bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of Exchange Coupling on the Spectra of Transition Metal Ions. The Ligand Field Spectrum and Crystal Structure of CsCrCl3
Authors: McPherson G. L. , Kistenmacher T. J. , Folkers J. B. , Stucky G. D.
Published: February 2004

The single crystal polarized absorption spectra of CsCrCl3 and of CsCrCl3 doped into CsMgCl3 have been measured and interpreted in terms of an octahedral ligand field. The intensities of the spin forbidden transitions are found to be very dependent on the chromium ion concentration. In addition, an absorption band which is strongly polarized along the crystallographic c axis is observed at approximately twice the energy of the 10 Dq band. Both of these effects are attributed to magnetic coupling between the chromium ions; the anomalous band is assigned to a ``double excitation'' of the E→55T2 transition on coupled pairs of ions. The crystal structure of CsCrCl3 has also been determined. The lattice is hexagonal with P63mc the most probable space group. The lattice constants are a=7.256(3) and c=6.224(3) Å, with ρobs=3.24(4) g/cm3 and ρcalc=3.40 g/cm3. Least-squares refinement, based on F, of counter data has led to a final R index of 0.042. The main structural feature is a linear array of [CrCl6]−4 octahedra sharing faces. The structure is similar, but not identical, to that of CsMgCl3 and CsNiCl3. A small static Jahn-Teller distortion of the chloride ions from trigonal symmetry about the Cr(II) ions or a Jahn-Teller active E1g vibrational mode is probably responsible for the structural differences.
Effects of Interionic Coupling on the Electronic Spectra of Transition Metal Ions. The Ligand Field Spectra of CsNiCl3 and CsNiBr3
Authors: McPherson G. L. , Stucky G. D.
Published: February 2004

The polarized crystal spectra of CsNiCl3, CsNiBr3, and nickel(II) doped into the isostructural magnesium salts, CsMgCl3 and CsMgBr3, have been investigated in the near infrared and visible region. The spectra can be satisfactorily interpreted using the Liehr and Ballhausen ligand field model for a d8 system in an octahedral environment. The intensities of the allowed, triplet→ triplet, transitions are essentially independent of nickel ion concentration and show the temperature dependence expected of a vibronic mechanism. On the other hand, the oscillator strengths of certain spin forbidden, triplet→ singlet, bands show a strong concentration dependence. These transitions are more intense in the spectra of the pure salts suggesting that their intensities are increased by exchange interactions between nickel ions. The intensity behavior of the multiplicity forbidden transitions in the nickel compounds is similar to that observed for a number of high spin manganese (II) salts.
Energy-Shift Theory of Low-Lying Excited Electronic States of Molecules
Authors: Simons Jack
Published: February 2004

The present research is directed toward understanding the low-lying excited electronic states of atoms and molecules. Such excited states play essential roles in atmospheric photochemistry, charge transfer processes, and chemical reaction dynamics. The excitation energies and oscillator strengths between these states and the ground state are the principal concerns of this investigation. A new theory, referred to as the energy-shift theory, is put forth as a means of directly calculating molecular electronic excitation spectra. Within this theory, four computationally tractable approximations are developed, two of which allow the use of experimental data on higher states to aid the calculations. The method is shown to have some advantages over other direct calculation schemes currently being used in this and other laboratories, the primary advantage being the separation of low- and high-energy excitations which arises naturally in the energy-shift formalism. The relation of this approach to Green's function theory is discussed.
Reconsideration of an Experiment on Water under Negative Pressure
Authors: Macdonald J. Ross
Published: February 2004

The experiment of Winnick and Cho on centrifuged water under inhomogeneous positive and negative pressure is re-examined. Several deficiencies in the data and analysis method are identified and corrected. A new, more complete theory of the experiment is derived and used to analyze corrected 20°C centrifuge data. From analysis of ordinary positive-pressure PV water data, it is established that best 20°C values of K0, the bulk modulus at the vapor pressure of water, and K0′, the bulk modulus pressure derivative, are about 21 800 bar and 5.2, respectively. Least squares analysis of the Winnick-Cho data, on the other hand, shows that no meaningful estimate of K0′ can be obtained and that the K̂0 values derived are from about 11% to about 18% higher than the above expected value, depending on the detailed analysis approach. These unexpectedly large values of K̂0 appear to be highly statistically significant. It is concluded that they most probably arise from remaining systematic errors in the data, rather than from a still inadequate fitting equation or from significant differences in water properties on going from the positive pressure region to the low negative pressure region between zero and about −75 bar covered by the Winnick-Cho data.
Ozone Ultraviolet Photolysis. V. Energy Distribution in the O(1D)+O3 Reaction
Authors: Webster Henry , Bair Edward J.
Published: February 2004

The experiments described in this paper concern the distribution of vibrationally excited O2† following flash photolysis of ozone under conditions which permit an improved extrapolation to find the distribution in which they are initially formed. The observed states v″=14–21 are found to have an initial distribution which can reasonably be described by a vibrational temperature, Tvib=23 400± 1260°K. The origin of the distribution and the proportionation of product between 2O2† and 2O(3P)+O2 is discussed in terms of a statistical model. This interpretation accounts for the observed distribution of O2i(i=14–21), it accounts for the predominance of 2O(3P)+O2 product in gas phase photolysis, and for the predominance of 2O2 products in photolysis of liquid Ar solutions. An inert gas pressure dependence of the reaction product distribution is predicted and observed qualitatively.
Microwave Spectrum, Structure, and Dipole Moment of Epoxybutyne
Authors: Collins Michael J. , Boggs James E.
Published: February 2004

