bluuebunny/crossref_metadata_2025 huggingface dataset:
Optical Spectra of Small Rings. II. The Unsaturated Three-Membered Rings
Authors: Robin M. B. , Basch Harold , Kuebler N. A. , Wiberg K. B. , Ellison G. B.
Published: February 2004

The electronic spectra of the unsaturated three-membered rings cyclopropene, 3,3-dimethylcyclopropene, and difluorodiazirine were recorded down to 85 000 cm−1 and are compared with calculations of their optical spectra performed using Gaussian-type orbitals, with limited configuration interaction. The π → π*(1A1 → 1B1) excitations are computed to fall in the region expected for olefins and azoalkanes, but with oscillator strengths only half their normal value, as observed. Calculations on diazirine and difluorodiazirine correctly predict the lower n → π*(1A1 → 1B2) excitation energies and, in addition, predict a second, forbidden n → π*(1A1 → 1A2) band about 3 eV higher than the lower, allowed one. This too has been observed experimentally. In the cyclopropenes, the analog of the lower n → π* bands of the diazirines is a valence shell σ → π*(1A1 → 1B2) excitation, which is computed to lie well below either π → π* or π → σ* excitations. A low-lying valence shell excitation which is observed in cyclopropene and 3,3-dimethylcyclopropene is tentatively assigned as a Berry-type σ → π* promotion. Just as in the spectra of the saturated rings, the unsaturated rings also display intense external σ → external σ* bands in the deep ultraviolet. Various ground-state properties were also calculated for the three-membered rings and compare favorably with experimental values where such are available. Adequate agreement was obtained with ground-state dipole, quadrupole, and second moments, but the agreement was poor for the components of the electric field gradient tensor at the N nuclei of diazirine. Ionization potentials and diamagnetic susceptibilities were also calculated.
De-excitation of Electronically Excited Sodium by Nitrogen
Authors: Bauer Ernest , Fisher E. R. , Gilmore F. R.
Published: February 2004

A semiquantitative calculation is made of the cross section for the quenching of Na(32P) by molecular nitrogen, as a function of initial kinetic energy and of final vibrational quantum number, υf, of the nitrogen molecule. The large observed cross section, which is of gas-kinetic order, can be explained in terms of an intermediate ionic state, involving Na+ and N2− (υ = υ−). This state is unstable at infinite separation of Na and N2, but because of the Coulomb attraction it becomes stable at collision distances below about 3 Å. As a result of the vibrational structure of both the intermediate and final states, we treat the reaction in terms of a diffusion of the probability flux through a two-dimensional network of potential-energy curves parametrized by both the electronic state and also the vibrational quantum numbers υ− and υf. At each potential-energy curve crossing we compute the transition matrix element for insertion into a Landau–Zener type of transition probability. The transition matrix element is represented as the product of an electronic interaction function (obtained from a correlation, due to Hasted and Chong, of results obtained from charge-transfer processes involving multiply charged ions) and a vibrational overlap integral or Franck–Condon factor. Results are also presented on the quenching of Na(42P) by N2, and on the quenching of Na(32P) by CO. All the results have the same general character: The total cross section is of gas-kinetic order and depends only weakly on kinetic energy. The partial cross sections for excitation of the different final vibrational levels vf show a rather broad distribution, with somewhat more than half the energy of electronic excitation ending up as vibrational excitation.
Magnetic-Field Effects on a Compensated Cholesteric Liquid Crystal
Authors: Laronge Thomas M. , Baessler H. , Labes M. M.
Published: February 2004

By measuring changes in the dielectric constant of a 1.75:1 cholesteryl chloride–cholesteryl myristate (CM) mixture in the presence of magnetic fields H up to 10 kG, the orientation of the molecular and helical axes with respect to the field direction can be deduced, and the possibility of a dependence of the alignment process on the pitch Z of the helix is explored. When the sample thickness is much greater than Z, there is no preferred direction for the helix axis in the bulk, and applying H either ⊥ or ‖ to the sample causes the helix axis to align | H. This effect only occurs at Z > 10 μ, and only partial ordering occurs even at Tnematic. The angle δ between the long molecular axis of cholesteryl chloride and a plane ⊥ to the helix axis is reduced by application of H. Since δ is determined by the internal mechanical energy of the cholesteric structure, the decrease in δ becomes more pronounced as Z approaches infinity. A comparison is also made of the effects of electric and magnetic fields on CM; an electric field of 500 V/cm counteracts a magnetic field of the order of 10 kG. The anisotropy of the diamagnetic susceptibility is deduced to be ∼10−9.
On Symmetric Multicomponent Diffusion Coefficients
Authors: Condiff D. W.
Published: February 2004

The symmetric set of isotropic multicomponent diffusion coefficients introduced by Curtiss in the context of the kinetic theory of dilute gases are given a general definition and shown to possess general symmetry properties by virtue of Onsager reciprocal relations. Their relationships to Onsager coefficients, Curtiss–Hirschfelder diffusion coefficients, and Bearman–Kirkwood multicomponent friction coefficients are established and employed in stability considerations to deduce results which include the vanishing of Curtiss–Hirschfelder thermal diffusion coefficients in dilute solution as a consequence of the second law of thermodynamics.
Electron Paramagnetic Resonance Study of the Stereochemistry of Nitrosylhemoglobin
Authors: Chien James C. W.
Published: February 2004

