Volume 63, Issue 12, 15 December 1975
Index of content:

Thermal expansion near a critical solution point
View Description Hide DescriptionWe report precise measurements of the density as a function of temperature in the one‐phase region near the consolute point in nitroethane + 3‐methylpentane. We find evidence of the critical anomaly in the thermal expansion, but are not able to determine a unique value for the critical exponent α. We have performed computer experiments to determine under what conditions α could be extracted from thermal expansion data.

Ion‐pair states of O_{2}
View Description Hide DescriptionValence states of O_{2} that correlate adiabatically to the ion pair asymptote, O^{+}(^{4} S)+O^{−}(^{2} P), have been calculated using the multiconfiguration description of the electronic structure. The production of the ion pair is attributed to valence–Rydberg mixing that predissociates Rydberg states of the excited O_{2} ^{+} ion such as those leading to the b ^{4}Σ^{−} _{ g } state. The electronic states required the calculation of excited states of a given symmetry. This is possible with the BISON‐MC codes if the rigorous restriction on orthogonality to the lower states is relaxed. For homonuclear valence states, in particular, this should be a good approximation.

Merging beams study of the H^{+} _{3}(D_{2},H_{2})HD^{+} _{2} reaction mechanism
View Description Hide DescriptionLaboratory energy distributions of HD^{+} _{2} product ions formed in the H^{+} _{3}(D_{2},H_{2})HD^{+} _{2}reaction were measured for reactants with relative energy W in the range 0.1 eV?W⩽11.0 eV. The results show that most of the product ions appear as a single peak, asymmetric with respect to the c.m. zero energy, and suggest predominantly direct interactions over the entire energy range studied. The relative energy of products, calculated from the position of the peak in the measured distribution, follows the predictions of the spectator stripping (SS) model for W?8 eV. Above W=9 eV the SS peak disappears and a product peak found at lower laboratory energies becomes dominant.

Electron paramagnetic resonance of triplet diphenylmethylene in single crystal benzophenone: Evidence for a low temperature phase transition
View Description Hide DescriptionThe angular variation of the field for resonance for diphenylmethylene in benzophenone at T?2 °K has been measured over the entire unit sphere. Both the values of the fine structuretensors T and g and the spatial relationships between the four substitutional sites in the unit cell change as the temperature is lowered from 77 °K to ∼2 °K. The geometrical changes are consistent with a benzophenone phase transition from P2_{1}2_{1}2_{1} to P2_{1} symmetry.

Positronium spin conversion in Li–ammonia solution
View Description Hide DescriptionThe time spectrum of positrons annihilated in lithium–ammonia solutions of various concentrations was analyzed. It was found that the decay rate of the longest lived component increases at the same time as the Li concentration increases; likewise its intensity increases until it attains 100% at high concentration. The results are interpreted in terms of electron exchange collisions which convert triplet positronium to singlet and vice versa. The triplet to singlet conversion rate γ was found proportional to the number of Li atoms dissolved per unit volume.

Crystal Coulomb energies. VI. Madelung energies of simple TCNQ salts. What is the electron affinity of TCNQ (7,7,8,8‐tetracyanoquinodimethan) ?
View Description Hide DescriptionThe Madelung energy E _{ C } (classical interionic crystal Coulomb energy in the fractional point‐charge approximation) has been calculated for Na^{+}TCNQ^{−}, Rb^{+}TCNQ^{−}(I), Rb^{+}TCNQ^{−}(II), and for NH_{4} ^{+}TCNQ^{−}. The values obtained are E _{ C }=−5.29, −4.71, −4.64, and −4.82 eV/molecule, respectively. When these values are coupled with available thermochemical data in a Born–Haber cycle, then the best recent experimental estimate of the electron affinity of TCNQ, 2.8 eV (obtained by Cs beam collisional ionization) is not verified, because apparently in these organic ionic crystals, as many as 2.5 eV of binding must be due to other interactions which our Madelung calculations ignore at present. It is likely that charge‐induced dipole interactions may supply the several electron volts that are missing from the calculated crystal binding energy.

