bluuebunny/crossref_metadata_2025 huggingface dataset:
Statistical effects in the thermal deazetization reaction of 2,3-diazabicyclo(2.2.1)hept-2-ene
Authors: Sorescu Dan C. , Thompson Donald L. , Raff Lionel M.
Published: July 2002

The deazetization reaction of 2,3-diazabicyclo(2.2.1)hept-2-ene-exo, exo-5,6-d2 is investigated using Monte Carlo classical variational transition-state theory, implemented by the efficient microcanonical sampling procedure. Comparison is made with the results of trajectory calculations performed on the same global potential-energy surface. The microcanonical reaction rates have been determined for both reaction channels, i.e., the stepwise and concerted cleavage of the two C–N bonds of the reactant. The results demonstrate that the thermal decomposition of 2,3-diazabicyclo (2.2.1)hept-2-ene-exo,exo-5,6-d2 is well described by statistical theories that assume equal weighting for all energetically accessible phase-space points. It is found that the rate coefficients of the statistical calculations are close upper bounds of the rates determined in trajectory calculations. Previously reported classical trajectory simulations have shown that the distribution of internal energy in the reactant configuration, at the transition state and beyond is very nearly microcanonical for the range of excitation energies analyzed (60–175 kcal/mol). Under such conditions, the agreement obtained between the present statistically computed rate coefficients and those extracted from the trajectory results is not surprising. It is suggested that nonstatistical post-transition-state events account for the nonunity ratio of the exo/endo reaction products observed experimentally. These events are not considered in the present statistical theories of the reaction rates.
The Hamiltonian for a weakly interacting trimer of polyatomic monomers
Authors: Xantheas Sotiris S. , Sutcliffe Brian T.
Published: July 2002

The body-fixed Hamiltonian for a trimer of polyatomic monomers is expressed in terms of the distances between the monomer centers of mass and of the orientation angles of the monomers relative to the body-fixed frame. The particular case where the monomers are water molecules is considered in some detail. In this work we attempt to assess the possibility of using a computer algebra system to aid in the evaluation of the angular matrix elements.
Rotational dynamics of axially symmetric solutes in isotropic solvents. II. The stochastic model
Authors: Polimeno Antonino , Moro Giorgio J. , Freed Jack H.
Published: July 2002

In paper I of this series, a molecular dynamics (MD) study of liquid chlorine was performed, and it includes the definition and observation of operational cage variables. These cage variables were used to describe the local environment of a rotating axially symmetric molecule, or probe. Probe and cage properties of interest, such as rotational correlation functions and momentum correlation functions, were computed, together with an effective distribution of librational cage frequencies. In the second part of this work, we develop a stochastic model which includes the relevant elementary relaxation processes previously identified by the MD study. This stochastic model is based upon a multi-dimensional Fokker–Planck equation for the coupled dynamics of the probe and cage orientations, the angular momentum of the probe, and the librational frequencies for the probe in the cage. Semi-analytical approximations, based upon a ‘‘Born–Oppenheimer’’-type separability of fast and slow variables, are used in order to calculate probe and cage correlation functions, and they are found to be in reasonable agreement with the MD results. In an appendix the Born–Oppenheimer approximation for stochastic operators is developed.
Transition-state optimization using the divide-and-conquer method: Reaction of trans-2-butene with HF
Authors: Zhao Qingsheng , Nicholas John B.
Published: July 2002

We present a study of transition state optimization with the divide-and-conquer (DC) density functional theory (DFT) method, using the addition of HF to trans-2-butene as an example. We present relative energies and geometries for each state in the reaction. We show that the DC method gives results that agree well with those obtained by conventional Kohn–Sham DFT methods, using the same exchange-correlation functional and basis set.
Core level binding energy shifts and polarization screening: A combined experimental and theoretical study of argon clusters
Authors: Björneholm O. , Federmann F. , Fössing F. , Möller T. , Stampfli P.
Published: July 2002

