Index of content:
Volume 64, Issue 4, 15 February 1976

Deactivation of CO_{2}(010) and CO_{2}(001) by hydrogen and deuterium
View Description Hide DescriptionThe probabilities of deactivating the bending and asymmetric stretching modes of CO_{2} by hydrogen and deuterium are calculated semiclassically over the temperature range of 300–2000 °K. It is assumed that the molecular interaction potential consists of a superposition of atomic pair repulsion forces. For the CO_{2}(010) –H_{2} case, an inverse temperature dependence of probability was found over the 300–500 °K range; no inverse temperature dependence was found for CO_{2}(010) –D_{2}. The calculations indicate that it is possible to attribute the vibrational relaxation to energy transfer between the vibrations of CO_{2} and the rotation as well as translation of H_{2}/D_{2}. The different temperature dependences are essentially due to the difference between H_{2} and D_{2} in rotational energy matching with CO_{2} vibrations. The calculated temperature dependence of the deactivation probabilities is in good agreement with experimental data. However, the measured absolute values are lower than the calculated ones.

Oscillations in chemical systems. XII. Applicability to closed systems of models with two and three variables
View Description Hide DescriptionMost computations on models of chemical oscillators have maintained constant concentrations of major reactants while concentrations of intermediates were allowed to vary. Such simplified computations are applicable to closed chemical systems only if reactants are depleted by small fractions during each cycle. Existing models that involve only two intermediate species are generally unsatisfactory for modeling closed systems. Thus, the Lotka mechanism (which does not generate a true limit cycle) can not model even infinitesimally small oscillations in a closed system unless the rate constant for predator–prey interaction is very large. The Brusselator model can not model closed system oscillations unless the various rate constants are confined to very restricted ranges. Any other model with only two intermediates must contain a step at least third order in those intermediates. By contrast, the Oregonator model with three variables and only first‐ and second‐order processes, can model a closed system in which the major reactants are depleted by only a small fraction during each cycle. By suitable coupling approximations, some of the mathematical advantages of models involving only two variables can be retained when the Oregonator is used as a model.

Negative ion formation in halocarbons by charge exchange with cesium
View Description Hide DescriptionA crossed molecular beam apparatus is used to study the formation of anions by charge exchange of fast cesium atoms with a variety of halocarbons. The branching ratios are determined from threshold to 350 eV. Comparison of the spectator stripping and informationtheoretical models to the data leads to the conclusion that this class of reactions is an example of an ultradirect impulsive mechanism. In the most favorable case, that of CF_{3}I, we have determined the near threshold relative cross sections for the anions CF_{3}I^{−}, I^{−}, CF^{−} _{3}, and IF^{−}. From these we determine the electron affinity of CF_{3}I^{−} to be 1.4±0.2 eV and the bond dissociation energy of CF_{3}‐I^{−} as 0.38±0.1 eV, which disagrees with values obtained in a previous experiment.

Spin–orbit coupling in the excited states of ferricytochrome c and deoxyhemoglobin studied by magnetic circular dichroism
View Description Hide DescriptionMagnetic circular dichroism of ferricytochrome c and deoxyhemoglobin has been measured through their Soret bands at 300 and 80 °K. The MCD of both molecules is found to be temperature dependent (as 1/T) and to have derivative shape in respect to absorption band. The observed MCD is concluded to be the sum of closely spaced paramagneticC‐type effects of opposite signs. The most striking observation is the sign difference for MCD spectra of low spin (S=1/2) ferric heme in cytochrome c and high spin (S=2) ferrous heme in deoxyhemoglobin. The obtained results are interpreted in terms of spin orbit coupling. Two possible mechanisms of spin orbit coupling are discussed. One of them is the usual interaction of the angular momentum of an electron with its own spin, the other is the interaction of circular orbital motion of an electron with a foreign spin located at the metal atom inside the circular current. The two mechanisms give spin orbit coupling constants of opposite sign. The observed MCD seems to indicate the former mechanism to be dominant in the deoxyhemoglobin and the latter one to be dominant in ferricytochrome c.

