bluuebunny/crossref_metadata_2025 huggingface dataset:
Rotational Spectra of Substituted Diazirines. III. Methylchlorodiazirine
Authors: Wollrab James E. , Scharpen LeRoy H.
Published: February 2004

Ground-state rotational spectra of CH335ClCN2 and CH337ClCN2 have been assigned, and the derived r0 structural parameters are: r(NN) = 1.241 ± 0.005 Å, r(CCl) = 1.743 ± 0.01 Å, r(CC) = 1.498 ± 0.01 Å, ∢C–C–Cl = 116.7° ± 1.0°, ∢CCN2 = 124.5° ± 1.0°, and ∢NCN = 50.2° ± 0.5°. The internal-rotation splittings for CH335ClCN2 yield a barrier to internal rotation of 1689 ± 10 cal mole−1. The Cl quadrupole coupling constants (megahertz) for CH335ClCN2 are χaa = −66.51, χbb = 32.91, χcc = 33.60 and for CH337ClCN2 are χaa = −53.07, χbb = 26.40, χcc = 26.67. The nitrogen quadrupole splittings were generally unresolved.
Vibrational Spectra of Liquid Crystals. I. Changes in the Infrared Spectrum at the Crystal–Nematic Transition
Authors: Bulkin Bernard J. , Grunbaum Dolores , Santoro Angelo V.
Published: February 2004

The infrared spectra of 4,4′-bis(alkoxy) azoxybenzenes (methyl, ethyl, butyl, pentyl, and hexyl) have been examined as a function of temperature through the crystal–nematic transition. Several bands are observed to disappear gradually as this transition is approached. The nature of these vibrational modes and the relative rates of their disappearance are discussed in terms of a lattice defect model of the crystal–nematic transition. Preliminary results for the crystal–smectic transition in 4,4′bis(heptyloxy) azoxybenzene are also reported.
Deuteron Quadrupole Coupling Constants in Deuterocarbons
Authors: Olympia P. L. , Wei I. Y. , Fung B. M.
Published: February 2004

The dependence of the deuteron quadrupole coupling constant (DQCC) on the nature of hybridization of carbon in deuterocarbons is examined in detail. A simple molecular orbital description of the C–D fragment, with various effective nuclear charges on carbon, is used. The calculated values are shown to be comparable to experimental results and other theoretical models which treat the entire molecules rather than simply the C–D fragment. It is shown that DQCC(sp) > DQCC(sp2) > DQCC(sp3) which may be attributed to the fact that the nuclear contribution to the electric field gradient decreases faster than the electronic part in the series sp, sp2, sp3. A collection of experimental data, establishing the same trend obtained theoretically, is presented including new deuterium NMR results on phenylacetylene-d and ferrocene-d10.
Thermodynamics of the Solubility and Permeation of Hydrogen in Metals at High Temperature and Low Pressure
Authors: Shupe D. S. , Stickney R. E.
Published: February 2004

The solubility and permeation of hydrogen in metals is analyzed on the basis of classical thermodynamics plus a minimum of kinetic theory. Expressions are derived for x, the number of H atoms absorbed per solid atom, and J, the permeation rate, in the limiting cases of continuum and free-molecule flow. For most conditions of temperature (T) and total pressure (pt = pH + pH2), the expressions reduce to the familiar equations for solubility and permeation, i.e., a curve for constant pt appears as a straight line on Arrhenius plots of logx vs 1 / T and of logJ vs 1 / T. However, for extreme conditions of high temperature and low pressure, the expressions predict that a curve of constant pt will become increasingly nonlinear as T increases, and it may even pass through a maximum when T becomes sufficiently high to produce almost complete dissociation of the gaseous H2. Also, under certain conditions neither x nor J is proportional to the square root of the total pressure. The analysis provides a simple explanation of existing experimental data on the solubility and permeation of hydrogen in tungsten and in molybdenum at high temperature and low pressure.
Viscosity of Isomeric Octyl Alcohols as a Function of Temperature and Pressure
Authors: Johari Gyan P. , Dannhauser Walter
Published: February 2004

The viscosity of 2- and 3-octanol and 6-methyl-3, 5-methyl-3-, and 2-methyl-3-heptanol has been measured as a function of temperature (−20°–80°C) and pressure (0–4 kbar). Specific structural differences exist in the temperature and pressure coefficients of isobaric viscosity; these differences are minimized, but still exist, when isochoric viscosities are compared. Viscous flow and dielectric relaxation of these liquids are compared, and it is concluded that the rate-determining steps are dissimilar. A previously proposed pressure-induced liquid structure transformation in these liquids could not be verified viscometrically.
Dissociation Energies of Ti2 and V2
Authors: Kant Arthur , Lin Sin-Shong
Published: February 2004

The Ti2 and V2 molecules have been shown to exist over liquid titanium and vanadium at temperatures above 2050°K. Using information derived from a combination of effusion and mass-spectrometric techniques, the dissociation energies of Ti2 and V2 have been estimated from the ratio of the dimeric and monomeric species in equilibrium with the liquid metals: The values as obtained by the second-law method are D°0(Ti2) = 28.3 ± 4 kcal and D°0(V2) = 57.5 ± 3 kcal and by the third-law are D°0(Ti2) = 32.8 ± 5 kcal and D°0(V2) = 56.9 ± 5 kcal.
Cartesian Coordinates of Molecular Models
Authors: Hilderbrandt R. L.
Published: February 2004

