bluuebunny/crossref_metadata_2025 huggingface dataset:
A unified molecular force field via a model theory of isoelectronic diatomic molecules
Authors: Laurenzi Bernard J.
Published: February 2003

Electronic energy curves for diatomic molecules come in a wide variety of shapes. More categorically, these correspond to stable, metastable or repulsive states. We propose that a great deal of this apparently diverse behavior might in fact be contained in a single isoelectronic energy surface E(R,Z,Z′). That is, each kind of curve can be thought of as a particular cross section of the surface corresponding to some range of the nuclear charges Z and Z′. Analytic representations for these surfaces have been given and their properties examined. We have found that they are folded and contain critical points. These features put limits on the number of isoelectronic species that can exist in a sequence and interconnect their properties. By this we understand that data gathered on even the repulsive states of one molecule of an isoelectronic sequence contains information about the stable and metastable members of that sequence.
Density functional calculations for atoms in the first transition series
Authors: Harrison Joseph G.
Published: February 2003

s-d interconfigurational energies and spin-flip energies for atoms in the first transition series are examined in the self-interaction-corrected local spin density approximation (SIC-LSDA). The results are substantially improved over previous LSDA calculations and SIC-LSDA calculations employing sphericalized orbital densities. The implications for accurate cohesive energy calculations and molecular bonding in iron-series dimers are discussed.
A new formulation of scaled atoms-in-molecules theory
Authors: Ellison Frank O.
Published: February 2003

In scaled atoms-in-molecules theory, variational scaling parameters are introduced into the A- and B-atom eigenfunctions from which molecular basis functions are constructed. All intraatomic contributions to matrix elements are determined rigorously in terms of scaling parameters and experimental atomic state energies. Approximate atomic state eigenfunctions are used to compute interatomic contributions. Ground and excited state energy roots of the resulting molecular eigenvalue problem can be independently minimized with respect to the scaling parameters. The original formulation of theory (SAIM.V) employed atomic potential energy operators as part of the intraatomic correction due to scaling. A new formulation (SAIM.T) using atomic kinetic energy operators in this role is tested in calculations on H2, HeH+, He+2, and HeH. The simpler SAIM.T formulation is found to give results of the same quality as the original SAIM.V version.
Rotational R-branch spectroscopy in CO2: Reply
Authors: Godfried H. P. , Silvera Isaac F.
Published: February 2003

A reply to the comments of Altman et al., regarding a previous measurement of the rotational Raman spectra of CO42 is given. The resolution and accuracy of measurements4 is stressed. (AIP)
Thermally induced nucleation. I. A new way to obtain activation energies for unimolecular reactions
Authors: Katz Joseph L. , Ruggiero Joseph G. , Partch Richard , Warren Dale , Ebetino F. H.
Published: February 2003

Thermal diffusion cloud chambers have been used to study a variety of physical chemical phenomena, particularly homogeneous nucleation. A new and potentially valuable application of the diffusion cloud chamber—the monitoring of thermally allowed reactions—is presented. It is shown that a low concentration of benzocylobutenol dissolved in nonane will induce temperature dependent nucleation. At constant temperature the rate of nucleation decays exponentially over time. Exponential decay constants obtained from rates of nucleation, when plotted in Arrhenius fashion, give an activation energy of 31 kcal/mol—the same value reported for the racemization of a chiral benzocyclobutenol derivative.
Vibrational relaxation and vibrational predissociation in DCl gas
Authors: Chesnoy J.
Published: February 2003

The vibrational relaxation lifetime of compressed deuterium chloride is measured at room temperature as a function of density using time resolved laser induced fluorescence. The results, showing a deviation from the simple gas behavior, are interpreted as giving evidence of the role played by vibrational predissociation of dimers in vibrational relaxation, according to recent theories of vibrational predissociation. The lifetime measurement of low density deuterium chloride, obtained between −95 and +55 °C is first described and the results are quantitatively interpreted and compared to HCl gas.
Total scattering cross sections for the helium–argon system at low relative velocities
Authors: York R. W. , Taylor W. L. , Pickett P. T.
Published: February 2003

Absolute total cross sections for He–Ar were measured in the relative velocity range of ∼800–2000 m/s by passing a supersonic nozzle beam through a cell containing the target gas. The beam attenuation was determined by passing the unscattered beam particles into a quadrupole detector. The absolute number density of target gas particles in the target cell was obtained by a calibrated-flow method of introducing gas into the cell during an experiment. A correction was applied for the angular resolution of the apparatus. The average experimental uncertainty was estimated to be ∼3% to 3.5%. The present results, as well as other available data, were deconvoluted in a two-step process to yield QI and QII, the first and second approximations to the total cross section, as a function of the relative velocity. The composite results of all experimental data were compared to seven interatomic potentials and the Hartree–Fock-dispersion (HFD) potential represented the data best.
Investigation of various ways of forming CsH by irradiating a Cs+H2 mixture with laser light
Authors: Visticot J. P. , Ferray M. , Lozingot J. , Sayer B.
Published: February 2003