Muonium chemistry in gases: Mu+Br_{2}
View Description Hide DescriptionWe report a precise measurement of the rate of reaction of muonium atoms with bromine molecules in an argon moderator gas at a atm and 23 °C. The bimolecular rate constant is k= (2.4±0.3) ×10^{11} l/mole‐sec, ten times higher than that for the analogous reaction of hydrogen atoms. Since muonium can properly be treated as a light isotope of hydrogen, this comparison has potential significance for the theory of reaction rates. The technique is described and the results discussed.

Intermolecular potential models for anisotropic molecules, with applications to N_{2}, CO_{2}, and benzene
View Description Hide DescriptionThree representations of the angle‐dependent intermolecular interactions of nonspherical molecules are compared with one another and with experimental data for nitrogen, carbon dioxide and benzene. The models are the atomic, Kihara core, and overlap. The effect of including quadrupolar terms in the potential functions is also considered. It is found that the theoretical virial coefficients are not greatly altered when the quadrupole terms are added, at least at the temperatures corresponding to the experimental ranges for the molecules chosen. However, the theoreticalsolid state heat of sublimation and, to a lesser extent, the crystal lattice parameter, do change significantly. Satisfactory fits to the experimental second virial coefficients and the solid state data were obtained by varying ε, σ, the well‐depth, and range parameters, while fixing the other constants appearing in the models (such as the parameter specifying the nonsphericity) at reasonable values. Quantities of interest, including the equilibrium distance, angular configuration, and dissociation energy of the van der Waals’ dimer, are estimated from the best‐fit potential functions. It is concluded that all three models are approximately equivalent when the molecules are only slightly nonspherical and that there is sufficient flexibility in the models to give good fits to the data even for strongly nonspherical molecules. It appears that the representations of the N_{2}, CO_{2}, and benzene interactions derived here are sufficiently realistic to warrant their use in future work.

Librational excitations in ordered and in partially ordered molecular crystals
View Description Hide DescriptionA projection operator technique is used to calculate the matrix of time correlation functions describing the librational dynamics of molecular crystals in ordered and partially ordered phases. Starting from a general multipolar potential and using the symmetry properties of the dynamic variables, coupled dynamicequations are derived for the angular displacements, their conjugate momenta and the energy density. The frequency matrix and memory kernels that enter the equations describe the libronic excitation frequencies and damping, respectively. The nonharmonicity of the kinetic energy is taken into account self‐consistently in evaluating the libron frequencies. New transport coefficients arising from the nonharmonicity of the kinetic energy are discussed. In addition, for solid CD_{4}II, coupling of the librational excitations to the rotationally disordered central molecules is taken into account. This coupling can lead to an overdamping of the librons. A new central resonance due to the relaxation of the central molecules is found. Comparison with recent neutron scattering experiments is made.

Electron impact investigation of electronic excitations in furan, thiophene, and pyrrole
View Description Hide DescriptionThe electronically excited states of furan, thiophene, and pyrrole have been studied by electron impact at scattering angles from 0° to 80°, and impact energies of 30 and 50 eV. Low‐lying features at 3.99 and 5.22 eV in furan, 3.75 and 4.62 eV in thiophene, and 4.21 eV in pyrrole are identified as singlet → triplet transitions. The locations and, for furan and thiophene, the energy splittings of these excitations suggest that they are analogous to the lowest π → π* singlet → triplet transitions in benzene, and that these heterocycles have appreciable aromatic character. A weak feature observed in pyrrole at 5.22 eV is attributed to an optically forbidden singlet → singlet transition. In all three molecules, transitions to several superexcited states are observed.

