bluuebunny/crossref_metadata_2025 huggingface dataset:
Interaction of lead atom with atmospheric hydroxyl radical. An ab initio and density functional theory study of the resulting complexes PbOH and HPbO
Authors: Benjelloun Adil Touimi , Daoudi Abdelali , Chermette Henry
Published: October 2004

The two potential hypersurfaces A′1 (ground state) and A″3 (excited state) have been studied through ab initio and density functional theory (DFT) methods for the Pb(OH) complex. Two processes have been identified. The first one concerns the hydrogen inversion process in the coordination of PbOH and the second one the isomerization of PbOH into HPbO. Eight stationary points have been found; four of them correspond to the stable structures with symmetries PbOH(2A′), PbOH(2A″), HPbO(2A′), and HPbO(2Π), and four correspond to transition states [TS] with the symmetries Π2 A′2, Σ2 +, and A″2. The hydrogen inversion process in PbOH exhibits the so-called Renner-Teller effect with a rather low barrier, whereas the isomerization process PbOH→HPbO exhibits a rather high barrier. The energetic, structural, spectroscopy, and thermodynamics results obtained at various levels through, e.g., DFT with BLYP, B3LYP exchange-correlation functionals, coupled clusters methods, namely CCSD (single and double excitations) and CCSD(T) (with triple excitations, by perturbation) are presented for the whole sets of the stationary points and their dissociation products. The relativistic effects, as well as spin-orbit interaction, taken into account in the case of the BLYP exchange–correlation functional, have been estimated and discussed in order to measure their importance in the case of system including heavy metals such as Pb. Reactions of lead (Pb) with oxidizing atmospheric molecules (OH, HO2, O2, and O3) have been studied at various levels of approximation in order to study the possible existence of PbOH in the atmosphere.
Characterization of the hyperline of D1/D conical intersections between the maleic acid and fumaric acid anion radicals
Authors: Takahashi Ohgi , Sumita Masato
Published: October 2004

The cation and anion radicals of symmetrical 1,2-disubstituted ethylenes are expected to have a symmetry-allowed conical intersection (CI) between the ground doublet state (D0) and the lowest excited doublet state (D1) near a 90°-twisted geometry. By the complete active space self-consistent field method, we characterized the hyperline formed by D1/D0 CIs between the anion radicals of maleic acid (cis) and fumaric acid (trans). An implication of the results for the known one-way cis→trans photoisomerization of the maleic acid anion radical and other related ion radicals is presented.
Studies of energetic electrons with space and time resolution in Mo and W X-pinches from measurements of x rays >9 keV
Authors: Kantsyrev V. L. , Fedin D. A. , Shlyaptseva A. S. , Mitchell M. D. , Song Byungmoo , Pikuz S. A. , Shelkovenko T. A. , Chandler K. M. , Hammer D. A. , Maxson L. M.
Published: October 2004

Electron beams in 400 kA peak current Mo and W X-pinches have been studied using 1 ns time-resolution Si diodes to monitor x rays >9 keV. Softer x rays were monitored by photoconducting detectors (PCDs). Three different types of higher energy x-ray bursts were observed. The first type appears to be produced by electrons generated starting at the moment of, or immediately after, the first thermal x-ray burst (typically 40–50 ns after the current start), and Si detector signals last 1–2 ns. The second type of harder x-ray burst occurs 50–80 ns after the current start, lasts 2.5–10 ns, and is typically not correlated with a thermal x-ray burst. These two types of bursts were generated near the cross-wire region. The third type of x-ray burst occurs 70–100 ns after the current start, and is also uncorrelated with PCD signals. The energetic electrons responsible for these x rays are generated for 10–30 ns, and the radiation is produced in the anode region.
Temporal and spectral deconvolution of data from diamond, photoconductive devices
Authors: Knauer J. P. , Gindele N. C.
Published: October 2004

Diamond, photoconductive devices (DPCDs) have proven to be stable detectors of x-ray emission from plasmas. An array of six DPCDs is used on the OMEGA target chamber to measure the temporal and spectral dependence of x rays emitted from targets. 100 ps laser pulses incident on high-Z targets are used to measure the time response of the detectors, bias circuits, cables, and digitizer. This information is used to deconvolve the measured data to extract the temporal shape of the x-ray emission. The x-ray spectrum is measured by placing absorber filters for a specific x-ray energy region in front of the DPCDs. The data are analyzed with a linear algebraic method to determine the spectral shape. This technique removes the need to apply smoothing to the spectral analysis and provides “confidence bands” for the x-ray spectrum.
Accurate prediction of heat of formation by combining Hartree–Fock/density functional theory calculation with linear regression correction approach
Authors: Duan Xue-Mei , Song Guo-Liang , Li Zhen-Hua , Wang Xiu-Jun , Chen Guan-Hua , Fan Kang-Nian
Published: October 2004

