bluuebunny/crossref_metadata_2025 huggingface dataset:
Cluster Expansion Method for the Density of Excited Electronic States of Substitutionally Disordered Crystals
Authors: Hoshen Joseph , Jortner Joshua
Published: February 2004

In this paper we introduce a cluster expansion method which is suitable for numerical calculations of the density of electronically excited states of substitutionally disordered molecular crystals. This method is based on an expansion of the density of states of the guest subband in terms of the localized densities corresponding to various clusters and is valid for a large scattering potential and for concentrations of guest molecules below the percolation concentration. The general features of the cluster expansion method are illustrated by calculations for a simple one-dimensional molecular crystal. These numerical calculations for the one-dimensional model are compared with the results obtained by the coherent potential approximation. The role of the localized states, which are not accounted for by the coherent potential method can be elucidated.
NMR Studies of a Series of Phenyl Substituted Ferric N-Ethyl-N-Phenyldithiocarbamates
Authors: Golding R. M. , Lukeman B. D. , Sinn E.
Published: February 2004

The proton NMR results of a series of phenyl substituted tris(N-ethyl-N-phenyldithiocarbamato)-iron(III) complexes have been successfully interpreted assuming the iron atom is in an intermediate crystal field of octahedral symmetry. The substituent in the phenyl ring has very little effect on the energy separation of the 6A1 and 2T2 electronic states. However, the substituent substantially influences the methylene proton isotropic hyperfine interaction constants in the 6A1 state. These results have been interpreted as arising from changes in the π character of the N-phenyl bond due to the degree of hindered rotation with the aromatic substitution.
Stochastic Model of Molecular Luminescence
Authors: Lin S. H.
Published: February 2004

A general kinetic mechanism for molecular luminescence including vibrational relaxation has been proposed and solved analytically for the case in which the vibrational relaxation is faster than the electronic relaxation and for the case in which the electronic relaxation is faster than the vibrational relaxation. The purpose of this paper has been to investigate the role played by the vibrational relaxation in molecular luminescence and how the vibrational relaxation affects the measurements of the lifetimes of electronic states under various experimental conditions. We have theoretically shown that if the rate of vibrational relaxation is much faster than that of electronic relaxation, the rate constants (radiative and nonradiative) experimentally determined represent the equilibrium microcanonical averaged rate constants or the equilibrium thermal averaged rate constants depending on whether the system is isolated (collision free) or thermal, and if the rate of electronic relaxation is much faster than that of vibrational relaxation, for small molecules the rate constants determined are the rate constants of single vibronic states if the system is selectively excited.
Single Ion Transitions in Ionic Diatomic Molecules
Authors: Hamill William H.
Published: February 2004

The conventional model for free-ion electronic transitions in KCl:Tl+ and related systems implies comparable transitions in diatomic M+X− for suitable M+, e.g., alkaline earths. Correlations are facilitated by comparing optical bands of M+X− with term values of the nearest isoelectronic neutral atom. The available data for the diatomic halides CaX, SrX, BaX, SnX, HgX, and PbX provide evidence for single ion transitions.
Electrostatics of Diffuse Anisotropic Interfaces. III. Point Charge and Dipole Image Potentials for Air-Water and Metal-Water Interfaces
Authors: Clay John R. , Goel Narendra S. , Buff Frank P.
Published: February 2004

Within the framework of macroscopic continuum theory, the point charge and point dipole image potentials in the presence of a diffuse dielectric interface are calculated. We consider the cases of both short and long range diffuseness for air-water interfaces. In the first category we propose a model profile for which the charge image potential is devoid of singularities and is an integral over elementary functions. The calculated image potential goes through a maximum and minimum as the charge is taken from the high to the low dielectric constant medium. The heights of the maximum and minimum may be estimated in terms of a physical cutoff, characteristic of the gradient of the dielectric constant, D, in the diffuse region, using the crude model of an abrupt interface. For a diffuse layer, the very large but finite electrostatic barrier for an ion is determined by the ratio of a distance characteristic of the bulk electrolyte and a distance characteristic of the maximum gradient of D. The image potential for a point dipole, normal or tangential to the diffuse interface, is exhibited in the repulsive region, for the long range diffuse interface. For metal-water interfaces, encountered in electrochemistry, the effects of diffuseness of the inner region, adjacent to the metal, are examined. One effect of diffuseness is to remove singularities in the image potential and to increase it in the relevant part of the inner region. The other effect is to decrease the effective attractive interaction between charges. For both the air-water and metal-water interfaces, the limitations of the commonly used constant dielectric profile are discussed.
Microwave Spectrum, Structure, and Dipole Moment of Cyclopropylacetylene
Authors: Collins Michael J. , Britt Chester O. , Boggs James E.
Published: February 2004

The microwave spectra of cyclopropylacetylene and one deuterated species, C3H5C≡ CD, have been analyzed. With certain assumed structural parameters, the C–C bond attaching the acetylenic group to the ring was found to have a length of 1.47 Å and the C≡ C bond was 1.19 Å. These bond lengths are essentially indistinguishable from the corresponding distances in methylacetylene; the difference, if any, is in the opposite direction from that which would be predicted on the basis of either conjugative interactions or differences in hybridization. The dipole-moment components are μa=0.89 D, μc=0.05 D, and μtotal=0.89 D, nearly identical with that observed for pentyne.
Dielectric Relaxation of Aliphatic Amines in Cyclohexane Solution
Authors: Tay Sing Pin , Crossley John
Published: February 2004

