Index of content:
Volume 37, Issue 6, 15 September 1962
Anomalous Chemical Shifts in the Proton Magnetic Resonance Spectra of the Dimethylcyclohexanes and Related Hydrocarbons37(1962); http://dx.doi.org/10.1063/1.1733259View Description Hide Description
High‐resolution protonmagnetic resonance spectra have been determined for the dimethylcyclohexanes and several related hydrocarbons between —130° and 130°C. All of the compounds which should undergo rapid ring inversion at room temperature produce spectra which change upon cooling because of ``freezing out'' of this motion. The assumption that appearance of the ring‐hydrogen resonances as a relatively narrow band is invariably a symptom of rapid ring inversion is shown to be unfounded. Several of the ring spectra differ drastically from what is predicted using a bond‐anisotropy model. A previously unrecognized effect must then make a significant contribution to the observed chemical shifts.
37(1962); http://dx.doi.org/10.1063/1.1733260View Description Hide Description
A model is developed for one‐dimensional steady‐state diffusion‐limited transport of solid between source and seed planes in the presence of a known chemical equilibrium. The transport depends on the reactionenthalpy, the separation and temperature difference between source and seed, and the departure from equilibrium at the crystal surfaces. A simple expression gives the dependence of transport on the gaseous binary diffusion coefficients, the reaction coefficients, and mean partial pressures of the various species present.
37(1962); http://dx.doi.org/10.1063/1.1733261View Description Hide Description
The method of Lever, which has enabled calculation of the steady‐state diffusion limited rate of transport of solid for the case of a single chemical equilibrium, is generalized to include the situation in which several equilibria are simultaneously operating. This is accomplished for the case of N equilibria by the introduction of (N — 1) ``coupling parameters,'' so defined that the steady‐state conditions become formally identical to those given previously. Following Lever, the results of these operations are combined with the usual thermodynamic relations for the temperature dependence of the equilibrium constants, finally resulting in a set of (N — 1) linear simultaneous equations in (N — 1) unknowns, the ``coupling parameters.'' These equations can be solved, in general, in terms of the ratios of (N — 1) dimensional determinants.
On the basis of the above, a straightforward procedure is devised for the calculation of the rate of transport of solid, based on knowledge of initial conditions as well as thermodynamic and diffusion coefficient data.
Spectral Solvent Shift. II. Interactions of Variously Substituted Hydrocarbons with Polynuclear Aromatic Hydrocarbons37(1962); http://dx.doi.org/10.1063/1.1733262View Description Hide Description
Nine electronic transitions in the chromophores naphthalene, azulene, and phenanthrene have been observed in 21 chlorinated hydrocarbons and chlorocarbons. The results support an interpretation of spectral shifts which considers interbond potentials between solvent and chromophore, involving bond polarizabilities and dipole moments. The results are generally in accord with the McRae solvent‐shift theory. Exceptional behaviors of some of the transitions in azulene are attributable to changes in the magnitude of static dipole moment on excitation.
The 1 La transition of naphthacene has been observed in some 64 substituted hydrocarbons, the substituents ranging over nine groups. The results are also interpreted by a statistical theory using interbond (or intergroup) potentials. They are compared to the McRae theory, especially with respect to variation of the weighted mean wavelength parameter L 0 and the quadratic Stark effect. The effect of the latter seems considerable while no effect of the former can be detected.
37(1962); http://dx.doi.org/10.1063/1.1733263View Description Hide Description
Self‐diffusion in liquid ethane, under its own vapor pressure, has been studied over most of the liquid range between the triple point and the critical point by means of the spin‐echo technique. The activation energy for self‐diffusion increases markedly from approximately 0.5 kcal/mole at 100°K to 3.5 kcal/mole at 300°K. The Stokes—Einstein relation is not well satisfied with either a constant molecular radius or a radius proportional to the cube root of the molar volume. The results are well correlated by a free‐volume expression in which the free volume, defined as an actual volume minus the volume of the hypothetical liquid at 0°K, replaces temperature as the independent variable. The value of the hypothetical 0°K liquid volume resulting in best fit of the experimental data is less than that determined by extrapolation of the liquid specific volume to 0°K or that based upon best fit of experimental liquidviscosity data, but it is between a value based on a hard‐sphere radius determined from gas viscosity and one derived from the second virial coefficient.
Proton spin‐lattice relaxation measurements on the liquid ethane sample over a similar temperature range indicate that the relaxation is controlled by mutual diffusion of ethane and small amounts of dissolved oxygen, although the situation is somewhat complicated by temperature dependence of the distribution of oxygen between the two phases.