Paramagnetic resonance spectra of single crystals of nitrosylhemoglobin (HbNO) have been obtained at − 195°C. There are four nonequivalent paramagnetic atoms which form two sets; the members of each set are related by the [010] dyad axis. The principal values of the g tensor are: gxx = 2.0820, gyy = 2.0254, and gzz = 1.9909; the isotropic g value is 2.039. The principal values of the hyperfine A tensors are: Tξξ = 77 MHz, Tηη = 54 MHz, and Tξξ = 18 MHz; the isotropic coupling constant has a value of 50 MHz. The calculated principal values for the dipolar T′ tensors are: tξξ′ = 27 MHz, tηη′ = 5 MHz, and tξξ′ = −32 MHz. The principal axes of the g tensor do not coincide with these of the T tensor; the results were interpreted to give a bent Fe–N–O angle of 110°. The N–O axes of the ligands of one set lie in the ab plane; those of the others are inclined at an angle of about 80° to the ab plane. The electrostatic interaction of the oxygen atom with the imidazolium group of the distal histidine and the nonpolar evnironment in the vicinity of the heme probably contribute to the stabilization of this particular Fe–N–O bond.
Quenching of Lyman-α Fluorescence
Authors: Wauchop T. S. , McEwan M. J. , Phillips L. F.
Published: February 2004

Intensities of scattered Lyman-α radiation have been measured for mixtures of partially dissociated hydrogen in argon with variable amounts of O2, N2, NO, N2O, CO, CO2, or SO2. Quenching cross sections measured for each of these gases, and obtained indirectly for H2 and Ar, have been corrected for radiation trapping using the theory of Holstein. We find σ2 values of 45 ± 15 (O2), 33 ± 10 (N2), 49 ± 25 (NO), 86 ± 30 (N2O), 52 ± 20 (CO), 120 ± 45 (CO2), 170 ± 50 (SO2), 30 ± 15 (H2), and 11 ± 5 (Ar) (all × 10−19 cm−2).
Photoionization Study of the Dissociation Energy of Fluorine and the Heat of Formation of Hydrogen Fluoride
Authors: Dibeler Vernon H. , Walker James A. , McCulloh K. E.
Published: February 2004

Mass spectra and photoionization yield curves are obtained for the molecular and atomic ions of fluorine and hydrogen fluoride. The atomic ions of both molecules are formed by ion-pair and by dissociative ionization processes. The F2+ curve has a weak onset at 15.58 eV, ascribed to a hot band, followed by an abrupt, intense onset at 15.69 eV, ascribed to the 0–0 transition. Several broad bands of autoionization are observed. The onset of the dissociative ionization process occurs at 18.76 eV, from which we calculate D°0(F2) = 1.34 ± 0.03 eV (30.9 kcal mole−1). The only observed ion-pair process is spin forbidden, producing F+(3P) + F−(1S), for which the thermodynamic threshold is not reached. The HF+ curve has a slightly tailing onset at 15.92 eV. No F+ ion is observed. The dissociative ionization process yielding H+ occurs at 19.34 eV, permitting the calculation of D°0(HF) = 5.74 eV, and ΔHf°0(HF) = −2.83 ± 0.03 eV (−65.3 kcal mole−1). Identical ion-pair curves have a threshold at 15.87 eV resulting in EA (F) = 3.47 eV. The indicated uncertainties are estimated limits of error.
Structure Studies of Solutions of Large Organic Molecules. I. C9H19I and C18H36I2 in Decalin
Authors: Brady G. W. , Cohen-Addad C. , Lyden E. F. X.
Published: February 2004

The x-ray scattering from solutions of C9H19I and C18H36I2 in decalin, C10H18, at various concentrations has been studied. The principal aim of the research was to develop a technique for the direct determination of end-to-end distances of polymers in solution. This was achieved, and the method is outlined in detail. In the course of the work some interesting clustering phenomena were discovered in the monoiodide solutions. A model is proposed which has as its main feature the formation of quasi-liquid-crystal regions, with the alkane chains lying parallel to each other and the I atoms in contact.
Effect of Charge on the EPR Hyperconjugative Interaction Parameter
Authors: Underwood Graham R. , Vogel Veronica L.
Published: February 2004

Extensive McLachlan MO calculations have been performed on a number of methyl-substituted radicals and radical ions to obtain a consistent set of parameters to describe the methyl group and to examine the effect of charge density on the parameter QCCH3H which relates the methyl hfsc (hyperfine splitting constant) to the spin density. It was found that there was a strong dependence of QCCH3H on charge, but it was concluded that other factors were involved and that caution should be exercised when employing β-hydrogen hfsc's as a measure of spin density.
Equilibrium Properties of a Superfluid Separator for Enriching the Concentration of 3He
Authors: Listerman Thomas W.
Published: February 2004

The equilibrium properties of a superfluid separator for enriching the concentration of 3He in a liquid 3He–4He solution have been investigated. Measurements were made at selected temperatures above 1°K and at equilibrium vapor pressures throughout the entire range of attainable 3He concentrations. The data were grossly describable by classical thermodynamics using previously reported experimental data in the calculations. Small systematic deviations between the thermodynamic predictions and the measurements were observed. These deviations agree with those observed by London, Clarke, and Mendoza.
Virial Coefficients of Methanol from 150 to 300°C and Polymerization in the Vapor
Authors: Kell G. S. , McLaurin G. E.
Published: February 2004

The second and third virial coefficients of methanol have been determined from 150 to 300°C. At most temperatures the estimated standard error of the second is less than 1%. These values of the second virial coefficient are less negative than results in which the third and higher virial coefficients were neglected. The virial coefficients are interpreted in terms of equilibria between monomer, dimer, and trimer; the enthalpies and entropies of dimerization and trimerization are in line with those found for steam.
Quantum Hard Disks and Hard Spheres at High Densities
Authors: Hubbard Joseph B. , Masinter Larry , Salsburg Z. W.
Published: February 2004