A detailed study of the formation of CsH by the laser photoexcitation of cesium atoms is presented. It is shown that, under our experimental conditions, CsH can be formed not only from the Cs(7P) state but also from the Cs(6D) state and that the 6D level appears to be even more efficient than the 7P level. CsH formation has also been observed when rubidium impurities present in the medium are photoexcited. In this situation, excitation transfer from excited Rb atoms to Cs atoms is not sufficient to explain the observed rate of CsH formation; consequently most of the CsH appears to be formed in an indirect process requiring no excited Cs atom and perhaps involving vibrationally excited H2 molecules.
Xα multiple scattering calculations on iron(II) porphine
Authors: Sontum Stephen F. , Case David A. , Karplus Martin
Published: February 2003

Spin-restricted and spin-unrestricted Xα multiple scattering calculations are reported for iron porphine with D4h symmetry in the absence of axial ligands. A 3A2g ground state is predicted when the radius of the porphyrin ‘‘hole’’ is less than 2.0 Å, whereas a quintet ground state is expected for larger Fe–N distances. A large asymmetry is found in the occupations of the three Fe 4p orbitals. This has important implications for the interpretation of Mössbauer quadrupole coupling constants. Magnetic susceptibility and NMR results can be understood in terms of a model that involves spin-orbit mixing of an excited 3Eg state with the ground 3A2g state; an essential element in explaining the NMR data is that the proton hyperfine constants are not the same in these two states. Tentative assignments are given for observed absorptions in the optical and near-infrared regions. It is shown that spin-unrestricted wave functions are particularly suited for the qualitative interpretation of electronic structure of metallophorphyrins.
A comparative study of the molecules Cu2 and Ag2 by all electron ab initio HF–CI methods
Authors: Shim Irene , Gingerich Karl A.
Published: February 2003

In the present work the molecules Cu2 and Ag2 have been investigated using all electron ab initio Hartree–Fock (HF) and configuration interaction (CI) calculations. The basis sets used were of double zeta quality in general, but the 3d orbital for Cu and the 4d orbital for Ag were represented by triple zeta functions. The wave functions for the Cu2 and the Ag2 molecules have been analyzed in detail. Calculated spectroscopic constants have been derived and compared to experimental data, where available. Both for the Cu2 and the Ag2 molecules the calculated spectroscopic data vary considerably as the CI calculations exclude or include excitations from the valence d orbitals. The chemical bonds for both molecules are single bonds almost entirely due to sσg molecular orbitals.
The density profile of a fluid bounded by a soft wall
Authors: Percus Jerome K. , Williams G. O.
Published: February 2003

An approximate relationship is developed between the density profile of a simple classical fluid bounded by a soft wall and that bounded by an idealized hard wall. This relationship is exact for a class of one-dimensional fluids, and also constitutes a simplification of the ‘‘shielding approximation.’’ It accurately reproduces several Monte Carlo calculations.
The application of the HNC/MSA approximation to bulk electrolytes
Authors: Henderson D. , Lozada-Cassou Marcelo , Blum Lesser
Published: February 2003

The HNC/MSA integral equation, where the hypernetted chain (HNC) kernel is replaced by the mean spherical approximation (MSA) kernel is applied to a model bulk electrolyte consisting of charged hard spheres in a uniform dielectric medium. The thermodynamic functions which result from this hybrid integral equation are better than those of the HNC equation at low concentrations and only slightly worse than those of the HNC equation at high concentrations. At low concentrations, the HNC/MSA radial distribution functions are qualitatively better than the HNC values. In particular, there is no peak in g++(r) near r=2σ. Quantitatively, the HNC/MSA g++(r) appears to be an overcorrection of the HNC g++(r).
On cooperative modes of reorientation in liquid crystals
Authors: Zientara Gary P. , Freed Jack H.
Published: February 2003

The role of cooperative modes of reorientation in liquid crystals for the spectral densities of importance in magnetic resonance relaxation is examined from the viewpoint of microscopic models that can be solved analytically. These include linear arrays of rods in an applied external field whose orientations are strongly coupled and whose motion is described by overdamped and coupled diffusion equations. Also considered are Cayley-tree models, which, at least for nearest neighbors, can mimic higher dimensions than one. All these models lead to the result that the qth normal mode of relaxation has a decay rate that depends upon q as sin2(qa/2), where a is the lattice spacing. Using simple scaling, it is argued that if a is replaced by a′≡La, so our model consists of blocks of L rodlike molecules with renormalized parameters, then molecular detail becomes less important and the model arrays become more physically representative. These results are then utilized to extend the standard hydrodynamic theory for long wave vectors q to short wave vectors approaching molecular dimension. The spectral densities obey scaling (i.e., are nearly independent of a′) and are nearly identical to the hydrodynamic result for lower frequencies. However, the new results do show small departures at the higher frequencies usually studied by NMR.
Critical ultrasonic attenuation in the binary liquid cyclohexane + nitroethane
Authors: Sanchez G. , Garland C. W.
Published: February 2003

The ultrasonic velocity and attenuation have been measured in the critical binary mixture cyclohexane + nitroethane over the frequency range 3–27 MHz and the temperature range 0.01≤T−Tc≤15 K. The sound velocity varies linearly with T and shows no critical dispersion in this frequency range. The critical attenuation is analyzed in terms of the dynamic scaling theory of Bhattacharjee and Ferrell. In general, the agreement between theory and experiment is good but the data conform to an empirical scaling function that differs in small but systematic ways from the theoretical scaling function. Universality in the critical acoustic behavior is demonstrated by a comparison of the scaled attenuation in three binary liquid systems with that in two pure fluids.
Kinetics of hydrogen absorption by α-zirconium
Authors: Naito Shizuo
Published: February 2003

