bluuebunny/crossref_metadata_2025 huggingface dataset:
High-Resolution Photoionization Study of the H2 Molecule near Threshold
Authors: Chupka W. A. , Berkowitz J.
Published: February 2004

Relative photoionization and absorption cross sections of H2 (ordinary and para) have been measured from 745–810 Å at 300 and 78°K, with a resolution width of 0.04 Å. The photoionization data show the presence of extensive structure due to autoionization of vibrationally excited Rybderg states. Analysis of the data on the D–X(6, 0) band in the region of the ionization threshold leads to the conclusion that the ionization potential of H2 lies between 124 418.2 and 124 393.5 cm−1 and is probably very near the former value. Analysis of the data on the B″–X(4, 0) band requires either that the ionization potential lies below 124 407.2 cm−1 (which is rather unlikely) or that autoionization of the R(2) and R(3) lines occurs in violation of one of the autoionization selection rules proposed by Beutler and Jünger. Some Rybderg states that lie just below their ionization limits and do not autoionize spontaneously can be made to ionize in weak electric fields and by collision with unexcited H2 molecules. Direct ionization of H2(J = 0) appears to begin about 23 cm−1 below the expected threshold, probably as a result of collisional ionization of Rydberg states lying just below the ionization threshold. A series of peaks appearing in a short region immediately above threshold apparently coincides with a series which Herzberg has observed in absorption and identified as the Rydberg series converging to the rotationally excited K = 2 state of the H2+ ion. The autoionization of these Rydberg states must occur by conversion of rotational to electronic energy. An intensity anomaly of this series is ascribed to channel interaction with a vibrationally excited Rydberg state. A large number of the autoionization peaks have been identified in the spectrum of parahydrogen at 78°K. By comparison of the absorption cross section with the photoionization cross section, it was found that all states that can autoionize with Δυ = 1 are strongly autoionized (∼40%–100%) while the autoionization probabilities of states that autoionize with Δυ > 1 vary widely. The dominance of autoionization with Δυ = 1 over predissociation extends to the very highest Rydberg states and leads to the absence of observable thresholds for formation of vibrationally excited H2+ ions. Some peaks that autoionize with Δυ = 1 have widths larger than the instrumental width, and the autoionization rates have been estimated. They are in fair quantitative agreement with recent calculations of Nielsen and Berry. The widths decrease rapidly with principal quantum number as Bardsley predicts. The autoionization rate decreases by about a factor of 10 or more when the Δυ of the transition changes from 1 to 2. For larger values of Δυ, the autoionization rate appears to decrease still more but not necessarily as a monotonic function of Δυ.
Vibrational Spectra of PClnF5–n
Authors: Beattie I. R. , Livingston K. M. S. , Reynolds D. J.
Published: February 2004

The frequencies of the title compounds are computed using a simple force-field transference scheme. On the basis of these calculations, and gas-phase Raman spectroscopy of PF5, ν7 for this molecule is assigned at 175 cm−1. An assignment of the species PF3Cl2 is suggested.
Model for the Dynamics of Some Primary Photochemical Reactions
Authors: Philpott Michael R.
Published: February 2004

A model similar to that proposed by Rice, McLaughlin, and Jortner [J. Chem. Phys. 49, 2756 (1968)] is examined in detail form the point of view of momentum space and is extended to include orthogonal decay channels. It is shown that a cutoff function in matrix elements between fragments and undissociated molecules is essential for convergent integrals over fragment energies. The explicit time dependence of molecular and fragment state amplitudes is investigated and formulas for the fragment energy profile given. The possibility of using this model to describe threshold and isotope effects is suggested.
Molecular g Values, Magnetic Susceptibilities, Molecular Quadrupole Moments, and Second Moments of the Electronic Charge Distribution in OF2, O3, and SO2
Authors: Pochan J. M. , Stone R. G. , Flygare W. H.
Published: February 2004

The rotational Zeeman effect has been observed in several transitions in OF2, O3, and SO2. The absolute values of the molecular g values (gaa, gbb, and gcc) and the signs and magnitudes of the magnetic susceptibility anisotropies [(2χaa − χbb − χcc) and (2χbb − χaa − χcc)] are obtained by high-resolution microwavespectroscopy with the sample in the presence of high magnitude fields. Arguments are presented to show that all of the molecular g values are negative leading to the molecular quadrupole moments, (Qaa, Qbb, and Qcc) also listed below. The a, b, and c axes are the principal inertial axes in each molecule. The b axis bisects the interatomic angle and the a axis is also in the molecular plane. The units of χ are 10−6erg/G2·mole and the units of Q are 10−26 esu·cm2.OF2O3SO2gaa− (0.213 ± 0.005)− (2.968 ± 0.035)− (0.6037 ± 0.0005)gbb− (0.058 ± 0.002)− (0.228 ± 0.007)− (0.1161 ± 0.0002)gcc− (0.068 ± 0.002)− (0.081 ± 0.006)− (0.0882 ± 0.0004)(2χaa − χbb − χcc)− (8.8 ± 1.4)+ (98.0 ± 5.6)+ (6.39 ± 0.53)(2χbb − χaa − χcc)− (4.4 ± 0.7)− (17.4 ± 4.4)+ (3.07 ± 0.34)Qaa− (1.6 ± 1.4)− (17.1 ± 5.2)− (5.3 ± 0.4)Qbb+ (2.1 ± 1.1)+(9.2 ± 4.7)+(1.3 ± 0.3)Qcc− (0.5 ± 1.9)+ (7.9 ± 7.9)+ (4.0 ± 0.6)The diagonal elements in the paramagnetic susceptibility tensor and the anisotropies in the second moment of the electronic charge distributions (〈a2〉 − 〈b2〉 and 〈b2〉 − 〈c2〉) are computed from the above results and the known molecular structures. The bulk susceptibilities are known for O3 and SO2 which leads to a direct determination of the diagonal elements of the total and diamagnetic susceptibilities and the individual values of 〈a2〉, 〈b2〉, and 〈c2〉 in these two molecules. The bulk susceptibility for OF2 is estimated from a reliable estimate of 〈c2〉 for this molecule.
Analytical Evaluation of Multicenter Integrals of r12−1 with Slater-Type Atomic Orbitals. V. Four-Center Integrals by Fourier-Transform Method
Authors: Kay Kenneth G. , Silverstone Harris J.
Published: February 2004

