine structure levels. The use of a relativistic correction on the fine structure which minimizes the error in comparison with the experiment is investigated.
37(1962); http://dx.doi.org/10.1063/1.1733102View Description Hide Description
Fluoro‐organic compounds were subject to irradiation by fast neutrons produced in a nuclear reactor; fluoroderivatives labeled with F18 were produced and their yield was determined. It was found that labeling yields of fluoro‐organic compounds were substantially higher than those of other organic compounds exposed to fluorine 18 produced extramolecularily, either by the O16(H3, n) or by the F19(n, 2n) reactions. The extent of substitution of a hydrogen atom by radiofluorine was found more than 100 times higher than that of a fluorine atom. It was shown that the labeling occurs mainly in the same part of the molecule which carried the original fluorine atom. In some cases it was shown that it is the same atom which is transformed to a radionuclide.
When fluoroorganic compounds were irradiated in solution, a constant molecular labeling yield was observed. This yield which was obtained in dilute solutions in water, acetic acid or ethanol, was found independent of solute concentration; moreover, it was not affected by the presence of scavengers. The labeling yield of the aliphatic solvents was lower when exposed to fluorine 18 produced from an organically bound fluorine than from a fluoride ion.
It is difficult to interpret the experimental results without assuming that a genuine retention of the transformed fluorine 18 in the parent molecule takes place. It is suggested that the (n, 2n) reaction proceeds, in part at least, as a spallation‐type reaction, forming an excited F18 nucleus which does not undergo any recoil. The excited F18 is subsequently converted to the ground state following gamma emission; the recoil energy of many of these gammas is not sufficient to cause an irreversible cleavage of the C–F bond.
Molecular Vibrations and Structure of High Polymers. III. Polarized Infrared Spectra, Normal Vibrations, and Helical Conformation of Polyethylene Glycol37(1962); http://dx.doi.org/10.1063/1.1733103View Description Hide Description
The polarized infrared spectra of highly oriented crystalline films of polyethylene glycol were measured in the region 3500–400 cm—1. Five parallel bands and eleven perpendicular bands due to the fundamental vibrations were observed in the region 1500–600 cm—1. From the analysis of these bands, the polyethylene glycol chain was found to belong to the dihedral group and have twofold axes intersecting the helix axis at right angles. The structural models of polyethylene glycol were discussed by the use of the equations for the helical parameters (the second paper of this series). The most likely model (TGT) contains seven repeating units and two helical turns per fiber period of 19.25 Å. The internal rotation angles for this model are calculated to be 60° for the C–C bond and 191.5° for the C–O bond. The A 1, A 2, and E(θ) normal vibrations of polyethylene glycol were calculated by the general method for treating helical polymers (the first paper of this series). The normal vibrations of p‐dioxane were also calculated. The modified Urey—Bradley force field was used for the calculations and the potential constants were transferred from polyethylene, dimethyl ether, and propyl alcohol. The frequencies calculated for the TGT model agreed well with the observed frequencies. The potential energy distributions were also calculated and the nature of the infrared bands was elucidated. The far‐infrared spectra of crystalline films were also measured in the region 600–50 cm—1 and two parallel bands and two perpendicular bands were observed. The skeletal normal vibrations of the TGT model were also calculated, taking into account the torsional potential field. The torsional constants correspond to a potential barrier of 3 kcal/mole. The calculated frequencies agreed well with the observed, providing substantial support for the TGT model. The infrared bands in the region of 3000 cm—1 were assigned to the C–H stretching modes and the infrared active combination vibrations.
