bluuebunny/crossref_metadata_2025 huggingface dataset:
Electronic spectra of dipolar solutes at liquid/liquid interfaces: Effect of interface structure and polarity
Authors: Michael David , Benjamin Ilan
Published: July 2002

Molecular dynamics computer simulations are used to elucidate the role of solvent polarity and interface structure in determining the electronic absorption and fluorescence line shapes for model dipolar solutes at the interface between water and one of four different organic liquids. The different organic liquids represent a range of molecular structure and polarity: 1-octanol, 1,2-dichloroethane, n-nonane, and carbon tetrachloride. The solute is represented by two rigidly connected Lennard-Jones spheres. The different electronic states correspond to different charges on the two Lennard-Jones centers. In each interfacial system, different choices of solute charge distribution and solute location relative to the interface (including the bulk region) are considered and provide insight into different microscopic factors that influence the electronic line shape. For the water/1,2-dichloroethane and water/CCl4 interfaces, all of the calculations are repeated while the interface is externally constrained to be smooth in order to investigate the role of surface roughness. The calculated electronic line shapes are Gaussians whose peak positions reflect solvent polarity, interface structure, and probe location. Their widths are in general agreement with the prediction of linear response theory. Although continuum electrostatic models predict qualitatively correct behavior, they miss interesting interfacial effects.
The dissociation of dimeric aliphatic acids: Energetics and relaxation kinetics
Authors: Wilcox C. F. , Bauer S. H.
Published: July 2002

After a brief listing of the available structural, spectroscopic, and thermochemical data for gaseous formic acid, we summarize the results of quantum mechanical computations of minimum energy structures and standard entropies for the ring and chain dimers and for the monomer. The computed increments in thermochemical parameters for interconversions between these species are compared with values derived from measured variations of equilibrium constants with temperature. Rate constants were estimated both for the limiting high and low pressure regimes; these relaxation times were computed for several possible mechanisms. At low pressures, the computed activation energy for one of several sequential processes is consistent with that derived from the temperature- dependet relaxation times.
Transition state theory and grand canonical partition functions
Authors: Kostin M. D.
Published: July 2002

A partial differential equation for the number density of a D-dimensional grand canonical ensemble is used to derive a simple expression for the reaction rate of transition state theory as a function of temperature and chemical potential. The reaction rate equation is valid for both Bose–Einstein and Fermi–Dirac statistics. In the limiting case where the chemical potential goes to minus infinity, it is shown that this reaction rate equation reduces to the well-known Eyring reaction rate equation of transition state theory. By expanding the reaction rate equation in a Taylor series about the equilibrium activity, it is shown how rate equations can be derived giving the approach to equilibrium.
Block copolymer micelles: Viscoelasticity and interaction potential of soft spheres
Authors: Buitenhuis Johan , Förster Stephan
Published: July 2002

Block copolymer micelles in the size range between 10 and 100 nm are investigated as model systems for soft spheres. The zero shear viscosity η0 and complex modulus G* of micellar solutions are studied via dynamic mechanical spectroscopy and shear viscosity measurements over a wide range of concentrations. Depending on their structure, block copolymer micelles exhibit the characteristic rheological behavior of hard spheres, soft spheres, or polymers. With increasing concentration, hard-sphere and most soft-sphere samples exhibit a sharp liquid–solid transi-tion as apparent by a divergence of zero shear viscosity η0 and the development of a fre-quency-independent elastic modulus. The transition occurs at a certain volume fraction which can be related to the softness of the particles. In the solid regime the elastic modulus G exhibits a characteristic concentration dependence which is related to the spatial variation of the soft sphere repulsive potential. We observe a G∝Z1.48r−2.46 relation between modulus, aggregation number Z and intermicellar distance r which is close to the theoretical prediction G∼Z3/2r−2 of Witten and Pincus derived for polymerically stabilized colloidal particles.
Orientational relaxation times of rhodamine 700 in glycerol-water mixtures
Authors: Megens Mischa , Sprik Rudolf , Wegdam Gerard H. , Lagendijk Ad
Published: July 2002

We determined orientational relaxation times for rhodamine 700 dye in glycerol-water mixtures using time-resolved fluorescence depolarization. It appears that the orientational relaxation time varies linearly with the viscosity of the solvent between 1 and 60 cP, in accordance with the Perrin-Stokes-Einstein model with stick boundary conditions. Previously others have found that for two anionic dyes in glycerol-water and a cationic dye in glycerol-ethylene glycol mixtures, the orientational relaxation time becomes less sensitive to the viscosity at very high viscosities (>25 cP at least). We discuss the influence of dye and solvent on the relation between orientational relaxation time and viscosity, which suggests that the relaxation time as a function of viscosity can be scaled on a common curve.
Analytical solutions for the dynamic Kerr effect: Linear response of polar and polarizable molecules to a weak ac electric field superimposed on a strong dc bias field
Authors: Déjardin J. L. , Déjardin P. M. , Kalmykov Yu. P.
Published: July 2002