The four-center integral of r12−1 with Slater-type atomic orbitals is evaluated analytically. The Fourier-transform convolution theorem is used to express the integral as an infinite sum in which the internuclear angles appear in spherical harmonics, and the internuclear distances in integrals over spherical Bessel functions and exponential-type integrals. These “radial” integrals are evaluated as convergent infinite expansions by contour integration techniques. The formulas are valid for general values of the n, l, m, ζ parameters of the orbitals and for general nonzero values of the internuclear distance vectors.
Analytic Optimization in Atomic SCF Calculations
Authors: Olive Joseph P.
Published: February 2004

For self-consistent field calculations by the expansion technique, equations were derived which incorporate the optimization of the exponents as well as the coefficients of the Slater-type basis functions. The equations were programed for a computer, and the program was tested for small atoms.
Effect of Pressure on the C–Fe Bond in Ferrocyanides and Ferricyanides
Authors: Fung S. C. , Drickamer H. G.
Published: February 2004

High-pressure studies have been made on Fe–C bonding in ferrocyanides and ferricyanides using Mössbauer resonance. For the Cu, Ni, and Zn ferrocyanides a partial conversion to high-spin Fe(II) was observed at high pressure and elevated temperature. This is explained in terms of reduced metal-to-ligand back donation at high pressure. The ferricyanides reduced to ferrocyanides at modest pressure and room temperature. At high pressure and high temperatures all these salts showed extensive conversion to high-spin Fe(II). The process was reversible, but with considerable hysteresis. In prussian blue the high-spin Fe(III) and low-spin Fe(II) were separately tagged with Fe57. The high-spin Fe(III) reduced strongly to high-spin Fe(II). The low-spin Fe(II) showed a partial conversion to low-spin Fe(III) at modest pressures. This conversion was independent of temperature. At higher pressures it converted to high-spin Fe(II) in an amount which increased sharply with temperature.
Effect of Pressure on the Fe–C Bond in Substituted Cyanides
Authors: Fung S. C. , Drickamer H. G.
Published: February 2004

The effect of pressure has been studied on the bonding in four nitroprussides and in Na3(CN)5NH3, using Mössbauer resonance. At atmospheric pressure, the spectrum consists of low-spin Fe(II) with a very high electron density at the iron nucleus and a large quadrupole splitting. At modest pressure and high temperature a single peak appears with approximately the isomer shift of the initial pair. This is attributed to a decrease in the asymmetry due to changes in the Fe–NO bond. At high pressures and high temperatures very large conversion to high-spin Fe(II) is observed. This is attributed to a large decrease in the back donation of Fe electrons to the ligands.
Coupling-Parameter Expansion in the Kirkwood Integral Equation for Dense Fluids
Authors: Weeks John D. , Rice Stuart A. , Katz Ira
Published: February 2004

A new treatment of the coupling-parameter dependence of terms in the Kirkwood integral equation is proposed. The effect of partial coupling ξ of molecule 1 on the pair correlation function g(13, ξ) = exp[− ξφ(13)]Y(13, ξ) is divided into two parts, the direct coupling term exp[− ξφ(13)] and the indirect coupling term Y(13, ξ). We argue, on physical grounds, that the indirect coupling term Y(13, ξ) is a relatively slowly varying function of ξ as compared with the direct coupling term over almost all the range 0 < ξ ≤ 1 except in a very small region near ξ ≤ 0. In the kernel of the Kirkwood integral equation, Y(13, ξ) is expanded in a Taylor series in the coupling parameter about ξ = 1. The first term of this series gives an integral equation which resembles both the Percus–Yevick and hypernetted chain equations, if we assume our approximation to be valid for the entire range 0 ≤ ξ ≤ 1. An interative procedure for higher-order terms in a resummation of the Taylor series in the coupling parameter is found which, in principle, permits calculations to any desired order in the coupling. However, for a strongly repulsive potential the coupling parameter representation encounters difficulty near ξ = 0. By subdividing the range of variation of ξ a new expansion is suggested. Preliminary numerical calculations indicate that this scheme gives a good representation of g(1, 3).
Nuclear Magnetic Relaxation of 19F in Transition-Metal Complexes. II. Complexes with Mn(II), Co(II), Fe(II), and Ni(II)
Authors: Eisenstadt Maurice
Published: February 2004

