bluuebunny/crossref_metadata_2025 huggingface dataset:
Electron Density Rearrangement Description of Ion–Molecule Reactions
Authors: Schaefer Jacob , Henis J. M. S.
Published: February 2004

A description is proposed for low-energy ion–molecule reactions of the form A(±)+BC→AB(±)+C, which permits a prediction of the reaction cross section as either large or small. The description, restricted to reactions involving carbon, nitrogen, and oxygen (or combinations of these), is based on the idea that the probability for a reaction is determined by the nature of the change of electron density around the nuclear centers involved. The low-energy cross sections for 1 ion–molecule reactions are predicted and found in agreement with obs erved values, the majority of which have been obtained or confirmed by ion cyclotron resonance techniques.
H2 Lyman-Band Oscillator Strengths
Authors: Hesser James E. , Brooks Neil H. , Lawrence George M.
Published: February 2004

Absorption oscillator strengths for thirty rotational lines in nine vibrational bands of the Lyman transition, B 1Σu+, of H2 have been obtained from equivalent widths measured photoelectrically under high resolution. Argon and helium continua were used in the third and first orders, respectively, of the grating. A curve-of-growth analysis of the measured equivalent widths of the Doppler-broadened lines yielded the f values. Our results are in agreement with electron-scattering data obtained by Geiger and Topschowsky; consequently, there is now strong evidence from absorption f value and emission lifetime data for a significant variation of dipole transition moment with internuclear separation in the Lyman transition.
Raman Spectrum of α Quartz at High Pressures
Authors: Asell Jean F. , Nicol Malcolm
Published: February 2004

Raman spectra of single crystals of α quartz at pressures as high as 40 kbar are reported. The frequencies of the Raman lines located at 128, 207, 265, 464, 697, 795, and 807 cm−1 at atmospheric pressure increase with increasing pressure by about 0.6, 1.8, 0.5, 0.9, 0.8, 0.8, and 0.8 cm−1·kbar−1, respectively, with a probable accuracy of about 20% due to an uncertainty about the pressure scale. These frequencies of the α-quartz spectrum also are significantly sensitive to temperature. The frequencies of all other Raman lines of α quartz, except three at 145, 450, and 509 cm−1 that could not be studied, are independent of pressure.
Self-Diffusion Coefficients and Rotational Correlation Times in Polar Liquids
Authors: O'Reilly D. E.
Published: February 2004

Self-diffusion coefficients and rotational correlation times have been measured in several polar liquids by pulsed nuclear magnetic resonance techniques. Diffusion coefficients were measured for HCCl3, ClC6H5, HF, and HCl and relaxation times for HF. The self-diffusion coefficient of a liquid is related to the rotational correlation time τ2 by means of the quasilattice model of liquids. The rotational diffusion equation and the Stokes–Einstein expression for the rotational diffusion coefficient are inadequate and are respectively replaced by the results of the large-step random-walk theory of rotational diffusion and an expression for τ2 based on the quasilattice model. An expression for τ2, obtained from the dynamical rotational coherence theory, is shown not to apply in the case of the spherical-top molecule CCl4 which would be anticipated to have a small friction constant. The correspondence between calculated values of τ2 based on the quasilattice model and the observed rotational correlation times is reasonably good except for liquid HCl for for which the rotational correlation times are anomalously short. The possibility that for short times the molecular rotation in HCl is dynamically coherent is examined and this effect is quantitatively taken into account. Water is also exceptional in that the activation energies for the rotational correlation times of deuterium and oxygen-17 are larger than the activation energy for proton self-diffusion. The Stokes–Einstein expression for τ2 is in approximate agreement with the observed values for water and liquid hydrogen fluoride.
Calculation of the NMR Spin Coupling Constant in HD
Authors: Armour Edward A. G.
Published: February 2004

The NMR spin coupling constant in HD is calculated by a perturbation-variation procedure using basis sets ranging from 2 to 12 functions. The simplicity of the HD molecule makes it possible for the coupling constant to be calculated exactly within a given basis set. It is thus possible to determine the effect on the coupling constant not only of extension of the size of the basis set but also of the inclusion of configuration interaction. Finally, the coupling constant is calculated using very accurate ground-state wavefunctions, and the values obtained are compared with the results of the earlier 2 to 12 basis function calculations. It is found that the main contribution to the coupling comes from the Fermi contact interaction. A value of 54.06 cps is obtained for the coupling constant. The experimental value is 42.7 ± 0.5 cps.
Mössbauer Investigation of Frozen Solutions of 129I2
Authors: Bukshpan S. , Goldstein C. , Sonnino T.
Published: February 2004

The Mössbauer effect in the 27.8-keV level of 129I was used to investigate frozen solutions of I2 in hexane, CCI4, and benzene at 88°K and in solid argon at 22°K. The quadrupole couplings and isomer shifts of I2 in hexane, CCl4, and solid argon (inert solvents) were found to be very similar, the average values being e2qQ = − 2255 ± 30 Mc/sec (Q ofI) 127and δ(ZnTe source) = + 0.094 ± 0.005 cm/sec. These values are interpreted as belonging to the free iodine molecule. For I2 in benzene a coupling of e2qQ = − 2412 Mc/sec and isomer shift δ = + 0.076 cm/sec were obtained. These values are in agreement with the electron transfer from the benzene ring to the iodine molecule.
Onsager's Reciprocal Relationships with Phenomenological Coefficients and Diffusivity Depending on the Intensive Properties of the System
Authors: Francesconi R. , Trevissoi C.
Published: February 2004

