bluuebunny/crossref_metadata_2025 huggingface dataset:
Microwave Spectrum and Ring-Bending Vibration of 3-Methyleneoxetane
Authors: Gibson James S. , Harris David O.
Published: February 2004

The microwave spectrum of 3-methyleneoxetane, O–CH2–C=CH2,|||–CH2–| in the ground vibrational state and first four excited states of the ring-bending vibrational mode has been observed and assigned. The features of the microwave spectrum and four previously observed far-infrared vibrational transitions have been used to determine the ring-bending potential function, V(Z) = 14;ν0(Z2 + 0.011Z4), where Z is a dimensionless reduced coordinate describing the ring bending and ν0 = 108.60 cm−1. This potential function is nearly harmonic with a single minimum, implying a planar configuration for the equilibrium ring structure. Assuming curvilinear motion of the atoms, an approximate model for the vibration has been developed which reproduces reasonably well the observed rotational constant variations. Using this model, the potential function has been interpreted in terms of the energy contributions from the torsional interactions and ring-angle deformations. The rotational constant data has been used to improve an electron diffraction structural determination using a least-squares technique. The dipole moment for the ground state of 3-methyleneoxetane has been measured as 1.63 ± 0.03 D.
Modification of the Significant Structure Theory of Liquids for Argon at High Pressures and Temperatures
Authors: Tung C. T. , Duvall G. E. , Band W.
Published: February 2004

The significant structure theory of liquids is applied to the shock and isothermal compression of argon. Three models and two types of intermolecular potential are considered. In Model 1, an Einstein oscillator model is used in the solid partition function. In Model 2, a Lennard-Jones and Devonshire cell model is used in the solid partition function. In Model 3, a cell model is used for the entire partition function (this version is independent of the significant structure theory). For the intermolecular potential, exp-6 and 12–6 are adopted separately with numerical parameters obtained from second virial coefficient and high-density shock wave experiments. The Einstein characteristic temperature and sublimation energy at absolute zero are calculated from the potential energy function based on the cell model. The experimental solidus is introduced for the molar volume of the solid. All the calculations are performed in a computer program without which analytical calculations are cumbersome or impossible. It is found that Model 1 with exp-6 potential provides the best fit to measurements. Calculated Hugoniot and isotherms agree well with experimental results.
On the Oscillator Strengths of Model Systems. I. The Double Well
Authors: Dancz John , Harris Robert A.
Published: February 2004

The exact and various molecular orbital formulations of the oscillator strength were investigated in one dimension using square and Dirac delta double-well potentials as models. The Dirac delta potential was abandoned as an unphysical model, but the square well did prove to possess many of the inherent difficulties of the molecular problem. Various representations of the oscillator strength in the molecular orbital approximation were examined and discussed, and the molecular orbital wavefunctions as a single-particle basis was examined and found to be poorer, particularly with respect to excited state properties, than was expected.
Statistical Mechanics of Linear Molecules. V. Pressure Virial Coefficients for the Hard-Core Model
Authors: Chen Yi-Der , Steele W. A.
Published: February 2004

The formalism necessary to evaluate the pressure equation of state for nonspherical hard-core molecules is developed. By making use of previous results, exact second and third pressure virial coefficients are calculated for a diatomic hard-core model. Also, the fourth pressure virial is calculated in the Percus–Yevick approximation. These results are compared with the corresponding compressibility virials and with virial coefficients calculated from the scaled particle theory as applied to spherocylindrical hard-core molecules.
Microwave Spectra and Intramolecular Hydrogen Bonding in the 2-Haloethanols: Molecular Structure and Quadrupole Coupling Constants for 2-Chloroethanol and 2-Bromoethanol
Authors: Azrak Raymond G. , Wilson E. Bright
Published: February 2004

A proposed intramolecular hydrogen bond in the 2-haloethanols has been investigated with analysis of the microwave spectra of 2-chloroethanol and 2-bromoethanol. For each moledule, only a form gauche about the CC bond has been observed. For the chloro compound nine isotopic species have been studied to yield the molecular coordinates of the chlorine, oxygen, hydroxyl hydrogen, and carbon atoms, and thus a molecular structure. For bromoethanol a molecular structure has been found based upon the coordinates of the bromine and hydroxyl hydrogen atoms. Principal structural conclusions are that, of the probable rotameric forms, the lowest energy form is the one allowing close approach of hydrogen and halogen—namely, the gauche–gauche for dihedral angles (CCX) (COH) close to 60°. The O–H bond length in chloroethanol is 1.008 Å, about 5% longer than in ethanol. The H···X distance is ∼ 0.5 Å less than the sum of the atom van der Waals radii for X = Cl, Br. The O···X distance is approximately equal to the sum of the van der Waals radii. The (CCX) (CCO) dihedral angles are quite similar (63.2°, 64.2°, respectively, for X = Cl, Br vs 72.2° for X = F). The OH and CCl bonds are nearly parallel. The quadrupole coupling constants of the chlorine nucleus have been determined from analysis of fortuitous second-order effects: χzz = − 70.0 ± 2 Mc/sec, ηb = + 0.02 ± 0.03, θz = 20′ ± 20′. Approximate coupling constants have been determined for 2-bromoethanol. These quadrupole data reveal that the charge distribution in the vicinity of the halogen atom is similar to that in the corresponding ethyl halide, despite the H···X interaction. Vibrationally exited states for 2-chloroethanol have been assigned to the C–C torsion (155 cm−1), the ∠CCCl bend (310 cm−1), and the C–O torsion (350 cm−1). The relative strengths of the hydrogen–halogen and deuterium–halogen interactions have been considered. These results clearly indicate a hydrogen–halogen interaction in the 2-haloethanols and strongly suggest that this interaction is electrostatic in nature—possibly of the dipole–dipole type.
Phase Equilibrium and Critical Behavior in a Two-Component Bethe-Lattice Gas or Three-Component Bethe-Lattice Solution
Authors: Wheeler John C. , Widom B.
Published: February 2004