The effect of Mn(II), Fe(II), Co(II), and Ni(II) on T1 and T2 of aqueous 19F has been measured from pH 0–6 at 25°C as a function of fluoride concentration and magnetic field, similar to previous work with Cu(II) [J. Chem. Phys. 48, 4445 (1968)]. Equilibrium constants for the first coordination complex MF+ were obtained from the data and ranged from 1 to 5; there was no evidence of higher complexes. The principal results were the association and dissociation rates of the complex. The latter is found to be of the form kd = τm−1 = ka + kb(H+) + kx(F−) + ky(HF) + kz(HF2−), where τm is the lifetime of the complex. These rates are given in the following table: Mn(II)Fe(II)Co(II)Ni(II)logka5.95.85.03.3logkb8.78.88.45.8logkx6.2···5.23.8logky7.1······4.7logkz6.9···6.3···The electron relaxation time T1e was >1 × 10−8 sec for Mn(II) and <5 × 10−12 sec for the other three metals, similar to results found for the aquo complex. The isotropic hyperfine interaction constants Ai for FeF+ and CoF+ were 8.7 × 108 and 2.1 × 108 rad/sec. The temperature dependence of T1 and T2 for Ni19F+ was similar to that of proton relaxation in the aquo complex.
Infrared and Raman Spectra of Substituted Hydrazines. II Unsymmetrical Dimethyl Hydrazine
Authors: Durig J. R. , Harris W. C.
Published: February 2004

The infrared spectra of unsymmetrical dimethyl hydrazine, H2NN(CH3)2, and unsymmetrical dimethyl hydrazine-d2, D2NN(CH3)2, have been recorded in both the liquid and gaseous states from 33 to 4000 cm−1. In addition the infrared spectrum of solid H2NN(CH3)2 has been measured from 33 to 4000 cm−1 and that of D2NN(CH3)2 from 33 to 400 cm−1. The Raman spectra of the liquid dimethyl hydrazines have been obtained from 200 to 4000 cm−1 and depolarization measurements were made. The spectra were interpreted on the basis of C1 symmetry, and all 30 vibrations have been assigned on the basis of the band positions, relative intensities, and isotopic shift. The methyl torsional fundamentals were observed, and the barrier hindering the rotation of the CH3 group was calculated to be 4.69 kcal/mole. A symmetrical threefold cosine-based potential function has been employed in calculating a barrier height of 3.2 kcal/mole for the amino torsional vibration. Thermodynamic functions have been calculated for the temperature range 200–3000°K on the basis of a rigid-rotator harmonic-oscillator model for all vibrations except those involving the internal rotation of the amino and methyl groups, for which appropriate corrections have been made.
Quasiequilibrium Treatment of Gas–Solid Reactions. II. Flash Desorption of Oxidation Products from a Tungsten Surface
Authors: Batty J. C. , Stickney R. E.
Published: February 2004

The quasiequilibrium treatment of heterogeneous reactions is applied to the flash desorption data of Ptushinskii and Chuikov [Surface Sci. 6, 42 (1967)] for the reaction of oxygen with a tungsten surface. Comparison of the theoretical and experimental results provides support for the view that, in the present case, flash desorption may be considered to be a quasistatic process (i.e., the adsorbate passes through a continuous series of equilibrium states). On this basis, the quasiequilibrium treatment is used to develop a semiquantitative explanation of the experimental observation that the amounts of the various oxide species desorbed per flash depend upon the adsorption temperature.
Chlorine-35 NMR Study of the Shifts and Line Shapes of Some Liquid Inorganic Chlorides
Authors: Johnson K. Jeffrey , Hunt J. P. , Dodgen H. W.
Published: February 2004

The chemical shifts and linewidths of a series of 18 liquid inorganic chlorides have been measured using 35Cl broadline NMR techniques. The chemical shifts vary from 120–950 ppm downfield from the chloride ion. Assuming the mean excitation energies of these compounds are equal, a rough correlation of shift with quadrupole coupling constants is obtained for the nontransitional metal chlorides. The shifts of the transitional metal chlorides do not correlate with the quadrupole coupling constants, suggesting dπ–pπ interactions. The full linewidths at half-maximum intensity vary from 0.15–48 G. The observed widths, corrected for viscosity and molecular dimensions, correlate reasonably well with the quadrupole coupling constants. The observed correlation times τc are about a factor of 10 shorter than those calculated with the Debye–Einstein equation. The line shapes and widths were determined using a computer program that resolved the experimental spectra in terms of overlapping Lorentzian functions.
ESR Study of the Kinetics of the Reactions of D Atoms and O Atoms with NH3
Authors: Kurylo Michael J. , Hollinden Gerald A. , LeFevre Henry F. , Timmons Richard B.
Published: February 2004

A wide temperature-range study using ESR atom detection has beem employed to measure the rates of the reactions of D atoms and O atoms with NH3. In the case of the deuterium reaction, evidence is presented which shows that the interaction with NH3 proceeds via an exchange process of the type: D+NH3→NH3D+H. This reaction was studied over the temperature range of 423–741°K. The specific rate constant for this exchange reaction was found to be 1.9 ± 0.2 × 1013exp(− 10 100 ± 150 / RT) in units of cubic centimeters mole−1·second−1. The reaction of O atoms with NH3 was studied over the temperature interval of 361–677°K. The specific rate constant obtained for the reaction O+NH3→OH+NH2 was found to be 4.0 ± 0.9 × 1012exp(− 6600 ± 130 / RT) in units of cubic centimeters mole−1·second−1. This Arrhenius pre-exponential factor depends on a stoichiometric factor of 3.50 atoms per NH3 molecule assumed from other work.
17O Nuclear Magnetic Relaxation in Aqueous Solutions of Diamagnetic Metal Ions
Authors: Connick R. E. , Wüthrich K.
Published: February 2004

