Volume 65, Issue 10, 15 November 1976
Index of content:

ESR study of oxidation product in irradiated α‐amino acids: Nitrogen centered π radical in α‐glycine
View Description Hide DescriptionThe structure of the oxidation product H_{2}Ṅ^{+}CH_{2}CO_{2} ^{−} in irradiated α‐glycine has been studied based on the hyperfine coupling tensors which have been determined at 4.2°K by single crystalESR measurements. It has been found that the radical initially possesses a nearly planar structure at the nitrogen atom with a framework similar to that of the undamaged molecule and then changes its conformation into the form previously found by Sinclair. The principal axes of the nitrogen coupling tensor suggest that the selective proton or hydrogen atom transfer from the NH_{3} ^{+} group takes place across the shortest hydrogen bond forming the radical.

Ground X ^{2}Σ^{+} state potential well, and excited state dynamics of diatomic XeF in solid Ne and Ar
View Description Hide DescriptionThe ’’shape’’ of the ground state potential well of XeF in solid Ne has been obtained from an analysis of D (1/2) →X ^{2}Σ^{+} resonance fluorescence. The constants ω_{ e } ^{″}=247 cm^{−1} and ω_{ e } x _{ e } ^{″}=10.2 cm^{−1} give an extrapolated Morse oscillatorD _{ e }?1500 cm^{−1}. The ground state well is only slightly deeper in the solid than in the gas phase (D _{ e }?1160 cm^{−1}). In solid Ar, the T _{ e } values for the excited D (1/2) and B (1/2) states undergo unprecedented red shifts of ?8% and ?13% respectively, apparently reflecting the charge transfer nature of these excited states. The extrapolated gas phase radiative lifetimes are τ_{ r }=13±1 nsec for D (1/2), and τ_{ r }=8±2 nsec for B (1/2). Polarized fluorescence studies prove that the gas phase 3500 Å lasing transition is B (1/2) →X ^{2}Σ^{+}. The influence of the T _{ e }red shift upon the excited state dynamics, and upon the X ^{2}Σ^{+} well depth in the solid, is discussed. Unassigned emission spectra are also observed from other van der Waals complexes among Xe and F atoms.

Decomposition of isolated molecules: A transition state treatment
View Description Hide DescriptionIt is argued that, contrary to the assumption of RRKM theory, reactant states near the critical surface frequently may not be at statistical equilibrium with the bulk of reactant states. The main objective of this work is to examine conditions under which the RRKM specific unimolecular rate constant expression nevertheless remains valid. The analysis proceeds by casting the general, time‐dependent, decomposition rate of isolated molecules into time‐independent form and applying a transition state approximation similar to that introduced by Miller for bimolecular reactions. In the process of carrying out this program, the following is achieved: (a) A potentially useful unimolecular rate expression, analogous to Miller’s transition state theory rate formula for bimolecular reactions, is obtained; (b) a criterion for choosing critical configurations similar to the Bunker–Pattengill recipe is derived when classical mechanics is obeyed; (c) features of a recent calculation of unimolecular reaction dynamics are explained.

Polarized Raman spectra and infrared reflection spectra of lithium potassium sulfate
View Description Hide DescriptionPolarized Raman spectra and infrared reflection spectra of LiKSO_{4} are observed in the frequency region lower than 2000 cm^{−1}. The frequencies of transverse and longitudinal modes for the internal and external vibrations are obtained directly from the polarized Raman spectra and from the classical dispersionanalysis of the infrared reflection spectra. The assignments of the observed bands are discussed.

