bluuebunny/crossref_metadata_2025 huggingface dataset:
Fluorescence and hole burning depletion spectra of 1-(9- and 1-anthryl)-3-phenylpropanes in supersonic free jet
Authors: Takasu Ryozo , Kizu Noriyuki , Itoh Michiya , Shinoda Hiroyuki
Published: July 2002

Fluorescence excitation and hole burning depletion spectra of 1-(1- and 9-anthryl)-3- phenylpropanes (1- and 9-An–Ph) in a supersonic free jet suggest two types of ground state conformers, which are dependent on the cooling conditions of supersonic expansion. The two major conformers were ascribed to trans/trans (t/t) and gauche/trans (g/t) concerning the torsional conformations of C1–C2 and C2–C3 bondings. In addition to the t/t conformation of 1-An–Ph, the anti-gauche/trans (anti-g/t) conformer was proposed to be most plausible. The t/t and g/t (anti-g/t) conformers exhibit different fluorescence decay times. The former (t/t) of both compounds shows excess vibrational energy dependence of decay times as well as dispersed spectra, while decay times of the latter g/t and anti-g/t are less dependent. The optimum geometries of these conformations in the ground state were obtained by the AM-1 calculation. The different fluorescent features of these conformations were discussed in terms of the intramolecular vibrational energy redistribution in geometrically restricted conformations of the g/t (9-An–Ph) and anti-g/t (1-An–Ph).
Relaxation of anisotropically oriented I=3/2 nuclei in the multipole basis: Evolution of the second rank tensor in the double quantum filtered nuclear magnetic resonance experiment
Authors: Dinesen T. R. J. , Sanctuary B. C.
Published: July 2002

The relaxation of an I=3/2 spin system in an anisotropic environment characterized by a finite residual quadrupolar splitting ωq is modeled by analytically solving for the density operator from Redfield’s relaxation theory. The resulting equations are cast into the multipole basis in order to describe the tensorial components of the spin density matrix. Included in the relaxation matrix are off-diagonal elements J1 and J2, which account for anisotropic systems with ωq values less than the width of the resonant line. With the Wigner rotation matrices simulating hard pulses, the response to an arbitrary pulse sequence can be determined. An analytical expression for the response to the double quantum filtered (DQF) pulse sequence (π/2)−(τ/2)−π−(τ/2)−θ−δ−θ−AQ for θ=π/2 is presented, showing explicitly the formation of a second rank tensor owing only to the presence of a finite ωq. This second rank tensor displays asymptotic behavior when the (reduced) quadrupole splitting is equal to either of the off-diagonal spectral densities J2 and J1. Line shape simulations for ωq values of less than a linewidth reproduce the general features of some recently reported 23Na DQF line shapes from biological systems. Distinct relaxation dynamics govern each of the tensorial components of the resonant signal revealing the influence of the experimental variables on the line shape.
Formation of striped surface phases by short-range forces
Authors: Meyer J. A. , Stranick S. J. , Weiss P. S.
Published: July 2002

Anisotropic Cu–O islands form striped periodic supergratings upon annealing an oxygen-exposed Cu(110) surface. While the formation of these striped phases has been attributed to long-range repulsive forces between stripes, we show using the one-dimensional lattice gas model and Monte Carlo simulations that short range adsorbate–adsorbate interactions are capable of producing striped phases.
Instabilities in homogeneous nonisothermal reactors: Comparison of deterministic and Monte Carlo simulations
Authors: Vlachos D. G.
Published: July 2002

A homogeneous nonisothermal continuous stirred tank reactor is modeled using the Monte Carlo method. The results are compared with deterministic bifurcation theory and time integration of the continuum unsteady state equations. Multiple solutions are determined for certain conditions and metastability is observed near ignition and extinction points. It is found that the amplitude of fluctuations increases near turning and Hopf bifurcation points. For finite size systems a supercritical Hopf bifurcation appears as a smooth transition from stationary solutions to oscillations, and near Hopf bifurcation point oscillations are more chaotic. Near a subcritical Hopf bifurcation, the stationary branch exhibits small amplitude oscillations of the same period as the oscillatory attractor, and metastability of both the stationary and the oscillatory attractors is observed. In the presence of isolas, metastability can cause a phase transition from the extinguished branch to the isola branch followed up by oscillations and subsequent extinction of the oscillations as the oscillatory attractor becomes metastable at high residence times.
An accurate computational model for the study of intermolecular interactions
Authors: Ro/eggen I. , Almlöf J. , Ahmadi G. Reza , Wind P. A.
Published: July 2002

A new computational method within the framework of extended group function models is introduced for describing intermolecular interactions between closed shell systems. The model is size extensive, applicable for any intersystem distance, and has a conceptual structure which facilitates interpretation. The basis set superposition error can be eliminated at the correlation level. Test calculations have been performed on the helium dimer and the helium trimer.
On the accuracy of gradient corrected density functional methods for transition metal complexes
Authors: Eriksson Leif A. , Pettersson Lars G. M. , Siegbahn Per E. M. , Wahlgren Ulf
Published: July 2002

