bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermal conductivity of the Lennard-Jones liquid by molecular dynamics calculations
Authors: Vogelsang R. , Hoheisel C. , Ciccotti G.
Published: July 2002

Precise results for the thermal conductivity of the Lennard-Jones liquid obtained by equilibrium molecular dynamics (MD) are presented. These are compared in detail with previous nonequilibrium MD results. Effects due to the truncation of the potential and the particle number dependence are considered. The contributions of the partial correlation functions to the total one were separately calculated. Main results are: (i) in contrast to the viscosity, the thermal conductivity is rather insensitive to the MD conditions, even for the state corresponding virtually to the triple point of argon. The reason for this originates from the simple short ranged time decay of the correlation function. (ii) For the states considered, the partial correlation function involving the ‘‘potential–potential’’ term governs the transport coefficient. (iii) Nonequilibrium MD and MD give consistent values, except for the nonequilibrium method devised by Heyes which generates data far off the range permitted by the error bars. (iv) The computed thermal conductivities fall well in line with experimental data for argon.
Molecular structure and puckering potential function of cyclobutane studied by gas electron diffraction and infrared spectroscopy
Authors: Egawa Toru , Fukuyama Tsutomu , Yamamoto Satoshi , Takabayashi Fujiko , Kambara Hideki , Ueda Toyotoshi , Kuchitsu Kozo
Published: July 2002

The electron diffraction intensity of cyclobutane was measured and analyzed conjointly with the rotational constant, B0=0.355 82(11) cm−1, determined by an analysis of FTIR spectra of the ν14 (CH2 scissoring, B1g) and ν16 (CH2 rocking, A2u) bands. The rz structure was determined to be rz (C–C)=1.552±0.001 Å, rz (C–H)=1.093±0.003 Å, αz (∠HCH)=106.4±1.3°, and the ring dihedral angle, θz =27.9±1.6°; the rg distances of the C–C and C–H bonds are 1.554±0.001 and 1.109±0.003 Å, respectively. The uncertainties represent estimated limits of error. The rocking angle βz between the bisectors of the adjacent H–C–H and C–C–C angles was found to be 6.2±1.2°, the axial C–H bonds in the 1,3 positions being tilted towards each other. The coefficient of coupling of the ring-puckering and CH2-rocking motions was estimated to be βz/θz =0.22±0.05. The combination and difference sideband structures appearing in the ν14 band due to the puckering mode ν6 were analyzed. The puckering energy levels thus obtained were consistent with the existing data. A double-minimum potential function for the puckering mode was determined by taking into account the coupling of the puckering motion with the CH2-rocking motion. The potential gave the zero-point vibrational average value of the ring dihedral angle, 27.5±1.1°, which agreed with that determined in the ED–IR joint analysis mentioned above. The puckering potential for C4D8 was found to be consistent with that for C4H8 when the coupling between the puckering and the rocking motions was considered.
Extended interactions in the ε phase of oxygen
Authors: Agnew S. F. , Swanson B. I. , Jones L. H.
Published: July 2002

The high pressure infrared and Raman spectra for the ε phase of a series of oxygen samples, 16O2, 16O2 with 10% 18O2, and 18O2 with 6% 16O 18O, are reported. Assignments are made for the fundamental, combination, and isotope-induced features of this phase of solid oxygen based on a simple model that incorporates a pairwise interaction betweeen nearest-neighbor oxygen molecules within the molecular oxygen plane, with pairs forming extended chains. Vibrational analysis on this chain with two interaction force constants reproduces all of the features that we have observed.
Studies of silicon cluster–metal atom compound formation in a supersonic molecular beam
Authors: Beck Steven M.
Published: July 2002

The first observation of a reaction between a metal atom and silicon in a supersonic jet to form metal atom silicon clusters is reported. Using the technique of laser vaporization supersonic expansion with metal carbonyl seeded carrier gas, clusters of the form MSin have been detected by ArF and KrF laser photoionization time-of-flight mass spectrometry. Three transition metals have been investigated, Cr, Mo, and W. The dominant product cluster peaks observed in the mass spectra obtained for all three metals corresponds to identical but remarkable cluster stoichiometries. The dominant peaks have formulas given by MSin where n=15 and n=16. The metal–semiconductor clusters are relatively more stable towards photofragmentation than the bare silicon cluster of the same size. The observation of these new species may be relevant to reactions which occur at the interface between a silicon wafer and deposited metals.
Primary and secondary basis set superposition error at the SCF and MP2 levels. H3N--Li+ and H2O--Li+
Authors: Latajka Zdzisl/aw , Scheiner Steve
Published: July 2002