A b i n i t i o effective core potentials: Reduction of all‐electron molecular structure calculations to calculations involving only valence electrons
View Description Hide DescriptionA formalism is developed for obtaining a b i n i t i o effective core potentials from numerical Hartree–Fock wavefunctions and such potentials are presented for C, N, O, F, Cl, Fe, Br, and I. The effective core potentials enable one to eliminate the core electrons and the associated orthogonality constraints from electronic structure calculations on atoms and molecules. The effective core potentials are angular momentum dependent, basis set independent, and stable against variational collapse of their eigenfunctions to core functions. They are derived from neutral atom wavefunctions using a pseudo‐orbital transformation which is motivated by considerations of the expected accuracy of their use and of basis set economy in molecular calculations. Then the accuracy is demonstrated by multiconfiguration Hartree–Fock calculations of potential energy curves for HF, HCl, HBr, HI, F_{2}, Cl_{2}, Br_{2}, and I_{2} and one‐electron properties for HF and HBr. The differences between valence‐electron calculations employing the present effective core potentials and all‐electron calculations are smaller than differences due to basis set choices, even though the basis sets are extended ones. Thus the effective core potentials are quite successful. In addition larger configuration mixing calculations are performed for HBr and Br_{2} (1637 and 3396 configurations, respectively) and again the effective core potentials are judged to perform well.

The low‐lying states of hydrogen fluoride: Potential energy curves for the x ^{1}Σ^{+}, ^{3}Σ^{+}, ^{3}Π, and ^{1}Π states
View Description Hide DescriptionGeneralized valence bond (GVB) and derived configuration interaction (CI) calculations have been carried out on the states of hydrogen fluoride (HF) which arise from ground state atoms: X ^{1}Σ^{+}, ^{3}Σ^{+}, ^{3}Π, and ^{1}Π. The GVB plus all single and double excitation calculations yield (with experimental constants in parentheses): D _{ e }=5.72 eV (6.12 eV), R _{ e }=0.920 Å (0.9168 Å), and ω_{ e }=4158 cm^{−1} (4138.73 cm^{−1}). The polarization CI (POL−CI) wavefunction is found to reproduce well the results of the more extensive calculation. The POL−CI wavefunction takes into account the internal and semi‐internal correlation effects of open‐shell many electron theory. A simple formula, involving Morse functions, is proposed to account for the remaining error in the ground state potential curve.

Kinetic studies of N_{2} and N_{2}–SF_{6} following proton excitation
View Description Hide DescriptionLow intensity proton pulses have been used to excite pure N_{2} and N_{2}–SF_{6} mixtures. By accumulating time and wavelength resolved information on the fluorescence that arises from a very large number of pulses, we have obtained data that are free from effects due to superelastic collisions with electrons or other nonlinear effects such as the interactions between ions or excited neutral species. For N_{2} the natural lifetimes of N_{2}(C ^{3}Π_{ u }) and N_{2} ^{+}(B ^{2}Σ_{ u } ^{+}) were determined by monitoring N_{2}(C ^{3}Π_{ u }→B ^{3}Π_{ g }) and N_{2} ^{+} (B ^{2}Σ_{ u } ^{+}→X ^{2}Σ_{ g } ^{+}) states, respectively. The quenching rates of N_{2}(C ^{3}Π_{ u }) and N_{2} ^{+}(B ^{2}Σ_{ u } ^{+}) in different vibrational states by pure N_{2} and by SF_{6} were obtained. The addition of small concentrations of SF_{6} altered the late time behavior of the N_{2}(C ^{3}Π_{ u }→B ^{3}Π_{ g }) transitions, and the addition of much higher concentrations of SF_{6} caused a large increase in the intensity of the 3371 Å (C→B) transition.

A theory of molecular velocity correlations in liquids
View Description Hide DescriptionIt is proposed that the frequency spectrum(Fourier transform) of the single particle velocitycorrelation function for a classical liquid is of the form ρ (ω) = (ω_{ c } ^{2}−ω^{2})^{1/2} f (ω) + (ω_{ c }ω−ω^{2})^{1/2} g (ω), where ω_{ c } is identified with the intermolecular collision frequency. The rationale for this construction is that the features of a cutoff frequency and nonanalytic points characteristic of the phonon spectrum of a solid should be reflected in the associated liquid: ω_{ c } defines the cutoff frequency and ρ (ω) is nonanalytic at ω=0. As a consequence, a general analytic expression for the correlation function as a superposition of Bessel functions is developed, whose damped oscillatory behavior parallels that deduced from neutron scattering experiments and molecular dynamics computer simulation studies of liquids. The Bessel function basis set for the correlation function implies an expansion of the frequency spectrum in terms of a Gegenbauer polynomial basis set, both of which can provide analytical tools for molecular dynamics investigations. For long times such that ω_{ c } t≫1 the correlation function decays as t ^{−3/2}[A+B sin(ω_{ c } t−π/4)]. The first term is identified with the shear viscosity from hydrodynamics, and it is argued that the second term should also exist as a residuum lattice contribution not deriveable from hydrodynamics. A specific two parameter model spectrum is considered to establish a connection between the present formulation and the Langevin theory of Brownian motion.

