Index of content:
Volume 30, Issue 4, 01 April 1959
30(1959); http://dx.doi.org/10.1063/1.1730116View Description Hide Description
The absorptionspectrum of single crystals of 1,3,5‐trichlorobenzene in polarized light was investigated at 4.2°K. The spectrum was found to consist of a forbidden molecular part and an allowed crystal field induced system. The two parts of the spectrum are characterized by different polarizations and the polarization behavior of the molecular spectrum leads to conclusions concerning the orientations of the molecular planes in the crystal.
30(1959); http://dx.doi.org/10.1063/1.1730117View Description Hide Description
The spectrum of single crystals of hexachlorobenzene in polarized light was investigated at liquid nitrogen and liquid helium temperatures, in the 3000 A region. The absorptionspectrum consists of a forbidden benzene‐type molecular spectrum with transition moment in the molecular plane and an allowed spectrum based on the 0–0 band which is polarized in a direction close to the molecular normals. This spectrum is concluded to be crystal induced. A prominent band at 790 cm—1 from the 0–0 band is assigned as involving the a 2g vibrational fundamental and its observation in the b‐axis polarized component suggests that the molecule is puckered and has symmetry D 3d in the crystal.
30(1959); http://dx.doi.org/10.1063/1.1730118View Description Hide Description
The absorption and fluorescence spectra of single oriented crystals of durene (symmetric tetramethyl‐benzene) were photographed at 20°K in the 2800 A region. The ab plane of the crystal was investigated and it was found that the absorption intensity in the two polarization components in this plane was very nearly equal. The crystal splitting is less than 3 cm—1. The polarization properties of the absorption require the assignment of B 2u for the lowest excited state of the parent molecule, benzene.
30(1959); http://dx.doi.org/10.1063/1.1730119View Description Hide Description
The rate of recombination of nitrogen atoms in the nitrogen afterglow has been measured in a flow system using NO as a titrant and determining the NO content continuously by mass spectrometer. Above about 3 mm of mercury the reaction is predominantly homogeneous and third order. The rate constant, 5.7×1015 cc2 mole—2 sec—1, is independent of temperature in the interval studied (195–450°K). The rate is reduced by substituting helium or argon for N2 as the third body, the ratios of the third‐order rate constants with these third bodies being about the same as those for recombination of iodine and bromine atoms. Below about 3 mm of mercury a pseudo‐first‐order wall recombination becomes important. The recombination coefficient was found to be 1.6×10—5 on a glass wall that undoubtedly was partly poisoned by water.
30(1959); http://dx.doi.org/10.1063/1.1730120View Description Hide Description
The line widths of the protonmagnetic resonanceabsorption spectra and the spin‐lattice relaxation times for polycrystalline samples of ferrocene, ruthenocene, and titanocene dichloride were measured as functions of temperature, and are interpreted in terms of reorientation of the cyclopentadienyl rings about their fivefold axes. At low temperatures such reorientation is interpretable in terms of an activated process governed by a single correlation time, and the barrier restricting reorientation is obtained. At higher temperatures for these substances, as well as for benzene, such an interpretation is not suitable, and a hypothesis involving the coupling and uncoupling of adjacent rings taking part in reorientation is proposed to account for the data.
Estimation of s‐Electron Promotional Energies and the Dependence of Orbital Hybridization on Ionicity30(1959); http://dx.doi.org/10.1063/1.1730121View Description Hide Description
A simple electron pairing picture of the chemical bond is used to relate experimental valence state energies to the ionic character, orbital hybridization, and s‐electron promotional energy of an atom. A number of s‐electron promotional energies have been estimated and, in every case, they are less than s‐electron promotional energies calculated from atomic spectral data. The cause of this discrepancy has been suggested as arising from a shifting of the energy levels of the valence electrons on bonding.
30(1959); http://dx.doi.org/10.1063/1.1730122View Description Hide Description
The infrared spectrum of the dimethylnitrilium ion is presented and discussed. Thirteen of the fifteen infrared active fundamentals expected for the ion on the basis of a C 3v ′ structure are assigned, and the frequencies of the two remaining fundamentals are estimated from observed combination bands. A calculation of the skeletal bond stretching and bending force constants is given.