A linear regression correction approach has been developed successfully to account for the electron correlation energy missing in Hartree–Fock calculation and to reduce the calculation errors of density functional theory. The numbers of lone-pair electrons, bonding electrons and inner layer electrons in molecules, and the number of unpaired electrons in the composing atoms in their ground states are chosen to be the most important physical descriptors to determine the correlation energy unaccounted by Hartree–Fock method or to improve the results calculated by B3LYP density functional theory method. As a demonstration, this proposed linear regression correction approach has been applied to evaluate the standard heats of formation ΔHfΘ of 180 small-sized to medium-sized organic molecules at 298.15 K. Upon correction, the mean absolute deviation for the 150 molecules in the training set decreases from 351.0 to 4.6 kcal/mol and 360.9 to 4.6 kcal/mol for HF/6-31G(d) and HF/6-311+G(d,p) methods, respectively. For B3LYP method, the mean absolute deviations are reduced from 9.2 and 18.2 kcal/mol to 2.7 and 2.4 kcal/mol for 6-31G(d) and 6-311+G(d,p) basis sets, respectively.
A refined model of the double exchange phenomenon: Test on the stretched N2+ molecule
Authors: Taratiel David , Guihéry Nathalie
Published: October 2004

The N2+ molecule is studied at different interatomic distances as a model molecule for the double exchange mechanism. The energy spectrum as well as the wave functions of the lowest states are analyzed and confronted both with the usual model of double exchange and with a recently proposed refined model. It is shown that the usual model fails to reproduce the energy spacings while the refined model is valid on a large domain of interatomic distances (in the magnetic regime). The study of a model molecule on a large domain of interatomic distances makes it possible to systematically investigate several regimes associated with different energetic state orderings. The perfect agreement between the refined model and the computed energies in the whole domain of stretched distances shows its applicability to a large number of real compounds. Finally, the respective contributions of dynamical and nondynamical correlations are analyzed.
Local order and dynamics in supercooled water: A study by IR spectroscopy and molecular dynamic simulations
Authors: Zasetsky Alexander Yu. , Khalizov Alexei F. , Sloan James J.
Published: October 2004

Micron-sized water droplets in a cryogenic flow tube were probed by IR spectroscopy. The analysis of the IR spectra suggests that there is a relative increase of about 30% in the fraction, fL, of low density domains in water on cooling over the temperature range between 300 and 240 K. The results derived from the experiments agree qualitatively with those of molecular dynamics (MD) simulations in terms of the increase in the fL values. The MD simulations show that the intensities of the mode at about 100 cm−1 for the molecules in the low density domains are reduced in comparison to the average, while the intensities and frequencies of the librational mode at 600 cm−1 are increased. Furthermore, the reorientations (dielectric relaxation times) in these domains are found to be somewhat slower, pointing to the fact that these low density “cages” live longer than the average local molecular environments in supercooled water.
Relativistic effects on nuclear magnetic shielding constants in HX and CH3X (X=Br,I) based on the linear response within the elimination of small component approach
Authors: Melo J. I. , Ruiz de Azua M. C. , Giribet C. G. , Aucar G. A. , Provasi P. F.
Published: October 2004

Numerical calculations of relativistic effects on nuclear magnetic shielding constants σ corresponding to all one-body operators obtained within a formalism developed in previous work [J. I. Melo et al., J. Chem. Phys. 118, 471 (2003)] were carried out. In this formalism, the elimination of small component scheme is applied to evaluate all quantities entering a four-component RSPT(2) expression of magnetic molecular properties. HX and CH3X (X=Br,I) were taken as model compounds. Calculations were carried out at the Hartree-Fock level for first-order quantities, and at the random-phase approximation (RPA) level for second- and third-order ones. It was found that values of σ(X) are largely affected by several relativistic corrections not previously considered in the bibliography. σ values of the H nucleus are in close agreement with four-component RPA ones. Overall relativistic effects on the shift of σ(X) from HX to CH3X are smaller than the nonrelativistic shifts.
Molecular dynamics simulations of the d(CCAACGTTGG)2 decamer in crystal environment: Comparison of atomic point-charge, extra-point, and polarizable force fields
Authors: Baucom Jason , Transue Thomas , Fuentes-Cabrera Miguel , Krahn J. M. , Darden Thomas A. , Sagui Celeste
Published: October 2004

Molecular dynamics simulations of the DNA duplex d(CCAACGTTGG)2 were used to study the relationship between DNA sequence and structure in a crystal environment. Three different force fields were used: a traditional description based on atomic point charges, a polarizable force field, and an “extra-point” force field (with additional charges on extranuclear sites). It is found that all the force fields reproduce fairly well the sequence-dependent features of the experimental structure. The polarizable force field, however, provides the most accurate representation of the crystal structure and the sequence-dependent effects observed in the experiment. These results point out to the need of the inclusion of polarization for accurate descriptions of DNA.
Liquid-gas coexistence and critical point shifts in size-disperse fluids
Authors: Wilding Nigel B. , Fasolo Moreno , Sollich Peter
Published: October 2004

Specialized Monte Carlo simulations and the moment free energy (MFE) method are employed to study liquid-gas phase equilibria in size-disperse fluids. The investigation is made subject to the constraint of fixed polydispersity, i.e., the form of the “parent” density distribution ρ0(σ) of the particle diameters σ, is prescribed. This is the experimentally realistic scenario for, e.g., colloidal dispersions. The simulations are used to obtain the cloud and shadow curve properties of a Lennard-Jones fluid having diameters distributed according to a Schulz form with a large (δ≈40%) degree of polydispersity. Good qualitative accord is found with the results from a MFE method study of a corresponding van der Waals model that incorporates size dispersity both in the hard core reference and the attractive parts of the free energy. The results show that polydispersity engenders considerable broadening of the coexistence region between the cloud curves. The principal effect of fractionation in this region is a common overall scaling of the particle sizes and typical interparticle distances, and we discuss why this effect is rather specific to systems with Schulz diameter distributions. Next, by studying a family of such systems with distributions of various widths, we estimate the dependence of the critical point parameters on δ. In contrast to a previous theoretical prediction, size dispersity is found to raise the critical temperature above its monodisperse value. Unusually for a polydisperse system, the critical point is found to lie at or very close to the extremum of the coexistence region in all cases. We outline an argument showing that such behavior will occur whenever polydispersity affects only the range, rather than the strength of the interparticle interactions.
A closed-shell coupled-cluster treatment of the Breit–Pauli first-order relativistic energy correction
Authors: Coriani Sonia , Helgaker Trygve , Jørgensen Poul , Klopper Wim
Published: October 2004