The primary basis set superposition error (BSSE) results from the artificial lowering of the energy of each subunit of a pair by the presence of ‘‘ghost orbitals’’ of its partner. In addition, these ghost orbitals perturb the one-electron properties of the molecule, causing a change in the interaction energy, an effect known as secondary BSSE which is not corrected by the counterpoise procedure. The primary and secondary BSSE are calculated for the interactions of NH3 and H2O with Li+, using a variety of different basis sets. It is found that the 2° BSSE can be quite large, comparable in magnitude to the 1° component at both the SCF and MP2 levels. There is no basis found for the supposition that 2° BSSE improves the calculated interaction energy, nor do the 1° and 2° effects cancel one another in general. While the MP2 BSSE tends to be smaller than the SCF analog, the former can be similar in magnitude to the ‘‘true’’ MP2 contribution to the interaction; failure to remove the BSSE can hence lead to a qualitatively incorrect interpretation of the effects of electron correlation. Comparison with a system in which basis set superposition is rigorously excluded suggests that subtraction of both the full 1° and 2° BSSE is appropriate and does not overcorrect the potential. Addition of a diffuse sp shell, especially if coupled with orbital exponent reoptimization, leads to a lowering of the 1° and 2° BSSE, which moreover take on opposite sign and cancel one another to some extent.
Optical study of the He++N2 charge transfer reaction in a flowing afterglow and in a low-pressure chamber coupled with flowing afterglow
Authors: Sekiya Hiroshi , Tsuji Masaharu , Nishimura Yukio
Published: July 2002

The He++N2 charge transfer (CT) reaction has been studied by optical emission spectroscopy utilizing a conventional flowing afterglow apparatus and a low-pressure apparatus. In the latter apparatus the He+ ions generated by a microwave discharge are expanded into a low-pressure chamber through a nozzle orifice. An intense structured continuum emission as well as the N+2(C 2∑+u–X 2∑+g), N+2(B 2∑+u–X 2∑+g), and N+2(D′ 2Πg–A 2Πu) emission systems has been observed in the 190–320 nm wavelengths region in the both experiments, although the continuum emission detected in the flowing afterglow is electronically quenched and/or vibrationally relaxed by collisions with He atoms. It has been found that the N+2(D′ 2Πg–A 2Πu) emission predominates in the 210–260 nm region, and occupies about a half of that of the total emission in the 200–320 nm region. The relative excitation rate constant for the D′ state has been measured to be 30% of that of the C state, suggesting that the process leading to the D′ state is an important exit channel in the He++N2 CT reaction at (near) thermal energy.
Molecular model for aqueous ferrous–ferric electron transfer
Authors: Kuharski Robert A. , Bader Joel S. , Chandler David , Sprik Michiel , Klein Michael L. , Impey Roger W.
Published: July 2002

We present a molecular model for studying the prototypical ferric–ferrous electron transfer process in liquid water, and we discuss its structural implications. Treatment of the nonequilibrium dynamics will be the subject of future work. The elementary constituents in the model are classical water molecules, classical ferric ions (i.e., Fe3+ particles), and a quantal electron. Pair potentials and pseudopotentials describing the interactions between these constituents are presented. These interactions lead to ligand structures of the ferric and ferrous ions that are in good agreement with those observed in nature. The validity of the tight binding model is examined. With umbrella sampling, we have computed the diabatic free energy of activation for electron transfer. The number obtained, roughly 20 kcal/mol, is in reasonable accord with the aqueous ferric–ferrous transfer activation energy of about 15 to 20 kcal/mol estimated from experiment. The Marcus relation for intersecting parabolic diabatic free energy surfaces is found to be quantitatively accurate in our model. Due to its significance to future dynamical studies, we have computed the tunnel splitting for our model in the absence of water molecules. Its value is about 1 kB T at room temperature for ferrous–ferric separations around 5.5 Å. This indicates that the dynamics of the electron transfer are complex involving both classical adiabatic dynamics and quantal nonadiabatic transitions. The dynamics may also be complicated due to glassy behavior of tightly bound ligand water molecules. We discuss this glassy behavior and also describe contributions to the solvation energetics from water molecules in different solvation shells. Finally, the energetics associated with truncating long ranged forces is discussed and analyzed.
The neon–argon potential revisited
Authors: Barrow David A. , Aziz Ronald A.
Published: July 2002

The neon–argon interaction has been reexamined in the light of new data, both theoretical and experimental. Two recent potentials have been analyzed as to their ability to predict both macro- and microscopic properties. Both of these fail to give a complete description of the interaction. A new simple potential is presented which accurately predicts a wide range of properties including high and low energy total cross sections, differential cross section, as well as second virials, viscosity, and diffusion. New combining rules have been tested and are found to be consistent with the new potential.
A low-energy quasiclassical trajectory study of O(3P)+OH(2Π)→O2(3Σ−g)+H(2S). II. Rate constants and recrossing, zero-point energy effects
Authors: Nyman Gunnar , Davidsson Jan
Published: July 2002

Quasiclassical trajectory calculations for the title reaction have been carried out using the recent double many body expansion III potential-energy surface by Varandas et al. (1988). Recrossing factors are calculated and found to depend strongly on how the vibrational zero-point energy is treated. Detailed and thermal rate constants for the temperature range 20–500 K are presented. Comparisons with experiments, trajectory calculations by Miller (1986), and by Quintales et al. (1988), and a previously proposed extended Langevin model are made. It is noted that the coupling of the electronic and nuclear motion in OH may have a large effect on the thermal rate constant below 200 K. Close complex formation rate constants are calculated and found to agree well with the experimental rate constants.
Coated droplet model of microemulsions: Optical matching and polydispersity
Authors: Ric̆ka J. , Borkovec M. , Hofmeier U.
Published: July 2002