The complete coexistence surface is found for a symmetric case of a two-component Bethe-lattice gas or three-component Bethe-lattice solution. It is shown to agree with previously obtained results in the limit of a two-component lattice solution and in the limit of the Bragg–Williams approximation to a three-component solution. The coexistence surface is found to be parabolic in the neighborhood of the plait point in this approximation, and the isothermal coexistence curve remains inside the composition triangle even at absolute zero of temperature. The behavior of fluctuations in concentration, density, and energy in the vicinity of the plait-point curve is investigated, and the density fluctuations are found to have the interesting property that they approach a finite constant at the plait point, the value of which depends upon the path of approach to the plait point. The problem is similar, but not equivalent, to that of the randomly dilute ferromagnet. A proof is given that the quasichemical approximation holds exactly for a Bethe-lattice gas of any number of components.
Emission of Radiation in the Vacuum Ultraviolet by Impact of Electrons on Carbon Monoxide
Authors: Aarts J. F. M. , de Heer F. J.
Published: February 2004

We have measured relative cross sections for emission of radiation in the vacuum ultraviolet in the case of 0.1–5-keV electrons on CO. The molecular transitions studied are the fourth positive band system A 1Π−X 1Σ+ and the Hopfield–Birge band systems B 1Σ+−X 1Σ+ and C 1Σ+−X 1Σ+. Absolute values for the cross sections are obtained by normalization on those evaluated from inelastic scattering experiments of Lassettre and co-workers. The apparent cross sections for the transitions which combine with the zero vibrational level of X 1Σ+ are found to depend strongly on the gas pressure. This is probably caused by trapping of resonance radiation. There are indications in our results, that the electronic transition moment is not constant for the different transitions in the fourth positive band system. Dissociative excitation processes have been studied by observation of atomic radiation of fragment atoms and ions. Applications in the intensity calibration in the vacuum ultraviolet wavelength region are discussed.
Sodium-Atom Excitation in Nitrogen Afterglows
Authors: Milne E. L.
Published: February 2004

The addition of Na metal vapor to nitrogen discharge afterglows in a fast flow system produced strong Na atomic excitation and ionization. Spectroscopic techniques were used to determine the relative populations of several Na atomic levels for Na excitation in the nitrogen pink and Lewis–Rayleigh afterglows. Three sets of Na f values were used. For nitrogen afterglow pressures of 2–10 torr, the populations of the Na 42P and several higher atomic levels appear related by a Maxwell–Boltzmann distribution. Measured Na excitation temperatures were 4500 ± 300°K in the N2 pink and 4000 ± 200°K in the Lewis–Rayleigh afterglows. A double-probe measurement of the electron temperature in the pink afterglow yielded a value of 5000°K. Comparison with earlier work indicates that the Na excitation temperature corresponds to the N2(X1Σ) vibrational temperature. These results support the hypothesis that the Na atomic excitation and ionization are produced by energy transfer between the N2(X1Σ) vibrational states and the Na-atom electronic states.
LCAO–MO–SCF Studies of HF2− and the Related Unstable Systems HF2 and HeF2
Authors: Noble Paul N. , Kortzeborn Robert N.
Published: February 2004

LCAO–MO–SCF calculations were performed on the isoelectronic linear species HF2− and HeF2, and also on the open-shell HF2 radical. Seventy-two Gaussian s- and p-type atomic orbitals were contracted to 33 basis functions for each calculation. Experimental bond lengths, hydrogen-bond energy, and vibrational potential function for HF2- were matched closely by the present calculations. The calculations predict a weak (about 3 kcal/mole) hydrogen bond for HF2 radical, and a long (4.0 a.u.) hydrogen-bond distance between the HF molecule and the F atom. HeF2 is predicted to be an unstable repulsive state, in agreement with other, earlier calculations.
ESR Relaxation Studies on Orbitally Degenerate Free Radicals. II
Authors: Stephen M. R. Das , Wagner B. , Freed J. H.
Published: February 2004

Careful linewidth and saturation studies were performed on several orbitally degenerate hydrocarbon free radicals in liquid solution as a function of temperature, solvent, and counterion, and these are compared with similar studies on nondegenerate hydrocarbon free radicals. The anomalous T1's and T2's attributed to the orbital degeneracy are found to be strikingly independent of all these variables. This is most clearly evidenced for the coronene and triphenylene anion radicals. The results for the benzene and cyclo-octatetraene anions (alkali metal prepared) are complicated by a further contribution to T2−1 which increases with temperature. This contribution is correlated with counterion and solvent dependent ion-pairing effects. The intrinsic temperature-independent T2−1's for the orbitally degenerate free radicals are surprisingly well correlated with the small but anomalous deviations of the g values for these radicals from the Segal, Kaplan, and Fraenkel experimental fit to Stone's theory of g values. This is taken as positive evidence for the role of anomalous spin–orbit interactions in the mechanism(s) yielding the anomalous relaxation times. Their independence of temperature, solvent, and counterion suggests that the relaxation mechanism(s) is largely intramolecular.
Infrared Double Resonance in Sulfur Hexafluoride
Authors: Steinfeld J. I. , Burak I. , Sutton D. G. , Nowak A. V.
Published: February 2004

