bluuebunny/crossref_metadata_2025 huggingface dataset:
Covalent Bonding in Octahedral (MnCl6)4− and Tetrahedral (MnCl4)2− Complexes
Authors: Tsay Fun-Dow , Helmholz Lindsay
Published: February 2004

Chlorine ligand hyperfine structure has been observed in the EPR spectrum of the octahedral complex ion (MnCl6)4−1, in dilute solution in a single crystal of K4CdCl6 at room temperature. From the isotropic and anisotropic components of the spectrum, it is estimated that fs = 0.40%, and (fσ − fπ) = 3.9%, respectively. No chlorine ligand hyperfine structure is observed in the tetrahedral complex ion (MnCl4)2− introduced as an impurity in either [N(CH3)4]2ZnCl4 or [(C6H5)3CH3As]2ZnCl4. A substantial reduction in the manganese nuclear hyperfine coupling constant As(Mn) is observed in both cases, however, As(Mn) having the value − 79.50 ± 0.50 G for [N(CH3)4]2ZnCl4, and − 79.30 ± 0.50 G for [(C6H5)3CH3AS]2ZnCl4 compared to − 86.40 ± 0.40 G found for MnCl64− in K4CdCl6. The failure to observe ligand hyperfine interactions in the tetrahedral ion is discussed in terms of the shorter bond distance in the four coordinated compound and the increased importance of π bonding.
Thermal Effects on the Fluorescence Lines of Sm2+ in Crystals
Authors: Birang Behzad , Alam A. S. M. Mahbub'ul , Bartolo Baldassare Di
Published: February 2004

The effects of temperature on the fluorescence lines of Sm2+ in CaF2, SrF2, and BaClF were studied. The results for the first two samples were obtained in the region 10°–100°K. For CaF2 a strong dependence of the linewidth on temperature was found; this width is ∼ 1.7 cm−1 at 15°K and ∼ 40 cm−1 at 95°K. For SrF2, the linewidth is constant and equal to ∼ 2.15 cm−1 up to ∼ 77°K. This difference in behavior is explained qualitatively as due to the fact that in CaF2 the metastable state belongs to the 4f55d configuration and therefore, the electron charge is strongly affected by the thermal perturbations of the environment. We also measured the widths and positions of several fluorescence lines of BaClF:Sm2+ in the 30°–600°K region. The thermal broadening of most of these lines is explained as due to microscopic strains and Raman scattering of phonons, the thermal shifts as due to emission, and absorption of virtual phonons. A Debye model of phonon distribution was used with the same TD for both the lineshift and linewidth.
Improved Equation for Lower Bounds to Eigenvalues; Bounds for the Second-Order Perturbation Energy
Authors: Miller William H.
Published: February 2004

Using the bounding theorems given by Eqs. (1) and (2), an improved equation for lower bounds to eigenvalues is obtained [Eq. (8)]. This improved result is a generalization of an earlier lower-bound equation given by Gay and another given by Miller. A numerical application to the ground state of the two-electron atom is presented. Upper and lower bounds for the second-order perturbation energy are also presented, and a general upper bound for inverse operators is obtained.
Magnetic Susceptibility of Nitric Oxide Molecules Adsorbed on Silica Gel
Authors: Huang N. L. , Van Vleck J. H.
Published: February 2004

The present paper consists of an attempt to interpret theoretically the experiments of Solbakken and Reyerson on the magnetic susceptibility of nitric oxide adsorbed on silica gel. A primitive model is used in which the axis of the NO molecule is regarded as frozen in space and the crystalline field mixes the 3II1/2 and 3II3/2 states. To explain the experimental data it is necessary that the size of the crystalline potential increase with temperature. Possible justification of this assumption is found in the fact that the observed differential heat of adsorption increases with temperature. More refined calculations do not seem warranted in the absence of magnetic data over a wider temperature range including possibly magnetic resonance measurements.
Vibrational Spectra of Molten Salts. VI. Raman Spectra of Indium Trichloride–Alkali-Metal Chloride Mixtures
Authors: Clarke J. H. R. , Hester R. E.
Published: February 2004

Raman spectra of molten mixtures of InCl3 with 1–6 moles LiCl, 1–6 moles KCl, and 1–12 moles CsCl have been obtained over the temperature range 400°–700°C. The 1:1 mixtures all gave essentially similar spectra, which are interpreted as arising from a definite and thermodynamically stable chemical species, InCl4−. The effect of adding further alkali-metal chloride to the 1:1 mixtures varied with the nature of the alkali metal. With KCl and CsCl, spectral changes are consistent with increasing the number of chloride ions associated with each indium, and the formation of InCl5−2 and InCl6−3 species appears likely, though environmental perturbations evidently are severe. However, with LiCl only small changes were observed, and InCl4− appears to remain as the dominant species. Possible interpretations of these data in terms of variations in a quasilattice structure of the melts are discussed. Comparative studies of selected solid and liquid systems are reported, and interpreted in terms of structural rearrangements accompanying fusion.
Theory of Trapped-Hole Centers in Rare-Gas Solids
Authors: Druger S. D. , Knox R. S.
Published: February 2004

Experimental evidence suggests that self-trapping of holes occurs in rare-gas solids. Formation of an intrinsic diatomic center, similar to the Vk center, is investigated as a possible mechanism. The energy of trapping a valence-band hole is discussed on the basis of a simple model for the rare-gas hole center. A method of calculating the relaxation and polarization energies is developed, appropriate parameters for the model estimated, and the method applied to solid argon, krypton, and xenon to determine the degree of relaxation and the contributions to the trapping energy. The results suggest stable trapped-hole centers at 0°K associated with only small lattice relaxations. The internal molecular binding of the center is found to be the overwhelming factor determining its configuration on this model.
Quadrupole Effects in Electron Paramagnetic Resonance Spectra of Polycrystalline Copper and Cobalt Complexes
Authors: Rollmann Louis D. , Chan Sunney I.
Published: February 2004