Dielectric constants and losses have been obtained for n-butyl-, n-hexyl-, n-nonyl-, 2-nonyl-, 5-nonyl-, n-decyl-, n-undecyl-, N-methyl-n-octyl-, N, N-dimethyl-n-octyl-, and N, N-diethyl-n-octylamines in cyclohexane solution at 25°C at up to 10 frequencies in the range 1.0–145 GHz. Static dielectric constants have also been measured at 2 MHz. Primary amines are characterized by short relaxation times which are almost independent of the size of the alkyl group and the location of the −NH2 group. Considerably longer values are obtained for the secondary and tertiary amines. The results are discussed in terms of the relative importance of dipole reorientation by intramolecular and whole molecule rotations.
Magnetic Structures of CeZn2 and TbZn2
Authors: Debray D. , Sougi M. , Meriel P.
Published: February 2004

CeZn2 and TbZn2 have the orthorhombic CeCu2-type crystal structure. CeZn2 becomes a simple antiferromagnet below the Néel temperature of 7.5°K. The magnetic unit cell is of the same size as the chemical cell. All moments are aligned along the b axis and the saturation moment per Ce at 4.2°K is 1.6± 0.2 μB. In the range 60–75°K, TbZn2 exhibits a linear-transverse-wave moment alignment propagating along the c axis and polarized along the b axis direction. The wavelength of this static magnetization varies from 2.28 to 2.54 in units of c in this temperature range. The moment amplitude per Tb at 62.4°K is 6.91± 0.17 μB. At below 60°K, a first-order magnetic transition takes place and the resulting antiferromagnetic structure has a magnetic unit cell twice the size of the chemical cell, the c axis being doubled. All moments are aligned along the b axis and the saturation moment per Tb at 4.2°K is 8.15± 0.10 μB. The magnetic structures of both CeZn2 and TbZn2 can be described as resulting from the stacking of ferromagnetic sheets of the rare-earth ions in some antiferromagnetic fashion.
Molecular Model for Prenucleation Water Clusters
Authors: Plummer P. L. M. , Hale B. N.
Published: February 2004

A molecular model applicable to prenucleation water clusters is described. As an illustration the model is applied to water clusters having clathrate-like structures composed of five-membered rings. This work was motivated by the apparent inadequacies of the corrected liquid drop model which (in addition to applying bulk properties to small clusters) predicts nucleation rates which may be as much as 1017 larger than experiment. We present the ``energy of formation'' at a temperature of 277°K for our molecular model for clusters ranging in size from 5 to 57 molecules. These results agree qualitatively with experiment and, we believe, provide a motivation for further development of the molecular approach.
Unitary Transformations and Pair Energies. III. Relation to Perturbation Theory
Authors: Davidson Ernest R. , Bender Charles F.
Published: February 2004

The effect of unitary transformations among the occupied orbitals of the beryllium atom is discussed. It is shown that pair-pair interaction energies become large for choices other than the canonical Roothaan-Hartree-Fock orbitals. The many-body perturbation theory diagrams responsible for these effects are discussed.
Configuration of the Water Molecule in Cupric Fluoride Dihydrate at 298°K
Authors: Prince E.
Published: February 2004

The crystal structure of cupric fluoride dihydrate at 298°K has been redetermined from three-dimensional, single crystal, neutron diffraction data. Least-squares refinement of 163 independent, observed reflections gave a final, weighted R index Rw = 0.038. The H–O–H angle in the water molecule is 110.6° ± 0.2°, which, in contrast to the value 115.5° ± 0.4° found in an earlier study, is not significantly different from the tetrahedral angle of 109.5°. Thermal vibration is highly anisotropic, with much larger amplitudes perpendicular to infinite, hydrogen-bonded sheets than parallel to the sheets.
Collisional Transition Probabilities for Rotational Levels of CN (B2Σ+)
Authors: Duewer W. H. , Coxon J. A. , Setser D. W.
Published: February 2004

Rotational relaxation of CN (B 2Σ+) formed by interaction of metastable argon atoms with CNBr was studied. The CN (B 2Σ+) initially is formed only in a few perturbed rotational levels for some vibrational states. Transfer out of the initially formed rotational levels via collision with argon took place with about the same rate for all rotational levels studied, although the magnitude of the rotational level spacings differed for the various cases that were investigated. Transition probability models were developed to simulate the observed pressure dependence of the spectrum. The best fit was obtained with a model having down transitions given by Pi → j = (0.2|i −j| + 0.05) (2j + 1)/(2i + 1). For all models, it was essential to include a large probability of multiquantum (|i−j| > 4) transitions in order to fit the data.
Thermalization Distance Distribution of Secondary Electrons
Authors: Tachiya Masanori
Published: February 2004

Ionic processes in irradiated substances are considered in terms of the thermalization distance distribution of secondary electrons. Based upon this consideration the thermalization distance distribution function is calculated from experimental data on these processes. Agreement between the distribution functions calculated from the temperature dependence of the free ion yield and from the scavenger concentration dependence of the geminate recombination yield is not good.
Nonretarded van der Waals Interaction between Anisotropic Long Thin Rods at All Angles
Authors: Parsegian V. Adrian
Published: February 2004