Photoelectron spectra of the argon 2p core level for free argon clusters of up to 4000 atoms are compared to detailed calculations. The comparison shows that the size-dependent shifts of the core level binding energy can be explained in a pure polarization-screening model. Important differences arise between the shifts for the bulk (interior) and the surface atoms. The agreement between experiment and theory allows the extrapolation of the cluster data to the ‘‘infinite’’ solid. In this way we obtain the shifts of the core level binding energy between the free atom, the surface atom and the bulk of argon. The relation between these shifts and those of the first ionization potential is discussed.
Femtosecond study of Xe-Cl2 reactions in clusters
Authors: Dedonder-Lardeux C. , Berdah M. , Jouvet C. , Martrenchard-Barra S. , Mestdagh J. M. , Solgadi D. , Visticot J. P.
Published: July 2002

The reaction of a xenon atom with a chlorine molecule has been studied in the femtosecond regime using the pump (395 nm) and probe (308 nm) technique in Xe-Cl2 clusters. The electronically excited reaction product XeCl was observed both after a simultaneous and a sequential (with a delay of ≊500 fs) absorption of the pump and probe photons.
Stochastic simulations of the trapping of ethane on Pt(111) from a realistic potential: The roles of energy transfer processes and surface corrugation
Authors: Stinnett James A. , Madix Robert J. , Tully John C.
Published: July 2002

Classical three dimensional stochastic trajectory simulations using an empirical pairwise additive Morse potential were employed to model the molecular adsorption of ethane on cold Pt(111). A single set of parameters was found which quantitatively represents the dependence of the initial adsorption probability on incident energy and angle and accurately reproduces scattering distributions of ethane from Pt(111). The simulations suggest that, on average, rotational excitation serves as an effective temporary energy storage mechanism which facilitates trapping. Excess rotational excitation into cartwheel motion, however, can cause ethane to scatter by a chattering collision. At moderate translational energies trapping is determined primarily by energy transfer from translational energy to cartwheel rotation and surface phonons for molecules incident along the surface normal, whereas cartwheel rotation combined with parallel translational energy retention determine trapping at glancing angles of incidence. As the incident translational energy is increased, trapping becomes more dependent on the excitation of cartwheel rotational excitation at normal incidence. Finally, the trapping probability of ethane on Pt(111) was found to be determined to within 10% by the fate of the first bounce.
Collapse of a polymer chain: A Born–Green–Yvon integral equation study
Authors: Taylor Mark P. , Lipson J. E. G.
Published: July 2002

A Born–Green–Yvon (BGY) type integral equation is developed for the intramolecular distribution functions of an isolated flexible polymer chain. The polymer is modeled as a linear array of n identical spherical interaction sites connected by universal joints of bond length σ. In particular we study chains composed of up to n=400 square-well spheres with hard-core diameters σ and well diameters λσ (1≤λ≤2). Intramolecular distribution functions and the resulting average configurational and energetic properties are computed over a wide range of temperatures. In the high temperature (good solvent) limit this model is identical to the tangent hard-sphere chain. With decreasing temperature (worsening solvent) the square-well chain undergoes a collapse transition identified by a sudden reduction in chain dimensions and a peak in the single chain specific heat. Extensive comparison is made between the BGY results and Monte Carlo results for square-well chains with λ=1.5. The BGY theory is extremely accurate for square-well 4-mers at all temperatures. For longer chains the theory yields reasonably accurate results for reduced temperatures greater than T*≊1 (expanded and theta states) and qualitatively correct behavior for T*<1 (collapsed state). Very accurate values for the theta temperatures for square-well chains with 1.25≤λ≤2.0 are also obtained.
Reaction of tungsten clusters with molecular nitrogen
Authors: Mitchell S. A. , Rayner D. M. , Bartlett T. , Hackett P. A.
Published: July 2002

