bluuebunny/crossref_metadata_2025 huggingface dataset:
Oscillatory instabilities during formic acid oxidation on Pt(100), Pt(110) and Pt(111) under potentiostatic control. II. Model calculations
Authors: Strasser P. , Eiswirth M. , Ertl G.
Published: July 2002

A kinetic model is developed for the electrocatalytic oxidation of formic acid on Pt under potentiostatic control. The model development proceeds stepwise via a simple model of the electrocatalytic CO oxidation. The full model consists of four coupled, nonlinear ordinary differential equations. The scanned and stationary current/outer potential (I/U) behavior, stationary current oscillations, two-parameter bifurcation diagrams and stirring effects are simulated using realistic model parameters. The numerical findings are found to be consistent with the experimental results given by Strasser et al. The model reproduces period-1 as well as mixed-mode oscillations. Furthermore, a mechanistic analysis of the model was performed: two suboscillators are identified whose characteristics allow a plausible interpretation of the observed dynamics. After a classification of the suboscillators into previously described categories, an attempt is made to identify the minimal mechanistic requirements for electrochemical current oscillations.
Comment on “Density functional theory study of some structural and energetic properties of small lithium clusters” [J. Chem. Phys. 105, 9933 (1996)]
Authors: Yannouleas Constantine , Landman Uzi
Published: July 2002

The Ionization Potentials of small LiN clusters are calculated with a Shell Correction Method. They are used to illustrate that, within the jellium approximation, deformed cluster shapes provide an adequate description of the observed systematic size dependence of the properties of simple metal clusters. Such deformation effects were overlooked in the analysis of Gardet et al.
Response to “Comment on ‘Density functional theory study of some structural and energetic properties of small lithium clusters’ ” [J. Chem. Phys. 107, 1032 (1997)]
Authors: Gardet Georges , Rogemond Frangois , Chermette Henry
Published: July 2002

It is shown that whereas the spherical and spheroidal jellium models are inadequate to describe lithium clusters, only the ellipsoidal jellium model is adequate. The corresponding result, obtained by Yannouleas and Landman, was unpublished at the time of submission of our paper.
Memory kernels and effective Hamiltonians from time dependent methods. I. Predissociation with a curve crossing
Authors: Desouter-Lecomte M. , Liévin J.
Published: July 2002

Non-Hermitian complex effective Hamiltonians resulting from bound-continuous partitioning techniques are built from time dependent methods. We treat predissociation processes with a curve crossing. The energy dependent shift and half-width matrices are obtained simultaneously by a generalization of the wave packet Golden Rule treatment, as the real and imaginary parts of the Fourier transform of a memory kernel matrix. The latter contains auto- and cross-correlation functions. They are overlap integrals among the projections on the continuum of bound states multiplied by the interchannel coupling function responsible for the predissociation. These wave packets are propagated by the propagator of the sole continuous subspace. An approximate analytical expression of this correlation matrix is established for the harmonic/linear model. The numerical method is applied to the electronic predissociation of the MgCl A 2Π state, to a Morse/exponential model and to a predissociation with two coupled repulsive decay channels. The comparison between the correlation time scales and the Golden Rule lifetimes is decisive so as to justify whether the memory kernel can be considered as an impulsive kernel. This Markovian approximation implies that the two time scales are well separated. In the energy domain, this corresponds to the introduction of a mean phenomenological effective Hamiltonian that neglects the energy variation of the discrete-continuous coupling elements. We observe that the separation of the time scales is effective for weakly open systems, but not for overlapping metastable states for which the perturbative-theory widths largely exceed the mean energy spacing. This confirms from a temporal viewpoint that a nonperturbative treatment should not neglect the energy dependence of the effective Hamiltonian, as currently assumed in the study of largely open systems.
HO
2
rovibrational eigenvalue studies for nonzero angular momentum
Authors: Wu Xudong T. , Hayes Edward F.
Published: July 2002

An efficient parallel algorithm is reported for determining all bound rovibrational energy levels for the HO2 molecule for nonzero angular momentum values, J=1, 2, and 3. Performance tests on the CRAY T3D indicate that the algorithm scales almost linearly when up to 128 processors are used. Sustained performance levels of up to 3.8 Gflops have been achieved using 128 processors for J=3. The algorithm uses a direct product discrete variable representation (DVR) basis and the implicitly restarted Lanczos method (IRLM) of Sorensen to compute the eigenvalues of the polyatomic Hamiltonian. Since the IRLM is an iterative method, it does not require storage of the full Hamiltonian matrix—it only requires the multiplication of the Hamiltonian matrix by a vector. When the IRLM is combined with a formulation such as DVR, which produces a very sparse matrix, both memory and computation times can be reduced dramatically. This algorithm has the potential to achieve even higher performance levels for larger values of the total angular momentum.
A new integral equation formalism for the polarizable continuum model: Theoretical background and applications to isotropic and anisotropic dielectrics
Authors: Cancès E. , Mennucci B. , Tomasi J.
Published: July 2002

We present a new integral equation formulation of the polarizable continuum model (PCM) which allows one to treat in a single approach dielectrics of different nature: standard isotropic liquids, intrinsically anisotropic medialike liquid crystals and solid matrices, or ionic solutions. The present work shows that integral equation methods may be used with success also for the latter cases, which are usually studied with three-dimensional methods, by far less competitive in terms of computational effort. We present the theoretical bases which underlie the method and some numerical tests which show both a complete equivalence with standard PCM versions for isotropic solvents, and a good efficiency for calculations with anisotropic dielectrics.
Detection of cyclic carbon clusters. II. Isotopic study of the ν12(eu) mode of cyclic C8 in solid Ar
Authors: Wang S. L. , Rittby C. M. L. , Graham W. R. M.
Published: July 2002