The nonretarded van der Waals interaction between anisotropic long thin rods in an isotropic medium is formulated for parallel and skewed orientations. The results are obtained by considering the interaction between two composite media in which arrays of rods are embedded. This interaction may be written directly using the theory of van der Waals forces between anisotropic materials. As a function of rod density, the leading term in the total interaction is that between pairs of rods in the two media. A test function for this rod-rod interaction is summed over all pairs and equated to the known total interaction of all rods. This result retains all many-body properties of the van der Waals interaction. A method for computation using dielectric birefringence data is also described. Two restrictive criteria are developed for deciding when the interaction of rods in an array will be the sum of individual rod-rod interactions.
Local Structures in Chemical Reactions with Heterogeneous Catalysis
Authors: Ortoleva Peter , Ross John
Published: February 2004

In the presence of heterogeneous catalysis, time independent local, undulatory inhomogeneities may occur (1) in chemical systems which are stable and may never have symmetry-breaking transitions to (global) dissipative structures, and (2) in chemical systems which may have such transitions, but then the local structure appears under far less restrictive conditions. The results of the theory are demonstrated with two reaction mechanisms, one due to Prigogine and Lefever, the other to Lotka and Volterra. Local structures may serve better in a role in morphogenesis than global ones. Experiments on spatial patterns must distinguish between global and local structure.
Absorption Spectrum of Ta Atoms Isolated in an Argon Matrix
Authors: Graham W. R. M. , Weltner W.
Published: February 2004

Absorption spectra have been recorded for Ta atoms isolated in an Ar matrix at 4.2°K. A complex spectrum consisting of more than 40 features is observed between 2288 and 5041 Å. With only a few exceptions, these features can be correlated with gas phase transitions involving the 4F3/2 ground state of neutral Ta atoms. There is qualitative agreement between the relative intensities of matrix features and intensity data for the gaseous state. Matrix shifts occur in the range 963–1451 cm−1 with an average shift of 1160 cm−1 to higher energy.
Ion-Pair Formation of the Benzene Anion Radical and Its Effect upon Previously Reported Measurements of Spin-Lattice Relaxation by the cw Saturation Technique
Authors: Jones M. T. , Komarynsky M.
Published: February 2004

The effects of various types of exchange processes upon the line-shape analysis procedure described by Jones, Komarynsky, and Rataiczak [J. Phys. Chem. 75, 2769 (1971)] are considered. Extended studies of the metal hyperfine splitting (hfs) in the benzene anion radical show that the metal hfs is independent of radical concentration and linearly dependent upon temperature. It is concluded that source of the hfs is the potassium ion. It is also shown that the fact the benzene anion radical resonance lines are slightly inhomogeneous leads to an error in the measurement of the spin-lattice relaxation times by the cw saturation technique. However, these errors can be reduced to an acceptable range, if one uses the correct spin-spin relaxation time, T2, in the measurement of the spin-lattice relaxation time, T1. This correction has been performed, and better values for benzene anion radical spin-lattice relaxation times are reported.
Dielectric Relaxation of Rigid Molecules in Supercooled Decalin
Authors: Johari Gyan P. , Smyth Charles P.
Published: February 2004

The equilibrium dielectric permittivity of pure decalin and the dielectric permittivity and loss of dilute solutions of chlorobenzene, bromobenzene, toluene, tetrahydrofuran, 1-fluoronaphthalene, 1-chloronaphthalene, 1-bromonaphthalene, and 1-methylnaphthalene in decalin have been measured from 50 Hz to 100 kHz at several temperatures between −135 and −155°C. The equilibrium dielectric permittivity of solutions shows no significant short-range order tending to align the dipoles preferentially. All the rigid molecules show a broad dispersion region which does not seem to be describable by any simple distribution function. It is shown that the introduction of the solvent as a viscoelastic medium, with frequency-dependent viscosity, can account for the broad dispersion region. An additional region of dielectric absorption at relatively high frequencies appears to be present in all of these solutions. The Arrhenius plots of the relaxation time are nonlinear, with the activation energy increasing from 25 kcal/mole to 70 kcal/mole with decreasing temperature. The relaxation behavior of all these solutions is remarkably similar to the α relaxation in amorphous organic polymers.
Absorption Spectrum and Reaction Kinetics of the HO2 Radical in the Gas Phase
Authors: Hochanadel C. J. , Ghormley J. A. , Ogren Paul J.
Published: February 2004

HO2 radicals were produced in the gas phase by flash photolysis of water vapor (3%) in an atmosphere of hydrogen, helium, or argon containing ∼ 2% oxygen. Water is dissociated in the first continuum to H and OH, and O2 converts the H atoms to HO2. Hydrogen nearly doubles the amount of HO2 produced by converting OH to H. The absorption spectrum of HO2 is a broad band with a peak at 2050 Å. The molar extinction coefficient, εmax, based on measurement of the H2O2 formed in the hydrogen system, is 1770 ± 150 M−1· cm−1. The rate constant for the bimolecular combination reaction, HO2+HO2 → H2O2 + O2, was evaluated as 5.7 ± 0.5 × 109 M−1· sec−1 at 298°K and for the reaction HO2+OH→ H2O+O2, k = 1.2 ± 0.2 × 1011M−1. sec−1. From auxiliary measurements of the rate of O3 formation it was also found that, in the flash photolysis of O2 (2%) in H2, hot O atoms react with H2 to form OH and H which are then converted to HO2.
High-Resolution Molecular Zeeman Measurements in CH2F2; Observation of 19F Chemical Shift Anisotropy
Authors: Kukolich Stephen G. , Nelson Albert C.
Published: February 2004