First-order relativistic corrections to the energy of closed-shell molecular systems are calculated, using all terms in the two-component Breit–Pauli Hamiltonian. In particular, we present the first implementation of the two-electron Breit orbit–orbit integrals, thus completing the first-order relativistic corrections within the two-component Pauli approximation. Calculations of these corrections are presented for a series of small and light molecules, at the Hartree–Fock and coupled-cluster levels of theory. Comparisons with four-component Dirac–Coulomb–Breit calculations demonstrate that the full Breit–Pauli energy corrections represent an accurate approximation to a fully relativistic treatment of such systems. The Breit interaction is dominated by the spin–spin interaction, the orbit–orbit interaction contributing only about 10% to the total two-electron relativistic correction in molecules consisting of light atoms. However, the relative importance of the orbit–orbit interaction increases with increasing nuclear charge, contributing more than 20% in H2S.
Ab initio and kinetic calculations for the reactions of Cl with SiHnCl4−n (n=1,2,3,4)
Authors: Pei Kemei , Li Haiyang
Published: October 2004

The direct hydrogen abstraction reactions of Cl atom with SiHnCl4−n (n=1,2,3,4) have been studied systematically using ab initio molecular orbital theory. Geometries have been optimized at the MP2 level with 6-311+G(d) basis set, QCISD(T)/6-311+G(d,p) has been used in the final single point energy calculation. The kinetic calculations of these reactions have been explored using the canonical variational transition (CVT) state theory method with small-curvature tunneling (SCT) effect correction over the temperature range of 200–2000 K. The CVT/SCT rate constants exhibit typical non-Arrhenius behavior and three-parameter rate-temperature formulas have been fitted for the reactions of Cl with SiH4, SiH3Cl, SiH2Cl2, and SiHCl3, respectively (in unit of cm3 molecule−1 s−1). The calculated CVT/SCT rate constants are in agreement with the available experimental values.
Periodic table of 3d-metal dimers and their ions
Authors: Gutsev G. L. , Mochena M. D. , Jena P. , Bauschlicher C. W. , Partridge H.
Published: October 2004

The ground states of the mixed 3d-metal dimers TiV, TiCr, TiMn, TiFe, TiCo, TiNi, TiCu, TiZn, VCr, VMn, VFe, VCo, VNi, VCu, VZn, CrMn, CrFe, CrCo, CrNi, CrCu, CrZn, MnFe, MnCo, MnNi, MnCu, MnZn, FeCo, FeNi, FeCu, FeZn, CoNi, CoCu, CoZn, NiCu, NiZn, and CuZn along with their singly negatively and positively charged ions are assigned based on the results of computations using density functional theory with generalized gradient approximation for the exchange-correlation functional. Except for TiCo and CrMn, our assignment agrees with experiment. Computed spectroscopic constants (re,ωe,Do) are in fair agreement with experiment. The ground-state spin multiplicities of all the ions are found to differ from the spin multiplicities of the corresponding neutral parents by ±1. Except for TiV, MnFe, and MnCu, the number of unpaired electrons, N, in a neutral ground-state dimer is either N1+N2 or |N1−N2|, where N1 and N2 are the numbers of unpaired 3d electrons in the 3dn4s1 occupation of the constituent atoms. Combining the present and previous results obtained at the same level of theory for homonuclear [Gutsev and Bauschlicher, J. Phys. Chem. A 107, 4755 (2003)] 3d-metal and ScX (X=Ti–Zn) dimers [Gutsev, Bauschlicher, and Andrews, in Theoretical Prospects of Negative Ions, edited by J. Kalcher (Research Signpost, Trivandrum, 2002), pp. 43–60] allows one to construct “periodic” tables of all 3d-metal dimers along with their singly charged ions.
Investigation of the radiation properties of L- and M-shell X-pinch plasma x-ray sources using a transmission grating spectrometer
Authors: Fedin D. A. , Kantsyrev V. L. , Shlyaptseva A. S. , Mitchell M. D. , Song B. M. , Pikuz S. A. , Shelkovenko T. A. , Chandler K. M. , Hammer D. A. , Maxson L. M.
Published: October 2004