A second-order theory, including quadrupole effects and transitions with ΔmI > 0, for the calculation of EPR spectra of polycrystalline samples of S = 12 transition-metal ion complexes possessing axial symmetry has been developed. With this theory, the EPR spectrum of Cuacac2 in Pdacac2 and that of Na4CoPTS in DMSO could be explained in detail. The effects of electric quadrupole interaction together with a large anisotropy in the nuclear hyperfine coupling are shown. The advantages of obtaining spectra at two frequencies for the determination of reliable spin-Hamiltonian parameters are demonstrated.
Proton Magnetic Resonance Studies of Pentaerythritol
Authors: Smith George W.
Published: February 2004

Proton magnetic resonance (PMR) studies of pentaerythritol, C(CH2OH)4, have revealed two line-narrowing processes in the solid. One, at temperatures just below the transition to the “plastic” phase, is associated with a rather general reorientation of molecules about their centers of gravity, with activation energy ER = 25.2 ± 1.0 kcal/mole. In reorienting, the molecules must overcome the considerable energy of intermolecular hydrogen bonds. The second process corresponds to molecular self-diffusion in the plastic phase with activation energy ED = 24.3 ± 0.5 kcal/mole. Spin–lattice relaxation studies indicate that impurity diffusion may be occurring in the pentaerythritol lattice and that the low-temperature line narrowing may be the result of a succession of small amplitude, lower-energy processes which lead to and culminate in the 25-kcal/mole general molecular reorientation. The rigid-lattice second moment (22.9 G2) agrees within experimental error with the value calculated from Hvoslef's neutron-diffraction studies (after his proton coordinates have been corrected for a tabulation error). The PMR measurements of ER and ED for pentaerythritol and eight different molecular crystals are compared to thermal parameters and correlated with the Pople–Karasz theory of melting for molecular crystals. Of the nine substances, only two (phosphorous, P4, and pentaerythritol) fail to give qualitative agreement with the Pople–Karasz theory. The dissimilarity of P4 with the other molecules and the intermolecular hydrogen bonding of pentaerythritol are assumed to be the sources of disagreement for these two substances.
Universal Parameter for the Extrapolation of Force Constants, Mean-Square Quantities, and Bond Energies of Halides
Authors: Yeranos W. A.
Published: February 2004

A general formula of the type Y = kω + C, which is simultaneously applicable to force constants, the reciprocals of the mean-square amplitude quantities σr, bond dissociation as well as thermochemical bond energies of halides, has been presented. ω, moreover, is given by the expression ω = 16 | χm − χx | + 3.5(χm − χx)2 + θf, where χm and χx are the Pauling electronegativities of the atoms forming the M–X bond, and where θf is a constant for each of the aformentioned molecular parameters and may have a nonzero value only in the fluorides. The θf values for the three parameters studied were found to be 0, − 11, and − 15 or − 30, respectively. The choice of the latter depends on whether χm > 1 (− 15), or χm ≤ 1(− 30), except for the NaX series for which θf = − 15.
Interfacial Tension of Near-Critical Cyclohexane–Methanol Mixtures
Authors: Warren Connie , Webb W. W.
Published: February 2004

We have measured the capillary length for the interface between the coexisting phases in the vicinity of the critical consolute temperature of mixtures of cyclohexane and methanol. The data on the capillary length a = 0.250 (Tc − T)0.44±0.02cm were combined with values of the difference in density Δρ between the coexisting phases calculated from the coexisting compositions to obtain the interfacial tension σ = a2Δρg / 2 = 100(1 − T / Tc)1.23 ± 0.05erg/cm2. Our measurements of σ for this system combine with the only existing measurements of the effective thickness of a critical interface in a new test of theories of critical interfaces. Data include 0.09 < Tc − T < 9°C.
Electron Spin Resonance of a Sigma-Electron Radical in X-Ray-Irradiated Malonamide
Authors: Cyr N. , Lin W. C.
Published: February 2004

Two radical species were formed when malonamide crystals were irradiated with x ray. One was a π-electron radical of common occurrence, NH2COĊHCONH2, and the other was interpreted as a σ-electron radical of the formula NH2COĊH2COṄH. The ESR spectra of the latter were studied in detail and the assignments were substantiated by ESR spectra of deuterated malonamide and 15N-malonamide.
Electronic De-excitation of O(1D) by Rare Gases—Vacuum-Ultraviolet Flash Photolysis of O2
Authors: Clerc M. , Reiffsteck A. , Lesigne B.
Published: February 2004

Far-uv photolysis of O2 produces excited atoms O(1D) whose relative quenching constants with argon, krypton, and neon are kAr = 0.015, kKr = 0.09, kNe = 0.007 if kXe is taken as unity. These values are in good agreement with Cvetanović's relative efficiencies [J. Chem. Phys. 45, 8, 2888 (1966) ].
Absorption Spectrum of Np4+ in Single-Crystal Thorium Oxide
Authors: Gruber J. B. , Menzel E. R.
Published: February 2004