Density functional theory with gradient corrections (DFTG) is applied to a series of second-row transition metal compounds. The mean absolute deviations from experiment for the atomic excitation energies and ionization potential are in the range 0.2–0.4 eV. In comparison with accurate multireference configuration interaction (MR-CI) and parametrized CI with parameter 80 (PCI-80) results for the binding energies of the MCH+2 cations, the mean absolute deviation is 6.7 kcal/mol to be compared with 17.8 kcal/mol at the modified coupled pair functional (MCPF) level. The DFTG result is more sensitive to the choice of asymptote with a mean deviation of 14 kcal/mol if the bonding atomic state is not used as asymptote. Product energies and transition states for the addition of methane to RhXL (X=H, Cl, L=H2O, NH3, PH3, and CO) also show the same trends as the PCI-80 comparison and represent an improvement over the MCPF results.
Potential energy surfaces of van der Waals complexes of water and hydrogen halides modeled using distributed multipoles
Authors: Wales David J. , Popelier Paul L. A. , Stone Anthony J.
Published: July 2002

Analytic first and second derivatives of the energy have been calculated for intermolecular potentials based upon distributed multipoles (to describe the electrostatics) and atom–atom Lennard-Jones terms (to describe dispersion and repulsion). This enables us to employ eigenvector-following to determine minima, transition states, and rearrangement mechanisms for a variety of van der Waals complexes. Where previous ab initio studies are available the agreement is usually satisfactory, but in some cases we find that a geometry corresponding to an ab initio minimum is a transition state with the model potential, or vice versa. Even in such cases the stationary points we identify will be useful in guiding more accurate calculations. The rearrangement mechanisms should be of particular interest in determining effective molecular symmetry groups and splitting patterns due to tunneling when low rearrangement barriers are present.
Theoretical prediction of the structure and infrared spectrum of the molecule–ion complexes NH3–H−, NH3–D−, and ND3–H−
Authors: van der Sanden G. C. M. , Reinsch E.-A. , van der Avoird A. , Wormer P. E. S. , Rosmus P.
Published: July 2002

The potential energy and dipole moment functions of the molecule–ion complex NH3–H−, and its isotopomers, NH3–D− and ND3–H−, have been calculated by the CEPA-2 method. From these functions we have computed the vibration–rotation–inversion states for J=0 and J=1, and the rovibrational transition probabilities between them. The complexes are found to have a nearly rigid structure in the ground state, with the H− or D− ion localized near a hydrogen (or deuterium) atom of the ammonia, and a small probability of tunneling between the three equivalent equilibrium positions. For the vibrationally excited states, however, the probability of this threefold tunneling increases considerably. The umbrella inversion of the ammonia is nearly quenched by the presence of the ion. The character of the rovibrational excitations is determined, and is found to be affected by the isotope substitutions. In order to investigate whether it is possible to evaluate the rovibrational transition probabilities for other total J transitions from the present calculations, an approximate expression for the vibrational transition probabilities has been derived. The formula appears to be valid for the ortho species, for the para species it is found to be rather crude.
Buoyant convection in the Belousov–Zhabotinsky reaction. I. Thermally driven convection and distortion of chemical waves
Authors: Zhang Dongmei , Peltier William R. , Armstrong Robin L.
Published: July 2002

A numerical study of the complete system of reaction-diffusion-convection equations that govern the dynamics of Belousov–Zhabotinsky (BZ) reactive fluid has been carried out in order to better understand the interaction of chemical waves with thermally driven buoyant convection in the BZ system. It was shown by direct numerical simulation that such convection may be responsible for inducing the abrupt and continuous changes of chemical wave velocity which have been routinely observed in experiments using magnetic resonance imaging techniques. Distortions of chemical waves propagating in upright cylindrical tubes, by the axisymmetric mode of convection, were predicted by the numerical simulation that strongly resemble those observed experimentally.
The complex H3N⋅⋅⋅Br2 characterized in the gas phase by rotational spectroscopy
Authors: Bloemink Hannelore I. , Legon A. C.
Published: July 2002

The ground-state rotational spectra of four isotopomers of the complex H3N...Br2 have been observed using pulsed-nozzle, FT microwave spectroscopy. Reaction of NH3 and Br2 was avoided by using a fast-mixing nozzle. The isotopomers H315N...79Br2, H315N...81Br79Br, H315N...79Br81Br, and H315N...81Br2 exhibited symmetric-top type spectra which were analyzed to give rotational constants B0, the centrifugal distortional constants DJ and DJK, the Br-nuclear quadrupole coupling constants χ(Brx) (x=i, inner or o, outer), and the Br spin-rotational coupling constants Mbb (Brx). The distances r(N...Bri)=2.72(2) Å and r(Br–Br)=2.335(10) Å in the complex were estimated from the rotational constants while the intermolecular stretching force constant kσ=18.5(5) N m−1 was determined from DJ. An interpretation of the magnitude of the χ(Brx) values and the difference Δχ(Br)=χ(Bri)−χ(Bro) allows the conclusion that the extent of intermolecular charge transfer is probably small and that a polarization of Br2 equivalent to the transfer of ∼0.1e from Bri to Bro accompanies formation of the complex. The properties of the series of complexes H3N...XY, where XY=Cl2, BrCl, and Br2 are compared.
Simulation of C60 through the plastic transition temperatures
Authors: Johnson Scott D. , Mountain Raymond D. , Meijer Paul H. E.
Published: July 2002

An interaction model is presented that accounts for the phase transitions in a crystalline C60 at 90 and 250 K. This was obtained by a molecular dynamics simulation of the C60 crystal. These transitions are used as indirect evidence of the appropriateness of the charge distribution. Additional support of the proposed charge distribution comes from the agreement of multipole energies with theoretical considerations.
Reaction of SiC+n (n=1–6) and SiC−n (n=2–4) with O2 molecules in a radio frequency ion trap
Authors: Negishi Y. , Kimura A. , Kobayashi N. , Shiromaru H. , Achiba Y. , Watanabe N.
Published: July 2002