The microwave spectra of epoxybutyne and one deuterated species have been analyzed. A reasonable set of assumptions indicates that the C–C single bond attaching the acetylenic group to the ring has a length of approximately 1.48 Å, appreciably longer than the 1.458 Å reported in methylacetylene. The dipole moment is 1.80± 0.02 D, and analysis of its components along the molecular axes indicates little effect of interaction between the acetylenic group and the ring.
Magnetic Circular Dichroism of Coronene Mononegative and Dinegative Ions
Authors: Zandstra P. J. , Scholtens D. J. , Koning R. E.
Published: February 2004

The magnetic circular dichroism (MCD) of the mononegative and dinegative ions of coronene has been measured. The experimental values of A/D and C/D were compared with the values obtained from MO calculations. For the average z component of the electronic orbital angular momentum of the ground state mononegative ion an experimental value of 1.3 ℏ has been estimated. This value should be compared with the theoretical value of 1.7 ℏ, which depends to some extent on the choice of the parameter values used for the calculation. This is the first case in which an electronic orbital angular momentum for a hydro-carbon in its ground state has been found. The degenerate excited states of both ions have electronic orbital angular momenta of the same magnitude. With the exception of the E1g2→2E2u transitions in the mononegative ion, the agreement between experimental and theoretical MCD values in generally very satisfactory. For the E1g2→2E2u transitions the experimental A/D is greater and the C/D smaller than the calculated values. When a set of orthogonalized atomic orbitals is used for the MO calculations or with a free electron model some improvement results. No indication of a Jahn-Teller effect is present.
Dimers, Clusters, and Condensation in Free Jets. II
Authors: Golomb D. , Good R. E. , Bailey A. B. , Busby M. R. , Dawbarn R.
Published: February 2004

Supersonic jets of Ar, Kr, N2, O2, NO, CO, SO2, and CO2 formed by expansion from sonic orifices and conical nozzles into a vacuum were studied with a mass spectrometer, ionization gauge, and free-stream velocity analyzer. As the source pressure was raised, the dimer, trimer, etc., signals increased until they attained maxima and then declined. Following the sharp rise of the clusters, the monomer velocity increased, indicating that some of the heat of condensation is transferred into translational energy of the remaining monomers. For source conditions where the dimers, trimers, etc., peak, it is postulated that the rate of growth of supercritical nuclei increases at the expense of subcritical clusters, with the consequence of a relative decline in the smaller cluster concentrations. The source pressure, at which the monomer velocity increases and the dimers peak, shifts with d−q (d = orifice diameter) and T0r (T0=source temperature), where q≃ 0.55 for most gases and r=2.2± 0.2 for argon and 2.7± 0.2 for nitrogen. The scaling law implies termolecular kinetics for dimer formation which may be the rate-determining process for homogeneous condensation in free jets.
Influence of the Host Lattice upon EPR Coupling Parameters and d-d Transitions of Planar Copper (II) Complexes
Authors: Ammeter J. , Rist G. , Günthard Hs. H.
Published: February 2004

The EPR and optical spectra of two similar square planar CuO2N2 chelates, copper hydroxyquinolinate (CuOx2) and copper picolinate (CuPic2) have been investigated. By studying both molecules in a series of several paramagnetic and diamagnetic single crystal matrices, powders and solutions, the dependence of the g tensor, metal hyperfine (hfs) tensor, nitrogen hfs tensor and the optical d-d transitions on the host lattice was determined. The strong interactions of polar matrix groups with the substituted metal complex cause the major change in the magnetic parameters and the d-d optical transitions of these centrosymmetric planar d9 complexes. In both series of host metal complexes, the copper ligands changed stepwise from a nearly octahedral arrangement (strong axial coordination) to nearly square planar arrangement (negligible axial coordination). This change is accompanied by a total decrease in the Δg parameters of 30%–35%, a total increase in the ligand spin density of 15%, and a total increase in the d-d transition frequencies of 25%. The observed increase in covalency of the singly occupied orbital with increasing axial distortion of the ligand octahedron is in qualitative agreement with results of semiempirical MO calculations. The orthorhombic distortion of the g and A tensors shows significant coorelation with the Fermi contact parameter κ for the copper nucleus. A 2p/2s hybridization ratio of 5.8 for the nitrogen lone pair component of the singly occupied σ orbital has been found for CuOx2, as well as for CuPic2. The nitrogen hfs principal axis (Bx) coincides with one of the in plane g axes (gx) for CuPic2, while the gx and Bx axes differ by 12°–17° in the case of CuOx2. The nitrogen spin density is significantly smaller for CuOx2 than for CuPic2 at equal g shifts, indicating a stronger interaction of the phenolic oxygen in CuOx2 relative to the carboxylate oxygen in CuPic2 with the metal ion. These experimental results show the shortcomings of the simple LCAO model generally used to describe the electronic structure of metal complexes. The use of free ion (or atom) wavefunctions for example leads to large overestimations of the π delocalization parameters and of the amount of g dependence on covalency. In order to obtain agreement with experiment, a scaling parameter (as proposed by K. Ruedenberg in 1962) for the metal 3d part of the involved MO's should be introduced to account for the virtual expansion of the atomic orbitals in antibonding orbitals. Variations of the effective charge appear to be negligible. This expansion results in a smaller spin-orbit constant and a smaller 3d parameter. This effect contributes substantially to the generally observed reduced spin-orbit and metal hfs interactions in covalent transition metal complexes.
Magnetic Properties of Rare Earth Tetraborides
Authors: Buschow K. H. J. , Creyghton J. H. N.
Published: February 2004