The optical absorption spectrum of Np4+ in single-crystal ThO2 has been investigated at 4°K, 77°K, and room temperature between 0.3 and 2.5 μ. Infrared spectra were recorded between 2.5 and 300 μ. Intermediate-coupling wavefunctions and eigenvalues for the ground-state electronic configuration 5f3 were generated by an IBM 360-67 program using the combined Coulombic and spin–orbit Hamiltonian. The best over-all agreement between calculated free-ion J levels and the observed spectrum of Np4+ are obtained with the Racah parameters, E1 = 3185, E2 = 18.50, E3 = 323.2, and the spin–orbit parameter ζ = 2009 in cm−1. A six-parameter calculation that includes the effect of configuration interaction yields the parameters E1 = 3160, E2 = 17.59, E3 = 323.2, ζ = 1983, α = 22.44, and β = − 575.9 in cm−1. Intermediate-coupling wavefunctions were used to calculate operator equivalent factors. A first-order crystal-field analysis was carried out for the six lowest J manifolds, where the separation of adjacent J levels is large compared to the crystal-field splitting of each J level. For Np4+ in an eightfold cubic environment, the crystal-field parameters B4 = − 410 cm−1 and B6 = 50.4 cm−1 predict the positions of the crystal quantum states in good agreement with experiment.
Rotational Relaxation and Chemical Exchange
Authors: Anderson J. E. , Fryer P. A.
Published: February 2004

This article couples chemical exchange to rotational relaxation in liquids. We found that the relative rates of exchange and molecular rotation have a profound effect on relaxation behavior. Systems exchanging more slowly than the rate of molecular rotation can be distinguished from systems experiencing faster exchange. This leads to a novel class of experiments using molecular rotation as a frequency standard for the exchange process. Since rotational correlation times are typically 10−10–10−12 sec. these experiments provide sensitivity on a time scale not easily achieved by other techniques. Analysis of these experiments is presented. Studies of NMR spin–lattice relaxation in iodine charge-transfer complexes substantiate this analysis.
Statistics of a Two-Dimensional Gas of Long Thin Rods
Authors: Lakatos Katja
Published: February 2004

A three-dimensional gas of long thin rods exhibits an order–disorder phase transition as a function of rod concentration. Theoretically, this transition has been found using the leading term in Mayer cluster expansion theory. It is shown here that if a phase transition occurs for a two-dimensional gas of long thin rods, it cannot be found within the same approximation. This is achieved by showing that the equation obeyed by the angular distribution function of the rods in this approximation has a unique constant or isotropic solution for all concentrations.
Structure of Iron (III) Chloride–Methanol Solutions
Authors: Wertz David L. , Kruh R. F.
Published: February 2004

The structure of the solute species in concentrated iron(III) chloride–methanol solutions has been examined by diffraction techniques. Solute association is established by colligative measurements and indirectly by diffraction results and Fe2Cl6 emerges as a realistic solute species. The radial-distribution functions of the solutions and those obtained from electron diffraction studies of vapor-phase Fe2Cl6 are comparable. The average iron(III)–chloride bond distance is 2.25 Å, and each iron atom has four chloride nearest neighbors; both facts are consistent with the dimer model. Crystal radii serve to interpret bond distances.
Scattering Function for a Simple Fluid near the Critical Point
Authors: McLennan James A.
Published: February 2004

The treatment by Felderhof of the scattering function for a simple fluid near its critical point is extended. Both the hydrodynamic equations and the scattering function are derived simultaneously; this makes it possible to avoid the difficulty in Felderhof's treatment that certain parameters are left undetermined.
Electron Paramagnetic Resonance Absorption in Oriented Triphenylene in Its Phosphorescent State at Low Magnetic Fields
Authors: Gerkin Roger E. , Szerenyi Peter
Published: February 2004

Electron paramagnetic resonance absorption in oriented triphenylene in its lowest triplet state has been investigated in static magnetic fields below 50 G at 77°K. The orienting medium was melt-grown single crystals of symmetric dodecahydrotriphenylene, the structure of which has been briefly investigated and compared with that of vapor-grown crystals. The lowest triplet state of triphenylene in this host has been shown to possess lower than threefold symmetry. Taking the normal to the mean molecular plane of triphenylene to be the C axis and the mean molecular plane to be the AB plane, the results have been described by the spin Hamiltonian H = g | β | H·S + S·T·S, where S = 1, g = 2.003 (isotropic), and, at 77°K, | TCC | / hc ≡ | D | / hc = 0.133526 cm−1, σ{| D | / hc} = 0.000048 cm−1, | TAA | / hc = | TBB | / hc ≡ | E | / hc = 0.000564 cm−1, σ{| E | / hc} = 0.000048 cm−1. The triplet-state lifetime and the angular dependence of the resonance absorption linewidths have also been studied at 77°K, and the temperature dependence of the zero-field splitting parameters has been determined in the temperature range ∼ 67°–∼ 90°K. For this host, the departure of the symmetry of the triphenylene triplet state from threefold symmetry has been found to decrease with decreasing temperature, in contrast to the behavior previously reported for distorted triplet states of other canonically threefold symmetric molecules in various environments. Preliminary exploration of a photochemical reaction induced in the triphenylene–dodecahydrotriphenylene mixed crystal by broad-band ultraviolet irradiation is described.
Spin–Lattice Relaxation and Elastic Properties for Rare-Earth Substituted LaCl3
Authors: Stedman G. E. , Newman D. J.
Published: February 2004