It is demonstrated that application of Onsager's reciprocal relations, with coefficients dependent on the intensive characteristics of the system, to ordinary diffusion and to thermal diffusion still in the range of linearity of the phenomenological relations, enables diffusivity of the components to be expressed as a sum of two terms, one purely thermodynamic and the other also containing the hydrodynamic characteristics of the system. Combination of the two factors is explicit and very simple in the case of isothermal diffusion of dilute polymer solutions. When the thermodynamic factor is predominant there is a linear law relating diffusivity with concentration, coinciding with the equation confirmed experimentally in various polymer systems. In the limit, when the hydrodynamic factor reaches zero, Wagner's equation for the diffusion of solid PbCl in AgCl is obtained.
Intrinsic Viscosity of Wormlike Polymer Chains
Authors: Ullman Robert
Published: February 2004

The intrinsic viscosity of wormlike polymer chains has been calculated using the Kirkwood–Riseman method. The chain statistics are taken from the Hermans and Ullman analysis of the Kratky–Porod model. Results are obtained for several degrees of chain extension for chains of varying molecular weight as a function of the degree of hydrodynamic interaction. The finite cross section of the chain is explicitly introduced in the model. The limiting results of the rigid rod and of the random coil are obtained at low and high degrees of chain coiling, respectively. Application of this model to DNA leads to a Kuhn statistical bond length of 1020 Å. The model was also applied to flexible polyelectrolytes in solutions of low salt concentration and to cellulose nitrate. A method of using the tabulated results for estimates of the degree of coiling of other polymer systems is described.
Ligand Hyperfine Interactions in Molybdenyl and Chromyl Halide Complexes
Authors: Manoharan P. T. , Rogers Max T.
Published: February 2004

Ligand superhyperfine interactions due to 35,37Cl and 19F have been observed in the ESR spectra of MoOCl52−, MoOF52− and CrOF52−. The presence of both isotropic and anisotropic ligand hyperfine splittings indicates considerable covalency in these complexes. The bonding coefficients, which have been calculated with resonable accuracy, reveal moderately strong in-plane π bonding involving the metal dxy orbital and the ligand py orbital. The spin–orbit interaction of the halide ligand is shown to be important in determining the g values. A configuration-interaction treatment has been used to account for the spin densities in the ligand px and s orbitals. The ESR spectra show that in solutions of KCrOF4 in aqueous hydrofluoric acid two species, CrOF4− and CrOF52−, are present.
Large Z Asymptotic Wavefunction for the Ground State of a Two-Electron Diatomic Molecule
Authors: Mendelsohn Lawrence B.
Published: February 2004

The asymptotic large Z method is applied to the ground state of a two-electron homonuclear diatomic molecule with total nuclear charge Z and internuclear separation R and the first-order basic asymptotic wavefunction is obtained. As R goes to zero, the molecular wavefunction is shown to go over to the atomic (united atom) result obtained previously. Electron–electron correlation effects are explicitly exhibited.
Determination of Bond Dissociation Energies in Hydrogen Cyanide. Cyanogen and Cyanogen Halides by the Photodissociation Method
Authors: Davis Douglas D. , Okabe H.
Published: February 2004

The photodissociation process yielding CN B 2Σ+ from various cyanogen compounds has been studied in the vacuum ultraviolet. Threshold energies of incident photon to produce the CN B 2Σ+ are measured by monitoring the fluorescence due to the transition CN B 2Σ+ − X 2Σ+. The upper bounds of bond energies can be obtained from these thresholds and the electronic energy of CN B 2Σ+. The heat of formation of CN, ΔHf0° (CN), is computed using these bond energies together with various ΔHf0°. ΔHf0° (CN) of 101 ± 1 kcal mole−1 (422 kJ mole−1) is obtained from the lowest two values measured. This value is within an estimated error in excellent agreement with the photoionization value recently obtained by Dibeler and Liston. The bond energies based on ΔHf0°(CN) = 101 kcal mole−1 for various cyanogen compounds are D(H–CN) = 120, D(Cl–CN) = 97, D(Br–CN) = 83, D(I–CN) = 73, D(NC–CN) = 128, D(C–N) = 184 all in kilocalories per mole with an over-all estimated error of ± 1 kcal mole−1. Because of the weak fluorescence intensity, no reliable bond energy was obtained for CH3CN. Other values obtained are D(F–CN) ≤ 111 kcal mole−1 and ΔHf0°(FCN) ≥ 7.4 kcal mole−1. A correlation of the dissociation process with the absorption spectrum is briefly discussed. A comparison is made of bond energies obtained by the photodissociation, photoionization, and electron-impact methods. Limitations of the photodissociation method to determine bond energies are discussed.
Shock-Tube Study of Vibrational Relaxation in Carbon Dioxide
Authors: Simpson C. J. S. M. , Bridgman K. B. , Chandler T. R. D.
Published: February 2004

Measurements of the variation of density behind incident shock waves in carbon dioxide have been made using a Mach–Zehnder interferometer at shock speeds of up to Mach 9 (2500°K). The results show that vibrational relaxation occurs to a state in which all the vibrational modes are excited to their equilibrium values with neither the symmetric stretching nor the asymmetric stretching modes having significantly longer relaxation times than the bending mode. It has been found that the density does not change exactly exponentially with distance behind a shock front as has been assumed in the past. It has been shown that it is this assumption which led to the idea that the rate of vibrational relaxation depends not only on temperature, but also on the departure of the system from equilibrium. The present results can be accounted for if the relaxation time is a function of temperature alone. Relaxation times have been calculated taking account of the changes in the translational temperature which occur during the relaxation process. The results cover the temperature range 330°–1600°K.
Adsorption and Decomposition of Ammonia on a Single-Crystal Tungsten (100) Surface
Authors: Estrup P. J. , Anderson J.
Published: February 2004