Optimization of diatomic state mixing in diatomics‐in‐molecules theory: The CH_{ n } potential‐energy surfaces
View Description Hide DescriptionThe diatomics‐in‐molecules (DIM) theory is extended to include larger valence bond (VB) basis sets and the optimization of mixing between the diatomic VB structures. This mixing is essential to the calculation of DIM potential‐energy hypersurfaces (PEH) using basis sets large enough to generate more than one diatomic state of a given symmetry. In keeping with the philosophy of DIM, the diatomic state mixing is treated as being independent of the number of atoms in the molecule. The method is used to calculate ground and excited state potential energies for CH_{2}, CH_{3}, and CH_{4}. These extended DIM results demonstrate a number of improvements over earlier DIM methods and are in good agreement with a b i n i t i o and experimental results.

Ultrasonic investigation of cholesteric liquid crystals
View Description Hide DescriptionThis paper reports experimental results for the velocity and absorption of ultrasonicwaves in the mesomorphic states of four cholesteric liquid crystals, namely, cholesteryl nonanoate, decanoate, oleate, and linoleate. The absorption has been measured by a pulse technique and velocity by an acoustic interferometer, both at a single frequency of 2 MHz. The results obtained for these materials furnish distinct similarities with the behavior of pure normal materials in the critical region. Anomalous increase in absorption and decrease in velocity observed across the isotropic–mesomorphic transition temperature in each case are interpreted in terms of pretransitional effect.

The effect of shape factor on the quantal collision integrals
View Description Hide DescriptionQuantal collision integrals taking into account the shape factor which represents a nonspherical repulsive part of molecular interactions have been evaluated by using the tabulations of collision integrals for the Lennard‐Jones (12:6) potential. The angular part of the interactions has been averaged by assuming that the colliding particles maintain the same relative orientation during an encounter and that all relative orientations have equal probability. The results show that the shape factor has significant influence on the quantal collision integrals. Attempts have been made to interpret the viscosity and thermal diffusion factor data for H_{2} and D_{2} gases. Another important result of these calculations is that, contrary to earlier assumptions, it is necessary to consider the effect of shape factor on viscosity for the precise determination of quadrupole moment of these substances from second virial and viscosity data.

Absolute elastic differential electron scattering cross sections in the intermediate energy region. II.—N_{2}
View Description Hide DescriptionUtilizing a recently developed method, the ratios of elastic differential cross sections (DCS) of N_{2} to those of He have been measured at electron impact energies of 5, 7, 10, 15, 20, 30, 40, 50, 60 and 75 eV covering an angular range of 20° to 135° at each energy. The absolute values of N_{2} DCS (which, in the present work, also include the DCS for pure rotational excitation) have been obtained from these ratios by the use of absolute He DCS of McConkey and Preston. From the differential cross sections, elastic integral and momentum transfer cross sections have been evaluated. The results of the present experiment are compared with some previous measurements.

Attractive well of He–He from ^{3}He–^{4}He differential elastic scattering measurements
View Description Hide DescriptionThe elastic differential cross section for ^{3}He–^{4}He was measured at a relative collision energy of 0.799×10^{−14} erg, approximately five times the well depth. The data are fitted to a multiparameter potential form with ε/k=10.57 °K and r _{ m }=2.97 Å. Comparisons with recent experimental and theoretical helium potentials are made. No evidence for a significant isotope effect in the ^{3}He–^{4}He and ^{4}He–^{4}He interactomic potentials is found in this work.

CI study of the water dimer potential surface
View Description Hide DescriptionThe potential energies for the water dimer in various geometrical configurations have been calculated with a configuration–interaction method. The computed dimerization binding energies corresponding to the potential minima for the linear, cyclic, and bifurcated configurations are −5.6, −4.9, and −4.2 kcal/mol, respectively; the correlation effects account for −1.1, −1.2, and −0.9 kcal/mol, respectively, of the total binding energy for these three dimeric forms. The correlation effects for the entire potential surface have been analyzed in terms of inter‐ and intramolecular effects; the substantial coupling found between these effects, particularly in the vicinity of equilibrium position, is discussed. The computational technique employed, in particular an analysis on the selection criteria for the configuration state functions, is discussed, and its reliability is assessed. Two analytical expressions for the water dimer potential surface obtained by fitting the calculated energies are presented. The potential surface given here is being used to determine the structure of liquid water (in the pairwise approximation and with Monte Carlo techniques); this latter work will be reported elsewhere.