We develop a new theory of spin–lattice relaxation in an effort to improve the agreement between detailed predictions and experiment. Our work is based on (a) a uniquely general and convenient formulation of the crystal field problem utilizing the superposition model developed by Newman and co-workers and (b) a knowledge of the detailed lattice motions, in particular the eigenvectors of long wavelength acoustic modes, in LaCl3. In conjunction with the latter, we give elastic constants, tables for the stress-induced crystal field, and sound velocities. While the velocities agree with experiment, they are sufficiently low to reveal the inadequacy of earlier spin–lattice relaxation calculations on these systems. Relaxation data due to Stapleton and co-workers permit a sensitive test of our model. It is possible to improve agreement with experiment and to achieve a consistent picture of direct and double direct (Orbach) relation with the dopants Pr3+, Nd3+, Tb3+, and to suggest an improved phenomenological estimated scheme. However, Raman rates are consistently overpredicted; also we find (as did earlier workers) special problems in connection with LaCl3:Er3+.
NMR of HBF4
Authors: Stungis George E. , Rugheimer John H.
Published: February 2004

Longitudinal and transverse relaxation times for all three species of nuclei in aqueous solutions of HBF4 have been measured by pulse techniques. Relaxation measurements were performed at room temperature as a function of HBF4 concentration. The relaxation times of each nuclear species exhibited a strong concentration dependence. In addition, cw measurements were performed in order to determine the nature of the electric and magnetic interactions. The results demonstrate that the 19F nuclei are participating in a chemical exchange process which produces a 11B spin multiplet collapse. The concentration dependence of the 1H nucleus was studied to determine the type of coupling that exists between the 1H nuclei and their environment. On the basis of the transverse relaxation and cw measurements, a spin–lattice relaxation model was constructed for the HBF4 system. Theoretical estimates of the relaxation parameters were made using the BPP model.
Moments of NMR Absorption Lines from the Free Induction Decay or Echo of Solids
Authors: Dianoux A. J. , Sýkora S. , Gutowsky H. S.
Published: February 2004

A new method is presented for determining the moments of nuclear magnetic resonance absorption lines from the shape of either the free induction decay or that of the echo. Unlike previously used techniques, this method does not require the assumption of an analytic function for the line shape or the fitting of the experimental decay with a polynomial. A fast, suitably precise and numerically stable algorithm has been developed for performing the integration required by the new method.
9Be Quadrupole Perturbed NMR Study of the Ferroelectric Transition in Deuterated Triglycine Fluoberyllate
Authors: Blinc R. , Slak J. , Stepišnik J.
Published: February 2004

The quadrupole perturbed magnetic resonance spectra of 9Be in a deuterated triglycine fluoberyllate single crystal have been studied as a function of temperature and crystal orientation, and the electric field gradient (EFG) tensors at the Be sites have been determined in the para- and in the ferroelectric phase. The data show that the main effect of the ferroelectric transition is a shift of the Be ion away from the twofold b axis where it lies in the paraelectric phase and a rotation of the two minor principal axes of the 9Be EFG tensor for θ = ± 25° away from the average, paraelectric direction. The results can be understood in terms of the dynamic order—disorder model, proposed by Hoshino, Okaya, and Pepinsky.
ESR Spectra from Paramagnetic Centers in Irradiated KBrO4
Authors: Byberg J. R.
Published: February 2004

ESR spectra of KBrO4 crystals irradiated with x rays at 10 or 26°K contain three ``families'' of intense signals. The signals observed at 10°K are assumed to derive from primary defects constituted by electrons and holes trapped on BrO4− ions. The families of signals are assigned to the species BrO4−, BrO4, and (BrO4)2−, each trapped in a number of distinct sites. Thermal processes starting at 20°K result in formation of the secondary defects O3− from BrO4−, and BrO2 from BrO4.
New Method for Constructing Solutions to Time-Dependent Perturbation Equations
Authors: Alexander Millard H. , Gordon Roy G.
Published: February 2004

A method is developed for obtaining solutions to the usual first-order perturbation equations arising from the presence of a harmonic perturbation. By means of piecewise polynomial approximations to the unperturbed potential and to the perturbation, the method yields the first-order wavefunction in piecewise analytic form. It is easy to obtain error bounds to the relevant second-order properties. The method can be extended to multidimensional problems or to problems involving nonlocal potentials. Numerical application is made to the calculation of the frequency dependent dipole polarizability of the hydrogen atom and of the helium atom within the uncoupled Hartree—Fock approximation.
EPR of Trapped H and D Atoms in γ-Irradiated Acidic Glasses. Anomalous Differential Saturation of Hyperfine Components via Cross Relaxation Induced by Paramagnetic Metal Ions
Authors: Wong S. K. , Wan J. K. S.
Published: February 2004

Cross relaxation is suggested as the mechanism for the observed anomalous differential saturation of the hyperfine components of trapped D atoms in γ-irradiated acid glasses. The theory of cross relaxation was examined. Further experimental confirmation was obtained from a detailed investigation of the abnormal saturation of trapped D and H atoms induced by the presence of paramagnetic transition-metal cations deliberately added to the system. The metal cations have a much shorter spin—lattice relaxation time, and their resonances overlap to different degrees with the hyperfine components of the trapped D and H atoms. The quantitative effect of concentration of these added paramagnetic metal cations on the cross relaxation process was also examined.
Equation of State for Dense Gases
Authors: Haar Lester , Shenker Stephen H.
Published: February 2004