The magnetic properties and the lattice constants of the rare earth tetraborides RB4 have been determined. The compound PrB4 is ferromagnetic. The compounds in which R represents Nd, Sm, Gd, Tb, Dy, and Ho are antiferromagnetic while ErB4 and YbB4 show metamagnetic behavior. The element Ce and to some extent also the element Yb show an abnormal valency in the tetraborides. The magnetic properties of the RB4 compounds are discussed in terms of the RKKY formalism. It is shown that the ferromagnetic behavior of PrB4 is probably the result of a strong participating of the orbital angular momentum to the indirect exchange coupling between the localized moments.
Calculation of the Isotope Separation Factor Between Argon Liquid and Vapor
Authors: Mandel Frederic
Published: February 2004

The Weeks-Chandler-Anderson perturbation theory and the Percus-Yevick theory of fluids are used to calculate 〈 ∇2U〉, the mean Laplacian of the intermolecular potential, from the Lennard-Jones 6–12 potential for liquid and gaseous argon, respectively. The 〈 ∇2U〉 's are then used to calculate the isotope separation factor, which is compared with experiment. The accuracy of the PY equation is estimated by a comparison of a calculation of the compressibility of low density gaseous argon with the experimental data of Levelt. The validity of the WCA approximation is established by the comparison of calculations of 〈 ∇2U〉 with those from molecular dynamics for the Lennard-Jones 6–12 potential.
Microdynamics of Methane Adsorbed on Magnesium Oxide
Authors: Anderson P. R. , Jones E. P.
Published: February 2004

The nuclear magnetic relaxation times of protons in methane and some of its deuterated modifications adsorbed on magnesium oxide have been measured as a function of temperature between 5 and 80°K. Coverages were 0.5 and 1.0 times a statistical monolayer. Two spin-lattice relaxation rate maxima are observed, a low temperature maximum due to anisotropic reorientation of the adsorbed methane on the magnesium oxide surface and a high temperature spin-lattice relaxation rate maximum due to both dipolar interactions between protons in neighboring methane molecules and between the protons in the adsorbed methane molecules and intrinsic surface paramagnetic centers. Correlation times and activation energies associated with both molecular reorientation and translational diffusion were measured, and an estimate of the magnitude of the electric field at the adsorption sites on the surface of MgO was obtained.
Absorption Spectrum of the 2000 Å System of Borazine in the Gas Phase
Authors: Bernstein E. R. , Reilly J. P.
Published: February 2004

The optical absorption spectrum of borazine vapor, at pressures between 0.1 and 5 mm Hg, has been obtained in the wavelength region 2015 Å–1800 Å on a 2.0 m vacuum spectrograph both photoelectrically and photographically. On the basis of vibronic analysis, observed absorption features are consistent with a single π-π*, dipole-forbidden, electronic transition, 1A1′←1A1′, assuming the D3h point group is a valid description for both ground and excited electronic states of borazine. Absorption and emission to the red of this state have been searched for but neither have been observed under current sensitivity conditions. It is concluded that the excited 1A1′ state is most likely the lowest singlet excited state of borazine. This spectrum is discussed in comparison with that of benzene; the 1A1′(D3h) state of borazine corresponds to the 1B1u(D6h) state of benzene. In particular, the 3B1u←1A1g spectrum is compared to the 1A1′←1A1′ absorption. Arguments are presented suggesting the Jahn-Teller nature of the 1A1′ excited state of borazine. In this framework, various possible excited state vibrational assignments are presented.
Proton Distribution in Ice and the Kirkwood Correlation Factor
Authors: Rahman Aneesur , Stillinger Frank H.
Published: February 2004

4096 molecules of water forming an ice Ic structure and 2048 molecules forming an ice Ih structure, with standard periodic boundary conditions in each case, have been used to calculate the dipole correlations which arise from proton allocation to the oxygen nuclei according to the Bernal-Fowler ice rules. The calculations reveal a substantial antiparallel correlation in the coordination shells at about twice the distance of the nearest neighbors in the lattice. The value of gK, the Kirkwood correlation factor, is found to be 2.07±0.02 for ice Ih. For ice Ic the value, determined with less precision, is 2.11±0.10. In connection with Kirkwood's dielectric formula the value of gK implies an effective dipole moment, μs, of the molecule in the solid ice Ih phase equal to 2.92 D.
Square well potential. IV. Use of a modification of the Kirkwood superposition approximation
Authors: Schrodt I. B. , Kozak John J. , Luks K. D.
Published: February 2004

The Yvon-Born-Green integral equation with the Kirkwood superposition approximation is modified via a truncation of dense fluid contribution to third-particle correlations at the boundary of the intermolecular potential wells of the interacting molecular pair. This de-emphasis of third-body contributions, manifesting itself in a ``truncated superposition approximation,'' improves a broad spectrum of equilibrium and transport property predictions significantly while simultaneously shortening the computational extent of the integral equation problem. The results are compared with earlier work performed by the authors using the Kirkwood superposition approximation and with experiment. The nature of both the Kirkwood and truncated superposition approximations is discussed in light of recent schemes used to shorten the computations required to obtain solutions to the Percus-Yevick integral equation.
Vibrational de-excitation of HF(v = 1) in HF + Ar: Importance of rotational transitions
Authors: Shin H. K. , Young A. W.
Published: February 2004

