Volume 35, Issue 2, 01 August 1961

Diamond‐Graphite Equilibrium Line from Growth and Graphitization of Diamond
View Description Hide DescriptionDiamond growth occurs at high temperatures and pressures in the presence of certain molten metals which serve as solvent catalysts. The zones of pressure and temperature in which diamond growth occurs have been determined for a number of metals. These zones are bounded on the low‐temperature side by the melting point of the metal‐carbon eutectic at pressure. They are bounded on the high‐temperature side by the diamond‐graphite equilibrium line. This experimentally determined equilibrium line agrees very closely with the theoretical extrapolation of the thermodynamically calculated line proposed by Berman and Simon, viz.,

Atomic Polarization. II. Vibrational Polarization of Liquids
View Description Hide DescriptionA theory applied specifically to the problem of the vibrational polarization of liquids is presented, and a detailed discussion is given of the vibrational polarization terms in isotropic condensed states.

Atomic Polarization. III. Vibrational Polarization Terms Associated with Forbidden Transitions in Polyatomic Molecules
View Description Hide DescriptionThe contribution, to the total vibrational polarization, of transitions forbidden by harmonic oscillator selection rules is examined for the case of polyatomic molecules in the gaseous state. Numerical calculations are given for a number of molecules. In connection with these results, criteria are developed for the completeness of the treatment, developed in a previous paper, taking into account the anharmonicity of the vibrations but limiting the contributions to those associated with transitions allowed by harmonic oscillator selection rules. The present work permits one to draw conclusions as to the applicability of this approximation to a given polyatomic molecule.

Atomic Polarization. IV. Temperature Dependence of the Vibrational Polarization of Gases
View Description Hide DescriptionA treatment of the temperature dependence of the vibrational polarization of gases is given in several degrees of approximation. Calculations are presented for a number of molecules.

Atomic Polarization. V. Normal Mode Polarizability and the Additivity of the Vibrational Polarization
View Description Hide DescriptionThe vibrational polarizability of a molecule is derived in several degrees of approximation and is calculated for a number of molecules. An extension of this treatment is then presented, terminating in the development of the properties of the normal modepolarizability. The criteria for the additivity of the vibrational polarization of a molecule in terms of the normal modepolarizabilities of its constituent segments are developed. Numerical calculations of the normal modepolarizability terms are given for a large number of molecules.

Atomic Forces and Thermal Effects in Solids
View Description Hide DescriptionIt is shown that the vanishing of an interatomic force constant in a solid can produce specific heat anomalies very similar to the familiar ``lambda point'' phenomena. Possible application of this to higher order phase transitions is discussed.

Effect of Pressure on the Spectra of Certain Complexes of Cu^{++}, Co^{+++}, and Fe^{++}
View Description Hide DescriptionThe effect of pressure is as measured on the spectra of four Cu^{++} salts, and on K_{3}COF_{6} and FeSiF_{6}·6H_{2}O. The four Cu^{++} salts have varying degrees of distortion from octahedral (or tetrahedral) symmetry. For the Tutton's salt (the more distorted octahedral complexes) there were measurably larger effects on the shift and especially on the intensity of the transition. The Co^{3+} and Fe^{++} systems were compared to investigate the possibility of crossover from spin free to spin paired arrangement in the former. From the data it is estimated that this should occur at 220–250 kbar.

Effect of Pressure on the Spectrum of Ruby
View Description Hide DescriptionThe effect of pressure has been measured on the spectrum of ruby, both parallel and perpendicular to the C axis, to 120 kbar. From these data it is possible to calculate the change in crystal field strength D_{q} in interelectronic repulsion B and in trigonal field distortion (). The crystal field increases with increasing pressure, while the interelectronic repulsion decreases, indicating increasing covalency. The trigonal distortion is constant to about 60 kbar, and then increases markedly at higher pressures. The fractional change in D_{q} with pressure follows the R ^{‐5} law closely to 30 kbar, the upper limit of the p‐v data.

Effect of Pressure on Tetrahedral Ni^{++} and Co^{++} Complexes
View Description Hide DescriptionThe effect of pressure has been studied on the spectra of five tetrahedral Co^{++} complexes and two tetrahedral Ni^{++} complexes. Calculations have been made for the change in the crystal‐field parameter 10 Dq and for the change in the interelectronic repulsion B. The increase in 10 Dq and the decrease in B correlates strongly with the polarizibility of the ligands. For the Co^{++} complexes the effect of pressure on the spin‐orbital splitting was studied. The splitting increased with pressure to an extent determined by the mass and polarizability of the ligand. For ZnS compressibility data were available. It was found that the fractional change in Dq for both Co^{++} and Ni^{++} was greater than that predicted from the bulk compressibility of ZnS. This is attributed to relaxation in the neighborhood of the foreign ion.