The molecular Zeeman effect was observed in methylene fluoride using a molecular beam maser spectrometer with the cavity in a 100-kG magnetic field. This combination of high fields and high resolution (4 kHz at 22 GHz) provides unusually high resolution for these measurements. The rotational transition 303→212 at 22 204 240 ± 2 kHz was observed in emission. The measured fluorine magnetic shielding anisotropies are 2σcc− σaa − σbb = − 335.0 ± 35 ppm and σaa − σbb = 9.0 ± 13 ppm. Accurate susceptibility anisotropies and molecular quadrupole moments were also obtained.
Evidence of Effective Potential Barriers in the X-Ray Absorption Spectra of Molecules
Authors: Dehmer J. L.
Published: February 2004

We interpret the enhanced lines and frequent failure to detect Rydberg structure in the x-ray absorption spectra of molecules in terms of ``inner-well'' and ``outer-well'' states which result from an effective potential barrier located near the electronegative atoms in the molecule. The K and LII, III spectra of sulfur in SF6 are discussed in detail and all prominent features are identified. X-ray absorption spectra of molecules in which a B, S, or Si atom is combined with two or more electronegative atoms are surveyed to illustrate further the influence of effective potential barriers. It is pointed out that ``inner-well'' states have been observed in emission experiments, and that such states should also be observed in electron scattering experiments on molecules and in x-ray absorption experiments on ionic crystals. Finally, cases are described in which x-ray absorption measurements may permit the determination of the geometry of molecules.
Microwave Spectrum and Centrifugal Distortion Effects of H2S
Authors: Helminger Paul , Cook Robert L. , De Lucia Frank C.
Published: February 2004

The rotational spectrum of H232S in the ground vibrational state has been studied in the 1 cm–0.39 mm wavelength region by means of tunable, high-resolution microwave spectroscopy. Thirty-one newly observed rotational transitions of H232S, 12 of which occur at frequencies higher than 600 GHz, have been combined with previous microwave measurements on this molecule in a rotation-distortion analysis of H232S which is based entirely on high precision microwave data. The results for the effective rotational constants and quartic and sextic distortion constants are (in megahertz): A = 310 182.24 ± 0.60, B = 270 884.05 ± 0.51, C = 141 705.88 ± 0.51, ΔJ = 49.851 ± 0.038, ΔJK = −159.696 ± 0.069, ΔK = 111.851 ± 0.068, δJ = −6.0191 ± 0.0050, δK = 262.17 ± 0.21, HJ = (2.813 ± 0.105) × 10−2, HJK = (−2.283 ± 0.027) × 10−1, HKJ = (4.59 ± 0.16) × 10−1, HK = (−2.765 ± 0.141) × 10−1, hJ = (−5.841 ± 0.081) × 10−3, hJK = (2.428 ± 0.052) × 10−1, and hK = 2.870 ± 0.039. In addition, a partial set of distortion terms of order P8 and P10 is found to be necessary in the analysis.
Vapor Pressures of Isotopic Krypton Mixtures. Intermolecular Forces in Solid and Liquid Krypton
Authors: Lee Myung W. , Eshelman Donald M. , Bigeleisen Jacob
Published: February 2004

The vapor pressures of normal krypton and a krypton sample containing 50% 80Kr have been measured from 94 to 130°K. New vapor pressure equations are derived for solid and liquid normal krypton. The triple-point pressures and (temperatures) of 80Kr and 84Kr are 547.565 (115.77) and 547.502 (115.78) int. mm Hg (°K), respectively. The triple-point pressure of normal krypton is 547.52 ± 0.10 int. mm Hg. The reduced partition functions, fc/fg, of 84Kr/80Kr have been derived for the solid and liquid from the vapor pressure data. The solid data lead to a θ0 (D) = 71.8°K in excellent agreement with heat capacity data. The change in ln (fc/fg) on melting is found to be 24% in good agreement with previous results on neon and argon. V 〈 ∇2 U 〉 is derived from the experimental values of ln (fc/fg) in the liquid. The repulsive potential in liquid krypton is less steep than that in neon or argon. Calculations of V 〈 ∇2 U 〉 from neutron scattering measurements show that the exp-6 potential to be superior to the LJ (12, 6) potential for liquid krypton. A corresponding states analysis through 〈 ∇2 U* 〉 leads to a quantitative correlation of the solid and liquid phase (fc/fg) data for Ne, Ar, and Kr.
Excitation of Na D-Line Radiation in Collisions of Sodium Atoms with Internally Excited H2, D2, and N2
Authors: Krause H. F. , Fricke J. , Fite W. L.
Published: February 2004

Excitation of D-line radiation in collisions of Na atoms with vibrationally excited N2, H2, and D2 has been studied in two modulated crossed beam experiments. In both experiments, the vibrational excitation of the molecules was provided by heating the molecular beam source to temperatures in the range of 2000–3000°K, which was assumed to give populations according to the Boltzmann expression. In the first experiment, a total rate coefficient was measured as a function of molecular beam temperature, with absolute calibration of the photon detector being made using the black body radiation from the heated molecular beam source. Since heating affects both the internal energy and the collisional kinetic energy, the first experiment could not determine the relative contributions of internal energy transfer vs collisional excitation. The second experiment achieved partial separation of internal vs kinetic energy transfer effects by using a velocity-selected molecular beam. Using two simple models for the kinetic energy dependence of the transfer cross section for a given change in vibrational quantum number, the data from both experiments were used to determine parameters in the models. Evidently the most probable situation is that most of the Na excitation energy comes from internal vibrational energy, with the remainder coming from kinetic energy. The paper discusses the consequences of the detailed results with respect to the excitation of sodium radiation in aurorae.
Pressure and Temperature Studies of NMR Translational Relaxation in Hydrogen Bonded Liquids
Authors: Fiorito R. B. , Meister R.
Published: February 2004

