bluuebunny/crossref_metadata_2025 huggingface dataset:
Dielectric Polarization and Alignment and the Structure of Polar Fluids
Authors: Ramshaw J. D. , Schaefer D. W. , Waugh J. S. , Deutch J. M.
Published: February 2004

An analysis is made of the information about the structure of dense polar fluids which resides in the dielectric constant, the Kerr constant, and the nuclear magnetic resonance (NMR) quadratic electric field effect. The inadequacy of the “local-field” model for liquids is discussed. The existence of a nonzero molecular hyperpolarizability is shown to destroy an equivalence which would otherwise exist between the Kerr and NMR experiments, and can easily account for apparent discrepancies between the reported Kerr and NMR data for nitrobenzene and nitromethane. A method is presented for removing dielectric boundary effects from statistical averages, so that the averages can be computed locally.
Infrared Spectra of the Argon Matrix-Isolated Cyanamide, Cyanamide-d2, and Carbodiimide
Authors: King S. T. , Strope J. H.
Published: February 2004

The infrared spectra of cyanamide and cyanamide-d2 isolated in solid argon at 20°K were studied. The NH2 and ND2 wagging frequencies were observed at 414 and 310 cm−1, respectively, and were interpreted in terms of “inversion” transitions. Cyanamide and cyanamide-d2 were also pyrolyzed in a hot nozzle under very low vapor pressure, and the carbodiimide produced from the pyrolysis was positively identified from the resulting infrared spectra.
Analysis of Viscous Relaxation in Critical Oxide Mixtures
Authors: Simmons Joseph H. , Macedo Pedro B.
Published: February 2004

An analysis of shear viscosity and ultrasonic measurements on a series of critical oxide mixtures shows that the supercritical, excess static viscosity is associated with a broadening in the distribution of structural relaxation times, rather than with an interaction between the long-wavelength components of the fluctuations and the shear flow processes. The broadening saturates near the critical point, and the excess static viscosity approaches a finite limit as a function of reduced temperature. These results are in disagreement with existing excess-viscosity theories. A new model is proposed which relates the distribution of relaxation times to microstructure in the melt resulting from the supercritical fluctuations in composition. It also limits the range of interaction associated with structural relaxation to a finite value. The resulting conclusions are consistent with the observed lack of divergence in the excess viscosity and width of the distribution of relaxation times at Tc.
Relative Rates and Their Dependence on Kinetic Energy for Ion–Molecule Reactions in Ammonia
Authors: Huntress Wesley T. , Mosesman Michael M. , Elleman Daniel D.
Published: February 2004

Ion cyclotron resonance techniques are used to measure the relative rates and their dependence on kinetic energy for the major ion–molecule reactions in ammonia. Charge transfer is shown to compete with proton transfer in the reaction of both NH2+ and NH3+ with ammonia over an energy range from thermal velocities to 50 eV: NH4++NH2k1↗NH3++NH3k4↘NH3++NH3, NH4++NHk2↗NH2++NH3k3↘NH3++NH2. The rate for charge transfer increases with increasing kinetic energy while the rate for proton transfer decreases with kinetic energy. At thermal kinetic energies, k2 / k1 = 0.6 and k3 / k2 = 1.0. Resonant charge transfer from NH3+ was observed only for translationally excited ions.
ESR of 17O-Labeled CO2− Radical Trapped in a Single Crystal of Sodium Formate
Authors: Schlick S. , Silver Brain L. , Luz Z.
Published: February 2004

The ESR spectrum of x-irradiated sodium formate enriched in 17O is interpreted in terms of the CO2− radical. Two magnetically inequivalent oxygens were observed. The hyperfine tensors of 13C and 17O were determined and the following principal values were obtained for 13C: 155, 150, and 195 G, respectively, along the axis perpendicular to the molecular plane, along the O–O direction, and along the C2 axis. These results agree with those previously reported. For 17O the magnitudes of the principal values for both oxygens are practically identical, the averages being 22.0, 23.2, and 51.0 G. The direction of the largest value is in the molecular plane inclined at 13° to the C2 axis. The results are used to derive the coefficients of the 4a1 molecular orbital containing the unpaired electron.
On the Critical Region of a Simple Fluid. I. Two Elementary Index Inequalities
Authors: Stephenson John
Published: February 2004

The continuity of the slopes of isochores (∂P / ∂T)V of a simple fluid at its critical point is shown to imply, and be implied by, the strict index inequality β + γ− > 1, which is not a consequence of known index inequalities based on thermodynamic stability. Under certain assumptions relating to the index inequality α− + β < 1, it is shown that CP diverges with index γ− on the phase boundary, and that the saturated specific heat diverges with index (1 − β)
Perturbation Theory in Terms of Reduced Density Matrices
Authors: Harriman John E.
Published: February 2004

A simple perturbation theory has been developed for reduced density matrices which are known to be N representable as functions of some set of independent parameters. Formulas are presented for single and double perturbations through second order. Extension to higher order would be possible, but involved. The results do not depend explicitly on the order of the density matrix, and when the N-particle density matrix is used, the formulas can be reduced to those of conventional Rayleigh–Schrödinger theory. Details are given for the special cases of spin-unrestricted or extended SCF density matrices and CI or optimized multiconfiguration density matrices which are known to be N representable.
Isothermal Compressibility of Deuterium Oxide at Various Temperatures
Authors: Millero Frank J. , Lepple Fred K.
Published: February 2004

