bluuebunny/crossref_metadata_2025 huggingface dataset:
Isotope effect on the hyperfine splittings of the deuteriobenzene anions: An example of the dynamic Jahn–Teller effect
Authors: Purins D. , Karplus M.
Published: February 2003

The vibronic wavefunctions and energy levels of the benzene, benzene−1−d, benzene−2,6−d2, benzene−1,4−d2, and benzene−1,3,5−d3 anion radicals have been obtained and used to determine the effect of deuterium substitution on the degenerate energy levels and spin densities of the benzene anion. It is shown that a dynamic Jahn−Teller effect treatment is required for these systems to obtain a quantitative interpretation of the consequences of deuterium substitution. The ground vibronic states of benzene− and benzene−1,3,5−d3− are degenerate. In benzene−2,6−d2−, the symmetric state (A1 under C2v) is 22 cm−1 below the antisymmetric state (B1 under C2v). In benzene−1−d− and benzene−1,4−d2−, the antisymmetric states are lower by 9 and 39 cm−1, respectively. The benzene−2,6−d2− and benzene−1,4−d2− hyperfine coupling constants and their temperature dependence calculated using the vibronic wavefunctions and energy splittings are in very good agreement with the experimental values. For benzene−1−d, the experimental splitting is estimated to be 20 cm−1, about twice as large as the calculated value, and agreement between the experimental and calculated hyperfine constants is less satisfactory. The vibronic energy splitting are interpreted in terms of the effect of deuterium substitution on the vibrational potential energies corresponding to the symmetric and antisymmetric electronic states. Calculations of the expectation values of the unpaired electron energy over the neutral molecule and the two anion Born−Oppenheimer state vibrational functions show that changes in the average values of the out−of−plane bending coordinates upon deuteration lead to a splitting of the vibrational potential energies. To a first approximation, the differences among the anions are a function of the unpaired orbital bond orders at the deuterated positions in the two electronic states. If the substituted bond order is large and negative, deuterium substitution increases the vibrational potential energy, but if it is approximately zero, the potential energy is essentially unchanged.
Recomputation of certain functions in the Peterlin–Stuart theory of flow birefringence and directions for the evaluation of experimental data in terms of molecular weights and molecular dimensions
Authors: Nakagaki M. , Heller W.
Published: February 2003

Determination of the physical dimensions of nonspherical rigid macromolecules or particles by means of the flow birefringence technique requires reference to theoretical data derived from the Peterlin−Stuart theory. The data universally used for this purpose are those computed nearly 25 years ago by Scheraga, Edsall, and Gadd. Since simplification and approximations mandatory at that time are no longer necessary with modern computer technique, more precise theoretical data were now obtained by recomputation of the pertinent ahk and bhk values. In addition, the considerable number of additional coefficient values that are required for the application of the Peterlin−Stuart theory to the dimensional analysis by means of flow light scattering were computed. The results are followed by a brief survey and discussion of explicit equations useful for the practical application of the flow birefringence technique for dimensional analysis.
Freezing of vibrational degrees of freedom in free-jet flows with application to jets containing CO2
Authors: Sharma P. K. , Young W. S. , Rodgers W. E. , Knuth E. L.
Published: February 2003

The primary objective of this study is to advance the development of a procedure for predicting the terminal vibrational temperatures in free−jet flows of either pure gases or gas mixtures. A secondary objective is to apply this procedure to free−jet flows of (a) pure CO2 and (b) dilute binary mixtures of CO2 in N2 and CO2 in H2. Predictions are based on the sudden−freeze model described by Knuth [Proceedings of the 1972 Heat Transfer and Fluid Mechanics Institute (Stanford U. P., Stanford, California, 1972)]. The pressure and temperature dependence of the vibrational relaxation time, required for application of Knuth’s freezing−point criterion, are deduced from available experimental data on relative probabilities for collisional de−excitation of a given species by impurity molecules. The predicted terminal vibrational temperatures are correlated to excellent approximation by a dimensionless scaling parameter based on source conditions. Measurements were made using the supersonic−molecular−beam technique suggested by Milne et al. [J. Chem. Phys. 56, 3007 (1972)]. This technique takes advantage of the temperature dependence of the fragmentation pattern realized when molecules are ionized by electron impact, which temperature dependence can be established quantitatively using an effusive molecular beam with a variable−temperature source. Good agreement between predicted and measured
A method analyzing the dielectric relaxation spectra of mixtures by decomposition into Debye elementary domains. Application to the study of the water spectrum
Authors: Bottreau André Marie , Moreau Jacques Marie , Laurent Jean Michel , Marzat Claude
Published: February 2003

The authors propose a method of spectral analysis which consists of decomposing the experimental dielectric relaxation spectra of mixtures into a sum of elementary spectra of the Debye type. The technique of this approach of spectral parameters yields a noticeable improvement on the usual methods. After dealing with a great number of theoretical examples, so as to put the computation programs through a test, the study of water was carried out at 20°C. In this case, the experimental spectrum was reconstituted from three Debye constituents.
An ESR analysis of a heat-stable radical in γ-irradiated single crystals of 1-methylthymine
Authors: Schmidt Jack
Published: February 2003