A transmission grating spectrometer (TGS) with one dimensional spatial resolution was used to investigate the radiative properties of X-pinch plasmas produced using wires made of NiCr and CoNiCr alloys, and Mo. Experiments were carried out on the Cornell XP machine. The TGS was used to estimate of the energy yields of L-shell Cr and Ni, and M-shell Mo radiation. The absolute energy yields in the wavelength range 10<λ<15 Å were 1–5% of the total energy delivered to the load (10–40 J for L-shell NiCr, and 20–50 J for M-shell Mo). An analysis of the spatial structure of X-pinch emission regions in different wavelength ranges shows that the source structure is changing with wavelength and the size varies from less than 0.1 up to 1 mm. For example, in one Mo X-pinch test shown, three separate soft x-ray sources radiating in the wavelength range λ<42 Å can be seen, but only one of those sources radiates in the wavelength range λ>42 Å. The results are compared with data obtained previously with the TGS on a 1 MA pulse powered machine Zebra.
X-ray line measurements with high efficiency Bragg crystals
Authors: Pak A. , Gregori G. , Knight J. , Campbell K. , Price D. , Hammel B. , Landen O. L. , Glenzer S. H.
Published: October 2004

We have studied the focusing properties of two highly oriented pyrolitic graphite (HOPG) spectrometers, which differ in the degree of the mosaic spread: ZYA with a low mosaic spread (γ=0.4°) and ZYH with a large mosaic spread (γ=3.5°). In order to asses the crystal performance for a variety of different experiments, various Kα and Kβ x-ray lines have been produced using a high-intensity (≳1017 W/cm2) short-pulse (∼100 fs) laser beam focused onto Ti, V, Zn, and Cu foils. The measured spectral resolution of the HOPG crystals in both first and second order diffraction has been compared with theoretical predictions. Using known values for the peak reflectivity of HOPG crystals, we have also computed Kα x-ray conversion efficiencies of Ti, V, Zn, and Cu. These results are important to estimate the optimal conditions under which different types of HOPG monochromators can be used for the detection of weak x-ray signals as the one encountered in x-ray Thomson/Compton scattering experiments.
The effect of kinematic parameters on inelastic scattering of glyoxal
Authors: Duca Mariana D.
Published: October 2004

The effect of kinematic parameters (relative velocity vrel, relative momentum prel, and relative energy Erel) on the rotational and rovibrational inelastic scatterings of 00K0S1 trans-glyoxal has been investigated by colliding glyoxal seeded in He or Ar with target gases D2, He, or Ne at different scattering angles in crossed supersonic beams. The inelastic spectra for target gases He and D2 acquired with two different sets of kinematic parameters revealed no significant differences. This result shows that kinematic factors have the major influence in the inelastic scattering channel competition whereas the intermolecular potential energy surface plays only a secondary role. The well-defined exponential dependence of relative cross sections on exchanged angular momentum identifies angular momentum as the dominant kinematic factor in collision-induced rotationally and rovibrationally inelastic scatterings. This is supported by the behavior of the relative inelastic cross sections data in a “slope-prel” representation. In this form, the data show a trend nearly independent of the target gas identity. Representations involving Erel and vrel show trends specific to the target gas.
Anisotropic fluctuations in turbulent shear flows
Authors: Jacob Boris , Biferale Luca , Iuso Gaetano , Casciola Carlo Massimo
Published: October 2004

An experimental analysis of small-scale anisotropic fluctuations has been performed in two different flows in order to assess possible universal features of shear dominated turbulence, such as the decay rate of anisotropy at small scales and the recovery of isotropy. The two data sets—a homogeneous shear flow and a turbulent boundary layer—are characterized by turbulent Reynolds numbers in the range Reλ=230−330. The measurements in the logarithmic region of the turbulent boundary layer extend from y+=90 up to y+=350. The explicit use of the Taylor hypothesis is avoided by resorting to two-point measurements, with separation in the spanwise direction, z. Streamwise (u) and wall-normal (v) velocity components are simultaneously measured in both positions by means of cross-wire probes. This procedure allows the construction of the mixed transverse structure functions, here addressed up to order 6. According to the SO(3) formalism, the observables we selected are free of any isotropic contribution, thus allowing a direct measurement of the fully anisotropic component of the field. The results are consistent with the “exponent-only” scenario for universality, i.e., all experimental data can be fitted by fixing the same set of anisotropic scaling exponents by changing only the prefactors, for different shear intensities and boundary conditions.
Optical spectroscopy of RuC: 18 000–24 000 cm−1
Authors: Lindholm Ned F. , Hales David A. , Ober Linnea A. , Morse Michael D.
Published: October 2004

The optical spectrum of diatomic RuC has been recorded from 17 800 to 24 200 cm−1. Three previously unidentified excited electronic states were analyzed and identified as having Ω′=0, Ω′=2, and Ω′=3. The Ω′=3 state was determined to be a Δ33 state that is suggested to arise from a mixture of the 10σ211σ25π32δ312σ16π1 and 10σ211σ15π32δ312σ26π1 electronic configurations. Three additional bands belonging to the previously observed [18.1] 1Π←X 1Σ+ system were analyzed to obtain Be′=0.558 244(48) cm−1, αe′=0.004 655(27) cm−1, ωe′=887.201(37) cm−1, and ωe′xe′=5.589(7) cm−1 for the Ru102 12C isotopomer (1σ error limits). A Rydberg-Klein-Rees analysis was then performed using the determined spectroscopic constants of the [18.1] 1Π state, and similar analyses were performed for the previously observed states. The resulting potential energy curves are provided for the Ru100 12C, Ru101 12C, Ru102 12C, and Ru104 12C isotopic species.
Structure and phase behavior of polyelectrolyte star solutions
Authors: Hoffmann Norman , Likos Christos N. , Löwen Hartmut
Published: October 2004