37(1962); http://dx.doi.org/10.1063/1.1733264View Description Hide Description
Thermodynamic properties of multicomponent systems can be simply generated from matrices whose elements represent basic data. Although this concept is quite general, it has been applied specifically to generating the entropy,enthalpy, and free energy of multicomponent systems, as functions of composition and temperature. Each of these properties can be generated from the product of three matrices: a row vector whose elements depend only on the composition, a rectangular matrix whose elements remain invariant for a given system, and a column vector whose elements depend only on the temperature. A given row of the rectangular matrix corresponds to one of the species in the system and may be regarded as a thermodynamic‐property vector for that species. Thermodynamic‐property vectors have been calculated and tabulated for 143 species which are stable at high temperatures. The matrix representation of thermodynamic data has proved to be particularly useful in formulating large multicomponent problems for a digital computer. Two numerical examples illustrate the utility of the method.
37(1962); http://dx.doi.org/10.1063/1.1733265View Description Hide Description
Photodetachment of I— ions has been observed in a crossed‐beam experiment. The apparatus used was essentially similar to that in previous photodetachment experiments. Use of four Sharp Cut filters provided good ``effective'' resolution close to threshold but did not permit an independent determination of the behavior of the cross section as a function of wavelength. A step‐function cross section, shown by Berry and co‐workers to approximate closely the true cross section behavior, was used to provide an upper energy limit to the electron affinity shown to be close to the actual value. The value of the electron affinity thus determined, 3.076±0.005 eV, is in excellent agreement with the shock‐wave determination of Berry, 3.078±0.005. The absolute magnitude of the cross section in the region 0.3 eV above threshold was determined to be 2.1±1.1×10—17 cm2.
37(1962); http://dx.doi.org/10.1063/1.1733266View Description Hide Description
Data for the specific heat of polyethylene as a function of crystallinity are collected from 1° to 420°K. Specific heats,entropies, and enthalpies of the completely amorphous and crystalline polyethylene are extrapolated. The glass transition is located for the amorphous at 237°K with a ΔCp of 2.1 cal deg—1 (mole CH2)—1. Specific‐heat values at constant volume are calculated for the crystalline polyethylene.
37(1962); http://dx.doi.org/10.1063/1.1733267View Description Hide Description
Using the specific heats of completely crystalline polyethylene as a basis, the vibrational spectrum is discussed. The vibrational spectrum is found to consist of three completely separate parts. (A) The high‐frequency CH2‐stretching vibrations between 2850 and 2930 cm—1 which contribute little to the specific heat below 350°K. (B) The low‐frequency optical vibrations between 720 and 1480 cm—1 which contribute above 150°K increasingly to the specific heat. (C) The acoustical vibrations which stretch from zero to approximately 500 cm—1 and are at 260°K to 90% excited. A and B are adequately known from infrared and Raman experiments. C can be approximated by a Tarasov‐type treatment leading to θ temperatures of 123° and 540° for the three‐dimensional and one‐dimensional parts, respectively.
More detailed spectra are discussed, and a semiempirical best fit is given. The one‐dimensional Debye function necessary for this discussion has been computed to five places and is tabulated in steps of hν/kT=0.01.
Measurement by Cyclotron Resonance of Molecular Cross Sections for Elastic Collisions with 295°K Electrons37(1962); http://dx.doi.org/10.1063/1.1733268View Description Hide Description
Cyclotron resonance absorption is used to obtain molecular scattering cross sections for electrons at 295°K. The electrons were present in active nitrogen obtained by passing N2 through a microwave discharge, and various gases were introduced into the active nitrogen downstream from the discharge. Very weak rf fields, with electric components parallel to the dc magnetic field, were used in order not to ``heat up'' the electrons. The observed width of the cyclotron resonance line is proportional to the molecular‐electronic cross section since electron—electron collisions are not significant in these experiments. But suitable averages over electron velocities must be carried out to account for the observed line shapes and to relate the line widths to the cross sections. The spectral lines are sometimes quite asymmetric and this asymmetry can then be used as a rather sensitive measure of electron concentration. Dipolar gases were observed to have large cross sections which exhibited approximately a power dependence on the dipole moment; theoretical calculations based on a partial wave treatment of dipole scatterers agreed well with experiments. The cross section for N2 is 4.96 Å2; for CO2 302 Å2; and for the dipolar gases N2O, NH3, SO2, HCF3, H2O, HCN, and CH3NO2, the cross sections are 56, 1560, 1840, 1760, 2360, 5280, and 6020 Å2, respectively. The dipolar cross sections are inversely proportional to the energy of the electrons, and both theoretical and experimental values of the cross sections refer to energies of 3k(295°)/2. The large cross section for CO2 probably arises from its large quadrupole moment.