The cyclic C8 cluster was detected and identified for the first time in Fourier transform infrared spectra of the products from the laser evaporation of C13/C12 rods trapped in Ar at ∼10 K. Measurements on spectra produced using both C12- and C13-enriched rods combined with ab initio predictions have resulted in the assignment of the most intense infrared active mode, ν12(eu)=1844.2 cm−1 of the cyclic C8 isomer with C4h symmetry. A new isotopic perturbation model (IPM) in which normal mode information from ab initio calculations is combined with experimental vibrational frequencies was developed in order to investigate the isotopic vibrational spectrum that is complicated by the fact that the mode is doubly degenerate and mixes with two infrared inactive modes lying nearby. The assignment of ν12(eu) is based on excellent agreement of the frequency, C13 isotopic shifts, and relative intensities with the predictions of earlier density functional theory (DFT) calculations and perturbation treated DFT/B3LYP/cc-pVTZ calculations using the IPM model carried out in the present study.
Understanding the activation energy trends for the C2H4+OH→C2H4OH reaction by using canonical variational transition state theory
Authors: Villà Jordi , González-Lafont Angels , Lluch José M. , Corchado José C. , Espinosa-Garcı́a Joaquı́n
Published: July 2002

The potential-energy hypersurface of the addition reaction OH+C2H4 was partially explored following two different approaches. First, the stationary points were located at the MP2(FULL)/6-31G(d,p) level and then the minimum energy path (MEP) was built starting from the MP2 saddle-point geometry. In order to improve the energetics along the MEP, single-point calculations were carried out at several higher levels, in particular, PMP2, MP4sdtq, PMP4sdtq, and QCIsd(t). In a different approach, the C–O bond length was assumed to provide an accurate parametrization of the reaction path in the vicinity of the transition state. The minimum energy structures at the MP4sdq/6-311+G(d,p) level for 16 points along the RC–O coordinate have been calculated, followed by a generalized normal-mode analysis at the MP2(FULL)/6-311+G(d,p) level for each point. The initial potential information from both approaches was used to calculate canonical variational transition state (CVT) association rate constants for the temperature range 200–1000 K. Our calculations at the PMP4sdtq/6-311+G(d,p)//MP4sdq/6-311+G(d,p)[MP2(FULL)/6-311 +G(d,p)] level reproduce the inverse dependence of the rate constant with temperature at T<565 K, in agreement with the experimental evidence that this reaction has a negative activation energy at room temperature. The analysis of the enthalpic and entropic contributions to the Gibbs free-energy profile has allowed us to understand those negative values of the activation energy.
Shear-induced dimerization
Authors: Evans G. T.
Published: July 2002

The Yamada–Kawasaki theory of nonlinear response was applied to the calculation of the frequency and shear-rate dependent shear viscosity of a dilute Brownian suspension of harmonically bound dumbbells. When the dumbbells were allowed to dimerize, as implemented by the Wertheim theory of association, the resulting fluid displayed exhibited shear-thinning and shear-thickening regimes. Head-to-head dimerization led to shear thinning, and for shear thickening to occur, the rotational correlation time of the dimers must be commensurate with those of the monomers. The present findings were evaluated in light of the observed shear-induced coagulation in micellar solutions.
Infrared absorption spectrum of Ar–HN2+ in a supersonic slit expansion
Authors: Speck Thomas , Linnartz Harold , Maier John P.
Published: July 2002

The first direct measurement of a high resolution infrared absorption spectrum of an ionic complex, Ar–HN2+, is presented. The complex is generated in a continuous supersonic slit jet expansion with electron impact ionization. There have been observed 28 rotationally resolved transitions of the ν1+νs combination band in the 2501–2510 cm−1 region using a tunable diode laser. The band origin is determined at ν0=2505.4998(4) cm−1.
Rotational analysis of the B̃2A′′−X̃2A′′ origin band of the CH2CFO radical
Authors: Wright Scott A. , Dagdigian Paul J.
Published: July 2002

The laser fluorescence excitation spectrum of the origin band of an electronic transition in the CH2CFO radical was recorded with partial rotational resolution using a supersonic, rotationally cold beam. The radical was prepared in a pulsed free jet by 193 nm photolysis of acetyl fluoride diluted in helium or argon. The rotational structure of the band is consistent with an in-plane electronic transition of this near oblate rotor. In analogy with the vinoxy radical, this transition is designated as B̃2A′′−X̃2A′′. Spectroscopic constants were derived from a fit to the assigned rotational transitions. The lower state rotational constants agree with those calculated from an ab initio CH2CFO equilibrium structure [M. Furubayashi, I. Bridier, S. Inomata, N. Washida, and K. Yamashita, J. Chem. Phys. 106, 6302 (1997)]. The present study thus provides confirmation of the assignment of the molecular carrier as CH2CFO and eliminates the alternative assignment to FCO [B. A. Williams and J. W. Fleming, J. Chem. Phys. 106, 4376 (1997)].
Comment on “Examining the influence of the [Zn(H2O)6]2+ geometry change on the Monte Carlo simulations of Zn2+ in water” [J. Chem. Phys. 105, 5968 (1996)]
Authors: Bernal-Uruchurtu M. I. , Hernandez-Cobos J. , Ortega-Blake I.
Published: July 2002