A procedure has been developed for calculating the Cartesian coordinates of atoms in a molecule in terms of a minimal set of geometric parameters. The superiority of this method arises from the use of symmetry operations to simplify the calculation. An additional advantage occurs through the definition of several types of standard connections which facilitate locating the atoms in the coordinate framework. Three illustrative examples are included. A listing of the program is available from the author upon request.
Atomic and Molecular Scattering from Solids. II. Comparison of Classical Scattering Models in Relation to Experiment
Authors: McClure J. Doyle
Published: February 2004

Characteristics of a classical two–dimensional (2d) model of atomic scattering from finite temperature, elastic solids are compared to similar characteristics of “quasi-1d” scattering models in light of pertinent experimental scattering studies of rare-gas atomic beams on metals. (The term “quasi-1d” is used to denote scattering models based on the assumption that tangential interaction is negligible, i.e., dynamical interaction is presumed to occur only in the direction normal to the local surface plane.) The 2d finite-range interaction field (denoted 2d FRI) is composed of additive Lennard-Jones (12, 6) pairwise potentials. Compared scattering characteristics include beam directivity and position as well as variation of mean tangential/normal momentum with scattering angle. It is shown that combined “structure” and “thermal” broadening associated with 2d FRI can account for systematic differences between experimentally observed lobe directivity and that predicted by quasi-1d “soft/hard cube” scattering models. For nongrazing incidence, strong angular variation of tangential momentum over the high-intensity portion of the scattered beams, as predicted by the 2d FRI model, is also in qualitative agreement with experiment. Dominant effects of finite “collision time,” as determined by 2d FRI, cast doubt on previous interpretation that observed differences in lobe position for effusive incident beams of different rare gases scattered from a particular hot metal crystal result from simple mass ratio effects. Effects of finite “collision time” are also invoked to interpret recent experimental findings on differences in lobe position for a given rare-gas beam scattered from different hot metal crystals. It is concluded that quantitative theoretical studies of gas–surface scattering must await development of tractable 3d FRI models.
Classical Calculations of Linear Reactive Systems: H+Cl2→HCl+Cl
Authors: Russell D. , Light John C.
Published: February 2004

Classical trajectory calculations were performed for the collinear H+Cl2 system using reaction coordinates and the potential-energy surfaces described previously for the quantum calculations. The resulting probabilities of reaction and the vibrational-energy distributions were compared with previous quantum calculations.
Lower-Energy Electronic States of HNCS, NCS−, and Thiocyanate Salts
Authors: McDonald J. R. , Scherr V. M. , McGlynn S. P.
Published: February 2004

The phosphorescence emission spectra and lifetimes of the thiocyanate salts of K+, Ba2+, Zn2+, Cd2+, Pb2+, and Ag+ have been measured. The converse absorption process has also been detected and studied. It has been established, as a result of these studies, that the lowest-energy excited state of the thiocyanate ion is of ···(π)3(π*)1; 3Σ+ nature in the linear NCS− ion. The 240-mμ transition of NCS−, previously assigned as a charge-transfer-to-solvent (ctts) transition, has been detected in all thiocyanate salt solutions and in gaseous HNCS. This absorptive transition has been reassigned as a ···(π)3(π*)1;1Σ− ← ···(π)4;1Σ+ transition of the isolated ion. This transition possesses some intramolecular charge-transfer nature and should be expected to exhibit the solvent effects previously adduced as evidence for its supposed ctts nature. A third transition has been detected at 197 mμ in HNCS gas and is tentatively assigned to the transition ···(π)3(π*)1;1Δ ← ···(π)4;1Σ+. A number of other topics are also broached. These are isomerism in thiocyanate salts of K+, Zn2+, Cd2+, and Ba2+; polymerization in thiocyanate salts of Ag+ and Pb2+; spin–orbit coupling in the SCN− ion and the effects of associated metal ion thereon; and energy levels of the SCN− ion. All conclusions drawn are based on experiment and/or computation and/or analogy with the known characteristics of some of the excited states of the isoelectronic CS2 and CO2 molecules.
Molecular Beam Kinetics: Reactions of K, Rb, and Cs with ICl and IBr
Authors: Kwei G. H. , Herschbach D. R.
Published: February 2004

Crossed-beam studies of the reactions of alkali atoms with ICl and IBr show that the main features are very similar to those found for the Cl2, Br2, and I2 reactions: (1) The reaction cross sections are extremely large, >̃150 Å2. (2) Most of the alkali halide product appears in the forward hemisphere with respect to the incident alkali-atom beam, although there is also considerable intensity (∼15%–25% of the forward peak) throughout the backward hemisphere. (3) The angular distribution of the alkali atoms scattered without reaction falls off much more rapidly at wide angles than for collisions between unreactive molecules of comparable size. (4) Most of the chemical energy released appears as internal excitation of the products. The surface ionization detectors used in these experiments do not reveal whether the product of the M + IX reactions is MI or MX (or both). However, kinematic analysis of the scattering and results of chemiluminescence experiments indicate that (5) MX is the principal product. The electron-jump model for alkali reactions again offers a plausible chemical basis for the observed reaction dynamics, including Property (5). A qualitative molecular-orbital argument predicts that the attacking alkali atom transfers its valence electron to an antibonding σ̄ orbital of IX which is made up primarily of the 5pz orbital of the I atom, but the charge shifts to the X atom as the intermediate (IX)− molecule–ion dissociates in the field of the incoming M+ ion.
Dissociative Thermal Electron Attachment to Some Aliphatic Chloro, Bromo, Iodo Compounds
Authors: Wentworth W. E. , George R. , Keith H.
Published: February 2004