30(1959); http://dx.doi.org/10.1063/1.1730123View Description Hide Description
A technique for studying rapid chemical exchange and reorientation reactions by nuclear magnetic resonance techniques is discussed. A treatment for the general case relating the changes in the proton line width with changes in the frequency of rotation about the O–N bond in the alkyl nitrites is developed. Such a treatent gave fairly accurate potential energy barriers to rotation for methyl, ethyl, n‐propyl, and isopropyl nitrite. A temperature controlling device for low temperatures applicable to proton resonance techniques is discussed.
30(1959); http://dx.doi.org/10.1063/1.1730124View Description Hide Description
Further expressions are given which relate the rotational constants of the rigid asymmetric rotator to experimental data through moments of the energy levels. These expressions remove the requirement that all energy levels for given J be known. Instead, the energy levels associated with one symmetry group of the rotational wave functions for given J are required.
30(1959); http://dx.doi.org/10.1063/1.1730125View Description Hide Description
The paramagneticsusceptibilities of CrF3, CrCl3, CrBr3, and CrI3 have been measured by the Gouy method from room temperature down to a temperature at which the susceptibility is field‐dependent. Above approximately 100°K these susceptibilities all obey the Curie‐Weiss law with respective constants as follows: θ(°K) = — 124, 31, 51, and 70; P eff (Bohr magnetons) = 3.85, 3.69, 3.94, and 4.03. CrF3 evidently becomes ferrimagnetic below 70°K.
30(1959); http://dx.doi.org/10.1063/1.1730126View Description Hide Description
Diffraction patterns obtained from atomically clean germaniumsurfaces contained half‐integral order beams in (110) azimuths for both (100) and (110) surfaces and in all azimuths for the (111) surface. These results are considered to be due to displacements of surface atoms from their normal bulk lattice positions in the surface plane. Adsorption of oxygen on all of these surfaces extinguished all of the diffraction beams which were not integral order.
In addition to the normal surface lattice spacings of clean (111) and (100) surfaces of silicon, there were surface structures with larger spacings, most of which depended on the conditions of ion bombardment and/or subsequent heat treatment. Two such structures have been observed for the (100) surface of silicon; one is a double‐spaced lattice in the (110) azimuth, similar to that for germanium, and the other has a spacing about 8% greater than that of normal silicon and was obtained only after radiation quenching of the crystal from 1000°C. Two large‐spaced structures were observed for the (111) surface. All of these structures were extinguished by exposure to oxygen. Evidence is presented which indicates that these structures were not due to contamination but to the silicon itself.
The method of determining the kinetics of gas adsorption from the low‐energy electron diffraction beams is outlined and the calculations of the fractional coverage and sticking probability are presented for oxygen. For silicon crystals, the calculations of surface coverages and sticking coefficients for oxygen were found to depend on the preceding treatments of the crystals. The rate of adsorption of oxygen was proportional to the pressure, at least for pressures below 10‐6 mm Hg, and depended on the preceding treatments of the crystals. After oxygen adsorption, the clean germaniumsurface could be regenerated by heating at 500°C for 30 min and for silicon it was regenerated by heating at 900°C for a few minutes. A comparison with results of other observers is given.
30(1959); http://dx.doi.org/10.1063/1.1730127View Description Hide Description
Thermodynamic and Spectroscopic Study of Vinylidene Chloride. I. Thermodynamic Properties of the Solid, Liquid, and Ideal Gas30(1959); http://dx.doi.org/10.1063/1.1730128View Description Hide Description
The low‐temperature heat capacity, heat of vaporization, and vapor pressure of vinylidene chloride have been measured. Data obtained from these measurements are as follows: the heat capacity of the solid from 13 to 150°K; the liquidheat capacity from 150 to 290°K; the heat of fusion, 1557 cal mole—1, at the melting point, 150.59°K; the entropy of the liquid at 298.15°K, 48.17 cal mole—1 deg—1; the heat of vaporization at 298.15°K, 6328 cal mole—1; and the vapor pressure,. The entropy of the ideal gas, evaluated from the calorimetric data, is in agreement with the entropy calculated from spectroscopic and molecular data. Thermodynamic properties of the solid, liquid, and ideal gas have been tabulated at selected temperatures.