Here we present a study to investigate further the influence of a model assuming a rigid hexahydrate for ion solvation, compared to fully relaxed studies having polarization and nonadditive effects. It is found that the model is suited for ions that really have a depletion layer after their first hydration shell, otherwise the model will impose it. In particular, small distortions in the Zn2+ hexahydrate lead to small modifications of the enthalpy, but there are other effects that lead to an appreciable difference in the predicted hydration energy of the ion.
Effect of the solvent density and species on the back-electron transfer rate in the hexamethylbenzene/tetracyanoethylene charge-transfer complex
Authors: Kimura Y. , Takebayashi Y. , Hirota N.
Published: July 2002

The back-electron transfer (b-ET) process in the hexamethylbenzene/tetracyanoethylene charge-transfer complex was studied by the transient absorption spectroscopy in several fluids (ethane, nitrous oxide, carbon dioxide, and trifluoromethane) from the critical density to twice that of it at 323.2 K. The b-ET rate was determined by the decay rate of the excited state absorption. The b-ET rate increased with the increase of solvent density in carbon dioxide and nitrous oxide. The b-ET rate also increased in the order of ethane, nitrous oxide, carbon dioxide, and trifluoromethane, compared at the similar reduced density divided by the solvent critical density. Based on the formulation by Marcus and Jortner, the reaction free energy and the solvent reorganization energy were estimated from the change of the absorption spectrum relative to the gaseous phase spectrum, simply by assuming that the intramolecular reorganization energy does not depend on the solvent density and the species. The reaction free energy and the solvent reorganization energy in fluids obtained in this way were almost linearly correlated, and the density dependence was larger in the lower density region. The b-ET rates in various conditions showed a good correlation with the reaction free energy estimated from the spectral simulation. The b-ET rate showed a significant isotope effect by perdeuteriation of hexamethylbenzene as is predicted by the fluorescence quantum yield [K. Kulinowski et al., J. Phys. Chem. 99, 17715 (1995)], which could not be reproduced by this model.
Conformational transitions in a lattice model of a three-component mixture of solvent, amphiphile, and soluble polymers
Authors: Jennings D. E. , Kuznetsov Yu. A. , Timoshenko E. G. , Dawson K. A.
Published: July 2002

We present a lattice model of amphiphile, solvent and polymer. The model is simulated in a hybrid Monte Carlo scheme using the grand canonical ensemble for solvent and amphiphile, and the canonical ensemble for the polymer. The model has been studied for a limited range of parameters, albeit consistent with the most elementary properties of surfactants and polymer. However, despite this apparently very simple set of microscopic interactions, a number of concentration-dependent effective interactions emerge, and cause conformational transitions of the polymer. We examine surfactant-polymer binding curves to relate these conformational changes of the polymer to binding. We have established the viability of using Monte Carlo simulations to study solutions of amphiphile, polymer and solvent.
The hindered rotor density-of-states
Authors: McClurg Richard B.
Published: July 2002

The density of states for hindered rotors is calculated analytically based on a previously published approximation to the canonical partition function [J. Chem. Phys. 106, 6675 (1997)]. The density is used to calculate the fraction of rotors freely rotating and to explain the trends in the heat capacity of hindered rotors.
HCO (N,Ka,Kc,J) distributions from near-threshold photolysis of H2CO (J,Ka,Kc)
Authors: Terentis Andrew C. , Waugh Siobhan E. , Metha Gregory F. , Kable Scott H.
Published: July 2002

The dynamics of the reaction H2CO+hν(λ≈330 nm)→H+HCO have been studied following excitation of formaldehyde into the Ã(1A2) state, just above the dissociation threshold of the X̃(1A1) state. Formaldehyde was excited via specific J, Ka, Kc rotational states and the ensuing rotational distribution of HCO measured by fully resolving N, Ka, Kc, and J=N±S of the fragment. When only the N and Ka quantum numbers of both formaldehyde and the formyl radical are considered, the distributions are generally modeled well by phase space theory (PST). Within ≈10 cm−1 of the threshold, however, the PST predictions consistently exceed the experimental populations. This was accounted for by the inclusion of a centrifugal barrier in the PST model. The attractive part of the effective centrifugal potential was modeled by a dipole-induced dipole plus dispersion interaction. The barrier is weak and long range (>5 Å). Resolution of Kc in the reaction, in both parent and product, gave large deviations from the PST model. The HCO population distributions separate according to whether Kc was the upper- or lower-energy state. Additionally, the upper/lower preference was sensitive to the choice of Kc in the parent. Insufficient data are currently available to quantify this observation. The product state distribution was also found to be independent of the spin–rotation state of HCO.
Analysis of absorption and luminescence spectra of U3+ doped Cs2NaYCl6 and Cs2LiYCl6 single crystals
Authors: Karbowiak M. , Drożdżyński J. , Hubert S. , Simoni E. , Strȩk W.
Published: July 2002