Reactions of tungsten clusters with molecular nitrogen have been investigated by using a fast-flow reactor equipped with a laser vaporization cluster source and time-of-flight mass spectrometer detector. Absolute rate coefficients are reported for reaction of Wn clusters in the range n=4–26, at temperatures 277, 300, and 370 K in He buffer gas at 1 and 2 Torr pressure. For smaller clusters with n<15, complexes with N2 are formed with binding energies near 16 kcal mol−1, and act as precursors to dissociation of N2 on the clusters. A sharp jump in the binding energy occurs at a cluster size of 15 metal atoms, and may signal the onset of atomic as opposed to molecular binding of N2 on the cluster. It is suggested that the change in reactivity at n=15 is correlated with a structural transition of the clusters, from relatively close-packed to more open structures. The reactivity of tungsten clusters with N2 is compared with that of molybdenum clusters and tungsten metal surfaces.
Theoretical study of the valence π→π* excited states of polyacenes: Benzene and naphthalene
Authors: Hashimoto T. , Nakano H. , Hirao K.
Published: July 2002

Multireference perturbation theory with complete active space self-consistent field (CASSCF) reference functions was applied to the study of the valence π→π* excited states of benzene and naphthalene. The eigenvectors and eigenvalues of CASSCF with valence π active orbitals satisfy pairing properties for the alternant hydrocarbons to a good approximation. The excited states of polyacenes are classified into the covalent minus states and ionic plus states with the use of the alternancy symmetry. The present theory satisfactorily describes the ordering of low-lying valence π→π* excited states. The overall accuracy of the present approach is surprisingly high. We were able to predict the valence excitation energies with an accuracy of 0.27 eV for singlet u states and of 0.52 eV or better for singlet g states of naphthalene. Our predicted triplet states spectrum provides a consistent assignment of the triplet–triplet absorption spectrum of naphthalene. For benzene we were able to predict the valence excitation energy with an accuracy of about 0.29 eV. The covalent minus states and ionic plus states exhibit different behavior as far as the electron correlation is concerned. The ionic plus states are dominated by the single excitations but covalent minus states include a large fraction of doubly excited configurations. The covalent minus states always give lower energy than the corresponding ionic plus states. This is true for triplet states. The dynamic σ–π polarization effects introduced by perturbation theory are significant for the ionic plus states while those on covalent excited states are usually of the same order as in the covalent ground state. The enlargement of the active space of the reference functions represents a great improvement of the description of the ionic states. The present approach with the pairing properties has proved to be of great value in understanding and predicting the experimental data of the alternant hydrocarbons.
Complex oscillations and global coupling during the catalytic oxidation of CO
Authors: Liauw M. A. , Plath P. J. , Jaeger N. I.
Published: July 2002

Rate oscillations during heterogeneous catalytic reactions often display self-similar mixed-mode oscillations that have also been found in various other experimental systems. Spatially nonuniform behavior is revealed by monitoring the surface temperature during complex oscillations in the kinetic regime of the CO oxidation on a Pd supported catalyst at atmospheric pressure. The experimental results demonstrate the influence of global coupling via the gas phase concentration of the deficient component. In addition, the local autonomous frequency appears to be spatially nonuniform. A simple discrete model, a set of globally coupled relaxation oscillators with a frequency distribution, captures qualitatively the experimentally observed behavior.
Adsorption of living polymers on a solid surface: A Monte Carlo simulation
Authors: Milchev Andrey , Landau D. P.
Published: July 2002

The effects of surface segregation in a polydisperse system of ‘‘living polymers,’’ confined between parallel solid surfaces, are studied by means of a lattice Monte Carlo (MC) simulation. For given density and temperature the system is characterized by exponential distribution of chain lengths established in the process of reversible equilibrium polymerization. The results of the calculations show that entropic effects cause a rearrangement of the species with different length into an ‘‘onion shell’’ structure. Thus for flexible chains preferential presence of shorter chains in the vicinity of the surface takes place, in contrast to the case of semi-rigid chains, when shorter chains deplete the region around the solid surface. In the case of adsorbing walls this surface segregation is enhanced. The study also reveals that isotherms of the adsorbed coverage and adsorbed amount are strongly influenced by the degree of chain stiffness.
Dissociative photoionization of H2 and D2 in the energy region of 25–45 eV
Authors: Ito Kenji , Hall Richard I. , Ukai Masatoshi
Published: July 2002