Using the recently developed effective interaction potentials between polyelectrolyte stars, we examine the structure and phase behavior of solutions of the same. The effective interaction is ultrasoft and density dependent, owing to the integration of the counterionic degrees of freedom. The latter contribute extensive volume terms that must be taken into account in drawing the phase diagram of the system. The structural behavior of the uniform fluid is characterized by anomalous structure factors, akin to those found previously for solutions of uncharged star polymers. The phase diagram of the system is very rich, featuring a fluid phase at low arm numbers of the stars, two reentrant melting regions, as well as a variety of crystal structures with unusual symmetry. The physical origin of these features can be traced back to the ultrasoft nature of the effective interaction potential.
Effect of macromolecular polymer structures on drag reduction in a turbulent channel flow
Authors: Kim K. , Islam M. T. , Shen X. , Sirviente A. I. , Solomon M. J.
Published: October 2004

This paper presents the influence of injected polymer solutions on turbulence in fully developed channel flows. In particular, it investigates the impact of concentration and mixing of the polymer solution on drag reduction. It is observed, via flow visualization and birefringence measurements, that for large injection concentrations macromolecular polymer structures exist in the flow. They are found to be mostly located in the neighborhood of the channel centerline. Laser Doppler velocimetry was used to characterize the mean and turbulent flow with and without the presence of macromolecular polymer structures.
Multiresolution quantum chemistry in multiwavelet bases: Hartree–Fock exchange
Authors: Yanai Takeshi , Fann George I. , Gan Zhenting , Harrison Robert J. , Beylkin Gregory
Published: October 2004

In a previous study [R. J. Harrison et al., J. Chem. Phys. (in press)] we reported an efficient, accurate multiresolution solver for the Kohn–Sham self-consisitent field (KS-SCF) method for general polyatomic molecules. This study presents an efficient numerical algorithm to evalute Hartree–Fock (HF) exchange in the multiresolution SCF method to solve the HF equations. The algorithm employs fast integral convolution with the Poission kernel in the nonstandard form, screening the sparse multiwavelet representation to compute results of the integral operator only where required by the nonlocal exchange operator. Localized molecular obitals are used to attain near linear scaling. Results for atoms and molecules demonstrate reliable precision and speed. Calculations for small water clusters demonstrate a total cost to compute the HF exchange potential for all nocc occpuied MOs scaling as O(nocc1.5).
Single molecule conductivity: The role of junction-orbital degeneracy in the artificially high currents predicted by ab initio approaches
Authors: Solomon Gemma C. , Reimers Jeffrey R. , Hush Noel S.
Published: October 2004

A priori evaluations, using Hartree–Fock self-consistent-field (SCF) theory or density-functional theory (DFT), of the current passing between two electrodes through a single bridging molecule result in predicted conductivities that may be up to one to two orders of magnitude larger than observed ones. We demonstrate that this is, in part, often due to the improper application of the computational methods. Conductivity is shown to arise from tunneling between junction states of the electrodes through the molecule; these states are inherently either quasi two-fold or four-fold degenerate and always comprise the (highest occupied molecular orbital) HOMO band at the Fermi energy of the system. Frequently, in previous cluster based molecular conduction calculations, closed-shell SCF or Kohn–Sham DFT methods have been applied to systems that we demonstrate to be intrinsically open shell in nature. Such calculations are shown to induce artificial HOMO-LUMO (LUMO—lowest unoccupied molecular orbital) band splittings that Landauer-based formalisms for steady-state conduction interpret as arising from extremely rapid through-molecule tunneling at the Fermi energy, hence, overestimating the low-voltage conductivity. It is demonstrated that these shortcomings can be eliminated, dramatically reducing calculated current magnitudes, through the alternate use of electronic-structure calculations based on the spin-restricted open-shell formalism and related multiconfigurational SCF of DFT approaches. Further, we demonstrate that most anomalies arising in DFT implementations arise through the use of hybrid density functionals such as B3LYP. While the enhanced band-gap properties of these functionals have made them the defacto standard in molecular conductivity calculations, we demonstrate that it also makes them particularly susceptible to open-shell anomalies.
Complete structural and magnetic characterization of biological radicals in solution by an integrated quantum mechanical approach: Glycyl radical as a case study
Authors: Ciofini Ilaria , Adamo Carlo , Barone Vincenzo
Published: October 2004

An integrated quantum mechanical approach for the structural and magnetic characterization of flexible free radicals in solution has been applied to a model of the glycyl radical engaged in peptidic chains. The hyperfine couplings computed using hybrid density functionals and purposely tailored basis sets are in good agreement with experiment when vibrational averaging effects from low frequency motions and solvent effects (both direct H bonding and bulk) are taken into the proper account. The g tensor shows a smaller dependence on the specific form of the density functional, the extension of the basis set over a standard double-ζ+polarization level, vibrational averaging, and bulk solvent effects. However, hydrogen bridges with solvent molecules belonging to the first solvation shell play a significant role. Together with their intrinsic interest, our results show that a comprehensive and reliable computational approach is becoming available for the complete characterization of open-shell systems of biological interest in their natural environment.
A semiclassical study of wave packet dynamics in anharmonic potentials
Authors: Yang Shilong , Cao Jianshu , Field Robert W.
Published: October 2004