Uranium (3+) doped single crystals of Cs2NaYCl6 and Cs2LiYCl6 with a 2.0% and 0.1% U3+ concentration have been obtained by the Bridgman-Stockbarger method. Luminescence spectra of the crystals were recorded at 160, 70, and 15 K. The emission bands observed in the visible and near infrared regions have been assigned to transitions from the lowest components of the I11/24, F3/24, and G7/24 multiplets to the crystal-field components of the I9/24 ground level. Absorption spectra were recorded from 4 000 to 25 000 cm−1 at 4.2 K. The zero phonon transitions were identified from an analysis of the vibronic side bands as well as in least-squares fits by applying a semiempirical Hamiltonian representing the combined atomic and crystal-field interactions for an ion of the 5f3 configuration and Oh symmetry. Eight energy level parameters were varied simultaneously in the least-squares adjustments yielding a mean error of 57 and 61 cm−1 for U3+:Cs2LiYCl6 and U3+:Cs2NaYCl6, respectively. The analysis of the spectra enabled the assignment of 25 crystal field levels for U3+:Cs2NaYCl6 and 27 for U3+:Cs2LiYCl6. The received total splitting of the ground level is equal to 625 and 595 cm−1, respectively.
A systematic study of water models for molecular simulation: Derivation of water models optimized for use with a reaction field
Authors: van der Spoel David , van Maaren Paul J. , Berendsen Herman J. C.
Published: July 2002

We have performed long molecular dynamics simulations of water using four popular water models, namely simple point charge (SPC), extended simple point charge (SPC/E), and the three point (TIP3P) and four point (TIP4P) transferable intermolecular potentials. System sizes of 216 and 820 molecules were used to study the dependence of properties on the system size. All systems were simulated at 300 K with and without reaction fields and with two different cutoff radii, in order to study the impact of the cutoff treatment on density, energy, dynamic, and dielectric properties. Furthermore we generated two special-purpose water models based on the SPC and TIP4P models, for use with a reaction field. The atomic charges and the Lennard-Jones C12 parameter were optimized to reproduce the correct energy and pressure using the weak coupling algorithm for parameters. Indeed, in simulations without parameter coupling of both new models the density and potential energy were found to be close to the experimental values. The other properties of these models that we called SPC/RF and TIP4P/RF (where RF stands for reaction field) are evaluated and discussed.
Electronic friction in proton tunneling at the electrochemical interface
Authors: Sebastian K. L.
Published: July 2002

We consider the effects of electronic friction (nonadiabaticity) on the tunneling of a proton from an H3O+ in water to the surface of a metal electrode. The final state is a hydrogen atom adsorbed on the surface of the metal. We use a simple model Hamiltonian for the description of the process and calculate the tunneling probability using the semiclassical approaches of Miller [J. Chem. Phys. 62, 1899 (1975)]. The continuum of electron-hole excitations is accounted for by treating them as bosons. We show that typically, tunneling probability can be decreased by one to two orders of magnitude by coupling to electron-hole excitations. We also report on the effect of isotopic substitution on this rate. While our analysis is specifically for an electrochemical situation, the results are of general validity and imply a decrease in tunneling probability, due to electronic friction for the proton in cases where tunneling occurs in the presence of a metal surface for example, atom transfer in STM experiments.
Picosecond real time study of the bimolecular reaction O(3P)+C2H4 and the unimolecular photodissociation of CH3CHO and H2CO
Authors: Abou-Zied Osama K. , McDonald J. Douglas
Published: July 2002

The bimolecular reaction of O(3P) with ethylene and the unimolecular photodissociation of acetaldehyde and formaldehyde have been studied using a picosecond pump/probe technique. The bimolecular reaction was initiated in a van der Waals dimer precursor, C2H4⋅NO2, and the evolution of the vinoxy radical product monitored by laser-induced fluorescence. The NO2 constituent of the complex was photodissociated at 266 nm. The triplet oxygen atom then attacks a carbon atom of C2H4 to form a triplet diradical (CH2CH2O) which subsequently dissociates to vinoxy (CH2CHO) and H. The rise time of vinoxy radical production was measured to be 217 (+75−25) ps. RRKM theory was applied and a late high exit barrier was invoked in order to fit the measured rise time. The structure and binding energy of the van der Waals complex have been modeled using Lennard-Jones type potentials and the results were compared with other systems. The unimolecular side of the potential energy surfaces of this reaction has been investigated by photodissociating acetaldehyde at the same pump energy of 266 nm. The resulting photoproducts, acetyl radical (CH3CO) and formyl radical (HCO), have been monitored by resonance enhanced multiphoton ionization (REMPI) combined with a time-of-flight mass spectrometer. The similarity in the measured evolution times of both radicals indicates the same photodissociation pathway of the parent molecule. The photodissociation rate of acetaldehyde is estimated from RRKM theory to be very fast (3×1012 s−1). The T1←S1 intersystem crossing (ISC) rate is found to be the rate determining step to photodissociation and increases with energy. The REMPI mechanism for the production of CH3CO+ is proposed to be the same as that of HCO+(2+1). The HCO product from the photodissociation of formaldehyde at 266 nm reveals a faster T1←S1 ISC rate than in acetaldehyde.
Quantum chemical study on the oxidation process of a hydrogen terminated Si surface
Authors: Teraishi Kazuo , Takaba Hiromitsu , Yamada Aruba , Endou Akira , Gunji Isao , Chatterjee Abhijit , Kubo Momoji , Miyamoto Akira , Nakamura Kazutaka , Kitajima Masahiro
Published: July 2002