Dissociative photoionization of molecular hydrogen and deuterium has been studied using synchrotron radiation and an electrostatic analyzer with a position sensitive detection system for the threshold region of the doubly excited states, 25–45 eV. The Q1 1Σ+u(1) autoionizing state plays an important role in the production of fast protons below 30 eV. It is found that, for the incident photon energy 30–40 eV, the dominant process for the production of energetic protons is autoionization of the Q2 1Πu(1) state which preferentially autoionizes to the H+22pσu state rather than to the H+21sσg state. The Q2 1Πu(2) and Q2 1Σ+u autoionizing states are also found to produce fast protons in this energy region. Above 42 eV, the kinetic energy distributions are dominated by direct ionization to the 2pσu and 2pπu states of H+2.
Structural evolution and microscopic interactions in a three-component amphiphilic microemulsion system
Authors: Komura S. , Seto H. , Takeda T. , Nagao M. , Ito Yuji , Imai M.
Published: July 2002

Structures and interaction parameters of a three-component amphiphilic microemulsion system consisting of n-decane, water, and an amphiphile AOT (sodium-di-2-ethylhexylsulfosuccinate) have been studied by means of small angle neutron scattering. The scattering curves are fitted with Teubuer and Strey’s formula, from which three structural parameters are extracted; the average periodic distance D between the oil and water, the structural correlation length ξ, and the area aH per amphiphile molecule. The same data are further examined in terms of Gompper and Schick’s formula, from which three interaction parameters are obtained; the strength of segregation J/T among the oil and water, the amphiphilicity L/T of the amphiphile toward the oil and water, and the critical surfactant volume fraction φcs. These interaction parameters have right order of magnitudes and their implications to structural evolution are discussed.
Smart Monte Carlo for accurate simulation of rare-event dynamics: Diffusion of adsorbed species on solid surfaces
Authors: Kumar P. V. , Raut Janhavi S. , Warakomski Steven J. , Fichthorn Kristen A.
Published: July 2002

We introduce a dynamical Smart Monte Carlo algorithm and assess its applicability for simulating the rare-event dynamics of adsorbate diffusion. Using the dynamical Smart Monte Carlo method, we simulate the self-diffusion of an adatom in the Cu/Cu(001) and Rh/Rh(111) systems and we compare the simulated diffusion coefficients to values arising from molecular dynamics and transition-state theory. We find that the accuracy of Smart Monte Carlo is sensitive to details of the potential-energy surface. For Cu/Cu(001), the agreement between dynamical Smart Monte Carlo, molecular dynamics, and transition-state theory is excellent. A similar comparison for the Rh/Rh(111) systems shows discrepancies between these three techniques. We find that the origins of the discrepancies in the Rh/Rh(111) system are transition-state recrossings, for small simulation time steps, and low escape rates of the adatom from the binding sites, at large time steps. We examine the sampling and dynamics in trajectories using a smaller time step for motion perpendicular to the surface than that for parallel motion. These studies show that low Smart Monte Carlo escape rates in the Rh/Rh(111) system can be correlated to excessive sampling, beyond the configurational space of the potential-energy minimum, at large time steps. Recrossings can be understood to arise from the absence of velocity correlations in the low-friction, transition-state region and can be minimized through the use of a large time step for parallel motion. With the appropriate choice of simulation time steps it is possible to improve the agreement between dynamical Smart Monte Carlo and more rigorous dynamical techniques.
Estimation of mutual diffusion coefficients in polymer/penetrant systems using nonequilibrium molecular dynamics simulations
Authors: Sunderrajan Suresh , Hall Carol K. , Freeman Benny D.
Published: July 2002