Free radicals formed in γ−irradiated single crystals of 1−methylthymine were studied by ESR spectroscopy. In addition to the known C6−hydrogen adduct radical, a second radical is formed which is very stable for short periods at 200°C, as well as being present in the crystals at room temperature before high temperature annealing. Computer simulation was used as an aid to identifying this radical, which is 1−methyl−5−methyleneuracil−10−y1, formed by net hydrogen abstraction from the methyl group. Certain ESR spectral features suggest molecular packing disorder about an axis constructed along C6 and N3. The principal values for proton hf couplings perpendicular to both the CH bond and the unpaired electron’s p orbital direction, along the C−H bond and along the p orbital direction, respectively, in gauss, are: H1(24.5, 8.1, 15.4); H2(H2−C10∥H−C6)(24.8, 6.9, 15.4); HC6(14.0, 5.2, 10.5); and the g tensor values parallel and perpendicular to the p orbital direction are, respectively, 2.0018 and 2.0027.
’’Exactly soluble’’ two-component lattice solution with upper and lower critical solution temperatures
Authors: Wheeler John C.
Published: February 2003

A decorated lattice model of a two−component liquid solution is presented which has closed−loop coexistence curves with both upper and lower critical solution temperatures analogous to the behavior found in the nicotine + water and m−toluidine + glycerol systems. The model can be transformed exactly into the spin−1/2 Ising model for which exact results are known in two dimensions and reliable estimates are available in three dimensions. The model exhibits nonclassical critical exponents at both upper and lower critical solution temperatures and has coexistence curves in qualitative agreement with those for real systems. The coexistence curves exhibit characteristic features found in most systems with closed−loop coexistence curves.
Vibrational energy transfer from the (00°1) level of 14N2O and 12CO2 to the (m, n
l, 1) levels of these molecules and of their isotopic species
Authors: Doyennette L. , Margottin-Maclou M. , Chakroun A. , Gueguen H. , Henry L.
Published: February 2003

We have studied versus temperature the fast vibvrational energy transfers which occur between two nitrous oxide molecules, or two carbon dioxide molecules, initially excited by a laser pulse into the (00°1) level. These transfers occur upon near−resonant collisions of the type M(m, nl, 1) + M(m′, n′l′, 0) ? M(m, nl, 0) + M(m′, n′l′, 1) with M = 14N2O) or 12CO2. The rates of these V−V transfers have been measured from room temperature up to 900 K by the laser−induced fluorescence method. These exchanges can play an important role in the kinetics of the fast V−V transfers which occur in gaseous mixtures involving these molecules. This is particularly the case for the transfers occurring between different isotopic species of CO2 and N2O, such as 12CO2−13CO2, and mixtures of 14N2O with 14N15NO, 15N14NO, and 15N2O, also studied in this work. The transfer rates for the isotopic mixtures of nitrous oxide have been measured versus temperature. The experimental results are compared with the values calculated on the basis of the long−range multipolar interactions, according to a first−order perturbation theory.
Condensation of a supersaturated vapor. II. The homogeneous nucleation of the n-alkyl benzenes
Authors: Katz Joseph L. , Scoppa Carman J. , Kumar N. Ganesh , Mirabel Philippe
Published: February 2003

The design of an improved version of the upward thermal diffusion cloud chamber is described. Such a chamber was constructed and used to measure the supersaturations required to observe a rate of homogeneous nucleation of 2−3 drops cm−3⋅sec−1. Measurements were made on benzene, toluene, ortho−xylene, and n−butylbenzene over as much as a 150°C range of temperatures. The results obtained are compared to the predictions of the classical theory of homogeneous nucleation (Volmer−Becker−Döring−Zeldovich) and of the Lothe−Pound theory. The classical theory is found to be in excellent agreement with the experimental results.
Theory of the interface between polymers or polymer solutions. I. Two components system
Authors: Roe Ryong-Joon
Published: February 2003

A theory is developed to determine the properties of the interface between two coexisting liquid phases, where one or both of the components comprising the system can be polymeric. The local free energy in the interfacial region of nonuniform composition is expressed in terms of a modified Flory−Huggins free energy of mixing, which was derived earlier and found successful in predicting the extent of adsorption from polymer solutions. The system is defined in terms of the chain lengths of the two components r1 and r2 and the segmental pair interaction parameter χ. When χ − χc is small, where χc is the value of χ for critical phase separation, the interfacial tension γ and the reciprocal of the interface thickness d vary with (χ − χc)α, the value of the positive exponent α being dependent on the chain lengths of the components. Agreement with available, limited experimental data is satisfactory. As an aid for more critical tests against experiment, the values of γ and d for polystyrene−cyclohexane solutions are computed with the use of realistic χ values.
Thermal diffusion in isotopic mixtures of polyatomic gases. II. Model calculations
Authors: Matzen M. Keith , Hoffman D. K.
Published: February 2003

Model calculations are used to study the dependence of the thermal diffusion coefficient on the molecular kinematic parameters for binary, isotopic mixtures of diatomic gases. The effects of a static magnetic field on thermal diffusion are also investigated. The basis of these studies is an analytic expression for DTα previously derived by perturbation techniques.
Spectrum of vanadium in rare gas matrices
Authors: De Vore T. C.
Published: February 2003

The spectrum of V isolated in Ar and Kr matrices at 10°K has been correlated with the gas phase spectrum. The electron configuration was found to be a major factor in determining the direction and the magnitude of the matrix shift for this system. A model calculation was performed using extended Hückel theory. A plausible explanation for the origin of the matrix shifts and the altered intensity relationships observed in the matrix has been developed.
Laser photoelectron spectrometry of the negative ions of silicon and its hydrides
Authors: Kasdan A. , Herbst E. , Lineberger W. C.
Published: February 2003