The relaxation of sulfur hexafluoride has been monitored by observation of infrared absorption intensities following passage of an infrared laser pulse through the gas. The laser pulse saturates a small number of rotational lines in the ν3 ← 0 transition of SF6 at 944 cm−1, producing a transient “hole” in the absorption spectrum at that frequency. This hole is filled in very rapidly by rotational relaxation processes. The specific vibrational excitation is, at essentially the same time, transformed into a vibrational temperature in excess of the gas's translational temperature, by means of very efficient vibration ↔ vibration energy transfer collisions. The energy released in this step also heats the gas translationally by the order of 25°K. The vibrational temperature then relaxes to the translational temperature by a binary collision process, with pτ = (122 ± 8) μsec·torr in pure SF6. The rate-controlling step is concluded to be the vibration→translation relaxation of the ν6 level at 363 cm−1. The mean efficiency per collision of this step has been measured for He, Ne, Ar, Kr, C2H6, (CH3)2O, CH4, CH3Br, CHF2Cl, H2O, H2, N2, and Cl2, in addition to SF6 itself. A transient absorption spectrum arising from the vibrationally hot SF6 is also given. The slowest relaxation observed in the system is the bulk cooling of the translational temperature of the gas, which occurs in the order of milliseconds under typical experimental conditions. The dominant mechanism for this relaxation appears to be cooling by thermal conduction.
Interference Function Approach to Knight Shift in Liquid Metals
Authors: Halder N. C.
Published: February 2004

It is shown in this paper that a structural dependence of the atomic interference function, i.e., the structure factor I(K), is of considerable importance in the study of the fractional change of the Knight shift ΔS / S (contact term) and nuclear spin relaxation time T1 with temperature in liquid metals. A perturbation expansion of the pseudowave function has been made to calculate the electron densities, where the matrix elements are expressed in terms of the pseudopotential and interference function. The predicted values of ΔS / S in liquid In, Tl, Pb, and Sn show a linear relationship with the temperature. When these results are used in the well-known Korringa relation, we get T1 values which are little higher than the experimental values. However, they compare reasonably well with those which can be obtained by using the Pines theory that is based on the many-body effect of the electrons for the spin paramagnetic susceptibility. Qualitatively, the predicted results for ΔS / S and T1 are in good agreement with the recent experimental results.
Exp-6 Cell Model Calculations for the Inert Gas Solids. I. Characterization of Pair Potential Parameters with and without the Triple-Dipole Term
Authors: Utting Barry D. , Walkley John
Published: February 2004

A quantum version of the Lennard-Jones and Devonshire cell model is developed for the exp-6 pair potential. The three characteristic reduction parameters of this potential are determined from zero-point thermodynamic data for solid neon, argon, krypton, and xenon. Calculations are made with and without the consideration of the long-range triple-dipole term in the static lattice potential. Consideration is then given to the available second virial low density gas data for argon and krypton. It is shown that those parameters determined with the inclusion of the three-body term given a good prediction of the second virial data. These parameter values are considerably different from those obtained by a direct fit of the potential to the virial data. It is nevertheless suggested that using these parameters the exp-6 potential acts as an acceptable pair potential.
Exp-6 Cell Model Calculations for the Inert Gas Solids. II. Thermodynamic Properties—The Effect of the Triple-Dipole Term
Authors: Utting Barry D. , Walkley John
Published: February 2004

Using the potential parameters developed in Part I of this series of papers we now present a comparison of the theoretically predicted and experimentally measured thermodynamic properties of the four inert gas solids, neon, argon, krypton, and xenon. It is shown that without the inclusion of any three-body term the exp-6 potential gives a good prediction of most available data. The predicted data are sensitive to the inclusion of the three-body term, but its inclusion in the calculations removes the good agreement with experiment. Some discussion is given to the usefulness of the “effective pair additive” potential, for which the exp-6 is obviously a better choice than is the commonly accepted Lennard-Jones (12–6). The calculations further show that the prediction of the solid state properties using a cell model, and including the triple-dipole nonadditive term, can indicate the unacceptability of a pair potential which nevertheless is able to accurately predict second virial gas data. The exp-6 potential, when parameters are characterized from solid state data with the inclusion of the three-body term in the calculation, is only partially successful in the prediction of either second virial or solid state data but at least shows the likelihood of a pair potential being successful in these two greatly disparate tasks.
Application of an Approximate Percus–Yevick Equation for Liquid Water
Authors: Ben-Naim A.
Published: February 2004

An approximate Percus–Yevick equation for liquid water is suggested. The approximation involves the splitting of the potential of average force for two particles into a sum of a direct, angle-dependent pair potential and an angle-independent indirect potential. The angle-dependent pair potential is chosen in such a way that the characteristic tetrahedral geometry for the relative orientation of two water molecules is favored. Employing this potential function in the integral equation produces a pair correlation function which carries the main features of the experimental pair correlation function for liquid water.
Theory of the Hyperfine Splittings of Pi-Electron Free Radicals. III. Methyl Radical in a Pyramidal Configuration: Temperature Dependence of the Hyperfine Splittings
Authors: Chang S. Y. , Davidson E. R. , Vincow G.
Published: February 2004