Measurements are reported on the transverse relaxation of oxygen-17 of water in solutions of diamagnetic electrolytes. For the 2+ ions it is shown that the effective correlation time for the quadrupole relaxation of waters in the first coordination sphere is appreciably shorter than the tumbling time for the hydrated ion. The data can be explained if the waters rotate randomly around the metal–oxygen axis in a time short compared to the tumbling time of the hydrated ion. This intramolecular rotation lowers considerably the effective quadrupole coupling. The model implies that corresponding effects should be present in proton and deuterium relaxation in water, and evidence is cited which is consistent with this prediction.
Far-Infrared Spectra of Inorganic Nitrate and Chloride Glasses, Liquids, and Crystals: Complex Ions or Optical Phonons?
Authors: Angell C. A. , Wong J. , Edgell W. F.
Published: February 2004

Polyethylene-sandwiched films of glassy and viscous liquid metal nitrate, nitrate–chloride, and chloride mixtures, sufficiently thin for their absorption spectra in the far ir to be determined in direct transmission studies, have been prepared. Comparisons of the spectra of these amorphous phases with spectra of the same thin-film samples after crystallization, have been made. Intense absorption bands arising from the vibrations of the quasilattices are observed in all instances, and in the case of the Cd(NO3)2 + TlNO3 glasses and liquids, the separate modes are almost as well defined as for the crystalline double salt TlNO3·Cd(NO3)2. The dependence of lattice vibration frequencies on cation charge and mass, and on cation mole fraction within a given binary system, have been studied. In the mixed systems frequency shifts to both high and lower values than those found in the individual components are observed. No reflections of the abrupt changes in thermodynamic and flow properties, which occur at the glass transition temperature, are found in the vibrational spectra. The nature of collective vibrational modes of motion in condensed phases and the relation of the common “complex-ion” interpretation of vibrational spectra in molten salts to the “quasilattice dynamics” view of the same spectra, are considered in some detail. Information on the internal modes of the NO3− ion, and evidence for a distribution of NO3− ion environments, in the glass, are also presented.
Comparison of Quantum and Classical Theories of an Idealized Three-Body Rearrangement Collision
Authors: Kleinman B. , Tang K. T.
Published: February 2004

Two theories, transition-state theory and collision theory, are available for the study of the reaction rate of a three-body rearrangement process. Classical mechanics plays an important role in both theories. We have made computations using a model of a linear, three-body rearrangement collision that is “solvable” in both the classical and quantum-mechanical formulations. A comparison of the results of these calculations indicates conditions under which the use of classical mechanics might be valid, and also yields information about the possible existence of an “activated complex.”
Nuclear-Spin Entropy and Librational Degeneracy. Application to Ethane and Diborane
Authors: Ko Hsien-Wen , Steele W. A.
Published: February 2004

A treatment of the problem of nuclear-spin entropy in symmetrical molecules is given. Symmetries of the acceptable nuclear-rotational states for molecules in the solid are calculated by applying the Pauli principle to the nuclear wavefunctions, and the numbers of such nuclear states are obtained from group theoretical considerations. Expressions for the nuclear-rotational partition functions and low-temperature entropies are given for rapid conversion of nuclear-spin symmetry species as well as the more usual case of nonconversion. After applying the general theory to ethane and diborane, it is shown that the results obtained for the low-temperature entropy depend sensitively upon the degeneracies and spacings of the librational states in the solid. When the symmetry of the potential field for librational motion in solid ethane and diborane is estimated from the known crystal structures, it is found that the librational degeneracies can exist which would give rise to sufficient randomness to cancel a calculated loss in nuclear entropy for the ethanes and to account for the experimental zero-point entropy in diborane. The implications of this work concerning the nature of molecular motion in these and similar solids at low temperature are briefly discussed.
Kinetic Analysis of the Concurrent Ion–Molecule Reactions in Mixtures of Argon and Nitrogen with H2, D2, and HD Utilizing Ion-Ejection–Ion-Cyclotron-Resonance Techniques
Authors: Bowers Michael T. , Elleman Daniel D.
Published: February 2004

A technique is developed for using the ion-ejection mode of ion-cyclotron-resonance spectroscopy for determining quantitative ion–molecule rate constants in concurrent reaction systems. Rate constants for the reaction of H2+, D2+, HD+, H3+, and D3+ with Ar and N2 are reported along with the concurrent reactions of Ar+ and N2+ with H2, D2, and HD. The results are discussed and compared with the Gioumousis and Stevenson theory and other experimental data where available. The energy dependence of the various reactions are reported as determined by double-resonance experiments. The results are discussed in terms of the phase-space theory of Light. Indications of the mechanism of reaction (complex formation or stripping) were made where appropriate and compared with the crossed-beam data.
First-Order Wavefunctions, Orbital Correlation Energies, and Electron Affinities of First-Row Atoms
Authors: Schaefer Henry F. , Klemm Richard A. , Harris Frank E.
Published: February 2004

The most strongly structure-sensitive part of the correlation energy of an atomic state (called here the orbital correlation energy) may be calculated using a configuration-interaction function we call the first-order wavefunction. First-order wavefunctions and orbital correlation energies are presented for all low-lying electronic states of B, C, N, O, F, Ne, and their positive and negative ions. These data provide ab initio ionization potentials and electron affinities for the atoms involved, which are used in conjunction with available experimental data to make estimates of unmeasured electron affinities to a probable accuracy of ± 0.10 eV. The results indicate the 3P ground state of B− to be stable by 0.187 eV, while predicting the instability of N− (3P) and the metastability of N−(1D) and N−(1S). In apparent disagreement with the conclusions of Seman and Branscomb, C−(2D) is predicted to be unstable relative to C(3P).
Magnetothermodynamics of CuK2(SO4)2·6H2O. I. Heat Capacity, Entropy, Magnetic Moment, from 0.5 to 4.2°K with Fields to 90 kG along the b Crystallographic Axis
Authors: Lemley J. R. , Giauque W. F.
Published: February 2004