Fixed−frequency laser photoelectron spectrometry has been utilized to study the ions Si−, SiH−, and SiH−2. In each case, the photodetached electron energy spectrum consists primarily of sharp, easily assignable peaks. The electron affinities of Si, SiH, and SiH2 have been determined to be (1.385±0.005) eV, (1.277±0.009) eV, and (1.124±0.020) eV, respectively. Two bound excited states attributed to 2D and 2P terms have been observed for Si−, and their binding energies determined. Similarly, two bound excited states, assigned to 1Δ and 1Σ+ symmetry, have been discovered in SiH−. The internuclear distance re in the ground state of SiH− has been determined by a Franck−Condon factor analysis to be (1.474±0.004) Å. In addition, spectroscopic constants of the excited states have been deduced. Transitions from the ground state of SiH− to both the 1A1 and 3B1 states of SiH2 have been observed; unlike the case of CH2, the 1A1 state is the lower in energy.
Calculation of ionization potentials from density matrices and natural functions, and the long-range behavior of natural orbitals and electron density
Authors: Morrell Marilyn M. , Parr Robert G. , Levy Mel
Published: February 2003

Given an exact eigenfunction ψ for some system with N electrons, a procedure is developed for determining ionization potentials of the system to various states of the corresponding system having N−p electrons. The long−range behavior of the electron density and of the natural spin orbitals is shown to involve a set of eigenvalues which are obtained by this procedure. Following is the procedure. First determine the pth−order natural functions for ψ, Xm(p), and their occupation numbers nm(p), by diagonalization of the pth−order density matrix Γ(p). Calculate the quantities where ? ≡ (x1x2⋅⋅⋅xp) and ?ξ ≡ (ξ1ξ2⋅⋅⋅ξp). Then diagonalize the Hermitian matrix μ(p)kl = λ(p)kl / (n(p)kn(p)l)1/2. The resultant eigenvalues μα(p) are approximations to the negative of the p−electron ionization potentials of the system, with successive μα(p) (from the highest to the lowest) being lower bounds to the negative successive p−electron ionization potentials (from the lowest to the highest). For an approximate eigenfunction ?, the same procedure is recommended, except that the Hermitian part of the approximate matrix ?(p)kl is diagonalized. For ? a Hartree−Fock approximation to Ψ, the procedure for the case p = 1 is the classic method of Koopmans. It is shown that in general all natural spin orbitals χm(x) in a system have long−range expontential fall−off governed by μ(1)max, i.e., χm(x) ∼ exp[−(−2μmax(1))1/2r], and that the electron density behaves similarly, π (x) ∼ exp[−2(−2μmax(1))1/2r]. The behavior naively expected, namely π (x) ∼ exp[−2(2Imin)1/2r], results if μmax(1) is equal to −Imin.
Nitroxide radical pairs in the plastic crystal camphene
Authors: Glarum S. H. , Marshall J. H.
Published: February 2003

Electron spin resonance is employed to study pairs of nitroxide radicals in the plastic phase of dl−camphene. Both Δm = 1 and Δm = 2 transitions are observed, and an adiabatic theory is presented for the dependence of the absorption intensity, second−order shift, and linewidth of the latter transitions upon radical pair distributions and motions. Results are interpreted by a model in which radicals are nearly randomly distributed over lattice sites, lattice vacancies are thermally generated, and radical diffusion occurs by a vacancy mechanism. The enthalpies for vacancy formation and radical diffusion are, respectively, 4.6 and 11 kcal/mole, while at the melting point D ? 10−7 cm2/sec.
Effect of an external electric field on the yield of free ions. IV. Epithermal electron scavenging in neopentane
Authors: Mozumder A. , Tachiya M.
Published: February 2003

It has been rigorously shown that, in the Onsager model, the probability of a pair of ions escaping geminate neutralization is independent of charge scavenging irrespective of the nature of the escaping ions provided only that scavenging follows thermalization. Thus the experimental evidence that a small concentration of SF6 drastically reduces the free−ion yield in neopentane in the presence of an external field (Schmidt) is taken to demonstrate epithermal electron scavenging in neopentane. Corroborative evidence is found in substantial reduction of free−ion yield in neopentane with relatively small concentrations of CCl4 and CS2 in the absence of an external field (Schmidt and Allen). On the basis of a theoretical treatment presented in this paper, the cross sections of epithermal electron attachment in neopentane are found to be 4×10−16, 5×10−17, and 1×10−18 cm2, respectively, for SF6, CCl4, and CS2. These values are obtained on analyzing the relevant experiments on free−ion yields according to the present theory. Comparison with gas−phase cross sections is attempted.
Theory of inhomogeneous polymers: Fundamentals of the Gaussian random-walk model
Authors: Helfand Eugene
Published: February 2003

In earlier work a theory of inhomogeneous polymers was developed from a mean field theory point of view. Applications were made to polymer−polymer interfaces and to the microdomain structure of block copolymers. Here a connection is made between the intuitive mean field arguments for these problems and more fundamental statistical mechanics. Functional integral techniques are employed extensively.
Infrared spectrum of matrix-isolated nitric acid
Authors: Guillory William A. , Bernstein Mark L.
Published: February 2003