The isothermal compressibilities of deuterium oxide solutions have been measured from 5–65°K. The measurements were made by a piezometric technique from 1–34 bar and extrapolated to 1 atm. The results are briefly discussed and compared to the values for normal water.
Study of the Interaction of Ammonia with Tungsten Surfaces by Thermal Desorption Spectrometry
Authors: Peng Y. K. , Dawson P. T.
Published: February 2004

The interaction of ammonia gas, at pressures between 10−7 and 10−2 torr, with a polycrystalline tungsten filament, at temperatures between 200 and 700°K, has been investigated by thermal desorption mass spectrometry. Several procedures have been adopted to overcome the problems caused by the persistence of ammonia gas in ultrahigh vacuum systems. The adsorbed phase obtained by interaction at 200°K produces a desorption spectrum with a single low-temperature hydrogen peak (peak maximum 450°K) and a single high-temperature β-nitrogen peak (peak maximum 1450°K) in agreement with earlier field emission observations. Increasing the adsorption temperature in the range 200–700°K causes the single hydrogen desorption peak to shift to higher temperatures and the nitrogen desorption to increase and shift to lower temperatures, eventually forming two well-resolved desorption peaks. The nitrogen desorption features resemble those obtained by adsorption of nitric oxide (ω), electron bombardment of γ nitrogen (λ), and by repeated ammonia adsorption at 300°K with intervening flashing to 800°K (δ). In these experiments, with the reaction vessel cooled to 77°K, little hydrogen desorption accompanies the nitrogen desorption. However, experiments carried out with the reaction vessel at room temperature (or coated with an ammonia layer at 77°K) show that for adsorption at the higher temperatures the low-temperature nitrogen desorption peak is accompanied by the simultaneous desorption of hydrogen. These clearly resolved desorption features are designated η nitrogen and η hydrogen; desorption occurs by first-order kinetics with peak maxima at 970 and 985°K, respectively. Characteristic behavior of the η-hydrogen peak indicates that it desorbs as hydrogen atoms. Surface coverage estimates show that ammonia interacts with tungsten at 700°K to form, successively, surfaces of stoichiometry W2N(β), WN(δ), and W2N3H (η). The initial sticking probability in the formation of the η species is 10−6 at 700°K. It is concluded that, except at extremely low reactant pressure, the catalytic decomposition of ammonia on tungsten has as its limiting step the desorption of the η species W2N3H(η)→W2N(β)+N2(g)+H(g) and not desorption of nitrogen alone as has been sometimes suggested. Moreover, since the η-desorption reaction is expected to involve the breaking of N–H bonds, the observed hydrogen isotope effect in this zero-order reaction can be readily understood.
General Expression for the Quantum Transport Cross Sections
Authors: Wood Herbert T.
Published: February 2004

Two expressions, one for even n and one for odd n, are developed for the quantum transport cross sections Q(n). In both cases the expressions involve sums over the angular momentum quantum number, and differences in the phase shifts due to collision.
Reactions of Metal Atoms. IV. The uv Spectra of CdBr, CdI, ZnBr, and ZnI
Authors: Gosavi R. K. , Greig G. , Young P. J. , Strausz O. P.
Published: February 2004

Ultraviolet absorption spectra were recorded for CdBr, CdI, ZnBr, and ZnI produced in the cophotolysis of the appropriate trifluoromethyl halide and dimethyl metal compounds. Both the Π3/22 − 2Σ andΠ1/22 − 2Σ systems were observed in all cases. Most of the bands analyzed are in agreement with the emission spectra reported earlier in the literature. The Π1/22 − 2Σ system of CdBr and ZnI are reported here for the first time. Complete vibrational analysis of the spectra was performed and the Π2 splitting values compared.
Opacity Analysis of Inelastic Molecular Collisions. VI. The Sudden Approximation for Rotational Excitation
Authors: Levine R. D.
Published: February 2004

The limited coupling domain (where only first-order-allowed transitions are important) for rotational excitation is considered using the quantum mechanical sudden approximation. The derivation of the approximation is discussed using a variational procedure and a simplified form, valid for large angular momentum, is introduced. A further simplification is achieved using the limited coupling form of the S matrix, which interpolates between the weak coupling (distorted-wave) limit and the dominant coupling regime. Explicit results are obtained for differential and total cross sections of specified first-order-allowed rotational transitions of diatomic molecules. The inelastic cross sections are expressed in terms of a coupling parameter and a common cross section, and the angular distribution, evaluated using a semiclassical summation, is peaked at a forward angle θ0, with Eθ0 increasing with increasing collision energy.
Wiener Integrals and Models of Stiff Polymer Chains
Authors: Freed Karl F.
Published: February 2004

In an attempt to obtain simple zeroth-order models for stiff polymer chains for which the relevant distributions can be obtained in simple analytical closed forms, models based upon Wiener integral representations are investigated. The simplest model, which is discussed in detail, is related to the prescription of Sait̂, Takahashi, and Yunoki for the wormlike chain. The model and its generalization can be considered to be continuous limit analogs of rotational isomeric-type models of polymer chains in which there are interactions between neighboring, next-nearest, etc., bonds. Thus these models represent multidimensional Markov processes, and their Wiener integral representations and properties are therefore briefly discussed.
High-Pressure Ion–Molecule Reactions in Carbon Monoxide and Carbon Monoxide–Methane Mixtures
Authors: Chong Shuang-Ling , Franklin J. L.
Published: February 2004