The initial oxidation process of a hydrogen terminated Si surface was investigated by molecular orbital calculations using the cluster models representing H–Si(100)-2×1 and H–Si(111)-1×1. Ab initio calculations using small cluster models revealed that as a Si atom is coordinated by more oxygen atoms, it increases the affinity toward another oxygen. Furthermore, the insertion of up to five oxygen atoms into Si–Si bonds of large models were traced by the semiempirical AM1 method, whose reliability was proven by comparison with ab initio results. The structural relaxation was suggested to be as important as the electronic effect on the stability of oxides, and on the H–Si(111)-1×1 surface oxidation was predicted to proceed to the second layer before its completion on the first layer to avoid a large strain which otherwise would be caused. It was also revealed that on the H-Si(100)-2×1 surface, the growth of the oxide island and the nucleation of oxide at a distant site have almost the same probabilities. In contrast, the lateral growth of the oxide island is preferred to the formation of an isolated oxide nuclei on the H-Si(111)-1×1 surface. These differences derive from the different Si–Si bond topology on each surface.
A new inhomogeneity parameter in density-functional theory
Authors: Becke Axel D.
Published: July 2002

Density-functional exchange–correlation approximations depending on spin densities and their gradients have proven remarkably accurate in recent thermochemical tests [e.g., A. D. Becke, J. Chem. Phys. 107, 8554 (1997)]. With the inherent limitations of first-order gradient corrections now in sight, however, we investigate here a class of inhomogeneity corrections based on a new second-order gradient parameter. The new parameter is logically motivated by previous work on Taylor expanded exchange hole densities, and generates exchange–correlation functionals more accurate than those containing first-order gradients only.
Electron bombardment matrix isolation of Rg/Rg′/methanol mixtures (Rg= Ar, Kr, Xe): Fourier-transform infrared characterization of the proton-bound dimers Kr2H+, Xe2H+, (ArHKr)+ and (ArHXe)+ in Ar matrices and (KrHXe)+ and Xe2H+ in Kr matrices
Authors: Fridgen Travis D. , Parnis J. Mark
Published: July 2002

Matrices formed by subjecting methanol vapor diluted in argon/krypton mixtures to electron bombardment and subsequent matrix isolation (EBMI) reveal a new feature at 885.3 cm−1 which is assigned to the v3 fundamental of the triatomic cation (ArHKr)+. In samples containing about 5% krypton, the v3 fundamental of Kr2H+ is also observed in a predominantly solid argon environment. These assignments are supported by annealing experiments and by density functional theory calculations reported in a separate paper. Similar experiments with xenon diluted in argon yield infrared spectra showing the v3 and v3+v1 bands of (ArHXe)+, and v3 of Xe2H+ in predominantly argon environments. EBMI of methanol diluted in krypton in the presence of xenon gives rise to infrared bands assigned to the v3 and v3+v1 fundamentals and combination bands of both (KrHXe)+ and Xe2H+ isolated in predominantly krypton environments. The energetics and plausible mechanisms to the formation of these homogenous and mixed proton-bound rare-gas dimers are considered.
Multidimensional solid-state nuclear magnetic resonance for determining the dihedral angle from the correlation of C13–H1 and C13–C13 dipolar interactions under magic-angle spinning conditions
Authors: Fujiwara Toshimichi , Shimomura Taiji , Ohigashi Yasunori , Akutsu Hideo
Published: July 2002

Multidimensional solid-state nuclear magnetic resonance (NMR) under magic-angle spinning (MAS) conditions has been developed to determine the dihedral angle for a Hα1–Cα13–Cβ13–Hβ1 moiety in powdered states. The pulse sequence for this experiment includes C113H dipolar evolution periods for Cα and Cβ, which are correlated through a coherent Cα1313Cβ dipolar mixing period. Theoretical analysis based on the symmetry of the spin system indicates that the dipolar correlation spectrum only due to the CαHα and CβHβ dipolar couplings is strongly dependent on the dihedral angle χ about the CαCβ bond axis, but two χ angles give the same spectrum in the χ range from 0° to about 140°, where χ=0° corresponds to the cis conformation. Inclusion of the CαCβ dipolar coupling together with the weak CαHβ and CβHα dipolar couplings, however, breaks the symmetry of the system with respect to χ in the range from 0° to 180°. These properties are confirmed by the spectra calculated for the pulse sequence as a function of χ and the root-mean-square deviation between them. The bond lengths, bond angles, and dihedral angle also alter the dipolar correlation spectrum differently. This enables us the experimental determination of all the structural parameters, which improves the accuracy of the dihedral angle determination. The high resolution due to C13 isotropic chemical shifts under MAS conditions in this multidimensional NMR permits its application to molecules having a number of C13-labeled sites. Experimental results are presented for powdered L-valine uniformly labeled with C13 and N15 nuclei. Effects of the structural parameters and noise on the dihedral angle determination are evaluated numerically. The accuracies of the determined structural parameters are discussed.
35Cl nuclear quadrupole resonance study of lattice dynamics in SnCl4⋅5H2O comprising uncoordinated water molecules
Authors: Oh Dong Keun , Kwon Young Man , Rhee C. , Lee Cheol Eui , Kang Jun-Kun
Published: July 2002

Lattice dynamics and bond characteristics in SnCl4⋅5H2O have been investigated using 35Cl nuclear quadrupole resonance (NQR). Two chemically inequivalent chlorine sites in SnCl4⋅5H2O were distinguished from the temperature dependences of the NQR frequency and from the spin–lattice (T1Q) and spin–spin relaxation times (T2Q). The anomalous temperature dependence of the NQR frequency of one of the two sites was attributed to a thermal weakening of the crystal field from uncoordinated water molecules. In addition, high temperature activated molecular motion was observed.
A non-Dyson third-order approximation scheme for the electron propagator
Authors: Schirmer J. , Trofimov A. B. , Stelter G.
Published: July 2002