A simple quantitative equation of state of the van der Waals type is proposed for dense fluids. The equation yields compressibility factors with a precision comparable to those from the more complicated temperature perturbation theory when tested against ``computer experiments'', including isotherms as low as 0.8 liquid-vapor critical temperature. The equation is capable of predicting compressibility factors for real gases as illustrated by calculations for argon and nitrogen at pressures up to 10 000 atm. The derivation of the equation is based on a rearrangement of the terms in the Ursell—Mayer density expansion of the configuration integral. The equation contains two temperature-dependent parameters, to be determined from the dilute gas second virial coefficients. A principal feature of our approach is that it does not require explicit knowledge of the details of the pair interaction.
Z Dependence of Coupling Constants between Directly Bonded Nuclei. The Value of J (73Ge–13C)
Authors: Dalling Don K. , Gutowsky H. S.
Published: February 2004

The Fermi contact contribution to the reduced, nuclear spin—spin coupling constant of directly bonded atoms, KXN, has been studied by determining the Z dependence of the electron spin density at the nuclei of the isolated atoms. Two methods of calculating the spin density from atomic spectra were tested and the results are employed to determine KXN. It is concluded that KXN for a series of isoelectronic, isostructural compounds varies as κZX, where κ is a relativistic correction which ranges from 1.0 to about 2.5 depending on ZX. The theoretical results are compared with experimental data for symmetrical compounds with bonds of the type XF, XH, and XC, and apparent core polarization contributions to KXN are noted. The directly bonded coupling constant between 73Ge and 13C has been measured in tetramethylgermane; it is 18.7 ± 0.9 Hz.
13C Hyperfine Interactions in Radicals from Some Carboxylic Acids
Authors: Laroff Gary P. , Fessenden Richard W.
Published: February 2004

The ESR spectra of four radicals of the type RĊHCO2−, four of the type RĊ (O−) CO2−, and one each of the types R2ĊCO2− and ROHĊCO2− have been studied and the 13C hyperfine constants determined. The radicals were produced by continuous, in situ radiolysis of basic aqueous solutions of the appropriate acids. In all cases the radicals bore at least two units of negative charge, and radicals labeled with 13C at the natural abundance level were detectable because of the lowering of the recombination rate by this multiple charge. A typical radical of the first type is ĊH (CO2−)2 (aαC = 31.44 G, aβC = 11.92 G) derived from malonate. Also studied were radicals from succinate, citrate, and the anion of 1,2,3-propanetricarboxylic acid. In all cases where aαC could be determined, the values are close to 30 G. The values of aβC are also rather constant, being in all cases in the range 10–13 G. The hyperfine constants of the γ carbons (substituents on β carbons) in this type of radical are consistent with an angular dependence aγ ≅ 20 Ccos2 θ. A typical radical of the second type is CH3Ċ(O−;) CO2− (a α = 14.09 CG, aβC = 8.19, 6.35 G). The small 13C hyperfine constants of this type of radical (∼ 7–14 G) demonstrate the planarity of these radicals at the α carbon. Also studied was the radical O2−CCH2OĊHCO2− for which aαC was found to be 26.45 G. The low value of aαC shows that this radical is planar at the radical site and hence can serve as a reference for the values of aαC in hydroxyalkyl and ketyl radicals. The hyperfine constants of the radicals ĊH2CO2−, ĊH (CO2−)2, and HĊ(O−)CO2− are compared with values from INDO molecular orbital calculations.
Evaluation of Dilute Gas Correlations in Quantum Statistical Mechanics by Inequalities
Authors: Bruch Ludwig W.
Published: February 2004

Upper and lower bounds are reported for correlation functions and cluster integrals occurring in the quantum statistical mechanics of dilute gases. This work is related to the Wigner—Kirkwood semiclassical approximation and may be used to extend the range of application of that approximation. A bound for the exchange second virial coefficient is presented. Numerical results are presented for a model of hydrogen vapor. Some analytical results for angle-dependent potentials are derived.
Cross Sections for the Collision of Thallium (62 P1/2 and 62 P3/2) with I2
Authors: Gedeon A. , Edelstein S. A. , Davidovits P.
Published: February 2004

The inelastic collision cross sections with I2 have been measured for the ground state (62P1/2) and the metastable state (62P3/2) of thallium. The thallium atoms were produced in the presence of a known amount of I2 by photodissociating TlI salt vapor with a short pulse of uv light. The cross sections were computed from the decay rate of the thallium density. We have observed that the collision of the metastable state with I2 results in the de-excitation of the state rather than in chemical reaction. The cross section for the process Tl(62P1/2)+I2→ TlI+I is 105 Å2 and for the process Tl(62P3/2)+I2→Tl(62P1/2)+I2 is 159 Å2. these results are in good agreement with theory.
Energy Loss Spectra of Inelastically Scattered Electrons from Tungsten
Authors: Edwards David , Propst F. M.
Published: February 2004

The energy loss spectra of electrons scattered from a W (100) surface were measured on both clean and gas covered (H2, N2, H2O, O2, CO) surfaces. Two additional peaks have been observed in the spectra from the clean surface and a 7.5−eV peak is observed upon H2O adsorption.
Limited Expansion of Diatomic Overlap (LEDO): A Near-Accurate Approximate Ab Initio LCAO MO Method. I. Theory and Preliminary Investigations
Authors: Billingsley Frank P. , Bloor John E.
Published: February 2004

A method is presented for approximating multicenter integrals which arise in LCAO MO calculations utilizing Slater-type orbitals as basis functions. Two-center charge distributions are projected onto one-center functions, allowing all difficult three- and four-center integrals to be expressed as linear combinations of rapidly calculated one- and two-center Coulomb integrals. Test calculations are reported for a wide variety of polyatomic molecules yielding excellent agreement with corresponding ab initio treatments. Integral generation rates are well over an order of magnitude faster than current accurate approaches. The systematic and reliable nature of the approximation is examined and comparison made with the recently reported PDDO (projection of diatomic differential overlap) technique.
Neutron Diffraction Study of the Trigonal and Cubic Phases of NaSH
Authors: Schroeder L. W. , de Graaf L. A. , Rush J. J.
Published: February 2004