Spin-lattice and spin-spin relaxation time have been measured using the spin echo method in three hydrogen bonded liquids: glycerol, 1,3-butanediol, and 2-methyl-2,4-pentanediol as a function of temperature through the region of the T1 minimum and at seven pressures in the range 0–3000 kg/cm2. The results of this study lead to the conclusion that the translational model for NMR relaxational presented by Torrey is satisfactory for representing the diffusive motion of molecules in these viscoelastic liquids. Agreement between the experimentally measured values of the self-diffusion constant and those theoretically predicted by the translational theory provides verification of this assumption. Recent measurements of quadrupolar-relaxation times as a function of temperature on deuterated glycerol as well as low frequency proton spin-lattice relaxation measurements also provide supporting evidence. The nuclear correlation time τC, the mean time between diffusive jumps τj, and the distance of closest approach d are produced from a fit of the T1 and T2 data to the translational relaxation theory. The values τC and τj are compared to the corresponding times obtained from dielectric, ultrasonic and light scattering experiments.
Excitation of the Werner Bands of H2 by Electron Impact
Authors: Stone E. J. , Zipf E. C.
Published: February 2004

Absolute cross sections for the excitation of the Werner band system H2(C 1Πu→ X 1Σg+) have been measured from energy threshold to 300 eV for electron impact on H2. The bands were observed in emission in the wavelength region 1100–1250 Å. The measured cross sections were compared with published transition probabilities, leading to the conclusion that the Werner bands are suitable as the basis for a relative spectral response calibration only when the bands are observed under sufficiently high resolution (FWHM ∼0.4 Å). The effect of the perturbation between the C 1Πu and B 1Σu+ states of the hydrogen molecule was clearly observed in anomalies in the rotational intensity distribution in bands of the (3, v″) progression.
Formation of Sodium Ions by Charge Transfer
Authors: Rutherford J. A. , Mathis R. F. , Turner B. R. , Vroom D. A.
Published: February 2004

Cross sections have been measured in the energy range 1–500 eV for the charge transfer of several ions with neutral sodium. The ions studied are O+, N+, N2+, NO+, O2+, H2O+, H3O+, N2O+, and Na+. The magnitude of these cross sections can be related to the availability of reaction paths which have a small internal energy defect. The smaller the energy defect the larger the cross section. Where possible the measured cross sections have been extrapolated to thermal energy. Comparison is made with other experiments.
Polarized Raman Spectra of Cesium Nitrate in Solid and Molten States: The Nature of the Disordered Phases
Authors: James David W. , Devlin J. Paul
Published: February 2004

The polarized Raman spectra of the room temperature (ordered) phase and the high temperature (disordered) phase of cesium nitrate have been recorded. The assignment of the spectral bands differs somewhat from earlier reports of the room temperature phase with a to/lo splitting of the ν3(E) band at ∼ 1350–1400 cm−1 reported. In the disordered crystal phase the discrete lattice spectrum is replaced by a broad band at ∼ 90 cm−1 superimposed on an anisotropic wing. However, it is found that the internal vibrational phonons retain their site group and polarization characteristics in the disordered solid. The spectrum obtained for the melt is very similar to that for the disordered crystal at elevated temperature (excepting polarization properties).
Magnetic Exchange in Transition Metal Complexes. VII. Spin Coupling in μ4-Oxohexa-μ-halotetrakis [(Triphenylphosphine Oxide or Pyridine) Copper(II)]: Evidence for Antisymmetric Exchange
Authors: Lines M. E. , Ginsberg A. P. , Martin R. L. , Sherwood R. C.
Published: February 2004

Magnetic susceptibility measurements are reported for a group of tetrameric Cu(II) complexes Cu4OX6L4 with X = Cl, L = (C6H5)3PO and X = Br, L = (C6H5)3PO or C5H5N. All samples show an effective magnetic moment μeff per copper ion which exhibits a maximum in the 40–60°K temperature range and which, in general, cannot be quantitatively accounted for by simple Heisenberg theory. The results contrast with those observed for the closely related anionic cluster [Cu4OCl10]4− which exhibits a monotonic variation of μeff with temperature and is compatible with simple Heisenberg theory. It is suggested that the difference may be due to a different relative arrangement of 3d Cu(II) energy levels in the anionic cluster, leaving an orbital singlet state lowest in the [Cu4OCl10]4− environment but an orbitally degenerate level lowest in the other complexes. To test the hypothesis the copper-pair exchange Hamiltonian is derived for the orbitally degenerate case and shown, for the tetramer environment, to contain a large antisymmetric component. Using this orbitally degenerate spin-pair Hamiltonian, the statistical problem for a tetramer of interacting spins is solved to give an effective magnetic moment per copper ion as a function of temperature. The resulting theory gives excellent agreement with the experimental results for the Cu4OX6L4 complexes. In addition, the required values for the exchange parameters within the theory are physically realistic and may be qualitatively understood in terms of orbital symmetry relationships.
Classical Mechanics Calculations of the Transport Properties of Gaseous Mixtures with Internal Degrees of Freedom
Authors: Ree J. VAN DE , Scholtes T.
Published: February 2004