The kinetics of hydrogen absorption by polycrystalline α-zirconium metals has been investigated by the use of spherical specimens in the pressure range 2×10−5–1×101 Pa and in the temperature range 673–1073 K. The experimental results are reasonably explained on the basis of the model in which the process is assumed to be composed of three successive steps: dissociative adsorption with lateral interaction between adsorbed hydrogen atoms; jump of the adsorbed hydrogen atom into the interstitial site; and finally its diffusion into the metal bulk. Both the first and third steps are rate controlling under the conditions of low pressures or low temperatures within the pressure and temperature ranges of this investigation. At low temperatures the absorption rate is significantly affected by the lateral interaction as the pressure is increased. Some parameters which characterize the derived rate equations have been determined: the activation energies are (1.7±0.5)×10−21 J/H atom for dissociative adsorption and (9.43±0.30)×10−20 J/H atom for desorption, and the heat for jump is (9.4±1.5)×10−21 J/H atom. The lateral interaction is repulsive and its energy is ∼4×10−20 J/H atom for 0≤α≤0.4, where α represents the so-called transfer factor.
Dynamic bond percolation theory: A microscopic model for diffusion in dynamically disordered systems. I. Definition and one-dimensional case
Authors: Druger Stephen D. , Nitzan Abraham , Ratner Mark A.
Published: February 2003

A dynamic bond percolation model is defined and studied. The model is intended to describe diffusion of small particles (ions, electrons) in a medium which is statistically disordered (as in ordinary bond percolation), but which is also undergoing dynamic rearrangement processes on a timescale short compared to the observation time. The model should be applicable to polymeric solid electrolytes, where the orientational motions of the polymer (which are responsible for configurational entropy) cause the dynamic motion of the medium (polymer) in which the small particles (alkali ions) diffuse. The model is characterized by three parameters: an average hopping rate w which appears in the master equation for hopping, a percentage of available bonds f, and a mean renewal time τ̄ren for dynamic motion of the medium to rearrange the assignments of closed and open bonds. We show that the behavior is always diffusive for observation times long compared to τ̃ren, in agreement with experiment on polymeric solid electrolytes. We also derive a closed-form expression for the diffusion coefficient. For observation times smaller than the renewal time there is no diffusion, again in accord with the behavior of polymeric solid electrolytes below the glass transition temperature. The diffusion coefficient is a monotonically increasing function of the inverse renewal time and hence of the free volume, the configurational entropy, and the temperature.
Turbulence induced change in the conformation of polymer molecules
Authors: Armstrong Rob , Jhon M. S.
Published: February 2003

An investigation into the conformation of a polymer molecule exposed to a stochastic flow field is made utilizing a cumulant expansion. The study begins by considering the full Kirkwood–Riseman n-bead chain with an arbitrary connector potential as a model for the polymer molecule, digressing to the two-bead dumbbell molecule for detailed calculations. It is found that the effect of the stochastic velocity field on the molecule can be interpreted as a ‘‘renormalization’’ of the connector potential. The introduction of this renormalized potential greatly simplifies the mathematics and lends an intuitive understanding to the effect of the turbulence on the polymer. It is shown that this analysis will recover the anomalous tendency of the radius of gyration to become infinite for the special case of a Hookean potential which has been observed by previous investigators. Further, this study involves the Eulerian velocity–velocity correlation function rather than the Lagrangian correlation used in the previous studies.
The ν3 vibrational spectrum of the free ammonium ion (NH4+)
Authors: Schäfer Eckhard , Begemann Marianne H. , Gudeman Christopher S. , Saykally Richard J.
Published: February 2003

The ν3 band of the ammonium ion was measured near 3.0 μm with a color center laser spectrometer using the technique of velocity modulated absorption spectroscopy. NH4+ was produced in an ac discharge containing 0.1 Torr of nH3 and 1.5 Torr of H2. Seventy individual transitions were measured with an accuracy of 1×10−6 and assigned to q0, P+, and R− branches of a tetrahedral molecule. The constants derived in a preliminary analysis are: νeff0=3342.540 2(97) cm−1, B0+B1−2B1χ3=11.110 8(35) cm−1, B1−B0=−0.050 71(45) cm−2u1, and D=8.5(2.6)×10−5 cm−1. (AIP)
Laser-induced fluorescence of PuF6 in the 3Γ4g→1Γ1g electronic band at room temperaturea)
Authors: Barefield J. E. , Rice W. W. , Tiee J. J. , Walters R. T.
Published: February 2003

When PuF6 is photoexcited at 797.3 nm, fluorescence is observed from the 1 μm electronic energy level. The lifetime τ of the fluorescence signal varies with pressure. The lifetimes cover a range of 655 to 243 ns for a pressure range of 0.5 to 72 Torr. A linear Stern–Volmer relationship was obtained over the complete pressure range considered. An extrapolation to zero pressure yields an intercept of τ0=663 ns and a slope of 3.62×104 Torr−1 s−1. Fluorescence and excitation spectra are reported and are in good agreement with the previously reported absorption spectrum of PuF6.
Rotational correlation times of a nitroxide spin probe in low temperature matrices determined by saturation transfer ESR: Their correlation to dynamic behavior of radiation products
Authors: Ito Yoshiro
Published: February 2003

