Volume 55, Issue 3, 01 August 1971

Compressibility of 18 Molecular Organic Solids to 45 kbar
View Description Hide DescriptionNew compressibility data on 18 organic solids are reported. Measurements on a number of these solids have been made for the first time. Compressibilities up to 45 kbar were determined by observing piston displacement in a static high‐pressure apparatus. Related compounds show a progressive decrease in compressibility with increase in molecular volume.

Calculation of the Zero Wave Vector Lattice Frequencies of Alpha‐ and Beta‐Oxygen
View Description Hide DescriptionThe optically active lattice frequencies of α‐ and β‐O_{2} have been calculated in the rigid‐molecule approximation using an atom–atom Lennard‐Jones [6–12] potential, with and without the addition of an intermolecular quadrupole–quadrupole interaction. Both models predict frequencies reasonably close to those observed experimentally for α‐O_{2} and support the assignment of the higher‐frequency librational mode to the torsion about the twofold axis in the crystal. Excellent agreement is obtained between the observed librational frequency and its temperature dependence in β‐O_{2} and the results calculated using the Lennard‐Jones potential. If it is assumed that the molecules in β‐O_{2} may be tipped from the crystallographic axis, an increase in the tilt angle as the temperature is raised is predicted, which is consonant with the temperature dependence of the lattice constants for this phase. It is concluded that the Lennard‐Jones atom–atom interaction is a satisfactory model for the potential function in solid oxygen, provided that the packing is such that electron overlaps are small.

Infrared Spectrum and Structure of the SO_{2} ^{−} Radical Ion
View Description Hide DescriptionWhen a sample of SO_{2} in a large excess of argon is codeposited at 4 or at 14°K with an atomic beam of an alkali metal, prominent new absorptions appear near 500 cm^{−1} and between 975 and 1100 cm^{−1}. The pattern of absorptions is somewhat dependent both upon the alkali metal and upon the temperature at which the sample is deposited. Product absorptions which appear at 495, 985, and 1042 cm^{−1} in samples containing a small concentration of cesium atoms have been demonstrated to be contributed by a single species, shown by isotopic substitution studies to contain one sulfur and two oxygen atoms. A slight splitting in the absorptions of the mixed oxygen isotopic species indicates that the two oxygen atoms are not symmetrically equivalent, either because of a site perturbation or because of a slight nonequivalence of the two S–O bonds. The absorptions have been assigned to the three SO_{2} ^{−} vibrational fundamentals of a charge‐transfer complex in which the residual interaction of the cation is sufficiently small that all of the vibrational data can be fitted within experimental error without considering the motion of the cation. The data are consistent with an O–S–O valence angle of 110° ± 5°. The S–O stretching force constant is significantly lower than that of SO_{2}, consistent with the addition of an electron to an orbital which is antibonding between the sulfur and two oxygen atoms.

Application of Conformal Solution Theory to Gas–Gas Equilibria
View Description Hide DescriptionThe conformal solution theory (CST) is applied to the problem of gas–gas immiscibility. It is found that the occurrence of this phenomena can be predicted in the system He–Xe; in particular, the calculated critical locus is in satisfactory agreement with the experimental data of de Swaan Arons and Diepen, provided a suitable reference is chosen. Furthermore, using CST as a guide, it was found that criteria could be developed, related to Temkin's criteria, which permit the prediction and classification of the three types of gas–gas equilibria known to occur in mixtures of nonpolar molecules.

Model Calculations of Transport Coefficients for Diatomic Gases in an External Magnetic Field. II. The Cartesian Tensor Perturbation Scheme
View Description Hide DescriptionModel calculations are used to examine the convergence of a Cartesian tensor perturbation scheme for computing transport coefficients of diatomic gases in a static magnetic field. Numerical results are given to fourth order in the “nonsphericity” for the thermal conductivity and viscosity coefficients. For the shear viscosity only the second‐order contributions are significant. However, for the thermal conductivity, bulk viscosity, and coupling coefficients third‐order contributions are important. It is found that the addition of Sonine polynomials in the linear and angular momenta in the expansion for the shear viscosity makes no substantial contribution.