Dilute water-in-oil microemulsions, such as the ternary system water–AOT–hexane, can be modeled as suspensions of spherical droplets coated with surfactant. There is a general agreement that the radius of the droplets is not uniform but the magnitude of the polydispersity is subject to discussions. The polydispersity influences strongly optical properties of the microemulsions, in particular it determines the characteristic features of scattering data observed in the vicinity of optical matching. We present here analytical expressions and data for the refractive index increment, the average scattering intensity, the apparent hydrodynamic radius, and the second cumulant of dynamic light scattering. Our model reproduces without any free parameters the dependence of the refractive index increment on the water-to-surfactant ratio ω. Only two free parameters are needed to fit the ω dependence of the other three quantities: δ represents approximately the thickness of the surfactant layer and γ=〈r 2〉/〈r〉2−1 reflects the radius polydispersity. Our results suggest a polydispersity index (γ)1/2 of only 12%. This value is much smaller than previously estimated by small angle scattering.
An ab initio study of the ionization of sodium superoxide
Authors: Marshall Paul
Published: July 2002

Structures and frequencies of 1A1 and 3Σ NaO+2 were calculated at the HF/6-31G level, and energies at the MP4/6-31G level. Predicted IPs for NaO2 are 8.86 and 7.35 eV, respectively. The barrier to exothermic dissociation of the 3Σ ion to Na++O2 is about 4 kJ mol−1.
Optical spectra and crystal-field analysis of europium double nitrates
Authors: Görller-Walrand C. , Hendrickx I. , Fluyt L. , Gos M. P. , D’Olieslager J. , Blasse G.
Published: July 2002

Locations and assignments of crystal-field levels in low-temperature spectra are reported for Eu3+ in the europium double nitrate system [Eu2M3(NO3)12⋅24H2O with M=Mg, Zn]. These energy levels are assigned from the polarized luminescence and the polarized absorption measurements on single crystals at 77 K. The arrangement of the oxygen atoms around the europium ion has approximate icosahedral symmetry and we have tried to find out whether the spectroscopic properties of Eu2M3(NO3)12⋅24H2O reflects this pseudosymmetry, even though the site symmetry of the europium ion is C3. Quantitative crystal-field calculations have first been performed assuming a C3v symmetry which is close to icosahedral. The levels are analyzed in terms of 20 free-ion and 6 crystal-field parameters. Afterwards it is examined how far the C3v parameters can be used to approximate the C3 symmetry. In this case, three additional imaginary crystal-field parameters are taken into account.
The nonadditive intermolecular potential for water revised
Authors: Dang Liem X.
Published: July 2002

The results of an improved version of a nonadditive intermolecular model for water that explicitly includes the nonadditive polarization energy are reported. The original polarizable water potential model (POL1), upon which the improved version is based, was developed by Caldwell, Dang, and Kollman [J. Am. Soc. Chem. 112, 9144 (1990)]. To improve the POL1 model, we developed a new set of atomic polarizabilities that reproduce the experimental molecular polarizability for water using the atom–dipole interaction model (Applequist, Carl, and Fung [J. Am. Soc. Chem. 94, 2952 (1972)]). Using the new atomic polarizabilities, we optimized the Lennard-Jones parameters for O–O interactions to improve the model. As expected, the new model has improved the radial distribution functions and the average potential energy for liquid water as well as the density and the average total dipole moment. The model is then used to compute the binding energies of Cs+–water clusters. Without the need for three-body forces (ion–water–water interaction), the agreement between the results of molecular-dynamics simulations and experimental energies of cluster formation is very good.
Molecular integrals with Slater basis. IV. Ellipsoidal coordinate methods for three-center nuclear attraction integrals
Authors: Rico J. Fernández , López R. , Ramírez G.
Published: July 2002

We develop and check an updated version of the ellipsoidal coordinate methods for the calculation of three-center nuclear attraction integrals with Slater-type orbitals (STOs). A first set of recurrence relations allows us to express these integrals in terms of some basic potentials. The basic potentials are classified in short- and long-range types, and then calculated in specific ways from auxiliary functions obtained by using a new set of both stable and fast recurrence relations. Numerical tests show that the procedure enables to reach very high accuracy in the integrals with a low computational cost.
Phonon-induced site relaxation of tetracene guests in p-terphenyl single crystals
Authors: Krüger Arnd , Kryschi Carola , Valkunas Leonas
Published: July 2002

Site relaxation processes of tetracene-h12 and tetracene-d12 guests in p-terphenyl crystals are examined using high-resolution fluorescence and fluorescence excitation spectroscopy. Two of the four sites, the so-called O3 and O4 sites, undergo this process and their phonon sidebands observed in the excitation spectra are characterized by an exceptionally long and well resolved progression of a low-frequency (pseudo-) local phonon mode superimposed on a broad background. Perdeuteration of tetracene results in a frequency reduction of the progression forming mode which identifies this local phonon as a guest librational mode. The latter is suggested to induce the site relaxation via guest reorientation processes. The intensity distribution in the phonon sideband of the O3 site indicates a reorientational angle of ∼21° about the L-axis for the tetracene guest in the excited singlet state.
On statistical mechanics of solvation
Authors: Helman A. B. , Keyes T.
Published: July 2002

An exact solution is obtained for the solvation free energy of a two-level system immersed into dielectric continuum. The solution is invariant with respect to the representation of the quantum electronic states.
Vibrational relaxation rate constants for HF(v=1–4) by CO, CO2, and HCN with product identification by infrared emission
Authors: Arunan E. , Raybone D. , Setser D. W.
Published: July 2002