Neutron diffraction powder patterns have been measured for the trigonal and cubic phases of NaSH at 296 and 379°K, respectively. Several possible models describing the orientations of the SH– ion in the trigonal phase were compared with the experimental results. A model in which the SH– ions are aligned along the trigonal axis gave the best agreement between calculated and observed structure factors. Thus the transition from the cubic to trigonal phase apparently results from the alignment of the SH– ions along one of the cube diagonals which then becomes shorter than the others resulting in the less symmetric trigonal structure. Several structural models were also considered for the cubic phase. The isotropic, free rotator mode describing the SH– orientations can be rejected in favor of a less randomly disordered model by comparison with preliminary neutron inelastic scattering data. The limited diffraction data in this phase, however, do not enable a distinction to be made between disordered models. The preliminary inelastic scattering results also indicate that the librational frequencies of the SH– ions are similar in both phases, but that reorientations of the SH– ion between equivalent directions occur at a much faster rate (∼1012 sec−1) in the cubic phase.
Localized Orbitals for Polyatomic Systems. III. Open Shell Extended Hückel Approximation
Authors: Schlosser Herbert
Published: February 2004

We present a rigorous derivation of the extended Hückel (EH) approximation for open shell polyatomic systems making use of the open shell generalization of the Adams—Gilbert (AG) equations for localized orbitals which we derived in a previous paper. The open shell EH theory is derived for the open shell filled band and open shell partly filled band cases from a systematic expansion of matrix elements of the one-electron Fock operator in powers of the overlap using distorted atomic orbital and polarized atomic orbital solutions of the AG equations as bases, respectively.
Photoionization and Absorption Spectrum of Formaldehyde in the Vacuum Ultraviolet
Authors: Mentall J. E. , Gentieu E. P. , Krauss M. , Neumann D.
Published: February 2004

Absorption and photoionization coefficients have been measured for H2CO in the 600–2000-Å region. Integrated oscillator strengths were determined for a number of strong Rydberg transitions above 1200 Å. From the photoionization curve the first adiabatic ionization potential was found to be 10.87±0.01 eV. As an aid in interpreting the absorption spectrum, theoretical calculations were made using a single-configuration self-consistent field procedure for the Rydberg states and a model which included mixing between the Rydberg and valence states. On this basis, weak absorption features between 1340 and 1430 Å have been assigned to the B11(σ → π *) valence state. The 1A1(π → π *) valence state is deduced to be strongly autoionized just above the 2B2 ionization limit.
Electron Transfer and Ion—Atom Interchange in C+–CO Collisions
Authors: Maher K. E. , McClure G. J. , Leventhal J. J.
Published: February 2004

Kinetic-energy distributions of CO+ and C2+ ions resulting from C+–CO collisions were measured over the incident ion kinetic-energy range 13–30 eV. These distributions were used for calculation of the reaction energetics. The CO+ kinetic-energy distributions provide strong evidence that this product is formed by both electron transfer and ion—atom interchange. In addition to the major (forward scattered) peak in the C2+ distribution small yields of C2+ were detected at 180° in the center-of-mass system.
Theory of Inelastic Collisions: Uniform Asymptotic (WKB) Solutions and Semiclassical S-Matrix Elements for Two-Channel Problems
Authors: Eu Byung Chan
Published: February 2004

Semiclassical scattering matrix elements for two-channel problems are obtained by means of uniform asymptotic (WKB) solutions and solving a set of coupled first-order differential equations generated from the radial Schrödinger equations. By solving such first-order coupled equations, Stueckelberg's mathematically unsatisfactory procedure of tracing solutions in a complex plane to obtain the S-matrix elements is completely removed and the expression thus obtained for the transition probability is devoid of physically unreasonable shortcomings of Stueckelberg's result. The S-matrix elements obtained are given in terms of well-defined quadratures easy to calculate and valid for any types of the interaction matrix elements, Vii (x) and w (x). The transition probability obtained from the S-matrix elements is shown to reduce essentially to the well-known Landau—Zener expression as a limiting case when w (x) is small.
Vapor Pressure Isotope Effect near the Triple Point
Authors: Jeevanandam M.
Published: February 2004

The large discontinuity of the vapor pressure isotope effect (VPIE) observed near the triple point is discussed with the application of Bigeleisen's theory of equilibrium isotope effects in condensed phases to Eyring's significant structure theory of liquids. The partition function ratios are calculated for monatomic, diatomic, and polyatomic systems, and they are in good agreement with the experimental values. The simple molar volume change (Vs/Vl) is found to be adequate to explain the change in the vapor pressure ratios of 36Ar/40Ar, 20Ne/22Ne, 12C16O/13C16O, 12C16O/12C18O, and 12CH4/13CH4 systems, especially near the melting region. When the structural change at the melting point of ice is also taken into account, the VPIE is satisfactorily explained in the H2O/D2O system. The ``crossover'' phenomenon observed in systems like the isotopic methanes can be explained when the difference in the rotational contribution to the ratio of the reduced partition function ratios between the liquid and solid states is also taken into account. For isotopic hydrogen with large quantum effects, the isotope effect on the molar volume ratio must be included.
Collisional Deactivation of O2(1Δg)
Authors: Findlay F. D. , Snelling D. R.
Published: February 2004