Reactions of Hydrogen and Deuterium Atoms Formed in the Photolysis of Methane and Perdeuterated Methane at 123.6 nm
View Description Hide DescriptionThe photolysis of methane and perdeuterated methane has been carried out at 123.6 nm, in the presence of interceptors (deuterium in the case of methane, and hydrogen in the case of deuterated methane). The hydrogen and deuterium atoms formed are statistically in thermal equilibrium with the reaction medium; it follows that the photodissociation into CH_{3} and H has a probability close to zero at 123.6 nm. In addition, it is confirmed that CH_{4} and CD_{4} have different photolytic behaviors insofar as the quantum yields of hydrogen and deuterium are concerned, with .

Far‐Infrared Collision‐Induced Absorption in CO_{2}. I. Temperature Dependence
View Description Hide DescriptionAccurate measurements of collision‐induced absorption in CO_{2} are made at a number of temperatures in the range from − 40 to 60°C in the wavelength region 7–250 cm^{−1}. Direct evidence for the separation of the pure translational band from the rotational–translational band is obtained at all temperatures. This and other aspects of the band shape are discussed. Over the entire temperature range, the experimentally determined Kramers–Kronig integral is found to be in good agreement with the theoretical value, i.e., the static dielectric constant. This agreement is achieved only when the contribution of the quadrupole–quadrupole energy in the radial distribution function, of particular importance for CO_{2} because of its large quadrupole moment, is calculated accurately. A value of the quadrupole moment is obtained, (4.5 ± 0.2)10^{−26} esu, which is in satisfactory agreement with that obtained by the method of Buckingham and Disch, which does not depend on a knowledge of intermolecular force constants. Induction due to higher multipole moments and the overlap interaction is considered.

Far‐Infrared Collision‐Induced Absorption in CO_{2}. II. Pressure Dependence in the Gas Phase and Absorption in the Liquid
View Description Hide DescriptionThe pressure dependence of the collision‐induced spectrum in CO_{2} at room temperature in the frequency region 7–250 cm^{−1} is measured throughout this region as a function of density, , from 0–85 amagat. At each frequency the density variation of the absorption is fitted by to obtain values of and . The coefficient and its temperature dependence is discussed in the previous paper of this series. The coefficient is negative in sign and has a band shape considerably sharper than that found for . The Kramers–Kronig integral is in reasonable agreement with the theoretical value of the third dielectric virial coefficient. Absorption in liquid CO_{2} is measured at 0°C and compared with the band spectrum obtained in the gas phase at the same temperature. The peak intensity in the liquid spectrum occurs at a frequency 25 cm^{−1} higher than in the gas phase. The integrated intensity in the liquid is 3.2 ± 0.1 × 10^{−4} cm^{−2}·amagat^{−2}, whereas that for the low density gas is 5.11 ± 0.20 × 10^{−3} cm^{−2}·amagat^{−2}.

Electron Distributions for the Helium Atom–Hydrogen Molecule System
View Description Hide DescriptionA study is made of the electron distribution generated by self‐consistent field wavefunctions for the hydrogen molecule–helium atom system. The electron density is studied by means of electron density contour plots and plots of the difference between the electron density for interacting hydrogen molecule and helium atom systems and the noninteracting hydrogen molecule and helium atom. A population analysis technique is applied to these wavefunctions. Though the population analysis is a rather unrealistic method of distributing the electrons among and between the nuclei, it appears to give a qualitatively correct description of the electron distribution. The wavefunction studied gives a potential surface on which the H_{2} molecule tends to contract when the He atom approaches. This is true even when He approaches the H_{2} at an angle of 90° to the internuclear axis. The population analysis reflects this effect by showing an increase in the H_{2}bond population as He approaches.

Excitation of Parity‐Forbidden Transitions in H_{2} by Energetic Electrons
View Description Hide DescriptionThe cross sections for energetic electron excitation of the parity‐forbidden transitions are calculated in the Born approximation. The intensities for these forbidden transitions are predicted to be comparable to the intensities of the allowed and transitions for scattering angles as small as 0.75° for 34‐keV electrons.