Nonempirical calculations for methyl radical (CH3·) in a pyramidal configuration (C3υ) were performed using two minimum basis sets of Slater-type orbitals, one in which orbital exponents were chosen according to Slater's rules (unoptimized) and the other in which they were optimized by minimization of the SCF energy. The spin-restricted SCF plus configuration-interaction method, including all spin-adapted configurations with single and double excitations of space orbitals, was employed. The temperature dependence of the contact hyperfine splittings was computed assuming that the variation with temperature arises from the out-of-plane bending motion. Agreement with experiment at various temperatures is good. The optimized-basis values of the temperature coefficient of the proton splitting, daH / dT, are about 20%–50% too large and the unoptimized-basis results are about 10%–25% too small. Temperature coefficients of the carbon-13 splitting calculated using the optimized and unoptimized basis sets also bracket the experimental values, from which they deviate by less than 10%, which is within the experimental uncertainty. The variation of the spin densities at the magnetic nuclei with the out-of-plane angle is analyzed in detail. Other results of the calculation are discussed, namely, the equilibrium molecular geometry, an approximate frequency for the bending motion, and the incomplete orbital following.
Crystal Structure of Luminescent ZnSiP2
Authors: Abrahams S. C. , Bernstein J. L.
Published: February 2004

ZnSiP2 crystallizes with the chalcopyrite structure: there is no detectable difference in x-ray scattering between luminescent and nonluminescent crystals. The level of impurity, either in the form of chemical dopant, physical defect, or a departure of less than about 1% from complete order at the Zn and Si sites hence determines if the crystal luminesces or not on excitation by an electron beam. The lattice constants of this tetragonal crystal at 298°K are a = 5.399 ± 0.001 and c = 10.435 ± 0.002 Å. The space group is I4̄2d, and there are four formulas in the unit cell. A total of 2023 reflections were measured with the luminescent crystal, and 1600 with the nonluminescent crystal, using pexrad. The final agreement factor, based on a combination of 197 symmetry-independent reflections from the first crystal and 176 from the second, is 0.049. Each Zn is tetrahedrally surrounded by 4 P atoms at 2.375 ± 0.001 Å distance, and each Si by 4 P atoms at 2.254 ± 0.001 Å. All phosphorus tetrahedra have four P–P edges of length 3.832 ± 0.002 Å. The two remaining independent P–P edges are of 3.968 ± 0.001 Å distance in the ZnP4 tetrahedra, and are of 3.684 ± 0.001 Å in the SiP4 tetrahedra.
Elementary Model of the Broadening of Localized Transitions in a Simple Liquid
Authors: Greer William L. , Rice Stuart A. , Morris Graeme
Published: February 2004

The line shapes corresponding to localized electronic or vibrational transitions in a simple liquid are studied using a model. The model is defined by a Hamiltonian taken to be linear in the instantaneous force acting on a molecule. One consequence of this assumption is the appearance of the force autocorrelation function in the line-shape formulas. It is shown that in the weak coupling limit, in the lowest order of approximation, the transition line shape is Lorentzian. In the next approximation the line shape has a frequency dependence given by the convolution of the lowest-order Lorentzian with the frequency-dependent force autocorrelation function. Some implications of the model are discussed, and a rationalization given to interpret variations in the line shapes of Raman transitions corresponding to totally symmetric and nontotally symmetric vibrations.
Molecular g Values, Magnetic Susceptibility Anisotropies, Second Moments of the Electronic Charge Distribution, and Molecular Quadrupole Moments in Pyridine
Authors: Wang J. H. S. , Flygare W. H.
Published: February 2004

The high-field linear and quadratic Zeeman effect has been observed in pyridine. The spectra is complicated by the presence of the 14N nuclear quadrupole coupling. Perturbation theory through second order is used to extract the molecular Zeeman parameters. The molecular g values are gaa = − (0.0770 ± 0.0005), gbb = − (0.1010 ± 0.0008), and gcc = 0.0428 ± 0.0004. The magnetic susceptibility anisotropies are (2χaa − χbb − χcc) = (54.3 ± 0.6) × 10−6 and (2χbb − χaa − χcc) = (60.5 ± 0.8) × 10−6 in units of erg/gauss2·mole. The a axis bisects the CNC angle and the b axis is also in the molecular plane. The molecular quadrupole moments are Qaa = − (3.5 ± 0.9), Qbb = 9.7 ± 1.1, Qcc = − (6.2 ± 1.5) all in units of 10−26 esu·cm2. Using the known molecular structure and the molecular g values gives the diagonal elements in the paramagnetic susceptibility tensor and the anisotropies in the second moment of the electronic charge distribution. These results are χaap = 241.5 ± 1.5, χbbp = 247.4 ± 2.0, and χccp = 393.9 ± 2.0 all in units of 10−6 erg/G2·mole and 〈a2〉 − 〈b2〉 = 0.92 ± 0.80, 〈b2〉 − 〈c2〉 = 48.28 ± 0.60, and 〈c2〉 − 〈a2〉 = − 49.19 ± 0.60 all in units of 10−16 cm2. Combining the above values with the known bulk magnetic susceptibility gives the individual diagonal elements in the total and diamagnetic susceptibility ternsors. The individual values of 〈a2〉, 〈b2〉, and 〈c2〉 are also obtained. The results are compared to other ring compounds.
Dynamic Nuclear Polarization in Phosphorus Compounds with Perchlorotriphenylmethyl and Other Radicals
Authors: Poindexter Edward H. , Neil George R.
Published: February 2004