The infinite hierarchy of differential-recurrence relations for ensemble averages of the spherical harmonics pertaining to the noninertial rotational Brownian motion of an ensemble of polar and anisotropically polarizable molecules in a strong external dc electric field is derived by averaging the underlying Langevin equation. This procedure avoids recourse to the Fokker–Planck equation, the solution of which involves complicated mathematical manipulations. By calculating the Laplace transforms of the relaxation functions for the dynamic Kerr effect of symmetric top molecules, two equilibrium correlation functions are established, thus allowing one to express the corresponding birefringence ac responses by using linear response theory. Exact analytic solutions for the spectra of these correlation functions and relaxation times are first calculated for two limiting cases, namely, pure induced dipole moments and pure permanent moments, using the continued fraction method. The general case where both types of moments are taken into account, is then considered using matrix continued fractions. Furthermore, exact analytical expressions for the Kerr effect relaxation time are also derived in terms of integrals (which are evaluated exactly) and compared with the matrix continued fraction result. Plots of the relaxation time are presented for various values of the parameters ξ and σ characterizing the permanent and the induced dipole moments. Features of the relaxation behavior are emphasized in figures showing the real and imaginary parts of the spectra of the birefringence function. Moreover, Cole–Cole diagrams are presented for various values of ξ and σ in order to see how they deviate from the Debye-like spectra.
The effective mass of excess electrons in condensed xenon: Toward methods for modeling metal-dielectric interfaces
Authors: Bowen H. F. , Space Brian
Published: July 2002

A molecularly detailed simulation method, designed to be efficacious for modeling conduction properties of closed shell atoms or molecules in solids, liquids, and at interfaces, has been developed. This approach successfully predicts the effective mass of a conduction electron in both solid xenon, and liquid xenon over a wide density range, as compared to experimental results. To model the electron-atom interaction, angular momentum and density-dependent semi-local pseudopotentials are employed. The pseudopotentials are first fit to reproduce the gas phase electron-xenon scattering phase shifts, and are then corrected to include many-body polarization effects in a reliable mean field fashion. The effective mass of a conduction electron is calculated by solving the Schrödinger-Bloch equation using Lanczos grid methods to obtain the Bloch wave vector (k) dependent energies in both the solid and the liquid. In the liquid phase, a representative sample of the fluid is replicated to form the “periodic” infinite system. This approximation is shown to be reliable as the effective mass does not depend on the system size or the particular configuration which is chosen. It is shown that the l=0 scattering in the condensed phase determines the k=0 ground state energies; these are coincident with the conduction band energy in this system. In contrast, the l=1 scattering is shown to determine the effective mass of the conduction electrons.
Solid-state N14 nuclear magnetic resonance techniques for studying slow molecular motions
Authors: Hill Edward A. , Yesinowski James P.
Published: July 2002

A variety of transition-selective solid-state N14 (I=1)NMR techniques are demonstrated for the first time to be useful for quantitatively describing slow molecular motions in the solid state. These techniques are validated by quantitative measurements of molecular reorientation by tetrahedral jumps in hexamethylenetetramine (HMT). A new four-site magnetization-exchange model, capable of being generalized to n-sites, which includes the effects of spin-lattice relaxation is developed. This model provides the limiting conditions under which the orientation dependence of spin-lattice relaxation values T1 can be safely neglected. The model is used to analyze results from a frequency-selective DANTE train used to burn a hole in the spectrum, that provide a direct indication of the existence of 4-site exchange. The measured correlation time for the motion in HMT of 103±6 ms at room temperature agrees well with previous studies by other techniques. In a novel application to molecular dynamics, the repeated hole-burning pulse trains of the SINK experiment are used to measure a magnetization recovery time constant due to N14 spin-lattice relaxation in HMT of 0.99±0.12 s. Both experiments employ frequency-selective as well as transition-selective radio frequency pulses on a relatively small frequency region (<100 kHz) of the entire quadrupolar powder pattern of HMT (NQCC=4.414 MHz, η=0). The Hahn spin-echo used for detection can be understood in terms of the fictitious spin-1/2 formalism. Quantitative dynamical information is obtained from measurements at only one frequency position of a wide inhomogeneously broadened powder pattern. Because we are operating in this unusual regime, the sensitivity can be significantly improved by replacing the DANTE hole-burning train with a series of π/2 pulses that saturate all observable magnetization. Results from such an experiment compare well with those obtained using DANTE trains.
The 129Xe nuclear shielding surfaces for Xe interacting with linear molecules CO2, N2, and CO
Authors: de Dios Angel C. , Jameson Cynthia J.
Published: July 2002

We have calculated the intermolecular nuclear magnetic shielding surfaces for 129Xe in the systems Xe–CO2, Xe–N2, and Xe–CO using a gauge-invariant ab initio method at the coupled Hartree–Fock level with gauge-including atomic orbitals (GIAO). Implementation of a large basis set (240 basis functions) on the Xe gives very small counterpoise corrections which indicates that the basis set superposition errors in the calculated shielding values are negligible. These are the first intermolecular shielding surfaces for Xe-molecule systems. The surfaces are highly anisotropic and can be described adequately by a sum of inverse even powers of the distance with explicit angle dependence in the coefficients expressed by Legendre polynomials P2n(cos θ), n=0–3, for Xe–CO2 and Xe–N2. The Xe–CO shielding surface is well described by a similar functional form, except that Pn(cos θ), n=0–4 were used. When averaged over the anisotropic potential function these shielding surfaces provide the second virial coefficient of the nuclear magnetic resonance (NMR) chemical shift observed in gas mixtures. The energies from the self-consistent field (SCF) calculations were used to construct potential surfaces, using a damped dispersion form. These potential functions are compared with existing potentials in their predictions of the second virial coefficients of NMR shielding, the pressure virial coefficients, the density coefficient of the mean-square torque from infrared absorption, and the rotational constants and other average properties of the van der Waals complexes. Average properties of the van der Waals complexes were obtained by quantum diffusion Monte Carlo solutions of the vibrational motion using the various potentials and compared with experiment.
Ab initio static polarizability and first hyperpolarizability of model polymethineimine chains. II. Effects of conformation and of substitution by donor/acceptor end groups
Authors: Jacquemin Denis , Champagne Benoı̂t , Kirtman Bernard
Published: July 2002