Computer simulation of anisotropic molecular fluids
View Description Hide DescriptionWe have extended the molecular dynamics method to permit the simulation of systems containing cylindrically symmetric molecules with arbitrary eccentricity. This extension is accomplished by means of a potential energy function which models the primary interaction effects of molecular anisotropy, and which is mathematically convenient for computer use. The method is then applied to two problems, one involving the stability of the nematic liquid crystal phase, and the other illustrating the effect of cooperative reorientation on spectral line shapes.

Cross sections for photodetachment of electrons from negative ions near threshold
View Description Hide DescriptionThreshold laws for the cross section for photodetachment of electrons from polyatomic negative ions are derived. The general form for the limiting behavior at threshold is derived using group theory and symmetry considerations. In addition, for energies within a few eV of threshold, a formulation using the Born approximation with partial orthogonalization is developed. The energy dependence of the photodetachment cross section for molecular negative ions is computed and shown to agree well with experimentally determined cross sections.

Cross sections for producing CO^{+} _{2} (A ^{2}Π–X ^{2}Π) and CO^{+} _{2} (B ^{2}Σ–X ^{2}Π) fluorescence by photoionization of CO_{2}
View Description Hide DescriptionCross sections have been measured with 0.5 Å resolution between 584 Å and respective thresholds for CO^{+} _{2} A and Bexcited states as they fluoresce to the CO^{+} _{2}ground state. Beutler–Fano resonance profiles are observed in both cross sections converging to the CO^{+} _{2} C ^{2}Σ ion limit at 640 Å. Characteristics of the initial and final states involved in the ionization process are discussed in detail. The ratio of the absolute magnitudes of the A:Bfluorescence cross sections reported in this work is in excellent agreement with results of previous workers near 584 Å, pointing up a fundamental discrepancy between initial and final state measurements of CO^{+} _{2} A and Bexcited state populations.

Laser magnetic resonance spectrum of HCO on the D_{2}O 108 μ laser line
View Description Hide DescriptionAn LMR spectrum on the 108 μ D_{2}O laser line has been observed and assigned to the 8_{2,6}←7_{1,7} rotational transition of HCO. The spectra of both π (parallel) and σ (perpendicular) polarizations have been fitted by the least squares method, giving excellent agreement between the calculated and observed magnetic fields. The rotational frequency obtained from the fittings has been used to determine the Arotational constant. In addition, the electron spin‐rotational splitting of each rotational level and the Fermi constant have been determined.

Linewidths in the vibrational spectra of polymers in dilute solution
View Description Hide DescriptionDamping of the vibrations of polymer chains due to friction with solvent, including hydrodynamic interaction between chain elements, is considered. The linear chain model is treated in detail. It is concluded that the damping constant is independent of the degree of polymerization. This conclusion is based on use of the high frequency hydrodynamic interaction. Only backbone vibrations of the chain are treated; localized group vibrations are not within the purview of the model.

Local phase and renormalized frequency in inhomogeneous chemioscillations
View Description Hide DescriptionInhomogeneous evolution of a chemically reacting system capable of a homogeneous oscillation is analyzed in terms of a nonsecular perturbation method. This ’’phase diffusion’’ theory takes account of the local phase shifts and frequency renormalization that occurs due to the interaction of reaction and diffusion. The theory is used to show that oscillator like plane waves are stable to small perturbations. Boundary value problems including those for a finite volume impermeable vessel, a ring shaped vessel (periodic boundary condition) and catalytic walls and membranes are shown to lead to stable oscillatory states some of which are inhomogeneous for all times. The structure of the theory allows for clear qualitative interpretations. Several experiments are suggested for the purpose of verification of the theory and the phenomena predicted.