Nuclear Magnetic Resonance Studies of B^{11} in Crystalline Borates
View Description Hide DescriptionThe coordination state of the boron atom in boron compounds may be correlated with the properties of the nuclear magnetic resonance(NMR) signals arising from the boron nuclei. Characteristicresonance line shapes occur when the NMR transitions are perturbed by interactions between the B^{11} nuclear electrical quadrupole moment and the electric field gradient at the boron site. In polycrystalline borates, boron in trigonal and tetrahedral coordination may be resolved by analysis of first and second‐order quadrupolar effects on the NMR transitions. The B^{11}quadrupole coupling constants have been measured for the simple trigonal BO_{3} and tetrahedral BO_{4} groups. The analysis has been extended to include polyborate structures having both trigonal and tetrahedral boron atoms. Results of the NMR study of simple, known bonding configurations are discussed and correlated with measurements of alkali‐borate glasses and of miscellaneous borate and mineral compounds.

Paramagnetic Resonance Absorption of the Dimesitylmethyl Radical
View Description Hide DescriptionThe isotropic hyperfine structure of the electron paramagnetic resonancespectrum of the dimesitylmethyl radical has been observed. Nearly 300 of the 910 theoretical lines have been resolved and analyzed in terms of the coupling constants of the various nuclear species. A spin‐density distribution throughout the molecule has been deduced from the measurements and is compared with recent theoretical calculations.

Cubic Potential Surfaces in the Transition‐State Theory of Unimolecular Reactions
View Description Hide DescriptionThe frequency factor of a unimolecular dissociation rate is enhanced if the molecular vibrations are loosened during the approach to the activated state; this loosening may be regarded as affecting the partition function or entropy in the nonreactive degrees of freedom. These ideas are illustrated here in terms of cubic potential surfaces, with examples of linear molecules and of the effect of freeing an internal rotation. As a more general but related point, the uniqueness of the reaction coordinate of transition‐state theory is discussed with some reference to isotope effects.

Vaporization of Aluminum Arsenide
View Description Hide DescriptionVaporization of aluminum arsenide was studied, using the Knudsen effusion method. Aluminum arsenide decomposes according to the reactionFor the reactionΔH _{298} is found to be —35.4±3.1 kcal.

Thermodynamic Properties of Trifluoroacetonitrile from 12°K to Its Boiling Point
View Description Hide DescriptionThe saturated heat capacities of trifluoroacetonitrile have been measured from 12°K to the normal boiling point 205.47°K. The vapor pressure of the liquid to 1 atm is represented by the equationThe solid‐liquid‐vapor triple‐point temperature is 128.73°K. The heat of fusion at the triple point and the heat of vaporization at the normal boiling point are, respectively, 1187.7 cal/mole and 4262 cal/mole. The experimental value of the entropy of the gas at the normal boiling point, 65.01±0.20 eu/mole, is in excellent agreement with the theoretical value of 64.96 eu/mole calculated from spectroscopic and molecular data assuming a symmetrical top molecule.

Anharmonic Potential Constants and Their Dependence upon Bond Length
View Description Hide DescriptionEmpirical study of cubic and quartic vibrational force constants for diatomic molecules shows them to be approximately exponential functions of internuclear distance. A family of curves is obtained, determined by the location of the bonded atoms in rows of the periodic table. Displacements between successive curves correspond closely to those in Badger's rule for quadratic force constants (for which the parameters are redetermined to accord with all data now available). Constants for excited electronic and ionic states appear on practically the same curves as those for the ground states. Predictions based on the diatomic correlations agree with the available cubic constants for bond stretching in polyatomic molecules, regardless of the type of bonding involved. Some implications of these regularities are discussed.

Determination of Excited‐State Dipole Moments of Azulene
View Description Hide DescriptionThe absorption spectra of three transitions of azulene were observed in a number of polar and nonpolar solvents. These frequency shifts were used to evaluate the constants in an equation developed by McRae representing solvent‐solute interactions, and the constants gave the change in dipole moment for each of the transitions.