Vibrational relaxation of HF(v=1–4) by CO, CO2, and HCN has been studied by infrared emission in a flow reactor. The vibrationally excited HF molecules were generated by the reactions of F atoms with CH4, CH3Cl, H2S, and HBr in 0.8 Torr of Ar carrier gas. By monitoring the rotational distribution in the lower v levels, the fraction of the relaxation proceeding by Δv=−1 and −2, V→R transfer was determined. Numerical integration of the rate equations and modified Stern–Volmer analysis of the HF(v) distribution vs reagent concentration were used to determine the relaxation rate constants and the mechanisms. The relaxation rate constants increase with v, but the magnitude of the rate constants depend upon the collision partner; the HF(v=3) values (in cm3 s−1) are 2.5×10−12 for CO, 1.1×10−11 for CO2, and 1.0×10−10 for HCN. The HF(v) relaxation mechanism by CO is dominated by V→R transfer (≥70%), giving HF(v−1, J=10–14). Relaxation by CO2 has significant Δv=−2 and −3, V→R (≊30%) and Δv=−1, V→R (∼20%) pathways, as well as a V–V pathway. Comparing the HCN(ν3) emission intensity to the loss in HF(v) emission intensity gave the efficiency of V→V transfer from HF(v) to HCN(ν3) as 55±15%; some Δv=−1, V→R transfer also was identified. The HF(v, J≥8) rotational relaxation rate constants increase in the CO, CO2, HCN series. R–V transfer from HF(J≥16) to the CO2 bending mode was identified as the fastest rotational relaxation process.
Theoretical and experimental structures of vinyl fluoride and vinyl alcohol
Authors: Smith Brian J. , Coffey Dewitt , Radom Leo
Published: July 2002

A new theoretical r0 structure for vinyl fluoride has been derived using ab initio molecular orbital procedures. In addition, new experimental r0 and rs structures have been obtained through a reanalysis of existing experimental microwave data. Significant discrepancies are found between the new theoretical (125.4°) and experimental (127.6°) r0 estimates of the CCHu angle involving the hydrogen atom geminal to the substituted center. This is attributed to the relative insensitivity of the experimental structural fits to the precise value of this bond angle, an insensitivity that is associated with the proximity of the relevant atoms to inertial axes. Because of these difficulties, the best estimate of the experimental r0 structure for vinyl fluoride is obtained by taking the theoretical r0 value for the CCHu angle and determining the remaining parameters by a weighted-least-squares fit to the experimental data. Weighted-least-squares fits are found generally to provide more consistency in the derived structural parameters than unweighted procedures. It is found that fitting of isotopic differences in rotational constants (as opposed to the conventional fitting of rotational constants themselves) provides the structure for vinyl fluoride most similar to the theoretical r0 structure. Similar observations are found to hold for vinyl alcohol, for which new theoretical and experimental results are presented.
Molecular dynamics simulation of liquid–plastic phase transition of cyclohexane in porous silica. II
Authors: Brodka A. , Zerda T. W.
Published: July 2002

The liquid–plastic phase transition of cyclohexane in small pores of sol-gel glass is studied by computer simulation. A cavity model is obtained by placing silica clusters at the corners of a cubic box. Two cavities of diameter of about 30 and 50 Å are considered. Cyclohexane is approximated by an assembly of six Lennard-Jones potentials. Translational and rotational motions of cyclohexane are studied in the temperature range from 190 to 333 K. Supercooling is observed and the freezing temperature is depressed in comparison to the bulk phase.
Collision-induced dissociation and charge transfer reactions of SF+x (x=1–5): Thermochemistry of sulfur fluoride ions and neutrals
Authors: Fisher Ellen R. , Kickel Bernice L. , Armentrout P. B.
Published: July 2002

Guided ion beam mass spectrometry is used to measure the cross sections for collision-induced dissociation of SFx+ (x=1–5) with Xe. The energy dependences of the cross sections are analyzed to give the following 0 K bond dissociation energies (BDEs): D°(SF4+–F)=4.60±0.10 eV, D°(SF3+–F)=0.36±0.05 eV, D°(SF2+–F)=4.54±0.08 eV, D°(SF+–F)=4.17±0.10 eV, and D°(S+–F)=3.56±0.05 eV. The ionization energies, IE(SF)=10.16±0.17 eV, IE(SF3)=8.18±0.07 eV, IE(SF4)=11.69±0.06 eV, and IE(SF5)=9.60±0.05 eV, are also measured from analysis of endothermic charge–transfer reactions. From these BDEs and IEs, we derive heats of formation for the sulfur fluoride ions and neutrals that provide a self-consistent set of thermochemical data for the sulfur fluoride species. In some cases, the thermochemical values determined here are considerably different from available literature values. These differences are discussed in detail.
Multiple-scattered, relativistic and correlated method (MS-RCΞ) for molecules
Authors: Vijayakumar M. , Gopinathan M. S.
Published: July 2002