Dynamic nuclear polarization has been examined in liquid solutions containing the radical perchlorotriphenylmethyl and 14 different phosphorus compounds. In addition, the radicals bisdiphenylenephenylallyl, tri-t-butylphenoxyl, and diphenylpicrylhydrazyl have been examined with five phosphorus compounds. The chlorocarbon radical shows on the average the least scalar coupling of the four. This is in accord with the well-shielded spin distribution on this radical and the great chemical stability of aromatic chlorocarbons. PCTM is found to be almost completely inert, both electronically and chemically, toward phosphines and phosphites. Exceptional cases of high coupling were observed with PCTM and dimethylphosphite, triphenylphosphine oxide, and hexamethylphosphoramide. The first case appears to be due to a transient hydrogen bond Cl···H–P, the second due to an exposed polarizable orbital system, and the last due to an obscure chemical complexation tendency. Exceptional cases with the other radicals serve to illustrate further stereospecific molecular couplings between sites of active chemical or electronic attraction for each other.
Observed Intensity and Speed Distributions of Thermal-Energy Argon Atoms Scattered from the (111) Face of Silver
Authors: Bishara M. N. , Fisher S. S.
Published: February 2004

The scattering of thermal-energy monatomic gases from metal surfaces is studied using the Ar–Ag system. The argon atoms are incident to the surface in the form of nearly monoenergetic beams from a gas-dynamic nozzle source. The target surface is the polished (111) face of a monocrystalline silver disk. In studying this interaction, essentially complete maps of the relative intensities and speeds of the scattered atoms are obtained using a time-of-flight method. The argon beams have energies ranging from 0.06 to 0.17 eV and are incident at angles varying from 40° to 60° from the target normal. The surface temperature ranges from 550 to 790°K. Due to surface roughness, approximately 80% of the incident atoms are scattered diffusely while the remainder are scattered into a crest-shaped lobe centered subspecularly within the plane of incidence. To a first approximation, it is shown that the speed distributions of the diffusely scattered components are independent of scattering angle and that they are nearly Maxwellian in form and exhibit mean speeds corresponding to high thermal accommodation to the surface. The lobularly scattered atoms show a clear tendency to conserve their momentum tangential to the surface. Moreover, it appears that the characteristics of the two component beams are relatively independent of the fraction scattered into each. General properties of the scattered beams are discussed and comparisons with applicable studies are offered.
Proton NMR Investigation of Hindered Methyl Rotation in Paramagnetic Cr(II) Complexes
Authors: La Mar Gerd N. , Van Hecke Gerald R.
Published: February 2004

It is shown that in a paramagnetic complex, methyl rotation can act as an effective correlation time for spin relaxation through the Fermi contact interaction. For unhindered methyl groups, this mechanism can lead to methyl line narrowing. In the limit of slow methyl rotation resulting from steric hinderance, methyl rotation is capable of modulating the average hyperfine interaction (contact shift), which leads to methyl line broadening similar to that observed from chemical exchange effects. The proton linewidths of a series of tris-Cr(II) chelates with methyl-substituted ortho-phenanthrolines have been investigated. The linewidths of the aromatic protons in all complexes are determined by primarily the dipolar interaction. The linewidths of methyl groups which are expected to be sterically hindered are considerably broader than accountable by either dipolar or hyperfine exchange relaxation. We conclude that the anomalous methyl linewidths arise from modulation of the average hyperfine interaction by methyl rotation. The temperature dependence of the observed linewidths yields the barrier to methyl rotation.
Electron Paramagnetic Resonance Study of Ordered Mn2+ in Mg2SiO4
Authors: Chatelain A. , Weeks R. A.
Published: February 2004

The electron paramagnetic resonance of Mn2+ in natural forsterite, both single crystal and powder, has been observed at 35 GHz. The spectrum is interpreted in terms of a spin Hamiltonian of orthorhombic symmetry with two equivalent sites related by a 67.5° rotation around the c axis. Hence, the Mn2+ ions occupy either the M(1) or the M(2) positions in the crystal. Neither heat treatment nor proton irradiation showed disordering of the Mn2+ ions up to 600°C and 3 × 1017 protons/cm3, respectively.
Inversion of the Partition Function. II. The Full Steepest-Descent Method
Authors: Hoare M. R.
Published: February 2004

The steepest-descent method for the computation for energy-level densities, g(E), described by Hoare and Ruijgrok [J. Chem. Phys. 52, 113 (1969)] is here extended to higher orders giving an asymptotic expansion of g(E) in inverse powers of the number of degrees of freedom N. The higher terms take the form of cumulant-type expressions related to the specific heat of the system. When tested against exactly counted levels for harmonic oscillators, the second-order term gives a useful improvement on the first-order prediction, the third-order term is unimportant, and fourth and higher corrections completely negligible. In all cases examined the series appears to diverge in the higher orders only for uninterestingly small energies.
Revised General A + BC Potential for Trajectory Studies
Authors: Bunker Don L. , Parr Christopher A.
Published: February 2004

A previously proposed adjustable potential-energy expression [D. L. Bunker and N. C. Blais, J. Chem. Phys. 41, 2377 (1964)] has been modified and extended. Besides its former properties, it now has variable cross section and provision for potential barriers and bound intermediates.
Calculation of the Energy Band Structures of Complicated Periodic DNA Models in the Semiempirical SCF–LCAO–Crystal-Orbital Approximation
Authors: Rozsnyai B. F. , Martino F. , Ladik J.
Published: February 2004

The electronic wavefunctions and energy bands of periodic DNA models consisting of two superimposed base pairs as unit cells have been calculated in the semiempirical SCF–LCAO–crystal-orbital approximation. The results obtained for poly(A–T; G–C), poly(A–T; C–G), poly(A–T; T–A), and poly(G–C; C–G) indicate that the bandwidths, in contrast to those previously calculated for poly(A–T) and poly(G–C) cases, are quite narrow.
Assignment of the Excited Electronic States of Two Methylazabenzyl Radicals
Authors: Johnson Philip M.
Published: February 2004