Following the theory of thermodynamics of irreversible processes for mixtures of reacting gases, expressions are derived for the diffusion and thermal diffusion coefficients of gases with internal degrees of freedom, in the two limiting cases of infinitely slow, and of infinitely rapid exchange between translational and internal energies. Classical mechanics trajectory calculations provide the necessary total inelastic cross sections for the transport collision integrals. For molecules with an asymmetric mass distribution the nonspherical interaction is treated with a realistic rotating potential field. Application to the systems D2-HD and D2-HT yield excellent agreement with experimental diffusion data. Thermal diffusion factors are extremely sensitive to nonspherical interactions through a correction term analogous with the Eucken correction in thermal conductivity. Derivations from the kinetic theory of Wang Chang, Uhlenbeck, and de Boer give the same correction term. At low temperatures the agreement with experiment is reasonable.
Vibrational Spectra and Barriers to Internal Rotation in Some Halogen Substituted Ethanes
Authors: Durig J. R. , Craven S. M. , Hawley C. W. , Bragin J.
Published: February 2004

The infrared spectra have been recorded from 33 to 4000 cm−1 for CH3CF2Br, CH3CBr2Cl, and CH3CBr3 and from 33 to 600 cm−1 for CH3CF2H and CH3CF2Cl for both the gaseous and solid states. The Raman spectra of CH3CF2Br and CH3CBr2Cl in the liquid state have also been recorded. The assignments of the 18 normal modes have been made on the basis of the band contours, Raman depolarization values, and characteristic ``group'' frequencies for the CH3CF2Br and CH3CBr2Cl molecules. The methyl torsional vibrations were observed for CH3CF2H, CH3CF2Cl, CH3CF2Br, CH3CBr2Cl, and CH3CBr3 at 248, 272, 275, 293, and 304 cm−1, respectively, in the solid state. From these assignments, barriers of 3.97, 4.80, 4.99, 5.66, and 6.08 kcal/mole were calculated, respectively. The effects of condensation on the torsional modes have been considered and the barrier values are compared with results obtained previously from similar molecules. Intermolecular fundamentals were observed for the CH3CF2H, CH3CF2Cl, and CH3CF2Br molecules in the far infrared spectra and possible assignments are discussed.
Outer Shell Overlap Integrals for Some Silver Halide Crystals
Authors: Hayns M. R. , Calais J. L.
Published: February 2004

Outer shell overlap matrix elements involving ns, npσ, npπ, ndσ, ndπ, and ndδ orbitals have been calculated for the three NaCl structured silver halides AgF, AgCl, and AgBr. Hartree-Fock-Roothaan wave-functions have been used in all calculations. The overlaps have been calculated for first (halogen-silver), second (halogen-halogen and silver-silver), and third (halogen-silver) neighbors at various internuclear separations. The exponential type variation of overlap with internuclear separation suggested by Hafemeister and Flygare has been utilized.
Square-Well Potential. I. An Yvon-Born-Green Square-Well Equation of State
Authors: Schrodt I. B. , Luks K. D.
Published: February 2004

The Yvon-Born-Green integral equation for the pair correlation function is solved for the square-well potential with σ2/σ1=1.85, using a θ=ε/kT expansion scheme previously devised by Kirkwood et al. The pertinent results are expressed in a power series of density by a least squares technique for the entire gas and liquid regimes. The coexistence region is determined, including the location of the critical point, and thermodynamic properties are calculated and tabulated as functions of state. The validity and utility of the results are examined in detail.
Kinetics of O(3P) +CO +M Recombination
Authors: Slanger Tom G. , Wood Bernard J. , Black Graham
Published: February 2004

Rate coefficients have been measured over the temperature range 250–370°K for the three-body recombination of O(3P) with CO. Earlier results at 300°K have been re-evaluated and were found to have been influenced by unknown impurities in the CO, which have now been removed by more elaborate purification methods. For CO as the third body, the rate constant is given by k = 6.5 × 10−33 exp(−4340 ± 550/RT) cm6 molecule−2·sec−1. For N2 and CO2, the 296°K rate constants are 2.3 and 6.2 × 10−36 cm6 molecule−2·sec−1, respectively.
Velocity Autocorrelation Function of a Large Cluster of Particles in a One-Dimensional Crystal
Authors: Rubin Robert J.
Published: February 2004

The velocity autocorrelation function of the center of gravity of a cluster of Q particles in a one-dimensional harmonic crystal is determined in the limit Q → ∞. In this limit, the autocorrelation decreases linearly from one to zero. In the continuum limit of the lattice equations of motion the same linear form is obtained for the velocity autocorrelation function. The coefficient of self-diffusion of the cluster of Q particles equals kT, a result which is exact and independent of cluster size.
Triplet-Singlet π*← n Absorption Spectra of 3,3-Chloromethyldiazirine and 3,3-Bromomethyldiazirine
Authors: Robertson Lawrence C. , Merritt James A.
Published: February 2004