The heat capacity and magnetic moment of a 3.912-cm-diam spherical single crystal of CuK2(SO4)2·6H2O have been measured over the range 0.5–4.2°K, with stabilized magnetic fields of 0, 0.5, 1.0, 2.5, 5, 10, 15, 25, 40, 65, and 90 kG directed along the b crystallographic axis. No evidence of magnetic irreversibility was found. At the lower temperatures, magnetic saturation was attained at 90 and 65 kG. The temperature-dependent saturation moment was found to be 6253 G·cm3/mole, corresponding to gb = 2.239. The zero of entropy for the electronic and lattice systems was located by the essentially zero heat capacity near 1°K and 90 kG. Thirty-eight temperature-field isentropes were measured to correlate the entropies throughout the various isoerstedic heat capacity series. The upper limit of the electronic entropy was found to be 1.379 gibbs/mole compared to the expected Rln2 = 1.377 gibbs/mole. The lattice heat capacity below 4°K is given by C = 3.66 × 10−4T3gibbs/mole. Smoothed values of the heat capacity, entropy, enthalpy, internal energy, magnetic moment, isothermal differential magnetic susceptibility, isoerstedic differential temperature coefficient of magnetic moment and its thermodynamic equivalent, the isothermal differential change of entropy with field, and the isothermal work of magnetization have been tabulated.
Application of Low-Field NMR to the Unambiguous Assignment of Spin-Coupling Parameters. III. Other Pentafluorobenzenes
Authors: Moniz William B. , Lustig Ernest , Hansen Elizabeth A.
Published: February 2004

The 6.14-MHz fluorine NMR spectra were employed in removing ambiguities in the signs and assignments of coupling constants in C6F5–Cl, –Br, –CN, –COOH, –COCl, –OH, –NH2, and –SO2Cl. Except for C6H5OH, selection of the correct parameter set entailed only a visual comparison of the low-field spectrum and computed spectra based on permutations of the nonassignable parameters from high-field studies. The correct parameter sets were subsequently computer refined to obtain best fits to the 6.14-MHz spectra. However, in the case of C6F5OH, the solvent dependence of the spectrum necessitated computer iterative analysis of the 6.14-MHz data to arrive at the correct parameter set. For C6F5COCl, high-precision 94.1-MHz spectra were of use in eliminating all but one uncertainty in the assignments. The Abraham–MacDonald–Pepper linear relationship involving the ortho–ortho substituent contribution and the Hammett σp constant was modified and extended to yield for the compounds C6F5–X the relationship J26(Hz) = − 9.5σp(X) − 2.3, from which several J26's and one σp were derived.
Configuration Interaction in the Simple Nonionic Valence-Bond Wavefunction for the H2I2 Reaction Complex
Authors: Raff L. M. , Porter R. N.
Published: February 2004

The symmetry-adapted MO configurations contained in the simple nonionic VB wavefunction for the H2I2 system in the H2+I2 and 2HI diatomic limits and in the C2v complex at the semiempirically determined saddle point are identified and their relative weights obtained as a function of the iodine orbital parameters. Implications of the results with regard to attempts to estimate the potential barrier height at the saddle point and to the mechanism of the hydrogen–iodine exchange reaction are discussed.
Electron-Impact Excitation Cross Sections of the Lowest-Lying Triplet States of Benzene
Authors: Matsuzawa Michio
Published: February 2004

Calculations of electron-impact excitation cross sections of the lowest-lying triplet states B1u3, E1u3, B2u3 of benzene are carried out, using the Ochkur and Ochkur–Rudge approximations. Electronic wavefunctions are constructed from the LCAO molecular orbitals for π electrons in benzene, and nuclear motions are disregarded. It is found that the differential cross section has a peak at a large scattering angle in the low-energy region. The angular distribution for the excitation of the B1u3, E1u3 states shows a broad subsidiary peak at a larger scattering angle. As the incident energy is lowered, the position of the peak in the angular distribution shifts towards larger scattering angles, and the subsidiary peak tends to vanish. The total excitation cross section for the lowest-lying triplet states is about πa02 at low incident energy. Results of calculations are discussed and compared with experimental findings.
Electron Spin Resonance and Structure of the BF2 Radical
Authors: Nelson William , Gordy Walter
Published: February 2004

The BF2 radical has been detected and its structure determined from observation of the electron spin resonance of >γ-irradiated molecules of BF3 trapped in the xenon matrix at 4.2°K. Hyperfine structure of both B and F was resolved and analyzed. The isotropic coupling constants are: for 10B, 277.98 Mc/sec; for 11B, 826.24 Mc/sec; and for 19F, 532 Mc/sec. Analysis of the hyperfine structure shows that the unpaired electron is in the σ system, that 93% of the electron spin density is in a nonbonding sp2 hybrid on the B, and that the remaining 7% is in hybridized bonding orbitals of the two F atoms. The sp2 hybrid of the unpaired electron on the B has 44% 2s character and 56% 2p character. This orbital is in the molecular plane. The bond angle derived from the hybridization is 112°.
Class of Potentials for the Description of the Interaction between Two Atoms
Authors: Kawatra M. P. , Lebedeff S. A.
Published: February 2004