In carbon monoxide at pressures above 0.2 torr, the principal product ion is C2O2+ formed by a three-body process. The rate constant for the reaction is 1.43 × 10−28 cm6 molecule−2·sec−1. Above about 0.8 torr, the reaction appears to approach equilibrium with an equilibrium constant of 1482 referred to 1 atm as standard. At pressures of methane above 0.2 torr and small additions of CO, the only reaction observed in the protonation of CO by CH5+ with a rate constant of 5.54 × 10−10 cm3 molecule−1·sec−1. All other reactions are endothermic and are not observed. At pressures of carbon monoxide above 0.2 torr, the addition of small amounts of methane results in an H-atom transfer reaction to CO+ with a rate constant of 13.7 × 10−10 cm3 molecule−1·sec−1. In addition, C2O2+ reacts to form HCO+, C2H3O+, and C3H3O+ with rate constants of 9.44, 4.37, and 0.76 × 10−10 cm3 molecule−1·sec−1, respectively.
Paradoxical Role of the Kinetic-Energy Operator in the Formation of the Covalent Bond
Authors: Feinberg M. J. , Ruedenberg Klaus
Published: February 2004

The origin of chemical binding is analyzed with the help of variational reasoning for the ground state of the hydrogen molecule–ion. The bond-parallel component of the kinetic-energy integral is shown to be the critical term. The effect of electron sharing on this term is such that the variation process yields, at all internuclear distances, a lower energy for the molecule than for the separated atoms. This appears to be paradoxical in as much as, at the equilibrium distance, the potential part of the binding energy is negative and the kinetic part is positive. The paradox is resolved by showing that the actual values of the potential and kinetic binding energy are the result of a scale variation which is similar to that found in the isolated atom and contracts the wavefunction towards the two nuclei. The variational origin of this redistribution of the kinetic- and potential-energy integrals, which establishes the virial relationship, is analyzed. It is concluded that the variational facts are inadequately described by the view that the chemical bond is formed “because the potential energy can be lowered through accumulation of charge in the bond.” Rather, the accumulation of charge in the bond is essential for chemical binding because it profoundly influences the behavior of the kinetic-energy integral under scale variations.
Cation K X-Ray Emission from Ionic Crystals
Authors: Best P. E.
Published: February 2004

The K x-ray emission spectra from the cations in KCl, CaF2, CaO, CaC2, CaCO3, and CaSO4(2H2O) are discussed. The energy separations between the outer cation and anion electronic shells are derived from the spectra. In the cases of the simple ions these energy separations can be calculated using a point ion model. The input data into these calculations is just the free ion ionization energies, and the Madelung potentials. The corrections that can be made to this simple model are discussed. Structure is observed in the cross-transition spectra from the salts containing complex anions or neutrals. It is shown that this structure most probably represents the one-hole valence spectrum of the complex species.
Repulsive Potential Curves from Molecular Continuum–Continuum Emission III. He2(X 1Σg+)
Authors: Chow Kwok-Wing , Smith Allan Laslett
Published: February 2004

A least-squares iterative procedure is developed for deducing a repulsive (lower) potential curve from a known attractive (upper) potential curve and the observed continuum–continuum emission spectrum. The RKR curve for the A 1Σu+ state of He2 is used in conjunction with the 600-Å band (A–X) data to determine that, for He2 (X 1Σg+), V(R) = (202 ± 10) exp(− 3.88 ± 0.05)R, 1.3 ≤ R ≤ 2.4 Å. It is shown that the 600-Å bands of 3He2 can be predicted from those of 4He2 without knowing the potentials. The Hopfield intensity distribution was recalculated to show that comparison with experiment still is hampered by lack of an accurate electronic transition moment function.
Ion–Molecule Reactions in Alpha-Particle Irradiated NH3, CD4, and D2O
Authors: Fluegge Robert A. , Landman Donald A.
Published: February 2004

We have investigated the time evolution of the various primary and secondary positive ion species formed in NH3, CD4, and D2O gases bombarded by 1- to 3.5-MeV alpha particles. The measurements were made with a pulsed time-of-flight mass spectrometer at pressures in the 10–40 micron range. Ion–molecule reaction schemes are proposed, the data analyzed for self-consistency, and a variety of reaction rates deduced. Comparisons are made with other experiments in which electron bombardment was used for primary ionization. Our results indicate the existence of a relatively long-lived [τ = 0.28(4)μsec] (C2D7+)* ion complex formed from the reaction CD3++CD4, paralleling previous results [R. A. Fluegge, J. Chem. Phys. 50, 4373 (1969)] for the CH4 system.
Correlation of Excess Energies of Dissociative Electron Attachment Processes with the Translational Energies of Their Products
Authors: DeCorpo J. J. , Bafus D. A. , Franklin J. L.
Published: February 2004

Appearance potentials and translational energies of the products resulting from several dissociative electron capture processes have been measured. Excess energies were determined for each process from the measured appearance potentials and known ground-state heats of formation of the products and reactants. As with positive ions, the results were found to agree with the predictions of classical statistical theory. Application of the method is demonstrated in determination of the electron affinity of radicals.
Experimental Determination of Oscillator Strengths for the CO A 1Π←X 1Σ+ Bands
Authors: Meyer Vincent D. , Lassettre Edwin N.
Published: February 2004