Dynamic orientational pair correlations in symmetric tops and static orientational pair correlations in anisometric molecules
View Description Hide DescriptionThe theory describing the depolarized spectrum of light scattered from symmetric tops is re‐examined. On the basis of molecular symmetry it is shown that, in the rotational diffusion limit, dynamic orientational pair correlations do not affect the depolarized spectrum of scattered light. Molecular symmetry arguments are also applied to the calculation of the depolarized spectrum of light scattered from asymmetric tops and binary solutions of symmetric tops. It is found that orientational pair correlations may affect the calculated spectrum in these two cases in a rather complex fashion. Effects which had previously been attributed to the affect of dynamic orientational correlations may actually be explained on the basis of static orientational correlations alone. The experimental data on nitrobenzene and binary solutions of hexafluorobenzene/benzene are discussed explicitly. The possibility of obtaining information about the static orientational pair correlations of more than one axis in anisometric molecules is suggested.

Nonequilibrium velocity distribution and reaction rate in hot‐atom reactions
View Description Hide DescriptionThe nonequilibrium velocity distribution and reaction rate in the hot‐atom reactions are studied by solving the time‐dependent Boltzmann equation with the Monte Carlo simulation. The explicit time‐dependent velocity distribution, temperature, and rate constant of hot atoms from initial to steady states are obtained for a simple model system, where hot atoms are dispersed in the heat bath of surrounding molecules without internal degrees of freedom and the cross sections are chosen as a simple hard sphere model. The low‐ or high‐temperature steady state exists in consistence with the prediction of Keizer with the Maxwell distribution approximation. The hot‐atom velocity distribution for the high‐temperature steady state is, however, different from the Maxwell distribution. The nonequilibrium velocity distribution yields a much smaller value of the lower limit of the ratio of reactive to elastic cross sections for the existence of the high‐temperature steady state and a smaller rate constant than those of the Maxwell distribution approximation by Keizer.

Sequential impluse model of direct reactions
View Description Hide DescriptionA model in which the reaction A(BC, C) AB occurs as the result of a sequence of two hard sphere elastic impulses between the A–B and B–C pairs is analyzed. It is shown that the limits of the product velocity vector distribution can be readily obtained from the analytic geometry of the impulse sequence, and that an analytical expression for the detailed product distribution for all mass combinations and energies can be deduced. The results show a propensity for processes in which the velocity of the C atom is changed very little and which therefore lead to products near the spectator stripping velocity. The angular distribution is dependent on, but relatively insensitive to, the ratio of the mutual hard sphere diameter of B and C to their bond distance. The predictions of the model are generally consistent with the features found in the experimental investigations of high energy ion–molecule reactions.

Ion induced luminescence of alkali halides with CN impurity
View Description Hide DescriptionDuring a study of luminescence induced by low energy (20 keV) light ion (H^{+}, He^{+}) impact on certain alkali halides (LiCl, NaBr, NaCl, KBr, KCl) we have observed a band spectrum in the region 230 to 350 nm. The bands are separated by 0.26 eV and have a half‐width of 0.12 eV; no fine structure is observed. Identical bands of much increased intensity are observed when samples deliberately doped with CN^{−} are bombarded. The phenomenon is not found with KBr crystals. We suggest that the observed bands are due to transitions in the CN radical, possibly the D ^{2}Π_{1}–X ^{2}Σ^{+} transition that is observed as a very weak transition in discharges. The mechanism whereby CN is formed on the samples remains unclear.