An efficient third-order propagator method to compute ionization potentials and electron affinities of atoms and molecules is presented. The development is based on the algebraic diagrammatic construction (ADC) representing a specific reformulation of the diagrammatic perturbation series of the electron propagator G(ω). In contrast with previous approximation schemes, relying on the Dyson equation and approximations for the self-energy part, the ADC procedure here is applied directly to the (N∓1)-electron parts G−(ω) and G+(ω), respectively, of the electron propagator. This leads to decoupled secular equations for the ionization energies ((N−1)-electron part) and electron affinities ((N+1)-electron part), respectively. In comparison with the Dyson-type approach, there is a substantial reduction of the secular matrix dimension opposed by a small additional expense in computing some second- and third-order contributions to the secular matrix elements. The relationship of the non-Dyson ADC(3) method to coupled cluster methods is outlined.
On the application of numerical analytic continuation methods to the study of quantum mechanical vibrational relaxation processes
Authors: Gallicchio E. , Egorov S. A. , Berne B. J.
Published: July 2002

A major problem still confronting molecular simulations is how to determine time-correlation functions of many-body quantum systems. In this paper the results of the maximum entropy (ME) and singular value decomposition (SVD) analytic continuation methods for calculating real time quantum dynamics from path integral Monte Carlo calculations of imaginary time time-correlation functions are compared with analytical results for quantum mechanical vibrational relaxation processes. This system studied is an exactly solvable system: a harmonic oscillator bilinearly coupled to a harmonic bath. The ME and SVD methods are applied to exact imaginary-time correlation functions with various level of added random noise, and also to imaginary-time data from path integral Monte Carlo (PIMC) simulations. The information gathered in the present benchmark study is valuable for the application of the analytic continuation of PIMC data to complex systems.
Theoretical studies of nitroamino radical reactions: Rate constants for the unimolecular decomposition of HNNO2 and related bimolecular processes
Authors: Chakraborty D. , Hsu C.-C. , Lin M. C.
Published: July 2002

The unimolecular decomposition of HNNO2 and the related bimolecular reaction of NH with NO2 have been studied by high-level ab initio molecular orbital and statistical theory calculations. The potential energy surface for the bimolecular association leading to the formation of HNNO2 and HNONO, the formation of exothermic products N2O and OH via the HNNO2 intermediate, and of HNO and NO via the HNONO intermediate have been computed with a modified GAUSSIAN 2 (G2M) method. The rate constants for these two bimolecular reaction channels have been calculated by means of the canonical variational Rice–Ramsperger–Kassel–Marcus (RRKM) approach over a broad temperature range. The predicted values at room temperature correlate reasonably well with the experimental overall rate and also with the experimental product branching probability for the formation of N2O vs HNO via the nitro and nitrite intermediates. The effect of multiple reflections within the potential well of the HN-ONO complex was found to be quite significant. It reduces the rate constant for the formation of the HNO+NO products by as much as 350%.
The electron-spin magnetic moments (g factors) of O3−, O3Li, and O3Na: An ab initio study
Authors: Bruna Pablo J. , Grein Friedrich
Published: July 2002

The electron-spin magnetic moments of O3−, O3Li, and O3Na, as parametrized by the g factors, are studied at the uncorrelated restricted open Hartree–Fock (ROHF) and correlated multireference configuration interaction (MRCI) ab initio levels. The present method, which uses a perturbative approach complete to second order, is based on a Breit–Pauli Hamiltonian. The calculated Δg values, with Δg=g−ge, are very similar for all three species, confirming that the O3− moiety is retained in the ozonides O3M. In the standard C2v notation, Δgyy>Δgzz≫|Δgxx|. The perpendicular component Δgxx is small and negative, while the in-plane components Δgyy and Δgzz are large and positive. The MRCI results for Δgxx, Δgyy, Δgzz (in ppm) are: −475, 16 673, 10 121 for O3−; −679, 13 894, 9308 for O3Li; and −494, 12 298, 8690 for O3Na. The ROHF values of Δgyy and Δgzz are smaller than the MRCI data, due to a general overestimation of the excitation energies. The MRCI Δg values for isolated O3− reproduce the experimental results for O3− trapped in crystals or adsorbed on MgO surfaces (in ppm, −500–1200 for Δgxx, 12 400–16 400 for Δgyy, and 6900–10 000 for Δgzz). For O3Na, the experimental Δgxx, Δgyy, Δgzz data (−100, 14 200, 9800 ppm) are again satisfactorily described by our correlated results. No experimental g shifts are available for O3Li. In all systems studied here, the Δgxx component is dictated by first-order terms (ground state expectation values); Δgyy is governed by the second-order magnetic coupling between X 2B1 and 1 2A1 [electron excitation from the highest a1 molecular orbital (MO) into the b1(π*) singly occupied MO]; and Δgzz, by the coupling with two B22 states (excitations from the two highest b2 MOs into π*).
Nuclear magnetic resonance and molecular dynamics study of methanol up to the supercritical region
Authors: Asahi Nobuyoshi , Nakamura Yoshio
Published: July 2002