The infrared spectra of four isotopic species of matrix−isolated nitric acid have been obtained. The vibrational assignments are in general agreement with previous gas and condensed phase observations with the exception of those assigned to the NO2 valence bend and NO′ single bond stretch. The results of diffusion experiments performed indicate that nitric acid may form cyclic dimers similar to the carboxylic acids.
Decay of an emitting dipole between two parallel mirrors
Authors: Chance R. R. , Prock A. , Silbey R.
Published: February 2003

A classical treatment is presented for the modification of the emission rate of an atom between two parallel mirrors. For the case of two perfectly reflecting mirrors, the results are identical with previous treatments of this problem using a quantum mechanical approach. The purely classical treatment has the advantage of being able to treat nonperfectly reflecting mirrors.
Adsorption of 4He on graphitized carbon in the submonolayer region between 2 and 15°K
Authors: Antoniou A. A.
Published: February 2003

The adsorption of 4He on graphitized carbon (Spheron 6) was studied at submonolayer coverages between 2 and 15°K. Heat capacities, heats of adsorption (measured calorimetrically), and equilibrium pressures are reported. The data indicate that adsorbed helium remains localized up to at least 15°K. A phase transition is observed at temperatures above 9°K for the 22.26 cm3 STP/g adsorbent coverage. Frequencies of three−dimensional asymmetric localized harmonic oscillators which can reproduce the experimental results are given. Excited levels are heavily populated at the temperatures of measurement. The zero point energy of adsorbed helium is approximately 200 J/mole. Another change in the state of adsorbed helium takes place below 7°K to the lowest temperature attained, 2°K. The rate of this change in the state of adsorbed helium follows first order kinetics. It is suggested that this process involves the ’’falling’’ of the adatoms from higher to lower levels as the temperature is lowered. The residual entropies are calculated for two coverages. The residual entropy is attributed to the distribution of helium atoms over the localized surface sites.
Molecular dynamics of octane in a gas liquid solution
Authors: Assink Roger A.
Published: February 2003

A gas/liquid solution was formed by applying up to 70 (MPa) of argon to n−octane. The molecular dynamics of octane in the solution were measured by NMR relaxation techniques. A relatively insoluble gas, helium, was also applied so that the effect of pressure could be contrasted with the effect of solution. The transport properties of octane compressed by argon possessed two distinguishing features. First, the behavior of its diffusion constant indicated that the viscosity of the fluid decreased with pressure as more argon dissolved into the liquid. Second, the pressure depencence of octane’s over−all rotational correlation time was much less than the estimated pressure dependence of the solution’s viscosity. This latter feature could be explained by a microviscosity theory which takes into account the effect of solvent size on the coupling between molecular rotation and viscosity.
Shock tube study of the thermal decomposition of O3 from 1000 to 3000°K
Authors: Center R. E. , Kung R. T. V.
Published: February 2003

Measurements have been made of the thermal decomposition of O3 behind incident shock waves. Both uv absorption and Schumann−Runge emission measurements were made in separate experiments to follow the O3 and O−atom concentrations, respectively. The high temperature data are interpreted to indicate a departure from the Arrhenius form for the rate constant of the unimolecular reaction O3 + M ? O + O2 + M. The rate constant for this reaction is measured to be approximately a factor of 2 smaller at 3000°K than predicted by the Arrhenius fit to the low temperature data (200°K ? T ⩽ 900°K).
Ultrasound propagation in NH3–H2O mixtures
Authors: Bowen D. E. , Priesand M. A. , Feighny S. R.
Published: February 2003

Data on the sound velocity and absorption in liquid mixtures of NH3 and H2O are presented. The sound velocity at 10 MHz was measured as a function of concentration and temperature. The absorption was measured as a function of frequency (between 10 and 85 MHz), concentration, and temperature. The results indicate that the NH3−H2O system behaves ultrasonically like mixtures of associated liquids with a relaxation due to the electrolytic nature of NH3.
Some observations on the continuum emission from NO2
Authors: Butler Shahla , Kahler Carol , Levy Donald H.
Published: February 2003

The fluorescence spectrum of NO2 produced by laser excitation in the visible region consists of a discrete banded structure superimposed on what appears to be a continuum. Previous experiments have implied that the intensity of continuum emission increases relative to that of the discrete emission as the pressure is raised, and that therefore the continuum emission is produced by some collision mechanism. In this paper we describe some magnetic quenching experiments which indicate that the continuum/discrete ratio is pressure independent. We also describe experiments which indicate that the continuum emission lifetime is longer than the discrete emission lifetime, and this allows a reinterpretation of previous observations which is not in conflict with the conclusion based on our magnetic quenching data. These observations suggest that either the continuum emission originates in the isolated molecule, or else, that the cross section for any bimolecular process is extremely large.
Total x-ray scattering by H2
Authors: Bentley John J. , Stewart Robert F.
Published: February 2003

From the Waller−Hartreee theory, the total x−ray scattering of H2 has been calculated with the first 10 natural orbitals of Davidson and Jones. Vibrational averaging has not been included. The calculated intensities differ by as much as 5% from a similar calculation with the Davidson and Jones SCF orbital. The accuracy of the calculations is demonstrated by agreement of 〈r12−1〉 from Tavard’s sum rule with 〈r12−1〉 values given by Davidson and Jones. The coherent intensities from the first 10 natural orbitals are in close agreement with the SCF results. By correcting the calculated inelastic scattered intensities for relativistic effects, the total scattering intensities from the correlated wavefunction are in excellent agreement with the experimental values of Wollan. Corresponding SCF values are significantly different. The results for molecular hydrogen encourage us to suggest more accurate gas phase measurements of total x−ray scattering by other diatomic molecules in order to determine the two electron expectation 〈r12−1〉.
Spin–lattice relaxation in cross relaxed systems: Anisole
Authors: Cutnell J. D. , Roeder S. B. W. , Tignor S. L. , Smith R. S.
Published: February 2003