An ab initio coupled Hartree–Fock study has been carried out on polymethineimine (PMI) oligomers with an increasing number of unit cells (up to N=20) to determine the effect of conformational differences, and of end-capping by a strong donor-acceptor pair, on the static electronic longitudinal polarizability (αL) and first hyperpolarizability (βL). For the unsubstituted polymer the differences in βL per unit cell among the four conformations examined—trans-transoı̈d (TT), cis-transoı̈d (CT), trans-cisoı̈d (TC) and helical (HEL)—are large. βL/N is positive for the CT polymer, in contrast with the other conformers and with μL/N, which is negative in all cases. The TC polymer has the largest |βL|/N despite having a relatively small αL/N. Substitution of a strong acceptor (NO2) at the carbon end of an oligomer and a strong donor (NH2) at the nitrogen end results in a large increase of βL/N for small/medium N. In the case of the TT and, particularly, the CT conformer this leads to a maximum in |βL|/N vs N at N=4–6, which is substantially greater in magnitude than the one at N=∞. The latter is unaffected by substitution. We compare the [βL(N)/N]max obtained at intermediate N for substituted PMI chains with those calculated for linear polyenes and other π-conjugated oligomers. The shape of all the βL/N vs N curves obtained is fit to a function which yields parameters characterizing the delocalization along the backbone, the unit cell asymmetry, and the asymmetry of the chain ends.
Ab initio molecular dynamics study of solid- to liquidlike transitions in Li9+, Li10, and Li11+ clusters
Authors: Bonačić-Koutecký V. , Jellinek J. , Wiechert M. , Fantucci P.
Published: July 2002

An ab initio Hartree–Fock molecular dynamics procedure is applied to study structural and dynamical properties of Li9+,Li10, and Li11+ clusters with eight and ten valence electrons, corresponding to “closed” and “open” shell systems. Gradients of the ground state energy are used to compute the forces acting on atoms at each geometric configuration along trajectories generated by solving classical equations of motion. Dynamics of different isomers for each cluster size have been investigated as a function of excess energy. It is shown that different isomers, even those similar in energy, can exhibit different structural and dynamical behavior. The analysis of the simulations leads to the conclusion that structures with a central atom, in particular the centered antiprism of Li9+, exhibit concerted mobility of the peripheral atoms at relatively low excess energy. In contrast, compact tetrahedral type structures show much more rigid behavior at low excess energy. However, the former ones need larger excess of internal energy to undergo isomerizations to geometrically different structures than the latter ones, at least in the case of Li9+ and Li11+ clusters. At the time scale of our simulations we found that for the intermediate excess energies it is “easier” to carry the cluster in the basin of the lowest energy isomer than in the reverse direction. Moreover, for different cluster sizes isomerization processes occur at different excess energies (temperatures), which is a consequence of the differences in the structural properties rather than in the number of the valence electrons. It has been found that the liquidlike behavior in small Li clusters becomes apparent at relatively high temperature in spite of large mobility of their atoms.
Analysis of the optical spectra of aromatic–alkane clusters
Authors: Wallenborn Ernst-Udo , Wild Urs P. , Brown Ross
Published: July 2002

Classical molecular dynamics simulations are applied to isolated tetracene/(n-heptane)n clusters (n=1–12). The spectroscopic shift of the aromatic molecule is calculated via evaluation of the dispersion terms in the perturbation expansion of the intermolecular interactions, using a multipole expansion of the interaction Hamiltonian. Stable structures, their energies and spectral shifts, optical lineshapes and vibrational modes are compared with two-color photo-ionisation spectra of such clusters in a molecular beam [Ben-Horin et al. Chem. Phys. Lett. 177, 153 (1991)]. Calculated shifts are in error by under 10% on average, enabling identification of most features of the experimental spectra.
Harmonic inversion of time signals and its applications
Authors: Mandelshtam Vladimir A. , Taylor Howard S.
Published: July 2002