Thermal electron attachment to several aliphatic fluoro, chloro, and bromo derivatives have been investigated in the range 30°–220°C and activation energies of 0–12 kcal have been determined. The relationship between the energy of activation (E*) and change in internal energy (ΔE) for the dissociative electron attachment reaction is uniquely linear with approximately a unit slope. Both chloro and bromo compounds apparently satisfy the same relationship. This empirical relationship can be used to predict bond dissociation energies for aliphatic chloro, bromo, and iodo compounds from the energy of activation for thermal electron attachment. Bond dissociation energies for 1,3-dichloropropane, 1,2-dibromoethane, and 1,3-dibromopropane have been so determined as 75.6, 69.8, 70.9 kcal/mole, respectively. An expression for a two-dimensional representation of the potential-energy curve for the negative ion is proposed with which the linear relationship between E* and ΔE can be derived. The proposed potential-energy curve is empirical, but contains only a single adjustable parameter characteristic of each compound.
Correlation of Electron Beam and Thermal Electron Attachment Studies for Some Chloro, Bromo, Iodo Aromatic Compounds
Authors: Steelhammer J. C. , Wentworth W. E.
Published: February 2004

The relationship between thermal electron attachment and electron beam studies for chlorobenzene, 0-dichlorobenzene, 0-chlorotoluene, 3′- and 4′-chloroacetophenone, 1-chloronaphthalene, bromobenzene, 0-bromotoluene, 1-bromonaphthalene, and iodobenzene is shown. The empirical negative-ion potential-energy function used to describe electron attachment to aliphatic halides is used for convenience in showing this relationship. Use of the two-dimensional potential-energy function permits a more quantitative representation of the mechanisms previously proposed. In some cases, the mechanisms have been altered from the earlier qualitative interpretation. In general the two types of measurement appear to be essentially in complete agreement. The agreement between the results further supports the empirical potential-energy function for the negative ion. Also, thermal electron attachment to additional aromatic halogen derivatives was investigated by the pulse sampling technique as a function of temperature to further support the mechanism. In particular, the thermal electron attachment of 3′- and 4′-chloroacetophenone shows a unique temperature dependence involving three different phenomena over the temperature range 30°–240°C, which adds further support to the proposed mechanisms. In all the other cases investigated (chlorobenzene, 0-chlorotoluene, 0-bromotoluene), the electron attachment shows only one temperature region, again in complete agreement with the proposed mechanisms.
Kinetics of the Homogeneous Exchange Reaction 12C18O+13C16O⇆13C18O+12C16O. Single-Pulse Shock-Tube Studies
Authors: Bar-Nun Akiva , Lifshitz Assa
Published: February 2004

The isotopic exchange reaction between two CO molecules was studied under homogeneous conditions in a single-pulse shock tube at high dilution in argon within the 2000–2675°K temperature range. From a total of 100 shocks, run at varying initial pressures and compositions, the following rate law was derived: rate of exchange= kb[CO]1.45[Ar]0.55, where kb = 1014.36±0.42exp{− (76 ± 5) × 103 / RT} cc mole−1·sec−1. The possibility of a chain mechanism was ruled out. A two-step mechanism composed of a vibrational excitation step and an atom-switching step is proposed. It was found that exchange occurs at high probability when one of the CO molecules is elevated to the 7th–10th vibrational level before colliding with another CO molecule. A dipole interaction stabilizing the transition state is proposed in explanation of the relatively low activation energy of this exchange reaction. Two alternative reaction systems are described, and the corresponding rate constants are computed and compared with the experimental results.
Effects of Atomic Interactions on the Absorption and Emission of Radiation
Authors: Hearn Dwight D. , Fontana Peter R.
Published: February 2004

The processes of absorption and emission of radiation by a system of two interacting atoms is investigated. Separation between the atoms is assumed to be much larger than the atomic diameter, and interactions considered are those between the individual atoms and the radiation field and the electrostatic interaction between the atoms. Calculations include those for the time-dependent probability amplitude of the intermediate and final states as well as that of the emission line shape. The line shape of the emitted radiation consists of two peaks whose amplitude, separation, and half-width are functions of both the internuclear separation R of the atoms and the energy difference of the excited states of the two atoms. For a large energy difference, the peaks are distinct; for small energy differences, the peaks are distinct at smaller separations, but merge toward the line shape characteristic of a single, isolated atom as R increases.
Ionization Constants of T2O and D2O at 25° from Cell emf's. Interpretation of the Hydrogen Isotope Effects in emf's
Authors: Goldblatt Maxwell , Jones Wesley M.
Published: February 2004

The ionization constants of T2O and D2O were determined to be 6.1 × 10−16 and 1.43 × 10−15 at 25°, corresponding to K(H2O) / K(T2O) = 16.4 and K(H2O) / K(D2O) = 7.1 (concentrations in aquamolalities m′, moles of solute per 55.51 moles of isotopic water). Emf's were measured at 25° with X = H, D, and T for cells of the types: Pt–X2(p) | NaOX(m2′)+NaCl(m3′) | AgCl–Ag and Pt–X2(p) | XCl(m1′) | AgCl–Ag; m1′ = m2′ = m3′ = 0.1. The isotope differences [E°(isotope) − E° (H2O)] in the alkaline cell are 45.8 ± 0.5 mV for D2O and 65.4 ± 0.3 mV for T2O. In the acid cell the differences are − 4.4 ± 0.1 mV for D2O and − 6.4 ± 0.2 mV for T2O. Published data on the H2O–D2O emf isotope effects from 5° to 50° in alkaline and acid cells are correlated with spectral and structural information by methods similar to those used in the treatment of vapor-pressure isotope effects. These methods and the H2O–D2O data are used to predict H2O–T2O isotope effects in emf's and a value of K(H2O) / K(T2O) which agree satisfactorily with experiment. Free energies of transfer of NaCl from D2O to H2O and from T2O to H2O are discussed.
Thermodynamic Functions in the Thomas–Fermi Atom Model
Authors: Gilvarry J. J.
Published: February 2004