Experimental studies of the diffusion of small molecules in polymers typically report mutual diffusivities while simulation studies report tracer diffusivities. The Darken equation has been used to relate measured mutual diffusivities to calculated tracer diffusivities. The current study uses grand canonical molecular dynamics (GCMD) to simulate a gradient in chemical potential and calculate the mutual diffusivity in a polymer/penetrant system. The penetrants are modeled as hard spheres while the polymer is modeled as a collection of stationary hard chains. Penetrant mutual diffusivity is a function of two competing factors, a thermodynamic factor and a friction factor. An increase in the chemical potential of the penetrant tends to increase the diffusivity, the thermodynamic factor effect, while a corresponding increase in the number of particles within the system tends to decrease the diffusivity, the friction factor effect. Mutual diffusivity differs from penetrant tracer diffusivity by almost an order of magnitude. Mutual diffusivities do not obey the Darken relation for the simulation model used in this study.
Kinetics measurements of CO photo-oxidation on Pt(111)
Authors: Tripa C. Emil , Arumaninayagam Christopher R. , Yates John T.
Published: July 2002

The photoactivation of chemisorbed O2 in the presence of chemisorbed CO on Pt(111) has been investigated for uv light in the range 3.87–4.77 eV (260–320 nm). Three photoprocesses first-order in O2 coverage have been separated and for the first time the cross sections for each are reported. The dominant process is O2 photodissociation (Qdiss=4.0±0.1×10−21 cm2). The second most probable process is photodesorption (Qdes=2.2±0.1×10−21 cm2). The least probable process is photoreaction with chemisorbed CO (Qrxn=0.35±0.03×10−21 cm 2). Previous studies of Qrxn have reported cross sections as high as 5×10−17 cm2.
Rotational diffusion of asymmetric molecules in liquid crystals: A global analysis of deuteron relaxation data
Authors: Dong Ronald Y. , Shen X.
Published: July 2002

A new global analysis of the relaxation data in the nematic and smectic A phases of 4-n-pentyloxybenzylidene-4′-heptylaniline [Phys. Rev. E 49, 538 (1994)] supports a model which includes director fluctuations and rotational diffusion of an asymmetric rigid rotor in a biaxial potential of mean torque. The molecular biaxiality of the molecule is found, based on the relaxation data, to give a small positive biaxial order parameter Syy–Sxx. In addition, the activation energy for the tumbling motion of the molecule is found to be larger than that for the spinning motion. The usefulness of globally analyzing the deuteron relaxation data at different temperatures and frequencies is demonstrated.
Cube to cage transitions in (H2O)n (n=12, 16, and 20)
Authors: Sremaniak Laura S. , Perera Lalith , Berkowitz Max L.
Published: July 2002

Molecular dynamics computer simulations were performed for (H2O)n (n=12, 16, and 20) followed by systematic quenching under a polarizable and a nonpolarizable model to determine the minimum energy structures each favored. Ab initio calculations were done on several minima for (H2O)12 to determine their relative energies. The polarizable model prefers cagelike structures for all cluster sizes, whereas the nonpolarizable model predicts minima of fused cubes for (H2O)12 and (H2O)16 but makes the transition to a cagelike minimum at (H2O)20.
Dielectric continuum model for calculating reorganization free energies of electron transfer in proteins
Authors: Zhou Huan-Xiang
Published: July 2002

A dielectric continuum model is developed for calculating polarization reorganization energies of electron transfer reactions that occur in proteins. The model is based on an earlier microscopic formulation of the Marcus electron transfer theory. The classical Marcus result, λ=ΔGop1-0−ΔG1-0, for the free energy of polarization reorganization is derived from the microscopic theory. Both ΔGop1-0 and ΔG1-0 denote the electrostatic free energy due to a positive unit charge (+e) distributed in the region representing the electron donor and a negative unit charge (−e) distributed in the region representing the electron acceptor. In calculating ΔGop1-0, the donor and acceptor as well as the environment surrounding them take the optical dielectric constant εop. In calculating ΔG1-0, the donor and acceptor keep the optical dielectric constant but the environment takes the static dielectric constant ε. The environment consists of the protein matrix (where εop=εpop and ε=εp) and the solvent (where εop=εsop and ε=εs). The polarization reorganization free energy can be approximated as the sum of two components λ1 and λ2. In calculating λ1, the protein region is extended outward to infinity. For the case where the donor and acceptor are modeled as spheres (with both radii equal to a and center–center distance at r) and the electron charge is put at either center, a Marcus result, λ1=[(1/εpop)−(1/εp)][(1/a)−(1/r)]e2, is found to be highly accurate (maximum error 4%). In calculating λ2, the protein region is extended inward to fill the donor and acceptor. The magnitude of λ2 is usually much smaller than λ1. A toy electron-transfer protein is studied both by the dielectric continuum model and by implementing the microscopic formulation through computer simulations. Agreement of the results from the two approaches demonstrates the accuracy of the dielectric continuum model. © 1996 American Institute of Physics.
Interfacial quantum well states of Xe and Kr adsorbed on Ag(111)
Authors: McNeill J. D. , Lingle R. L. , Jordan R. E. , Padowitz D. F. , Harris C. B.
Published: July 2002