Spin−lattice relaxation has been monitored as a function of temperature in liquid anisole and its partially deuterated analogues C6D5OCH3 and C6H5OCD3, as pure liquids and in solution in C6D5OCD3. Conventional pulse and Fourier transform techniques have been used. The high resolution spectrum of C6H5OCH3 was found to contain three groups of lines, one associated with the methyl protons, one associated with the meta phenyl protons, and one associated with the ortho and para phenyl protons. The two groups of aromatic lines were found to have, within experimental error, identical spin−lattice relaxation times and activation energies. The contribution of intermolecular interactions to the relaxation mechanism was found to decrease with increasing temperature and was ? 36% for T ? 312°K. A method is discussed for determining the cross relaxation rate between the differently relaxing methyl and phenyl spins. This method utilizes average T1 values for C6H5OCH3, C6D5OCH3, and C6H5OCD3 and yields for anisole a positive cross relaxation rate as a function of temperature. The cross relaxation is found to proceed primarily by intramolecular interactions which cause methyl and phenyl spins to flip in the same sense. Correlation times for over−all molecular tumbling are calculated and compared to results from dielectric relaxation experiments. The comparison indicates that the over−all molecular tumbling may be near to the limit of large diffusive steps discussed by Ivanov [Sov. Phys.−JETP 18, 1041 (1964)]. A correlation time for the threefold methyl reorientation is also calculated. Activation energies were found for the species C6H5OCH3, C6D5OCH3, and C6H5OCD3 to be 2.77±0.08, 2.78±0.23, and 3.32±0.26 kcal/mole, respectively.
Group theoretic prediction of configuration mixing effects due to Coulomb repulsions in atoms with applications to doubly-excited spectra
Authors: Sinanoğlu Oktay , Herrick David R.
Published: February 2003

A group theoretical method which predicts Coulomb repulsion mixing coefficients of doubly−excited atomic states is presented. Good agreement with calculated mixings in helium is found. Two new quantum numbers and three rules classify and predict the energy level orderings. The exact mathematical construction of 1/r12 in the group theoretical configuration−mixed basis is given. It leads naturally to a formulation of the corresponding many−electron problem of configurational mixings.
A quantum kinetic theory of moderately dense gases. I. Wigner triplet distribution function
Authors: Olmsted R. D. , Curtiss C. F.
Published: February 2003

The quantum mechanical BBGKY (Bogoliubov−Born−Green−Kirkwood−Yvon) hierarchy is truncated and solution of the third BBGKY equation effected, within the ternary collision approximation, to yield an approximate Wigner triplet distribution function. This triplet distribution function is obtained as a functional of the pair and singlet distribution functions by the imposition of a generalization of the molecular chaos hypothesis. The potential is assumed to be pair−wise additive, but the discussion is not restricted to purely repulsive interactions. The resultant triplet distribution function may be used in the development of a quantum mechanical kinetic theory of moderately dense gases.
Neutron scattering from Fermi-resonant vibrational modes in carbon dioxide
Authors: Berney C. V.
Published: February 2003

Calculations have been carried out to account for the relative intensities of the Fermi−resonant modes in the observed neutron−scattering spectrum of carbon dioxide. Qualitative agreement is achieved if the scattering is assumed to be predominantly incoherent.
The alkaline earth intercalates of molybdenum disulfide
Authors: Somoano R. B. , Hadek V. , Rembaum A. , Samson S. , Woollam J. A.
Published: February 2003

Molybdenum disulfide has been intercalated with calcium and strontium by means of the liquid ammonia technique. Chemical, x−ray, and superconductivity data are presented. The x−ray data reveal a lowering of crystal symmetry and increase of complexity of the structure upon intercalation with the alkaline earth metals. The Ca and Sr intercalates start to superconduct at ∼4 and 5.6°K, respectively, and show considerable anisotropy regarding the critical magnetic field.
Theory of resonance Raman scattering. An improved formulation of the vibronic expansion method
Authors: Mingardi M. , Siebrand W.
Published: February 2003

The vibronic expansion theory of Raman intensities in polyatomic molecules is reconsidered so as to improve its accuracy and extend its validity into the resonance region. Several fundamental improvements are introduced: (i) Terms which couple adiabatic Born−Oppenheimer states are shown to be potentially important sources of Raman intensity even when the electronic level spacing is large. (ii) The conventional asymptotic expansion of the energy denominators is replaced by an expansion which remains convergent in the resonance region and allows direct summation over the intermediate vibrational states. (iii) The vibrational linewidth is shown to be an important parameter in the theory and is fully taken into account by treating the energy denominator consistently as a complex quantity. The over−all treatment results in analytic formulas expressed in terms of spectroscopic parameters. A preliminary comparison with general spectroscopic properties is presented. Far from resonance the new results are shown to reduce to the conventional ones if the appropriate approximations are introduced. A special effort is made to present the results in such a way that they clarify the relation between resonance fluorescence, ordinary Raman, and resonance Raman scattering.
Quantum states of intramolecular nuclear motion with large amplitudes: Pseudorotation of trigonal bipyramidal molecules
Authors: Russegger Peter , Brickmann Jürgen
Published: February 2003