The importance of rotational transitions in the vibrational de-excitation of HF(1 → 0) in HF + Ar collisions has been investigated by a semiclassical three-dimensional approach. Because of the inclusion of rotational transitions, this study gives vibrational relaxation times which are in reasonable agreement with experimental data and are very large compared to results of conventional vibration-to-translation energy transfer theories.
Thermodynamic model for tricritical points in ternary and quaternary fluid mixtures
Authors: Griffiths Robert B.
Published: February 2004

The properties of a phenomenological model of the Landau type (free energy expanded in powers of an order parameter) are worked out with special reference to published experimental data on tricritical points in three- and four-component fluid mixtures. When properly interpreted, the model is in good qualitative agreement with experiment. Suggestions are made for possible experimental measurements of the tricritical exponents. The model is also used to discuss the relation between these tricritical points (which are characterized by three phases becoming identical simultaneously) and other ``higher-order'' critical points, in particular the tricritical points observed in metamagnets, He3–He4 mixtures, etc.
Microwave spectrum of the HCO radical
Authors: Austin Jared A. , Levy Donald H. , Gottlieb C. A. , Radford H. E.
Published: February 2004

The microwave spectrum of the K -doublet transitions of the free radical HCO has been observed in the N = 1, 2, and 3 rotational levels. The spectrum was analyzed and the following spectroscopic parameters were determined from the data: B − C = 2836.38 ± 0.45 MHz, ε = 224.62 ± 0.65 MHz, |εz z| = 8.12 ± 0.48 GHz, τ = 5.06 ± 0.13 MHz, D = 0.049 ± 0.019 MHz. The newly measured rotational constant was used along with previously measured constants to determine that the geometry of the HCO radical is rCH = 1.11 Å, rCO = 1.17 Å, θHCO = 127°.
Physical cluster free energy from liquid-state perturbation theory
Authors: Abraham Farid F. , Lee Jong K. , Barker J. A.
Published: February 2004

Toxvaerd's extension of the Barker-Henderson perturbation theory to a nonuniform fluid is used to calculate the total free energy of clusters of Lennard-Jones 12–6 atoms. The theory predicts a minimum free energy density profile for two chosen cluster sizes, 43-atoms and 87-atoms, and the free energy values and profiles agree very well with recent Monte Carlo experiments for such clusters.
Formation of electronically excited CO2+ in collisions of H+, H, He+, and Ne+ with CO2
Authors: Haugh Michael J. , Birely John H.
Published: February 2004

Electronic excitation in collisions of 200 eV–25 keV H+, H, He+ and Ne+ with CO2 has been investigated under single-collision conditions in an atomic beam experiment. For each of these projectiles, the B 2Σu+ and A2πu states of CO2+ are the dominant electronically excited species emitting in the 200–500 nm wavelength range owing to target excitation. Although dissociative electron capture by He+ and Ne+ to form electronically excited CO+ minimizes the energy defect, no emission is observed from CO+ A2π, and formation of CO+B2Σ+ is of only minor importance. The latter observations are in direct contradiction to predictions derived from the adiabatic criterion and demonstrate that, in the competition among allowed excitation processes, details of the electronic structure of the collision partners override energy considerations. The qualitative features of the emission spectrum excited by H+, H, and He+ are insensitive to relative velocity over the energy range covered. The ratio of the cross sections for excitation of CO2+ B−X and A−X emissions is about 0.20 for H+ and 0.40 for He+. In Ne+ impact, the CO2+ A state vibrational distribution changes significantly, the (B−X)/A−X) cross section ratio decreases from 0.46 to 0.07, and the relative importance of Ne and Ne+ emissions diminishes markedly as the energy is reduced from 4 keV to 200 eV. Our results are compared with those for electronic excitation produced by photoionization, electron impact, and Penning ionization of CO2.
Tunnel model of liquid diffusion
Authors: Zwanzig Robert , Bishop Marvin
Published: February 2004

A tunnel model of liquid diffusion, analogous to Barker's thermodynamic model, is constructed by separating particle motion into components parallel and perpendicular to a tunnel axis. By combining exact results for hard rod systems, one is able to predict both the tunnel diffusion coefficient and the corresponding velocity correlation function. The ratio of the tunnel diffusion coefficient to the Enskog value at the same density and temperature is obtained by matching the virial and the temperature. The value predicted lies outside the range investigated by Alder et al; therefore direct comparison to molecular dynamics results is not possible. However, the velocity correlation function has the negative region characteristic of dense fluids. It is suggested that the derivation of the tunnel model dynamics may be valid at close packed densities.
Optical potential for elastic scattering when rearrangements are present
Authors: Lin Jeong-long , Kouri Donald J.
Published: February 2004

The problem of constructing an optical potential for the elastic scattering when rearrangement processes can occur is considered in two stages. First an ``arrangement channel optical potential'' is constructed using a coupled channel T operator formalism describing a general scattering process. This complex potential is shown to have the properties necessary to describe nonreactive scattering in the presence of rearrangements. Second the usual elastic and inelastic nonreactive projectors P and Q are employed in obtaining an optical potential for just elastic scattering. An explicit expression for the channel optical potential is obtained which permits easy interpretation. The two-stage approach thus has the advantage of completely avoiding nonorthogonality difficulties.
Molecular Zeeman spectra of 6,7Li 79,81Br
Authors: Cecchi Joseph L. , Ramsey Norman F.
Published: February 2004