New methods of high resolution spectral analysis of short time signals are presented. These methods utilize the filter-diagonalization approach of Wall and Neuhauser [J. Chem. Phys. 102, 8011 (1995)] that extracts the complex frequencies ωk and amplitudes dk from a signal C(t)=∑kdke−itωk in a small frequency interval by recasting the harmonic inversion problem as the one of a small matrix diagonalization. The present methods are rigorously adapted to the conventional case of the signal available on a sparse equidistant time grid and use a more efficient boxlike filter. Various applications are discussed, such as iterative diagonalization of large Hamiltonian matrices for calculating bound and resonance states, scattering calculations in the presence of narrow resonances, etc. For the scattering problem the harmonic inversion is directly applied to the signal cn=(χf,Tn(Ĥ)χi), generated by the dynamical system governed by a modified Chebyshev recursion, avoiding the usual recasting the problem to the time domain. Some challenging numerical examples are presented. The general filter-diagonalization method is shown to be stable and efficient for the extraction of thousands of complex frequencies ωk and amplitudes dk from a signal. When the model signal is “spoiled” by a moderate amount of an additive Gaussian noise the obtained spectral estimate is still superior to the conventional Fourier spectrum.
Potential energy surfaces and branching ratio of the dissociative recombination reaction HCNH++e−: Anab initiomolecular orbital study
Authors: Shiba Yoko , Hirano Tsuneo , Nagashima Umpei , Ishii Keisaku
Published: July 2002

Two-dimensional potential energy surfaces for the dissociative recombination reaction, HCNH++e− → (HCNH)* → HNC/HCN+H, have been calculated by the CASSCF-MRSDCI ab initio molecular orbital method. Near the HCNH+ ground state surface, which represents the upper limit of neutral HCNH Rydberg state surfaces, only two dissociative valence states of HCNH are located. One is the 2Σ+ state resulting in H–C bond dissociation to produce HNC, and the other is the 2Σ+ state resulting in H–N bond dissociation to produce HCN. The seam of intersection of these two potential surfaces almost bisects the zero-point vibrational wave function of the Rydberg-limit state of the neutral HCNH species, indicating that, through descending Rydberg ladder, transition from the Rydberg-limit state to each dissociative surface occurs with almost the same probability. Thus, branching ratio to HNC and HCN is predicted to be of the order of one or slightly higher, explaining the thermochemically unrealistic interstellar HNC/HCN abundance ratio.
Spin locking of I=3/2 nuclei in static and spinning samples: A description by abstract spins and Floquet formalism
Authors: Jeschke Gunnar
Published: July 2002

Abstract spin formalism is introduced to simplify the quantum mechanical description of spins I=3/2. The approach is based on a factorization of Hilbert space that can be applied to any spin for which 2I+1 is not a prime number. For I=3/2, a description in terms of two coupled abstract spins 1/2 is obtained that allows one to make use of the well-established product operator formalism. This approach is applied to the analysis of spin lock of the central transition in a static sample. The treatment is extended to the case of magic angle sample spinning by using Floquet operator formalism [A. Schmidt and S. Vega, J. Chem. Phys. 96, 6895 (1987)]. It is demonstrated that a proper spin lock requires a sample spinning frequency that is larger than the radio frequency field strength. The theoretical results are confirmed by numerical simulations and experiments. The concept of spin locking in the fast-spinning, low-power regime is utilized in a cross polarization experiment on sodium nitrite. Cross polarization from Na23 to N15 at a distance of more than 2.6 Å results in an about sevenfold better signal-to-noise ratio than direct polarization.
Reversal of radio-frequency-driven spin diffusion by reorientation of the sample spinning axis
Authors: De Paul Susan M. , Tomaselli Marco , Pines Alexander , Ernst Matthias , Meier Beat H.
Published: July 2002

The dipolar Hamiltonian in a rapidly rotating sample is scaled by the second Legendre polynomial of the cosine of the angle between the rotation axis and the static magnetic field. It is, therefore, possible to refocus the spatial polarization-transfer process, often termed spin diffusion, in extended spin systems by reorienting the rotor axis such that the dipolar interaction Hamiltonian changes sign. We present experimental results which demonstrate that a rapid mechanical sample reorientation leads to a time reversal of the “radio-frequency-driven” spin diffusion among C13 spins.
Ab initio band structure of polymethineimine isomers
Authors: Jacquemin Denis , Champagne Benoı̂t , André Jean-Marie
Published: July 2002

Ab initio crystal orbital calculations have been performed on stereoregular polymethineimine (PMI) at the restricted Hartree-Fock/6-31G level. Since the optimized geometries of the parent oligomers show only small energy differences, four isomers, namely, trans-transoı̈d (TT), cis-transoı̈d (CT), trans-cisoı̈d (TC) and helical (HEL) have been studied to point out the geometry-dependences of the band structure, density of states (DOS) and polarizability. Some of these could be helpful for identifying the isomers. It turns out that the DOS makes the TT and CT isomers distinguishable, whereas the TC and HEL isomers present very similar DOS. Moreover, it is shown that the longitudinal polarizability per unit cell increases in the order HEL, TC, CT and TT.
Translational/rotational coupling of the hindered CH3 quantum-rotor in lithium acetate dihydrate
Authors: Schiebel P. , Kearley G. J. , Johnson M. R.
Published: July 2002

The multiplet structure of the rotational-tunneling peaks in the inelastic neutron-scattering spectrum of lithium acetate is widely regarded as evidence of coupled pairs of CH3 quantum rotors. However, our molecular-mechanics study, which has no adjustable parameters, reveals that rotor/rotor coupling is less important than translational/rotational coupling. In order to remain at a potential-energy minimum during CH3 group reorientation the center-of-mass of this group follows an almost circular path around its time-average position. The energy levels from this dynamical model reproduce the observed tunneling-transitions reasonably well, and the predicted rectangular density distribution of the three methyl H-atoms is in good agreement with that measured by single-crystal neutron diffraction.
Two-photon fluorescence: Resonance energy transfer
Authors: Allcock Philip , Andrews David L.
Published: July 2002