Crystal Coulomb energies. VII. The electrostatic binding energy defect in Tetrathiofulvalinium 7,7,8,8,‐tetracyanoquinodimethanide
View Description Hide DescriptionThe Madelung energyE _{ C } of TTF TCNQ was computed by Ewald’s method using the 300 and 100 °K crystal structures and several charge models based on CNDO/2, PPP, and INDO atom charge densities, and as a function of charge transfer. In all cases E _{ C } compares unfavorably, by several electron volts, with the cost of ionizing the lattice. This had been noted previously for NMP TCNQ. This binding energydefect is too large to be explained by metallic binding or exchange forces, or by electron correlation, even in the Wigner crystal limit. We speculate that polarization forces, or covalent effects may play a large role in stabilizing the lattice. The Madelung site potentials in the TTF^{+1}TCNQ^{−1} lattice are sufficient to explain the observed core level splittings in the photoelectron spectrum.

Pressure and temperature dependences of the Raman‐active phonons in dichlorobis(pyridine)zinc(II) crystal
View Description Hide DescriptionThe effects of pressure and temperature upon the eight Raman‐active phonons of the [ZnPy_{2}Cl_{2}] crystal were investigated. Two techniques were used to measure the presssure dependence of the phonon frequencies, which allowed us to determine the isothermal compressibility and the mode Grüneisen parameters. The isobaric temperature dependence of each frequency was separated into thermal strain and anharmonic ’’self‐energy’’ contributions and discussed in terms of the cubic and quartic anharmonicities.

Derivation of stochastic equations for nonequilibrium Ising mean field model
View Description Hide DescriptionPhenomenological master equations are useful for discussions of various aspects of the kinetics of phase transitions. The transition rate in such master equations is of a form proposed by Langer; the rate is an exponential of the change of the free energy of the system due to a change in a thermodynamic variable. We present a derivation of this form from a microscopic theory for an Ising model which exhibits first and second order phase transition. Using Zwanzig’s formalism we derive a master equation for a spin system which interacts with a phonon heat bath. Thus, we obtain the transition rate on a time scale of one microscopic event (one spin flip). This form differs from the phenomenological one in that it depends on details of the dynamics of the system, the heat bath, and the interaction between them. These details are removed by a further averaging (coarse‐graining). If a Markoffian master equation is assumed to exist on a phenomenological time scale large compared to the microscopic time scale, yet small compared to the relaxation time of the system, then we show that the transition rate in that master equation has the form proposed by Langer.

Vacuum ultraviolet absorption spectra of solid OCS at 53 K
View Description Hide DescriptionPhotoabsorptionspectra of thin films of solid OCS were obtained in the 110–300 nm wavelength region. The absorption intensities and excitation energies of the ^{1}Δ←^{1}Σ^{+} and ^{1}Π←1Σ^{+} transitions were changed in the solid, and higher energyRydberg absorption was reduced and diffuse. Three absorption continua not normally observed in the gas phase were found at 4.3, 4.7, and 6.9 eV and assigned to the ^{3}Σ^{+}←^{1}Σ^{+}, ^{1}Δ←^{1}Σ^{+}, and ^{3}Π←^{1}Σ transitions, respectively.

Diffusion theory for crystal growth at arbitrary solute concentration
View Description Hide DescriptionThe influence of the solute concentration upon the kinetics of crystal growth is studied for various cases with equilibrium and nonequilibrium boundary conditions, constant cooling rate, and temperature‐dependent diffusion coefficient. A perturbation expansion is used to calculate first‐order corrections in the low concentration region. Complete numerical solutions for nonequilibrium crystal–liquid interface conditions are obtained by a finite difference method. The influence of the various parameters upon the time‐dependent growth rates is discussed.

Ultrasonic investigation of viscosity coefficients in the nematic liquid crystal, EBBA
View Description Hide DescriptionThe attenuation of ultrasonic compressional waves in the nematic liquid crystal,N‐ (p‐ethoxybenzylidene) –p‐butylaniline (EBBA), has been measured as a function of frequency, temperature, and the angle between an aligning magnetic field and the direction of wave propagation. The results are interpreted in terms of theories of nematic liquid crystals based on hydrodynamics and micropolar continuum mechanics. It is shown that the correct expression for the attenuationanisotropy can also be derived from Leslie’s theory of anisotropic fluids. It was found that shear viscosity coefficients had the usual exponential dependence on inverse temperature with an activation energy of 10 kcal/mole whereas volume viscosity coefficients had a more complex temperature dependence. The attenuation of compressional waves in the unoriented nematic phase is compared with that of the oriented crystal.