The oscillator strengths for three bands (υ′ = 0, 1, 2) in the transition A 1Π ← X 1Σ+ of carbon monoxide have been determined from electron impact spectra (taken at zero scattering angle) of mixtures of carbon monoxide and helium using the known oscillator strength for the 11S → 21P transition in helium as a reference. The oscillator strength for the υ′ = 2 excited state (υ″ = 0 in the ground state) is 0.045 ± 0.005. This is to be compared with the value 0.0429 obtained by an entirely different method [E. N. Lassettre and A. Skerbele, J. Chem. Phys. 54, 1597 (1971)].
Quantum Calculations of Collinear Reactive Triatomic Systems. III. H+Cl2→HCl+Cl
Authors: Miller G. , Light J. C.
Published: February 2004

The collinear H+Cl2→HCl+Cl reaction is treated quantum mechanically for a variety of potential energy surfaces. Those features of the potential energy surface leading to vibrational excitation of the product are identified, and vibrational excitation of the product comparable to that observed experimentally is obtained.
Diffusion and Permeation of He, Ne, Ar, Kr, and D2 through Silicon Oxide Thin Films
Authors: Perkins W. G. , Begeal D. R.
Published: February 2004

A technique for the preparation of pinhole-free glass films on porous substrates is described. The permeabilities of He, Ne, Ar, Kr, and D2 and the diffusivities of Ne, Ar, and D2 in the films are reported. The temperature range studied for He, Ne, and D2 was 25–500°C, while the range for Ar and Kr was 650–920°C. Activation energies reported for the permeabilities are: He, 5.22 kcal/mole; Ne, 9.13 kcal/mole; Ar, 25.2 kcal/mole; Kr, 44 kcal/mole; and D2, 8.32 kcal/mole. Activation energies reported for diffusivities are: Ne, 9.55 kcal/mole; Ar, 28.7 kcal/mole; and D2, 9.26 kcal/mole. A technique of nonlinear least squares curve fitting for the calculation of diffusion coefficients is described, and various interpretations of the temperature dependence of the diffusivity preexponential factor are discussed.
Application of the Minimax Approximation to the Reduced Partition Function of Isotopic Molecules
Authors: Németh G. , Gellai B. , Jancsó G.
Published: February 2004

The minimax approximation to the reduced partition function ratio of isotopic molecules is worked out. The applied approximation is an even power series in hν / kT which is convergent for all values of the argument. Its error function has as many extrema as possible and oscillates with uniform amplitudes of the smallest possible value. The rate of convergence and the utility of the minimax method are discussed through numerical calculations for various polyatomic molecules and the results are compared with those obtained by other approximations.
Crossed-Beam Study of the Reaction N++O2→NO++O
Authors: Tully John C. , Herman Zdenek , Wolfgang Richard
Published: February 2004

The reaction N++O2→NO++O has been studied as a function of collision energy using the crossed-beam apparatus EVA. This process is of considerable importance in the upper atmosphere. The system is also of particular interest because it may involve strong interactions between several intersecting or close-lying electronic surfaces of NO2+. This is not the case for previously well-studied ion–molecule reactions (e.g., Ar++D2), the dynamics of which are closely represented by simple models dominated by long-range electrostatic forces (e.g., “polarization stripping”). Beams of N+ in the ground (3P) state were crossed with thermal O2 beams and the angular and velocity distributions of the NO+ product measured. The reaction exhibits the asymmetric distribution characteristic of a direct process. Its translational exoergicity (Q) declines with energy, but above 5 eV relative energy, becomes constant. These data were then corrected for angular and energy spreads of the beams. The position of the plateau in Q appears consistent only with production of O(3P) and a state of NO+ which, with excess energy, dissociates to N(4S0) and O+(4S0). Consideration of the correlation diagram for NO2+ then indicates the following mechanism as probable: at moderately large separation N+(3P) and O2(3Σg+), originally on a surface correlating with linear NO2+ (excited Σ+1), experience on electron jump leading to a surface correlating similarly with NO2+ (1Σ−orΠ)1. Upon closer approach an O atom is transferred leading adiabatically to O(3P)+NO+(3Σ+). [The latter will dissociate to N(4S0) and O+(4S0) if sufficient energy is available.] Unlike the reaction to give ground-state products, this process is only slightly exoergic. This accounts for the fact that so little of the potentially available energy appears as translation.
Application of the RPA and Higher RPA to the V and T States of Ethylene
Authors: Shibuya Tai-Ichi , McKoy Vincent
Published: February 2004

We have applied our proposed higher random-phase approximation (HRPA) to the T and V states of ethylene. In the HRPA, unlike the RPA, one solves for the excitation frequencies and the ground-state correlations self-consistently. We also develop a simplified scheme (SHRPA) for solving the equations of the HRPA, using only molecular integrals sufficient for the usual RPA calculations. The HRPA removes the triplet instability which often occurs in the RPA. The excitation energy for the N → T transition is now in good agreement with experiment. The N → V transition energy increases by 15% over its RPA Value. The N → V oscillator strength changes only very slightly. These results are also useful in explaining the appearance and ordering of states obtained in recent direct open-shell SCF calculations.
Nuclear Magnetic Double Resonance in Chemically Exchanging Systems
Authors: Yang Ping P. , Gordon Sidney L.
Published: February 2004

A density matrix description of nuclear magnetic double resonance on chemically exchanging systems is developed using a double resonance basis set. The chemical exchange coefficients have many properties analogous to the relaxation coefficients. This allows the density matrix computation to be expressed in terms of a symmetric array. The solutions can then be obtained in terms of an eigenvalue procedure. This procedure is more efficient than standard Gauss–Jordan reduction and automatically takes into account the constraints introduced by the summation properties of the relaxation and chemical exchange coefficients and by the symmetry of the spin system. The formalism is illustrated by self-exchange in the AB2 system, 2,2,2-trichloroethanol. Comparison of theoretical and experimental double resonance spectra allow the determination of the chemical exchange lifetime and the relaxation parameters even when the exchange rates are slow compared with the experimental linewidths.
Interpretation of Photoionization Threshold Behavior
Authors: Guyon Paul M. , Berkowitz Joseph
Published: February 2004