Keeping in view the difficulty of a useful single convenient mathematical form of the interatomic potentials, we, in the present paper, have proposed a combination of potential forms to represent the potential between two atoms. These potential forms have the great advantage that, first, they have a fairly small number of parameters and, second, they are very convenient for further analysis. The potential forms are such that the corresponding Schrödinger equations can be solved exactly except in one case. The convenient mathematical form of the potential would enable us to interpret and analyze the energy and the angular momentum dependence of the scattering data with great ease. We have also attempted with a good degree of success to fit these potentials in the case of H2, H2+, and CO and have also studied the possible relationship between the various potential parameters and the spectroscopic constants.
Lower Bounds for Expectation Values
Authors: Wang P. S. C.
Published: February 2004

The low-bound formula for expectation values proposed recently by the author is applied to various powers of r1 and r12 in the normal helium atom, and the results are compared with those obtained previously by Jennings and Wilson and by Weinhold for the same problem.
Noniterative Solutions of Integral Equations for Scattering. II. Coupled Channels
Authors: Sams W. Neal , Kouri Donald J.
Published: February 2004

The recently developed method of homogeneous integral solutions for solving integral equations of scattering is extended to include coupled channels. The formalism is developed in matrix notation and applies without change both to purely local interactions and to combined nonlocal and local interactions. A numerical quadrature method for solving the integral equations is presented. This method does not require iteration or matrix inversions. Further, during the entire calculation of the wavefunction and T (or R) matrix elements, only one or two matrix inversions need be performed (in the case of purely local or local+nonlocal interactions, respectively). It is expected that the method will provide a particularly efficient means of solving coupled equations for scattering problems.
Surface Thermodynamic Functions for Noble-Gas Crystals
Authors: Allen R. E. , de Wette F. W.
Published: February 2004

Surface thermodynamic quantities have been calculated as functions of temperature between 0°K and the melting temperature for the (111), (100), and (110) surfaces of the noble-gas solids Ne, Ar, Kr, and Xe. The method of calculation differs from the methods used in previous treatments of surface thermodynamic functions in that the atomic vibrations are taken into account, and the vibrational frequencies are properly calculated rather than obtained from a Debye model or an Einstein model. The quantities calculated are the static surface energy, vibrational surface energy, surface entropy, vibrational surface free energy, and surface specific heat. In addition, a surface frequency-distribution function f8(ω) has been calculated; f8(ω) is positive at low frequencies, because of the presence of surface modes of vibration, and negative at higher frequencies. This behavior of f8(ω) produces a narrow peak in the graph of the surface specific heat as a function of temperature.
Evaluation of Transport Properties of Gases: The Bruch–McGee Potential for Helium
Authors: Keller J. M. , Taylor W. L.
Published: February 2004

The transport properties and second virial coefficient of helium have been calculated for two hybrid intermolecular potentials recently proposed by Bruch and McGee, and the results are compared with experimental values. Tables of transport collision integrals for the helium isotopes which are accurate to several parts per thousand were prepared for the temperature range 0.8–2500°K in order to adequately span the existing data. The collision integrals were computed by an exact quantum phase-shift calculation at low temperatures followed by a JWKB approximation and classical calculation at intermediate and high temperatures, respectively. The bulk properties were then evaluated using the prepared tables. The agreement with experimental measurements for the various properties at low temperatures is rather poor when compared with the results calculated using the Lennard-Jones (12–6) potential. We conclude that this is due to excessive depth of the potential well. On the other hand, the calculations above approximately 150°K agree quite well with experimental data which leads us to the conclusion that the low-energy repulsive region of either hybrid potential is a good representation of the helium–helium interaction. Since the potentials are essentially identical in this region and independent of the long-range attractive terms, it is suggested that for practical applications the Morse-VDD potential be truncated to the much simpler Morse potential with the given parameters and its region of validity restricted to temperatures above 150°K.
Heat Capacity and Other Thermodynamic Functions of Xenon Hexafluoride from 5 to 350°K
Authors: Schreiner Felix , Osborne Darrell W. , Malm John G. , McDonald Geraldine N.
Published: February 2004

Heat-capacity measurements have been made on a 53-g sample of XeF6 between 5 and 350°K. Melting-point measurements indicated 0.16 mole % liquid-soluble, solid-insoluble impurity. Two regions of anomalously high heat capacity and long equilibrium times were observed, near 253.8°K and near 291.8°K, which are believed to arise from structural changes in solid XeF6. The excess enthalpies associated with these regions are 972 and 1226 J mole−1, respectively. The melting point is 322.63 ± 0.10°K, and the enthalpy of fusion is 5743 J mole−1. The vapor pressure of the liquid is given by log10PmmHg = − 6170.88 / T − 23.67815 log10T + 80.77778 from 323 to 350°K. The second virial coefficient was found to be − 955 cm3 mole−1 at 346°K. Densities of the solid were determined at several temperatures by helium displacement. The high entropy of vaporization (136.9 J°K−1·mole−1) at the boiling point (348.72°K), as well as the high and rapidly rising heat capacity of the liquid, suggest that the liquid is associated. Thermodynamic functions of the condensed phases are tabulated at various temperatures. At 298.15°K the values of CP°, S°, H° − H0°, and (G° − H0°) / T for solid XeF6 are 171.59 J°K−1·mole−1, 210.38 J°K−1·mole−1, 30 999 J mole−1, and − 106.41 J°K−1·mole−1, respectively. At 355°K the standard entropy S° is 255.5 J°K−1·mole−1 for liquid XeF6 and 402.6 J°K−1·mole−1 for the gas.
Structural Influences on Adsorbate Binding Energy. I. Carbon Monoxide on (100) Palladium
Authors: Tracy J. C. , Palmberg P. W.
Published: February 2004