In applications, the important energy associated with the Thomas–Fermi atom model is the difference u of the total energy from that corresponding to an infinite atom. That part u′ of the energy u that depends on the difference of the initial slope of the Thomas–Fermi function from its value for an infinite atom limits the accuracy possible in energies on the model, over the intermediate range of atom radius xb. Values of the difference in initial slope accurate to four significant figures for xb up to 16 and accurate to three figures for xb up to 50 have been computed, essentially by integrating the pressure. These data have been fitted by an analytic function showing the proper limiting forms through the two leading terms in each case as xb approaches zero and as xb approaches infinity. The accuracy of the fit is 0.1%, in general. In conjunction with a fitted function given by Gilvarry and March, the present results yield thermodynamic functions accurate to a few parts in a thousand as analytic functions of the radius, over essentially the entire span of this variable.
Mean-Square Intrachain Distances in a Self-Avoiding Walk
Authors: Domb C. , Hioe F. T.
Published: February 2004

An investigation is undertaken of the second-order moments associated with the distribution of intrachain elements in a self-avoiding-walk model of chain polymer. The mean-square distance of an element of the chain from an end point of the chain, 〈Qn2〉, and the mean-square distance of an element of the chain from the centre of mass, 〈Sn2〉, are enumerated exactly on various two- and three-dimensional lattices for short chains of n links. It is conjectured that as n → ∞, the limiting values of 〈Qn2〉 / 〈Rn2〉 and 〈Sn2〉 / 〈Rn2〉, 〈Rn2〉 being the mean-square end-to-end distance of the chain, depend on dimensionality rather than on lattice structure. Estimates for these limiting values in two and three dimensions are given.
Size Effect in Ionic Charge Relaxation following Auger Effect
Authors: Wertheim G. K. , Guggenheim H. J. , Buchanan D. N. E.
Published: February 2004

The endpoint of ionic charge relaxation following an Auger cascade in rutile fluorides is empirically related to the size of the impurity ion relative to that of the lattice site. Possible mechanisms for defect stabilization are discussed.
Studies of the Photoproduction of Electrons in Inert Solid Matrices. The Electronic Spectrum of the Species C2−
Authors: Milligan Dolphus E. , Jacox Marilyn E.
Published: February 2004

The vacuum-ultraviolet photolysis of acetylene isolated in an argon matrix at 14°K in the presence of a small concentration of a photoelectron source leads to a marked enhancement in the intensity of the 5206-Å band system previously assigned to the Swan transition of C2. Cesium atoms have been found to provide an especially abundant electron source for these studies. The vibrational band spacings observed for the 5206-Å band system agree much more closely with those recently reported by Herzberg and Lagerqvist for the 5416-Å band system of a species tentatively identified as C2−, produced upon flash discharge through methane, than with the band spacings observed in the gas phase for the Swan transition of C2. Accordingly, it is suggested that the 5416-Å band system of the gas-phase studies and the 5206-Å band system of the argon-matrix experiments correspond to the same transition. The enhancement of this band system in the presence of a photoelectron source supports the hypothesis that the species which contributes it is negatively charged.
Magnetothermodynamics of α-NiSO4·6H2O. V. Proton Spin Polarization Rate and Activation Enthalpy as a Function of Temperature and Field to 90 kG along the a Axis
Authors: Fisher R. A. , Hornung E. W. , Brodale G. E. , Giauque W. F.
Published: February 2004

The rate of proton polarization in α-NiSO4·6H2O has been measured as a function of magnetic field and temperature by means of the spontaneous heat evolution and change of temperature during polarization. Fields of 90, 65, 40, 25, 15, and 11 kG were directed along the a crystal axis of the 4-cm-diam spherical single crystal at temperatures over the range 1.5–0.4°K. The rate observations have been represented by equations of the form, kA = Z exp(− ΔH / RT). The activation enthalpies ΔH were found to correspond to the electron terms, W1–W0, previously derived from the authors' magnetothermodynamic measurements on this crystal. The proportionality factors, Z, in the above equation were field dependent with a minimum near 50 kG. The procedure involved cooling the high-temperature proton distribution in zero field with subsequent application of the field at temperatures sufficiently low so that the rate of polarization was very small.
Zeeman Effect of Cr3+ in ZnAl2O4
Authors: Wood D. L. , Burke W. E. , Van Uitert L. G.
Published: February 2004

The Zeeman effect for Cr3+ in the trigonally distorted octahedral site of ZnAl2O4 has been measured for applied fields up to 32 kOe. The lines whose splittings were observed correspond to the transitions E(Ē)2 and E(2Ā)→42A2(Ē) and A24(2Ā) (the R lines) in fluorescence. For H applied along the crystal [001] axis all four sites in the unit cell are equivalent and one of the upper-state g values is zero. Under these circumstances some of the observed lines correspond to a direct observation of the ground-state splittings in the nonlinear region of level crossing. The anisotropy of the upper and lower state g factors was determined from experiments in which the sample was rotated around a [110] direction with the magnetic field applied perpendicular to the rotation direction. The g values determined from the spectra are compared with calculated values from crystal-field theory given in the literature.
Quadrupole Coupling in Dicobalt Octacarbonyl
Authors: Mooberry E. S. , Spiess H. W. , Garrett B. B. , Sheline R. K.
Published: February 2004