A high-density asymptotic expression for the ground-state energy per particle for a system of d-dimensional hard spheres at density ρ is given by E0 ≈ lim ρ→ρ0; (ℏ2K0/2m) (ρ−1/d−ρ0−1/d)−2, where 2πℏ = h is Planck's constant, m is the particle mass, ρ0 is the highest attainable density, and K0 is a constant that depends on the type of hard-sphere packing. Rigorous upper and lower bonds on K0 of 27.0 and 1.63 are established for hard spheres in fcc or hcp lattices, and an estimate for the lower bound on K0, believed to be accurate to within 5%, is 5.94. A cell-cluster expansion is used to approximate FN, the Helmholtz free energy, for a hard-sphere system at high density and low temperature, and the low-temperature limit for disks in a triangular lattice is N−1FN→ lim T→0E0 ≈ lim a→σ h2(5.3667 + 1.881)/2m(a − σ)2, a being the nearest-neighbor lattice spacing and σ the hard-disk diameter. The second term in the constant factor is the cell-cluster correction to the free volume approximation due to pair correlations. As an aid in formulating the general problem for hard spheres at high density it is proved that in the asymptotic limit π → ρ0, the eigenvalue problem for the region of accessible configurations becomes that of determining the frequencies of a dN-dimensional resonance cavity whose boundary consists of intersecting (dN−1)-dimensional hyperplanes.
Ionic Reactions in Gaseous Cyclobutane
Authors: Hughes B. Mason , Tiernan Thomas O.
Published: February 2004

Ionic reactions in gaseous cyclobutane have been examined at pressures up to 1 torr in a specially designed ion source fitted to a time-of-flight mass spectrometer. It was observed that C4H8+ and C4H7+ are unreactive with cyclobutane, although both of these ions appear to be produced by reactions of other fragment ions. These conclusions were confirmed by experiments in a tandem mass spectrometer in which the reactions of individual ions of the cyclobutane system were identified at impacting ion kinetic energies as low as 0.3 eV. It was shown that ethylene and acetylene ions charge transfer to cyclobutane with high efficiency while the remaining fragment ions react principally by a hydride-transfer mechanism. Isotopic techniques were employed to aid in establishing the reaction mechanisms. The interactions of cyclobutane ions with various molecules either employed as additives or formed as decomposition products in radiolysis experiments have also been examined by mass-spectrometric techniques. It was observed that the addition of small quantities of NO, (CH3)3N, C2H4, or cis-2-C4H8, for example, leads to the production of the corresponding parent ion–additive complex at higher pressures. The H2S, on the other hand, was observed to intercept C4H7+ ions. Rate constants for charge-transfer reactions of parent cyclobutane ions with a series of molecules have also been determined. Data from these and related experiments are consistent with rearrangement of cyclobutane parent ions to linear butene structures prior to reaction. The implications of these findings for the radiolysis of cyclobutane are discussed.
Electronic Structure of Luminescence Centers of ZnS Phosphors Activated with Impurity Ions of s2 Configuration. II. ZnS:Pb (Zinc-Blende) Phosphors
Authors: Uehara Yasuo
Published: February 2004

A, B , and C bands with peaks at about 4910, 4220, and 3990 Å (mean values for split subbands at 77°K), respectively, were found in the excitation or reflection spectra of visible region of ZnS:Pb (zinc-blende) phosphors. These bands were analyzed by means of Sugano's molecular-orbital theory and identified unambiguously as those due to the resonance transitions from the ground state (A1, 1S0) to the excited states (T2, 3P1; T1 and E, 3P2; and T2, 1P1, respectively) in the localized Pb2+ center at lattice site. The observed orange (6400 Å), green (5000 Å), and yellow (5850 Å) emission bands were also unambiguously correlated to the inverse transitions to the A, B, and C absorption transitions, respectively. The assignment for the excitation bands or lines in the uv region were performed satisfactorily. The observed singlet, doublet, and triplet structure of C, B, and A excitation bands, respectively, or the triplet structure of the green emission band were interpreted in detail in terms of dynamic JT effect, leading to satisfactory understanding of the preparation conditions for the green- and orange-emitting phosphors.
Electronic Structure of Luminescence Centers of ZnS Phosphors Activated with Impurity Ions of s2 Configuration. III. ZnS:Pb (Wurtzite) Phosphors
Authors: Uehara Yasuo
Published: February 2004

In the accurate measurement of excitation and reflection spectra of Pb-activated wurtzite phosphors, A(A1, A2, A3), B, and C absorption or excitation bands with peaks at about 5070 (5360, 5050, 4800), 4240, and 4060 Å (room-temperature values), respectively, were found in the visible region of the spectra. Quantitative analysis of these bands justify again the ligand-field model established by the present author for impurity-activated ZnS phosphors, such as ZnS:Bi or Pb-activated (zinc-blende) phosphors. A number of bands or lines observed in the uv region of excitation spectra were also analyzed satisfactorily in terms of charge-transfer and exiton or phonon-coupled exciton transitions. Unambiguous assignment for the observed green, yellow, orange, and red emission bands was given. Interpretation for the origin of glow was also given.
Far-Infrared and Raman Spectra of Phosphonium Bromide and Phosphonium Bromide-d4
Authors: Durig J. R. , Antion D. J. , Pate C. B.
Published: February 2004