Fluorescence resonance energy transfer (FRET) is a technique now widely applied to probe biological and other complex systems for the determination of fluorophore separation and structure. Recently the theory behind the anisotropy of fluorescence has been extended to include the case of a residual polarization following energy transfer between fluorophores, and here the theory is further extended to accommodate two-photon excitation. This reveals not only novel polarization characteristics but also a distance dependence whose analysis does not require a priori knowledge of the donor–acceptor spectral overlap. The two-photon FRET anisotropy results mirror their one-photon counterparts, in terms of fluorophore separation characteristics and also their relationship to the anisotropies for isolated fluorophores. Moreover, the two-photon results are not restricted to a plane polarized input, results being given for both plane and circularly polarized pump radiation.
Study of the NH4 tunneling manifold by energy level matching in the proton spin rotating frame
Authors: Choi C. , Damyanovich A. , Peternel J. , Pintar M. M.
Published: July 2002

Matching of Zeeman and tunneling levels in the proton spin rotating frame is used to detect splitting of the torsional ground state due to rotational tunneling of the NH4 group in solid NH4I. When highly polarized Zeeman splitting matches tunneling splitting a fast resonant transfer of polarization between Zeeman and tunneling states occurs. This in turn causes a change of the proton magnetization. When the magnetization change is monitored as a function of the magnetic field strength in the rotating frame, peaks are observed whenever two levels match. The model spectrum of the NH4 group with a split F-symmetry state, indicating that the symmetry of the lattice potential at the site of the NH4 group is lower than tetrahedral, reproduces the observed spectrum. The energy differences between the A state and the nondegenerate, lower F state and between this F state and the degenerate, higher F state, as measured by level matching in units kHz, are 82±4 and 41±2, respectively. The magnetization evolution spectrum shows peaks at 78±3 and 116±4 kHz which equal the anticipated A to lower F and the lower F to higher F energy differences.
Effects of local reactant concentration perturbations in oscillatory catalysis
Authors: Lund C. D. , Surko C. M. , Maple M. B. , Yamamoto S. Y.
Published: July 2002

Reaction-rate oscillations in CO oxidation over heterogeneous platinum thin-film catalysts were studied at atmospheric pressure where spatial coupling is due to gas-phase diffusion of reactant concentration gradients. The catalyst was perturbed locally by the introduction of CO directly above the surface, while the resulting spatiotemporal behavior was monitored using infrared imaging. The transient response of the system was studied in both the steady-state and oscillatory regimes, and comparisons are made between the observed behavior and predictions from models for the oscillations. Using these perturbations, global behavior was initiated using a localized effect. Similarities and differences between the induced oscillations and those occurring naturally are discussed, as are tests to vary the phase of the oscillatory cycle.
Photoinduced electron transfer in mixed-valence compounds: Beyond the golden rule regime
Authors: Evans D. G. , Nitzan A. , Ratner M. A.
Published: July 2002

The short-time charge transfer evolution following photoexcitation in mixed valence compounds is studied using path integral calculations. Due to the large nonadiabatic coupling, path integral calculations using direct path summation techniques are inadequate, and charge transfer dynamics can only be computed using a transfer matrix technique developed by Makri and Makarov. The resulting relaxation is considerably slower than that predicted by low-order perturbation theory. The effects of the solvent on the decay process, and the validity of the golden rule to predict the dynamics of the decay process are investigated. The effects of preparing an initial state that is not a rovibrational state of the acceptor potential energy surface is also examined. These exact calculations show that the large electronic mixing gives rise to very fast oscillations in the electronic state population as the wave function oscillates coherently between the donor and acceptor. This is followed by a slower relaxation induced by the coupling to the dissipative solvent modes, which occurs on time scales ⩽100 fs. This information provides insight into the mechanism for oscillations observed in time-resolved transient spectra of these compounds, and suggests substantial limitations of the golden rule picture.
An application of distributed approximating functional-wavelets to reactive scattering
Authors: Wei G. W. , Althorpe S. C. , Kouri D. J. , Hoffman D. K.
Published: July 2002

A newly developed distributed approximating functional (DAF)-wavelet, the Dirichlet–Gabor DAF-wavelet (DGDW), is applied in a calculation of the state-to-state reaction probabilities for the three-dimensional (3-D) (J=0)H+H2 reaction, using the time-independent wave-packet reactant-product decoupling (TIWRPD) method. The DGDWs are reconstructed from a rigorous mathematical sampling theorem, and are shown to be DAF-wavelet generalizations of both the sine discrete variable representation (sinc-DVR) and the Fourier distributed approximating functionals (DAFs). An important feature of the generalized sinc-DVR representation is that the grid points are distributed at equally spaced intervals and the kinetic energy matrix has a banded, Toeplitz structure. Test calculations show that, in accordance with mathematical sampling theory, the DAF-windowed sinc-DVR converges much more rapidly and to higher accuracy with bandwidth, 2W+1. The results of the H+H2 calculation are in very close agreement with the results of previous TIWRPD calculations, demonstrating that the DGDW representation is an accurate and efficient representation for use in FFT wave-packet propagation methods, and that, more generally, the theory of wavelets and related techniques have great potential for the study of molecular dynamics.
Reactive removal of unstable mixed NO+CO adlayers: Chemical diffusion and reaction front propagation
Authors: Tammaro M. , Evans J. W.
Published: July 2002