Analytical expressions are derived for treating the threshold regions of photoionization efficiency curves. The effects of slitwidth, the initial thermal energy of the molecular system, and Franck–Condon factors are considered. In the dissociative ionization of small molecules such as F2 and HCN, discrepancies can arise from failure to take these factors into account. It is also shown that for large polyatomic molecules, the use of a Watanabe-type plot to determine the adiabatic ionization potential can lead to large errors. The paper concludes with detailed applications to HF, O2, S2, Se2, Te2, S6, and S8, and some general observations applicable to the diatomic halogens, hydrogen halides, and alkali halides.
EPR Observations of Temperature Dependent Level Crossing for Gd3+ in LaCl3
Authors: Reynolds R. W. , Boatner L. A.
Published: February 2004

Observations by electron paramagnetic resonance of LaCl3 single crystals containing very small concentrations of Gd3+ have revealed previously unresolved structure in the Gd3+ spectrum at temperatures between 150 and 294°K. A multiple crossing of the S7/28 ground doublets occurs in this temperature range, and the ability to make correct M8 assignments to the EPR transitions results in values for the spin-Hamiltonian parameters which differ from those reported previously.
Vibrational Anisotropy and Gol'danskii–Karyagin Effect in Trimethyltin Fluoride
Authors: Herber Rolfe H. , Chandra Subhas
Published: February 2004

The vibrational anisotropy of the tin atom in trimethyltin fluoride has been studied by means of 119Sn Mössbauer spectroscopy in the temperature range 7 < T < 296°K. From the Gol'danskii–Karyagin effect asymmetry of the doublet spectrum, it is inferred that while in the low-temperature limit the metal-atom vibrations are essentially isotropic, at room temperature the difference in the mean-square amplitudes of vibration parallel and perpendicular to the (nearly) threefold symmetry axis defined by the ···F···Sn···F framework, is given by 〈x‖2〉 − 〈x⊥2〉 = − 3.41 × 10−18cm2. The infrared data at room and liquid-nitrogen temperature in the 600–450-cm−1 region are consistent with a small departure from true C3υ symmetry for this molecule, and it is shown that the assumption of an oblate charge distribution about the metal atom (Vzz positive) with an asymmetry parameter η ∼ 0 is only valid in the first approximation. Using free-tin-atom masses and the high-temperature limiting slope of the area under each component of the Mössbauer spectrum doublet, it is possible to calculate a Mössbauer temperature for the π and σ transitions, which for trimethyltin fluoride are 105 ± 2°K and 114 ± 2°K, respectively.
Extensions of the Hulthén Orbital Concept
Authors: Weber Thomas A. , Weare John H. , Parr Robert G.
Published: February 2004

Three extensions of the Hulthén representation of atomic orbitals are investigated. In earlier work it was shown that radial functions 0s = N1[exp(− αr) − exp(− βr)](1 / r) and 1p = N2[exp(− γr) − exp(− δr)] excellently represented 1s and 2p radial functions for atoms. In the present paper, investigations are reported (a) using Hulthén orbitals as the starting point for a perturbation calculation of exact Hartree–Fock orbitals, (b) employing a full Dirichlet-type series [Σiλiexp(− μir)](1 / r) to represent a 1s orbital, and (c) using the Laplace transformation representation for a 1s orbital, 1s(r) =  ∫ 0∞ a(x) exp(− xr)dx.
Ionization Potentials of the Difluoroamino Radical by Photoelectron Spectroscopy and INDO Calculations
Authors: Cornford A. B. , Frost D. C. , Herring F. G. , McDowell C. A.
Published: February 2004

The photoelectron spectrum of the NF2 radical has been measured. The spectrum shows evidence for three vertical ionization potentials at 12.10, 14.60, and 16.38 eV. The adiabatic first ionization potential was found to be 11.62 eV. LCAO–INDO calculations have been employed to deduce the excitation energies of the NF2+ ion, and these are compared with the experimental values. The calculations indicate that the A11 state of the bent NF2+ ion is lower in energy than the linear Σ−3 state. Vibrational structure is clearly seen on the first band (1250 cm−1) and on the second (520 cm−1), and there is evidence that both progressions belong to the symmetric stretching mode ν1(a1).
Calculation of Upper and Lower Bounds to Oscillator Strengths
Authors: Weinhold Frank
Published: February 2004

A procedure is described for calculating rigorous upper and lower bounds to dipole oscillator strengths and is illustrated numerically with various simple applications to the hydrogen molecular ion. The procedure appears to give a good account of the actual errors which may arise in the conventional method based on energy minimization of the two states. In addition, specific regions of configuration space which are important for the oscillator strength, but perhaps not for the energy, are singled out and emphasized automatically.
Electron Affinities of the Halogen Molecules by Dissociative Electron Attachment
Authors: DeCorpo J. J. , Franklin J. L.
Published: February 2004

The electron affinities of the halogen molecules have been determined mass spectrometrically. This was accomplished by measuring both the appearance potentials and average translational energy of the negative halogen molecular ion resulting from dissociative electron attachment reactions. The values reported in this work agree favorably with those found in the recent literature. Also reported are the heats of formation of several free radicals used as auxiliary data.
Photolysis of Propyne at 1470 Å
Authors: Stief L. J. , De Carlo V. J. , Payne W. A.
Published: February 2004