Ion-molecule reactions of silicon-carbon binary cluster ions SiC+n (n=1–6) and SiC−n (n=2–4) with O2 molecules were studied by using an rf ion trap. For all selected positive and negative parent ions, only SiCnO+ or SiCnO− were produced, respectively. The rate constants for the oxidation of these clusters were determined for the first time. The size dependence of the reactivities are discussed, and the presence of the ring form positive ions for n=3 and 5 are suggested.
Decane under shear: A molecular dynamics study using reversible NVT-SLLOD and NPT-SLLOD algorithms
Authors: Mundy Christopher J. , Siepmann J. Ilja , Klein Michael L.
Published: July 2002

In a recent article [J. Chem. Phys. 102, 3376 (1995)] molecular dynamics (MD) was used to calculate the shear viscosity of liquid n-decane using an intermolecular potential that reproduces the liquid–vapor coexistence as well as the critical temperature. Semi-quantitative agreement with experiment was obtained. To further test the model, n-decane is examined under shear using reversible nonequilibrium molecular dynamics (NEMD) in both the isothermal–isobaric (NPT) and the canonical (NVT) ensembles. The algorithm proposed herein, for studying a molecular system, is an atomic version of the so-called SLLOD algorithm originally introduced by Edberg, Evans, and Morriss [J. Chem. Phys. 84, 6933 (1986)]. Results obtained by Daivis and Evans [J. Chem. Phys. 100, 541 (1994)] indicating the disappearance of a high shear-rate (γ) thickening regime when the simulations are performed in the NPT ensemble are corroborated. The ‘‘plateau’’ region, where the viscosity is essentially independent of shear-rate, γ is examined in detail in order to interpolate back to the Green–Kubo value.
Surface migration as a determinate factor of the sticking coefficient derived from the chemisorption model
Authors: Izawa M. , Kumihashi T.
Published: July 2002

A chemisorption process model has been developed and a sticking coefficient has been formulated by taking the surface migration of physisorbed particles into account. This model incorporates Langmuir’s model for use when the migration barrier is infinite, and the standard precursor model for use when dealing with a bare surface. The migration of incident particles was found to make the sticking coefficient independent of the surface coverage at low temperatures (≊100 K). At higher temperatures, the migration appears to be a hidden parameter in the chemisorption process. The effect of migration can probably be confirmed, even at room temperature, by analyzing the chemisorption using free radicals.
The search for the low-lying states of the silicon carbide cluster cation Si2C+2
Authors: Ignatyev Igor S. , Schaefer Henry F.
Published: July 2002

Recent mass-spectroscopic experiments on the silicon carbide cluster cations have pointed out an urgent need for reliable structural and energetic information. For this reason the doublet potential energy surface of the disilicon dicarbon cation was investigated at several levels of theory, in order to locate low-lying minima. Three low-lying minima were found. The slightly distorted rhombic structure of the cation, (2Ag) which is the ground state for the neutral cluster, is predicted to lie 25 kcal/mol higher than the linear isomer (2Πg). An intermediate minimum corresponds to the distorted rhomboid structure (2A′) and lies 17 kcal/mol above the global minimum. The first two structures appear to correspond to those identified in the experiment and the theoretical difference between the ionization potentials of these two isomers agrees satisfactorily with that estimated by experimental methods. The best estimate of the individual ionization potential values are 0.5 eV lower than the lower limits of the experimental estimates.
Constant pressure molecular dynamics simulation: The Langevin piston method
Authors: Feller Scott E. , Zhang Yuhong , Pastor Richard W. , Brooks Bernard R.
Published: July 2002

A new method for performing molecular dynamics simulations under constant pressure is presented. In the method, which is based on the extended system formalism introduced by Andersen, the deterministic equations of motion for the piston degree of freedom are replaced by a Langevin equation; a suitable choice of collision frequency then eliminates the unphysical ‘‘ringing’’ of the volume associated with the piston mass. In this way it is similar to the ‘‘weak coupling algorithm’’ developed by Berendsen and co-workers to perform molecular dynamics simulation without piston mass effects. It is shown, however, that the weak coupling algorithm induces artifacts into the simulation which can be quite severe for inhomogeneous systems such as aqueous biopolymers or liquid/liquid interfaces.
Observations of rotational magnetic moments in the ground and some excited vibrational Σ states of C2H2, C2HD, and C2D2 by magnetic vibrational circular dichroism
Authors: Tam Cheok N. , Bour Petr , Keiderling Timothy A.
Published: July 2002