This paper reports determinations of collisional deactivation rate constants for O2(1Δg) by minor atmospheric constituents and other gases. The benzene–oxygen photochemical system was used to produce O2(1Δg), and relative concentrations were measured by monitoring the intensity of its emission at 1.27 μ. The rate constants for the reaction O2(1Δg)+M→O2+M were 4.53 × 10−18, 5.60 × 10−18, and (0.5–1.0) × 10−19 (cm3 molecule−1·sec−1) for M=H2, H2O, and N2O, respectively. Upper limits were obtained for deactivation by He, Ar, CO2, and SF6 of 8.0 × 10−21, 8.3 × 10−21, 1.5 × 10−20, and 1.2 × 10−20, respectively. The temperature dependence of the deactivation of O2(1Δg) by oxygen has been investigated. The rate constant can be expressed in the form kO2 = 2.22 ± 0.09 × 10−18 (T / 300)n, where n = 0.78 ± 0.32 and T is the temperature in degrees Kelvin.
Determination of the Absolute Values of the Rate Constants of the Reactions of O(3P) Atoms with Alkenes by a Modulation Technique
Authors: Atkinson R. , Cvetanović R. J.
Published: February 2004

A modulation technique has been used for the production of O(3P) atoms and the absolute values of the rate constants at 25.0 ± 0.5°C for their reactions with ethylene, propylene, butene-1, and isobutene have been determined from phase shift measurements. The values obtained are fully consistent with the relative rate constants determined earlier and also show excellent agreement with some absolute values in the literature. A value for the rate constant of the reaction of O(3P) atoms with NO has also been obtained.
Planar Vibrations of Carbon Skeleton of Polyenes. II. A Hartree–Fock Molecular Orbital Calculation of the Stable Configuration and Vibrational Frequencies of Polyenes
Authors: Tric C.
Published: February 2004

The configuration of polyenes is studied in terms of the alternance (difference between bond lengths of alternating “single” and “double” bonds) and of the average elongations of all the bonds. A self-consistent calculation of energy allows for the determination of the stable configuration and of the vibration frequencies of the uniform modes, for alternated (cyclic) polyenes. The stability of the alternating configuration is reinforced by taking into account both exchange correlation effects and nuclear displacements, and a good agreement is obtained with the observed frequencies.
Relaxation Processes and Molecular Motion in Liquids and Gases
Authors: Morawitz H. , Eisenthal K. B.
Published: February 2004

In this paper we extend the time correlation function approach to infrared absorption in an organic liquid or dense gas. We include an interaction energy (coupling) between the ir active vibrational mode and the coupled translational–rotational degrees of freedom to account for collision-induced de-excitation of the vibration (damping). The additional time dependence of the dipole correlation function is a damped exponential and modifies the line shape obtained by earlier treatments. The appearance of a damping term also changes the interpretation of the first few terms in the short-time expansion of the correlation function, which are related to the molecular moment of inertia and the mean-square torque. Our treatment is based on many-body perturbation theory (renormalization of the vibrational energy to second order in the coupling) and can be extended to more general cases.
Heat Capacities of Cubic, Monoclinic, and Vitreous Arsenious Oxide from 5 to 360°K
Authors: Chang S. S. , Bestul A. B.
Published: February 2004

Experimentally determined heat capacities, Cp, precise to better than 0.1%, and derived thermal data are reported for cubic and monoclinic crystalline and vitreous arsenious oxide at temperatures from 5 to 360°K. Cp for the glass is higher than for either crystal at all temperatures. Cp's for the cubic crystal are lower than those for the monoclinic between about 20 and 310°K and higher at temperatures below and above this range. Cp / T3 for the glass is still increasing with decreasing temperature at 5°K, although Cp / T3 for both crystals peaks a few degress above this. Cp's reported here for the monoclinic crystal are grossly different from those previously reported by Stranski's school. The corresponding entropy difference between the two crystals at 298.15°K reported here is 5.9 ± 0.2 J deg−1·mole−1, whereas that previously reported from Cp measurements by Stranski's school was 14.4 J deg−1·mole−1. Combined with a value newly obtained elsewhere for the enthalpy difference between the two crystals at 298.15°K, the value reported here for the entropy difference yields a transition temperature between them of 311 ± 30°K, which is in the range currently cited on other bases. Cp values reported here for the cubic crystal and the glass are consistently 4%–5% lower than those previously reported by Tarasov's school, suggesting a systematic proportional effect of the magnitude in those previous values. The proportionality of Cp to T for the cubic crystal from 65 to 160°K, previously erroneously attributed to the monoclinic crystal by Tarasov on the basis of Anderson's Cp data, are discussed.
Structure of Carbon Monoxide Chemisorbed on Platinum
Authors: Lin Kenneth C. , Witt J. D. , Hammaker R. M.
Published: February 2004

C–O stretching frequencies for 12C16O, 13C16O, and 12C18O chemisorbed on silica supported platinum at 25° and 200° are used to demonstrate that the bonding to the surface is via the carbon atom. Empirical force constant correlations suggest that the best single structure approximation is Pt=C=O.
Defect Phosphorescence of Benzophenone Crystals
Authors: Simpson J. D. , Offen H. W.
Published: February 2004