Pressure dependence of rotational relaxation times T _{1} and T _{2} in the J=0−1 transition of OCS
View Description Hide DescriptionTime‐domain experiments on the OCS J=0−1 ΔM=0 microwave transition have been used to measure the pressure dependence of two relaxation times: for radiation from a coherently excited sample, 1/T _{2}=37.66±0.10 μsec^{−1}⋅Torr^{−1}, and for the population difference between the J=0, M=0, and J=1, M=0, states, 1/T _{1}=38.10±0.18 μsec^{−1}⋅Torr^{−1}. These results imply that not all collisions induce dipole‐allowed transitions. These values are consistent with previous linewidth values, beam maser experiments, and four‐level double resonance experiments.

Analysis of Monte Carlo results on the kinetics of lattice polymer chains with excluded volume
View Description Hide DescriptionMonte Carlo calculations by Verdier e t a l. on the kinetics of polymer chains on a lattice have shown a large increase of relaxation times in the presence of excluded volume restrictions, i.e., when two beads of the chain cannot occupy the same lattice site. We show that these long relaxation times must be attributed to the specific choice of the kinetics rather than to the intrinsic nature of the excluded volume interaction. A simplified analytic model which preserves the essential characteristics of the kinetics of Verdier’s model reproduces qualitatively the Monte Carlo results for the realxation of the squares of the Rouse coordinates.

Use of percolation theory to estimate effective diffusion coefficients of particles migrating on various ordered lattices and in a random network structure
View Description Hide DescriptionTwo computational strategies were developed to estimate from percolationtheory the relative diffusion coefficients D _{ r } of particles executing random walks on various ordered lattices of different coordination numbers z and in random networks, characterized by a uniform density of nearest‐neighbor sites around a given site, beyond a certain minimum separation of the sites R _{ c }. Relative solubilitiess _{ r } also were estimated for these systems, using literature data. The way the computed D _{ r } and s _{ r } depend on the probability that a given site is accessible to the migrating particle, and on z or on R _{ c }, respectively, when compared to corresponding experimental results, may allow one to characterize the ordered or random network structure of the substrate used.

Reactions of germanium vapor with oxidizers: Photon yields and a new GeO band system
View Description Hide DescriptionGermanium was reacted with N_{2}O, O_{2}, NO_{2}, and NO in a flow system, which produced GeO. Chemiluminescence was observed from three electronic band systems of GeO: A ^{1}Π→X ^{1}Σ^{+} in the uv, a ^{3}Σ^{+}→X ^{1}Σ^{+} in the blue, and b ^{3}Π_{1}→X ^{1}Σ^{+} (a previously unreported system) in the near uv. Vibrational constants and T _{ e } values have been obtained for the a ^{3}Σ^{+} and b ^{3}Π_{1} states, and evidence is presented for a perturbation between the b ^{3}Π (v=8) and A ^{1}Π (v=0) states. Photon yields for each of the three GeO band systems were measured to be small (<0.1% in all cases) and to be strongly pressure dependent in the 0.5‐ to 20‐Torr region. Reaction of Ge vapor with F_{2} produced GeF (A ^{2}Σ^{+}→X ^{2}Π) chemiluminescence with a measured photon yield of 6.8×10^{−4}; the flames from Ge+Br_{2} and Ge+Cl_{2} were too weak to measure.