Classical and semiclassical methods are developed to calculate and invert the wave packet motion measured in pump-probe experiments. With classical propagation of the Wigner distribution of the initial wave packet created by the pump pulse, we predict the approximate probe signal with slightly displaced recurrence peaks, and derive a set of first-order canonical perturbation expressions to relate the temporal features of the signal to the characteristics of the potential surface. A reduced dynamics scheme based on the Gaussian assumption leads to the correct center of mass motion but does not describe the evolution of the shape of the wave packet accurately. To incorporate the quantum interference into classical trajectories, we propose a final-value representation semiclassical method, specifically designed for the purpose of computing pump-probe signals, and demonstrate its efficiency and accuracy with a Morse oscillator and two kinetically coupled Morse oscillators. For the case of one-color pump probe, a simple phase-space quantization scheme is devised to reproduce the temporal profile at the left-turning point without actual wave packet propagation, revealing a quantum mechanical perspective of the nearly classical pump-probe signal.
Calculation of infrared absorption spectra using Gaussian variational wave packets
Authors: Buch Victoria
Published: October 2004

Formulas are developed for calculation of infrared absorption spectra with the help of Gaussian variational wave packets. The excitation is viewed as occurring to a linear combination of basis states corresponding to a Gaussian multiplied by Hermite polynomials of coordinates. Application of the formulas is examplified on HDO isolated in a D2O matrix. A simple approximation is proposed for the fundamental transitions.
Gas phase detection of cyclic B3: 2 2E′←X 2A1′ electronic origin band
Authors: Cias P. , Araki M. , Denisov A. , Maier J. P.
Published: October 2004

The rotationally resolved origin band in the 2 2E′←X 2A1′ electronic spectrum of cyclic B3 has been observed by cavity ring down spectroscopy in the gas phase. The B3 molecule was generated in a supersonic planar plasma containing decaborane (B10H14) and neon as a carrier gas. The rotational structure pattern is that of a cyclic molecule. It is analyzed assuming an equilateral triangle in both electronic states. The band origin is determined to be 21 853.52 cm−1, and the bond lengths 1.603 77(106) Å in the ground and 1.619 07(96) Å in the excited electronic state are inferred from analysis of the rotational structure.
New Rydberg-Rydberg transitions in N2. Identification of the d3 1Σg+ state
Authors: Cossart D. , Cossart-Magos C.
Published: October 2004

Use of a special Penning-ionization source allowed Fourier-transform recording of two previously nonobserved IR emission bands spectra at 5480 and 7630 cm−1 arising from neutral N2. The first of these bands is the c4′ 1Σu+-a″ 1Σg+ (0,0) transition, both states involved being previously known by direct vacuum (UV) absorption spectroscopy. The second band corresponds to a d3 1Σg+-c4′ 1Σu+ (0,0) transition, in which the upper level belongs to an up to now unidentified Rydberg state. Both the upper and lower levels are perturbed by neighboring valence state levels.
Evidence for the coexistence of two different mechanisms for the occurrence of anti-Kasha S2 (1 1Bu) fluorescence from α,ω-diphenylpolyenes
Authors: Itoh Takao
Published: October 2004

Fluorescence, fluorescence excitation, and absorption spectra have been measured for diphenylpolyenes with the polyene double bond number (n) from three to seven in solvents with different polarizabilities at room temperature. These polyenes exhibit anti-Kasha S2 (1 1Bu) fluorescence beside conventional S1 (2 1Ag) fluorescence in room temperature solution. It is shown that there are two different mechanisms for occurrence of the S2 (1 1Bu) fluorescence of diphenylpolyenes. S2 fluorescence occurs through the thermal population of the S1 state for the shorter diphenylpolyenes (n=3 and 4), while it occurs as prompt S2 fluorescence due to inefficient S2→S1 internal conversion for the longer diphenylpolyenes (n=6 and 7). With diphenyldecapentaene (n=5), S2 fluorescence based on both mechanisms is observed depending on the polarizability of the solvent used. The present results provide deeper insight into the mechanism for the occurrence of anti-Kasha S2 fluorescence of polyenes.
Insights into dynamics of the S2 state of thiophosgene from ab initio calculations
Authors: Zhang Feng , Lin Ling , Fang Wei-Hai
Published: October 2004

The S2 potential energy surface for Cl2CS dissociation has been characterized with a combined complete active space self-consistent field and multireference configuration interaction method. The S3/S2 minimum-energy intersection has been determined with the state-averaged complete active space self-consistent field method. The S2 direct dissociation was found to have a barrier of 6.0 kcal/mol, leading to formation of Cl(X̃2P)+ClCS(Ã2A″) in the excited electronic state. Dynamics of the S2 state of Cl2CS can be summarized as follows: (1) The S2–S0 fluorescence occurs with high quantum yield at low excess energies; (2) Both the S2 dissociation and the S2→S3 internal conversion cause the loss of the S2–S0 fluorescence upon photoexcitation at 235–253 nm; (3) The S2→S3 internal conversion (IC) followed by the direct IC to the ground electronic state results in the fragments produced in the ground state, while the S2 dissociation leads to formation of the fragments in excited electronic states.
Rescaled density expansions and demixing in hard-sphere binary mixtures
Authors: López de Haro M. , Tejero C. F.
Published: October 2004