The infrared spectra of thin films of PH4Br and PD4Br have been recorded from 33 to 4000 cm−1 at − 170°C. Raman spectra of polycrystalline samples were also recorded at − 170°C. Three librations and two translations have been assigned for both compounds, and eight internal fundamentals were observed. Other bands found in the infrared and Raman spectra were attributed to overtones and combinations involving external and internal fundamentals. Static field and correlation field splittings of 25 and 11 cm−1, respectively, were observed for PH4Br. The barriers restricting phosphonium ion rotation in the respective lattices were calculated from the observed librational frequencies and the barrier height for the PH4 ion is 7.8 kcal/mole (ν = 354 cm−1) and for the PD4 ion is 7.9 kcal/mole (ν = 254 cm−1).
Random Matrix Theory and Nuclear Magnetic Resonance Spectral Distributions
Authors: Schaefer Jacob , Yaris Robert
Published: February 2004

The complicated 13C and proton NMR spectra of the cyclic tetramer of propylene oxide have been interpreted by an analysis of the spin Hamiltonian in terms of the statistical theory of energy levels (random matrix theory). Predicted Poisson and Wigner spectral distributions (corresponding physically to uncorrelated and correlated spin energy levels of the propylene oxide tetramer) are confirmed experimentally. This success of the random matrix analysis suggests that, in general, qualitative chemically significant information about the internal correlation of a system can be extracted from NMR spectra (especially 13C NMR spectra) too complicated to analyze in any other way. Such an analysis seems particularly suitable to test the suggestion that certain biopolymers exist in coherent or correlated states.
Near-Infrared Absorption and Circular Dichroism Spectra of Ferrocytochrome c: d→d Transitions
Authors: Eaton William A. , Charney Elliot
Published: February 2004

Transitions to the lowest crystal field singlet states of horse heart ferrocytochrome c have been identified from absorption and circular dichroism spectra in the wavelength region 5700–10 000 Å. The results are correlated with crystal field and molecular orbital theoretical predictions. The Dq value for the porphyrin ring of ferrocytochrome c is found to be between 1700 and 1900 cm−1, in excellent agreement with the extended Hückel calculations of Zerner, Gouterman, and Kobayashi.
Raman Spectra of Solid Chlorine and Bromine
Authors: Cahill J. E. , Leroi G. E.
Published: February 2004

Librational lattice modes have been observed at 15°K in solid Cl2 at 83, 100, and 118 cm−1, and at 55, 74, 86, and 101 cm−1 in solid Br2. Their intensities relative to one another can be qualitatively accounted for by the oriented gas model. The stretching frequencies [ν(1–0)] of Cl2 and Br2 are shifted −13 and −20 cm−1, respectively, from their gas phase values and show structure due to isotope splittings and inter-molecular coupling. The spectra of the lattice modes, the Br2 stretching motion, and the Cl2 stretching motion indicate, respectively, strong, intermediate, and weak intermolecular coupling relative to the isotope splittings. Both lattice and internal frequencies indicate stronger intermolecular forces in solid Br2 than in solid Cl2.
Thermoluminescence in Sodium Silicate by uv Excitation
Authors: Winer S. A. A. , Chen R.
Published: February 2004

Thermoluminescence (TL) in sodium silicate, known to be excitable by x rays and gamma irradiation, was found to be excited by uv light. Only the glow peak appearing below room temperature (RT) is excitable whereas additional peaks that had been found above RT after γ irradiation do not appear. This fact indicates that γ rays may create new defects while uv light can only excite existing imperfections. The observed peak at about 130°K is found to consist of many overlapping components corresponding to a continuum of activation energies between 0.16 and 0.38 eV. The emission, which was found to be in the visible, peaked at ∼500 mμ. The dependence of TL intensity on the uv excitation dose and the decay of phosphorescence were also studied.
Resonance Fluorescence and Xenon Sensitization of the CO(A 1II→X 1Σ+) System
Authors: Slanger Tom G. , Black Graham
Published: February 2004

Methods of excitation of the CO(A 1Π) system have been investigated, and it has been demonstrated that resonance fluorescence is an extremely efficient process. Even the very low intensity CO fourth-positive (A 1Π→X 1Σ+) contaminant emission observed in rare-gas resonance lamps gives strong fluorescence when minute amounts of CO are irradiated. It has been established that the Xe 1470-Å resonance line does not produce CO(A1Π). On the other hand, Xe sensitization gives intense fourth-positive emission by a collisional process involving the resonant Xe(3P1) state. Previous work had shown that three CO triplet states (d 3Δ, e3Σ−, and a′3Σ+) are produced by Xe sensitization. Of the total singlet plus triplet emission, a fraction of 0.06 ± 0.15 is from the A 1Π state.
Calculated Radiationless Transition Rates for Benzene and Deuterobenzene
Authors: Burland D. M. , Robinson G. W.
Published: February 2004

Radiationless rates have been calculated for the B1u3⇝A1g1 transitions of benzene and perdeutero-benzene as well as for the B2u1⇝A1g1 transition of benzene. The rates were calculated using a model that considers the radiationless transition as a tunneling process between two multidimensional potential surfaces and assuming both harmonic and anharmonic vibrational potentials. The importance of anharmonicity for the radiationless transition rate provides a possible explanation of environmental effects on these rates. Whenever possible, experimental parameters were used in the calculation. To this end we have obtained experimental values for the anharmonicities of the carbon–carbon and carbon–hydrogen vibrations and the size of the lowest triplet state of benzene.
Generalized X-Ray Scattering Factors
Authors: Stewart Robert F.
Published: February 2004