Magnetic vibrational circular dichroism (MVCD) spectra of acetylene and its deuterated isotopomers have been recorded for the following Σ symmetry combination and overtone bands of C2H2: ν4+ν5; C2HD: ν4+ν5, 2ν4, 2ν5; C2D2: ν4+ν5, the ν3 fundamental for C2HD and C2D2; and the ν4→2ν4+ν5 and ν5→ν4+2ν5 hot bands for C2H2. For a Σg→Σu transition, the MVCD A terms observed must arise primarily from the rotational Zeeman effect. These negative A1/D0 values for low J″ transitions confirm that the sign of the rotational g-value for acetylene is positive. The rotational magnetic moments in both the lower and upper vibrational states were determined by comparison of moment analyses of experimental and simulated MVCD spectra obtained with a model Hamiltonian for acetylene. The gJ values in all the excited bending combination and overtone vibrational levels observed are smaller than those in the ground and the first excited stretching vibrational levels. This observation has been confirmed by theoretical simulation of the MVCD spectra of the ν4+ν5 combination band of C2H2. From these MVCD results, for C2H2, gJ(ground)=+0.0535±0.0033 and Δg(ν4+ν5)=−0.0061±0.0004; for C2D2, gJ(ground)∼gJ(ν3) =+0.0363±0.0048, Δg(ν4+ν5)=−0.0052±0.0031; and for C2HD, gJ(ground)∼gJ(ν3)=+0.0409 ±0.0069. These are the first quantitative, MVCD determinations of nondegenerate excited state g values distinctly different from the ground state. The decrease in g value correlates with off-axis deformation of the linear C2H2 rotation.
The span of one-dimensional multiparticle Brownian motion
Authors: Sastry G. Madhavi , Agmon Noam
Published: July 2002

A closed-form expression is obtained for the time evolution of the territory covered by N independently diffusing particles starting from the origin in one-dimension, with and without bias. For the latter case, the transcendental-approximation derived is essentially exact for any number of particles.
The transport properties of fluid mixtures near the vapor–liquid critical line
Authors: Luettmer-Strathmann J. , Sengers J. V.
Published: July 2002

We apply the mode-coupling theory for the dynamics of critical fluctuations to binary fluid mixtures near the vapor–liquid critical line and derive a set of coupled equations for the mode-coupling contributions to the diffusion coefficients of the relevant hydrodynamic modes. An approximate solution of the mode-coupling integrals yields representative equations for the effects of the critical fluctuations on the thermal conductivity, the viscosity, the mutual diffusivity and the thermal-diffusion coefficient, which are valid not only in the asymptotic critical region but also describe the crossover to regular behavior far away from the critical line. The ‘‘crossover’’ functions depend on the thermodynamic properties of the mixtures, background values of all the transport coefficients and two concentration-dependent cutoff wave numbers. We compare the proposed crossover model with experimental thermal-conductivity data for mixtures of carbon dioxide and ethane in the critical region and find good agreement between theory and experiment.
Auger spectrum of octatetraene by a two-particle propagator method
Authors: Liegener Christoph-Maria
Published: July 2002

A matrix eigenvalue formulation is presented for the pole search problem occurring in the renormalized two-particle Green’s function method. Emphasis is on the method’s suitability for calculations on large molecules. Thus it employs a first-order irreducible vertex part. The reformulation is achieved by a transformation from the doubly-occupied orbital space to a space spanned by orbital and satellite indices. The method is applied to the calculation of the Auger spectrum of octatetraene. Evaluation of the average hole–hole separation for every state shows that the Auger spectrum of octatetraene should in general already provide a model for the Auger spectrum of polyacetylene. However, the results also predict that several single-state peaks at low binding energy can be expected in the Auger spectrum of octatetraene. These should be due to delocalized states which are characteristic for long finite polyenes.
Spin-unrestricted time-dependent Hartree–Fock theory of frequency-dependent linear and nonlinear optical properties
Authors: Karna Shashi P.
Published: July 2002

A self-consistent time-dependent unrestricted Hartree–Fock (TDUHF) theory of linear and nonlinear optical properties is presented. Expressions are derived to calculate the elements of the linear polarizability tensor α, the first-hyperpolarizability tensor β, and the second-hyperpolarizability tensor γ, in terms of spin-adapted perturbed density matrices. For the hyperpolarizability tensors, β and γ, expressions are also derived from the lower-order solutions to the TDUHF equations. A novel feature of the present formulation is that it automatically allows for the separation of the contributions to (hyper)polarizability tensors from individual spin. Results obtained from the calculations of α(ωσ;αa) for H, C, N, O, F, Si, P, S, Cl, O2, NO, and OH and of β(−ωσ;ωa,ωb) corresponding to various second-order nonlinear optical processes for NO and OH radicals are presented. The present results for α show excellent agreement with the literature data. The calculated result for β∥ in the case of the NO radical is too small and has a different sign in comparison to the published experimental data. For both NO and OH, the β tensor for different second-order nonlinear optical processes show the order: β(−2ω;ω,ω)≳β(−ω;0,ω) ≡β(0;ω,−ω)≳β(0;0,0).
Diaelastic pressure-induced effects on spectral holes in crystals
Authors: Kikas J. , Leiger K.
Published: July 2002

A statistical theory is developed in order to describe the pressure-induced shift and broadening of spectral holes in pressure-tuning experiments in crystals. The theory accounts for the defect-related diaelastic effect (induction of internal inhomogeneous strain fields by the applied hydrostatic pressure due to the host-defect compressibility and/or size mismatch). General results are specified and analyzed in the case of similar defects and for two different types of point defects. The former case yields no hole broadening, while the latter one does. A similar consideration applies to electric- and magnetic-field-induced effects on spectral holes in crystals as well.
Pattern formation on a nonuniformly active ring
Authors: Liauw Marcel A. , Ning Ju , Luss Dan
Published: July 2002