The structural arrangement and binding energy of CO adsorbed on a (100) surface of palladium have been determined and related to each other over the submonolayer coverage range. There are four distinct equilibrium structural arrangements of CO: (1) At low coverages (< 0.4 monolayers) a random lattice gas configuration is observed in which each CO molecule is localized on a specific adsorption site. (2) Just below 0.5 monolayers a liquidlike short-range order arrangement is found. (3) At exactly 0.5 monolayers a c(4 × 2) − 45° overlayer structure forms which is in complete registry with the substrate. (4) For coverages greater than 0.5 monolayers, the c(4 × 2) − 45° structure is uniaxially compressed to form an overlayer which is out of registry with the palladium substrate. The observed linear decrease in binding energy with coverage up to 0.5 monolayers suggests that the only important CO–CO interaction occurs between nearest neighbors. Near 0.5 monolayers an almost discontinuous decrease in the binding energy of 0.35 eV is attributed to loss of registry between the overlayer and substrate. As the coverage increased above 0.6 monolayers the binding energy again decreased sharply as a consequence of neighboring CO orbital overlap.
Nuclear Magnetic Resonance Studies of Solutions of the Rare-Earth Ions and Their Complexes. III. Oxygen-17 and Proton Shifts in Aqueous Solutions and the Nature of Aquo and Mixed Complexes
Authors: Reuben Jacques , Fiat Daniel
Published: February 2004

A nuclear magnetic resonance survey has been undertaken in order to gain a better insight into the nature of the complex species present in aqueous solutions of the rare-earth ions. Oxygen-17 and proton chemical shifts of water are reported and the effects of concentration and added anions upon them discussed. Nuclear magnetic resonance evidence is presented, showing that the shifts observed in solutions of the perchlorate salts are due to a single hydrated species persisting over a wide range of concentrations and that no “inner-sphere” complex is formed with perchlorate. It appears, also, that all the water molecules in the first solvation shell are equivalent with respect to their contribution to the shift, owing to their relatively short residence time. The sign, magnitude, and temperature dependence of the oxygen-17 shifts as contrasted to those of protons support the suggestion that the former are mainly due to contact interaction, whereas the latter are dominated by dipolar interaction with the unpaired electrons of the rare-earth ions. Examination of the chemical shifts of nuclei of the anions unequivocally demonstrates that “inner-sphere” complexes are formed with nitrate and acetate. It is suggested that both nitrate and acetate act as bidentate ligands. A marked decrease of the number of water molecules contributing to the observed oxygen-17 shift is noted in presence of acetate, nitrate, and sulfate. Chloride also gives rise to some dehydration of the rare-earth ion, although in this case the mechanism of the process is as yet unclear. An approach to the estimation of equilibrium quotients for complex formation using water chemical shift data is outlined in an Appendix. Equilibrium quotients are estimated for the formation of nitrato–dysprosium complexes. A comparison of water and other ligand proton shifts of the acetate and tropolonate complexes along the lanthanide series suggests that in both complexes there is a similar ligand arrangement around a given cation and that a change of configuration occurs at erbium.
Crystal Structure of Pyroelectric Paramagnetic Barium Manganese Fluoride, BaMnF4
Authors: Keve E. T. , Abrahams S. C. , Bernstein J. L.
Published: February 2004

BaMnF4, pyroelectric at room temperature and paramagnetic above 30°K, crystallizes in the orthorhombic system with space group A21am and lattice constants a = 5.9845 ± 0.0003, b = 15.098 ± 0.002, and c = 4.2216 ± 0.0003 Å at 298°K. The integrated intensities of 5085 reflections within a reciprocal lattice hemisphere of radius (sinθ) / λ = 1.02 Å−1 were measured using pexrad. The crystal structure has been solved and refined by a combination of Patterson and Fourier series and the method of least squares, using 455 independent and significantly nonzero F(meas). The final agreement factor R, for atoms with anisotropic temperature coefficients, is 0.0466. The structure consists of MnF6 octahedra sharing corners to form puckered sheets parallel to (010). These sheets are linked by Ba2+ ions only, giving rise to excellent (010) cleavage. Within the sheets there are nearly linear –Mn–F–Mn–F– chains parallel to c. Parallel to a there is a second set of planar zigzag –Mn–F–Mn–F– chains, formed by adjacent shared fluorine atoms with Mn–F–Mn angles of 138.6°. The F–Mn–F angles in this chain are 98.3°. The remaining two fluorine atoms in each octahedron are adjacent. These atoms, on the outside of the sheets, are bonded to one Mn atom only. The octahedra are distorted with Mn–F distances ranging from 2.037 to 2.151 Å, with an average of 2.098 Å. Each barium ion is surrounded by a distorted trigonal prism of fluorine atoms, with two more located in the equatorial plane through rectangular prism faces, all Ba–F distances being less than 2.88 Å. A model is presented to account for the observed ferroelectricity in BaZnF4 and the absence of ferroelectricity in isomorphous BaMnF4, based on the separation of the Zn or Mn atoms in the puckered sheets. The experimental absolute polarization sense is in agreement with that calculated. The theoretical polarization for BaMnF4 of 11.5 ± 1.5 μC cm−2 may be compared with the average value of 8.0 μC cm−2 for the ferro-electric members of the BaMF4 series.
Attachment of Slow Electrons to Oxygen Molecules
Authors: Herzenberg A.
Published: February 2004