A lattice-gas model is developed to describe the reactive removal of a preadsorbed, mixed NO+CO adlayer covering a Pt(100) surface, via reduction of NO with CO, and behavior of the model is analyzed. Since NO dissociation requires an adjacent empty site, the NO+CO covered surface constitutes an unstable steady state. The creation of vacancies leads NO dissociation, the reaction of CO with the O formed by dissociation, the subsequent creation of more vacancies, and thus the autocatalytic removal of the adlayer. The high mobility of most adspecies leads to an initial “disperse stage” of adlayer removal, characterized by an exponential increase in the number of highly dispersed vacancies. Thereafter follows a transition to a “reaction front propagation” stage of adlayer removal, where a chemical wave develops that propagates into the NO+CO covered region of the surface with roughly constant velocity, and leaves in its wake a surface populated only by excess reactant. We provide a suitable rate equation formulation for the initial disperse stage, but focus on a reaction-diffusion equation analysis of reaction front propagation, examining, in detail, behavior for long times where the front is nearly planar. We emphasize that it is necessary to incorporate the coverage-dependent and tensorial nature of chemical diffusion in the mixed adlayer. Both these features reflect the interference on the surface diffusion of each adspecies by coadsorbed species. Thus, a key component of this work is the development of an appropriate treatment of chemical diffusion in mixed layers of several adspecies.
Generalized integral-screening for efficient calculations of nonlinear optical properties of large molecules
Authors: Ruud Kenneth , Jonsson Dan , Norman Patrick , Ågren Hans , Saue Trond , Jensen Hans Jo/rgen Aa. , Dahle Pål , Helgaker Trygve
Published: July 2002

We describe the implementation of integral screening in the integral-direct SCF calculation of cubic response functions. The screening is applied to all computational steps that involve the construction of Fock matrices or one-index transformed Fock matrices. We apply this implementation to study the length dependence of the polarizability and second hyperpolarizability of diphenylpolyenes. It is demonstrated that calculations of the second hyperpolarizability of molecules containing more than 140 atoms are now accessible by ab initio methods on a time scale that makes it of interest in real applications.
An assessment of theoretical procedures for the calculation of reliable free radical thermochemistry: A recommended new procedure
Authors: Mayer Paul M. , Parkinson Christopher J. , Smith David M. , Radom Leo
Published: July 2002

The ability to predict reliable thermochemical properties of molecules and ions has led to an ever increasing application of ab initio molecular orbital theory. Methods such as G2 theory have been shown to generally give accurate heats of formation (ΔfH) for closed-shell molecules and ions. Open-shell systems have been less thoroughly examined to date and the present paper attempts to redress this situation through a detailed assessment of the performance of a variety of levels of theory in calculating ΔfH values for free radicals. Representatives of three families of theoretical procedures have been studied: the infinite basis set extrapolation techniques of Martin, the CBS procedures of Petersson et al., and the G2 methods of Pople et al. Among the specific influences investigated are choice of geometry, zero-point vibrational energy, high level electron correlation treatment and basis set size. We recommend a new procedure called CBS-RAD for the treatment of free radicals. CBS-RAD is a modification of the CBS-Q method in which the geometry and zero-point energies are obtained at the QCISD/6-31G(d) level, and coupled-cluster theory is used in place of quadratic configuration interaction in single-point energy calculations. We find that for free radicals with low spin contamination G2 theory also performs adequately, but as 〈S2〉 increases the results of G2 calculations can become increasingly unsatisfactory. The recommended CBS-RAD procedure should yield more reliable results over a broader range of free radicals.
Global analytical potential hypersurfaces for large amplitude nuclear motion and reactions in methane. I. Formulation of the potentials and adjustment of parameters to ab initio data and experimental constraints
Authors: Marquardt Roberto , Quack Martin
Published: July 2002

Analytical representations of the global potential energy surface of XYn molecules are developed and applied to model the potential surface of methane in the electronic ground state. The generic analytical representation allows for a compact, robust, and flexible description of potentials for XYn systems irrespective of the specific nature of the atomic interactions. The functions are global in that structures near several minima of the potential hypersurface as well as saddle points and dissociation limits are well described. Clusters of atoms Yn can be represented as well by this type of function. Care is taken to implement conditions resulting from the symmetric group Sn and to construct positive definite bilinear forms of special functional forms of certain coordinates (such as bond lengths and bond angles), in order to avoid artifacts in exceptional ranges of the potential hypersurface. These special functional forms include intrinsic, symmetry allowed couplings between coordinates such as bending and stretching. We include linear potential terms in bond angle coordinates, which result in effectively quadratic potential terms for highly symmetric structures. True logical multidimensional 01-switching functions Ssw(r) of bond lengths r are used to interpolate between limiting ranges in the hypersurface. The particular form Ssw(r)∼exp(−(rsw/r)nsw) allows us to describe the potential as a multipole expansion representation in the limit of large r(→∞). In the application to methane, first the representations are fitted to data from high level ab initio calculations using multireference configuration interaction techniques. Additional conditions which help to improve the description of experimental data are considered during the fit. Typically, these conditions involve some parameters or parameter groups and refer to the equilibrium geometry and harmonic force field. Other constraints apply to the energies of dissociation channels. We describe the model potentials METPOT 1 to METPOT 4 in the present work.
High level ab initio molecular orbital study of the structures and vibrational spectra of CHBr+ and CBr+
Authors: Li Z. , Francisco J. S.
Published: July 2002