The pure nuclear quadrupole resonance spectrum of 59Co in dicobaltoctacarbonyl was studied in the temperature range of − 196 to − 30°C. At liquid-nitrogen temperature, the quadrupole coupling parameters for the two independent crystallographic sites are (A)e2qQ / h = 90.18 ± 0.15 MHz, η = 0.3149 ± 0.001 and (B)e2qQ / h = 89.30 ± 0.15 MHz, η = 0.4837 ± 0.001. The asymmetry parameters for the two sites approach each other with increasing temperature. To understand the observed parameters, the environment of the cobalt atom in Co2(CO)8 is described in terms of an octahedron with distortions along its three and fourfold axes. Defining two corresponding axially symmetric field gradient tensors with z components qa and qb, a quadratic equation relating the ratio qa / qb to the observed asymmetry parameter η is derived. The calculated field gradient with the z axis along the Co–Co direction is in reasonable agreement with the observed quadrupole splitting in Fe2(CO)9. The difference in the η values for the two sites and the temperature dependence is explained assuming additional distortions of the molecule in the solid. The observed quadrupole coupling constant is shown to be almost equal to the contribution of the tetragonal distortion of the molecule. This part of the field gradient is attributed to a difference in occupation number between those bonding orbitals directed towards the position not occupied by a bridging carbonyl and other bonding orbitals. Values of 0.16–0.18 are obtained for the population difference for a single molecular orbital, depending on the degree of 4p participation in the bonding.
121Sb Isomer Shifts in Antimony Halides
Authors: Bowen L. H. , Stevens J. G. , Long G. G.
Published: February 2004

Mössbauer spectra of 121Sb in all the binary antimony halides have been obtained, as well as in cubic Sb2O3. The isomer shifts relative to the 121SnO2 source are − 11.3 mm/sec (Sb2O3), − 13.8 (SbCl3), − 13.9 (SbBr3), − 15.9 (SbI3), − 14.6 (SbF3), + 2.2 (SbF5), and − 3.12 (SbCl5). Quadrupole splittings were obtained for most of the antimony(III) compounds. The linear variation of isomer shift with ligand–metal electronegativity difference for Sb2O3, SbCl3, SbBr3 and SbI3 is interpreted as due to a constant s character in the ligand–metal bonds. The SbF3 does not obey this relationship due to its radically different geometry.
Fluorescence Spectra Resulting from vuv Photon Impact on COS
Authors: Judge D. L. , Ogawa M.
Published: February 2004

The COS+Ã2Π–X̃2Π emission bands between 3500 and 6500 Å have been observed following vuv photon impact on COS. Only the υ′ = 0 progression is found to occur for incident vuv photons in the energy range 15.11–20.3 eV. The absence of emission from the higher vibrational levels is interpreted as evidence of predissociation of the Ã 2Π state into CO(X1Σ+) + S+(2D0) components.
Photodissociation of Methane
Authors: Karplus Susan , Bersohn Richard
Published: February 2004

A valence-bond calculation is carried out on the ground and six excited energy levels of methane. The molecule is deformed in a C2v fashion so as to produce incipiently CH2 and H2, and in a C3v fashion to obtain CH3 and H. Potential surfaces are obtained which permit one with classical mechanics to determine the shape of the molecule during its dissociation. The crude calculation leads to three principal results: (1) the lowest excited state is a forbidden E state; (2) in the next-highest allowed T2 state there is a steeper potential gradient in the CH3 and H direction than in the CH2 and H2 direction; the reverse is true for the E state; and (3) the CH2 derived from the allowed T2 state is produced in an electronically excited state. It follows that, unless there is a crossover from T2 to E, the red bands of CH2 should be observable in emission when methane is photolyzed.
Elastic Scattering of Low-Energy Electrons by the Hydrogen Molecule
Authors: Tully John C. , Berry R. Stephen
Published: February 2004

Electron–H2 scattering at low energies is treated by an efficient numerical solution of the two-dimensional Schrödinger equation including the nonlocal exchange interaction. Correlation within the scatterer is included but polarization and short-range free–bound correlation are omitted. The elastic differential cross section is well described for scattering angles larger than about 30°–45°, and for energies up to at least 5 eV. The scattering function is analyzed in terms of spherical harmonics; the expansion is slowly convergent in the region 0.6–1 Bohr radii, with significant contributions as high as l = 10. The position of the observed broad resonance at about 3 eV is accounted for by the model in terms of the pσ wave, but it is inferred that free–bound correlation is necessary to account quantitatively for the width.
Localized Bonds in SCF Wavefunctions for Polyatomic Molecules. I. Diborane
Authors: Switkes Eugene , Stevens Richard M. , Lipscomb William N. , Newton Marshall D.
Published: February 2004

Molecular SCF orbitals of B2H6 have been computed from optimized minimum basis sets which employ isotropic or anisotropic atomic 2p orbitals. These SCF wavefunctions have been transformed to localized MO's which maximize the self-energy, D = Σi(φiφi | φiφi). This objective procedure strongly supports the three-center bond for each BHB bridge. The resulting hybrids are sp2.5, with ∠Ht–B–Ht=125° and ∠Hb–B–Hb=93° for terminal and bridge H's, respectively.
Geometry and Vibrational Spectra of Alkaline-Earth Dihalides. II. CaF2, SrF2, and BaF2
Authors: Calder V. , Mann D. E. , Seshadri K. S. , Allavena M. , White David
Published: February 2004

The infrared spectra of 40CaF2, 44CaF2, 86SrF2, 88SrF2, and BaF2 trapped in solid krypton matrices at 20°K are reported. From precise measurements of the shifts of the vibrational modes on isotopic substitution upper and lower limits for the apex angles of these molecules can be established. The mean values of the apex angle based on these limits are found to be 140° for CaF2, 108° for SrF2, and a reanalysis of the earlier data for MgF2 gives 158°. For BaF2 the apex angle has been estimated at 100°. Using a valence force field a set of force constants have been obtained from the measured fundamental frequencies, as well as approximate values for the anharmonic corrections to these vibrational modes. The statistical entropies calculated from the vibrational frequencies and molecular geometries are in excellent agreement with the thermodynamic measurements.
Effect of Internal Excitation on the Collision-Induced Dissociation of 2000-eV Diatomic Molecule Ions
Authors: Moran T. F. , Petty F. C. , Hedrick A. F.
Published: February 2004