A method for the variational calculation of multidimensional eigenstates of a Hamiltonian for the nonharmonic bounded nuclear motion in a molecule is presented. For the case where the potential energy along an internal reference coordinate ξ is strongly nonharmonic, while perpendicular to this coordinate only harmonic terms with ξ−dependent force constants and displacements appear, the secular matrix elements can be calculated from general selection rules with respect to vibrational quantum numbers and one−dimensional integrals. The diagonalization was performed by an iterative procedure. The formalism was applied to the Berry pseudorotation of the trigonal bipyramidal CH3PF4 molecule where the energy surface was obtained from specially adapted semiempirical calculations. The rate of the rearrangement process was calculated as a function of temperature using a simplified model for the time dependence of the system. Nonadiabatic effects are included.
Carbon tetrafluoride—phase II: Far-infrared spectrum and crystal structure
Authors: Sataty Y. A. , Ron A. , Herbstein F. H.
Published: February 2003

The far−infrared spectrum of crystalline CF4 shows a discontinuity at the transition between Phases I and II; the spectrum of the low−temperature Phase II shows three lattice modes. Published x−ray diffraction data for Phase II have been reinterpreted in terms of a unit cell with space group C2/c, 4 molecules per cell, to give a crystal structure consistent with the observed spectrum.
X-ray spectra and electronic band structures of PdCl2 and CdCl2
Authors: Sugiura Chikara
Published: February 2003

The Pd Lβ2,15 emission, Pd LIII absorption, Cd Lβ2,15 emission, Cd LIII absorption, Cl Kβ emission, and Cl K absorption spectra of PdCl2 and CdCl2 have been obtained with a 50−cm bent−quartz−crystal vacuum spectrograph. The electronic band structures are determined empirically from the emission and absorption spectra. The valence band of PdCl2 consists of the overlap of the 4d band of palladium and the 3p band of chlorine; the upper part arises primarily from the d band and the lower part predominantly from the p band. In CdCl2 the valence band consists of the 3p band of chlorine and the bottom of the conduction band arises from the empty 5s state of the Cd++ ion. A tentative interpretation for the Pd LIII, Cd LIII, and Cl K absorption spectra is made in terms of the core−level−to−band transition. It is indicated that the chemical bonds of these metal chlorides are largely covalent.
A new computational approach to Slater’s SCF–Xα equation
Authors: Sambe Hideo , Felton Ronald H.
Published: February 2003

A new computational scheme is presented for the performance of LCAO−MO calculations in the SCF−Xα model. The scheme is intended to be applicable for large systems and to be more accurate than the scattered−wave SCF−Xα method. The Xα potential is fitted by least−squares to a linear combination of Gaussians, and the approximated SCF−Xα equation is solved by the conventional Rayleigh−Ritz variational method. The muffin−tin approximation is avoided, and matrix elements are calculated analytically in contrast to the discrete variational scheme. Some illustrative results are given for the ionization energies and equilibrium geometries of small molecules. It is found that over−all performance of the method is satisfactory for both ionization energies and equilibrium geometries.
Infrared and Raman spectroscopy of carbohydrates. Paper V. Normal coordinate analysis of cellulose I
Authors: Cael J. J. , Gardner K. H. , Koenig J. L. , Blackwell J.
Published: February 2003

A normal coordinate analysis of cellulose I has been performed for an isolated chain using a valence force field and atomic coordinates from an x−ray refinement analysis. The calculated frequencies show good agreement with the observed infrared and Raman data, while the computed potential energy distribution is compatible with previous assignments, based on deuteration etc. The atomic displacements show that all the predicted modes in the region below 1500 cm−1 result from complex motions of a number of atoms or groups, rather than from single group vibrations. Most of the modes above 1200 cm−1 are localized within the residue and are very modes above 1200 cm−1 are localized within the residue and are very modes above 1200 cm−1 are localized within the residue and are very similar to those predicted for the monomer, β−D−glucose. Below 1200 cm−1 inter−residue coupling becomes more appreciable. Negligible splitting is predicted between equivalent A and B modes which suggests that the observed spectral lines are due to superimposed A and B components.
A self-consistent theory of rotational diffusion
Authors: Berne Bruce J.
Published: February 2003

The effect of long range Coulomb and dipolar forces on the collective tumbling of rigid linear dipoles is considered. It is shown that the Debye relaxation times are modified by the presence of these long range forces in important ways. Explicit expressions for these relaxation times are derived for polar and electrolyte solutions. It is shwon that for dense solutions there are significant deviations from the Debye semicircle in the Cole−Cole plot.
Hydroxyl formation accompanying defect center production in proton and deuteron bombarded aluminum oxide
Authors: Gruen D. M. , Wright R. B. , McBeth R. L. , Sheft I.
Published: February 2003

The defect center produced in Al2O3 by energetic neutron or electron bombardment which gives rise to a 205 nm (6.03 eV) absorption band appears with two to three orders of magnitude higher intensity in proton and deuteron bombarded samples compared to either neutron, electron or heavier ion bombardments. The infrared spectra of this system is used to show that the formation of OD or OH provides an additional mechanism of defect production. (AIP)
Classification of instabilities in chemical reaction systems
Authors: Tyson John J.
Published: February 2003