The equilibrium structures and harmonic vibrational frequencies for CHBr+, CBr+, and their neutrals have been determined using second-order Mo/ller–Plesset perturbation theory, singles and doubles configuration interaction theory, and coupled-cluster theory in conjunction with the triple-zeta double-polarized and 6-311++G(3df,3pd) basis sets. The coupled-cluster results are in good agreement, for the experimental geometries and harmonic frequencies, with available experimental data. The adiabatic ionization potentials are predicted to be 205.7 kcal mol−1 for CHBr and 197.5 kcal mol−1 for CBr.
Infrared spectra of cis and trans-(NO)2− anions in solid argon
Authors: Andrews Lester , Zhou Mingfei , Willson Stephen P. , Kushto Gary P. , Snis Anders , Panas Itai
Published: July 2002

Laser-ablation of over 20 different metal targets with concurrent 10 K codeposition of Ar/NO mixtures produces metal independent infrared bands at 1589.3 cm−1 due to (NO)2+, a new absorption at 1221.0 cm−1, and a band set at 1300.3, 1222.7, 884.4 cm−1. The latter bands decrease more on annealing than the 1221.0 cm−1 band. Isotopic substitution (14NO,15NO, 15N18O, and mixtures) shows that these new vibrations involve two equivalent N–O oscillators, which identifies two new (NO)2 species. The excellent agreement with frequencies, intensities, and isotopic frequency ratios from density functional theory calculations substantiates assignment of the 1221.0 cm−1 band to trans-(NO)2− and the three band set to cis-(NO)2−. The observation of a weak combination band at 2492.0 cm−1 further substantiates assignment of the two N–O stretching modes in cis-(NO)2−.
A novel form for the exchange-correlation energy functional
Authors: Van Voorhis Troy , Scuseria Gustavo E.
Published: July 2002

A new approximate form for the exchange-correlation energy functional is developed. The form is based on the density matrix expansion (DME) for the exchange functional [R. M. Koehl, G. K. Odom, and G. E. Scuseria, Mol. Phys. 87, 835 (1996)]. The nonlocal portion of the correlation energy is assumed to have the same general form as that derived for exchange, while the local portion is taken to be that of the uniform electron gas. The resulting formula does not resort to the use of exact-exchange mixing. A Kohn-Sham implementation of this functional is constructed and the parameters within the functional are adjusted to minimize the difference between the theoretical and the experimental data for a large set of atomic and molecular systems. The results are found to compare favorably with existing functionals, even those which include exact-exchange mixing.
Resonant coherent hyper-Raman scattering (CHRS). II. A theory on the general relationship between CHRS and coherent Raman scattering (CRS) processes
Authors: Cho Minhaeng
Published: July 2002

A theoretical description of the resonant coherent hyper-Raman scattering for an arbitrary system–bath interaction is presented. Particularly the relationship between the coherent hyper-Raman scattering (CHRS) and coherent Raman scattering (CRS) signals is established without alluding to any specific model of optical broadening. The permanent dipole moment difference between the excited and ground states of a given chromophore is shown to be proportional to the ratio of the CHRS to CRS signals, and it is proved that this result is valid regardless of the model for the chromophore–bath interaction.
Small angle neutron scattering experiments from colloidal dispersions at rest and under sheared conditions
Authors: Dux Ch. , Musa S. , Reus V. , Versmold H. , Schwahn D. , Lindner P.
Published: July 2002

Neutron diffraction from charge stabilized shear ordered colloidal dispersions at rest and under sheared conditions are presented. A newly designed shear cell is used to generate a linear shear profile. Hexagonal scattering patterns were observed both at rest and under sheared conditions. The stacking probability A is determined by measuring the intensity dependence of the Bragg spots as a function of the angle between the incoming neutron beam and the sample cell. The shear experiments are discussed in terms of a continuous distortion [W. Loose and B. J. Ackerson, J. Chem. Phys. 101, 7211 (1994)] at small shear rates, and shear melting at higher shear rates.
The correlation functions of hard-sphere chains: Monodisperse chains as a complete association limit
Authors: Chang Jaeeon , Kim Hwayong
Published: July 2002