The interaction of ammonia with a single-crystal tungsten (100) surface has been investigated at pressures below 10−7 torr with a combination of techniques, including low-energy electron diffraction (LEED), flash desorption, mass spectrometry, and work-function measurements. The results indicate that adsorption of NH3 at room temperature is nondissociative, occurs with a sticking probability >0.45, and lowers the work function φ by ∼1 eV. Raising the temperature to ∼800°K after saturation results in dissociation of NH3 and desorption of H2, leaving behind half a monolayer of NH2 in a C(2 × 2) structure with a Δφ of ∼−0.4 eV. This structure can be destroyed rapidly at ∼1375°K, with desorption of H2 and N2. The NH2 density can be increased to ∼1 monolayer by exposure to ammonia at ∼800°K. During this process Δφ returns to a value of ∼0, the structure changes from a C(2 × 2) to a(1 × 1), and there is evidence that the rate of hydrogen desorption is controlled by the breaking of a N–H bond. Adsorption and desorption of NH3 occurs readily, but without decomposition, when all the NH2 sites have been filled. The (1 × 1)–NH2 layer desorbs as N2 and H2 in two stages, at ∼1150°K and ∼1375°K, each with the equivalent of half a monolayer of NH2. There are no indications that adsorbed NH or N are formed as intermediate products.
Steady-State Properties of Ising Lattice Membranes
Authors: Gordon Richard
Published: February 2004

If a finite strip of an Ising lattice is placed between two baths, a membrane is formed in which the particles being transported can undergo a phase transition. The steady-state properties of this system are investigated using computer simulation (by a new, more efficient Monte Carlo procedure) and Bragg–Williams approximations for two kinds of kinetics, based on nearest-neighbor attractions between the particles. A comparison with the steady-state behavior of the H/Pd system indicates that H–H interactions are not responsible for the α − β phase transition. It is suggested that an allosteric mechanism is involved.
Vibrational Spectra and Force Constants for Isotopic Species of Nitrosyl Chloride
Authors: Jones Llewellyn H. , Ryan Robert R. , Asprey Larned B.
Published: February 2004

From infrared absorption studies in the gaseous phase, fundamental harmonic vibrational frequencies have been determined for 16O14N35Cl, 16O15N35Cl, 18O14N35Cl, and 18O15N35Cl. With the aid of centrifugal-distortion data from the literature, a unique general quadratic valence-force-constant solution was obtained. The resulting values are: FNO = 15.26 mdyn Å−1, FNCl = 1.27 mdyn Å−1, Fα = 1.32 mdyn Å−1·rad−2, FNO,NCl = 1.53 mdyn Å−1, FNO,α = 0.1 mdyn rad−1, FNCl,α = 0.12 mdyn rad−1.
Application of Electron Spin Resonance in the Study of Triplet States. III. Extinction Coefficients of Triplet–Triplet Transitions
Authors: Brinen Jacob S.
Published: February 2004

Simultaneous ESR and triplet–triplet absorption measurements have been performed on several aromatic hydrocarbons. The ESR measurement is used to determine concentration of triplet molecules at steady state. This concentration is used to calculate extinction coefficients and oscillator strengths for triplet–triplet transitions and the radiative lifetimes of the upper triplet state. Extinction coefficient values are compared with available literature data.
Microwave Spectrum, Internal Rotation, Dipole Moment, Quadrupole Coupling, and Structure of Nitrosomethane
Authors: Coffey Dewitt , Britt Chester O. , Boggs James E.
Published: February 2004

The microwave spectra of CH3NO and CD3NO have been observed and analyzed. The barriers to internal rotation are 1137 ± 5 and 1095 ± 7 cal/mole for CH3NO and CD3NO, respectively, a difference of 42 cal/mole which is thought to be real. The barriers were calculated by the internal-axis method with retention of higher terms in the usual Fourier expansion of the rotational energy and computation of the torsional integrals in the harmonic-oscillator approximation. The theoretical parameters were fitted to the data by a least-squares method and the uncertainties reported are standard deviations. Transitions having comparable Stark and quadrupole energies were used to calculate the dipole moment. Secular equations of second or third order were solved for each of the data points. The dipole moment components are μa = 2.240 ± 0.001 D, μb = 0.522 ± 0.006 D, and μtotal = 2.300 ± 0.002 D. The quadrupole coupling constants of CH3NO are 0.50 ± 0.16, −6.016 ± 0.031, and 5.518 ± 0.031 for the components along the a, b, and c axes, respectively. The corresponding values for CD3NO are 0.60 ± 0.07, −6.007 ± 0.020, and 5.411 ± 0.020. The N=O and C–N distances are not well determined by the data. The CNO angle is 112.6 ± 1.0°.
Electric-Field-Induced Spectra: Excited-State Dipole Moment from Line-Shape Analysis
Authors: Haner David A. , Dows David A.
Published: February 2004