37(1962); http://dx.doi.org/10.1063/1.1733104View Description Hide Description
Octadecyl disulfide irradiated at — 196°C in the absence of oxygen exhibits radicals reflecting breakage at the sulfur group to form RS·radicals as well as breakage in the alkyl group. During storage of the samples at from 0° to 40°C an increase in RS·concentration is observed while reduction of radicals in the alkyl group takes place. After initial rapid reactions occurring immediately after warm‐up, a lower rate is observed for the increase in RS·content and decay of the radicals associated with the alkyl group. Both of these slower processes appear to occur as a result of a common reaction with an activation energy of about 13 kcal/mole. It is believed that this reaction involves the combination of a disulfide link with a radical in the alkyl group and occurs as a result of molecular diffusion in the crystalline material. Calculations based on a diffusion model yield theoretical curves which can be made to fit the experimental results within the limits of experimental error.
37(1962); http://dx.doi.org/10.1063/1.1733105View Description Hide Description
The drift velocity of F centers in KCl and KI as a function of temperature is determined. The electric field producing this motion in the colored portion of the crystal is measured directly. The electric field in the uncolored region is found to be appreciably higher than the field in the colored portion.
The thermal ionization energy for F centers is found to be 2.4 eV for KCl and about 2.0 eV for KI.
Empirical Relations of Thermodynamic ``Constants'' to the Activation Parameters of Polymer Relaxations37(1962); http://dx.doi.org/10.1063/1.1733106View Description Hide Description
Experimental data are analyzed to show that for some polymer relaxations observed in dynamic mechanical experiments, the activation enthalpy and entropy appear to follow a series of empirical relations. These relations, which also apply to inorganic solids, indicate that melting temperature, thermal volume expansion coefficient, glass transition temperature, and the relaxations are interrelated in polymers. Also, the modulus and internal friction of polypropylene and polyoxymethylene are reported. Measurements made between —65° and 165°C with longitudinal waves at a frequency of 12 Mc show relaxations to occur at 60°C in polypropylene and at 0° and 145°C in polyoxymethylene.
37(1962); http://dx.doi.org/10.1063/1.1733107View Description Hide Description
Azide ion solutions containing mercuric chloride were photolyzed with ultraviolet light from a mercuric arc. The products were nitrogen and mercurous azide, both of which were produced by a chain mechanism. Assumptions that the reaction is initiated by photochemicalcharge transfer from an azide ion to water and that the chain carriers are the lowest triplet state nitrogen molecule and the excited azide radical can account for the reaction of azide ion plus mercuric chloride system.
Based on the same assumption, the photolysis of pure azide ion and hydrogen azide solutions have been successfully reviewed.
37(1962); http://dx.doi.org/10.1063/1.1733108View Description Hide Description
Molecular‐orbital calculations of the pi‐electron spin densities in a series of aromatic and aliphatic nitrile anion radicals have been performed using the Hückel—LCAO method and the approximate configuration‐interaction correction of McLachlan. Coulomb and resonance integrals for the nitrile group were estimated by comparing calculated spin densities with proton and carbon‐13 hyperfine splittings obtained from electron‐spin resonance measurements. The predicted spin densities were in generally good agreement with the experimental results, but for some of the compounds it is impossible to get an exact fit between theory and experiment within the framework of the simple valence‐theory calculations if the sigma—pi interaction parameters relating splittings to spin densities are taken as fixed quantities. Semiempirical treatments of the N14 and C13 splittings in the cyano groups give excellent correlations of the experimental splittings with the calculated spin densities, and estimates have been made of some of the sigma—pi interaction parameters relating to the N14 splitting. Polarographic half‐wave potentials have also been compared with calculated pi‐electron energies. A discussion is given of the relation between the spin densities predicted by the valencebond and molecular‐orbital theories.