Rotational correlation times τc of the spin probe, di-tert-butyl nitroxide, in several organic solvents cooled to temperatures lower than their melting points have been measured using a saturation transfer (ST) ESR technique. Since the ST spectra changed markedly depending on microwave power, we have introduced a scaled ST parameter to eliminate this effect. The temperature dependence of τc in a noncrystalline phase can be described by the Arrhenius relation. The activation energies are 17.6 kJ mol−1 for methanol containing 5% water, 20.8 kJ mol−1 for ethanol, and 18.9 kJ mol−1 for n-propanol. The values are close to the activation energies reported for the solvation and decay rates of electrons in the solvents. The rotational correlation time can be expressed in terms of a viscosity using the Stokes–Einstein equation. However, a macroscopic viscosity fails to reproduce the observed τc, especially at low temperatures. On the other hand, we have evaluated the solvation rate of an electron in n-propanol and the diffusion coefficients of radicals in methanol and 3-methylpentane on the basis of the tentative viscosity derived from τc using the same expression. The results are in good agreement with the experimental data. The temperature and phase dependences of τc will also be discussed.
ESR spectra and structures of radical cations of some branched alkanes: β-proton couplings in C–C σ cations
Authors: Nunome Keichi , Toriyama Kazumi , Iwasaki Machio
Published: February 2003

The systematic ESR studies of radical cations of five kinds of methyl substituted butanes show that the unpaired electron is rather confined to one of the C–C σ bonds in contrast to the linear alkane cations, in which the unpaired electron delocalizes over the entire σ molecular chain. Analyses of the origin of the trans C–Hβ proton couplings indicate that spin transfer due to hyperconjugation plays an important role in these branched alkane cations. It is also shown that the hyperconjugative effect in these branched alkane cations is two times higher than that in the neutral alkyl π radicals. The large difference of the front and back lobe interactions in the β-proton couplings in these C–C σ radicals is attributable to the bent structure of the radical carbon atom. The front lobe interaction is about one-half of that of the back lobe interaction as is the case of vinyl radicals.
Algebraic approach to molecular rotation-vibration spectra. II. Triatomic molecules
Authors: van Roosmalen O. S. , Iachello F. , Levine R. D. , Dieperink A. E. L.
Published: February 2003

The algebraic approach to molecular rotation-vibration spectra introduced in a previous article is extended from di- to tri- and polyatomic molecules. The spectrum generating algebra appropriate to triatomic molecules U(4)⊗U(4) is explicitly constructed. Its dynamical symmetries and their relation to rigid, nonrigid, linear, and nonlinear structures are discussed. Applications to the spectra of HCN, CO2, and H+3 are considered. In particular, it is shown that the algebraic description can account for the Fermi resonances occurring in CO2. Some remarks are made on the spectrum generating algebras appropriate to polyatomic molecules.
Beam-gas study of chemiluminescent reactions of Sn, Ge, and Si with F2
Authors: Rosano W. J. , Parson J. M.
Published: February 2003

Electronic chemiluminescence (CL) has been observed for the reactions of tin, germanium, and silicon with fluorine utilizing a beam-gas configuration. Only electronic states of the metal monofluorides which are energetically allowed in a single reactive collision of the metal atoms SnF(A 2Σ+), GeF(A 2Σ+), and SiF (A 2Σ+, a 4Σ−) have been observed, in contrast with the results of previous flame studies in which other high lying electronic states have been reported. Pressure and temperature variations of the CL indicate a first-order dependence with respect to metal and fluorine. Simulation of the A 2Σ+–X 2Π systems has been obtained by nonlinear least-squares fit of the experimental spectra. Vibrational populations of the A 2Σ+ states are found to be inverted, with the appearance of double maxima. Adiabatic correlation, with respect to overall electronic symmetries of the reactants and products, indicates all reactant surfaces lead to ground-state products. However, molecular orbital arguments indicate the presence of one diabatic surface connecting reactants and electronically excited products.
Vibrational energy exchange between carbon dioxide and hydrogen sulfide
Authors: Hoke Jeffrey , Preses J. M. , Weston Ralph E.
Published: February 2003

A mixture of CO2 and H2S was irradiated with a low-pressure Q-switched CO2 laser, and the infrared emission from CO2(00°1) was observed. The rates of collisional energy transfer to H2S were computed from the observed infrared fluorescence. (AIP)
Density of liquid ytterbium
Authors: Seerveld J. , Van Till S. , Van Zytveld J. B.
Published: February 2003

We have measured the density ρ of high-purity liquid Yb up to about 100 K above its melting temperature Tm. We find ρ(Tm)=(6.179±0.006)×103kg/m3 and dρ/dT=−1.02±0.16 kg/m3 K. We also compare dρ/dT with value characteristics of other liquid metals.
First and second anharmonicities of the MCSCF energy
Authors: Simons Jack , Jo/rgensen Poul
Published: February 2003

The dependence of the electronic energy of a multiconfigurational self-consistent field wave function on molecular geometry is currently receiving a great deal of attention. In this manuscript, analytical expressions are derived which permit the evaluaton of the third and fourth derivatives of the energy the respect to geometrical displacements. Although the resultant expressions do requires higher order derivatives of the atomic-orbital-level one- and two-electron integrals, there are no other substantial increases in computational difficulty when compared to the first (gradient) and second (Hessian) energy derivatives obtained earlier. (AIP)
Comment on ‘‘Nuclear magnetic resonance spectroscopy in cholesteric liquid crystals. I. Orientational order parameter measurements’’
Authors: Collings Peter J.
Published: February 2003