A theoretical development, including instrumental effects, is presented for the shape of a single experimental line in the electric-field-induced spectrum (EFS). The absorption coefficient of a line is defined by incorporating the Stark shifts and transition probabilities (to second order in the electric field) and a shape function for the degenerate components of the line. This treatment results in a model that explicitly contains the molecular parameters: ground- and excited-state rotational constants and dipole moments and the half-width at half-height of the line. This explicit dependence on the excited-state dipole moment leads naturally to employing a search procedure which compares the theoretical EFS line to the experimentally determined line shape. The rotational lines pQ3(4) and pP5(5) of the 3390-Å band of formaldehyde were measured under various experimental conditions and fit using this technique, yielding a value of 1.53 ± 0.11 D for the excited-state dipole moment. The accuracy of this result was found to be limited by the experimental accuracy of measuring the field strength, which is amenable to significant improvement. The accuracy of a method, previously used, which determines excited-state dipole moments from ratios of line intensities, is discussed in the light of the present results.
Theory of Chemical Reactions in Mixtures with Inert Gases
Authors: Schlag E. W. , Valance W. G.
Published: February 2004

Chemical gas reactions are often studied in the presence of inert gases, particular examples being shock-tube dissociations and unimolecular reactions in the low-pressure limit. A linear-mixture rule is usually invoked to separate contributions to the rate constant from the two types of possible collision processes. This separability assumption is investigated in this paper. A sufficient condition is that reactants below the critical energy are essentially in thermal equilibrium. If this equilibrium is seriously disturbed by the reaction, the linear mixture rule will break down. The observation of nonlinearity is suggested as evidence for nonequilibrium contributions to the rate constant. Unimolecular reactions and reactions with tunneling contain complications which must be carefully separated from nonequilibrium effects. Systems in which the reverse reaction cannot be ignored lead to predictable simple modifications in this mixture rule, it still being additive but nonlinear. If nonequilibrium effects are important, gross changes in the rate constants are expected depending on which collision partner dominates energy transfer in the region below the critical energy. In particular, if molecules determine energy transfer near E0 at low temperatures but atoms control this region at high temperatures, both cases may give linear plots, but the effective efficiencies in these two regions will differ.
Kinetics of the Thermal Dissociation of N2F4 in Shock Waves
Authors: Modica A. P. , Hornig D. F.
Published: February 2004

The equilibrium dissociation of N2F4⇌2NF2 was studied in shock waves, yielding an equilibrium constant identical to those obtained previously in static measurements. The thermal dissociation rate for N2F4 diluted with argon, helium, and SF6 was studied between 350° and 450° at total pressures of 0.58–2.7 atm. The reaction M + N2F4⇌ lim kMM + 2NF2 was found to be first order in M and first order in N2F4, indicating a bimolecular mechanism. The rate constant for argon could be expressed either by kAr = 1.5 × 1012T0.5exp ( − 15 200 / RT) liter/mole·sec or kAr = 3.0 × 10.T0.5 (E0 / RT)4.0exp ( − E0 / RT) liter/mole·sec, where E0 is the dissociation energy of N2F4, here taken to be 18.4 kcal/mole. For the other gases, the activation energy was unchanged, the relative rate constants being given throughout the temperature range by kHe / kAr = 1.2 ± 0.1, kSF6 / kAr = 1.7 ± 0.2, and kN2F4 / kAr = 1.8 ± 0.2. The rate constants are discussed in terms of collision theory.
Theory of Diffusion-Controlled Chemical Reactions in Porous Media I. Single-Phase Diffusion
Authors: Reck R. A. , Reck G. P.
Published: February 2004

Rigorous upper and lower bounds on the effective reaction-rate constant k are obtained for a diffusion-controlled chemical reaction which occurs in a porous medium. A reactive species B* is formed at a constant rate σ everywhere in the void region and reacts via competing finite surface and volume reactions. The diffusion coefficient of B* is set equal to zero within the solid phase. The results are stated in terms of a correlation function P(ε)dε which gives the probability that a point r, chosen at random, will be in the void region and at a distance “ε” from the surface of the nearest solid particle. Numerical calculations are presented for three special cases for a model in which the solid phase is constituted by randomly placed interpenetrating spheres.
High-Resolution Energy-Loss Spectrum of Molecular Oxygen
Authors: Geiger J. , Schröder B.
Published: February 2004

The spectrum of molecular oxygen has been studied in the energy-loss range 6.8–21 eV by means of a high-resolution electron energy-loss spectrometer using 25-keV primary electrons. The resulting energy-loss spectrum has been compared with the uv absorption spectrum. In contrast to the results for nitrogen, fair agreement is stated disregarding some deviations in the Schumann–Runge region. At high energy (E > 17 eV) the resolution in the energy-loss spectrum exceeds that of the absorption spectrum. Some bands could be grouped new.
Electron Spin Resonance Spectra of Er3+ and Yb3+ in Single Crystals of SrCl2
Authors: Boatner L. A. , Reynolds R. W. , Abraham M. M.
Published: February 2004

The ESR spectra of Yb3+ and Er3+ in cubic sites of SrCl2 (Fluorite structure) have been observed at a frequency of 10 Gc/sec and at 4.2° and 20°K. The spectra were isotropic with g = 3.417 ± 0.001 for Yb3+ and g = 6.754 ± 0.005 for Er3+ indicating that a Γ7 doublet is the ground state in both cases. The Er3+ Γ7 ground state shows that a large variation in the B4 / B6 ratio consistent with the point-charge model is not present in the more inflated SrCl2 lattice relative to other fluorite-type hosts. The Yb3+Γ7 ground state is in agreement with previous measurements on Yb3+ in eightfold coordinated cubic sites and no behavior inconsistent with predictions of the point-charge model was in evidence. Hyperfine structure was resolved with A(171Yb) = 881 ± 1 × 10−4cm−1, A(173Yb) = 242.9 ± 0.3 × 10−4cm−1, and A(167Er) = 233.5 ± 0.3 × 10−4cm−1. Axial spectra were also observed but not measured due to their low intensity and linewidth variation with magnetic-field orientation.
Hydrazine Cation Radical Formation in γ-Irradiated Single Crystals of Hydrazinium Hydrogen Oxalate
Authors: Edlund O. , Lund A. , Nilsson Å.
Published: February 2004