The demixing transition of a binary fluid mixture of additive hard spheres is analyzed for different size asymmetries by starting from the exact low-density expansion of the pressure. Already within the second virial approximation the fluid separates into two phases of different composition with a lower consolute critical point. By successively incorporating the third, fourth, and fifth virial coefficients, the critical consolute point moves to higher values of the pressure and to lower values of the partial number fraction of the large spheres. When the exact low-density expansion of the pressure is rescaled to higher densities as in the Percus-Yevick theory, by adding more exact virial coefficients a different qualitative movement of the critical consolute point in the phase diagram is found. It is argued that the Percus-Yevick factor appearing in many empirical equations of state for the mixture has a deep influence on the location of the critical consolute point, so that the resulting phase diagram for a prescribed equation has to be taken with caution.
Laser-induced fluorescence of cyclohexadienyl (c-C6H7) radical in the gas phase
Authors: Imamura Takashi , Zhang Weijun , Horiuchi Hiroaki , Hiratsuka Hiroshi , Kudo Takako , Obi Kinichi
Published: October 2004

A laser-induced fluorescence spectrum was observed in the 500–560 nm region when a mixture of 1,4-cyclohexadiene and oxalyl chloride was photolyzed at 193 nm. The observed excitation spectrum was assigned to the Ã 2A2←X̃ 2B1 transition of the cyclohexadienyl radical c-C6H7, produced by abstraction of a hydrogen atom from 1,4-cyclohexadiene by Cl atoms. The origin of the Ã←X̃ transition of c-C6H7 was at 18 207 cm−1. From measurements of the dispersed fluorescence spectra and ab initio calculations, the frequencies of several vibrational modes in both the ground and excited states of c-C6H7 were determined: ν5(C–H in-plane bend)=1571, ν8(C–Hin-plane bend)=1174, ν10(C–C–C in-plane bend)=981, ν12(C–C–C in-plane bend)=559, ν16(C–C–C out-of-plane bend)=375, and ν33(C–C–C in-plane bend)=600 cm−1 for the ground state and ν8=1118, ν10=967, ν12=502, ν16=172, and ν33=536 cm−1 for the excited states.
High resolution infrared spectra of helium clusters seeded with isotopic carbon monoxide, HeN-13C 16O and HeN-12C 18O
Authors: McKellar A. R. W.
Published: October 2004

Infrared spectra of isotopically substituted HeN-CO clusters (1<N<19) have been studied in order to extend the original results on the normal isotope [J. Tang and A. R. W. McKellar, J. Chem. Phys. 119, 754 (2003)]. The same two series of R(0) transitions were observed, correlating with the a- and b-type transitions of He1-CO, with only small shifts in relative position. The previously obscured a-type line for He6-CO was detected. Examination of the small shifts among isotopomers showed remarkably smooth behavior, except in the “unstable” regions around N=7 (b-type series) and 15 (a-type series). The overall results firmly support the assignments and analysis given for the normal isotope.
On the performance of molecular polarization methods. I. Water and carbon tetrachloride close to a point charge
Authors: Masia Marco , Probst Michael , Rey Rossend
Published: October 2004

The three main methods to implement molecular polarization (point dipoles, fluctuating charges, and shell model) are tested against high level ab initio calculations for a molecule (water, carbon tetrachloride) close to a point charge (at the distance of a lithium or magnesium ion). The goal is to check whether an approximation (linear polarization) strictly valid at large intermolecular distances is sufficiently accurate for liquid state molecular dynamics simulations, where strong polarization effects are to be expected at short separations. The monitored observable is the molecular dipole moment as a function of the charge-molecule distance for selected molecular orientations. Analytic formulas are derived for the components of the molecular polarization tensor, facilitating the optimization of the performance for each polarization method as a function of its underlying parameters. Overall, the methods studied provide a remarkably good representation of the induced dipole, with no divergences appearing even at the shortest distances. For water close to a monovalent point charge the point dipole model, implemented with one or three dipoles, accurately reproduces the water dipole moment at all distances. Deficiencies appear as the molecular polarizability and/or charge increase: basically, the ab initio induced moments grow faster at intermediate distances than the linear increase characteristic of the phenomenological polarization methods, suggesting that nonlinear effects (hyperpolarizability) cannot be neglected in these cases. Regarding the capabilities of each method, the point dipole method is the one that performs best overall, with the shell model achieving acceptable results in most instances. The fluctuating charge method shows some noticeable limitations for implementations of comparable complexity (in terms of the number of sites required).
Two-component relativistic methods for the heaviest elements
Authors: Kȩdziera Dariusz , Barysz Maria
Published: October 2004

Different generalized Douglas-Kroll transformed Hamiltonians (DKn, n=1, 2,…,5) proposed recently by Hess et al. are investigated with respect to their performance in calculations of the spin–orbit splittings. The results are compared with those obtained in the exact infinite-order two-component (IOTC) formalism which is fully equivalent to the four-component Dirac approach. This is a comprehensive investigation of the ability of approximate DKn methods to correctly predict the spin–orbit splittings. On comparing the DKn results with the IOTC (Dirac) data one finds that the calculated spin–orbit splittings are systematically improved with the increasing order of the DK approximation. However, even the highest-order approximate two-component DK5 scheme shows certain deficiencies with respect to the treatment of the spin–orbit coupling terms in very heavy systems. The meaning of the removal of the spin-dependent terms in the so-called spin-free (scalar) relativistic methods for many-electron systems is discussed and a computational investigation of the performance of the spin-free DKn and IOTC methods for many-electron Hamiltonians is carried out. It is argued that the spin-free IOTC rather than the Dirac-Coulomb results give the appropriate reference for other spin-free schemes which are based on approximate two-component Hamiltonians. This is illustrated by calculations of spin-free DKn and IOTC total energies, r−1 expectation values, ionization potentials, and electron affinities of heavy atomic systems.
Smooth transition from metastability to instability in phase separating protein solutions
Authors: Shah Mrinal , Galkin Oleg , Vekilov Peter G.
Published: October 2004