Collision-induced dissociations of 2000-eV N2+, O2+, CO+, and NO+ ions have been studied as a function of reactant ion internal energy. Relative cross sections and product ion velocity distributions for dissociative scattering processes through angles less than 2° have been examined as a function of reactant ion internal energy by changing the kinetic energy of the ionizing electrons. Product ion velocity distributions are consistent with a mechanism that assumes Franck–Condon-type transitions between electronic states of the reactant species. These dissociative reactions involving diatomic molecule ions have been used to probe the internal energy content of NO+ and CO+ ions resulting from the unimolecular decomposition of triatomic molecule ions formed by electron-impact ionization.
Simultaneous and Induced Electronic Transitions in Oxygen. II
Authors: Krishna V. G. , Cassen T.
Published: February 2004

A general theory of the oscillator strengths of the three bimolecular transitions, Σg+31Σg−←3Σg−3Σg−, Σg+11Δg←3Σg−3Σg−, and Σg+11Σg+←3Σg−3Σg−, in oxygen is given. The intensity of these three transitions is assumed to be derived from the allowed transitions due to mixing of the electronic terms through the intermolecular interaction. The theoretical techniques developed in Paper I of this series are used. No particular geometry for the O2···O2 pair is assumed. All electrons are included in the theory and all the interactions, allowed in the absence of intermolecular overlap, are considered. Two experimentally significant points emerge from this work. (1) Each of these three transitions should be considerably weaker than either of the two transitions Δg1 3Σg←3Σg−3Σg− and Δg1 1Δg←3Σg−3Σg− discussed in Paper I. This is because while the latter transitions can borrow from several allowed transitions, including the strongly-allowed π–π transitions, the former can borrow only from transitions involving π–σ excitation which are expected to be at higher energy and to have relatively small oscillator strengths. (2) The relative oscillator strengths of these three transitions considered here agree fairly well with experiments.
EPR Studies of the Rotation of Phenyl Rings in Benzophenone Ketyls
Authors: Takeshita Takuo , Hirota Noboru
Published: February 2004

Temperature dependence of the EPR spectra of various benzophenone ketyls and related compounds was studied in detail. Complete ranges of spectra from the spectra in the limit of rapid rotation of phenyl ring to the spectra in the limit of slow rotation of phenyl ring were obtained for benzophenone free ions and ion-pair ketyls and explained in terms of Freed–Fraenkel relaxation matrix theory, using a four-jump model. Unequivalent ortho and meta hyperfine splittings, rates, and activation energies for the rotation of phenyl rings were determined. Effects of ion-pair formation and higher aggregation on the rates of rotations of rings have been studied in several systems.
Fine-Structure Studies of Diatomic Molecules: Two-Electron Spin–Spin and Spin–Orbit Integrals
Authors: Matcha R. L. , Kern C. W. , Schrader D. M.
Published: February 2004

The two-electron integrals that contribute to the spin–spin and spin–orbit interactions in diatomic molecules are evaluated for wavefunctions constructed from Slater orbitals. For arbitrary combinations of quantum numbers, expressions are derived in tensor–operator form for all the one- and two-center Coulomb, hybrid, and exchange terms that occur. An attempt is made in the analysis to maximize compatibility with some of the previous treatments of interelectronic repulsion integrals.
Vibrational Relaxation in CO2 and CO2–Ar Mixtures Studied Using a Shock Tube and a Laser–Schlieren Technique
Authors: Simpson C. J. S. M. , Chandler T. R. D. , Strawson A. C.
Published: February 2004

Vibrational relaxation has been studied in CO2 and CO2–Ar mixtures over the temperature range 360°–3000°K using a laser–schlieren method. The purpose of this investigation was to compare the results obtained by this technique with those deduced using a Mach–Zehnder interferometer and to provide further evidence as to whether relaxation times depend upon how far the system is from equilibrium as well as on the translational temperature. Relaxation times measured by these two methods agree well. The laser–schlieren method is the more satisfactory for measuring the rate of relaxation and the Mach–Zehnder for finding the total density change during the process. These new results support the view that relaxation times do not depend upon how far the system is from equilibrium. The measurements using CO2 and CO2–Ar mixtures show that CO2 is about five times as effective as Ar in causing relaxation at 360°K and two times as effective at 2000°K.
Fluorescence Quenching in Aromatic Hydrocarbons by Oxygen
Authors: Parmenter C. S. , Rau J. D.
Published: February 2004

The role of the possible fluorescence quenching process S1 + O2(1Σg−) → T + O2(1Δg) which involves energy transfer to oxygen is examined in a number of aromatic hydrocarbons. It is observed that quenching is just as efficient in aromatics where the S1–T energy gap is too small (< 7900 cm−1) for excitation of oxygen in the above process as in aromatics where it is allowed energetically. This process cannot therefore account for the general phenomenon of fluorescence quenching by oxygen in aromatics. Evidence bearing on other quenching mechanisms is discussed.
Nonequilibrium Unimolecular Decomposition of Ethylene
Authors: Kirk A. W. , Tschuikow-Roux E.
Published: February 2004