All destabilizing processes in chemical reaction systems may be classified as (1) direct autocatalysis, (2) indirect autocatalysis (positive feedback), or (3) end−product inhibition (negative feedback). This classification scheme gives some perspective to models of sustained oscillations and multiple steady states which abound in the literature.
Ion–molecule reactions in the systems CF4–CH4 and CF4–C2H6
Authors: Heckel Edgar , Hanrahan R. J.
Published: February 2003

A mass spectrometric study of ionic reactions in CH4−CF4 and C2H6−CF4 mixtures was undertaken at pressures as high as 0.52 torr. It was found that CF3+ ions react in a hydride transfer process with ethane with a rate constant of 3.3×10−10 cm3 molecule−1⋅s−1. A rate constant of 1.6×10−10 cm3 molecule−1⋅s−1 was determined for the reaction of C2H5+ ions with C2H6. In mixtures of CH4 and CF4, both CH3+ and CH5+ react with CF4, apparently by fluoride ion transfer, for which we calculated rate constants of 8.6×10−10 and 2.3×10−10 cm3 molecule−1⋅s−1, respectively. CF3+ ions do not attack molecular methane. There is good agreement with published rate constants for the disappearance of CH2+, CH3+, and CH4+ ions in pure methane. However we observed a decreasing CH5+ yield above 0.1 torr which may be due either to impurities or to excess energy in CH5+ ions as a result of a relatively high repeller field strength within the ion source in our experiments.
Method of averaging wavefunction corrections in scattering theory
Authors: Malik David J. , Weare John H.
Published: February 2003

A convergent perturbative method is presented for strong interaction scattering problems. The procedure resembles the method of averaging functional corrections and can be related to a distorted wave perturbation scheme. The method relies on the introduction of a finite linear basis set to approximately describe the wavefunction. This leads to a Lippmann−Schwinger type integral equation with a reduced interaction strength. An iterative solution is obtained similar to the Born series. Improvement of the basis set improves the reliability of the low order terms in the series. The convergence of the series is discussed, and convergence criteria given. The method can also be used to determine the singularities of the T matrix. The wavefunctions associated with these singularities may be used to remove the divergent behavior of the iterative solution to the integral equation. To illustrate the convergence behavior we apply the method to an integral equation with a seperable kernel. The method is applied to several simple examples, and the results are compared to the usual Born series. Significant improvements in accuracy are noted over the Born series as well as an extension of the region of convergence. Comparison with other similar procedures is discussed.
The synthesis and transport properties of the complex salt (TMPD) (TCNQ)2
Authors: Somoano R. , Hadek V. , Yen S. P. S. , Rembaum A. , Deck R.
Published: February 2003

The syntheses and transport properties of the complex salt (TMPD) (TCNQ)2 are described. At high temperatures (T≳100°K), the complex is a magnetic semiconductor with transport properties intermediate between those found in the highly conducting and poorly conducting TCNQ salts. The complex undergoes a transition below 50−60°K to a state exhibiting singlet−triplet behavior with weakly alternating exchange coupling.
Photoionization measurement of the heat of formation of allyl cations
Authors: Buttrill S. E. , Williamson A. D. , LeBreton P.
Published: February 2003

The photoionization thresholds measured for the formation of C3H5+ from propylene and cyclopropane are 11.78 and 11.47 eV, respectively. Our best value of the heat of formation of allyl cation is 224.5±1.0 kcal/mole. Lossing’s [Can. J. Chem. 49, 357 (1971)] value of 8.07 eV for the ionization energy of allyl radical combined with our results gives 38.4±1.7 kcal/mole for the heat of formation of allyl radical.
Resonance studies on x-irradiated L-histidine HNO3
Authors: Rustgi Surendra N. , Box Harold C.
Published: February 2003

The paramagnetic species NO32− and NO2 are detected using ESR in single crystals of L−histidine HNO3 x irradiated at a temperature of 4.2°K. (AIP)
Transferred hyperfine magnetic fields at nonmagnetic solutes in iron and solute–solvent overlap integrals
Authors: Sondhi I.
Published: February 2003

The 3d°−ns overlap integrals are computed for the atom pairs FeIn, FeSn, FeSb, FeTe, FeI, and FeXe at different internuclear distances. The results obtained for the overlap integrals are used to compute the contributions to the hyperfine magnetic field at the 4d10 5s2 5pk solutes in iron due to overlap effects. According to a theory proposed by Shirley these contributions would explain the large positive fields observed at Te, I, and Xe in iron. However our results show that the overlap contributions are much smaller than Shirley’s preliminary estimates.
Vibration–rotation relaxation in He–H2
Authors: Rabitz Herschel , Zarur George
Published: February 2003

Vibration−rotation relaxation calculations for He4−(para−H2) are presented over the range 200 ? T ⩽ 700°K. The relaxation equations are solved for up to 12 vibration−rotation states. The rate constants were obtained from previously calculated quantum mechanical collision cross sections. It is shown that the eigenvalues and eigenvectors of the relaxation matrix may be physically interpreted in terms of different relaxation modes. Details of the relaxation are followed as a function of time. A rich variety of rotational relaxation behavior is shown to be possible at short times followed by a quiescent induction period and finally vibrational relaxation. Breathing sphere calculations were also performed, and they were found to be inadequate for describing the vibrational relaxation. The rotational states significantly participate in the vibrational relaxation of He−H2.
Abstraction versus exchange in the reaction of H with DBr
Authors: Su H. Y. , White John M. , Raff L. M. , Thompson Donald L.
Published: February 2003