37(1962); http://dx.doi.org/10.1063/1.1733109View Description Hide Description
Electron spin resonance studies are reported on the anion radicals of single‐ring aromatic ompounds containing aldehyde, acetyl, or amide groups, as well as other substituents. The radicals were generated by electrolytic reduction in N,N‐dimethylformamide solution. Many of the radicals have spectra which indicate that the carbonyl group is locked in a conformation planar with the ring for times of the order of a microsecond or longer. The para dicarbonyls and the 3‐cyanoacetophenone anion were found to be present in both the cis and trans modifications. A simple modification of conventional molecular‐orbital theory has been used with considerable success to account for the loss of symmetry in the pi‐electron spin density for compounds with a locked carbonyl group, and the calculated energy differences for the cis and trans isomers are in good agreement with experiment. Molecular‐orbital calculations of spin densities were made for most of the radicals, often with excellent results, and comparisons are made with the predictions of valence‐bond theory. The benzaldehyde, acetophenone, and 4‐fluoroacetophenone anions have spectra with abnormally small ring‐proton splitting constants, and no satisfactory explanation of these anomalous results has been found. The appearance or nonappearance in all the radicals but these three of twofold symmetry in the pattern of splitting constants is interpreted qualitatively in terms of competing effects determined by the bond order of the bond between the ring and the carbonyl group, and steric factors in the neighborhood of the carbonyl group. A number of features of the experimentally determined spin‐density distributions have been correlated with the relative electron‐withdrawing effects of the substituents.
37(1962); http://dx.doi.org/10.1063/1.1733110View Description Hide Description
It has been shown that a simple model can account for the available data on the solvent dependence of the hyperfine splittings in the ESR spectra of organic free radicals. We have assumed that the changes in splittings arise entirely from a redistribution of the pi‐electron spin density, and that the spin density is affected only by localized complexes between the solvent and polar substituents or heteroatoms in the radical. This model predicts that the magnitudes of the changes in proton splittings should often be small, although large fractional changes at positions of small spin density can sometimes occur. The large variations found for the nuclei of many electron atoms are shown to arise because their splittings are very critical functions of the spin density.
The effect of the solvent on the protonhyperfine splittings in the semiquinones has been treated by assuming that the solvent alters the electronegativity of the oxygen atoms and by performing molecular‐orbital calculations to estimate spin density changes. These calculations reproduce both the directions and magnitudes of the changes in proton splittings with solvent and are in agreement with the very large fractional changes observed at some positions with small splittings.
An analysis of the effects on the ESR spectra of the exchange reactions between the different solvent complexes has shown that the spectra normally observed result from systems undergoing rapid exchange. A simple model for a radical with only one functional group which can interact with the solvent (e.g., the nitrobenzene anion), is shown to account very well for the observed variations in hyperfine splittings as a function of the composition of a binary solvent mixture. The treatment of the exchange reactions by use of the modified Bloch equations is compared with the spectral density method. The contribution to the linewidth from solvent‐induced fluctuations in the spin‐density distribution is calculated for a simple two‐jump case, and other factors affecting the linewidths of radicals subjected to random solvent interactions are discussed.
37(1962); http://dx.doi.org/10.1063/1.1733111View Description Hide Description
The formal theory of the steady state of thermogravitational thermal diffusion in binary liquid mixtures is developed with the object of using a thermogravitational column of small annular separation to determine thermal diffusion factors quantitatively. Although the general approach of Furry, Jones, and Onsager is followed, full use of all hydrodynamic and thermodynamic transport equations permits the explicit statement and the subsequent re‐examination of the simplifying approximations. The resulting differential and integral equations, which are expressed in cylindrical coordinates, are solved subject to boundary conditions imposed by the vanishing of all velocities at the apparatus walls. Inclusion of the temperature and composition dependences of the thermodynamic properties (density, viscosity, etc.) of the fluid leads to the appearance in the final equations of a term which quantitatively takes account of the ``forgotten effect'' as well as of ordinarily negligible correction terms. The final working equation is valid for the arithmetic mean experimental temperature and the initial uniform composition of the mixture. It is shown that an easily measured composition difference can be substituted for the steady‐state vertical composition gradient.