Some of the experimental data in the literature on the nonequilibrium decomposition of ethylene have been examined in the light of calculations using a simple Rice–Ramsperger–Kassel–Marcus treatment. An activation energy of approximately 80 kcal mole−1 is indicated for the molecular elimination of hydrogen. Of the energy available to the products of the initial processes by which excited ethylenes are formed, the fraction retained by ethylene does not appear to be constant, as was previously concluded using the classical RRK theory of unimolecular reactions. In the case of ethylene produced in the photolysis of 3-methyldiazirine, an alternative explanation for the limiting yield of acetylene at low pressure is offered, based upon an arbitrary energy distribution selected to match the experimental data.
Triplet Energy Migration in Pure Fluids
Authors: Holzman P. , Jarnagin R. C.
Published: February 2004

Delayed excimer emission in liquid naphthalene, β-methylnaphthalene, and α-methylnaphthalene was used to estimate the specific rate of triplet–triplet interaction and triplet yield. The large values of the interaction rate are consistent with a rapid energy migration in these pure fluids.
Effect of Oxygen and Hydrogen Atoms on the Vibrational Distribution of N2(B 3Π) in the Nitrogen Afterglow
Authors: Oldman R. J. , Broida H. P.
Published: February 2004

The effect of oxygen, hydrogen, and deuterium atoms and of molecular O2, D2, and SF6 on the vibrational population distribution of N2(B 3Π) in nitrogen afterglows has been investigated for pressures between 0.01 and 2.0 torr. At N2 pressures less than 0.1 torr, these atoms cause preferential population in the lower vibrational levels, and all levels appear to have a high rotational temperature. Emission from the N2 first-positive system is increased by as much as a factor of 60 by the addition of oxygen atoms, and there is a pronounced population maximum at υ′ = 6 with smaller maxima at υ′ = 2, 4, 8, and 12. Hydrogen and deuterium atoms decrease the populations in high vibrational levels and have small population peaks at υ′ = 2 and 12.
Phosphorescence Decay of Benzene and Methylbenzene Derivatives
Authors: Rabalais J. W. , Maria H. J. , McGlynn S. P.
Published: February 2004

Phosphorescence lifetimes of benzene and methylbenzene derivatives, deuterated and undeuterated, have been measured in glassy matrices at 4.2 and 77°K. Spin–orbit coupling calculations have been performed for benzene (D6h), distorted benzene (D2h), and toluene; these calculations included spin–own-orbit, spin–other-orbit, and vibronic considerations and were extended to all the methylbenzenes by a vector-sum semiempirical method. Franck–Condon calculations were performed for all methylated, deuterated, and distorted benzene derivatives. The results of these calculations lead to a general understanding of phosphorescence lifetimes, at least insofar as these are affected by either methylation or deuteration or both. Matrix effects and temperature effects on the phosphorescence lifetimes are largest in the case of benzene and toluene and decrease with increasing methylation of benzene and with increasing size of the polynuclear hydrocarbon. We attribute this sensitivity to the small size of the space available to the π subsystem in benzene, to the forbiddenness of the electric-dipole T1 → S0 process in this system, and to the ease with which matrix effects may perturb the rather “naked” skeleton of this molecule and its less-methylated derivatives. A concept of solvent-assisted distortions is introduced in order to explain the direction of solvent and temperature effects and the means whereby these effects become operative. This last is a very tenuous suggestion, its primary merit being that it provides some rationalization of the observed phenomenology.
Photoelectron Spectroscopy of Autoionization Peaks
Authors: Berkowitz J. , Chupka W. A.
Published: February 2004

A vacuum ultraviolet monochromator has been combined with a retarding-field electron-energy analyzer to study the distribution of photoelectrons from autoionizing states of hydrogen and nitrogen. In most cases involving hydrogen, the photoelectrons produced are monoenergetic and correspond to autoionizing transitions in which Δυ is a minimum, i.e., the ejected electrons have the minimum kinetic energy. There is one possible exception to this generalization. Different mechanisms appear to be operative in the nitrogen autoionizations. In general, several bundles of electrons, corresponding to the formation of several ionic states, are observed. Whereas vibrational relaxation of the core appears to explain the results with hydrogen, an alternative mechanism involving configuration interaction has been examined in an attempt to rationalize the nitrogen results. This proposed mechanism implies that relative transition probabilities in the autoionization process are dominated by Franck–Condon factors connecting the quasidiscrete state and the ionic state. The ratio of Franck–Condon factors is very sensitive to the choice of internuclear distance for the quasidiscrete state. In many cases, it is possible to find a plausible internuclear distance for which the ratio of Franck–Condon factors agrees with experiment. If the Franck–Condon factors are indeed the major determinant of relative transition probabilities, photoelectron spectroscopy offers a method for determining the internuclear distances of the quasidiscrete states. One autoionizing transition in nitrogen, as yet unassigned, does not seem to fit this mechanism.
Bipolar Expansion for r12nYlm(θ12, φ12)
Authors: Kay Kenneth G. , Todd H. David , Silverstone Harris J.
Published: February 2004

Explicit formulas for the radial functions Vl1l2l3l(n)(r1, r2, R) in the bipolar expansion for r12nYlm(θ12, φ12), r12nYlm(θ12, φ12) = Σ(2λ + 1)1/2(2l3 + 1)1/2cλ( l1m1)c3l(λ, m − m1; l2m2) × Yl1m1(θ1, φ1)Yl2m2(θ2,φ2) Yl3m−m1−m2(θR, φR)Vl1l2l3l(n)(r1, r2, R), where r12 = r1 − r2 − R, are derived with the use of the theory of generalized functions and Fourier transforms. When n ≤ − 4 and n − l is odd, there are delta-function terms. In this approach the delta-function terms and the four-region form of the expansion are obtained from a single, unified formula valid in all regions. Recurrence formulas for the Vl1l2l3l(n) are given.
Diamagnetic Susceptibility of Hydrazinium (1 +) and Hydrazinium (2 +) Compounds
Authors: Zupan J. , Glavič P. , Drofenik M.
Published: February 2004