Resonance Raman scattering from metastable O_{2} molecules in γ‐irradiated NaClO_{3}: Excitation profile, polarization measurements, and decay kinetics
View Description Hide DescriptionResonance Raman effects have been observed with the 1544 cm^{−1} O–O stretching mode of a metastable O_{2} species (O_{2}*) in γ‐irradiated NaClO_{3}. An excitation profile of the 1544 cm^{−1} band was determined from the scattering intensity measured with exciting lines from 457.9 to 625.6 nm, and the overtones, 2ν to 4ν, were observed with 457.9 nm excitation. Linear depolarization ratios (ρ_{ l }) were measured as a function of exciting line wavelength from 457.9 to 514.5 nm. An average value of ρ_{ l }∼0.42 was found from z (x z) y and z (y x) y scattering, but a pronounced dispersion was observed in ρ_{ l } for z (y z) y scattering. The change in intensity of the 1544 cm^{−1} band observed over a period of days showed that the O_{2}* species decays by a first order process at 300 K with a half‐life of 1.4 days. A model of O_{2}* consisting of a weakly bound complex [X,O_{2}], where X is a radiation produced radical or ion, is discussed.

Anisotropic fluorine and proton hyperfine interaction and motional effects in the electron spin resonance spectrum of the β,β,β‐trifluoroethyl radical (CF_{3}CH_{2}⋅) in an argon matrix at 4.2 K
View Description Hide DescriptionThe electron spin resonance spectrum of the β,β,β‐trifluoroethyl (CF_{3}CH_{2}⋅) radical has been measured in a polycrystalline argon matrix at 4.2 K. The radical was produced i n s i t u by photolysing the corresponding iodide. An analysis of the spectrum leads to a complete evaluation of the anisotropies in the g and hyperfine interaction tensors of the spin Hamiltonian for the radical. The analysis is confirmed by a computer simulation of the observed line shapes, and the following magnetic parameters were evaluated: g _{ x }=2.0021, g _{ y }=2.0020, g _{ z }=2.0026, A_{ x }H(α) =23.7±0.1, A_{ y }H(α) =22.3±0.1, A_{ z }H(α) =30.9±0.1, A_{ x }F(α) =17.7±0.1, A_{ y }F(α) =20.4±0.1, and A_{ z }F(α) =48.0±0.1 G. A discussion of the above results provides an understanding of the mechanism of electron spin delocalization to the β‐fluorine atoms, as well as additional information on the conformation of the α‐substituent nuclei and the effects of their motion on the line shapes of the spectrum.

High temperature thermodynamics of palladium–hydrogen. II. Temperature dependence of partial molar properties of dilute solutions of hydrogen in the range 500–700 K
View Description Hide DescriptionThe thermodynamic properties of hydrogen in dilute solutions in palladium have been studied in the temperature range 500–700 K both by the equilibrium method and by calorimetry. In this range the partial enthalpies and excess entropies of hydrogen increase very significantly with temperature. This suggests the existence of a ’’soft’’ transition for the dissolved hydrogen atoms. The experimental values of the partial excess entropies of hydrogen in palladium may be explained by assuming occupancy by hydrogen of both tetrahedral and octahedral interstitial sites at room temperature or by the modification of the vibrational spectrum of palladium. The further increase of the excess entropy above room temperature may be due to the gradual accumulation of a communal entropy of R between room temperature and 700 K.

Vibrational energy exchange between H_{2}(v=1) and D_{2}, N_{2}, HCl, and CO_{2}
View Description Hide DescriptionThe vibrational relaxation of H_{2}(v=1) by D_{2}, N_{2}, HCl, and CO_{2} was studied at 295°K by a laser‐induced fluorescence technique. HF added in small quantities to the gas mixtures and vibrationally excited by a pulsed HF TEA laser produced H_{2}(v=1) by V–V transfer; HF also served as the tracer for H_{2}(v=1). The results were interpreted to be primarily V–V rates; the values being (1.25±0.15) ×10^{−3}, (3.14±0.4) ×10^{−4}, (2.1±0.3) ×10^{−6}, and (4.4±0.9) ×10^{−4}(μsec Torr)^{−1} for H_{2}(v=1) transfer to HCl, D_{2}, N_{2}, and CO_{2}, respectively. The data for D_{2} and N_{2} were correlated with the V–V exchange probabilities for other diatom–diatom collisions by the parameter μ^{1/2}ΔE, where μ is the reduced mass of the collision partners and ΔE the energy mismatch. This parameter also correlates V–V exchange probabilities for CO_{2}–diatom collisions. A separate correlation of V–V exchange probabilities for hydrogen halide–homonuclear diatom collisions was obtained by plotting the probabilities versus ΔE/hν.