The energies and dispersions of the image states and quantum well electronic states in layers of Xe and Kr on a Ag(111) substrate were determined by angle-resolved two-photon photoemission (ARTPPE). For Xe, we measured binding energies of unoccupied electronic states for 1–9 layers and their parallel dispersion out to 4 layers. We measured the binding energies for a monolayer of Kr and dispersions for one and two layers. The n=2 and n=3 image states of the bare metal evolve into quantum well states of the layer (states of the Xe conduction band discretized by the boundary conditions of a 2-D slab) at higher Xe thicknesses, where the n=2,3 states exhibit both a perpendicular and parallel dispersion similar to that of the bulk Xe conduction band. The n=1 state appears to evolve with coverage as an image state screened by the Xe layer, with appreciable electron density in the vacuum. A continuum dielectric model (modified image state picture) reproduces the gross trends in the data, while an explicit quantum well analysis is used to extract the bulk Xe conduction band dispersion. A simple model which takes into account the band structures of the substrate and the overlayer, as well as the image potential, gives good agreement with the binding energy data. The combination of high energy and momentum resolution along both the surface parallel and surface normal yields very precise measurements of the bulk Xe conduction band as well as information about the behavior of conduction band electrons at interfaces.
Gas-phase niobium cluster reactivity with isobutane
Authors: Lafleur Rick D. , Parnis J. Mark , Rayner David M.
Published: July 2002

A laser ablation fast-flow reactor apparatus, equipped with a laser ionization time-of-flight mass spectrometer detector, has been used to investigate the reactivity of gas-phase Nbn with isobutane. Absolute second-order rate coefficients k(2) for the removal of Nbn by isobutane were measured at 279, 300, and 371 K in a continuous flow of He carrier gas held at 1 or 2 Torr total flow tube pressure. The rate coefficients for a given cluster species were found to be pressure independent, and became larger with increasing temperature. A gradual increase in k(2) with increasing cluster size has been observed. This increase is simply explained by a direct correlation of k(2) with the hard-sphere collision cross section. Pseudo-first-order kinetic plots for all cluster reactions with isobutane were linear with no bimodal character evident, indicating an insensitivity to structural forms of Nbn. Activation energies have been estimated, and their values were found to be close to zero for all Nbn cluster reactions with isobutane. The small magnitudes of k(2) relative to khs have been attributed to a reaction mechanism involving a ‘‘tight’’ transition-state complex. Product masses, after the reaction of a bare Nb cluster with one or more isobutane molecules, clearly show dehydrogenation to be an active process. A pseudo-first-order kinetic model for a consecutive bimolecular reaction scheme has been solved for the Nb2/isobutane system, and is in good agreement with the experimental data.
Intermolecular state dependence of the vibrational predissociation of N2HF
Authors: Tsang Susy N. , Chuang Cheng-Chi , Mollaaghababa Reza , Klemperer William , Chang Huan-C.
Published: July 2002

Three new combination bands, at the second overtone of the HF intramolecular stretch, 3ν1, with each of the three low frequency intermolecular modes, have been spectroscopically characterized by intracavity laser-induced fluorescence for the N2HF complex. The van der Waals stretching, HF and N2 bending frequencies at vHF=3 are determined to be 98.6, 328.6, and 68.5 cm−1, respectively. State dependent vibrational predissociation is observed in these three bands. The variation in the vibrational predissociation rate in these three bands suggests a strong angular dependence of the intermolecular potential with the HF internuclear distance in the complex.
Microwave spectrum of silylidene H2CSi
Authors: Izuha Mitsuaki , Yamamoto Satoshi , Saito Shuji
Published: July 2002