The radicals 3,5-dimethyl-4-azabenzyl and 3-methyl-2-azabenzyl have been prepared from s-collidine (2,4,6-trimethylpyridine) and 2,6-lutidine (2,6-dimethylpyridine) by photolysis in a rigid organic solution and studied by means of three-step photoselection. Two excited states of the radicals are found, one at about 2.6 eV and the other at about 4.1 eV. These are assigned the symmetries 2A2 and 2B2 (in C2υ), respectively. The phosphorescence of s-collidine in an organic matrix is described. The novel technique of “changing-axis” photoselection is described and is used in the assignment of the 3-methyl-2-azabenzyl radical. This technique also substantiates the in-plane character of the transition moment of 2,6-lutidine in its 270-nm absorption band.
Dissociation Energies and Chemical Bonding in the Alkaline-Earth Chlorides from Mass Spectrometric Studies
Authors: Hildenbrand D. L.
Published: February 2004

Gaseous equilibria involving the mono- and dichlorides of Mg, Ca, Sr, and Ba were studied by mass spectrometry, and the results were used to derive the dissociation energies of the diatomic and triatomic metal chlorides. Values of D0° (MCl) were found to be 75.1, 94.2, 95.9, and 105.0 kcal for MgCl, CaCl, SrCl, and BaCl, respectively, all ±3 kcal. For the reaction M(g) + MCl2(g) = 2MCl(g), the heat of which is equivalent to D(ClM–Cl)–D(M–Cl), ΔH0° = 35.0, 21.5, 16.7, and 7.2 kcal for Mg, Ca, Sr, and Ba, respectively. The dissociation energies of the diatomic metal chlorides are in general substantially higher than those inferred from electronic band spectra, but the thermochemical and spectroscopic D0° values can be very satisfactorily reconciled if one considers the degree of ionic character and its effect on the rate of convergence of the higher (unobserved) vibrational energy levels. As expected, predictions of the Rittner electrostatic model agree well with the measured dissociation energies of the heavier metal chlorides, whereas the beryllium and magnesium chlorides must be relatively covalent.
Mass Spectrometric Study of Ion–Neutral Reactions in Radio-Frequency Discharges in Carbon Dioxide
Authors: Schildcrout Steven M. , Collins J. G. , Franklin J. L.
Published: February 2004

Abundances of positive ions and neutral molecules emanating from an active radio-frequency discharge in CO2 at pressures from 0.01 to 1 torr have been determined mass spectrometrically. The data are used to evaluate rate constants for ion–neutral reactions occuring in the discharge. The results are in agreement with previous results for reactions occuring in an electron-impact ion source and in thermalized flowing afterglows. The large yield of O2+ at the higher discharge pressures is attributed mainly to charge transfer to O2, the extent of which is enhanced by the long retention (and hence reaction) time resulting from the low mobility of CO2+ at the higher pressures.
Experimental Identification of the Ground State of BN in a Rare-Gas Matrix
Authors: Mosher Oren A. , Frosch R. P.
Published: February 2004

The free radical BN has been obtained in Ne and Ar matrices by photolyzing H3BNH3. The radical was identified by observing the A 3Π ← X 3Π transition which is analogous to the C2 Swan bands. It was found that the ground state of BN in an Ne matrix is 3Π. No significant vibrational perturbation was observed. The 0–0 band is at 27 931 and 27 721 cm−1 in Ne and Ar, respectively, compared with a gas-phase value of 27 776 cm−1. The value of ΔG1/2 in the upper state in Ne is 1288 cm−1 which, within experimental error, is identical with the gas-phase value.
Absorption Spectrum of H2 in the Vacuum-uv Region. II. Rydberg Series Converging to the First Six Vibrational Levels of the H2+ Ground State
Authors: Takezawa Sanzo
Published: February 2004

The absorption spectrum of H2 at liquid-nitrogen temperature has been further investigated with a 6.65-m vacuum spectrograph in the 800–600-Å region. Eleven distinct groups, each consisting of 8–30 absorption lines, have been observed at wavelengths converging to the levels υ = 0–5 of the H2+ ground state. Evidently these individual groups are Rydberg series. Consideration of l-uncoupling effects permits these series to be assigned as R(0), R(1), and Q(1) series in npπ ← X transitions and as R(0) series in npσ ← X transitions. A new value of the ionization energy of H2, 124 418.4 ± 0.6 cm−1, has been obtained from Rydberg series converging to the υ = 0 level of the H2+ ground state.
Chlorine NMR in CsCoCl3 and CsMnCl3 and ESR in CsMg(Co)Cl3
Authors: Rinneberg H. , Hartmann H.
Published: February 2004

Analysis of the 35Cl NMR in single crystals of CsMgCl3, CsMnCl3 at 300°K and of CsCoCl3 at different temperatures yielded the chlorine quadrupole coupling and for the latter two compounds the paramagnetic shift tensors. We found at 300°K: for CsMgCl3, νQ = 4.465 Mc/sec, η = 0.235; for CsMnCl3, νQ = 5.2645 Mc/sec, η = 0.2683 (bent bridge), νQ = 9.1835 Mc/sec, η = 0.0234 (linear bridge); for CsCoCl3, νQ = 6.87925 Mc/sec, η = 0.4293. From the ESR spectrum of CsMg(Co)Cl3, the g and hyperfine tensors were determined (g‖ = 7.37, g⊥ = 2.51, A‖(Co) = 339 × 10−4cm−1, A⊥(Co) = 21 × 10−4cm−1). Using these data and Dq = 660 cm−1, EP = 12 000 cm−1 inferred from the optical spectrum, the spin expectation values 〈S‖〉 and 〈S⊥〉 were calculated for CsCoCl3. The following superhyperfine constants parallel (A‖) and perpendicular (A⊥) to the Me–Cl bond are computed from the measured paramagnetic shifts: A‖ = 1.59, A⊥ = 1.40 (CsMnCl3, ClI, linear bridge), A‖ = 1.68, A⊥ = 1.70 (CsMnCl3, ClII, bent bridge), A‖ = 4.74, A⊥ = 1.65 (CsCoCl3), all in units 10−4 cm−1. No transition to the ordered magnetic state has been observed for CsMnCl3 for temperatures down to 20°K, contrary to ESR data in the literature.
Kinetics and Thermodynamics of the CN Molecule. I. Decomposition Mechanism for CN Molecules
Authors: Slack M. W. , Fishburne E. S.
Published: February 2004