37(1962); http://dx.doi.org/10.1063/1.1733269View Description Hide Description
A study of molecular motion during the process of polymer formation has been performed with measurements of proton magnetic relaxation. The presence of two distinct relaxation processes for the protons of the monomer molecules and those of the polymer chains has been observed. During the polymerization the spin‐lattice relaxation timeT 1 of the monomerprotons decreases very slowly while the flowviscosity of the sample increases greatly; this reveals that the motions responsible for the relaxation are mainly rotational. These motions can be considered to be thermally activated: the activation energies increase as the polymerization proceeds. The results we have obtained can be explained by assuming that there is a motion which has a predominant effect upon the relaxation and that a distribution of correlation frequencies, the width of which increases as the polymerization proceeds, is present. By making certain assumptions it has been possible to obtain some information about this distribution.
Mass Spectrometric Studies of Atomic Reactions. III. Reactions of Hydrogen Atoms with Nitrogen Dioxide and with Ozone37(1962); http://dx.doi.org/10.1063/1.1733270View Description Hide Description
The reaction of H atoms with excess NO2 results in the production of one NO molecule for every NO2 molecule consumed and the consumption of 1.5±0.1 NO2 molecules per H atom initially present. The secondary reactions are rapid and make the over‐all reaction very suitable for H‐atom titration. A mechanism is proposed to explain these results. The reaction of H atoms with excess O3 results in the consumption of 3.1±0.2 O3 molecules for each H atom initially present. Addition of H2 and N2O to the system showed that vibrationally excited OH formed in the initial reaction decomposes O3. Other mechanisms for the additional O3 consumption are discussed. The rate constant for the primary step was found to be (4.8±0.5) × 10—11 cm3 molecule—1 sec—1 for the NO2reaction and (2.6±0.5) × 10—11 cm3 molecule—1 sec—1 for the O3reaction.
37(1962); http://dx.doi.org/10.1063/1.1733271View Description Hide Description
A Langmuir probe was used to study the ionization produced during the oxidation of acetylene in shock waves. While the results were ambiguous at low applied potentials, positive probes were shown to measure approximately the free electron concentrations. These were found to be several orders of magnitude higher than those of thermodynamic equilibrium. The ionization behaves very similarly to the emission of short uv radiation from shock waves in acetylene—oxygen mixtures described by Kistiakowsky and Richards: after an induction period it rises exponentially and then decays. The values of the induction periods and of the exponential time constants agree with those obtained from uv emission studies. Emission of visible and near uv radiation has also been studied now and very similar results obtained. The temperature coefficient of the maximum ionization and maximum radiation intensities is the same, equivalent to 23 kcal/mole. The decay of ionization is second order. The mechanisms of chemiluminescence and chemi‐ionization are discussed, and it is suggested that the following reactions are involved:or The effect of small additions of lead tetramethyl and acetone on the kinetics of the reaction was investigated. Both additives reduce sharply the induction periods in the reaction, without affecting strongly the rate of exponential growth of the reaction rate as evidenced by ionization and radiation.
37(1962); http://dx.doi.org/10.1063/1.1733272View Description Hide Description
The theory of neutron scattering by paramagnetic rare‐earth ions has been given by Trammell and more recently by Odiot and Saint‐James. Unlike the case for the iron‐series transition metals, for which the orbital angular momentum of the 3delectrons is almost completely quenched, the magnetic scattering of neutrons from the rare earths (except for gadolinium and europium) arises from both the spin and orbital contributions to the magnetization. Using recently determined Hartree—Fock wave functions for some of the trivalent rare‐earth ions we have determined the functions 〈jn 〉 and 〈gn 〉 (for n=0, 2, 4, and 6) which are necessary for the theoretical evaluation of the spin and orbital contributions, respectively, to the magnetic form factor. A comparison of these theoretical results with the experiments of Koehler and Wollan for Nd3+ and Er3+ and of Koehler, Wollan, and Wilkinson for Ho3+ is presented. Large differences are found between Trammell's theory and experiment and some possible reasons for this are discussed. X‐ray atomic scattering factors for these ions are also given and found to agree very closely with the results of the Thomas—Fermi—Dirac statistical method.
37(1962); http://dx.doi.org/10.1063/1.1733273View Description Hide Description
At low temperatures the vibrational relaxation time τv is much shorter than the dissociation time τd. The O2–Ar results of Camac [J. Chem. Phys. 34, 448 (1961)] and Camac and Vaughan [J. Chem. Phys. 34, 460 (1961)] yield τd/τv=60 at 5000°K and, upon extrapolation, τd/τv=1.4 at 18 000°. According to these extrapolations, dissociation at high temperatures would proceed significantly before vibrational equilibration would occur. The purpose of this investigation was to determine how the dissociation rate will be affected by a lack of vibrational equilibrium. Studies of the dissociation rate of dilute O2–Ar mixtures were made in a 24‐in. diam shock tube from 5000°—18 000°K. The O2 concentration was monitored by its absorption of 1470 Å radiation. An Arrhenius plot of the data yielded a straight line from 5000°—11 000°K, the rate constant being given by k d=2.9(±12%)×1014 exp (—D/RT)cc/mole‐sec. Above 11 000° the data deviate from the line given by this equation—at 18 000° k d being 0.45 times the calculated value. An incubation time Δt was observed during which dissociation does not proceed to a significant extent. The ratio of this incubation time to the vibrational relaxation time (obtained by extrapolating Camac's low‐temperature results) when plotted against translational temperature displays a slight negative temperature dependence. At 18 000° Δt/τv=0.4, at 8000° Δt/τv=1, and at 5500° we estimate that Δt/τv≈2.