We have recently proposed a ‘‘fully’’ correlated relativistic local density method called RCΞ method [M. Vijayakumar, N. Vaidehi, and M. S. Gopinathan, Phys. Rev. A 40, 6834 (1989)] for atoms and it has been shown that the results are of near Dirac–Hartree–Fock (DHF) accuracy. In this paper, the extension of the RCΞ method to molecules by modifying the standard multiple-scattering Xα (MS-Xα) method is presented. Then, the modified method is applied to calculate various molecular properties such as the molecular correlation energy, the molecular orbital energy levels, the molecular orbital ionization energies, expectation value of 1/r, and magnetic property like isotropic shielding constant for atoms in molecules. Such calculations are made for several molecules including some of them containing heavy atoms. The calculated molecular properties are compared with the available experimental results and with results calculated by other methods. The effects of relativistic and correlation corrections on these molecular properties are discussed.
Boron oxides: Ab initio studies with natural bond orbital analysis
Authors: Nemukhin A. V. , Weinhold F.
Published: July 2002

We employ ab initio theory and natural bond orbital (NBO) analysis to describe the structure, energetics, vibrational properties, and bonding in small boron oxides, BmOn, supplementing recent studies on isovalent aluminum oxide clusters, Al2On, in order to extend the overview of bonding tendencies in group IIIA metal oxides. The comparison of analogous boron and aluminum species reveals many surprising differences, such as the V-shaped (M=B) vs linear (M=Al) geometry of M2O3, the altered tendency toward cyclic structures (higher for M2O2 with M=Al, for M3O3 with M=B), and the diminished role of electron correlation in the boron congeners. Correlation effects are examined by a recently introduced selective, localized multiconfigurational approach. Differences in bonding patterns are traced to basic hybridization and electronegativity shifts (reduced ionic character of B–O vs Al–O bond). Detailed comparisons with recent experimental BmOn data (including isotopomer ir shifts) provide additional support for theoretical assignments in corresponding aluminum species, where significant disagreements between theory and experiment persist.
Calculation of the Verdet constants for H2, N2, CO, and FH
Authors: Parkinson William A. , Sauer Stephan P. A. , Oddershede Jens , Bishop David M.
Published: July 2002

The Verdet constant (V), which governs magnetic optical rotation, has been evaluated, at several frequencies, as a quadratic response function for H2, N2, CO, and FH, in the random-phase approximation (RPA). At the same time, the ‘‘normal’’ Verdet constant (VN) has been found from the linear response function in both RPA and second-order polarization propagator approximations (SOPPA), though in the latter case only for a single frequency. For H2 and N2, V is in good agreement with experiment, and the ratio V/VN accords very well with other accurate results. For CO the effect of correlation turns out to be more important. For FH there are no other results for V with which we can compare; however, SOPPA calculations on the normal Verdet constant show a dramatic effect for electron correlation. It appears that V/VN (which in all cases is only slightly dependent on frequency) is close to unity for FH, thus ruling out a connection between this ratio and molecular polarity. Zero-point-vibrational averaging is taken into account in the calculations and the effect of shifting the origin of the gauge is discussed.
Effects of critical fluctuation in the theory of microphase separation in symmetric systems
Authors: Stepanow S. , Schulz M.
Published: July 2002

We consider the effects of critical fluctuations in systems, which are undergoing microphase separations, such as diblock copolymers and interpenetrating networks. We predict the critical regime which, governed by the elastic constant C0, prevents the system from global separation. We obtain that in two dimensions there is no narrowing of the critical fluctuations for large degrees of polymerization studied within the field theoretic renormalization group. The scattering function S(p) in the homogeneous phase is obtained to behave as (∂[p2S−1(p)]/∂p2)p→0 ≂m0 p*(γ−1/ν), where γ and ν are the critical exponents and p*=2π/λ with λ being the dimension of the microphase.
Fast Hartree–Fock calculations by simulated dynamics. II. An efficient, stable, and accurate algorithm for molecules
Authors: Vianna Reinaldo O. , Chacham Hélio , Mohallem José R.
Published: July 2002

We have developed a computational routine for performing Hartree–Fock calculations in molecules by simulated dynamics. For fixed molecular geometry calculations, the routine is faster and more stable than the standard self-consistent-field approach. It further admits the simultaneous optimization of the electronic wave functions and geometry, using reliable Hellmann–Feynman forces with nuclei-centered Gaussian orbitals for the nuclear motion. We also investigate the role of nearly linear dependence of the basis set on this type of calculation, so far unrecognized. We present applications to some diatomic hidrides and compare our results with those from GAUSSIAN92.
Cross correlation of chemical shift anisotropy and dipolar interactions in methyl protons investigated by selective nuclear magnetic resonance spectroscopy
Authors: Müller Norbert , Bodenhausen Geoffrey
Published: July 2002

A group spin approach is used to describe the interconversion of magnetization modes in systems of three equivalent spins 1/2. It is shown that the most prominent effect of cross correlation of the fluctuations of chemical shift anisotropy and dipolar interactions is the transformation of two-quantum coherences of second rank into third rank coherences. A novel experiment is described which allows one to observe such a process.
Optical laser Stark spectroscopy of the yttrium monosulfide molecule
Authors: James Andrew M. , Simard Benoit
Published: July 2002

The yttrium monosulfide molecule was produced in a pulsed molecular beam apparatus by chemical reaction in a laser-generated plasma. The permanent electric dipole moments of the X 2Σ+ and B 2Σ+ states were determined by molecular beam Stark spectroscopy to be 6.098(64) D and 4.572(92) D, respectively (2σ error bounds). Results are interpreted in terms of a molecular orbital description of the bonding. The experimental data are compared with dipole moment measurements for related molecules, and with the predictions of ab initio theory.
Nonadiabatic effects in the photodissociation of H2S in the first absorption band: An ab initio study
Authors: Heumann Bernd , Weide Klaus , Düren Rudolf , Schinke Reinhard
Published: July 2002