Room-temperature γ irradiation of hydrazinium hydrogen oxalate single crystals yields the hydrazine positive ion, which has been studied by electron spin resonance. The complex is planar, symmetry D2h, with the unpaired electron delocalized over the two nitrogen nuclei, which have the axial hyperfine coupling tensor AN = (2.5, 2.5, 27.6)G, aN = 11.0 G. The principal components of the proton coupling tensors are AH = (+2, −20.6, −13.0)G, aH = −10.4 G. The relative signs of the latter principal elements have been ascertained by comparing experimental and calculated spectra, for orientations of the crystal where ΔMI = 1 transitions are observed. The radical has a different orientation in the crystal compared with that of the parent ion. The reorientation is thought to be due to the influence of the hydrogen bonds between the hydrazine positive ions and neighboring oxygen atoms.
Dielectric Saturation Effect in the Continuous Dielectric Model for the Solvated Electron
Authors: Fueki Kenji
Published: February 2004

The effect of dielectric saturation on the electronic properties of solvated electrons has been considered within the framework of the continuous dielectric model with the adiabatic approximation. An approximate potential energy function including dielectric saturation has been derived for the solvated electron in water. A calculation has been made of the energy levels, the transition energy, and the charge distribution of the hydrated electron on the basis of this potential energy function.
Significant-Structure Theory Applied to Molten Salts
Authors: Lu Wei-Chen , Ree Taikyue , Gerrard Verl G. , Eyring Henry
Published: February 2004

The significant-structure theory of liquids has been successfully applied to calculate the thermodynamic properties, viscosities, and specific conductance of molten salts, taking account of the coexistence of monomers and dimers in the gaslike part of the partition function.
Pressure Dependence of the Xenon Continuum Radiation Decay Rate
Authors: Nichols L. L. , Vali W.
Published: February 2004

The lifetime of the continuum radiation of xenon was determined over the pressure range of 1–280 torr. The experimental approach involved the use of the delayed coincidence counting technique and pulsed-proton beam to excite the xenon atoms. The continuum is interpreted as the decay of an excited Xe2 bound state to a dissociative ground state. This continuum cannot be attributed to free–free or free–bound transitions or any recombination phenomena. The natural lifetime of the Xe2 molecule is greater than 100 nsec.
Reaction of Ketene with Atomic Hydrogen and Oxygen
Authors: Carr R. W. , Gay I. D. , Glass G. P. , Niki H.
Published: February 2004

The reactions of atomic hydrogen and oxygen with ketene have been studied in a discharge-flow system coupled to a time-of-flight mass spectrometer. The principal products of the H-atom reaction are CH4 and CO, and a mechanism consistent with the kinetics of their formation is H + CH2CO = CH3 + CO, CH3 + H + wall = CH4 + wall. The bimolecular rate constant for addition of H to CH2CO was determined to be: k1 = 1.3 × 10−13cm3molecules−1·sec−1 at 298°K. The reaction of O atoms with ketene is quite complex. Considerable amounts of CO2, CO, H2O, and H2 are formed, and atomic H and H2CO were also detected. The data are consistent with an initial addition of O to ketene, O + CH2CO = CH2CO2, followed by rapid decomposition and the consumption of an additional O atom. The bimolecular rate constant was found to be k1 = 8.8 × 10−13cm3molecules−1·sec−1 at 298°K.
Classical Theory for the Interaction of Gas Atoms with Solid Surfaces
Authors: Logan R. M. , Keck James C.
Published: February 2004

A classical theory for the interaction of gas atoms with solid surfaces is presented. The principal assumptions of the model used are: (1) The surface atoms involved in the collisions can be represented as independent one-dimensional oscillators; (2) the gas atoms interact with the surface through a stationary square-well attractive potential and an exponential repulsive potential; (3) the surface is flat so that the tangential velocity component of the gas atom is unchanged; (4) the surface oscillators have an equilibrium energy distribution at the temperature of the solid. This model represents a logical successor to the “hard-cube” model introduced by Logan and Stickney and allows the important effects involving the collision time and the natural frequency of the surface atoms to be taken into account. An approximate closed-form expression is obtained for the angular distribution of particle flux for a monoenergetic atomic beam scattered from a surface. For physically reasonable values of the well depth and range of the exponential potential, good quantitative agreement is obtained with available experimental data for the angular position of the maximum of the scattering distribution. Satisfactory agreement with experiment is even obtained in a case where the gas atom is heavier than the surface atom (Xe on Ag); in a case such as this the hard-cube model would not give reasonable results. Values are also obtained for the fraction of incident gas atoms which are initially trapped on the surface.
Magnetic Properties of Polyatomic Molecules. I. Magnetic Susceptibility of H2O, NH3, CH4, H2O2
Authors: Arrighini G. P. , Maestro M. , Moccia R.
Published: February 2004