The molecular Zeeman spectra of the four isotopes of lithium bromide have been investigated using a molecular beam electric resonance apparatus augmented with a magnet in the resonance region. Values for the rotational magnetic moment, nuclear shielding anisotropies, diamagnetic susceptibilities, bromine nuclear magnetic moment, and quadrupole moment of the electron distribution for each isotope in the ν = 0 vibrational level and J = 1 rotation level were obtained from these data. For 7Li79Br, gJ = 0.112056(64), σBrT = − 0.000321(82), σLiT = − 0.0000480(42), ξT = 2.35(50) × 10−30 erg G−2 · molecule−1, μBr = 2.1049(8) nuclear magnetons, and 01 = 1.00(10) × 1015 cm2. Values of these parameters for 6Li79Br, 6Li81Br, and 7Li81Br are also reported. Unlike most previous experiments of this type, no static electric field is applied to the resonance region. The motional electric field which the molecules see as they travel through the magnetic field serves as the parity mixing electric field which is necessary for the transition process. The linear velocity dependence of this field tends to cancel the inverse velocity dependence in the interaction time. The theoretical line shape has been investigated by computer.
Localized orbitals and short-range molecular interactions. I. Theory
Authors: Magnasco V. , Musso G. F.
Published: February 2004

The separation of the electronic energy into Coulombic and penetration components [V. Magnasco, G. F. Musso, and R. McWeeny, J. Chem. Phys. 47, 4617 (1967)] in the case of a (closed-shell) single determinant wavefunction of nonorthogonal localized spin bond orbitals (BO) is studied on the basis of a convenient density matrix formulation. The main correction that results when overlap is admitted consists of two terms, the first describing the energy of the density ``perturbation'' in the field of the ``unperturbed'' one-electron operator F̂0, the second the Coulomb-exchange self-interaction of the density ``perturbation.'' When the groups interacting at short range are atoms or molecules, compact closed formulas for the interaction energies can be derived, and the connection with first-order perturbation theory is discussed. The present formalism is suitable for analyzing both nonadditivity of short-range intermolecular forces and the nature of barriers to internal rotation in molecules.
Localized orbitals and short-range molecular interactions. III. Rotational barriers in C2H6 and H2O2
Authors: Musso G. F. , Magnasco V.
Published: February 2004

Ab-initio calculations for eclipsed and staggered C2H6 and for several conformers of H2O2 are performed using a single determinant of doubly occupied nonorthogonal localized bond orbitals constructed from a minimal set of Slater AO's. The energies calculated with optimized BO's closely follow the pattern of the full SCF calculations. A component analysis of the rotation barriers is given along the lines suggested in Paper I [V. Magnasco and G. F. Musso, J. Chem. Phys. 60, 3744 (1974)]. The barrier in ethane (2.67 kcal) is primarily due to the penetration of nonorthogonal electron pairs belonging to vicinal C–H bonds into the core provided by nuclei and unperturbed electrons of all other groups. Although the minimal set is not suited to account for the small trans barrier in H2O2 both in BO and full SCF calculations, the calculated cis barrier in H2O2 (13.79 kcal) is seen to have a nature similar to that found in ethane, except for an enhanced importance of the Coulombic contribution. Delocalization effects are found to be of little importance in determining the conformational behavior of both molecules.
Electronic structure of VS
Authors: England W. B. , Liu S. H. , Myron H. W.
Published: February 2004

Energy bands, density of states, Fermi surfaces, and wavefunction populations are calculated for VS in the hexagonal phase. Purely metallic, mixed metal-nonmetal, and purely nonmetallic bands are observed. The Fermi level intersects the almost pure vanadium 3d bands so the compound should exhibit electrical conductivity like typical d band metals. The vanadium s and p and sulfur p bands are hybridized and lie well below the Fermi energy, and they give rise to the strong covalent bonds and the nonmetallic mechanical properties of the compound.
Intermolecular interaction in formaldehyde dimers
Authors: Del Bene Janet E.
Published: February 2004

Ab initio SCF calculations with a minimal STO-3G basis set have been performed on dimers of formaldehyde. Two equilibrium dimers have been found on the intermolecular surface. In one of these, dimer stabilization is due to the formation of a hydrogen bond, while in the other, the dipole-dipole interaction is primarily responsible for stabilization. These two equilibrium dimers are comparable in energy and easily interconverted. Upon dimer formation, only a minimal change in electron distribution is observed. Configuration interaction calculations have also been carried out to determine the effect of dimer formation on the n→π* transition energy. These calculations indicate that the n→π* band would be shifted to higher energy by only a few nanometers, relative to the monomer.
Intermolecular energy transfer in liquid benzene: Raman spectra, linewidth measurements, picosecond spectroscopy, and vibrational relaxation times
Authors: Griffiths J. E. , Clerc M. , Rentzepis P. M.
Published: February 2004