The mixture of associating hard spheres with two random association sites is considered to model freely jointed tangent hard-sphere chains of fixed length. In the case of the complete association limit with infinite association strength, the associating fluid becomes the hard-sphere chain fluid. The multidensity Ornstein–Zernike equation is applied to this limiting case, and an analytical solution is obtained within the polymer Percus–Yevick (PPY) approximation. In doing so, we imposed connectivity constraints between bonded segments in order to avoid numerically inconvenient forms. Explicit expressions for the contact values of the correlation functions are obtained, and the correlation functions for the region beyond the hard core are calculated from a set of integral equations involving only finite quantities. Predictions of the theory for 4- and 8-mer fluid are compared to computer simulation results. For overall correlation functions accurate predictions are obtained over the whole density range. For the inter- and intramolecular correlation functions, a significant improvement is found at low density compared to our previous theory with the PPY ideal-chain approximation. As chain length increases, the theory overestimates the intermolecular correlation functions, and underestimates the intramolecular correlation functions. It is concluded that the good accuracy for the overall correlation functions is due to the cancellation of errors between the inter- and intramolecular correlation functions.
Molecular dynamics study of the Ag6 cluster using an ab initio many-body model potential
Authors: Garzón I. L. , Kaplan I. G. , Santamaria R. , Novaro O.
Published: July 2002

A general approach to construct a model potential with parameters fitted to ab initio energy surfaces, including many-body nonadditive effects, developed in our previous works is applied to the Ag6 cluster. A molecular dynamics study of structural and dynamical properties of this cluster is performed using such a potential. Two new stable two-dimensional isomers with C2v and C2h symmetries are identified as local minima of the potential surface using the simulated quenching technique. An analysis of the thermal stability as a function of the cluster temperature reveals interesting features in the meltinglike transition of Ag6. A two-step isomerization phenomenon is observed: at temperatures around 300 K, the cluster structures fluctuate among two-dimensional isomers, at higher temperatures (500 K), fast transitions occur between two- and three-dimensional cluster configurations. The simulation was extended up to the cluster fragmentation which is observed through dimer evaporation.
Millimeter-wave spectroscopy of chromium monochloride (CrCl)
Authors: Oike Tomoko , Okabayashi Toshiaki , Tanimoto Mitsutoshi
Published: July 2002

The rotational spectrum of chromium monochloride in the 6Σ+ ground electronic state has been observed with a source-modulated microwave spectrometer in the millimeter-wave region. The CrCl radical was produced in a free space absorption cell by a dc glow discharge in a mixture of AlCl3 vapor and He using stainless steel electrodes. Chromium atom was supplied from the electrodes by the sputtering reaction. Observed spectral lines were analyzed by a least-squares fit and the molecular constants including the rotational and centrifugal distortion constants and the spin-spin and spin-rotation interaction constants were determined. The experimental rotational constants B0 for 52Cr35Cl and 52Cr37Cl are 5 009.345 69(69) and 4 847.851 00(32) MHz, respectively, where the uncertainties attached to the last digit correspond to one standard deviation.
A new phosphorus bearing derivative of the methyl radical, CH2CP, studied by microwave spectroscopy and ab initio calculation
Authors: Ahmad Imtiaz K. , Ozeki Hiroyuki , Saito Shuji , Botschwina Peter
Published: July 2002

The microwave spectrum of the CH2CP radical in its 2B1 ground electronic state was detected for the first time using a source-modulated microwave spectrometer. The radical was generated in the cell by a dc-glow discharge in a mixture of PH3 and C2H2. Fine structure was observed but no hyperfine structure pertaining to the phosphorus and hydrogen nuclei could be resolved. In total 110 spectral lines were measured for the K-structure and fine structure of eight rotational transitions in the 300–380 GHz region. The rotational, centrifugal distortion, and spin-rotation coupling constants were obtained by a least-squares analysis of the measured frequencies. On the basis of accompanying coupled cluster calculations an accurate equilibrium structure could be established; re(CH)=1.0816(5) Å, αe(HCH)=118.22(5)°, R1e(CC)=1.3418(10) Å, and R2e(CP)=1.5889(10) Å. The equilibrium dipole moment is predicted to be μe=−1.15 D, with the negative end at the phosphorus site.
An ab initio molecular orbital study of even-membered hydrogen cluster cations: H6+, H8+, H10+, H12+, and H14+
Authors: Kurosaki Yuzuru , Takayanagi Toshiyuki
Published: July 2002

Geometries and energetics for even-membered Hn+ (n=6–14) clusters that have the H2+ core in the geometrical structures have been theoretically studied using ab initio molecular orbital methods. It was found that the H2+-core H6+ cluster has D2d symmetry and that the geometrical structures of H2+-core clusters of larger size are composed of unperturbed H2+-core H6+ and outer H2s weakly bound to it. It was predicted for both the H6+ and H8+ clusters that the H2+-core clusters are more stable in energy than the corresponding H3+-core ones. However, the energy difference between H2+- and H3+-core H8+s was calculated to be significantly smaller than that between H2+- and H3+core H6+s. The binding energies of outer H2 in H2+-core clusters were predicted to be 0.7, 0.4, 0.1, and <0.1 kcal mol−1 for H8+, H10+, H12+, and H14+, respectively, at the PMP4(SDTQ)/cc-pVTZ//MP2/cc-pVTZ+ZPE level, suggesting that H8+ and H10+ are stable enough to be detected, but H12+ and H14+ are less stable. This result is consistent with the experimental observation of Kirchner and Bowers [J. Chem. Phys. 86, 1301 (1987)] that only Hn+ clusters with n⩽11 are formed with sufficient intensity to be detected.