The photolysis of propyne (methylacetylene) has been examined at the 1470-Å Xe resonance line and at low pressures (0.2 torr) in order to minimize polymerization. The major products observed in decreasing order of importance are H2, C2H2, C2H6, and CH4. Evidence is presented for the occurrence of primary processes forming atomic and molecular hydrogen. The following are energetically possible at 1470 Å: CH3C≡CH+hΛ→C3H3+H →C3H2+H2 →C3H+H2+H →C3+2H2. The dominant process is atomic hydrogen formation via A. The formation of acetylene and ethane is attributed to the reaction sequence H+CH3C≡CH→CH3+C2H2, 2CH3→C2H6. This is consistent with the observation that the ratio of C2H2 to C2H6 is close to two and that, in the photolysis of CD3C≡CH, the acetylene is 75% C2H2 and the ethane is 95% C2D6.
Determination of Intermolecular Potentials by the Inversion of Molecular Beam Scattering Data. I. The Inversion Procedure
Authors: Buck U.
Published: February 2004

A practical solution of the inversion problem within the realms of the WKB approximation is reported. The basic input information is only the measured relative differential cross section, from which the WKB phase function or the classical deflection function is constructed. The absolute potential curve is then determined by the application of a modified Firsov method. The problem of which part of the differential cross section is readily measurable and also suitable for the inversion procedure is studied. A practical procedure is proposed and tested by using a simulated example.
Determination of Intermolecular Potentials by the Inversion of Molecular Beam Scattering Data. II. High Resolution Measurements of Differential Scattering Cross Sections and the Inversion of the Data for Na–Hg
Authors: Buck U. , Pauly H.
Published: February 2004

High resolution differential scattering cross-section measurements for Na–Hg were made using the method of crossed molecular beams. The experimental arrangement, which has a resolution Δυ / υ in the relative velocity of better than 2%, is described. The measured differential scattering cross-section curves show up to eight rainbows. The interaction potential between 3 and 9.5 Å was determined from these data by using the inversion procedure described in Part I. The potential well depth is 8.79 × 10−14 erg, and the minimum is at 4.72 Å. The shape of this potential deviates significantly from that of the simple multiparameter potentials commonly used.
Approximations for the Functions Fm(z) Occurring in Molecular Calculations with a Gaussian Basis
Authors: Schaad L. J. , Morrell G. O.
Published: February 2004

Approximations, accurate to ± 1 × 10−8, have been developed for the functions Fm(z) which arise in molecular calculations using Gaussian basis functions. These are found to reduce integral computation time by about a factor of 5 over other methods. Approximations are listed for 0 ≤ m ≤ 16.
Exact Finite Method of Lattice Statistics. V. The Thermodynamic Phases of a Triangular Lattice Gas
Authors: Runnels L. K. , Craig J. R. , Streiffer H. R.
Published: February 2004

We present phase diagrams showing the stable phases of a two-dimensional lattice gas. The molecules reside on the triangular lattice and have hard cores which exclude other molecules from the first- and second-neighbor positions. An interaction w (positive or negative) is postulated for molecules separated by the third-neighbor distance, and no interaction is experienced at still greater separation. For attractive third-neighbor interactions (negative w) the phase diagram possesses only two regions: solid and fluid. The transition between them is first order at all temperatures, but more strongly first order at low temperatures. For positive w (soft repulsions), the phase diagram is topologically similar to that of helium. In addition to a solid region and a gaseous region, there are areas identified as a “normal liquid” and an “ordered liquid,” the latter being the stable phase as T → 0 at moderate pressures.
Electrolytic Preparation and EPR Study of Crystalline Pyrene, Perylene, and Azulene Perchlorates
Authors: Chiang T. C. , Reddoch A. H. , Williams D. F.
Published: February 2004

The preparation of crystalline pyrene, perylene, and azulene perchlorates is reported. Details of the novel method, growth on the anode in an electrolytic cell, are given. Crystalline samples of each compound give EPR resonances with very narrow linewidths, about 0.1 G. The resonances from the pyrene and perylene salts each consist of two components. One follows a Curie law and dominates at low temperature. It is attributed to “impurities,” probably lattice defects. The other component, dominating at high temperature, arises from thermally generated Wannier triplet excitons. The excitation energies are 0.18 and 0.27 eV for the pyrene and perylene salts, respectively. The azulene salt yielded only the “impurity” signal within the available temperature range.
Nuclear Quadrupole Resonance of 14N of the –CN Group in Some para-Substituted Benzonitriles
Authors: Colligiani A. , Ambrosetti R. , Guibé L.
Published: February 2004

The NQR of 14N of the –CN group in some benzonitriles with electron-releasing as well as electron-withdrawing para-substituent groups, has been studied. The experimentally obtained values of (e2qQ)z, the quadrupole coupling constant, and of Γ, the asymmetry parameter of the electric field gradient, are discussed on the basis of the Townes and Dailey theory. The discussion may be developed further, with some assumptions, to obtain the electron populations of the two p-type orbitals describing the π bonds between the nitrogen and the carbon atoms of the cyano group. The variations of such electron populations when changing the para substituent is obtained. A good correlation was also found between the values of the electron populations of the p-type orbital orthogonal to the benzene ring and the variations of the electron distribution on the ring, as described by the Hammett σ values for the corresponding para-substituent groups.
Hard-Sphere Pressure Bound from a Reformulation of Solid-Phase Thermodynamics
Authors: Hoover W. G. , Salsburg Z. W.
Published: February 2004