The phosphorescence from benzophenone crystals has been studied as a function of pressure at 77°K. Squeezing the crystal irreversibly replaces exciton phosphorescence by defect phosphorescence, characterized by a ∼ 1000-cm−1 trap depth and a lifetime of 3.7 msec. The pressure effect on the defect phosphorescence is Δν = − 490 cm−1 and Δτ = − 0.5 msec for ΔP = 30 kbar.
Calculation of the Zero Wave Vector Lattice Frequencies of Alpha- and Beta-Oxygen
Authors: Laufer J. C. , Leroi G. E.
Published: February 2004

The optically active lattice frequencies of α- and β-O2 have been calculated in the rigid-molecule approximation using an atom–atom Lennard-Jones [6–12] potential, with and without the addition of an intermolecular quadrupole–quadrupole interaction. Both models predict frequencies reasonably close to those observed experimentally for α-O2 and support the assignment of the higher-frequency librational mode to the torsion about the twofold axis in the crystal. Excellent agreement is obtained between the observed librational frequency and its temperature dependence in β-O2 and the results calculated using the Lennard-Jones potential. If it is assumed that the molecules in β-O2 may be tipped from the crystallographic c axis, an increase in the tilt angle as the temperature is raised is predicted, which is consonant with the temperature dependence of the lattice constants for this phase. It is concluded that the Lennard-Jones atom–atom interaction is a satisfactory model for the potential function in solid oxygen, provided that the packing is such that electron overlaps are small.
NMR Relaxation in Cholesterol and Cholesteric Liquid Crystals
Authors: Dybowski Cecil R. , Wade Chas. G.
Published: February 2004

The proton nuclear spin lattice relaxation has been measured in cholesterol and its esters (n-propionate, n-decanoate, myristate, and oleate) in the solid, mesophase, and liquid regions using conventional pulsed NMR techniques at a frequency of 30 MHz. All solid phases and the mesophases of all the esters except the oleate exhibited relaxation which could be fit within experimental error with a single exponential. Nonexponential relaxation, which was analyzed assuming two relaxation times, was observed in all liquid phases and throughout the mesophase of the oleate ester. Relaxation in the mesophases of all the esters had very little temperature dependence and this, plus its behavior at the phase transitions, indicated that the relaxation mechanism might be similar to that in nematic liquid crystals. It was not possible to determine the primary source of the nonexponential relaxation behavior but it did not require the presence of an ester group.
Two Modes of Diffusion in Liquids
Authors: Nir S. , Stein W. D.
Published: February 2004

We propose that in the case of diffusion in liquids two modes of diffusion are present. The first mode, a flow mechanism, resulting from the movement of the solvent molecules, is the only one available to large diffusants having molecular weights at least an order of magnitude larger than that of the solvent. Smaller diffusants can use this mode but also move by activated diffusion within the lattice of the solvent matrix. The form of the variation of the activation energy for diffusion with size of the diffusant is discussed. Semiempirical calculations of diffusion coefficients and activation energies for several (small to medium-sized) diffusants in water are performed and the results are compared with available data.
Effect of Pressure on the Electronic Structure of Protoporphyrin IX, Hemiporphyrins, and Related Compounds
Authors: Grenoble D. C. , Frank C. W. , Bargeron C. B. , Drickamer H. G.
Published: February 2004

Studies have been made on the effect of pressure to 170 kbar on the electronic structure of protoprophyrin IX, several hemiporphyrins, and related compounds using optical absorption and Mössbauer resonance. The optical absorption peaks which correspond to π–π* transitions shifted to lower energy and decreased in intensity with increasing pressure. This could be explained in terms of changes in the configuration interaction involving a shift of the electrons to the outer portions of the porphyrin ring. Imidazole protohemichrome is a low spin ferric compound. At room temperature and pressures above about 45–50 kbar it reduced apparently to an intermediate spin (or mixed spin) ferrous state. At 110° it reduced at moderate pressure to a low spin ferrous state, but with increasing pressure the Fe(II) transformed to intermediate spin. Hemin and hematin are high spin ferric compounds. With increasing pressure they reduced apparently to intermediate spin (or mixed spin) ferrous compounds. The amount of reduction with pressure was consistent with the location and shift of the observable metal to ligand-charge transfer bonds. Two ferric compounds with square pyramidal symmetry like hemin and hematin but with planar neighbors of oxygen or sulfur instead of nitrogen were studied. Both reduced with increasing pressure, in one case with a probable change of spin state. All reductions observed were reversible.
Dimerization of Methylenes by Their Least Motion, Coplanar Approach: A Multiconfiguration Self-Consistent Field Study
Authors: Basch Harold
Published: February 2004

The one-dimensional path of reaction in the least motion, coplanar approach of two methylenes to form ethylene has been calculated using the nonempirical multiconfiguration self-consistent field (SCF) method in a large Gaussian basis. Orthogonality between open- and closed-shell molecular orbitals of the same symmetry is constrained by a “partitioned basis set” technique which is found to give rapid and uniform convergence to the proper SCF solution. The results obtained for the reaction path suggest that the ground state of bent (∼ 120°) methylene is the triplet. Therefore, in this study, it is the triplet states (and not closed-shell singlet states) of two appropriately oriented bent methylenes that correlate with the normal (ground) state of ethylene. For two closed-shell singlet-state methylenes the reaction path is found to be purely repulsive.
Pressure Dependence of Effective Ionic Charge of Solids
Authors: Mitra S. S. , Namjoshi K. V.
Published: February 2004

The pressure dependence of the Szigeti effective ionic charge of some rock salt, cesium chloride, and zinc-blende type crystals is evaluated from the pressure dependence of their long-wavelength optic phonon frequencies and the high- and low-frequency dielectric constants. Comparison is made with values predicted by indirect methods.