Index of content:
Volume 46, Issue 9, 01 May 1967
46(1967); http://dx.doi.org/10.1063/1.1841211View Description Hide Description
The melting curve of NaClO3 initially increases with pressure and reaches a maximum at 6 kbar and 285°C. A very slight break in the curve at 8.2 kbar and 282°C may indicate the NaClO3 I—II—liquid triple point and is in reasonable agreement with the extrapolation of Bridgman's I—II boundary. The melting curve further decreases with pressure and meets Bridgman's II—III boundary in a triple point at 31 kbar and 100°C. Bridgman's NaClO3 IV is actually the liquid phase. The freezing of NaClO3 exhibits considerable supercooling at high pressure, and above 20 kbar liquid NaClO3 appears to be very viscous and behaves like a glass.
46(1967); http://dx.doi.org/10.1063/1.1841212View Description Hide Description
Liquid—vapor phase compositions for the system neon—argon are reported at five temperatures in the range 95.82°—129.92°K, and at pressures up to 9000 psia. The liquid—vapor critical curve has been determined for mixtures containing up to 60 mole % neon. The density of the liquid phase has been measured in the compressed‐liquid region at temperatures of 101.94°, 110.78°, and 121.36°K, at pressures up to 8000 psia.
46(1967); http://dx.doi.org/10.1063/1.1841213View Description Hide Description
The nonreactive scattering of K by HBr and DBr has been measured in crossed molecular beams as a function of relative kinetic energy and for two configurations of the beams and the detector. The effect of averaging over residual energy distributions in the two beam and detector configurations is studied. It is shown that the density of the cross beam must be kept low because attenuations of 50% of the K beam by the cross beam affect the observed rainbow cross section appreciably. From the rainbow scattering and a kinematic restriction, the parameters for an assumed exp−6 potential (α=12) are found to be K+HBr: ε=0.565 kcal/mole, rm =4.2 Å; K+DBr: ε=0.564 kcal/mole, rm =3.9 Å. Chemical reaction affects the nonreactive scattering at large scattering angles. Probabilities of reaction, threshold energies, and total reaction cross sections are obtained from the nonreactive scattering with a simple model used previously. The total reaction cross sections are: K+HBr: 35.0±9.0 Å2, K+DBr: 26.0±8.0 Å2 in the energy range 2.6 to 5.8 kcal/mole.
Scattering of Potassium by a Series of Reactive and Nonreactive Compounds in Crossed Molecular Beams46(1967); http://dx.doi.org/10.1063/1.1841214View Description Hide Description
The scattering of potassium by the nonreactive compounds C(CH3)4, C6H6, C6H12, (CH3)2C=C(CH3)2 and the reactive compounds CH3I, CCl4, SiCl4, SnCl4, SF6 has been studied in experiments with crossed molecular beams. In each case the nonreactive scattering of velocity‐selected K was measured as a function of laboratory scattering angle at a number of initial relative kinetic energies. Except for K+SnCl4, rainbow scattering is observed. The differences which appear between the measuredscattering in this region for the various nonreactive compounds show the influence of the molecular structure on the intermolecular potential. Potential parameters of an assumed spherically symmetric potential (exp−6, α=12) are calculated from the rainbow scattering. The scattering of K from the various reactive compounds differs markedly at larger angles and is an indication of a correspondingly large variation of reaction cross sections. Probabilities of reaction, threshold energies, threshold distances, and the energy dependence of the total reaction cross sections are obtained from an interpretation of the measurements.
46(1967); http://dx.doi.org/10.1063/1.1841215View Description Hide Description
The elasticscattering cross section is evaluated by the method of stationary phase for two assumed forms of complex phase shifts. The results are applied to the interpretation of measurements on the scattering of K by SiCl4 to yield a value for the total reaction cross section.
46(1967); http://dx.doi.org/10.1063/1.1841217View Description Hide Description
An investigation of the continua (2900–7000 Å) radiated from positive columns in xenon, krypton, and neon is described for discharges in the pressure range of 5 to 40 torr (neon, 20 to 120 torr) and current range of 0.7 to 5 mA, conditions chosen for normal glow, unconstricted discharges. Experimental results show definitely that these continua are not attributable to free—bound, free—free transitions and indicate that they are of molecular origin, the radiating states being populated by electron excitation of metastable molecules formed by three‐body conversion of metastable atoms.
46(1967); http://dx.doi.org/10.1063/1.1841219View Description Hide Description
A general method for carrying out population analysis of wavefunctions calculated with arbitrary basis sets is presented. The difficulties in defining orbital populations is discussed. Results are presented for the sequence BF, CO, and N2 which indicate that back‐transfer of charge in the π bond cancels the normal transfer of the charge in the σ bond. Contrary to popular belief, the more electronegative element has the larger degree of hybridization in each case. The amount of promotion of 2s electrons is greater on the less‐electronegative element, as expected.