Thermogravitational Thermal Diffusion in Liquids. II. Experimental Thermal Diffusion Factors for Carbon Tetrachloride—Cyclohexane at 25°C37(1962); http://dx.doi.org/10.1063/1.1733112View Description Hide Description
An experimental investigation of thermogravitational thermal diffusion in carbon tetrachloride—cyclohexane at 25° is described, and thermal diffusion factors for this system are calculated on the basis of the formal theory derived in the preceding paper. The procedure for determining the required compositions involves a special weighing technique and Rayleigh interference refractometry. A new method for using the latter is presented. The design, construction, and use of a cyclindrical thermogravitational apparatus are discussed. Small numerical corrections are made for the production and thermal diffusion of impurities, one of which is apparently cyclohexylhydroperoxide. Calculated thermal diffusion factors agree well for most compositions with results of pure thermal diffusion measurements.
37(1962); http://dx.doi.org/10.1063/1.1733113View Description Hide Description
The results of an LCAO—MO calculation for the boron hydrides and hydride ions B2H6, B4H10, B5H9, B5H11, B6H10, B9H15, B10H14, B10H16, BH4 —, B3H8 —, B9H14 —, B10H10 —2, B10H14 —2, B12H12 —2 are reported. Charge distributions and overlap populations are calculated from the wavefunctions for real distances and for idealized molecules with all distances equal. The three‐center bond theory is extended to incorporate unsymmetric equivalent structures with concomitant improvement in charge distributions. These are compared with the presumably better LCAO—MO charges. The valence structure of a new boron hydride, B18H22, is discussed.
37(1962); http://dx.doi.org/10.1063/1.1733114View Description Hide Description
Differential elasticscattering cross sections have been measured for collisions of potassium with krypton and hydrogen bromide at different energies in the thermal energy range. In all qualitative details the results can be explained assuming an interaction potential of type exp‐6 (Buckingham). Rainbow scattering is observed and potential parameters are evaluated.
Even though several hundred partial waves contribute to the scattering the experimental evidence appears to indicate the need for the semiclassical rather than the classical treatment of wide‐angle scattering in the region of the rainbow maximum.
37(1962); http://dx.doi.org/10.1063/1.1733115View Description Hide Description
The reaction of K+HBr→KBr+H is studied by measuringelastic and reactive scattering in crossed molecular beams. The K beam is well collimated and made nearly monoenergetic by a mechanical velocity selector. The HBr effuses from a low‐temperature source as a diverging beam.Angular distributions of KBr at fixed K energy and the variation of the total chemical reaction cross section with initial relative energy indicate a reaction threshold energy below 0.4 kcal/mole and a nearly constant reaction cross section at higher energies up to about 4.5 kcal/mole. The difference between the measuredscattered K and the K intensity calculated classically for hypothetical nonreacting molecules scattered elastically leads to a determination of the variation of the probability of reaction with impact parameter or with the potential energy at the distance of closest approach in the collision. The influence of the requirement of conservation of angular momentum on the probability of reaction is evident. At impact parameters below about 3.5 Å, 90% of the collisions lead to reaction. Above threshold the reaction cross section is found to be about 34 Å2.
37(1962); http://dx.doi.org/10.1063/1.1733116View Description Hide Description
The methyl proton spectrum of CH3CHF2 in the gas phase consists of three broad peaks, with a sharp doublet superimposed on the central broad peak. The proton spectrum of CH2F2 also consists of three broad features, with a single sharp peak superimposed on the central broad peak. Proton—fluorine double resonance measurements show that the width of the broad features in the proton spectra is determined by fluorine relaxation processes, and that the broad peaks arise from transitions between states with K(F)=1, while the sharp lines must be assigned to transitions between states with K(F)=0. The relative widths of the broad peaks are consistent with the assumption that the collision‐modulated spin—rotation interaction provides the dominant fluorine relaxation mechanism, but that this interaction is not important for proton relaxation. The addition of oxygen to CH3CHF2 causes the broadening of the sharp central doublet in the methyl proton spectrum. However, it is shown by double resonance that the dominant effect of the addition of oxygen to CH3CHF2 is to decrease the protonrelaxation time, rather than to increase the fluorine singlet—triplet conversion rate.