Two different ways of describing the work necessary to distort an elastic solid result in alternative microscopic expressions for the pressure and the elastic constants. In the case of a hard-sphere solid with nearest-neighbor interactions a lower bound on the pressure can be obtained by comparing the two pressure expressions. This lower bound is identical with the pressure derived from Kirkwood's self-consistent free-volume theory.
13C-Hyperfine Splitting in Semidiones
Authors: Russell Glen A. , Lawson David F. , Malkus Herbert L. , Whittle Philip R.
Published: February 2004

A number of ketyls and semidiones containing 13C have been examined by ESR spectroscopy. Grossly different values of aCOC in the ketyls (∼ 50 G) and the semidiones (∼ 1 G) allow for an easy differentiation of the two species. This is important because strained ring ketyls can undergo a decomposition reaction leading to the semidione with one additional carbonyl group added to the ketyl. Values of aCOC, aαC, and aβC for keyls and semidiones are reporduced by the Karplus–Fraenkel equation if the term QCC′C is allowed to increase by 15% when C′ is altered from methyl to t-butyl. No evidence for the nonplanarity of di-t-butyl ketyl could be deduced. Some unusual long-range 13C hfsc (aγC) have been detected in certain bicyclic semidiones possessing a nearly coplanar, zigzag arrangement of three carbon–carbon bonds with a pz orbital.
Models of a Degrading Polymer Network
Authors: Shaw Montgomery T.
Published: February 2004

Regular, two- and three-dimensional networks of linear analogs to elastic polymeric chains were degraded randomly in an attempt to establish a relationship between the force produced by a constantly strained network and the number of chain scissions. This behavior is important for the quantitative interpretation of chemical stress relaxation data. The models showed that the initial modulus of the three-dimensional networks should vary as the number of crosslinkages (tetrafunctional) to the 4/3 power. The initial rate of decay of the relative force for random main chain cleavage was (in the limit of infinite network size) equal to twice the rate of decay of the relative number of chains between the crosslinkages. At high degrees of degradation (relative number of chains between crosslinkages equal to 1 / e) the models indicated that the relative force would approach zero in the limit of infinite network size. The behavior of the models showed that the force decay of a degrading network is considerably more rapid than had been generally thought.
Integral Equations for Classical Fluids Using Bridge Diagrams: Hard-Sphere Solutions
Authors: Samec Augustine
Published: February 2004

Approximate integral equations for the pair correlation function and the partial of the pair correlation function with respect to density are derived for a classical fluid. The equations are solved for the hardsphere potential using second- and third-order bridge diagrams. Comparisons are made with the thermodynamic results of the Percus–Yevick (PY) and the hypernetted-chain (HNC) integral equations and with the Ree–Hoover Padé P(3, 3) approximant. Comparisons are also made of the pair correlation function, the direct correlation function, and the partial of the pair correlation function with respect to density at a density of ρΣ3 = 0.8. The results of the integral equations with bridge diagrams are found to improve on the PY and HNC equations.
Experimental Relative Franck–Condon Factors for the Ionization of Methane, Ethane, and Propane
Authors: Stockbauer Roger , Inghram Mark G.
Published: February 2004

Relative Franck–Condon factors, determined from threshold electron photoionization experiments, are presented for methane, ethane, and propane. The data are compared with those obtained by photoelectron spectroscopy and photoionization mass spectrometry. Differences are attributed to autoionization, cross section variations with energy above threshold, and, in photoelectron spectroscopy experiments, variations in transmission of the energy analyzer.
Number and Density of Bound States in Ion–Atom Systems
Authors: Tani Smio , Inokuti Mitio
Published: February 2004

Gross features of bound states are studied for a model of ion–atom systems. The model potential is taken to be of the form r−4 with a hard core. With the aid of an extensive tabulation, the number of bound states for a given rotational quantum number can be estimated quickly from the dipole polarizability of the atom, the reduced mass, and the hard-core radius. The energy density of bound states near zero energy is treated approximately by the WKB method. The effect of additional interactions—i.e., departures from the hard-core potential—is also discussed. The results are applicable to analyses of molecular–ion spectra and are relevant to ion–atom scattering at low energies.
Electron Paramagnetic Resonance of Mn2+ in Ni(CH3COO)2·4H2O and NiSO4·7H2O Single Crystals
Authors: Janakiraman R. , Upreti G. C.
Published: February 2004

Electron paramagnetic resonance of Mn2+ has been studied in nickel acetate tetrahydrate [Ni(CH3COO)2·4H2O] and nickel sulfate heptahydrate (NiSO4·7H2O) at 300°K. The Mn2+ was found to substitute for Ni2+ in both the systems and exhibit inequivalent magnetic complexes, two for the bimolecular Ni(CH3COO)2·4H2O and four for the tetramolecular NiSO4·7H2O. Besides, the Mn2+ resonance lines exhibited a direct dependence of linewidth on the intensity of Zeeman field and a shift in g value towards the lower side from the free-spin g value. These features have been explained in terms of the interaction of the Mn2+ ion with Ni2+ host ions.
Polymer Dynamics. V. The Shear Dependent Properties of Linear Polymers Including Intrinsic Viscosity, Flow Dichroism and Birefringence, Relaxation, and Normal Stresses
Authors: Noda Ichiro , Hearst John E.
Published: February 2004