Heat capacity and entropy of MnF_{2} from 10 to 300 °K. Evaluation of the contributions associated with magnetic ordering
View Description Hide DescriptionThe heat capacity of a sample of MnF_{2} consisting of crystals larger than 2 mm has been measured between 10 and 300 °K. The peak in heat capacity associated with the antiferromagnetic ordering is at 67.30 °K. Measurements with small temperature rise show the shape of the heat capacity curve near the anomaly. The corresponding states approximation for evaluating the lattice heat capacity is tested in regions where the magnetic contributions are calculable. Magnetic contributions to the heat capacity and entropy are tabulated between 10 and 100 °K. Smooth values of the total heat capacity,entropy,enthalpy, and Gibbs energy are tabulated between 10 and 300 °K. Values at 298.15 °K are C°_{ P }=15.96 cal °K^{−1} mole^{−1}, S°=22.04 cal °K^{−1} mole^{−1}, H°−H°_{0}=3106 cal mole^{−11}.

A molecular‐statistical theory of the temperature‐dependent pitch in cholesteric liquid crystals
View Description Hide DescriptionAn interaction between chiral molecules is derived in terms of the orientations of their long molecular principal axes. In analogy with Maier–Saupe and Goossens the derivation is based on the electric multipole expansion. The molecules are assumed to behave as if they were cylindrically symmetric, i.e., the system is locally nematic. As an example the chiral molecules are represented by Kuhn models, and the relevant coupling constants are calculated explicitely. An expression for the free energy is obtained in the molecular field approximation. It is shown that, if only the induced dipole–dipole and the dipole–quadrupole dispersion energies are taken into account, a temperature‐independent cholesteric pitch is obtained. This result is traced back to the symmetric character of the interaction. In order to explain the experimental situation one has to introduce, in analogy with Keating, an asymmetry producing interaction. The proposed model is discussed and its thermodynamic properties are calculated in the molecular field approximation. It is found that the magnitude of the reciprocal pitch varies nearly linearly with temperature in agreement with experiment. Interpretational difficulties related to the use of the multipole expansion are discussed.

Stability of the NO^{+}⋅N_{2} ion cluster
View Description Hide DescriptionThe equilibrium constantK for the reaction NO^{+}+2N_{2}?NO^{+}⋅N_{2}+N_{2} has been determined mass spectrometrically from 178–273 °K at N_{2} pressures of 3–12 Torr. Small quantities of O_{2} were added to the gas stream to catalyze the charge exchange between N_{4} ^{+} and NO. By analysis of the K data it was found that ΔH°=−5.04±0.09 kcal/mole and ΔS°=−18.5±0.3 cal/mole °K at 298 °K. By extrapolation of the enthalpy data to 0 °K, the NO^{+}–N_{2} bond energy was found to be 4.98±0.12 kcal/mole.

The visible emissions of molecular oxygen in rare gas solids
View Description Hide DescriptionThe vacuum ultraviolet‐excited emission spectra of molecular oxygen in low temperature rare gas solids have been recorded and analyzed. The observed transitions have been reassigned using isotopic substitution and are found to be C ^{3}Δ_{ u }→X ^{3}Σ_{ g } ^{−} in argon and krypton and c ^{1}Σ_{ u } ^{−}→a ^{1}Δ_{ g } in xenon. These transitions have not been observed in the gas phase and the transition origins established here confirm previous tentative assignments of vibrational numberings of the upper states by Herzberg and Degan, respectively. The emission in xenon is very unusual for a solid phase spectrum in that emission occurs from several excited vibrational levels, which are apparently being populated directly by intersystem crossing. It is also unusual in that the lower state of the emission is not the ground state. No significant chemical interaction between the molecule and the rare gas is seen for any of the observed states.