A rich variety of spatiotemporal patterns exists on reactive rings having a nonuniform activity. Simulations of a case with a nonuniformity indicate that when the local phase-plane features change along the ring, novel motions evolve which cannot form on a uniformly active ring. An example is a rotating pulse which can move only in one direction but not in the other. The nonuniformity increases the number of possible patterns which can exist on a specific ring relative to that existing on a uniformly active one. Thus the nonuniformity increases the sensitivity of the observed patterns to changes in the initial conditions or perturbations.
Lattice model of a hydrogen-bonded polymer blend
Authors: Hobbie E. K. , Han C. C.
Published: July 2002

A mean-field lattice model is used to investigate some possible aspects of the low-temperature behavior of a lower critical solution temperature (LCST) polymer blend made miscible by a dilute distribution of a hydrogen-bonding comonomer. The nature of the binding interaction is examined for both a random comonomer distribution and for the case of identically end-modified chains. For the latter case, competition between the interaction responsible for the immiscibility of the homopolymer components and the attraction due to hydrogen bonding leads to a stable intermediate phase. For random distributions, polydispersity in the binding interaction restricts this intermediate phase to an extremely narrow window in reduced temperature, consistent with a ‘‘smeared’’ transition. The free-energy landscape contains local minima separated by energy barriers.
Transport properties of a binary gas mixture of molecules with internal energy. II. Thermal conductivity
Authors: Singh G. S. , Prasad N. , Kumar B.
Published: July 2002

General expressions in terms of various square-bracket integrals for the thermal conductivity λ and its first-order approximation [λ]1 of a classical dilute binary gas mixture of nonspherical molecules have been obtained by extending the Taxman classical kinetic theory of transport phenomena within the framework of the Chapman–Enskog second approximation method for the mixture of spherical molecules. In the limit, [λ]1 formula tends to the corresponding formula for a dilute simple gas of nonspherical molecules or a dilute binary gas mixture of spherical molecules. The dimensionalities of the square-bracket integrals belonging to [λ]1 for the mixture of rigid spheroids having C∞v symmetry have been reduced by integrating analytically the velocity and angular velocity parts of the integrals. The resulting reduced quadratures are over the orientational coordinates of two colliding like or unlike spheroids. Two limiting checks are discussed which confirm that the quadrature formulas are correct.
Dynamics of electron-hole pair creation at semiconductor surface via charge transfer between molecule and solid
Authors: Masato Suzuki
Published: July 2002

In order to clarify the effects of the nonadiabaticity on the inelastic scattering between molecule and solid, we numerically investigate the dynamical processes in which the electron-hole (e-h) pairs are created at semiconductor surfaces by the charge transfer (CT) between the molecule and solid. Using the Fano–Anderson model that is extended to the multi-electron systems together with the real time Trotter’s formula, we take into account the effects of the quantum mechanical back reaction from the electronic transition to the motion of molecule, and the effects of the nonadiabaticity and the irreversibility for the CT processes. We also clarify the nature of the energy transfer from the motion of molecule to the e-h pairs as a function of the four parameters; the band gap energy in solid, the incident energy of the molecule, the resonant transfer energy between solid and molecule, and the electron–electron repulsive energy in the molecule. It is concluded that the energy transfer to the e-h pairs increases as increasing the lifetime of the intermediate states wherein the charge is separated between solid and molecule by the CT. Thus, the effects of the above four parameters on the energy transfer are clarified based on the lifetime of the CT intermediate states.
The SiOH–HSiO system: A high level quantum mechanical study
Authors: Yamaguchi Yukio , Xie Yaoming , Kim Seung-Joon , Schaefer Henry F.
Published: July 2002

The potential energy hypersurface of the SiOH–HSiO system has been investigated using ab initio electronic structure theory. The geometries and physical properties including dipole moments, harmonic vibrational frequencies, and associated infrared (IR) intensities for the two equilibrium and isomerization (1,2 hydrogen shift) transition state structures have been determined employing self-consistent-field (SCF) and configuration interaction with single and double excitations (CISD) methods. At the CISD optimized geometries, single point energies of the three stationary points were evaluated using coupled cluster with single and double excitations (CCSD) and CCSD with perturbative triple excitations [CCSD(T)] levels of theory. In the correlated procedures three different frozen core schemes (6 frozen core, 2 frozen core, and 0 frozen core) have been applied to examine the importance of 1s, 2s, and 2p core electrons. With the SCF method two isomers (A and B) were found for HSiO. However, at the CISD level of theory structure B with the bond angle of about 93° has collapsed to structure A with the bond angle of about 122°, confirming the findings of lower level studies. At the highest level of theory, CCSD(T) with triple zeta plus double polarization (TZ2P) augmented with higher angular momentum and diffuse functions TZ2P(f,d)+diff basis set, TZ2P(f,d)+diff CCSD(T), the energy separation between SiOH and HSiO is predicted to be 12.1 kcal/mol. This energy separation becomes 9.8 kcal/mol with the zero-point vibrational energy (ZPVE) correction. With the same method the classical energy barrier for the exothermic isomerization reaction (HSiO→SiOH) was determined to be 25.8 kcal/mol and the activation energy (with the ZPVE correction) becomes 24.1 kcal/mol. The two frozen core approximations have generated 0.005 Å (6 frozen core) and 0.001 Å (2 frozen core) in error for the SiO bond length compared to no frozen core method. In energetics these two frozen core schemes have produced errors of ±0.40 kcal/mol for the CCSD and CCSD(T) methods and error of ±0.95 kcal/mol for the CISD method.
Spectral line shapes in systems undergoing continuous frequency modulation
Authors: Agrawal Paras M. , Sorescu Dan C. , Kay Ronald D. , Thompson Donald L. , Raff Lionel M. , Conrey J. Brian , Jameson A. Keith
Published: July 2002