Two different reaction mechanisms, CN+CN→C2+N2 and also CN+M→C+N+M, have been reported for the decomposition of CN molecules behind shock waves. The current analytical study demonstrates that these two observed mechanisms are special cases of the generally applicable CN decomposition chain, CN+M→C+N+M, C+CN→C2+N, N+CN→N2+C, in which the first reaction or a combination of the second and third reactions can dominate, depending on temperature and CN concentration.
Reactions of F2O in Shock Waves. II. Kinetics and Mechanisms of the F2O–CO Reaction
Authors: Henrici Hans , Lin M. C. , Bauer S. H.
Published: February 2004

The kinetics of the reaction between oxygen difluoride with carbon monoxide were studies in shock tubes between 800 and 1400°K. Some of the experiments were done with a single pulse tube; these provided data on the production of CO2, O2, and F2CO. In addition time-resolved optical measurements were made behind incident shock waves. The rate of formation of CO2 was monitored through its emission at 4.3 μm and, simultaneously, the decrement in F2O through its absorption at 220 nm, for the reaction OF+CO→ lim k4CO2+F. Computer analysis of the experiments yielded k4 = 7.5 × 1010cm3mole−1·sec−1, independent of temperature. This value is believed to be correct within a factor of 2 between 900 and 1400°K. Combining of our earlier data on the reaction F2O+M→ lim k1F+OF+M with the high-temperature results from this work and the results from another independent work, we obtain: k1 = 1016.7 exp(−40 500 / RT) cm3mole−1·sec−1 from 770 to 1230°K. The error in activation energy is about ±3 kcal/mole. The reaction between F2O and CO leads to the production of significant amounts of F2CO, which results in a temperature increase which becomes more important at the lower temperatures. The heating effect, on the basis of the proposed mechanism, was found to be strongly dependent on the concentration of the FCO radical, which is believed to be the intermediate in the formation of F2CO. The thermochemistry of the FCO radical is discussed. We also very briefly investigated the possibility that Reaction (4) produces vibrationally excited CO2, with inverted (001)/(100) populations, by measuring the transmitted intensity of 10.6-μm radiation from a CO2 laser. No gain was observed, and we concluded that if the initially formed CO2 is highly vibrationally excited, under the conditions of these experiments the extent of inversion was not sufficient to permit detection with our technique.
Theory of Vibronic Coupling in the Polariton States of Molecular Crystals
Authors: Philpott Michael R.
Published: February 2004

A theory of the vibronic coupling in molecular exciton states in the presence of a radiation field is developed. Electronic and nuclear motion in isolated molecules is assumed to be separable, crystal states with two or more molecular excitations on different sites are excluded, and exciton–lattice phonon interactions ignored. The coupling process is examined, without using perturbation theory, from three points of view. In the first method solutions of a classical oscillating dipole model are derived. The second method develops the equivalent quantum theory, without the restriction to dipole interactions, by a one-step diagonalization of the crystal Hamiltonian. In the third method explicit solutions of Agranovitch's polariton theory are obtained. The latter is also extended to include photon Umklapp processes and a connection established with calculations which use retarded dipole sums. It is shown that the polariton branches, corresponding to a stack of vibronic levels belonging to the same electronic state of the molecule, are coupled by a cooperative (or collective) interaction. As a result for intense transitions (f≃3) the lowest Coulombic exciton and the highest k = 0 polariton can be pushed far away from the other states and transformed into cooperative states. The exciton carries with it practically all the intensity of the original transitions and the k = 0 polariton leads to the phenomenon of metallic reflection. The connection with previous treatments of the transition from weak to strong vibronic coupoing is discussed and the concept of very strong coupling defined as an extreme Simpson–Peterson limit.
Scattering of Potassium from Halogen Compounds at Hyperthermal Energies
Authors: Kempter V. , Kneser Th. , Schlier Ch.
Published: February 2004

A potassium atomic beam with energies from 0.5 to 12 eV has been produced by sputtering. The differential elastic cross sections for scattering from C6H12, CCl4, SiCl4, SnCl4, Cl2, and Br2 have been measured. The known behavior of these systems at thermal energies is reproduced to some extent, but there are distinct differences at higher energies for SnCl4, Cl2, and Br2. These may possibly be explained by the onset of nonadiabatic transitions between almost-crossing energy surfaces.
Crystal Structure of the Solid Electrolyte [(CH3)4N]2Ag13I15
Authors: Geller S. , Lind M. D.
Published: February 2004