The magnetic susceptibility of H2O, NH3, CH4, and H2O2 molecules has been evaluated by the perturbed HF scheme. Several wavefunctions, all of LCAO SCF MO type, have been considered. By using a double perturbation technique we derive some useful quantities which allow one to evaluate diamagnetic and paramagnetic contributions to the susceptibility for any gauge. The results corresponding to the largest basis sets are in satisfactory agreement with experimental data. The dependence of the evaluated quantities upon the gauge transformation corresponding to a change of the origin of the vector potential has been examined in detail.
van der Waals Potentials for Simple Polar Molecules Interacting with Graphite
Authors: Crowell A. D.
Published: February 2004

A simple model for calculating the interaction potentials of simple (nearly spherical) polar molecules with graphite based on a lattice summation of the (6–12) portion of a Stockmayer potential and the effects of a classical dipole image is proposed. Results are obtained and discussed for NH3 and SO2.
NMR Study of Internal Rotation in CHClBr–CHBrI: Direct Measurement of the gauche Proton–Proton Coupling Constants
Authors: Govil G. , Bernstein H. J.
Published: February 2004

NMR and thermodynamic parameters for the two gauche forms of 1-chloro-1,2-dibromo-2-iodoethane have been measured directly. Reaction field theory has been used to calculate the NMR coupling constant for the trans isomer, and the gas-phase enthalpies have been estimated. The reaction-field theory has been found to break down in polar solvents, presumably because of hydrogen bonding. Values of coupling constants have been discussed in light of the values predicted by theoretical and semiempirical methods. The barrier to internal rotation between gauche forms has been estimated from line-shape analysis.
Scaling and Relaxation of Self-Consistent-Field Wavefunctions of Isoelectronic Systems
Authors: Davis Daniel L. , Goodisman Jerry
Published: February 2004

Scaling an SCF wavefunction of a chemical system to produce an approximate wavefunction of an isoelectronic system is shown to correspond to minimizing the zero- plus first-order energy of the second system within the formalism of self-consistent perturbation theory; allowing the linear coefficients to relax to achieve self-consistency is shown to correspond to solving the perturbation problem to infinite order. A scaled SCF wavefunction for neon is used as the zero-order function to generate an SCF wavefunction for the spherical model of methane. The minimum total energy found by this method, for which only a few one-electron integrals need be evaluated, is −39.504 a.u. at a carbon–hydrogen bond distance of 2.082 a.u. Holding the bond distance at its experimental value 2.0665 a.u., the minimum energy obtained by optimization of all parameters in the set of basis functions (full SCF calculation) is −39.527 a.u.
Mössbauer Spectroscopy of Cytochrome c
Authors: Lang George , Herbert Denis , Yonetani Takashi
Published: February 2004

The cytochrome c of T. utilis grown in a 57-enriched medium has been investigated by Mössbauer spectrometry over a wide temperature range. The ferric cytochrome, in both lyophilized and frozen-solution form, shows magnetic hyperfine interaction at low temperatures and resembles some of the low-spin homoglobins. Spectra are in good agreement with calculations based on g values of horse-heart ferricytochrome c. The ferrous cytochrome exhibits a simple temperature-independent quadrupole splitting of 1.18 ± 0.05 mm/sec. Measurements in applied field indicate that Vzz > 0, and η ∼ 0.5.
Light Scattering from a Multiply Relaxing Fluid
Authors: Nichols W. H. , Carome E. F.
Published: February 2004

Formulas are derived for the roots of the dispersion equation for temporally damped hydrodynamic waves in a viscous, thermally conducting liquid having also a frequency-dependent viscosity with one or more relaxation times. Values of the roots are then computed for CCl4 at 25° for practically the entire range of wavenumbers, under the assumption that the extra viscosity has a relaxation time of 5.43 × 10−11 sec. Next, the values of the roots corresponding to k ≈ 2 × 105cm−1 (for 90° scattering of He–Ne laser light) are used to calculate both the unbroadened and the instrumentally broadened Rayleigh–Brillouin spectrum. A similar calculation under the assumption that CCl4 is doubly relaxing shows radically different velocity dispersion curves according to the relaxation time τ2 assigned to the longitudinal viscosity. For τ2 = 10−13sec, viscous overdamping reduces the sound velocity to zero for a narrow range of wavenumbers above about k = 1.2 × 107cm−1. For τ2 = 10−12sec, the velocity is an increasing function of frequency with points of inflection at about 3 and 100 GHz, as anticipated for a process having two widely separated relaxation frequencies.
Ultrasonic and Light-Scattering Studies of Carbon Tetrachloride
Authors: Carome E. F. , Nichols W. H. , Kunsitis-Swyt C. R. , Singal S. P.
Published: February 2004

A detailed examination of laser light-scattering data in CCl4 at 25°C was undertaken to determine which of reported relaxation times, determined from velocity-dispersion data in the gigahertz region, is consistent with both ultrasonic and hypersonic data. Assuming total relaxation of the vibrational specific heat, two relaxation times are shown to be consistent with ultrasonic absorption data: τ″ = 5.4 × 10−11sec with the bulk viscosity ηυ = 43ηs, the shear viscosity, and τ″ = 3.7 × 10−11sec with ηυ = 10(43ηs). For both cases, velocities were calculated from the dispersion equation for temporally damped waves and were adjusted for instrumental effects and the presence of the second unshifted and antisymmetric Brillouin components. Spectra were also calculated on a digital computer by convoluting the exact expression for the light-scattering spectrum of a singly relaxing fluid with an appropriate instrumental function. It is concluded, by comparison of the theoretical results with experimental velocities and spectra, that if there is just one relaxing specific heat, then its relaxation time τ″ ≈ 5.4 × 10−11sec and ηυ ≈ ηs.
Behavior of Excited Molecules of Aromatic Hydrocarbons in the Liquid State. I. Benzene and p-Xylene: Aggregation Effects
Authors: Yguerabide Juan
Published: February 2004