Generalized x-ray scattering factors have been determined from the evaluation of Fourier transforms of atomic-orbital products. Self-consistent-field atomic orbitals for first-row atoms have been studied. Analytical scattering expressions for rapid evaluation on digital computers have been developed. Scattering factors from small GTO expansions have been compared with both Clementi and Huzinaga SCF atomic orbitals. For one-center orbital products, the three-set GTO atomic orbitals agree with full SCF scattering factors to within five parts per hundred or better; the four-set GTO expansions have relative differences of 1.5% or less. For the two-center scattering cases, three or more Gaussians per atomic orbital yield relative errors less than 1%. The generalized x-ray scattering factors can serve as basis functions in the analysis of charge densities from x-ray diffraction data.
Nuclear Resonance Determination of the Magnetic Space Group of CsMnCl3·2H2O
Authors: Spence R. D. , de Jonge W. J. M. , Rao K. V. S. Rama
Published: February 2004

The Cl, Cs, and H nuclear magnetic resonance are used to examine the magnetic structure of antiferromagnetic Cs Mn Cl3·2H2O. The magnetic space group is P2bc′ca′, and the structure consists of eight sublattices with the sublattice magnetization along the b axis. The linear chains invoked by other authors to explain the high-temperature anomaly in the susceptibility and specific heat are shown to be antiferromagnetic. The large intrachain exchange postulated by these authors and the rather small observed Néel temperature are shown to be reconciled by the Green's-function theory of the Néel temperature, provided the exchange between the chains is assumed to be relatively small.
Low-Temperature Infrared Spectra between 1200 and 20 cm−1 and Normal-Coordinate Analysis of Silicates with Olivine Structure
Authors: Oehler O. , Günthard Hs. H.
Published: February 2004

Infrared spectra of powdered samples of Mg2SiO4 (olivine) and γ-Ca2SiO4 (space group D2h16, Z + 4) were measured at liquid-N2 and liquid-He temperature between 1200 and 20 cm−1. The low-temperature spectra do not show noticeable band sharpening. Using a simple force field with five stretching-type force constants and symmetry-adapted coordinates, a normal-coordinate analysis is carried out for both crystal compounds. It is shown that satisfactory agreement between observed band frequencies and normal frequencies may be obtained through few iteration cycles. A number of features of the conventionally used group-vibration approach are confirmed by the normal-coordinate result, which in turn leads to a complete and fairly detailed assignment of the olivine-type infrared absorption spectra.
Rapid Evaluation of Coulomb Integrals
Authors: Harris Frank E.
Published: February 2004

The properties of Fourier transforms and single-center expansions of Slater-type orbitals are used to obtain formulas for general Coulomb integrals as finite linear combinations of overlap and nuclear attraction integrals, thereby permitting the explicit expression of Coulomb integrals in terms of the auxiliary functions normally used in one-electron integral calculations. The formulas are relatively simple and have a form which indicates the avoidance of numerical difficulties found in other evaluation methods. Working formulas are given for all integrals arising from orbitals of principal quantum numbers 1 and 2.
Planar Vibrations of Carbon Skeleton of Polyenes
Authors: Tric C.
Published: February 2004

Planar vibrations of polyenes are described by a model of a zigzag chain with three force constants (two for alternating bonds, one for the angles); the principal features of experimental vibrational spectra are explained, and the contribution of angular deformations is emphasized.
LCAO–MO–SCF Studies of Carbon–Carbon Bond Cleavage in Ethylene Oxide
Authors: Hayes Edward F.
Published: February 2004

Nonempirical LCAO–MO–SCF studies of carbon–carbon bond cleavage in ethylene oxide indicate the presence of two open forms. In one form (the 90, 90 form) the terminal methylene groups are perpendicular to the C–O–C plane. In the other (the 0, 0 form), the terminal methylene groups are in the C–O–C plane. On the basis of molecular correlation diagrams such as those of Hoffmann and Woodward, one would expect that either of these open forms could add stereospecifically to olefins. Previously Huisgen had suggested that the 0, 0 open form was an intermediate in the 1, 2 cycloaddition of tetracyanoethylene oxide to olefins; however, a 90, 90 form is also consistent with the experimental data.
Paramagnetic Resonance Spectrum of O− Trapped in KCl, RbCl, and KBr
Authors: Brailsford J. R. , Morton J. R.
Published: February 2004

The impurity ion O− has been introduced into KCl, RbCl, and KBr and detected at 4°K by EPR spectroscopy. The center has orthorhombic symmetry. A quantitative understanding of the g factor and of the O17 hyperfine structure was obtained with the aid of equations derived in a recent analysis of the an-alogous S− impurity. The hyperfine splitting was described by the atomic parameters ψ2(0), 〈r−3〉l, and 〈r−3〉s. The present results indicate that in an earlier investigation of O− trapped in alkali iodide crystals erroneous values were obtained for these parameters.
Distribution of the Excess Energy in CN B 2Σ Produced in Photodissociation of Cyanogen Halides and Hydrogen Cyanide
Authors: Mele A. , Okabe H.
Published: February 2004