NMR spectra in cholesteric liquid crystals cannot be analyzed in the same way as in nematic liquid crystals, due to the random orientation of helical axes in the sample and due to the effects of diffusion along the helical axes. This comment points out that the authors of a previous article did in fact analyze their NMR spectra without taking these two considerations into account.
The simulated ab initio molecular orbital (SAMO) method. VII. Extended basis sets for molecules and polymers
Authors: Duke Brian J. , O’Leary Brian
Published: February 2003

The simulated ab initio molecular orbital SAMO method has been extended to study the use of basis sets larger than minimum basis sets for hydrocarbon molecules and polymers. Inclusion of polarization functions leads to no new features but the diffuse nature of double zeta basis sets requires, for the simulation of molecular properties, larger pattern molecules than was previously the case. For polymers, the approximations made within the ab initio crystal orbital method are comparable to those made in the SAMO method and the results are correspondingly closely similar. The problem of near linear dependence in the basis set for polymers is thoroughly discussed. This study opens up the SAMO method to the utilization of a wider range of extended basis function.
Dynamic twinning: Polymorphism of thallium thiocyanate studied with x-ray diffraction
Authors: Lippman Robert , Rudman Reuben
Published: February 2003

TlSCN melts at 230 °C and has a phase transition at 90 °C, with ΔSf=9.56 e.u./mol and Δstr=0.15 e.u./mol. Lattice parameter determination at several temperatures reveals that (ΔV/V) ⋅ 100 =0.6% at the phase transition. The crystal structure of phase II (stable below 90 °C) has been refined based on the structure reported by Büssem, Günther, and Tubin [Z. Phys. Chem. Abt. B 24, 1 (1934)]. At 24 °C, TlSCN is orthorhombic Pbcm, a=6.783(1), b=6.816(1), c=7.606(1) Å, Z=4, Dx=4.96 g/cc. Least-squares analysis resulted in R=0.040, with S–C=1.622(9) Å, C–N=1.173(10) Å, and ∠S–C–N=178.7(7)°, in agreement with interatomic distances and angles reported for other univalent metal–thiocyanate salts. Above 90 °C, phase I is tetragonal, with (at 130 °C) a=6.815(1), c=7.773(1) Å, Z=4, Dx=4.82 g/cc. Single-crystal data were refined in space group I4/mcm using group-refinement least-squares analysis of six variable parameters to R=0.067, where the group refined was the TlSCN moiety found in the structure of phase II. The small entropy and volume change of transition together with the successful group refinement reveal that the structure of the two phases are quite similar. The initial analysis of phase I indicates that Tl+ is in a fourfold special position and that SCN− reorients 180° (head to tail) in a dynamic mode. However, such a model results in very short Tl ⋅⋅⋅ S distances and does not refine to a satisfactory agreement factor. The use of the phase II structure in group refinement of phase I removes these difficulties, and reveals an unusual mode of dynamic twinning, wherein the crystal has an average structure of higher symmetry than the instantaneous structure due to dynamic ionic reorientation accompanied by a dynamically changing lattice reference system.
Establishing the absence of any influence by an external magnetic field upon the intrinsic rate of reduction of magnetite by hydrogen
Authors: Rowe M. W. , Gallagher P. K. , Gyorgy E. M.
Published: February 2003

Analysis of the water formed during isothermal reductions of single crystal fragments or sintered polycrystalline samples of Fe3O4 by H2, in the temperature range from 350 to 450 ° C, clearly demonstrated that there was no effect of an external magnetic field (6.9 kOe) upon the observed reaction rate. When powdered samples of Fe3O4 were used, however, it was possible to observe slight enhancements of the apparent rate of reduction presumably due to alignment of the particles in the presence of an external field in such a manner as to present greater exposed surface area to the gaseous reactant.
Sensitivity analysis of surface structure determination by low energy electron diffraction
Authors: Olszewski Gerard B. , Bernasek Steven L.
Published: February 2003

The determination of surface structures by low energy electron diffraction (LEED) has reached a stage of development where the assignment of atomic positions is no longer sufficient. An estimate of the accuracy of the derived structural information is necessary. A sensitivity analysis of the entire structural determination procedure provides this estimate. Several questions are addressed by this analysis. The effects of choices of the nongeometrical parameters on the geometrical results are studied. An estimate of the accuracy of structural assignments is inferred from the results. It appears that a simple least squares methodology (with an ultimate accuracy of ±0.05 Å) is the most logical choice for these problems.
Radiation chemistry of purines in the solid state: ESR and ENDOR studies of x-irradiated xanthosine dihydrate single crystals
Authors: Nelson William H. , Close David M.
Published: February 2003