Energy Value of the Octahedral‐Tetrahedral Coordination Change
View Description Hide DescriptionThe equilibrium ^{oct}(CoCl_{2}Py_{4}) = ^{tet}(CoCl_{2}Py_{2}) +2Py for cobaltous chloride solutions in pyridine is followed over a temperature range, yielding ΔH = +13.4 kcal/mole. At 38°C the equilibrium constant is estimated as (CoPy_{2}Cl_{2}) (Py)^{2}/(CoCl_{2}Py_{4}) = 0.04, and the ΔS for the reaction is about 36.7 eu. It is pointed out that the average bond strength in the tetrahedral species is about 17 kcal greater than for the same groups in the octahedral configuration, and that the strength of binding and the dissociation energy for the two ligands released according to the equation above are significant factors in determining the equilibrium reaction.
Detailed arguments are given against the view that the relative stability of octahedrally and tetrahedrally coordinated complexes, such as the pair discussed, reflect principally the difference in ``ligand‐field stabilization'' of the nonbonding d electrons between octahedral and tetrahedral fields.

Tait Coefficients and λ Transition of Helium I and II
View Description Hide DescriptionHelium I and helium II are both found to obey Tait's law. The constant J is constant, within experimental error, with temperature for He I making this substance a fluid of the first kind. Helium II is a fluid of the second kind inasmuch as J is constant along an isotherm, but J varies with the temperature. The isotherms which cross the λ transition can be fitted with two straight lines, one for the He I region and one for the He II region. Plots of J, L, and J/L vs temperature are given. While the behavior of the J and L curves is abnormal, apparently the abnormalities are parallel in both constants, since the J/L curve is much more regular. The number average degree of association, the number of particles, and the volume of holes is calculated along the 2.00°K isotherm. There is a sudden jump of these quantities at the λ point. In going from He I to He II the degree of association increases; the number of particles and the volume of holes decreases. Apparently the structural change that occurs is an inversion. In He I the structure is that of a normal liquid like water, where it consists of larger particles joined by defects consisting of holes and smaller particles. At the λ point due to the lowering of the pressure the defect ``continuum'' becomes tenuous due to the increase in the volume of holes, and the bonds suddenly break. The skeleton of the structure of He II then becomes one of the large particles forming a loose network with the remains of the small particles which formerly formed the ``continuum'' occupying the free space in the network. These small particles are then the superfluid component of He II.

Microwave Spectrum, Dipole Moment, Structure, and Internal Rotation of Dimethyl Sulfide
View Description Hide DescriptionThe microwave spectra of five isotopic species of dimethyl sulfide are reported. Changes in rotational constants with isotopic substitution yield the following structural parameters: CS 1.802 A; CSC=98°52′; CH 1.901 A; HCH=109°34′; 2θ=104°22′, where 2θ is the angle between the symmetry axes of the methyl groups. The equilibrium conformation of both methyl groups is the staggered one, i.e., staggered with respect to the adjacent CS bond axis. From Stark effect measurements the dipole moment of dimethyl sulfide is found to be 1.50±0.01 debye.
Fine structure in the ground‐state rotational spectrum of (CH_{3})_{2}S and an excited torsional state of CH_{3}SCD_{3} has been resolved and analyzed. This fine structure results from coupling of internal and over‐all rotation and is affected by top‐top coupling terms in the kinetic and potential energy portions of the Hamiltonian. Neglecting only the potential energy coupling terms, the (CH_{3})_{2}S and CH_{3}SCD_{3} splittings yield as the barrier to internal rotation 2132±6 and 2118±3 cal/mole, respectively. Estimates of the potential energy coupling parameters are made. They are found to be an order of magnitude smaller than the main term of ∼2100 cal/mole in the Fourier expansion of the potential energy.

Formulation of a Cell Model Using Periodic Boundary Conditions
View Description Hide DescriptionA cell‐type model for the liquid state, based upon the consideration of small systems with periodic boundary conditions, is introduced. The equation of state for rigid‐sphere molecules is then calculated for a tetragonal cell. By properly choosing the dimensions of the tetragonal cell, the equation of state for a system of rigid spheres which form a face‐centered cubic lattice in their regular lattice configuration is obtained. The model yields the correct second virial coefficient. The results are compared with the Monte Carlo calculations of Wood and Parker, the ``averaged model'' results of Buehler et al., and the Lennard‐Jones and Devonshire theory.