Self-diffusion coefficients and chemical shifts of the methanol OH proton referred to the CH3 proton have been measured in a wide temperature range from 289 to 580 K. The reduced densities ρr (ρr=ρ/ρC, ρC=272 kg m−3) of methanol studied are 0.183, 0.256, 0.372, 0.622 and 1.008 in the supercritical condition. Molecular dynamics (MD) simulations have been performed in the same temperature and density range. The observed self-diffusion coefficients are in a good agreement with the Chapman-Enskog kinetic theory in the supercritical region. They are consistent with the MD simulation results over the whole range studied. MD calculations show that hydrogen-bonded clusters of methanol are chain like both at room temperature and in the supercritical state. The formation energy and entropy of hydrogen bonding were obtained from the temperature dependence of the OH chemical shifts for supercritical methanol. The thermodynamic model which takes account of cluster distribution provides the degree of hydrogen bonding in methanol. The number of hydrogen-bonded clusters decreases with increasing temperature and decreasing density. In the supercritical region, the calculated results for the cluster size distribution from the thermodynamic model are in a good agreement with the MD calculation.
Structures and binding enthalpies of M+(H2O)n clusters, M=Cu, Ag, Au
Authors: Feller David , Glendening Eric D. , de Jong Wibe A.
Published: July 2002

Structures and incremental binding enthalpies were determined for the M+(H2O)n ionic clusters, M=Cu, Ag, Au; n=1–4 (5 for Cu) using correlated ab initio electronic structure methods. The effects of basis set expansion and high-level correlation recovery were found to be significant, in contrast to alkali and alkaline earth cation/water complexes, where correlation of the d electrons is unimportant. The use of a systematic sequence of one-particle basis sets permitted binding enthalpies in the complete basis set limit to be estimated. Overall, the best theoretical binding enthalpies compared favorably with the available experimental data for copper and silver. No experimental data is available for gold/water clusters. The largest deviation was noted for Ag+(H2O)2, where theory predicts an incremental binding enthalpy of 28 kcal/mol and experiment measures ∼25 kcal/mol. However, the uncertainty associated with one of the two experimental values is quite large (±3 kcal/mol) and almost encompasses the theoretical result. Results were also obtained with the more cost-effective 6-31+G* basis set and calibrated against the estimated complete basis set limits.
Dynamical transition for the resonant diffusion in correlated fluctuating obstacles
Authors: Bénichou Olivier , Gaveau Bernard , Moreau Michel
Published: July 2002

We consider the diffusion of a reactive degree of freedom in a fluctuating potential constituted by two barriers which can be present or absent. When the coupling between the barriers is sufficiently strong, we show that the reaction probability presents a maximum at a certain frequency of fluctuation of the obstacles, while the phenomenon disappears if the coupling is too weak.
Local mode behavior in the acetylene bending system
Authors: Jacobson Matthew P. , Silbey Robert J. , Field Robert W.
Published: July 2002

The bending eigenfunctions of the acetylene X̃1Σg+ state, as represented by our recently reported effective Hamiltonian [J. Chem. Phys. 109, 121 (1998)], are analyzed up to Evib=15 000 cm−1. A transition from normal to local mode behavior is observed around 8000–10 000 cm−1, such that above these energies, the eigenstates are better described in terms of local mode quantum numbers. The local mode behavior in the bend degrees of freedom of acetylene that is described here is in many ways analogous to the local mode behavior that has been observed in the stretching degrees of freedom of many ABA molecules. However, the local mode behavior in the acetylene bend degrees of freedom, because it involves two two-dimensional rather than two one-dimensional vibrational modes, encompasses a richer range of motions. Specifically, in the “local” limit, the bending eigenfunctions are describable in terms of a continuum of motions ranging from local bend (one hydrogen bending) to counter-rotation (the two hydrogens executing rotations in opposite directions).
The range of validity of the lattice cluster theory
Authors: Quinn Brian , Gujrati P. D.
Published: July 2002

We consider various tests of the lattice cluster theory to ascertain its range of applicability. We investigate this theory by applying it to an incompressible binary system. All versions of the theory predict a spurious phase separation and an associated critical point(s) in the miscible region. Higher than second-order versions also predict two nearby but distinct critical points for phase separation in the immiscible region. Various other problems are also discovered. All these problems are present when the theory is applied to other systems also, thus severely restricting the predictability and the reliability of the theory.
Infrared cavity ringdown spectroscopy of methanol clusters: Single donor hydrogen bonding
Authors: Provencal R. A. , Paul J. B. , Roth K. , Chapo C. , Casaes R. N. , Saykally R. J. , Tschumper G. S. , Schaefer H. F.
Published: July 2002

Infrared cavity ringdown laser absorption spectroscopy has been used to study the O–H stretching vibrations of jet-cooled methanol clusters in direct absorption. Rovibrational bands for (CH3OH)2, (CH3OH)3, and (CH3OH)4 have been measured. Both bonded and free O–H stretches were measured for the dimer, indicating that its structure is linear. Five bands were assigned to the methanol trimer, indicating the presence of a second cyclic isomer in the molecular beam. A detailed study of the free O–H stretching region shows that methanol clusters larger than dimer must exist in cyclic ring configurations. In order to facilitate spectral assignment, harmonic frequencies and infrared intensities were calculated for the methanol monomer, dimer, and trimer with second order Mo/ller–Plesset perturbation theory. Using the theoretical infrared intensities and measured vibrational band absorptions, absolute cluster concentrations were calculated. Results agree with previous experimental and theoretical work.
Theory of proton relaxation induced by superparamagnetic particles
Authors: Roch Alain , Muller Robert N. , Gillis Pierre
Published: July 2002