The triplet-singlet π*← n vapor absorption spectra of 3,3-chloromethyldiazirine and 3,3-bromomethyldiazirine in the 3800–4300 Å region are presented. A vibrational analysis is given for the spin forbidden A″3(nπ*)←A′1 electronic system of each molecule, and a 0–0 band is assigned at 24 005 cm−1 for 3,3-chloromethyldiazirine and at 23 951 cm−1 for 3,3-bromomethyldiazirine. The ν3′ N=N symmetric stretch vibration is the predominant feature in each molecule.
Two-Level Fluorescence of Ovalene
Authors: Johnson P. C. , Offen H. W.
Published: February 2004

Ovalene fluorescence has been studied as a function of pressure (0–30 kbar) in condensed media at room and liquid nitrogen temperatures. The red shift of the α and p bands are 8 and 22 cm−1/kbar, respectively, in polymethylmethacrylate. The fluorescence kinetics are consistent with two-level emission; the greater pressure dependence of the lifetimes compared to the relative intensities being attributed to differential level shifts and its influence on the thermal population of the adjacent singlet level.
Translational Spectra Due to Collision-Induced Overlap Moments in Mixtures of He with CO2, N2, CH4, and C2H6
Authors: Bar-Ziv Ezra , Weiss Shmuel
Published: February 2004

Enhancement of the absorption, upon addition of He to CO2, N2, CH4, and C2H6, has been observed in the frequency region 200–450 cm−1. It is shown that in this frequency region absorption due to multipole moment induction is weak and particularly so because of the very low polarizability of He. The observed spectra are thus due to induction by overlap forces and similar in origin to spectra observed for rare gas mixtures. The observed spectra are indeed found to resemble those for mixtures of He with rare gases in position and shape as well as in intensity. The spectra are analyzed in terms of a hard sphere potential and an overlap moment varying exponentially with intermolecular separation.
Effect of Molecular Shape and Flexibility on Gamma-Ray Directional Correlations
Authors: Marshall Alan G. , Werbelow Lawrence G. , Meares Claude F.
Published: February 2004

The perturbed angular correlation for a gamma-ray cascade from a nucleus rigidly bound to an asymmetric-top molecule in solution has been calculated, using a diffusional model for rotational reorientation. When molecular rotation is rapid [all diffusion constants Di ≫ (1/6τn), where τn is the lifetime of the intermediate nuclear state of the cascade], a treatment analogous to that in nuclear magnetic resonance leads immediately to the desired attenuation factor. On the other hand, when molecular rotation is slow [all Di≪ (ω 0/6), where ω0 is the fundamental quadrupole frequency], the derived form of the angular correlation may not contain a separable attenuation factor. These calculations are illustrated by several plots of the integral anisotropy for symmetric-top molecules of fixed shape and varying size, for several choices of the characteristic ``angle of attachment'' between the molecular symmetry axis and the principal axis of the electric field gradient at the nucleus. Finally, it is shown that the case of a tracer bound through a single flexible link to a spherical molecule is formally identical to the case of a tracer rigidly bound to a symmetric-top molecule; the resultant integral anisotropy is plotted for several fixed rates of internal rotation in spherical molecules of varying size, for several choices of the ``angle of attachment'' between the electric field gradient and the axis about which internal rotation occurs. The conclusions of practical significance are (1) the perturbed angular correlation depends only weakly on the shape of the molecule, while (2) in contrast, the correlation can be markedly affected by the presence of even relatively slow internal rotation at the site of attachment of the tracer to the molecule.
Note on the Poisson-Boltzmann Equation
Authors: Parlange Jean-Yves
Published: February 2004

An analytical scheme is presented which yields an approximate solution of the Poisson-Boltzmann equation. The result is more precise when the singularity of the solution is further away from the origin and when the dimensionality of the problem is lower. The solution is compared with the results of numerical integration in the three-dimensionless case.
Possible Effects of Magnetic Fields on the Quenching of Triplet Excited States of Polynuclear Hydrocarbons by Oxygen
Authors: Geacintov Nicholas E. , Swenberg Charles E.
Published: February 2004

A theory of the magnetic field effect on the rate constant γ of the quenching of aromatic hydrocarbon triplets by molecular oxygen is developed. Based on theoretical considerations of Kawaoka, Khan, and Kearns and on experimental evidence obtained by other workers, it is assumed that the final state of the quenching reaction gives rise to oxygen in a singlet excited state. The conclusions should, however, be applicable to other cases in which the initial state involves two triplets and the final state two singlets. The theory takes the intramolecular spin-spin interaction and the charge-transfer interaction in the bimolecular intermediate complex state into account. The distribution of singlet character among the nine possible collision complex states is considered. The theory predicts that as long as the splitting among the singlet, quintet and triplet like pair states due to charge-transfer interaction is negligible compared to the spin-spin interaction, a negative effect on γ in a quantizing magnetic field is expected. If the charge-transfer induced splitting ΔETS between singlet and triplet like pair states and ΔEQS between quintets and singlets is large compared to the zero-field spin-spin interaction and also the Zeeman energy, no effect of magnetic field on γ will be observed. Both positive and negative field effects on γ are predicted if ΔETS and ΔEQS are of the order of the Zeeman energy or if ΔETS∼0, but ΔEQS is larger than the spin-spin interaction. The magnetic field dependence of γ may then display minima and maxima, which may be very broad if ΔEQS and ΔETS are a function of the collision complex geometry which may assume many configurations. Experiments on partially quenched phosphorescent samples of chrysene, a,h-dibenzanthracene and coronene adsorbed on a polystyrene matrix were carried out at a field of 145 kG. Within the experimental error of 1%–2%, no effect of magnetic field on γ was detected. The experimentally observed null effect is interpreted assuming either (1) that the ratio ε of the transition rate to final singlet states to the rate of dissociation of the collision complex is small, or (2) that the effect of charge transfer mixing in the complex is significant. It is tentatively concluded that the results are better explained in terms of the latter hypothesis. It is estimated that the energy spread due to charge-transfer interaction between singlet and triplet-like complex states is greater than 30 cm−1.
Intramolecular Triplet-Triplet Energy Transfer in Polyadenylic Acids: The Influence of Triplet Energy Trapping on the Decay Kinetics of Delayed Fluorescence Produced by a Triplet-Triplet Annihilation
Authors: Helene Claude , Longworth J. W.
Published: February 2004