30(1959); http://dx.doi.org/10.1063/1.1730129View Description Hide Description
The Raman spectra of vinylidene chloride liquid excited by the 5461‐A line and the 4358‐A group of lines of the mercury arc have been recorded photoelectrically and some depolarization measurements have been made. The infrared spectrum of the vapor in the 700–320 cm—1 region has also been recorded. These data have enabled some uncertainties in the published vibrational assignment to be resolved but they do not indicate the need for major changes in this assignment.
30(1959); http://dx.doi.org/10.1063/1.1730130View Description Hide Description
Various valence bond structure functions are projected onto approximate ground‐state wave functions of butadiene. The most important structure, C=C–C=C, contributes approximately 70% of the total. The next most important structure corresponds to two attractive dipoles and contributes over 40% to the ground state, while structures with double bonds between the central carbons contribute relatively little. It appears that ionic contributions may be important to the central carbon‐carbon bond length, with the dipole‐dipole interaction behaving like an effective repulsion because of the variation of the electronic wave function. The problem of the nonorthogonal valence bond representation is examined: it is found that conjugation is strongly inherent even in a ``nonconjugated'' structure C=C–C=C, and that conjugation and dipolar interaction are nearly separable.
30(1959); http://dx.doi.org/10.1063/1.1730131View Description Hide Description
The prediction of the Volmer theory that the critical supersaturation for nucleation from the vapor phase on a flat surface depends upon , where φ is the contact angle of the condensate on the surface, has been tested experimentally. Critical supersaturations for visible fogging on test surfaces coated with various transparent plastics were found to agree within observational error with the theoretical values computed for the observed contact angles.
30(1959); http://dx.doi.org/10.1063/1.1730132View Description Hide Description
Energy levels and allowed transitions are calculated for a system of five spin‐½ particles of the type ABX 3. An extension to the more general system ABC 3 is made by perturbation theory. The results are applied to an analysis of the proton resonance spectrum of trans‐propenylbenzene, and it is found that at least one spin‐spin coupling constant is negative. An ``anomalous'' spectrum previously reported, which shows superficial resemblances to the one studied here, is explained by assuming the presence of an impurity.
30(1959); http://dx.doi.org/10.1063/1.1730133View Description Hide Description
The nuclear magnetic relaxation times of protons in aqueous solutions of paramagnetic ions have been measured as a function of temperature. For Mn++ ions, the protonT 1 and T 2 were measured at 10, 20, and 28 Mc/sec, and for Co++, Cu++, and Gd+++ ions, 20 Mc/sec. The temperature effects observed can be explained by incorporating the proper temperature dependences of the several correlation times into the general Bloembergen‐Solomon relaxation expressions.
In the case of Mn++solutions, which have a T 1/T 2 ratio of about 5.5 at room temperature and 20 Mc/sec, chemical as well as electron exchange and dipolar interactions are found to be important. Analysis of the data by means of the equations developed provides information concerning the chemical exchange of solvent protons with those in the hydrated Mn++ ion. An activation energy of 8.4 kcal/mole and other parameters for this process are evaluated.
The temperature dependence of T 1 and T 2 in the Co++, Cu++, and Gd+++solutions can be explained by the temperature dependence of the dipolar interaction alone. Varying the anion in Cu++solutions had little effect on T 1 or its temperature dependence; also, T 1 for Cu++solutions was found to vary inversely with concentration over the whole temperature range studied.
Pure Quadrupole Resonance Determinations of Molecular Charge Distributions. II. Substituted Pyrimidines30(1959); http://dx.doi.org/10.1063/1.1730134View Description Hide Description
The amount of double‐bond character of the carbon‐chlorine bond at the 2, 4, and 6 positions in twelve substituted chloropyrimidines has been estimated from the chlorine pure quadrupole resonance frequencies. If it is assumed that the nitrogens have a weak inductive effect upon adjacent ring positions, then the calculated values for the amount of double‐bond character agree quite well with data obtained from experiments employing Zeeman splitting techniques applied to pure quadrupole resonance spectra.