The vibrational and rotational excitations of CN B 2Σ produced in photodissociation of cyanogen halides and hydrogen cyanide in the vacuum ultraviolet have been measured as a function of incident wavelengths. More than 70% of the total population is in levels up to υ′ = 3. No population inversion was observed, indicating the photodissociation takes place without much change in the equilibrium C–N bond distance. The percent conversion of the excess energy to vibration is approximately 20% for cyanogen halides and about 12% for hydrogen cyanide, irrespective of the amount of excess energy, which is much less than that expected from the equipartition of energy in all vibrational degrees of freedom of the molecule. The rotational levels of CN B 2Σ are highly excited for all molecules. The extent of conversion of the excess energy to rotation is of the order of 10%–20% for ICN and ClCN, and only several percent for BrCN and HCN. The remaining energy must be distributed as kinetic energy between CN B 2Σ and atoms. The vibrational and rotational distributions are discussed in terms of the equilibrium geometries of the ground and excited molecules and of the final products. The CN A 2Π state is also formed in photodissociation and apparently not directly but through the CN B 2Σ. The intensity of the CN red emission is much less than that of the violet. Various other photodissociation processes are discussed. In general, the extent of conversion of the excess energy to vibration is much less than that expected from the equipartition theory for most triatomic molecules. No vibrational population inversion has been observed.
Noniterative Solutions of Integral Equations for Scattering. I. Single Channels
Authors: Sams W. Neal , Kouri Donald J.
Published: February 2004

A new method (homogeneous integral solution) for solving collision problems utilizing an integral equation formalism is presented. The approach is noniterative and applies equally well to purely local interactions or problems with combined local and nonlocal interactions. The method involves transforming the integral equation of scattering into a Volterra integral equation of the second kind. It follows that even for nonlocal interactions, the wavefunction may be determined noniteratively from knowledge of its value at a single point (the origin). A simple numerical procedure is proposed for solution of the Volterra equation which allows one to avoid matrix inversions completely. At the end of the calculation, the scattering T or R matrix is obtained directly from the solutions. The method is illustrated by a calculation of the singlet and triplet s-wave Hartree–Fock phase shifts for electron–H-atom scattering.
Pure Rotational Spectrum and Electric Dipole Moment of CH3D
Authors: Ozier Irving , Ho W. , Birnbaum George
Published: February 2004

The pure rotational spectrum of CH3D in the ground vibrational and electronic state has been observed using a far-infrared grating spectrometer. This spectrum is of particular interest because the electric dipole moment μ arises entirely from an isotopic substitution. Ten distinct lines have been observed in the frequency range from 40 to 120 cm−1, and have been identified with the (J → J + 1) transitions for the rotational levels J = 5 − 14. For the (6 → 7) line, the absolute intensity and the linewidth were determined by a curve of growth method. It was found that, for this line, the dipole moment μ = (5.68 ± 0.30) × 10−3D and the broadening parameter γ0 = (0.075 ± 0.012) cm−1/amagat. This measurement constitutes the first correct experimental determination of the dipole moment of CH3D. By making further intensity measurements on other lines, it was shown that, to within 6%, μ is independent of J in the range 5 ≤ J ≤ 12. A detailed account of the experimental methods used to make the intensity measurements is given. It was also determined that the rotational constant B0 = (3.882 ± 0.002) cm−1 and the distortion constant Dj = (7.7 ± 0.3) × 10−5cm−1.
Nuclear Magnetic Resonance Studies of Solutions of the Rare-Earth Ions and Their Complexes. IV. Concentration and Temperature Dependence of the Oxygen-17 Transverse Relaxation in Aqueous Solutions
Authors: Reuben Jacques , Fiat Daniel
Published: February 2004

The concentration and temperature dependence of oxygen-17 linewidths in solutions of rare-earth perchlorates have been investigated. Empirical models are proposed to account for the observed behavior. An equation relating the apparent relaxation rate to the concentration and to the NMR parameters of the nuclei under conditions of fast chemical exchange has been derived. It was used in the interpretation of the experimental results. Lower limits have been estimated for the rate constants of water exchange between the aquo complexes of Tb3+, Dy3+, Ho3+, Er3+, and Tm3+ and the solvent. These are found to be in the range 0.3–2.6 × 107 sec−1 and to follow the trend of the ionic radii. The upper limit for the activation enthalpy of the exchange process is estimated to be 5 kcal/mole. The upper limits of the electron relaxation times are in the range 2.8–5.0 × 10−13 sec. The temperature dependence of the oxygen-17 transverse relaxation rates in these systems is discussed in terms of the microdynamic behavior of the molecules in water and electrolyte solutions. The observed trends comply with a model according to which there are two kinds of water molecules differing in their activation enthalpies for rotation. The relative contribution of the “hydrogen-bonded” and coordinated molecules prevails at lower temperatures, whereas that of the “free” molecules becomes pronounced at higher temperatures. It is suggested that spherically shaped samples be preferred for studies of 17O line width in paramagnetic solutions in order to minimize inhomogeneous broadening. It is shown that this may be a source of some inconsistencies found in the literature.
Analytical Theory of Multiquantum Vibrational Excitation in Molecular Collisions
Authors: Curry Bill P.
Published: February 2004

Multiquantum vibration–translation energy transfer is considered within the framework of the distorted-wave approximation. The restriction to harmonic-oscillator selection rules is removed by treating any transition as a sequence of virtual one-quantum transitions, permitting retention of all the vibrational states, but only the initial and final translational states, in the eigenfunction expansion. Vibrational matrix elements are the same as those of Rapp and Sharp, but translational wavefunctions include the effect of energy transfer; consequently, excitation probabilities display more complicated dependence on final vibrational quantum number than do the classical results. Cross sections for CO compare favorably with Marriott's numerical studies. Relaxation times compiled from the cross sections exhibit a three-part temperature dependence: (1) at very low temperature, the relaxation time is exponential in 1 / T, (2) in a large range of intermediate temperatures, relaxation is described by a multiquantum generalization of the Landau–Teller theory, exact within the restriction of weakly coupled states, (3) at very high temperature, the relaxation time is given by an impulsive formula.
Electron Spin Resonance Absorption Spectra of Hole-Trap Centers Associated with Phosphorous in Irradiated Single Crystal Calcite
Authors: Serway R. A. , Marshall S. A. , McMillan J. A. , Marshall R. L. , Ohlsen W. D.
Published: February 2004