The photolysis of DBr−H2−Br2 mixtures at 185 nm furnishes data which can be analyzed to provide the abstraction to exchange rate coefficient ratio k8/k9 for reactive collisions of H with DBr where H is a translationally hot atom with an average energy of about 1 eV. The results of such experiments are reported here and are analyzed using (1) an assumed mechanism, (2) available independent experimental data, (3) some quasiclassical trajectory data, and (4) an isotope effect model based on collision frequencies to give 0.2 ? k8/k9 ? 0.8. In addition quasiclassical trajectory calculations have been used to generate model data for direct comparison with the experimental data. These calculations, using potential energy surfaces which have been very successful in accounting for a variety of other experimental data, give k8/k9 = 0.24, as well as values for the other rate coefficients in the mechanism. With a single adjustable parameter used to fit the intercept in one experiment, these trajectory calculations are in quite satisfactory agreement with the experimental data provided D−atom exchange with DBr is incorporated as a significant mechanism for thermalization of hot D atoms. It is also shown that the photolysis data are consistent with trajectory calculations that give an abstraction to exchange ratio for thermal H atoms that lies in the range 0.2−1.0.
Magnetic susceptibility of BkO2 in a thorium dioxide matrix
Authors: Karraker D. G.
Published: February 2003

Measurements of the magnetic susceptibility of BkO2 in a ThO2 matrix from 2.2 to 220°K showed two regions of temperature−dependent paramagnetism: from 10 to 95°K, μeff = 7.66 μB, ϑ = 12°K; from 95 to ∼220°K, μeff = 5.8 μB, ϑ = −34°K. Below 10°K, the temperature−dependent behavior was anomalous, caused by a possible antiferromagnetic transition for BkO2 at 3°K. By fitting the data to a two−level theoretical expression, g = 5.04 for the Γ6 ground level, about 10% higher than the accurate EPR value of 4.488. A Γ6−Γ8 energy difference of 80 cm−1 was calculated, and the total splitting of the Bk4+ 8S7/2 ground state in ThO2 was estimated to be about 300 cm−1.
A semiclassical, nonperturbative approach to the description of molecular collisions
Authors: Mehrotra Suresh C. , Boggs James E.
Published: February 2003

Using the effective potential formulated by Rabitz [J. Chem. Phys. 57, 1718 (1972)], rotational energy transition probabilities for OCS molecules colliding with other OCS molecules are calculated by solving coupled Schrödinger equations under the classical straight line path approximation. These results are compared with similar transition probabilities calculated from second−order perturbation theory. The results are quite different for strong collisions. For weak collisions, the discrepancies are less for low values of J. On the basis of these results, qualitative explanations of rotational energy transfer for weak and strong collisions are offered.
Charge transfer cross sections for metastable N+(1D) in collision with krypton and carbon monoxide
Authors: Rutherford J. A. , Vroom D. A.
Published: February 2003

Cross sections for the exothermic charge transfer processes involving ground and metastable N+ ions and Kr and CO have been obtained in the energy range from 1−500 eV. For both target species, the cross section for the metastable ion is larger than for the ground state in the low ion energy region. This difference is explained by the availability of excited product state which can carry away the exothermicity of the reaction. The method used to dfetermine the fraction of metastable N+ ions present in the primary ion beam is discussed.
Electronic properties of metal clusters
Authors: Baetzold R. C. , Mack R. E.
Published: February 2003

Extended Hückel calculations are reported for clusters of Pd, Ag, Cu, Au, Pd−Ag, and Ni−Cu containing up to 55 atoms. Electronic properties important in catalysis are calculated and include the width and position of conduction−band states. Silver particles have a conduction band that shows increasing spread with increasing size. It is 5 eV wide at 55 atoms and the d states are 5 eV below the Fermi level. Palladium clusters have a high density of d states at the Fermi level, which is −7.9 eV for Pd19. Fewer d holes are present on surface than interior Pd atoms in Pd19. Clusters of Cu/Ni and Ag/Pd show d holes on the Group VIII metal atom for compositions up to and greater than 60% Ib metal, in contrast to the rigid band model prediction.
Valence electron studies with Gaussian-based model potentials and Gaussian basis functions. IV. Application to molecular systems containing first row atoms
Authors: Switalski Jurgen D. , Schwartz Maurice E.
Published: February 2003

The previously developed simple valence−only electronic structure theory based on atomic core model potentials is extended to many−electron systems containing first row atoms. For a given atom, two additional parameters have been incorporated into the theory to deal with the electron−electron repulsion in the valence shells. Any molecular studies using the atom are then well defined without further calibration. The molecules considered in this study are CH4, NH3, H2O, HF, C2H4, C2H2, N2H2, N2, H2O2, CO, N2O, H2CO, HCONH2, CH3F, LiCN, and LiNC. The valence basis sets in the LCAO SCF MO studies were of ’’double−zeta’’ quality. Calculated orbital energies and dipole moments for the molecules compare favorably with those from analogous all−electron calculations of Synder and Basch. Some molecular conformational calculations have also been done: the relative stability of LiNC is correctly predicted compared to LiCN; the bond angle of H2O is predicted at 105; the bond lengths of HF and N2 predicted at 1.65 and 2.44 a.u., respectively; and the barriers to internal rotation in H2O2 found to be high compared to experiment.