A collision-induced intermolecular vibrational energy transfer process in liquid C6H6 has been studied using detailed measurements of Raman line shapes and linewidths and picosecond spectroscopy. Lifetimes of molecules in the upper vibratonal energy level of the Raman transition ν2(ν =0→1) in C6H6 and C6D6 have been evaluated using both frequency and time measurements. By varying the absolute number of C6H6–C6H6 collisions over a wide range by dilution of C6H6 with C6D6, the lifetime of the ν′2 state can be altered from 4.7−7.0×10−12 sec. The higher value corresponds to an intrinsic lifetime of ν′2 in the liquid state while the other is a lower value because of a near resonance collision induced intermolecular vibration to vibration energy transfer process which occurs in pure C6H6. Direct measurement of the ν2(a1g) vibrational lifetimes by picosecond spectroscopy gives values of 5×10−12 sec for pure liquid C6H6 and 8×10−12 sec for pure liquid C6D6.
Hydrogen bond studies. 85. A very short, asymmetrical, intramolecular hydrogen bond: A neutron diffraction study of pyridine-2,3-dicarboxylic acid (C7H5NO4)
Authors: Kvick Åke , Koetzle Thomas F. , Thomas Robert , Takusagawa Fusao
Published: February 2004

A neutron diffraction study of pyridine-2,3-dicarboxylic acid (quinolinic acid) has been carried out. The intensities for 3780 reflexions were measured at the Brookhaven High Flux Beam Reactor. The structure was refined to an R(F2) of 0.054 using starting parameters from a previous x-ray study. The short intramolecular hydrogen bond [O ··· O: 2.398(3) Å] has no symmetry restriction operating on the hydrogen atom position and is asymmetric. The two O ··· H distances are 1.163(5) and 1.238(5) Å, respectively, and the O–H–O angle is 174.4(4)°. The asymmetry of the bond can be explained by the intramolecular environment. The strain in the molecule caused by the short hydrogen bond results in long carbon-carbon bonds to the carboxyl groups [1.516(2) and 1.541(2) Å]. The C–O bond lengths [1.219(3), 1.263(2), 1.222(3), and 1.270(2) Å] vary depending on the hydrogen-bond involvement of the oxygen atoms. The mean C–H distance is 1.087 Å and the N–H distance is 1.036(4) Å. The nitrogen atom is involved in an intermolecular N–H ··· O hydrogen bond [N ··· O: 2.725(2), H ··· O: 1.845(4) Å and angle N–H ··· O: 140.5(2)°]. There are a number of short intermolecular O ··· H–C contacts. The refined neutron scattering length for the nitrogen atom is b̄N=0.925(4)×10−12 cm.
Orthonormalized local energy method. I. Description and a simple example
Authors: Scott Michael W.
Published: February 2004

A new variation of the local energy method of Frost is described. It is applied to the Hartree-Fock equations for the ground state of the beryllium atom, both as a test of the method and as an example of the application of a local energy method to the determination of orbitals.
Total energy distribution method for classification of normal modes of vibration
Authors: Rytter E.
Published: February 2004

A new method is described for classification of normal modes of vibration of molecules which takes into account the distribution of both potential and kinetic energy. Different classification methods are compared for several molecules.
Potential surface dependence of vibrationally inelastic collisions between He and H2
Authors: Alexander Millard H. , Berard Edward V.
Published: February 2004

We have carried out a careful study of He–H2 vibrationally inelastic collisions based on five different analytic fits to the Gordon-Secrest [J. Chem. Phys. 52, 120 (1970)] ab initio interaction potential. Within a model which freezes the atom-diatom orientation, we solve the collision dynamics fully quantum mechanically. Both for the 0→1 and 3→4 vibrational transitions, we find that the use of different analytic surfaces leads to substantial qualitative and quantitative differences in the calculated transition probabilities. In particular, we observe an anomalous angular dependence in the case of the original Gordon-Secrest analytic surface. We show that this arises from difficulties in the extrapolation of the ab initio points in the classically forbidden region at small center-of-mass separation. These results have important implications for future theoretical study of molecular collisions.
Evaluation of dielectric permittivity and conductivity by time domain spectroscopy. Mathematical analysis of Fellner-Feldegg's thin cell method
Authors: van Gemert Martin J. C.
Published: February 2004

A mathematical analysis is presented of Fellner-Feldegg's thin cell method from which the permittivity and conductivity of dielectric materials can be obtained in the time domain. It is proved that the thin cell response has to be interpreted as a Taylor expansion of the step response around l=0. The first term in the expansion (Fellner-Feldegg's thin cell relation) can be found by integration over the ordinary Laplace contour (c − i ∞, c + i ∞). The other coefficients can be obtained from complex integration over a contour W enclosing all singularities of the integrand. Restrictions on the thickness of the sample are derived from the requirement that the second term in the expansion must be small relative to the first one. The step response is also calculated with asymptotic methods and with numerical methods for three currently used permittivity relations. Also, conductivity is included. For Debye materials a relation is found that tends to give a better fit to the exact response than the thin cell relation. For such materials the relaxation time found experimentally, using the thin cell relation, is always too large. An iteration procedure is suggested to obtain the correct relaxation time. For conductive materials the thin cell approximation leads to an incorrect value of the offset of the base line. The correct relation is derived.
Configuration interaction calculations of some observed states of NO−, NO, NO+, and NO2+
Authors: Thulstrup P. W. , Thulstrup E. W. , Andersen A. , Öhrn Y.
Published: February 2004