An irradiation-produced defect center associated with phosphorous impurities and corresponding to S = 12, I = 12 has been observed in natural single crystal calcite. Its electron spin resonance absorption spectrum consists of six doublets each exhibiting orthorhombic symmetry with spin-Hamiltonian parameters given by gξξ = 2.0024, gηη = 2.0382, gζζ = 2.0090, Aξξ = − 17.25 MHz, Aηη = 14.27 MHz, and Aζζ = 17.33 MHz, where the coordinates ξ, η, and ζ are canted with respect to the calcite crystal coordinates. In addition to providing the nuclear magnetic moment of 31P, ENDOR data were used to determine the signs of the hyperfine splitting tensor components which in turn yield unambiguous values for the Fermi contact term and the traceless dipole–dipole interaction. On the basis of the small contact term in comparison to the dipole–dipole interaction strength and upon g-value comparisons, it is proposed that the defect center is a CO3− molecule ion in close association with a PO23− molecule ion, probably mutual charge compensators. The ENDOR spectrum of the PO42− molecule ion in calcite was reinvestigated, and the resulting data were used to provide relative signs of the hyperfine structure tensor components. From an analysis of the latter, it is concluded that the unpaired electron spin is localized primarily in an oxygen nonbonding orbital.
Self-Consistent Theory of Bond Alternation in Polyenes: Normal State, Charge-Density Waves, and Spin-Density Waves
Authors: Harris Robert A. , Falicov L. M.
Published: February 2004

The unrestricted Hartree–Fock theory is used to investigate bond alternation in C4N+2H4N+2 cyclic polyenes. The possible ground states are examined as functions of the parameters in the Pariser–Parr–Pople model Hamiltonian. We show that charge-density-wave ground states do not exist for physical values of the input parameters. The normal, or molecular-orbital, ground state does give rise to a negligible bond alternation at large N, in agreement with previous workers. Spin-density-wave ground states exist for most values of the parameters and have lower energy than the normal ground states. These states do not exhibit bond alternation when the sigma-bond compression is the same as that used to obtain the normal energy.
Raman Scattering Tensors for Single Crystals of Naphthalene
Authors: Hanson David M. , Gee A. Roger
Published: February 2004

The Raman scattering tensor for each gerade vibrational mode of crystalline naphthalene is evaluated from polarized Raman spectra of oriented single crystals. For molecular crystals, where the intermolecular interactions are weak, the form of the tensors is determined by the properties and symmetry of the molecule rather than the crystal; consequently, the symmetries of the gerade vibrations are readily identified. A comparison of these data with the phosphorescence and fluorescence spectra allows the fundamental vibrational frequencies to be assigned empirically. Except for one b2g mode and one b3g mode, the empirical assignments support those in the literature made on the basis of force field calculations. It is found that crystal effects on the gerade vibrations are small: frequency shifts are 2–4 cm−1, exciton splittings are less than 1 cm−1, and intensities are described qualitatively by the oriented gas model, although an unambiguous example of crystal-induced Fermi resonance is evident. Disagreement between the observed exciton splittings and some calculated splittings by N. Rich and D. Dows [Mol. Cryst. Liquid Cryst. 5, 111 (1968)] based on an atom–atom potential model may indicate that some finer features of the intermolecular potential are not well represented by such a model. The temperature dependence of the Raman linewidth and line shape is documented.
Electronic States of HNCO, Cyanate Salts, and Organic Isocyanates. I. Luminescence Studies
Authors: Rabalais J. W. , McDonald J. R. , McGlynn S. P.
Published: February 2004

The phosphorescence spectra and lifetimes of the cyanate salts of Na+, K+, Cd2+, Ag+, Hg2+, and Pb2+ have been measured. Similar measurements have been made on CH3NCO, CH3CH2NCO, C6H5NCO, and HNCO. The results of these studies indicate that the lowest-energy excited state of the cyanate ion and the covalent isocyanates is of ··· (π)3(π*)1 MO excitation type: This state is Σ+3(C∞υ) in NCO−, A′(Cs3) in HNCO and the alkyl isocyanates, and A13(C2υ) in phenyl isocyanate. Fluorescence is observed only from the phenyl isocyanate and is assumed to initiate in an excited state which has dominant excitation amplitude on the phenyl moiety. Vibrational structure observed in the phosphorescence of NaOCN and C6H5NCO is analyzed, and similarities in the spectra are pointed out. A heavy-atom spin–orbit coupling effect is observed in the cyanate salt series. Spin–orbit coupling calculations have been performed on NCO−, linear and bent. A phosphorescence lifetime of ∼ 0.12 sec, in good agreement with experiment, is predicted for the Γππ3* → 1Γ1 transition.
Electronic States of HNCO, Cyanate Salts, and Organic Isocyanates. II. Absorption Studies
Authors: Rabalais J. W. , McDonald J. R. , McGlynn S. P.
Published: February 2004