By means of ESR and ENDOR spectroscopy, we were able to detect and characterize four products of the purine riboside xanthosine in x-irradiated xanthosine dihydrate single crystals. Product I was identified (at 65 K) as the π cation of the xanthine base deprotonated at N3. Product II was identified (at 65 K) as the π anion of the xanthine base protonated at O6. Product III was identified as the result of the hydrogen abstraction from C5′ of the sugar group (probably from a primary cation of the ribose). Product IV was identified as the result of hydrogen addition at C8 of the purine ring. Although studied in detail at 65 K, products I and II were present after irradiation and study at 15 K. Slow warming of the crystal beyond 100 K led to gradual and irreversible decay of the signals from products I and II. No clear-cut evidence for new products appeared until the crystals were warmed beyond 250 K, where spectra from the C8 H adduct (product IV) appeared. At 65 K, signals from product III were present, but obscured by the more intense ones from products I and II. For that reason, product III was studied in detail at about 210 K. At 295 K, signals from product III decayed to nil in about 2 h.
The vibrationally averaged, temperature-dependent structure of polyatomic molecules. II. SO2
Authors: Mawhorter R. J. , Fink M.
Published: February 2003

The thermally averaged molecular structure parameters of gaseous SO2 have been measured by electron diffraction. Due to the anharmonicity of the molecular potentials and the resulting coordinate interactions in the potential the experimental parameters are temperature dependent. The data reveal this function. A comparison of our results with the theoretical ones of Kohl and Hilderbrandt show good agreement for the change of the structure values with temperature. As in the previous studies the measured mean square amplitudes of all atom pairs are consistently about 0.002 Å larger than the theories predict.
A systematic study of the convergence and additivity of correlation and basis set effects on the force constants of small molecules: HF, HCN, and NH3
Authors: Pulay Péter , Lee Jung-Goo , Boggs James E.
Published: February 2003

The quadratic and cubic force field, as well as the dominant quartic force constants of HF, HCN, and NH3 have been evaluated ab initio, using SCF, CI (singles and doubles) and cluster-corrected CI wave functions. Five different basis sets have been used, ranging in quality from small double-zeta to triple-zeta with two sets of polarization functions. Our main conclusions are as follows: (1) Transferability of the correlation contribution between different basis sets is quite poor; in particular, basis sets without polarization functions yield exaggerated correlation contributions. (2) Basis set and correlation effects on the force constants can be separated into an intrinsic and a geometry contribution, with the latter dominating for stretchings. Using an accurate reference geometry (e.g., from a high-quality calculation) in conjunction with a less accurate wave function for the evaluation of the force fields yields improved results. (3) Correlation contributions to cubic and quartic force constants, particularly to stretchings, are small. These quantities can, therefore, be evaluated at the less costly SCF level. (4) Double-zeta-plus polarization basis sets may have quite significant residual basis set errors at the SCF level; these errors were generally attributed to correlation effects in the past.
Semiempirical calculations of diamagnetic susceptibilities of acyl chlorides and alkyl β-chloro-vinyl ketones
Authors: Gupta R. R. , Kumar M. , Kishan
Published: February 2003

The diamagnetic susceptibilities of acyl chlorides and alkyl β-chloro-vinyl ketones have been calculated by using semiempirical method in which diamagnetic susceptibility is considered to be contributed by atoms, bonds, and bond–bond interactions. An excellent agreement is reported between experimental and semiempirically calculated diamagnetic susceptibility values irrespective of various structural parameters even in compounds containing polyfunctional groups.
Anharmonic coupling of vibrational modes in atom–polyatomic collisions: A time-correlation function treatment
Authors: Vilallonga Eduardo , Micha David A.
Published: February 2003

The role of molecular anharmonicity in hyperthermal collisions of atoms with polyatomic targets is investigated using a time-correlation function approach. By means of a cumulant expansion, the vibrational correlation is expressed in terms of displacement–displacement correlation functions, which are evaluated from the corresponding double-time Green functions; these in turn are obtained from their hierarchy of equations of motion. For a general intramolecular potential that contains anharmonic couplings between all the vibrational modes, the hierarchy is closed by means of a decoupling procedure that conserves the first few frequency moments of the spectral function. Consequently, the correlation functions can be evaluated analytically without need of internal state expansions, and they include the effects of anharmonic forces to infinite order. As examples, the targets CO2, N2O, and OCS are studied in detail, using spectroscopically derived vibrational potentials that contain quadratic through quartic terms; the roles of intra- and intermode anharmonic couplings are carefully considered. Differential cross sections for vibrational excitation calculated with the anharmonic potential are compared with those obtained from the harmonic model, for the systems Li+/CO2 and Li+/N2O at the collision energy E=4.72 eV. Results indicate that anharmonic forces make a small (15% to 25%) contribution to the vibrational energy transferred to these targets in hyperthermal collisions.
Supercooled molecules in molecular beams of ICl and IBr
Authors: Keil D. , Lübbert A. , Schügerl K.
Published: February 2003

The properties of molecular beams of ICl and IBr built by means of the seeded beam technique were investigated experimentally. The axial velocity distribution was measured in a pseudorandom time-of-flight experiment and the rotational state distribution was estimated from the focusing properties of the beam in an alternate gradient focusing system. From a comparison between the computer simulation of a focusing experiment and actual measured results, it can be stated that the population of the very lowest rotational states with rotational quantum numbers J=0, 1, 2 do not deviate significantly from a Boltzmann distribution. The rotational temperature is not very different from the translational temperature and has a value between 1 and 3 K. Collision numbers which are a measure of the translational rotational relaxation, are estimated from the experimental results.
The correlated pi-Hamiltonian of trans-butadiene as calculated by the ab initio effective valence shell Hamiltonian method: Comparison with semiempirical models
Authors: Lee Yoon S. , Freed Karl F. , Sun Hosung , Yeager Danny L.
Published: February 2003