Evaluating and understanding the performances of magnetic colloids as contrast agents for MRI requires a theory describing their magnetic interactions with water protons. The field dependence of the proton longitudinal relaxation rate (nuclear magnetic relaxation dispersion profiles) in aqueous colloidal suspensions of superparamagnetic particles is based on the so-called Curie relaxation, which essentially accounts for the high field part of the NMRD profiles (B0>0.02 T). The low-field part of the NMRD profiles can only be explained by the crystal’s internal anisotropy energy, a concept which clarifies the important difference between superpara- and paramagnetic compounds: the anisotropy energy modifies both the electronic precession frequencies and the thermodynamic probability of occupation of the crystal magnetic states. Our theory clearly explains why a low-field dispersion exists for suspensions of small size crystals, and why it does not for large crystals’ suspensions. This important effect is due to the Boltzmann factors depending on the anisotropy energy, which is itself proportional to the particle volume.
Theory and experiment on the cuprous–cupric electron transfer rate at a copper electrode
Authors: Halley J. W. , Smith B. B. , Walbran S. , Curtiss L. A. , Rigney R. O. , Sutjianto A. , Hung N. C. , Yonco R. M. , Nagy Z.
Published: July 2002

We describe results of experiment and theory of the cuprous–cupric electron transfer rate in an aqueous solution at a copper electrode. The methods are similar to those we reported earlier for the ferrous–ferric rate. The comparison strongly suggests that, in marked distinction to the ferrous–ferric case, the electron transfer reaction is adiabatic. The model shows that the activation barrier is dominated by the energy required for the ion to approach the electrode, rather than by the energy required for rearrangement of the solvation shell, also in sharp distinction to the case of the ferric–ferrous electron transfer at a gold electrode. Calculated activation barriers based on this image agree with the experimental results reported here.
The relativistic Dirac–Coulomb–Fock effect on atomization energies
Authors: Kedziora Gary S. , Pople John A. , Rassolov Vitaly A. , Ratner Mark A. , Redfern Paul C. , Curtiss Larry A.
Published: July 2002

We have used a second-order perturbation treatment of the Dirac–Coulomb–Hartree–Fock method to estimate relativistic contributions to energies in the G2/97 test set. The one-electron relativistic effect on atomization energies of molecules containing first- and second-row atoms nearly always reduces binding. When the relativistic corrections are included in G3 theory and assessed on the G2/97 test set, there is little change in overall performance.
Vibrational spectroscopy of 2-pyridone and its clusters in supersonic jets: Structures of the clusters as revealed by characteristic shifts of the NH and C=O bands
Authors: Matsuda Yoshiyuki , Ebata Takayuki , Mikami Naohiko
Published: July 2002

Vibrational spectroscopy of the key functional vibrations of 2-pyridone and its hydrogen-bonded clusters with water, methanol, dioxane, dimethylether, as well as its dimer, has been carried out by using infrared-ultraviolet (IR–UV) and stimulated Raman–UV double resonance methods combined with fluorescence detection. The characteristic spectral changes upon the cluster formation have been observed for the NH and C=O stretching vibrations of the bare molecule and also for the OH stretching vibrations of the solvent molecules. The cluster structures were investigated by comparing the observed spectra with the simulated ones of the energy-optimized structures obtained by ab-initio molecular orbital calculations. It was found that the “ring-type” hydrogen-bonded structure is appropriate for the cluster with water or methanol, while the “linear-type” hydrogen-bonded structure is appropriate for the cluster with dioxane or dimethylether. The symmetric form of 2-pyridone dimer was confirmed by the observed spectra, as well as the ab-initio calculation. A clear correlation between the observed frequency shifts of the NH stretching vibrations and the calculated NH⋯O hydrogen-bond angles was obtained indicating that the hydrogen-bond angle distortion reduces the local hydrogen-bond strength. Also it was found the blue shifts of the origin bands of the S1←S0 electronic transition strongly depends on the type of the cluster structures.
High-resolution x-ray photoelectron spectroscopy study of the ethanol oxidation reaction on Pd(110)
Authors: Holroyd R. P. , Bennett R. A. , Jones I. Z. , Bowker M.
Published: July 2002

The adsorption, decomposition, and oxidation of ethanol on Pd(110) has been studied using high-resolution x-ray photoelectron spectroscopy (XPS) and temperature-programmed XPS. The decomposition pathways of ethanol on the clean surface (to methane, hydrogen and carbon monoxide; and to methane, hydrogen, and carbon and oxygen adatoms) previously studied using molecular beam and thermal desorption spectroscopy were confirmed by this study. The presence of an overlayer of oxygen did not significantly alter the major or minor decomposition pathways observed on the clean surface, except for the production of water and, at temperatures above 380 K, carbon dioxide as oxidation products. It also resulted in the formation of acetate, which was first seen during temperature-programmed desorption as coincident carbon dioxide and hydrogen desorption, and was confirmed by XPS. Two C 1s peaks, one assigned to the methyl carbon in acetate and the other to the carboxylate carbon, developed simultaneously during TPXPS. The disappearance of these peaks in XPS occurred at a similar temperature (400 K) to that seen during temperature-programmed desorption.