The absorption spectra of HNCO, CH3NCO, CH3CH2NCO, C6H5NCO, and cyanate salts of Na+, K+, Cd2+, Hg2+, and Pb2+ have been determined in the near-ultraviolet region. Those of NaOCN and CH3CH2NCO have also been measured in the vacuum-ultraviolet region. The energy ordering of the observed electronic states of NCO− (C∞υ) is 3Σ+ < 1Σ− < 1Δ < 1Π < 1Σ+. A heavy-atom spin–orbit coupling effect is observed in the Σ+3 → 1Σ+ transition of the cyanate salts. The electronic states observed in CH3CH2NCO (C8) are correlated with those of NCO− and are discussed in terms of a C8 perturbation superposed on a zero-order C∞υ Hamiltonian. Continuous absorption of a predissociative nature underlies the absorption spectrum of CH3CH2NCO in the vacuum uv. Molecular orbital and configuration-interaction calculations have been performed for HNCO, NCO−, and phenyl isocyanate. The results are correlated with experimental data.
ESR Study of Morpholinium TCNQ Complexes
Authors: Bailey J. C. , Chesnut D. B.
Published: February 2004

Electron paramagnetic resonance studies have been carried out on three morpholinium–TCNQ charge resonance salts of varying stoichiometry. The three salts which have been studied include the previously known 1:1 and 2:3 complexes as well as a new, relatively unstable salt believed to be a 3:4 complex. All three materials exhibit resonance behavior similar to that found in other triplet-exciton-containing systems. A phase transition in the 1:1 salt has been discovered at 151°C which is accompanied by only minor changes in the zero-field splitting tensor parameters and principal axes. The morpholinium salts are characterized by an anomolous “impurity” resonance which occurs along with the zero-field doublet(s). The behavior of this “impurity” signal as a function of temperature suggests that it may be a characteristic property of the system.
Nuclear Magnetic Resonance of 27Al and 1H in Zoisite, Ca2Al3Si3O12(OH)
Authors: Brinkmann D. , Staehli J. L. , Ghose Subrata
Published: February 2004

The electric field gradient tensors at the two nonequivalent aluminum sites in zoisite, Ca2Al3Si3O12(OH), have been determined by means of nuclear magnetic resonance. The quadrupole coupling constants and the asymmetry parameters are 8.05 MHz and 0.46 for site I and 18.50 MHz and 0.160 for site II. The second coupling constant is the largest value measured so far for 27Al. The 10.4-MHz difference of the coupling constants is the greatest value known for Al in nonequivalent sites with the same number of nearest neighbors and reflects the excessive distortion of the aluminum–oxygen octahedron of site II. Furthermore, the directions of maximum and intermediate field gradients at site II are nearly parallel to the directions of the shortest and largest Al–O bond length, respectively. These facts seem to indicate that in a highly distorted Al–O octahedron the electric field gradient tensor is predominantly determined by the octahedron itself. The second moment of the proton resonance line has been measured. A comparison with calculated values confirms the tentative hydrogen position as determined by x-ray diffraction.
Calculation of Fermi Contact Hyperfine Splitting for Small Atoms and Molecules
Authors: Meyer Wilfried
Published: February 2004

Hyperfine splittings by the Fermi contact term are calculated for the atoms Li, B, C, N, O, F, Na, and the radicals OH, NH2, and CH3. The results of all atom calculations agree well with experiment. The errors are about 10% for the atoms and the radical OH; for the polyatomic radicals they are about + 30% consistently. The results are obtained using (1) an extended Hartree–Fock method approximated in this paper by the use of the natural orbitals of UHF calculations; (2) properly prepared Gaussian functions instead of Slater-type functions. Errors in frequently used methods are discussed and specified by calculations. Simple formulas are given which relate the p-type splittings to the atomic number.
Collision Dynamics and the Statistical Theories of Chemical Reactions. I. Average Cross Section from Transition-State Theory
Authors: Morokuma K. , Eu B. C. , Karplus M.
Published: February 2004

By recasting the transition-state-theory formula for the reaction rate of a general bimolecular reaction and making use of Laplace transform techniques, an expression for the average reaction cross section as a function of the total system energy is derived; the result for three-dimensional collisions have been given previously by Marcus. For the collinear model, a reaction probability replaces the cross section. Cross sections and probabilities for a number of special cases are given. It is shown that under certain conditions, improper high-energy limits result. The relation of the present conclusions to unimolecular rate theory is indicated.
Theory of Reactive Scattering. I. Homogeneous Integral Solution Formalism for the Rearrangement τ-Operator Integral Equation
Authors: Kouri Donald J.
Published: February 2004

The integral equation for the rearrangement τ operator is used as the basis for a discussion of reactive scattering. The concept of the amplitude density introduced by Johnson and Secrest is extended to reactive collisions by means of the τ operator. The homogeneous integral solution method of Sams and Kouri is used to develop a noniterative solution to the integral equation satisfied by the reactive-scattering amplitude density. This method is characterized by very stable behavior and is capable of quite accurate solutions with a relatively large step size. We consider the formulation of the method first for a general reactive collision. Both open and closed channel contributions to the reactive scattering amplitude density are considered. We employ the Feshbach projection operator formalism to treat the effects of complex formation due to virtual excitation of internal degrees of freedom. After discussing the general reactive collision, we next deal with the specific problem of atom–diatom reactions. Notation and techniques developed by Miller are used to treat problems associated with coordinates. Unlike Miller's discussion of reactive scattering, the present approach does not require inclusion of square integrable functions since the integral equation for the τ operator (and the equation for the amplitude density) already takes explicit account of the effects described by such terms. Further, the present approach allows one to easily study the effects of vibrational and/or rotational excitation on the reaction rate since the initial state appears explicitly as an inhomogeneity in the integral equation for the reactive scattering amplitude density. Finally, we discuss the manner in which the T matrix may be obtained from the reactive scattering amplitude density. It results that the reactive T matrix may be expressed in terms of the nonreactive T matrix.