Molecular Orbital Model for Antimony Luminescent Centers in Fluorophosphate
View Description Hide DescriptionA detailed semiempirical molecular orbital description of the low‐lying excited states of the antimony luminescent center in fluorophosphate is presented. The MO model is used to interpret both the excitation and emission spectra. Good agreement with experiment is found. The model also explains differences in the characteristics of the excitation and emission spectra which are observed when the phosphor is prepared in the presence or absence of charge‐compensating oxygen.

Continued Factorization Method for van der Waals Interactions
View Description Hide DescriptionA continued factorization procedure is introduced to telescope the infinite series of the dynamic polarizability into a finite sum. The effective oscillator strengths can also be continually factorized. Both the upper and the lower bounds of the dynamical polarizability can be obtained from expressions identical in form. A single change of the last effective excitation energy of the continually factorized series turns it from an upper to a lower bound. The rest of the effective excitation energies are the natural resonance frequencies of the system. When the continued factorized series is substituted into the Casimir–Polder formula, we obtained the usual London formula for the van der Waals force with finite number of terms. Expressed in terms of oscillator strength sums and the natural resonance frequencies, all quantities in the London formula are given in closed forms. While it has the same formal structure of the Padé approximant method, the present method eliminates the entire process of constructing the approximant and gives tighter bounds.

Electron Diffraction Investigation of Dimethyl Diselenide
View Description Hide DescriptionThe molecular structure of dimethyl diselenide has been determined by an electron diffraction investigation of the vapor. A computerized background correction routine, satisfying the positivity and area criteria, was employed to reduce the data and obtain a final molecular intensity curve. The values for a set of nine distances and amplitudes were obtained from a least‐squares fit to the final molecular intensity curve. The bonded distances are . The corresponding amplitudes are The ∠CSeSe = 98.9 ± 0.02° and the ∠HCSe = 108.4 ± 0.8°. The errors are estimated to be at the 99% confidence level. The methyl groups are unsymmetrically placed with respect to the CSeSe planes. They are rotated about the C–Se bonds such that one of the HCSe planes in each CH_{3}Se moiety makes an angle of 36.1 ± 6° with the respective CSeSe plane. Dimethyl diselenide assumes a stable skew conformation with a dihedral angle of 87.5 ± 4°. The molecule contains a twofold axis of symmetry passing midway between the two selenium atoms in the plane bisecting the dihedral angle.

Calculation of Collision‐Broadened Linewidths in the Infrared Bands of Methane
View Description Hide DescriptionAnderson–Tsao–Curnutte theory of collision‐broadened spectral lines has been extended to octopolar molecules, and the results are applied to the rotational lines in the band of methane. Calculated half‐widths are in excellent agreement with the high‐resolution measurements of Varanasi for self‐broadening and broadening by N_{2}, O_{2}, H_{2}, and He. The theory has also been applied to CO lines broadened by CH_{4} and to CH_{4} lines broadened by HCl. The excellent agreement with experimental data in both the cases leads to the estimate of for the octopole moment of CH_{4}.

Magnetic Susceptibility of Yb^{3+} in an Octahedral Environment
View Description Hide DescriptionThe magnetic susceptibility of Yb^{3+} in Cs_{2}NaYbCl_{6} was measured from 2.5 to 100°K and showed Curie–Weiss temperature dependence from 2.5 to 20°K and from 45 to 100°K , with a transitional region from 20–45°K. From a comparison of the experimental results with theoretical results for a ion in an octahedral field, the two crystalline field levels observed were identified as a ground level, and a level about 60 cm^{−1} higher. The energy separation between the and levels and a ratio of the crystalline field parameters derived from other measurements were employed to deduce and . From these parameters, a level at ∼ 160 cm^{−1} is predicted. A covalent contribution to the ground level is considered.