37(1962); http://dx.doi.org/10.1063/1.1733274View Description Hide Description
Two photosensitive electron paramagnetic resonance signals A and B are found in self‐activated ZnS phosphors of polycrystalline form. Both these signals appear at 77°K when illuminated with uv light, and can be quenched with visible light of longer wavelength. The center responsible for the A signal is a hole trap and is found to be the luminescent center of the self‐activated emission. The atomic structure of this center is concluded to be an associate of zinc‐ion vacancy and a coactivator ion at one of the nearest possible sites. The center responsible for the B signal is an electron trap. A simple sulfur vacancy is proposed to be the structure of this center. In one case, the A signal was traced using a crude single crystal. Owing to the crudeness of the crystal, the result is not complete, but is found to be sufficient enough to substantiate the accuracy of the powder pattern analysis. Also described in this report are the results of investigating the effects of oxygen incorporated into ZnS by both the electron paramagnetic resonance and the thermoluminescence techniques. One of the observed effects is that the incorporation of oxygen leads to formation of self‐activated luminescent centers whose coactivator ions are not at the nearest possible sites.
37(1962); http://dx.doi.org/10.1063/1.1733275View Description Hide Description
An improved continuous approximation to the number of states of a collection of harmonic oscillators of different frequencies has been developed and applied to the quasi‐equilibrium theory of mass spectra. The classical approximation previously used in this theory has been shown to be invalid at low energies. The difficulties in the application of the quasi‐equilibrium theory of mass spectra are to a great extent resolved by the use of this more reasonable approximation.
37(1962); http://dx.doi.org/10.1063/1.1733276View Description Hide Description
Temperature dependence of the ESR spectrum in irradiated polyethylene has been studied using stretched specimens. Among the observed spectra the septet spectrum with doublet substructure is known to be due to an ``allyl type'' –CH2–ĊH–CH=CH–CH2– radical. Although the spectrum observed at 142°C is such a septet (splitting 21.3 G) with doublet substructure (splitting 5.5 G), it changes with temperature in a complicated way. Positions of the peaks are displaced, and their apparent number changes, while the spectrum below — 180°C remains unchanged. Such changes were found completely reversible with temperature, and the magnitude of the over‐all splitting as well as the outermost doublet remained constant. The analysis of this complicated temperature dependence has been carried out on the assumption that the hyperfine coupling constant of the β protons changes with temperature, while that of the α protons remains constant. By choosing an appropriate set of coupling constants (A β) of β protons, the observed spectrum could be reproduced. From the spectrum at — 180°C were deduced A β1 = 30 G and A β2 = 11 G. Such inequivalence of the coupling might suggest that the methylenic group in the ``allyl type'' radical should be twisted with respect to the nodal plane of the unpaired π electron. As the temperature rises, a hindered oscillation of the methylenic group will take place between two equivalent twisted positions with respect to the same nodal plane. Such behaviors have been interpreted using a theory of time‐dependent phenomena and the activation energy was obtained to be 410 cal/mole, the frequency factor being 4.6×108 sec—1.
37(1962); http://dx.doi.org/10.1063/1.1733277View Description Hide Description
Diffusion coefficients for pairs of simple gases have been determined from the experimental viscosities of binary mixtures. The calculational procedure is essentially independent of any intermolecular‐force model. The resulting diffusion coefficients are in good agreement with directly measured experimental diffusion coefficients, even in the temperature range where quantum effects are important.
37(1962); http://dx.doi.org/10.1063/1.1733278View Description Hide Description
The infrared spectra of polycrystalline benzene and fully deuterated benzene have been examined under high resolution. All ``u'' fundamentals (except ν16 of C6D6) have been observed and most crystal multiplets have been resolved, many to the full extent predicted. Accurate frequencies at two low temperatures are presented.
Absolute intensities have been measured at two temperatures for all fundamentals observed. An interference method has been used to measure sample thickness. The results are compared to existing data on three phases of benzene. The sum of all integrated intensities for a given molecule is empirically found to be independent of state of aggregation, and a sum rule expressing this fact is derived and criticized.