Internal rotation in ethylamine: A treatment as a two‐top problem
View Description Hide DescriptionThe infrared absorption spectra of eight isotopic ethylamine molecules, i.e., CH_{3}CH_{2}NH_{2}, CD_{3}CH_{2}NH_{2}, CH_{3}CD_{2}NH_{2}, CD_{3}CD_{2}NH_{2}, CH_{3}CH_{2}ND_{2}, CD_{3}CJ_{2}ND_{2}, CH_{3}CD_{2}ND_{2}, and CD_{3}CD_{2}ND_{2}, have been examined in the vapor phase in the 300–100 cm^{−1} region. Several Q‐branch peaks were observed for each isotopic species and assigned to the torsional oscillations of the methyl and amino groups of the t r a n s and g a u c h e isomers. The energy levels were calculated on the basis of a coupled two‐top system. The analysis yielded a probable potential function for the internal rotation about the C–N bond of the form V (α) = (316.5/2)(1−cosα)−(11.3/2)(1−cos2α)+(713.7/2)(1−cos3α) −(25.0/2)(1−cos4α)+(25.0/2)(1−cos5α)−(3.7/2)(1−cos6α). The difference between the potential energy minima of the t r a n s and g a u c h e conformations is about 230 cm^{−1}, the t r a n s being the more stable form. It has been concluded that the axis of internal rotation of the amino group does not coincide with the N–C bond but is along a line about 4.5° from the N–C bond and in the plane bisecting the NHH triangle.

NMR and NQR study of ClF_{5} in its solid phases
View Description Hide DescriptionThe ^{19}F NMR line shape and relaxation have been studied in solid ClF_{5} from the melting point down to liquid nitrogen temperature. The corresponding results, completed by relaxation data concerning ^{35}Cl NMR and NQR, have yielded information about phase transitions and molecular motions. In particular, a new solid–solid transition was found at 117 °K. ^{35}Cl wide‐line and ^{19}F high resolution NMRspectra were obtained in the high temperature phase S _{I}, which is shown to be a plastic phase where molecular reorientations (τ_{ϑ}=8×10^{−12} sec) are nearly as fast as in the liquid. In addition to the reorientation there is a much slower translational diffusion, the main characteristics of which are derived from ^{19}F T _{1} and T _{1ρ} measurements using Torrey’s theory. In the low temperature phases S _{II} and S _{III}, the ^{19}F T _{1} is due to rotations of the molecules by jumps around their C _{4} axis (τ_{ r }=10^{−8} sec at 110 °K). In solid II the NQR relaxation timeT _{1Q } is due to another mechanism which is assumed to be jumps of the C _{4} axis between two equilibrium positions. The dipolar energy relaxation timeT _{ D } was also measured; between 123 and 153 °K, the Cl–F dipolar coupling modulated by the chlorine relaxation appears to be the main T _{ D } mechanism, for which a theory is described.

Study of the temperature dependence of chlorine‐35 nuclear quadrupole resonance in ClF_{5}
View Description Hide DescriptionThe ^{35}Cl nuclear quadrupole resonance frequency ν_{ q } has been studied as a function of temperature between 4.2 and 135 °K. No break is seen on either the ν_{ q }(T) curve or its derivative in spite of a phase change observed in the fluorine NMR at 117 °K. The quadrupole coupling constant observed is discussed in terms of the occupancy of the 3p orbitals on chlorine as determined by molecular orbital calculations and compared to the value found by microwave spectroscopy. The temperature dependence of ν_{ q }(T) is interpreted in the light of Bayer’s theory and a value of the lowest libration frequency is obtained (ν_{ l }?30 cm^{−1}) which is also found consistent with the value of the relaxation time,T _{1}, of ^{35}Cl nuclei.

The effect of pressure on the near‐neighbor distance in water
View Description Hide DescriptionWhen liquid water is compressed, the O−−O hydrogen bonds tend to contract because of the direct effect of the pressure, and the O−−O−−O angles tend to deviate more from the tetrahedral. The coupling between the O−−O stretching and O−−O−−O angle bending is such that the change of the angles tends to increase the bond lengths. The net effect of pressure is the resultant of the direct contraction and the indirect expansion. It is suggested that the actual contraction can be measured by the effect of pressure on the O–H or O–D stretching frequencies of HDO in dilute solution in D_{2}O or H_{2}O. The application of this suggestion to literature data shows that the bond contracts only about 0.3 of the rate that the hydrogen bond in ice contracts. No doubt the contraction due to the direct effect of pressure is largely compensated by the expansion due to the distortion of the O−−O−−O angles.