The pi-electron Hamiltonian ℋπ of trans-butadiene is calculated using the effective Hamiltonian quasidegenerate many-body perturbation theory formalism to include ‘‘correlation’’ contributions. When four tight, valencelike orbitals are employed, the calculated ℋπ reproduces experimental and calculated (by configuration interaction) valence state excitation energies, but fails to obtain the diffuse Rydberg-like pi-electron states which interleave the valence spectrum. The addition of a pair of Rydberg molecular orbitals to the valence shell leads to an effective Hamiltonian ℋv which accurately describes both valence and Rydberg states simultaneously. These results provide an explanation as to why semiempirical pi-electron theories have required the use of different parameters, particularly resonance integrals, to calculate different properties. Our calculated ℋπ contains hybrid, exchange, and multicenter two-electron integrals which are customarily ignored to varying degrees in zero differential overlap (ZDO) approximation based methods. The ℋπ integrals often differ considerably from their ‘‘theoretical,’’ uncorrelated counterparts, and our ℋπ calculation provides further insight into these ZDO approximations. A more direct comparison is made with the traditional Pariser–Parr–Pople ℋPPP by averaging our ℋπ to generate a form ℋ̄π similar to ℋPPP. Certain aspects of ℋ̄π are very close quantitatively to ℋPPP, but there are other strong differences which are easily understood physically. The full ℋπ with all repulsion integrals and even effective three-electron interactions can now be used as a testing ground against which new types of semiempirical approximations may be tested.
Charge distributions and chemical effects. XXXIII. Valence and core region energies of atoms in Thomas–Fermi theory
Authors: Fliszár S.
Published: February 2003

A physically meaningful partitioning into valence and core regions of ground-state atoms can be achieved in Thomas–Fermi theory. The valence region energy Ev is uniquely defined provided the repulsion between the (Z-Nc) valence and Nc core electrons is one half the total electron–electron potential energy. It is found that Ev =(Z-Nc)E/14 Nc, where E=total energy of the atom with nucleus Z. Hartree–Fock densities indicate that Nc≂2 electrons for first-row atoms.
Measurements of mobilities and longitudinal diffusion coefficients by a flight-distance scanning method: Comparison with the flight-time scanning method
Authors: Iinuma K. , Takebe M. , Satoh Y. , Seto K.
Published: February 2003

Mobilities of Na+ ions in Ne, Ar, CH4, H2, and D2 have been measured at room temperature with gas pressures in the range 0.1–1.2 Torr by a flight-distance scanning method using a continuous guard ring. The zero-field reduced mobilities are found to be 7.93±0.2 for Ne, 3.02±0.07 for Ar, 2.59±0.06 for CH4, 11.92±0.3 for H2, and 8.81±0.2 cm2/V s for D2. These values have been compared with the mobility data measured by the authors in each ion–gas combination with the conventional flight-time scanning method. The agreement is considered quite good for all these combinations except for slight deviations at high E/N regions. Longitudinal diffusion coefficients have also been measured by the same method and compared with the values obtained by the arrival time spectra.
Site selective laser spectroscopy of defects in PbF2, a superionic conductor
Authors: Mho Sun-Il , Wright John C.
Published: February 2003

Site selective laser spectroscopy has been used to identify the defect sites in PbF2:Er3+ and follow their distribution as a function of dopant concentration and annealing temperature. Three sites labeled S1, S2, and S3 are found to correspond to single Er3+ ions with nearby F′i charge compensation, three sites labeled D1, D2, and D3 are found to correspond to clusters (dimer or higher) of Er3+ ions with nearby F′i charge compensation, and a cubic site labeled C where the Er3+ ion is compensated distantly. The cubic site is found to continually increase relative to S1 as the Er3+ concentration is raised. This behavior is contrary to that expected from simple mass action relationships, but is similar to that found in other fluorites. As the annealing temperature is raised, the cubic site increases markedly while the other sites decrease markedly over a small temperature range around the superionic transition temperature. This change is direct evidence for the sudden increase in charge screening effects which have been suggested as the driving force for the transition. It is suggested that the additional interactions caused by strain fields exaggerate the Debye–Hückel nonideality corrections and make the screening effects more effective.
The second virial coefficient of polymeric globules. A simple model
Authors: Rabin Yitzhak
Published: February 2003

A simple model for the second virial coefficient of polymeric globules is developed based on the assumption that the interpolymer attraction is driven by surface forces. The second virial coefficient is expressed as a series in the scaled temperature parameter N1/2 ‖ τ ‖. A discontinuity in the second virial coefficient across the transition is predicted.
Sorption in inhomogeneous slabs
Authors: Jhon M. S. , Frisch H. L.
Published: February 2003

We have introduced a projection operator technique to solve the sorption problem in nonhomogeneous slabs and give a general, but formal, expression for the space- and time-dependent activity. Furthermore, we provide an explicit calculation for the special case of slow spatial variation in k(r) and D(r) giving results for both the long- and short-time limits. For practical computations, we suggest a diagonal approximation to deal with complicated situations. This approximation is virtually exact for the long-time behavior of the system in contrast to the short-time based theory of the WKB approximation introduced by previous investigators.
Intramolecular excimer formation of oligostyrenes from dimer to tridecamer: The measurements of rate constants for excimer formation, singlet energy migration, and relaxation of internal rotation
Authors: Itagaki Hideyuki , Horie Kazuyuki , Mita Itaru , Washio Masakazu , Tagawa Seiichi , Tabata Yoneho
Published: February 2003