The photodissociation of H2S through excitation in the first absorption band (λ≊195 nm) is investigated by means of extensive ab initio calculations. Employing the MRD-CI method we calculate the potential energy surfaces for the lowest two electronic states of 1A″ symmetry varying both HS bond distances as well as the HSH bending angle. (In the C2v point group these states have electronic symmetry 1B1 and 1A2, respectively.) The lower adiabatic potential energy surface is dissociative when one H atom is pulled away whereas the upper one is binding. For the equilibrium angle of 92° in the electronic ground state they have two conical intersections, one occurring near the Franck–Condon point. Because of the very small energy separation between these two states nonadiabatic coupling induced by the kinetic energy operator in the nuclear degrees of freedom are substantial and must be incorporated in order to describe the absorption and subsequent dissociation process in a realistic way. In the present work we treat the coupling between the two electronic states in a diabatic representation extracting the coordinate-dependent mixing angle from the CI coefficients of the electronic wave functions. The nuclear motion is treated in three dimensions in an exact quantum mechanical approach by propagation of a two-component time-dependent wave packet. The calculated absorption spectra for H2S and D2S satisfactorily agree with the measured spectra. In particular, the calculations reproduce the diffuse structures with energy spacing of about 1200 and 850 cm−1 for H2S and D2S, respectively. Furthermore, the calculated rotational- and vibrational-state distributions of the HS and DS fragments reproduce recent measurements in a convincing way. The photodissociation of H2S is a prototype for very fast electronic predissociation. The photon preferentially excites the binding (diabatic) state. This state, however, is quickly depleted by strong coupling to the dissociative (diabatic) state with the complex finally breaking up into products H and HS. The electronic quenching takes place on the time scale of one internal vibrational period only.

Our calculations unambiguously confirm that the diffuse structures superimposed to the broad background are caused by symmetric stretch motion—in the binding state—and not by activity in the bending mode as originally assumed.
Raman scattering by high-density dispersions. II. Calculation of surface-enhanced Raman scattering from CO and benzene physisorbed on pairs of particles of Ag, Pt, Ge, and SiO
Authors: Xu Muliang , Dignam Michael J.
Published: July 2002

In part I, a mathematical formalism was developed for calculating Raman scattering intensities from a collection of spherical particles uniformly coated with molecules, and applied to the special case of two CO-coated Ag particles in contact, with their centers aligned with the laser field and under parallel polarization conditions. In this paper, the additional relations required to perform the calculations for arbitrary orientation and polarization conditions are developed and used in calculations for CO and benzene physisorbed onto pairs of particles of Ag, Pt, Ge, and SiO. The large increase in Raman radiation intensity per molecule, reported in part I on going from a one-particle to a two-particle system, is shown here to apply only to the Raman radiation component polarized parallel to the interparticle axis and due to the incident radiation component similarly polarized. For all other components, the excitation spectra are similar for the two systems. The large increase for the parallel–parallel case is due to the process in which the primary Raman multipoles centered on one particle induce Raman dipoles on the pair of particles.
Nucleation, domain growth, and fluctuations in a bistable chemical system
Authors: Gruner Daniel , Kapral Raymond , Lawniczak Anna
Published: July 2002

Phase separation and nucleation processes are investigated for a bistable chemical system. The study utilizes a reactive lattice-gas cellular automaton model to provide a mesoscopic description of the dynamics. Simulations of steady-state structure, wave propagation, and critical nucleus size using this model are compared with results based on the deterministic equations of motion. The dynamic structure factor is computed for evolution from the unstable state and the effects of correlations are examined for early and late times. The study provides insight into these processes in a fluctuating, extended medium and also provides a test of the ability of the reactive lattice-gas method to describe the fluctuations in the system.
Monte Carlo investigation of surface self-diffusion: The role of anisotropic next-nearest neighbor interactions
Authors: Pekalski A. , Ausloos M.
Published: July 2002

The asymmetric next-nearest neighbor (NNN) Ising model has been studied for describing the self-diffusion of adsorbate atoms at low temperature, furthermore adding a relevant ingredient in such a regime, i.e., a finite value Q for the trapping potential barrier. The Monte Carlo technique leads to the components of the (tracer) diffusion coefficient. An interesting feature is seen: a ‘‘V–Λ transition’’ occurs at half-coverage (c=0.5) when the temperature is lowered. The activation energy (obtained from an Arrhenius plot) is seen to be varying as a function of coverage, and is not symmetrical with respect to c=0.5. The anisotropy of the diffusion and the change in its pattern as a function of the temperature and coverage are presented and discussed. Monte Carlo snapshots are shown at various coverage and temperature values.
Electronic structure of polyparaphenylene vinylene copolymers: The relationship to light-emitting characteristics
Authors: dos Santos D. A. , Quattrocchi C. , Friend R. H. , Brédas J. L.
Published: July 2002