Experimental results of the effects of increasing aromatic concentration on the fluorescence spectrum and intensity of benzene and p-xylene in cyclohexane are presented. Other authors have studied these systems but there is disagreement as to results and interpretations. In agreement with previous results, an increase in aromatic concentration results in the appearance of a new fluorescence band in benzene but has only a small effect on the spectrum of p-xylene. The fluorescence spectrum of benzene at high concentrations, however, cannot be simply resolved into monomer and excimer spectra. Further, the increase in excimer fluorescence could not be correlated with the decrease in monomer fluorescence as predicted by a simple monomer–excimer mechanism. The luminescence decay time for both p-xylene and benzene remains constant within 10% in going from very dilute solutions to undiluted aromatic. This constancy in decay time can be correlated with changes in the monomer fluorescence intensity by means of a monomer–excimer mechanism in which an approximate rapid equilibrium exists between the processes of excimer formation and excimer dissociation. Some discrepancies reported by other authors between experimental data and predictions of a monomer–excimer mechanism do not appear in our results, or we can remove some of these discrepancies, as shown in this paper, by proper choice of values for rate parameters. The results of this paper are applied to the analyses of external quenching data in Paper II.
Behavior of Excited Molecules of Aromatic Hydrocarbons in the Liquid State. II. Benzene and p-Xylene Quenching by O2 and CCl4
Authors: Yguerabide Juan
Published: February 2004

Results are presented on the efficiency of quenching of excited molecules of benzene and p-xylene by O2 and CCl4 as a function of the concentration of these aromatics in cyclohexane. At each aromatic concentration, the aromatic decay-time ratio τ(0) / τ(CCl4) and steady-state fluorescence intensity ratio I(0) / I(CCl4) [where τ(0) and I(0) refer to the aromatic decay time and fluorescence intensity at zero quencher concentration] are linear functions of quencher concentration, i.e., a Stern–Volmer relation is observed. Further, the quenching constant determined from decay-time and steady-state data are equal to each other at each aromatic concentration. Finally, the efficiency of quenching increases with aromatic concentration, the quenching constant for the undiluted aromatic being about two times greater than the quenching constant for very dilute solutions. The results can be explained in terms of the monomer–excimer mechanism discussed in Paper I. This explanation, however, is not unique and does not eliminate the possibility of excitation migration, but indicates that excimers may play an important role in excitation transfer.
Spectrum of Light Scattered from Optically Anisotropic Macromolecules
Authors: Pecora R.
Published: February 2004

Equations have been derived for the spectrum of light scattered from monodisperse, infinitely dilute macromolecular solutions. For rigid molecules it is shown that when the molecules are not very large and/or the scattering is observed at low angles the isotropic part of the scattering spectrum consists of a single Lorentzian line with full-width at half-maximum Δν1 / 2 proportional to the translational diffusion coefficient D, Δν1 / 2 = (16π / λ2)D sin2(θ / 2). For asymmetric molecules whose principal axes for polarizability and diffusion coincide, the anisotropic part of the spectrum consists of a weighted sum of two Lorentzians each with half-width dependent on both the translational diffusion coefficient and combinations of components of the rotational diffusion tensor. For molecules with cylindrical symmetry, the anisotropic part of the scattering spectrum becomes proportional to a single Lorentzian with half-width dependent on both the translational diffusion coefficient and the component of the rotational diffusion tensor describing rotation of the long axis of the cylinder. When the spectrum is integrated over all scattered frequencies, the results reduce to the standard scattering expressions. It is emphasized that the rotational effects described here are due to molecular optical anisotropy and should not be confused with rotational effects in optically isotropic molecules resulting from large molecular size as described in previous papers.
Simple Bond-Charge Model for Potential-Energy Curves of Homonuclear Diatomic Molecules
Authors: Parr Robert G. , Borkman Raymond F.
Published: February 2004

For a homonuclear diatomic molecule near its equilibrium internuclear distance Re, in some bound electronic state, a potential-energy function W(R) of the form W = W0 + W1 / R + W2 / R2 has previously been shown to be a good approximation to the true potential. From this equation and the molecular virial theorem, there follow expressions for the total electronic potential energy V(R) and the total electronic kinetic energy T(R), V = 2W0 + W1 / R, T = −W0 + W2 / R2. The R-dependent, Coulombic part of V is modeled by locating a positive charge Ze at each nucleus and a negative charge −qe at the bond center, with q = 2Z. The Rdependent, free-electron-like part of T is modeled by assuming that the charge q moves freely in a one-dimensional box of length νR. Thus W1 / R = e2(Z2 − 4Zq) / R, W2 / R2 = h2q / 8mν2R2, and W = W0 + (e2 / R) (Z2 − 4Zq) + (1 / R2)[(h2 / 8m) (q / ν2)]. For 17 molecules in 63 different electronic states, parameters q and ν are given that reproduce exactly the experimental equilibrium distance Re and harmonic force constant ke. The ν values obtained vary little from state to state in a given molecule, or through a given row of the periodic table. The average ν values are ν = 1.0, 0.80, 0.75, 0.65 for first-, second-, third-, and fourth-row homonuclear diatomics, respectively. A relation between Re and q is derived, Re(Å) = 2.98 / qν2, and this, together with the observed trends in the q values, shows that q is a reasonable measure of the charge accumulated in the bond region of these molecules. It is suggested that the formula q = (4Re3ke / 7e2)1 / 2 may be a useful definition of the bond order for a given state of a homonuclear diatomic molecule. For fixed ν, this simple point-charge model, and certain generalizations of it, predict Re to be proportional to (1 / q), and the quantity Re5ke to be constant. The one-dimensional-box interpretation is given a justification based on separate virial theorems for the parallel and perpendicular components of the kinetic energy.
Perturbation-Theoretic Approach to Potential-Energy Curves of Diatomic Molecules
Authors: Parr Robert G. , White Ronald J.
Published: February 2004