The emission properties of styrene oligomers from dimer to tridecamer have been investigated by picosecond pulse radiolysis at room temperature. The oligomers are precisely fractionated and are considered to be models of atactic polystyrene. The rate constant for excimer formation kDM of the order of 108 s−1, increases with increasing number of monomer units n for 2≤n≤8, then levels off at the value for polystyrene. The rate constant for singlet energy migration between adjacent chromophores in polystyrene was determined to be 3×1010 s−1 (∼30 ps). An average time of 7.2 ns was found to be necessary for two adjacent chromophores to assume the excimer conformation (g−t or tg− in a racemo dyad; tt in a meso dyad) from their initial equilibrium distribution of conformations. The present result is an indication that one may investigate the dynamics of internal rotation by photophysical techniques. Moreover, from a comparison of the results of photostationary measurements with those of transient methods, it is found that there is a discrepancy between them. This discrepancy is assumed to be caused by a molecular weight dependence either of the rate constant for the radiative deactivation from the excited monomer singlet kFM or of intramolecular self-quenching.
Low frequency dispersion in the electrophoretic mobility of DNA and mononucleosomes
Authors: Schmitz Kenneth S.
Published: February 2003

The technique of applying a sinusoidal electric field across the sample in the course of a quasielastic light scattering experiment (QELS-SEF) is similar in design to dielectric relaxation methods except the spectral density of the scattered light is monitored instead of the electrical properties of the system. The QELS-SEF technique was employed in a study of T7 DNA and mononucleosomes for driving frequencies of less than 100 Hz. These data suggest a dispersion in the electrophoretic mobility similar to that reported for dielectric relaxation on polynucleotides. These data are interpreted in terms of collective motions of the polyions as mediated by fluctuations in the distribution of small ions.
Implementation of the CCGM approximation for surface diffraction using Wigner R-matrix theory
Authors: Lauderdale Jack G. , McCurdy C. William
Published: February 2003

The CCGM approximation for surface scattering proposed by Cabrera, Celli, Goodman, and Manson [Surf. Sci. 19, 67 (1970)] is implemented for realistic surface interaction potentials using Wigner R-matrix theory. The resulting procedure is highly efficient computationally and is in no way limited to hard wall or purely repulsive potentials. Comparison is made with the results of close-coupling calculations of other workers which include the same diffraction channels in order to fairly evaluate the CCGM approximation which is an approximation to the coupled channels Lippman–Schwinger equation for the T matrix. The shapes of selective adsorption features, whether maxima or minima, in the scattered intensity are well represented in this approach for cases in which the surface corrugation is not too strong.
Measurement of vibration-rotation line strengths of HO using a tunable diode laser
Authors: Podolske James R. , Johnston Harold S.
Published: February 2003

Line strengths were measured for four transitions of the HO radical near 2.93 μm in the fundamental vibration-rotation band using a tunable infrared diode laser coupled to a molecular modulation spectrometer. The hydroxyl radical was produced by modulated photolysis of O3 at 2537 Å in the presence of H2O, and its number density obtained from numerical simulation of the complete photochemical system. The strongest line observed was at 3407.607 cm−1 with a measured line strength of 3.3±1.5×10−20 cm2 molecule−1 cm−1.
Chlorine K x-ray absorption spectra from SnCl2 and SnCl2 ⋅ 2H2O
Authors: Sugiura Chikara
Published: February 2003

The K absorption spectra of chlorine in SnCl2 and SnCl2 ⋅ 2H2O have been measured with a 50 cm bent quartz-crystal vacuum spectrograph. The spectra are quite similar to each other and have two intense absorption maxima near the K edge. The first maximum is ascribed to the transition to the lowest conduction band which results from the overlap of the 5p states of the Sn2+ ion and the 4p states of the Cl− ion, and the second maximum is attributed to the transition to the higher-lying p-like conduction band arising mainly from the 6p states of the Sn2+ ion. Furthermore, the spectrum of SnCl2 is compared with the chlorine K absorption spectrum of PbCl2 which is isomorphous with SnCl2, and a significant difference between them has been found for the absorption structures just above the K edge. This difference is explained in terms of the difference between the lowest conduction-band structures of SnCl2 and PbCl2.
Electron attachment to oxygen in supercritical hydrocarbon fluids
Authors: Nishikawa Masaru , Holroyd R. A.
Published: February 2003

The rate of attachment of electrons to oxygen was studied in methane, ethane, and propane gas above the critical temperatures up to pressures of ∼200 atm. For all three gases, the rate increases with density and levels off above N=3×1021 molecules/cm3. For ethane, there is little or no effect of temperature on the rate between 25 and 75 °C and the rate is independent of electric field (E) up to E/N=1.2×10−18 V cm2, but increases at higher values of E/N. To explain the results, it is suggested that the energetics of the attachment reaction change with density; i.e., the energy level of the electron in the media (V0) and the polarization energy (P−) of O2− change with N. The rate of attachment increases with the energy term ET=V0−P−+Ek, where Ek is the electron kinetic energy, similar to the way the attachment rate increases with electron kinetic energy in the dilute gas.