46(1967); http://dx.doi.org/10.1063/1.1841220View Description Hide Description
It is pointed out that the plait‐point (isothermal critical‐mixing‐point) phenomenon in a three‐component liquid system of fixed pressure and temperature is identical in character to that in a two‐component liquid system of fixed temperature but variable pressure. A lattice‐gas model of a two‐component liquid system is described. The intermolecular forces are all postulated to be infinitely strong repulsions, as between hard cores, but the model system nevertheless undergoes first‐order phase transitions with an associated plait point. The behavior of the system with varying densities ρ1 and ρ2 of its two components, at fixed temperature, is shown to be exactly transcribable, with only a change of language, from the behavior of a reference one‐component lattice gas of varying density and temperature, in which the interaction between neighboring molecules is attractive. The critical point of the reference one‐component system is mapped into a plait point in the isothermal ρ1, ρ2 plane of the binary system. The two‐phase coexistence curve (binodal curve) in the isothermal ρ1, ρ2 plane of the binary system is found to be of algebraic degree (1—α′)/β near the plait point, where α′ and 1/β are the index of the divergence of the constant‐volume specific heat in the two‐phase region of the reference system, and the algebraic degree of the reference system's temperature—density coexistence curve, respectively. Fluctuations in ρ1 and ρ2 in the one‐phase region of the binary system are found to diverge as the —γ/(1—α) power of the distance from the plait point, where α and γ are the indices of the divergence of the constant‐volume specific heat and of the compressibility, respectively, in the one‐phase region of the reference system.
46(1967); http://dx.doi.org/10.1063/1.1841221View Description Hide Description
Emission spectra of chlorophyll‐a, dissolved in ethanol and various nonpolar solvents, were measured at 300°, 77°, and 4°K, in both dilute and concentrated solutions. For this purpose, a new technique utilizing fiber optics was employed. In dilute ethanolic solutions, the observed fluorescence bands emanate from chlorophyll‐amonomers, solvated at the central magnesium. At 4°K the fluorescence yield in this solvent is 0.85. In highly concentrated ethanolic solutions, a low‐temperature band occurs at about 724 nm, which may be attributed to solvated dimers, formed by a π—π interaction between the chlorophyll monomers. In nonpolar solvents, a low‐temperature luminescence band is observed at about 755 nm. This emission is attributed to π—π*phosphorescence from unsolvated monomers. Also in nonpolar solvents, highly concentrated solutions have a weak band in the neighborhood of 733 nm at 77°K, which originates from chlorophyll dimers formed by interaction between the magnesium of one monomer with the cyclopentanone carbonyl of another. The 733‐nm band is either π—π*phosphorescence or fluorescence. In CCl4 and benzene the dimers are believed to be unsolvated. At room temperature unsolvated dimers fluoresce feebly at 713 nm. Cooling dilute solutions of chlorophyll a in benzene and 1,2‐dichloroethane causes dimer formation, but this is less in CCl4 and absent in ethanol. A calculation of the upper limit for the rate constant for intersystem crossing to the triplet state, in ethanol, gives a value of 9.0×106/sec.
46(1967); http://dx.doi.org/10.1063/1.1841222View Description Hide Description
This paper presents an interpretation of the chemical binding found in the first‐row homonuclear diatomic molecules. The interpretation is based upon the one‐electron density distribution and the forces which it exerts on the nuclei. The general topographical features of the density distributions are discussed in relation to ``molecular size'' and the manner in which the total charge is partitioned between different spatial regions. The binding in these molecules is discussed in terms of the density difference distributions which picture the redistribution of charge which results from the formation of the molecule. It is proposed that the density difference distribution, or Δρ map, may be taken as the pictorial representation of the ``bond density.'' The forces exerted on the nuclei in the molecule are related to the changes in the charge distribution pictured in the ``bond density'' and a quantitative discussion of the manner in which electrostatic equilibrium is attained to give a stable molecule is given in terms of the forces. The concepts of bonding and antibonding are compared with the terms binding and antibinding, terms which are defined in terms of the forces exerted on the nuclei. In particular, both vertical and adiabatic ionization processes are viewed from the standpoint of the change in the electronic force contribution between the molecule and the molecule ion.
A definition of ionic and covalent binding based on the density difference distributions is presented. The Δρ map (or bond density) for covalent binding is shown to be characterized by a density increase located between the nuclei and shared equally by each. The Δρ map for ionic binding exhibits an increase in charge density which is localized on a single nucleus. A partitioning of the total electronic force in accordance with the Δρ maps demonstrates that in covalent binding, the nuclei are bound by the density which is shared between them, while in ionic binding the nuclei are bound by the density which is localized on a single nucleus.
46(1967); http://dx.doi.org/10.1063/1.1841223View Description Hide Description
Molecular vibrations of the PO4 3− ion at a site having D 2d symmetry are treated theoretically. Symmetry coordinates and G‐matrix elements are derived. Analytical F‐matrix elements corresponding to a generalized valence force field are given. Two of the four observed infrared absorptions are assigned to the B 2 and two to the E species and corresponding F matrices calculated. The distribution of potential energy among the internal symmetry coordinates for each infrared‐active normal mode is derived. The results indicate a marked increase in the force constant for bending along the S 4 axis for the B 2 species which is interpreted as evidence for the formation of P–O–Y bonds.