A perturbation theory is developed whereby the diatomic molecular potential energy W(R) as a function of the internuclear distance R is expressed, for R near Re, as a power series in the parameter λ = 1 − (Re / R), W(λ) = w0 +  ∑ n = 1∞ (wn − wn−1)λn. Truncations of this series have the form of finite power series in R−1. The quantities wn are obtained simply as perturbation energies for a purely kinetic-energy perturbation at Re, by setting up the problem in confocal elliptic coordinates, in which the kinetic-energy part of the Hamiltonian is R−2 times an R-independent operator and the potential-energy part is R−1 times an R-independent operator. Expressions for the successive vibrational force constants ke, le, me, ···, are given, and it is shown how it happens, through cancellation of effects in the molecule near Re against effects in the separated atoms, that truncation of the power series in λ at the λ2 level is often a good approximation, as has been shown empirically.
Hydrogen-Bond Studies. XXX. The Crystal Structure of Hydrogen Bromide Tetrahydrate, (H7O3)+(H9O4)+2Br−·H2O
Authors: Lundgren Jan-Olof , Olovsson Ivar
Published: February 2004

The crystal structure of hydrogen bromide tetrahydrate (HBr·4H2O) has been determined from three-dimensional single-crystal x-ray data recorded at −180°C. Crystals of this compound are monoclinic, space group P21, with four formula units in a cell with the dimensions: a = 7.043, b = 11.466, c = 7.339 Å, β = 110.85°. The structure determination shows that the compound may be formulated as (H7O3)+(H9O4)+2Br−·H2O. The hydrogen-bond lengths within H7O3+ are 2.465 and 2.498 Å, and within H9O4+2.496, 2.587, and 2.588 Å. Other O–H···O bonds in the structure are about 2.75 Å.
Relaxation by Vibration–Vibration Exchange Processes. Part I. Pure Gas Case
Authors: Fisher Edward R. , Kummler Ralph H.
Published: February 2004

The relative relaxation of harmonic and anharmonic oscillators due to vibration–vibration exchange processes is compared through both a numerical and a steady-state analysis to the master equation. The results of this study can be summarized for the case of anharmonic oscillators as: (1) the steady-state vibrational distribution due to vibrational exchange processes is non-Boltzmann in which the upper levels are overpopulated relative to an energetically equivalent Boltzmann distribution; (2) the time to reach a steady-state distribution increases with increasing vibrational level; (3) total vibrational energy is not conserved on the steady-state time scale; and (4) under conditions of high vibrational “temperature” and low kinetic temperature, population inversion in the upper levels is predicted. These conclusions are markedly different than those found in the harmonic model. A discussion is also presented on the vibrational exchange probabilities and it is shown that these processes satisfy an adiabatic theory and, consequently, follow the Landau–Teller temperature dependence.
Magnetic Properties and Dipole Moment of CO
Authors: Stevens R. M. , Karplus M.
Published: February 2004

An extensive coupled Hartree–Fock calculation of the second-order magnetic properties of CO has been made. The susceptibility, shielding, rotational magnetic moment, and spin rotation constants were determined and are in reasonable agreement with the available experimental data. The calculations were carried out for four different internuclear distances; it was found that the error caused by not reoptimizing the basis set for the first-order wavefunction at the changed distances was small. The results for the susceptibility were used to determine the effect of vibrational averaging on the dipole moment obtained from the isotope shift in the rotational magnetic moment. The effect was found to be negligible and the previously determined sign was confirmed.
Vibrational Spectra of Liquid and Crystalline CF4
Authors: Fournier R. P. , Savoie R. , Bessette F. , Cabana A.
Published: February 2004

The Raman and infrared spectra of CF4 have been recorded in its liquid and two crystalline phases. In the liquid, rotational wings along side some of the fundamentals and the appearance of some forbidden infrared bands reveal the high degree of molecular disorder characteristic of this phase. Very little change occurs in the spectra at the liquid–solid transition; the spectra of Phase I thus appear to be those of a plastic crystal. Marked changes are observed upon cooling to Phase II. The splitting of the fundamentals in the Phase II spectra of CF4 is best interpreted in terms of a S4 tetragonal structure, with the molecules lying on sites of S4 symmetry.
Quenching of the Excited Singlet State of Benzene (1B2u) by Mono-olefins and Diolefins
Authors: Morikawa A. , Cvetanović R. J.
Published: February 2004

Fluorescence of benzene vapor has been measured at room temperature in the presence of a number of hydrocarbons in order to determine the rates of collisional deactivation of the excited benzene singlet (1B2u). A distinct although small efficiency is shown by mono-olefins with larger number of alkyl substituents on the double bond, such as trimethylethylene and tetramethylethylene. The rates of fluorescence quenching parallel approximately the corresponding rates of energy transfer from the benzene triplet (2B1u). Conjugated dienes are much more efficient quenchers both of the singlet (1B2u) and of the triplet (3B1u) benzene. Nonconjugated dienes, on the other hand, show in both cases an efficiency which is approximately comparable to that of the mono-olefins.