Minimal STO basis has been used in configuration interaction calculations on the molecule NO and some of its ions. Several observed states of these systems have been calculated with full potential energy curves, and comparisons have been made with results from photoelectron spectroscopy, double charge transfer spectroscopy, and photodetachment spectroscopy.
Helical pitch in mixtures of cholesteric liquid crystals
Authors: Wulf A.
Published: February 2004

The behavior of the helical pitch in binary mixtures of cholesteric liquid crystals is studied by means of a simple molecular model (an extension to mixtures of a model introduced by Priest and Lubensky). The model exhibits helical inversion, and with some reasonable assumptions is shown to be capable of giving order of magnitude agreement with experiments performed by Baessler and Labes. It is found that helical inversion is driven by the variation with temperature of the nematic order parameters of the two components of the mixture. A general argument is given for the linearity of inverse pitch wavenumber versus concentration difference in the special case of a mixture of the left- and right-handed versions of a single cholesteric liquid crystal. (Such a system was studied experimentally by Dolphin et al.)
Chemiluminescent spectra of alkali-halogen reactions
Authors: Oldenborg R. C. , Gole J. L. , Zare R. N.
Published: February 2004

Using a beam-gas arrangement, we have obtained the chemiluminescence spectra of alkali-halogen reactions at pressures less than 1 mtorr. The spectra in general are characterized by a broad molecular feature on which are superimposed sharp atomic alkali transitions. Under higher resolution (better than 5 Å) the molecular feature consists of a single progression of regularly spaced peaks extending over several thousand angstroms. Spectra are presented for the chemiluminescent reactions M2+X2(XY) →MX*+MX(MY), where M2=K2, Rb2, Cs2 and X2(XY)=Cl2, Br2, I2, IBr, ICl, ClF. The MX* emitter is identified by comparison of the vibrationally resolved spectra with previous absorption and emission studies of the alkali halides. The spectra consist of transitions from the bound or quasibound vibrational levels of a shallow homopolar excited state to high-lying vibrational levels of the ionic ground state. Differences in the onsets and cutoffs between our spectra and those of flame studies are noted and rationalized in terms of the different effective rotational potentials describing the MX* emitters produced under the single-collision conditions employed here and under the conditions prevailing in flame studies. For the mixed halogen reactions, the MY ground state is the more stable salt molecule that can be formed from the halogen atoms of the XY pair. A four-center mechanism is deduced from the dependence of the chemiluminescent intensity on reagent concentrations. The observed atomic emission apparently results from a different mechanism. While the alkali reactions with F2 do not yield appreciable molecular emission, atomic transitions from high Rydberg levels are prominent for these reaction systems. Both atomic and molecular chemiluminescence are shown to be consistent with electron-jump mechanisms.
Relaxation in simple quantum systems
Authors: Nitzan Abraham , Silbey Robert J.
Published: February 2004

The decay of an oscillator and a two level system into a general heat bath is studied using a truncated cumulant expression for the time evolution operator. Expressions for the decay rate in the weak coupling and rotating wave approximations are derived, and the temperature dependence of the rate examined in all cases. We find the temperature dependence to be strongly dependent on the nature of the system and bath.
Work functions of binary compounds
Authors: Yamamoto Shigehiko , Susa Kenzo , Kawabe Ushio
Published: February 2004

A new semiempirical model for predicting the work function of binary compounds is proposed. The model is a modification of the Gordy and Thomas relation and is applicable to binary compounds. The work functions of binary compounds, φAB are found to be primarily determined by the properties of the element of low work function, A, perturbed by the other element of high work function, B, according to φAB=φA + (1.41/dA)[(2φA − φB − 0.34)/(φA + φB − 0.68)]. Experimental values tabulated by Fomenko are compared with the predicted work functions, and the quantitative agreement is good in spite of the simplicity of the model.
Calculation of dense fluid transport properties via equilibrium statistical mechanical perturbation theory
Authors: Ely James F. , McQuarrie Donald A.
Published: February 2004

The viscosity and thermal conductivity coefficients of dense fluid argon have been calculated using the modified Enskog theory and the equilibrium statistical mechanical perturbation theory of Barker and Henderson. Agreement with experimental transport data is shown to be, in general, quite good. The results of these calculations are also compared to those obtained using the modified Enskog theory and an experimental equation of state. In this case, the results are seen to be excellent, which indicates that this approach provides us with a method of predicting transport properties of simple dense fluids from fundamental molecular theory.
Quantitative laser measurement of very small absorptions: Studies of the O + CS→CO(V) + S reaction
Authors: Djeu N.
Published: February 2004

An intracavity laser technique has been developed to detect quantitatively very small absorptions by measurements of the laser oscillation range. The technique has been applied to the measurement of the absolute densities of the individual vibrational levels of CO produced from the reaction between O and CS in a fast flow reactor prior to any significant relaxation. Optical losses (or gains) as small as 0.01% were detectable with the present apparatus. The detectivity may be further improved by an order of magnitude with lower loss optics and a more stable laser.
Vibration-rotation absorption spectra of a rotator hindered by fields of cubic symmetry
Authors: Beyeler H. U.
Published: February 2004

The rotation-vibration spectrum of a hindered rotator has been computed for a variety of hindering potentials of cubic symmetry. Spectra are displayed for potentials that are combinations of cubic harmonics up to sixth order for various potential strengths and temperatures. Agreement is found between the computed transition intensities for large hindering potentials and those given by the appropriate ``tunneling model.''
Vibrational relaxation of fluorine by a shock tube schlieren method
Authors: Diebold Gerald J. , Santoro Robert J. , Goldsmith George J.
Published: February 2004