The crystal structure of [(CH3)4N]2Ag13I15 has been determined from single crystal x-ray diffraction counter data. The structure belongs to space group R32-D37 and contains one (three) [(CH3)4N]2Ag13I15 in a rhombohedral (hexagonal) unit cell with dimensions a = 11.52 ± 0.02 Å, α = 67.35 ± 0.15° (a = 12.77 ± 0.03, c = 26.54 ± 0.05 Å). All iodide ions are at the corners of face-sharing tetrahedra. These tetrahedra are arranged in two kinds of channels, one of which ends at the crystal surfaces; the other is a 10-tetrahedra channel beginning at one tetramethylammonium ion and ending at another. The former are probably the more important for the diffusion of the Ag+ ions through the crystal. The Ag+ ions may diffuse from one channel to another via interconnecting face-sharing tetrahedra. One out of eight crystallographically nonequivalent tetrahedra is not in either kind of channel, but also links channels. There are 123 tetrahedra and 39 Ag+ ions in each triply primitive hexagonal cell. The distribution of Ag+ ions over the tetrahedral sites is significantly nonrandom. The lower symmetry of [(CH3)4N]2Ag13I15, the larger volume required for the [(CH3)4N]+ ions, and the smaller ratio of iodide tetrahedra to Ag+ ions are probably among the reasons that the average ionic conductivity of [(CH3)4N]2Ag13I15 is lower than that of RbaG4I5.
Ethylene Adsorption on Pt(111)
Authors: Smith Donald L. , Merrill Robert P.
Published: February 2004

The scattering of helium and deuterium beams has been used along with LEED to establish the cleanliness and order of a Pt surface under ultrahigh-vacuum conditions and to study the adsorption of ethylene on the (111) face. Ethylene adsorbs irreversibly according to first-order Langmuir kinetics with a sticking coefficient of unity, and dissociates upon adsorption into an acetylenic species which occupies four platinum sites and two mobile hydrogen atoms. At high coverages the ethylene is ordered, producing a (2 × 2) LEED pattern. Above temperatures of 200°C irreversible degradation of the adsorbate occurs. The use of both atomic scattering and LEED seems to be uniquely complimentary for the study of adsorption on clean surfaces of single crystals.
Thermal Conductivity of Gaseous and Liquid Hydrogen
Authors: Roder H. M. , Diller D. E.
Published: February 2004

The thermal conductivity of gaseous and liquid hydrogen has been measured with a guarded horizontal flat-plate calorimeter at temperatures between 17 and 200°K and at pressures to 15 MN/m2. The data have been analyzed as a function of density at fixed temperatures and as a function of temperature at fixed densities. Outside the critical region the thermal conductivity of both the gas and the liquid increases continuously with temperature and density. In the compressed liquid the temperature derivative at fixed density is positive and unusually large compared to that for most other simple liquids. In the critical region the thermal conductivity increases rapidly with both temperature and density as these parameters approach their critical values. The thermal conductivity of the dilute gas is consistent with the kinetic theory expression, K0(T) = fME(T)Cυ0(T)η0(T) / M, and the dilute gas viscosities to better than 3%.
Linewidths in High-Resolution NMR Spectra. Homogeneous Broadening Due to Intermolecular Relaxation
Authors: Sýkora Stanislav
Published: February 2004

The effects connected with the intermolecular relaxation in the coupled nuclear systems are studied theoretically. It was shown that, because of the general properties of nuclear-spin systems, the number of parameters necessary to describe the relaxation process is reduced substantially. Moreover, the structure of nuclear system and relaxation process parameters are to a far extent separated. This fact enables one to draw quite detailed conclusions on the energy-level widths even without any specification of the type of intermolecular relaxation process.
New Partitioning Perturbation Theory. I. General Formalism
Authors: Certain Phillip R. , Hirschfelder Joseph O.
Published: February 2004

By the use of partitioning techniques, a general formalism is developed for considering degenerate, almost-degenerate, and electron-exchange perturbation problems. In effect, we generalize the Van Vleck–Kirtman approach to arbitrary orders and arbitrary normalization and obtain three types of approximations: In the modified Kirtman treatment the functions through the Nth order are fully normalized and the energies are obtained as the roots of the secular equation. The DE–FOP–VIM approximation is the same except that the normalization of the functions is energy optimized. The Kirtman approximation uses the same functions as the modified Kirtman but the energies are obtained as the roots of a much simpler secular equation which results from a factorization of the original secular equation (except for terms of order 2N + 2). The Kirtman energies are not upper bounds. Löwdin's formalism is equivalent to the Modified Kirtman with the exception that Löwdin uses intermediate normalization. Electron exchange problems are considered more explicitly in a companion paper with the use of symmetry considerations.
Shock Wave Compression of Benzene, Carbon Disulfide, Carbon Tetrachloride, and Liquid Nitrogen
Authors: Dick Richard D.
Published: February 2004

Hugoniot data to several hundred kilobar have been obtained for benzene, carbon disulfide, carbon tetrachloride, and liquid nitrogen. Standard high explosive techniques were used for generating the shock waves. Experimentally measured quantities were transformed to pressure and volume data by the impedance match method. The shock-particle velocity data for the liquids are described by a linear relationship, however, a quadratic in particle velocity also provides an adequate representation of the data for carbon tetrachloride and liquid nitrogen. Benzene undergoes a transition at 133 kbar and carbon disulfide at 62 kbar. These transitions are accompanied by a volume decrease of approximately 16%. A double shock-wave structure, observed in many solids which undergo a transition, was not observed in benzene and carbon disulfide. There is some evidence that carbon tetrachloride and liquid nitrogen undergo a transition at 165 and 135 kbar, respectively. Hugoniot curves calculated from a Lennard-Jones and Devonshire (6-9) and a modified Buckingham exp-6 intermolecular potential fit the liquid nitrogen experimental Hugoniot curve between 20 and 170 kbar.