The diamagnetic susceptibilities of a number of solid hydrazinium salts have been determined by the Gouy method. The results have been compared with the diamagnetic susceptibilities of the corresponding alkali and ammonium salts. It has been confirmed that the relationship between the molar susceptibility of alkali, ammonium, and hydrazinium (1 +) halides and the effective atomic number of the cation is a periodic one. The diamagnetism of the hydrazinium (1 +) and hydrazinium (2 +) ions was estimated to be (− 20.3 ± 0.3) × 10−6emu/mole. The magnetic susceptibility of the hydrazinium (2 +) ion has been treated theoretically in the LCAO MO approximation and a good agreement with the observed susceptibility value of this ion has been found. A theoretical prediction about the diamagnetic anisotropy of the hydrazinium (2 +) ion was made. The values calculated by the Van Vleck method are about 30% smaller in the x and y directions as compared with the z one. Only a 10% difference was obtained by Pople's MO theory.
Spin–Orbit Coupling in Aromatic Hydrocarbons. Calculation of the Radiative Triplet Lifetimes of Naphthalene, Anthracene, and Phenanthrene
Authors: Henry Bryan R. , Siebrand Willem
Published: February 2004

A formalism is developed to calculate spin–orbit interactions in aromatic hydrocarbons. The theory is based on σπ interactions and involves singlet and triplet σπ* and πσ* states and their interactions with both the ground and lowest triplet states. CH and CC σ electrons are treated separately, and their wavefunctions are written as linear combinations of bonding and antibonding CH and CC orbitals. An estimate is made of the σ-state energies on the basis of spectroscopic and theoretical data. The calculations are restricted to one-center, one-electron terms in the matrix elements of the spin–orbit Hamiltonian. Radiative lifetimes obtained for naphthalene, anthracene and phenanthrene are in reasonable agreement with experiment. The calculations show the importance of including all of the mixing mechanisms considered here in any calculation of spin–orbit coupling in these molecules.
Field Theoretical Methods in Classical Statistical Mechanics. I
Authors: Paul R.
Published: February 2004

Equations of motions and a diagram technique are developed for the classical-mechanical Green's functions. The Boltzmann equation and a classical reaction-matrix operator analogous to the quantum-mechanical T matrix are derived.
Angular Relaxation of the Symmetrical Top
Authors: Fixman M. , Rider K.
Published: February 2004

The calculation of 〈Pn[cosθ(t)]〉 is formulated for the symmetric top, and evaluated for several special models; Pn is the Legendre polynomial and θ(t) the angle made by the symmetry axis with its initial direction. The collision operator is presumed to arise from a time independent transition probability, and includes the Boltzmann integral operator or Langevin differential operator as special cases. Diffusional relaxation is recovered only if the collision frequency is large, and is not the general behavior for large t.
Scattering of 0.05–5-eV Argon from the (111) Plane of Silver
Authors: Romney Michael J. , Anderson James B.
Published: February 2004

The scattering of atomic beams of argon from the (111) plane of silver has been measured for incident energies in the range of 0.05–5 eV. Nearly monoenergetic beams were produced by the use of nozzle sources with pure argon and mixtures of argon with helium or hydrogen. Epitaxially grown crystal surfaces were obtained by vapor deposition of silver on mica. The observed scattering patterns were single lobes with the maxima lying near the specular angle at all energies. Results were similar to those of Smith, Saltsburg, and Palmer at energies below 0.3 eV with the maxima shifting toward the tangent to the surface with increasing energy. At higher energies the maxima were observed to shift back toward the normal as predicted by the theoretical calculations by Oman. Comparison of results with the simple hard- and soft-cube models for scattering indicates limitations of these models at hyperthermal energies.
Molecular Structure of Thionyltetrafluoride, SOF4
Authors: Gundersen Grete , Hedberg Kenneth
Published: February 2004

A gaseous electron-diffraction investigation of SOF4 has led to the discovery of four models in excellent agreement with experiment, all with molecular symmetry C2υ corresponding to replacement of an equatorial fluorine atom of a trigonal bipyramid with oxygen. The models differ largely in the relative magnitudes of the F(eq)···F(eq) and F(eq)···O distances and the F(ax)···F(eq) and F(ax)···O distances. The favored model has the following distance (ra), angle, and root-mean-square amplitude (la) values (parenthesized errors are 2σ): S=O=1.403 Å (0.0032), S–F(eq) = 1.552 Å (0.0043), S–F(ax) = 1.575 Å (0.0038), F(eq)···F(eq) = 2.545 Å (0.0259), F(ax)···O = 2.121 Å (0.0073), F(eq)···O = 2.621 Å (0.0139), F(ax)···F(eq) = 2.204 Å (0.0050), F(ax)··· = 3.150 Å (0.0076), ∠F(eq)SF(eq) = 110.17°(1.82), ∠F(ax)SO = 90.65°(0.42), ∠F(ax)SF(eq) = 89.63°(0.24), ∠F(eq)SO = 124.91°(0.92), lS = O = 0.0367Å (0.0050), lS–F(eq) = lS–F(ax) = 0.0540Å (0.0029), lF(eq)...F(eq) = 0.0468Å (0.0124), lF(ax)...O = 0.0431Å (0.0055), lF(eq)...O = 0.0758Å (0.0192), lF(ax)...F(eq) = 0.0962Å (0.0074), and lF(ax)...F(ax) = 0.0611Å (0.0122).