The power spectrum line shapes for oscillators undergoing a continuous modulation of the vibrational frequency are investigated. It is shown that the single, sharp line normally characteristic of such systems broadens and exhibits a wealth of fine structure components. The characteristic fine structure pattern is one of decreasing amplitude and spacing. This continuous frequency modulation (CFM) effect has been examined for a series of model oscillators that includes harmonic systems with linear and exponential variation of the frequency without amplitude damping, a harmonic system with exponential damping of both the resonant frequency and the amplitude, and a Morse oscillator whose kinetic energy is being exponentially damped. An analytic expression for the power spectrum of a harmonic oscillator whose frequency is varying linearly with time is derived. This result demonstrates that the position of the fine structure extrema depends linearly upon the initial oscillator frequency and the square root of the absolute value of the modulation rate. The peak-to-peak spacing is shown to be proportional to the square root of the absolute value of the modulation rate. It is suggested that the CFM effect is the fundamental explanation of many previous empirical observations concerning power spectra. The CFM effect for a harmonic system with an exponentially modulated frequency is very similar to that observed for linear modulation. When amplitude depression is included, there is a significant intensity decrease of many of the spectral lines. Investigation of a Morse oscillator shows that energy transfer in an anharmonic system produces a CFM effect. By assuming that the analytic result for a harmonic oscillator with a linear modulation is transferable to the anharmonic case, an expression is obtained that relates the peak-to-peak fine structure spacing to the Morse potential parameters, the initial oscillator energy and the IVR rate coefficient. An experimental example of a CFM effect is presented by taking an NMR spectrum of H2O and HCCl3 in DCCl3 while the main B0 field is varying with time. The CFM effect is used to extract energy transfer rate coefficients for a diatomic molecule isolated in an argon matrix at 12 K and for total IVR rate coefficients for relaxation of the N=O and O–H local modes in cis-HONO. It is also shown that instantaneous energy transfer rates in small molecules can be determined by using local frequency analysis to compute the temporal variation of the CFM band spacings. It is concluded that line shape analysis can be effectively used as a probe of energy transfer rates.
Quantitative adsorbate orientation from vibrational spectra: Ethoxides on Cu(111)
Authors: Street Shane C. , Gellman Andrew J.
Published: July 2002

Fourier-transform infrared reflection absorption spectra have been obtained for a set of deuterium and isotopic carbon (13C) labeled ethoxides adsorbed on the Cu(111) surface at saturation coverages. The frequency shifts upon 13C isotopic replacement indicate that the proper description of the skeletal stretches is in terms of CCO modes and not local modes of CO and CC bonds. Comparison of the spectra of the deuterated ethoxides with gas phase and multilayer spectra of the corresponding ethanols allowed the determination of the orientation of the adsorbed ethoxide. A solution for the average structure of ethoxide shows that the plane of the species is tilted 17±2° from the plane of the surface normal and that the C–C bond, is tilted 70±2° from the surface normal. The tilt of the C–O bond from the surface normal is 17±2°. At elevated temperatures the ethoxides undergo β-hydride elimination and desorb as acetaldehydes. Temperature programmed reaction spectra for β-hydride elimination of the selectively deuterated ethoxides reveal both a primary kinetic isotope effect and a measurable secondary isotope effect.
Hyperfine structures of the 7Li2 b3Πu, 23Πg, and 33Πg states: Continuous wave perturbation facilitated optical–optical double resonance spectroscopy
Authors: Li Li , Yiannopoulou A. , Urbanski K. , Lyyra A. M. , Ji B. , Stwalley W. C. , An T.
Published: July 2002

The hyperfine structure of the b 3Πu, 2 3Πg, and 3 3Πg states of 7Li2 has been studied by continuous wave perturbation facilitated optical–optical double resonance fluorescence excitation spectroscopy. The b 3Πu state has case bβJ coupling scheme. The hyperfine splittings of the two perturbed b 3Πu v=19, N=10, J=11e, s, F1 and v=19, N=5, J=4e, a, F3 levels were resolved and hyperfine constants determined to be +9.90 MHz and −20.94 MHz, respectively. Fermi contact is the main source of the hyperfine interaction and bF(b 3Πu)=+107 MHz has been obtained. The coupling schemes of different 2 3Πg levels are different: N=4, 6 levels observed from the b 3Πu v′=19, N′=5, J′=4e, a, F3 intermediate level and the N=10 levels observed via the b 3Πu v′=19, N′=10, J′=11e, s, F1 intermediate level have case bβJ coupling; the N=5 levels observed via the b 3Πu v′=19, N′=5, J′=4e, a, F3 intermediate level and the N=9, 11 levels observed from the b 3Πu v′=19, N′=10, J′=11e, s, F1 intermediate level are closer to case bβS coupling. Hyperfine constants have been calculated and the Fermi contact constant, bF(2 3Πg)=60 MHz is obtained. The 3 3Πg state is a Rydberg state and has case bβS coupling with bF(3 3Πg)≊100 MHz.
A 193-nm-laser photofragmentation time-of-flight mass spectrometric study of dimethylsulfoxide
Authors: Zhao H.-Q. , Cheung Y.-S. , Heck D. P. , Ng C. Y. , Tetzlaff T. , Jenks W. S.
Published: July 2002