We investigate theoretically the electronic structure of regular and statistical copolymer chains containing segments of poly(p-phenylene vinylene) (PPV) and its dimethoxy-substituted derivative, poly(2,5-dimethoxy-1,4-phenylene vinylene) (DMeOPPV). Such copolymers have recently been shown to present remarkable electroluminescence properties. The calculations on the copolymer chains are performed on the basis of the negative factor counting technique coupled to the valence effective Hamiltonian method. The eigenfunctions of the upper occupied and lower unoccupied levels are calculated explicitly using the inverse iteration technique, and the degree of localization is determined via the inverse participation number approach. It is shown that the electronic structure of PPV/DMeOPPV copolymers is strongly affected by both the concentrations and relative locations of the parent polymer moieties along the chains. The appearance of clusters of dimethoxy-substituted units leads to energy gaps that are smaller than those of the parent polymers and to a spatial localization of the upper occupied levels within the DMeOPPV clusters, while the lower unoccupied levels remain smoothly delocalized over the unsubstituted regions. Our results thus indicate that electron–hole recombination processes should take place preferentially at the interfaces between PPV and DMeOPPV segments. The evolution of the PPV/DMeOPPV copolymer band gaps as a function of the concentration in dimethoxy substituted rings is compared to experimental photoluminescence and absorption data.
Viscosity of dilute suspensions of rodlike particles: A numerical simulation method
Authors: Yamamoto Satoru , Matsuoka Takaaki
Published: July 2002

The recently developed simulation method, named as the particle simulation method (PSM), is extended to predict the viscosity of dilute suspensions of rodlike particles. In this method a rodlike particle is modeled by bonded spheres. Each bond has three types of springs for stretching, bending, and twisting deformation. The rod model can therefore deform by changing the bond distance, bond angle, and torsion angle between paired spheres. The rod model can represent a variety of rigidity by modifying the bond parameters related to Young’s modulus and the shear modulus of the real particle. The time evolution of each constituent sphere of the rod model is followed by molecular-dynamics-type approach. The intrinsic viscosity of a suspension of rodlike particles is derived from calculating an increased energy dissipation for each sphere of the rod model in a viscous fluid. With and without deformation of the particle, the motion of the rodlike particle was numerically simulated in a three-dimensional simple shear flow at a low particle Reynolds number and without Brownian motion of particles. The intrinsic viscosity of the suspension of rodlike particles was investigated on orientation angle, rotation orbit, deformation, and aspect ratio of the particle. For the rigid rodlike particle, the simulated rotation orbit compared extremely well with theoretical one which was obtained for a rigid ellipsoidal particle by use of Jeffery’s equation. The simulated dependence of the intrinsic viscosity on various factors was also identical with that of theories for suspensions of rigid rodlike particles. For the flexible rodlike particle, the rotation orbit could be obtained by the particle simulation method and it was also cleared that the intrinsic viscosity decreased as occurring of recoverable deformation of the rodlike particle induced by flow.
Fine-structure transitions in metastable Ne*(3P0,2) colliding with ground state rare gases at thermal energies
Authors: Ferkel H. , Koch A. , Feltgen R.
Published: July 2002

The emission of XUV photons from collision-induced fine-structure transitions Ne*(3P0,2)+R→Ne*(1,3P1)+R→Ne(1S)+R+hν(74 nm) has been measured for R=He, Ne, Ar, and Kr(1S) in a beam-cell experiment. The metastable Ne* velocity ranges from 550 to 1600 m/s. No emission was found for Ne*(3P0)+R. An appropriate detector system enables the determination of absolute 3P2 → 3P1 transition cross sections. The results of Ne*(3P2)+He and Ne are compared with theoretical calculations.
Vibrational circular dichroism intensities by ab initio second-order Mo/ller–Plesset vibronic coupling theory
Authors: Yang Danya , Rauk Arvi
Published: July 2002

The vibronic coupling theory (VCT) of Nafie and Freedman for evaluating vibrational circular dichroism (VCD) intensities has been implemented at the level of ab initio second-order Mo/ller–Plesset perturbation theory (MP2). This method, in conjunction with the MP2 force field, is applied to calculate VCD spectra of (S,S)-2,3-dideuteriooxirane and (S)-NHDT. Several basis sets, namely 6-31G*, 6-31G*(0.3), vd/3p, and vd/3p(u) are used in the calculations. Comparisons with the experimental results for oxirane and alternate high level calculations in the case of NHDT show that VCT/MP2 with a near-Hartree–Fock limit basis set will predict nearly quantitatively values of vibrational rotatory strengths. The inclusion of electron correlation at the MP2 level into calculation of the atomic axial tensors is important for obtaining reliable vibrational properties for the conventional 6-31G* basis set. The 6-31G*(0.3) basis set, which has previously proved to be highly successful in predicting VCD spectra with the VCT-SCF method, yields only modest improvement at the MP2 level.
Kinetics of the reaction of HSO with O3 at temperatures 273–423 K
Authors: Lee Yin-Yu , Lee Yuan-Pern , Wang Niann Shiah
Published: July 2002

The rate coefficient of the reaction of HSO with O3 at five temperatures in the range 273–423 K was determined by means of the discharge-flow/laser-induced-fluorescence technique. The laser-induced fluorescence of HSO was detected by the gated photon-counting technique; a detection limit ∼4×109 molecules cm−3 was achieved. The reaction was carried out under pseudo-first-order conditions with [O3]≫[HSO]; the rate coefficient was determined to be k=(4.7±1.0)×10−14 cm3 molecule−1 s−1 at 298 K, approximately 30% greater than, but within experimental uncertainty of the only previous measurement by Wang and Howard by means of the discharge-flow/laser-magnetic-resonance technique. The temperature dependence of the rate coefficient is expressed in the Arrhenius form, k(T)=(2.1±0.7)×10−12 exp[−(1120±320)/T] cm3 molecule−1 s−1.
Complete active space self-consistent field and density functional study of FNO
Authors: Dibble Theodore S. , Francisco Joseph S. , Deeth Robert J. , Hand Michael R. , Williams Ian H.
Published: July 2002