For insight into the structure and dynamics of phases emerging upon crossing the metastability/instability boundary we monitor with optical microscopy, in real time and in real space, the generation of a dense liquid phase in high-concentration solutions of the protein lysozyme after temperature quenches into thermodynamically defined metastable and unstable regions. We show with this system, which is a poor fit to mean-field assumptions, that the evolution of the structure factor during nucleation is similar to that during spinodal decomposition and reveals no singularity predicted upon crossing the metastability boundary. We introduce two kinetic definitions of the metastability/instability boundary that yield values within ∼1.5 K, i.e., the boundary appears as an area rather than a line, which is near and above the thermodynamic prediction. Delay times for the appearance of the new phase in the unstable regime are significant, i.e., new-phase growth is hindered by kinetic barriers. While our results agree with predictions of the non-mean-field theories of phase transformations, the experimentally observed behavior is richer than the one envisioned by theory.
Interpreting nonlinear vibrational spectroscopy with the classical mechanical analogs of double-sided Feynman diagrams
Authors: Noid W. G. , Loring Roger F.
Published: October 2004

Observables in coherent, multiple-pulse infrared spectroscopy may be computed from a vibrational nonlinear response function. This response function is conventionally calculated quantum-mechanically, but the challenges in applying quantum mechanics to large, anharmonic systems motivate the examination of classical mechanical vibrational nonlinear response functions. We present an approximate formulation of the classical mechanical third-order vibrational response function for an anharmonic solute oscillator interacting with a harmonic solvent, which establishes a clear connection between classical and quantum mechanical treatments. This formalism permits the identification of the classical mechanical analog of the pure dephasing of a quantum mechanical degree of freedom, and suggests the construction of classical mechanical analogs of the double-sided Feynman diagrams of quantum mechanics, which are widely applied to nonlinear spectroscopy. Application of a rotating wave approximation permits the analytic extraction of signals obeying particular spatial phase matching conditions from a classical-mechanical response function. Calculations of the third-order response function for an anharmonic oscillator coupled to a harmonic solvent are compared to numerically correct classical mechanical results.
Fragmentation and conformation study of ephedrine by low- and high-resolution mass selective UV spectroscopy
Authors: Chervenkov S. , Wang P. Q. , Braun J. E. , Neusser H. J.
Published: October 2004

The neurotransmitter molecule, ephedrine, has been studied by mass-selective low- and high-resolution UV resonance enhanced two-photon ionization spectroscopy. Under all experimental conditions we observed an efficient fragmentation upon ionization. The detected vibronic peaks in the spectrum are classified according to the efficiency of the fragmentation, which leads to the conclusion that there exist three different species in the molecular beam: ephedrine-water cluster and two distinct conformers. The two-color two-photon ionization experiment with a decreased energy of the second photon leads to an upper limit of 8.3 eV for the ionization energy of ephedrine. The high-resolution (70 MHz) spectrum of the strongest vibronic peak in the spectrum measured at the fragment (m/z=58) mass channel displays a pronounced and rich rotational structure. Its analysis by the use of a specially designed computer-aided rotational fit process yields accurate rotational constants for the S0 and S1 states and the transition moment ratio, providing information on the respective conformational structure.
Excited electronic state calculations by the transcorrelated variational Monte Carlo method: Application to a helium atom
Authors: Umezawa Naoto , Tsuneyuki Shinji
Published: October 2004

We have implemented the excited electronic state calculations for a helium atom by the transcorrelated variational Monte Carlo (TC-VMC) method. In this method, Jastrow-Slater-type wave function is efficiently optimized not only for the Jastrow factor but also for the Slater determinant. Since the formalism for the TC-VMC method is based on the variance minimization, excited states as well as the ground state calculations are feasible. It is found that both the first and the second excitation energies given by TC-VMC are much closer to the experimental data than those given by the variational Monte Carlo method with using the Hartree–Fock orbitals. The successful results in the TC-VMC method are considered to be due to the nodal optimization of the wave functions.
Structure of Lennard-Jones fluids confined in square nanoscale channels from density functional theory
Authors: Yang Xaioning , Ding Junqing
Published: October 2004

The density distribution of Lennard-Jones fluids confined in square nanoscale channels with Lennard-Jones walls has been studied using the nonlocal density functional theory (DFT) based on the Tarazona model. The effect of channel lengths on the density profiles with various chemical potentials was discussed. It was found that there is an apparent layering phenomenon for the confined fluids due to the combining influences of the enhancing solid-fluid interaction and the excluded volume effect. The pronounced density peaks were observed at the corners of square channels due to the strong fluid-solid interactions. The grand canonical ensemble Monte Carlo simulation (GCEMC) was applied to test the nonlocal DFT results. The DFT calculations are in relatively good agreement with the GCEMC simulations. The adsorption isotherms in a series of square channels were evaluated based on the obtained density distributions. The adsorption mechanism within the square pores was investigated. A comparison between the adsorptions of the square pores with those of the corresponding slit-size pores has been given.