The photodissociation of dimethylsulfoxide [(CH3)2SO] at 193.3 nm has been investigated using the molecular beam time-of-flight (TOF) mass spectrometric technique. In addition to CH3 and SO, CH3SO is also observed as a stable primary product, indicating that CH3SO+CH3 is an important product channel for the 193.3 nm photodissociation of (CH3)2SO. The analysis of the TOF data provides evidence that SO is formed via a stepwise mechanism: (CH3)2SO+hν (193.3 nm)→CH3SO+CH3→2CH3+SO. The analysis also indicates that ≈53% of the primary CH3SO radicals undergo further dissociation to produce CH3+SO, yielding a quantum yield of ≈1.53 for CH3. Within the sensitivity of our experiment, the product channel of CH3SCH3+O is not found. The angular distribution for the formation of CH3SO+CH3 is found to be isotropic, an observation consistent with a predissociation mechanism, in which the dissociation of photoexcited (CH3)2SO is slow compared to its rotational period. The energetics for selected dissociation reactions of (CH3)2SO have also been investigated by ab initio calculations at the G2(MP2) level of theory. The experimental dissociation energy at 0 K (53±2 kcal/mol) for the CH3–SOCH3 bond obtained here is in excellent agreement with the theoretical prediction of 52.6 kcal/mol.
The microwave spectrum of the NH2 radical: The hyperfine structure of the 2B1 ground electronic state
Authors: Tonooka Mizuho , Yamamoto Satoshi , Kobayashi Kaori , Saito Shuji
Published: July 2002

The pure rotational spectrum of the NH2 radical in its X̃ 2B1 ground electronic state was measured in the frequency region of 230–470 GHz by microwave spectroscopy. The radical was generated in a free space cell by dc-glow discharge of NH3. Seventy nine fine and hyperfine components of four rotational transitions were measured, and were analyzed by least squares methods. The hyperfine coupling constants for both the nitrogen and hydrogen nuclei were redetermined with higher precision than those of the previous analysis for microwave optical double resonance (MODR) data. The nuclear spin–rotation constants for the hydrogen nucleus as well as the nitrogen nucleus were found to be anomalously large. The anomalous constant of Caa(H) is interpreted by the same reason for the anomalous large value for Caa(N) originating from the large A constant and the relatively low-lying Ã 2A1 electronic state.
A perturbation molecular orbital theory of electron-transfer rates
Authors: Dougherty Ralph C.
Published: July 2002

Perturbation molecular orbital (PMO) theory is used to approximate the electronic matrix element in the semiclassical expression for the rate of nonadiabatic electron transfer (ET). The resulting expression gives a satisfactory account of the intramolecular ET rate data reported by Closs, Miller, and co-workers. We develop the idea of electron-transfer efficiency for the contribution of electron transfer to the observed kinetics of ion–molecule collisions followed by electron transfer. Electron-transfer efficiency comes from the calculated ET rate divided by the maximum calculated ET rate. Electron-transfer efficiency values are also obtained by dividing the observed reaction rate by the collision rate, calculated by the PMO treatment of ion–molecule collision rates. We applied this approach to data on electron transfer from sulfurhexafluoride or perfluoromethylcyclohexane anions to aromatic acceptors. The structural reorganization energies, λs, for these reactions were 0.016 and 0.046 eV, respectively. The vibrational reorganization energies, λv, for the reactions were 1.01 and 1.00 eV, respectively. Electron transfer from either of the donor anions to fluoranil occurs in the inverted region.
Prediction of the temperature dependence of K-changing rotational collisional processes in CH3Cl
Authors: Pape Travis W. , De Lucia Frank C. , Skatrud David D.
Published: July 2002

The complexity of rotational energy transfer (RET) in polyatomic molecules has in general precluded prediction of nontrivial temperature and energy related effects. In this paper, experimental measurements of ΔK=3n RET in CH3Cl between 200 K and 400 K are successfully compared with predictions of an earlier model.
Reactions of laser-ablated Zn and Cd atoms with O2: Infrared spectra of ZnO, OZnO, CdO, and OCdO in solid argon
Authors: Chertihin George V. , Andrews Lester
Published: July 2002

Laser ablated Zn and Cd atoms contain sufficient excess energy to react directly with O2 to produce the subject molecules. A zinc isotopic triplet at 769.2, 766.8, 764.5 cm−1 gave a doublet with statistical isotopic oxygen confirming the identification of ZnO. A previous resonance Raman fundamental at 769 cm−1 is reassigned to ZnO, which suggests a green visible absorption for the zinc oxide molecule. A zinc isotopic triplet at 748.2, 744.4, 740.9 cm−1 produced a triplet with statistical isotopic oxygen and isotopic shifts appropriate for the linear OZnO dioxide molecule. Cadmium is less reactive than zinc; infrared absorptions at 645.1 and 626.6, 625.4 cm−1 exhibit proper oxygen isotopic behavior for assignment to CdO and OCdO and the latter showed partially resolved cadmium isotopic splittings.
Low energy (<5 eV) F+ and F− ion transmission through condensed layers of water
Authors: Akbulut Mustafa , Madey Theodore E. , Nordlander Peter
Published: July 2002