The molecular structure and bond dissociation energy of FNO are studied using complete active space self-consistent field (CASSCF) and density functional methods (DFT), and the results are compared to experimental. Nitrogen–fluorine bond lengths obtained from DFT using the local density approximation are fairly accurate but N–F bond energies are overestimated by more than 40 kcal mol−1. Application of gradient corrections reduces the error in the bond dissociation energy to ∼15 kcal mol−1 but yields bond lengths that are significantly too long. Basis set effects in density functional results are compared to those observed previously in FNO2. The CASSCF method produces accurate bond lengths only with very large (10 in 8 or 12 in 9) active spaces; smaller active spaces yield N–F bond lengths that are short by 0.07 Å or more. The 12 in 9 active space underestimates the bond energy by ∼14 kcal mol−1. The inclusion of certain orbitals in the active space leads to much improved structures and significantly lower absolute energies, but the SCF configuration dominates the wave function and occupation numbers are essentially 2 and 0. From these results, we conclude that the challenge FNO presents to ab initio calculation is due to dynamic electron correlation rather than any multireference character.
Probing dynamics in the Franck–Condon and exit channel regions of dissociating H2S: Emission spectra upon tunable excitation from 199–203 nm
Authors: Browning P. W. , Jensen E. , Waschewsky G. C. G. , Tate M. R. , Butler L. J. , Hessler J. P.
Published: July 2002

This study uses emission spectroscopy of H2S at excitation energies near 200 nm to probe the dissociation dynamics from a conical intersection in the Franck–Condon region to the H+SH product exit channel. Photoexcitation accesses these coupled surfaces near the transition state region of the lower adiabat, a potential surface for the excited state H+SH→HS+H reaction. Excitation wavelengths from 199–203 nm tune through the first of the resonances in the absorption spectrum assigned to recurrences in the motion along the symmetric stretch orthogonal to the reaction coordinate and also access energies just above and at the conical intersection. We disperse the emission from the dissociating molecules at each of five excitation wavelengths in this region to probe several features of the reaction dynamics on the coupled potential energy surfaces. The resulting emission spectra cover the range of final vibrational eigenstates from 500 to 11 000 cm−1 above the initial ground vibrational state for all five excitation wavelengths, and go out to 16 500 cm−1 for the 199 and 201 nm excitation wavelengths. The resulting spectra, when considered in conjunction with recent scattering calculations by Heumann and Schinke on ab initio potential energy surfaces for this system, evidence a progression of emission features to low vibrational eigenstates in the SH stretch that result from coupling of the nuclear motion from the bound to the dissociative region of the potential energy surfaces.

This emission, into local mode eigenstates such as 00+1, 11+0, 11+1, 21+0, 21+1, evidences the antisymmetric dissociative motion and bending induced near the conical intersection, and dominates the spectrum at excitation wavelengths only near 200 nm. We analyze the excitation wavelength dependence of these features and also of the n0+0 progression for n≥4, which reflect the exit channel dynamics. The excitation wavelength dependence shows that while the emission spectra do not reveal any dynamics unique to scattering states that access a symmetric stretch resonance in the Franck–Condon region, they do reveal the energy location of and the dynamics at the conical intersection. A reanalysis of other workers’ measurements of the SH product vibrational state distribution shows that v=0 products are strongly favored at excitation wavelengths near the conical intersection.
Nonequilibrium molecular dynamics simulation of diffusion and flow in thin microporous membranes
Authors: MacElroy J. M. D.
Published: July 2002

A new nonequilibrium molecular dynamics method for simulating flow and diffusion within membranes is presented. The method involves two fixed volumes, separated by the permeable medium, which are maintained at fixed, though different, chemical potential. By monitoring the flow and diffusion of the fluid particles through the nonequilibrium interstitial region one can simulate mass transfer processes in a manner which parallels real laboratory experiments. The method is applied to a simple microporous membrane system and it is shown that slip flow, rather than viscous shear, is the predominant mechanism governing the permeation of moderately dense hard-sphere fluids in very fine pores.
Time-resolved Fourier transform infrared study of the photodissociation of 1,1-difluoroethene at 193 nm
Authors: Hall Gregory E. , Muckerman James T. , Preses Jack M. , Weston Ralph E. , Flynn George W. , Persky Avigdor
Published: July 2002

We have measured vibrational and rotational population distributions for the excited HF product of the 193 nm photolysis of CH2CF2. Approximately 15% of the available energy is released as vibrational energy of HF, and the estimated nascent HF rotational energy corresponds to about 10% of the total available energy. The relative vibrational populations can be fit to a linear surprisal. We have compared the vibrational distribution to that observed from this reaction with different amounts of available energy, and to the HF vibrational distributions produced in the dissociation of other fluoroethenes. While results differ among the various experiments considered, overall more than the amount of vibrational energy expected statistically is deposited in HF. The rotational population distributions we observe fit both a Boltzmann distribution and an information theoretic model corresponding to a nearly statistical energy distribution.