Under the physical requirement that the contour length of a polymer molecule remains constant irrespective of shear rate in the Rouse and Zimm theories and the Harris–Hearst theory, the shear rate dependence of intrinsic viscosity, flow birefringence, flow dichroism, normal stresses, the relaxation of flow birefringence and flow dichroism, and some quantities at zero shear rate have been calculated for molecules of different stiffness ranging from the rod to the flexible coil in θ solvents. Changes of hydrodynamic interaction with shear rate are not included. Comparing the present theory with the experimental results there is satisfactory agreement for stiff molecules such as the cellulose derivatives and DNA at low shear.
Ferroelectric Tungsten Bronze-Type Crystal Structures. III. Potassium Lithium Niobate K(6−x−y)Li(4+x)Nb(10+y)O30
Authors: Abrahams S. C. , Jamieson P. B. , Bernstein J. L.
Published: February 2004

Ferroelectric K(6−x−y)Li(4+x)Nb(10+y)O30, with x ≃ 0.07, y ≃ 0.23, and a Curie temperature of 613°K, crystallizes with a tungsten bronze-type structure in the tetragonal system. The lattice constants are a = 12.5764 ± 0.0002 and c = 4.0149 ± 0.0001 Å at 298°K. The space group is P4bm. There is one formula per unit cell. The integrated intensities of 6578 structure factors, inside a reciprocal hemisphere of radius (sinθ) / λ = 1.02 Å−1, were measured with PEXRAD. There are 998 symmetry-independent Fmeas significantly above background. Isomorphism with BaxSr(5−x)Nb10O30 allowed the metal atom positions to be deduced. Oxygen atom positions were obtained by Fourier series methods. Refinement of structural parameters by the method of least squares resulted in a final agreement factor R = 0.0401. The structure is generally like that of other tungsten bronzes of formula (A1)2(A2)4C4(B1)2(B2)8O30, with the A1 site occupied by 87% K and 13% Li, the A2 site by 99% K and 1% Li, and the C site by 94% Li and 6% Nb. The B sites are fully occupied by Nb. X-ray spectrochemical analysis and the present diffraction study suggest x ≃ 0.07 and y ≃ 0.23 for the nonstoichiometric formula above. Evidence for major positional disorder, excluding occupancy disorder, similar to that observed in BaxSr(5−x)Nb10O30 and in Ba(4+x)Na(2−2x)Nb10O30 is not present for K(6−x−y)Li(4+x)Nb(10+y)O30. The K/Li atom at the A1 site is 12 coordinated with average K/Li–O distance of 2.833 Å. The K/Li atom at the A2 site is nine coordinated with average K/Li–O distance of 3.024 Å. The Li/Nb atom at the C site has three nearest oxygen neighbors at an average distance of 2.153 Å and six additional next-nearest neighbors at an average distance of 2.542 Å. The two independent irregular NbO6 octahedra have an average Nb–O separation of 1.973 Å. The Nb atom displacements, of 0.167 and 0.160 Å from the nearest mean oxygen plane, may be compared with the value of 0.175 Å predicted from the parabolic relation with Curie temperature. All metal atoms are displaced from the mean planes of oxygen in the same sense, making the positive polarization direction parallel to the sense of this displacement.
Study of the Electronic Structure of Molecules. XIV. Point Charge Perturbation on Periodic Systems
Authors: Clementi Enrico
Published: February 2004

We have simulated the perturbation involved by a solvent (or by other periodic perturbants) on a polymer (polyethylene) and we have analyzed (1) the effect of the perturbation on the energy bands of the polymer and (2) the internal charge transfer occurring on the polymer as a response to the external perturbation. The band structure is computed in the tight binding approximation using a self-consistent field model originally proposed by Ladik and subsequently improved and programmed by Andre. The model is an ab initio model, aimed at Hartree–Fock solutions for the band structure of periodic systems. All the many center integrals of coulomb and exchange type are accurately computed (the computation is an all-electron computation, and with no approximation for the atomic potential of the atoms in the unit cell). We have found that by surrounding the polymer with positive or negative point charges, the entire band system raises or lowers its entire energy spectrum. The band structure (and the energy gap) was left relatively stationary when the polymers were surrounded by alternate layers of point charges with different signs. The raising (or the lowering) of the band structure upon perturbation suggests that positive (or negative) charges on a polymeric medium can alter the band structure of the medium in the vicinity of the point charge, and that these perturbed sites can act as traps (or as sources) of electrons. We have in addition found that whereas the internal charge transfer for the polymer is not greatly altered by external perturbation of point charges of the same sign, it is highly altered by placing positive charges on one side of the polymer, and negative charges on the opposite side of the polymer (Stark effect simulation). The band energy shifts due to the external charge perturbation are sufficiently large to suggest that ESCA type experiments could be used to determine the structure of solvent molecules surrounding the polymers. The presentation of this work demonstrates that band structure computations of polymers are feasible and we expect that this type of computation will become a familiar aspect in quantum chemical work as computations on small molecules are today. As known, ab initio computations on the band structure of polymers are nearly unknown in today's scientific literature.
Identification of the C̃ State of Ammonia by Electron Impact Spectroscopy
Authors: Harshbarger William R.
Published: February 2004

The relative intensities of the B̃ ← X̃ and C̃ ← X̃ transitions in ammonia have been measured by electron impact spectroscopy. Spectra were obtained for electrons of 90- and 250-eV incident energies with scattering angles of 0° to 3°. Analysis of the relative intensities, in conjunction with the ultraviolet data previously reported by Douglas, indicates that the C̃ state is a separate excited electronic state of ammonia, identified by the term symbol A11′.