The attachment of electrons to O2 molecules to form stable O2− ions is discussed by supposing that the electron is first captured to form an unstable negative molecular ion in a compound state, which is later stabilized in a collision with another molecule. The rate of initial capture is described by a Breit–Wigner formula and the rate of formation of stable O2− ions obtained by multiplying by the probability that a stabilizing collision should occur before the ion re-emits its extra electron. The lifetime of the compound state, which would be difficult to calculate reliably, cancels out of the resulting expression. If one supposes all collisions of unstable ions to be stabilizing, one obtains an upper bound of the order of 10−30 cm6 sec−1 for the rate constant for the formation of stable O2−. The experimental value is of this order.
Effect of Oxygen in the Surface-Catalyzed Excitation of Nitrogen
Authors: Weinreb Michael P. , Mannella Gene G.
Published: February 2004

The emission of the nitrogen first positive bands which follows the recombination of nitrogen atoms at certain surfaces (surface-catalyzed excitation) requires the presence of a small amount of electrically discharged oxygen. We find that the oxygen interacts in the gas phase to produce the emitting N2(B 3Πg) from the vibrationally excited N2(A 3Σu+) which had been formed by N-atom recombination at the surface. This interaction is very rapid, with a rate coefficient estimated at 5 × 10−12 cm3 sec−1. The species of oxygen which is involved is most likely an excited molecule. These results imply that N2(A 3Σu+) can be formed in high concentrations by the surface-catalyzed excitation of nitrogen, provided that care is taken to insure an oxygen-free system.
Ab Initio Calculation of Harmonic Force Constants. II. Application to Gaussian Wavefunctions for H2+
Authors: Bishop David M. , Macias A.
Published: February 2004

The method of Bishop and Randič for evaluating force constants by direct differentiation and using only the ground-state wavefunction for the molecule in its equilibrium configuration is extended so as to handle wavefunctions built from a nonorthogonal basis set. The results are applied to a series of Gaussian wavefunctions for H2+.
Theoretical Investigation of the Transition Probabilities in the Hydrogen Molecule
Authors: Wolniewicz L.
Published: February 2004

The electronic transition moments for the B–X, C–X, and E, F–B transitions in the hydrogen molecule are given for several internuclear separations, R≤12 a.u. The electronic transition moments are used for the computation of the transition moments for individual vibrational and rotational lines. The results show that, except for the Werner bands, the relative intensities cannot be accurately represented by the Franck–Condon factors. It is not possible to make a definite quantitative comparison of the present results with experiment due to a significant R dependence of the electronic moments. Qualitatively, the results agree well with experiment. However, they do not explain the fact that the lowest vibrational levels in the F state have not yet been observed.
Quantum Vibrational Transition Probabilities in Atom–Diatomic Molecule Collisions. III. Reactive Scattering
Authors: Wilson David J.
Published: February 2004

The methods of Diestler and McKoy and Cheung and Wilson are combined to yield a procedure for calculating vibrational transition probabilities for inelastic reactive and nonreactive collisions. The model treated is that of a collinear collision between an atom and a diatomic molecule. The procedure should result in marked reductions in the computer time and core requirements for such calculations. An alternative method based solely on the approach of Cheung and Wilson is also developed; this method requires that the potential-energy surface in the vicinity of the transition state be approximated by a quadratic form and should permit extremely rapid computation of reactive-scattering transition probabilities. A second alternative method based on the use of the Born approximation in the vicinity of the transition state is also discussed.
Crystal Field Effects on Triplet States of Molecular Solids: The Electronic States of Quinoxaline
Authors: Clarke Richard H. , Hochstrasser Robin M. , Marzzacco Charles J.
Published: February 2004

A description of the effects of the crystalline environment on the paramagnetic excited states of a guest molecule substitutionally dissolved in a molecular solid is presented in the deep-trap limit. Perturbations by host electronic states on the individual spin substates of the guest are explored and shown to make contributions to detailed experimental studies of the triplet state in all mixed crystal systems to a degree dependent on both the geometry and electronic structure of the host crystal. The effect of the crystal field on such experimental measurements as the polarization of singlet–triplet transitions, zero field splittings, and the Zeeman effect in mixed crystal systems is considered. Finally, the results of this study are applied to the reported anomalous polarization measurements of the phosphorescence of quinoxaline in durene. Within the framework of our description of the crystal field, it is shown that there are no special intermolecular effects in the durene mixed crystal, but that the differences in polarization properties of quinoxaline in durene and in naphthalene (in which quinoxaline exhibits a behavior quite close to that predicted from the oriented gas ratios) reduce simply to differences in crystal geometry.
Ion–Molecule Reactions, He++O2 and He++N2, at Thermal Energies and Above
Authors: Heimerl J. , Johnsen R. , Biondi Manfred A.
Published: February 2004

The rate coefficients for the reactions of He+ ions with N2 and O2 have been measured in a drift tube–mass spectrometer from thermal energies (300°K) to ∼ 0.1 eV. In addition to reaction rate determinations by the usual product-ion “arrival spectrum” method, a new technique involving “additional residence time” for the parent ion is described which is applicable even when parent and product ions have equal mobilities. The total rate coefficient (i.e., the sum of dissociative and the ordinary charge transfer) for He++N2 reaction was found to be (1.0 + 0.3, − 0.2) × 10−9 cm3/sec at 300°K, decreasing to ∼ 7 × 10−10 cm3/sec at 0.1 eV. The corresponding coefficient for the He++O2 reaction was measured to be (8.5 + 2.5, − 2.0) × 10−10 cm3/sec and to be independent of energy over the same range. The ratio of dissociative charge transfer (with N+ or O+, respectively, as reaction products) to ordinary charge transfer (with N2+ or O2+, respectively, as products) was found to be 0.55 / 0.45 for He++N2 and 0.8 / 0.2 for He++O2.