A delayed fluorescence is observed from both polyriboadenylate and polydeoxyriboadenylate helices, but not from a random conformer of polyrA. The delayed fluorescence depends approximately on the square of the triplet concentration and is believed to originate from a triplet-triplet annihilation occurring within the polymer helices. The decay of the delayed fluorescence is nonexponential, with a short-lived component of 70 msec decay, and a longer-lived component of 670 msec. Increasing temperature from 77°K decreases the fractional contribution of the short component. The failure of phosphorescence intensity to increase linearly with exciting light intensity and its having a shorter rise time to steady state levels than decay indicate that the emission occurs from an exciting-light-saturable site. As there was no evidence for a depletion of the ground state by optical pumping to the triplet level at light intensities which gave appreciable reduction in rate of approach to steady state, all the phosphorescence emissions must originate from an intrinsic, lower energy trap site. Previous studies of the Stokes' shift of phosphorescence from the polymers have found an unexplained addition to the Stokes' shift of 15 mm−1 which is not present in oligomer helices. These observations were consistent with a simple trapping model for the production of delayed fluorescence. The triplet excitons became rapidly trapped by these lower energy sites, and delayed fluorescence is formed by thermal activation to the polymer host level, followed by a diffusive migration and anihilation with a trapped triplet exciton. This model is fully consistent with the transition metal quenching of phosphorescence, where, although extensive reduction in phosphorescence yield occurred, no alteration in lifetime was found.
Entropy and Chemical Change. I. Characterization of Product (and Reactant) Energy Distributions in Reactive Molecular Collisions: Information and Entropy Deficiency
Authors: Bernstein R. B. , Levine R. D.
Published: February 2004

The present paper considers optimal means of characterizing the distribution of product energy states resulting from reactive collisions of molecules with restricted distributions of initial states, and vice versa, i.e., characterizing the particular reactant state distribution which yields a given set of product states, at a specified total energy E. The S-matrix, or reaction probability matrix P(E), ``global'' in nature, contains much more detail than necessary to reproduce the results of any single specific experiment or computer simulation thereof (via classical mechanical trajectory calculations). Moreover, since reactant and/or product state resolution is always experimentally limited (to a greater or lesser degree), data are necessarily coarse-grained accordingly. Many quantal features are thereby lost and the results are often at a level appropriate for comparison with classical calculations (e.g., in the form of low-resolution contour maps of energy disposal). Such contour plots of the yield function Y or the averaged transition probability ω (the ``poor-man's'' P-matrix) nevertheless contain the essence of the dynamical results. It is suggested to represent the energy dependence of global-type results in the form of square-faced prism plots (contour maps vs E), and of data for specific-type experiments (or computer simulations) as triangular-faced prismatic plots (contour maps vs E). The essential parameters defining the internal state distribution are isolated, and the information content I(E) of such a distribution (for a microcanonical ensemble) is put on a quantitative basis. The relationship between the information content, the surprisal, and the entropy of the continuous distribution is established, thereby making the link between microscopic collision theory and nonequilibrium statistical mechanics. The concept of an ``entropy deficiency'' ΔS′, which characterizes the specificity of product state formation, is suggested as a useful measure of the deviance from statistical (``phase-space dominated'') behavior. The degradation of information by experimental averaging is considered, leading to bounds on the entropy deficiency. The present analysis is useful for the characterization of both experimental results and theoretical models.
Heteronuclear Associative Ionization of Argon with Nitrogen and Carbon Monoxide
Authors: Holcombe Norman T. , Lampe F. W.
Published: February 2004

Pulsed mass-spectrometric studies of the bimolecular formation of ArN2+, ArCO+, and Ar2+ in argon-nitrogen mixtures and argon-carbon monoxide mixtures subjected to the impact of 30 eV electrons show that all three reactions proceed with essentially the same specific reaction rate of 1.5×10−9 cm3/sec. The effective lifetime of the argon atoms producing ArN2+ is longer than that of the argon atoms forming Ar2+, while the reverse is true in a comparison of the effective lifetimes for ArCO+ and Ar2+ formation. On the basis of the dependence of the ratio ArN2+/Ar2+ on electron energy and the time delay between electron pulse and ion-drawout pulse, it is concluded that ArN2+ has a threshold energy lower than Ar2+. The same is probably true of ArCO+ but the effect of delay time in this case is complicated by dissimilar shapes of the pertinent ionization-efficiency curves. It is suggested that Penning ionization of CO becomes progressively more important for the higher-lying states of argon.