We report on the transmission of F+ and F− ions through ultrathin films of condensed water at 20 K, and compare the experimental results with theoretical calculations. The F+ and F− ions are produced by ESD (electron stimulated desorption) of a PF3 monolayer adsorbed on a Ru(0001) surface (PF3/Ru(0001) surface). We find two surprising results: (a) the off-normal F+ signal is attenuated to ∼1% by only ∼2.5 monolayer (ML) of H2O, while a much thicker layer, ∼10 ML of H2O, is necessary for equivalent attenuation of the F− ion emission, and (b) 1 ML of H2O increases the emission of F− ions and causes a dramatic change in the ion angular distribution. The striking changes in the angular distribution of F− ions transmitted through condensed H2O films indicate that elastic scattering is an important process in determining the attenuation of F− by H2O. No direct evidence for any kind of ion–molecule chemical reaction or collision induced dissociation reaction has been found. The strong attenuation of F+ without substantial changes in angular distribution suggests that charge transfer processes are important in limiting the transmission of F+ ions. Our quantum mechanical calculations indicate that the increase in F− emission upon adsorption of⩽1 ML of H2O is mainly due to a decrease in the neutralization probability of F− with the substrate, by a dielectric screening mechanism. The calculations also show that the increase in the F− survival probability saturates after a water bilayer (1 ML H2O) is formed, which is in excellent agreement with the experiment. Our measurements show no evidence for diffusion of H2O on the PF3/Ru(0001) surface between 20 and 60 K; the lack of diffusion, together with exponential attenuation of F+ and F− with H2O thickness, indicates that H2O vapor condensed on the PF3/Ru(0001) surface at 20 K grows statistically.
Depolarized dynamic light scattering from three low molecular weight glass forming liquids: A test of the scattering mechanism
Authors: Patkowski A. , Steffen W. , Nilgens H. , Fischer E. W. , Pecora R.
Published: July 2002

Depolarized dynamic light scattering (DDLS) experiments are performed on the glass forming materials ortho terphenyl (OTP), bis-methyl-methoxy-phenyl-cyclohexane (BMMPC), and bis-methyl-phenyl-cyclohexane (BMPC). Depolarized spectra are obtained for the bulk liquids and for solutions of varying concentration in CCl4. Effective optical anisotropies of the molecules are obtained from the solution spectra. A detailed analysis of the DDLS spectra, as well as a comparison with the results of quasielastic neutron scattering (QENS) experiments, indicates that the DDLS spectra in the bulk liquids result from at least two physical mechanisms: (i) The low frequency part of the spectrum, in the frequency range of the α peak, arises mainly from the molecular reorientation mechanism. The integrated intensity of this contribution is proportional to the square of the effective molecular anisotropy. (ii) At higher frequencies (but still below the Boson peak), the predominant contribution to the DDLS spectrum arises from interaction-induced scattering. The intensity of this contribution increases with increasing temperature (decreasing density) in the way expected for the cancellation effect. The implications of these results for the comparison of DDLS and QENS experiments and for the comparison of the data to the predictions of mode coupling theories of glass forming liquids are discussed.
A unified framework for quantum activated rate processes. II. The nonadiabatic limit
Authors: Cao Jianshu , Voth Gregory A.
Published: July 2002

A recently proposed unified theoretical framework for quantum activated rate constants is further developed and explored. The case of electronically nonadiabatic rate processes is considered, and the weak coupling limit explicitly investigated by an expansion of the rate constant expression. By virtue of this approach, a semiclassical Golden Rule expression is derived after a series of steepest descent approximations. The semiclassical analysis in turn reveals a closed form path integral expression for the quantum activated rate constant in the nonadiabatic (Golden Rule) limit which is free of harmonic and/or classical approximations for the many-dimensional nuclear (vibronic) modes. The latter expression is amenable to direct calculation in realistic systems through computer simulation.
The interpretation of vibrational spectra of ionic melts
Authors: Pavlatou Evangelia A. , Madden Paul A. , Wilson Mark
Published: July 2002

Computer simulations of the short-time, vibrational dynamics of the network-forming ionic melts, LaCl3 and ZnCl2, and of their mixtures with network-breaking alkali halides are described. In the mixtures, high frequency peaks in the vibrational density of states are shown to be describable in terms of the normal coordinates of vibration of transient molecular ion species, like LaCl63− and ZnCl42−. Novel simulation methods are presented which allow this association to be established. In the pure melts, the vibrational motions retain a strong aspect of this local polyhedral unit vibrational character, but the effects of network-induced coupling between the vibrations of different units become pronounced, particularly in ZnCl2. The calculated vibrational spectra are compared with extensive Raman data on these systems, and with infrared and neutron spectra in pure ZnCl2. For the mixtures, remarkably good agreement with experiment is found, confirming the high quality of the representation of the interionic interactions obtained with the polarizable ion model potentials used. For the melts, there are discrepancies between the peak frequencies observed in the vibrational DOS and the Raman spectra. These discrepancies are likely to be due to the network-induced couplings, whose effect on the Raman (and infrared) spectra is not fully included in the calculated DOS.
Semiclassical wave packet calculations on ion–molecule reactions: Studies on B+(3Pu)+H2 reaction
Authors: Vijay Amrendra , Billing G. D.
Published: July 2002

We present the investigations of nonadiabatic scattering processes (reactive as well charge-transfer) occurring in B++H2 reaction in the gas phase on the triplet electronic surfaces utilizing a mixed quantum-classical approach to scattering of three particle systems in hyperspherical coordinates. The time-dependent Schrödinger equation is solved in diabatic representation using wave packet propagation method on a grid in two quantum dimensions. The potential-energy surfaces have been obtained using the valence-bond diatomics-in-molecule approach.