Photodissociation of O_{3} in the Hartley Band. Reactions of and with O_{3} and O_{2}
View Description Hide DescriptionThe ultraviolet photolysis of O_{3} in the presence and in the absence of O_{2} has been investigated in the wavelength region ∼ 2375–2625 Å using flash photolysis and kinetic emission spectroscopy with a time resolution high enough to allow unambiguous identification of photodissociation products. The production of was established by observing the forbidden emission at 6300 Å. The decay of the emission provided a value of the rate constant for the reaction of (2.5 ± 1) × 10^{−10} cm^{3} molecule^{−1}·sec^{−1} at 25°C. Analysis of the decay showed that the rate of chain reactions in which could be reproduced must be smaller than the quenching rate by more than an order of magnitude. No emission was observed from when O_{3} alone was photolyzed, indicating that the primary yield for production is smaller than 1/20 of the production. was very efficiently quenched when N_{2} and O_{2} were added. was observed by the emission when O_{3}–O_{2} mixtures were photolyzed. The emission disappeared when N_{2} was added. The production and decay rates were measured at various O_{3} and O_{2} concentrations. The results indicated that was formed by energy transfer from to ground state O_{2}. The decay of the emission provided a value of the rate constant for the quenching of (2.5 ± 0.5) × 10^{−11} cm^{3} molecule^{−1}·sec^{−1}. The relative quenching rate of by O_{3} and O_{2} was about 5, yieding an rate constant of 5 × 10^{−11} cm^{3} molecule^{−1}·sec^{−1} with an uncertainty of a factor of 2. The dependence of the emission on the O_{2} pressure showed that the primary quantum yield of is at least 20 times smaller than that of , in agreement with the previous result.

Crystal Structure Refinement of SrMoO_{4}, SrWO_{4}, CaMoO_{4}, and BaWO_{4} by Neutron Diffraction
View Description Hide DescriptionThe crystal structures of CaMoO_{4}, SrMoO_{4}, SrWO_{4}, and BaWO_{4} have been refined from neutron diffraction data taken in the and zones. Oxygen position refinement is started from the parameters proposed by Sillén and Nylander and results in an improvement of over an order of magnitude in the oxygen coordinates. A comparison of coordinates for the different compounds shows several systematic differences.

Hei and Heii Photoelectron Spectra and the Electronic Structures of XeF_{2}, XeF_{4}, and XeF_{6}
View Description Hide DescriptionThe high‐resolution photoelectron spectra of XeF_{2}, XeF_{4}, and XeF_{6} have been obtained essentially free from any impurity features, and the ionization potentials up to 28 eV have been measured. The results are used, in conjunction with recent ab initio calculations, to assign the molecular orbital electronic structure of these molecules. Agreement between experiment and calculation is fairly good.

Radial Distribution Functions and Superficial Quantities for Rigid Sphere Fluids; Method of “Point” Chemical Potential Using Scaled Particle Theory
View Description Hide DescriptionThe radial distribution of rigid spheres of diameter about a single fixed solute rigid sphere of diameter was calculated using the method of “point” chemical potential and effective local density, together with results from the scaled particle theory of rigid sphere fluids. This method, which avoids the superposition approximation, gives rise to a nonlinear integral equation which was solved numerically for various values of in the interval and for intermediate fluid densities. An apparent freezing transition was observed; the density required for this transition was a minimum for and increased as was decreased. The superficial density of matter referred to the surface of radius , was found, using these distribution functions, to be small and positive. Knowledge of allowed the unambiguous location of the equimolecular Gibbs dividing surface.

ESR Studies of an Intrinsic Trapped‐Electron Center in X‐Irradiated Alkali Borate Glasses
View Description Hide DescriptionElectron spin resonance studies have been carried out at X‐band frequencies on a large number of glasses spanning the lithium,sodium, and potassium borate glass systems, following x irradiation at cryogenic temperatures . A new, intrinsic paramagnetic defect center characterized by a sizeable hyperfine interaction with boron was observed, in addition to the more familiar boron–oxygen hole centers. The new center, which is readily destroyed by visible light or thermal annealing above ∼ 100°K, is shown to be of the trapped‐electron type. By means of computer simulations of the spectra, a reasonably complete set of spin–Hamiltonian parameters is derived in spite of the obscuring effects of a substantial distribution in hyperfine coupling constants. Possible models for the new defect are considered and its potential usefulness as a “probe” of the structure of alkali borate glasses is discussed.