46(1967); http://dx.doi.org/10.1063/1.1841224View Description Hide Description
A report is given of derivations of useful formulas for the electrostriction and the nonlinear polarization of a gas of neutral molecules in a static electric field. The derivations begin with the definitions from quantum statistical mechanics for the grand‐ensemble averages of the two quantities in question, and the resulting formulas are obtained by means of the linked‐diagram method of Kaufman and Watson.
46(1967); http://dx.doi.org/10.1063/1.1841225View Description Hide Description
The Mössbauer effect of the 23.8 keV 119Sn and 26.8 keV 129I transitions has been studied in SnI4 crystals. From the temperature dependence of the recoilless fraction in tin and iodine experiments, two significantly different characteristic temperatures have been found, namely 166°K for the Sn resonance and 85°K for the I resonance. From the intensities of the peaks in the iodine spectrum, the Gol'danskii effect has been calculated. From the quadrupole‐coupling and isomer‐shift measurements in the iodine experiments the bond structure of the Sn–I bond is derived.
46(1967); http://dx.doi.org/10.1063/1.1841226View Description Hide Description
The argon‐matrix infrared spectra of (CH3)3N, (CD3)3N, and (SiH3)3N are used to deduce a more complete assignment of the fundamental vibrational modes of these molecules than has previously been possible. Evidence is presented in support of a C 3v rather than C 3h point‐group symmetry for trisilylamine. Barriers to internal rotation are calculated from the observed asymmetric torsional fundamentals of (CH3)3N in the solid and matrix phases and of (CD3)3N in the gaseous phase.
46(1967); http://dx.doi.org/10.1063/1.1841227View Description Hide Description
A partial assignment of the fundamental vibrational modes of (CH3)2SiH3N and CH3(SiH3)2N is proposed based on the infrared spectra of the matrix‐isolated molecules. Calculated and observed vibration—rotation band envelopes are compared and are found to be indicative of a pyramidal skeleton for the monosilyl molecule and a planar skeleton for the disilyl molecule. Compelling evidence for the intermolecular association through p→d bonding in (CH3)2SiH3N is presented in the form of radical changes in the infrared absorptionspectrum as the concentration of this molecule in an argon matrix is increased. Although the structure of the associated molecules can not be deduced from the complicated spectrum of the solid, several conceivable geometries are discussed.
Hydroxyl‐Radical Kinetics by Kinetic Spectroscopy. II. Reactions with C2H6, C3H8, and iso‐C4H10 at 300°K46(1967); http://dx.doi.org/10.1063/1.1841228View Description Hide Description
Gas‐phase, room‐temperature reactions of the OH radical with C2H6, C3H8, and iso‐C4H10 were studied with the flash‐photolysis—kinetic‐spectroscopy (KS) technique. Rate constants (in units of cubic centimeters per mole·second) were obtained as follows: It is shown that relative theoretical values for frequency factor ratios and Evans—Polanyi activation energies are inconsistent with the ratios of the observed rate constants for H abstraction by OH from these alkanes.
46(1967); http://dx.doi.org/10.1063/1.1841229View Description Hide Description
The present paper deals with experiments on the nuclear relaxation of CHFO from −142° to −20°C. The relaxation is analyzed in terms of intra‐ and intermolecular interactions that induce the relaxation. The deuterated compound has likewise been investigated as the pure liquid and in dilute solution in hexadeuteroacetone.
Spin—rotation interaction is found to be the dominant relaxation mechanism of fluorine in the high‐temperature region. At the lowest temperatures the relaxation is practically exclusively dipolar in origin; intramolecular dipolar interactions are dominant.
Quadrupolar relaxation of the deuterium nuclei manifests itself in the transverse relaxation rates of the fluorine spins.
On the Magnetic Optical Rotation and Magnetic Circular Dichroism of Polymers. I. Time‐Dependent Correlation Functions46(1967); http://dx.doi.org/10.1063/1.1841230View Description Hide Description
A response‐function theory of the Faraday effect is presented. The response function for a polymer made up of coupled harmonic oscillators is obtained. A criterion for the origin independence of the magnetic circular dichroism and magnetic optical rotation is presented.
Vibrational Spectra of Molten Salts. I. Infrared Spectra of Calcium Nitrate in Alkali‐Metal Nitrate Solutions46(1967); http://dx.doi.org/10.1063/1.1841231View Description Hide Description
Infrared spectra of thin films of Ca(NO3)2 dissolved in molten KNO3 and molten NaNO3 have been obtained over the spectral range 4000–200 cm−1. The changing appearance of the spectra as a function of calcium content indicates clearly an asymmetric perturbation of the nitrate ion by the calcium ion in these mixtures. The magnitude of the perturbation is, however, similar to that found in concentrated aqueous calcium nitrate solutions, consistent with the existence, in both types of systems, of contact ion pair complexes. A new band at 332 cm−1 in the spectra of melts containing Ca2+ characterizes the Ca–ONO2 stretching frequency. Addition of chloride ion in the form of KCl to KNO3/Ca(NO3)2 molten mixtures was found to restore the appearance of the spectrum towards that of pure KNO3, showing that Cl− ion displaced NO3 − from the Ca2+–ONO2 − complexes.