The vibrational relaxation of fluorine in argon has been studied in the temperature range 500–1300°K behind incident shock waves. A laser schlieren system utilizing a fast, quadrant photodiode and difference amplifier was used to record density changes behind the shock waves. Data from 10% and 20% fluorine in argon mixtures were reduced to relaxation times using the Blackman technique and relaxation times for pure fluorine, and fluorine infinitely dilute in argon deduced using a curve fitting technique. The fluorine-fluorine and fluorine-argon relaxation times, in μsec · atm, were found to be best described by ln(Pτ)F2–F2 = −6.972+65.20T−1/3,ln(Pτ)F2–Ar = −8.650+96.97T−1/3,where T is the temperature in degrees Kelvin. The results of this investigation agree well with the low temperature results of Shields for pure fluorine relaxation.
Electron paramagnetic resonance of divalent nickel in ferrous fluosilicate
Authors: Rubins R. S.
Published: February 2004

The EPR spectrum of Ni2+ in single crystals of ferrous fluosilicate FeSiF6·6H2O was studied at liquid helium temperatures in the frequency range 14.0–16.5 GHz. The six-line spectrum, associated with six inequivalent Ni2+ sites, was fitted to the spin Hamiltonian H = gβH·S + DSz2 + E (Sx2 − Sy2), with S=1, g=2.255±0.005, D=−(3.05±0.02) cm−1, and E=(0.170±0.001) cm−1. The magnitudes of g and E and the sign of D were deduced, respectively, from the frequency dependence and the temperature dependence of the single line spectrum observed in the parallel orientation. The magnitude of D was obtained from measurements in the perpendicular plane. The perpendicular spectrum showed a sixfold rotational symmetry and consisted of two sets of three lines related by a rotation of (13.0±0.5)° about the c axis.
Low-energy (< 1 eV) electron attachment to molecules in very-high pressure gases: C6H6
Authors: Christophorou L. G. , Goans R. E.
Published: February 2004

Benzene has been found to capture slow (≲ 0.3 eV) electrons in high densities of N2 (and Ar) with a rate that increases with increasing N2 density and decreasing electron energy at near-thermal energies. The dependence of the attachment rate on the N2 density and the electron energy has been studied in detail, and a model is presented that accounts for the experimental results. On the basis of this model the autodetachment lifetime of C6H6-* has been estimated and found to vary from ∼1 psec at 0.04 eV to ∼0.2 psec at 0.18 eV. From the high-pressure experiments the ``liquid state behavior'' has been predicted also. The finding that benzene negative ions are formed in the gas phase forces the conclusion that the benzene molecule has a positive (>0 eV) electron affinity, (EA)B, contrary to the accepted view that (EA)B <0 eV. The short autodetachment lifetime of C6H6-*, however, indicates that (EA)B is small. Benzene was also studied in mixtures with C2H4 and C2H4–N2. The results of these latter experiments strongly indicated that the probability of electron detachment in C6H6-*-C2H4 collisions is large.
ESR investigations of the aromatic triplet state in polymethylmethacrylate
Authors: Bramwell Fitzgerald B. , Laterza Marie E.
Published: February 2004

The use of polymethylmethacrylate (PMMA) as a host medium for the study of the aromatic triplet state by electron spin resonance spectroscopy (ESR) has been explored. The influence of photolytically induced host free radical formation on the triplet lifetimes and triplet spectral intensities is discussed. For naphthalene and phenanthrene the thermal dependence of the zero field splitting parameters D, E, and D* have been measured between 87 and 380 K. An unusual variation of D* with temperature at about 210 K is consistent with the presence of a second order phase transition of PMMA.
Temperature dependence of the 14N quadrupole coupling constants in pyrazine
Authors: Péneau A. , Gourdji M. , Guibé L.
Published: February 2004

The frequency of the 14N nuclear quadrupole resonances ν+ and ν− in pyrazine has been measured at several temperatures between 4.2 and 324 K. Below 300 K, only one ν+ line and one ν− line are seen, indicating the existence of a unique sort of crystalline site for nitrogen atoms in the crystal of pyrazine. At 27° and 35°C crystalline phase transitions are observed through a splitting of the lines and a discontinuity of the frequency of resonance. An interpretation of the temperature dependence of the resonance frequencies, ν+ and ν−, between 4.2 and 300 K is proposed using Bayer's theory and the molecular motion frequencies reported by Ito and Shigeoka; however, the assignment of these frequencies to specific rotations about the different axes of symmetry of the molecule, which fits nuclear quadrupole resonance data, is different from that given by Ito and Shigeoka.
Effect of an external electric field on the yield of free ions. I General results from the Onsager theory
Authors: Mozumder A.
Published: February 2004

Onsager's equation for the effect of an external electric field on the probability of an isolated ion pair escaping neutralization in a dielectric medium has been rearranged for evaluation in a consistent way. The present treatment allows the escape probability to be written as a single series valid for arbitrary field strengths and initial separations. In a given case, procedure is outlined for calculating this probability in the most systematic manner. Although expansion of the escape probability in powers of the external field is not advisable at high field strengths because of the slowness of convergence, the field coefficients are provided for evaluation and comparison at relatively low fields. At high fields possibility of a temperature inversion effect is indicated; that is, the field-dependent escape probability may actually decrease with temperature.