Rotational spectral lines of the H2CSi molecule are observed in the millimeter-wave and submillimeter-wave regions. The molecule is produced in a glow-discharge plasma of a gaseous mixture of SiH4 and CO. Rotational constants and centrifugal distortion constants have precisely been determined from observed frequencies of 32 a-type transitions with J=5–4 to J=12–11 and Ka=0 to Ka=4. From the observed inertial defect, the vibrational frequency of the CH2 rocking mode is estimated to be as low as 331 cm−1, which is consistent with the large quartic centrifugal distortion constants, DJK and d2. Higher-order centrifugal distortion constants up to the octic terms are necessary to obtain a good fit between the observed and calculated frequencies within experimental uncertainties. The low vibrational frequency and the necessity of the higher-order centrifugal distortion terms indicate a floppy nature of the CH2 rocking mode. A preliminary radioastronomical search for H2CSi in space has been carried out toward a few astronomical sources without success.
Precursor of the I
aq− charge-transfer-to-solvent (CTTS) band in I−⋅(H2O)n clusters
Authors: Serxner David , Dessent Caroline E. H. , Johnson Mark A.
Published: July 2002

Ultraviolet charge-transfer-to-solvent (CTTS) bands are general spectral features of inorganic salts dissolved in dipolar (e.g., aqueous) solvents. The bands originate from excitations of the anions, and the excited states are thought to involve a delocalized electronic state supported by the arrangement of solvent molecules around the ion in the ground state. These diffuse states have fascinated physical chemists since the 1930s, who first used continuum models to explain the phenomenon and then later refined the treatment to include local molecular interactions. In this paper, we report that CTTS bands evolve from features present in very small gas phase clusters, I−⋅(H2O)n, n=1–4, and discuss how localization of the continuum wave function of the ejected electron acts as a precursor to the bound excited state in the liquid.
On a theoretical model for the Renner–Teller effect in tetra-atomic molecules
Authors: Perić M. , Ostojić B. , Engels B.
Published: July 2002

A model for the ab initio treatment of the Renner–Teller effect in tetra-atomic molecules is elaborated. It is based on the approach developed by Petelin and Kiselev [Int. J. Quantum Chem. 6, 701 (1972)]. Particular attention is paid to Π electronic states. Perturbative formulas are derived for several coupling cases. The model is checked by means of ab initio calculations at various levels of sophistication. Results of computations of various quantities related to the model are presented for the X 2Πu states of B2H+2 and C2H+2. The reliability of the basis assumptions is demonstrated by comparing the results obtained in the framework of the model considered with those of independent ab initio calculations.
Structure and nucleation of pores in polymeric bilayers: A Monte Carlo simulation
Authors: Müller M. , Schick M.
Published: July 2002

Using a polymeric model, we study via Monte Carlo simulation the structure and stability of an amphiphilic bilayer against pore formation. The simulations yield the absorption isotherm of the amphiphile in solution. At a critical micelle concentration around 1.5×10−3, a bilayer forms, and the composition and orientation profiles across it are obtained. The bilayer tension is estimated via the spectrum of undulations. Pores in the bilayer are caused to nucleate by reducing the chemical potential of the amphiphile, and small ones form by peristaltic fluctuations of the bilayer thickness. In large pores, the amphiphile heads rearrange so as to shield the tails from solution. Composition profiles across pores of various sizes are obtained. An effective edge tension is extracted from the distribution of sizes and agrees with a simple model of a hydrophilic pore.
193.3 nm photodissociation of acetylene: Nascent state distribution of CCH radical studied by laser-induced fluorescence
Authors: Hsu Yen-Chu , Chen Fu-Tang , Chou Liang-Chien , Shiu Ying-Jen
Published: July 2002

The nascent rovibronic distribution of CCH radicals in the 193.3 nm photolysis of acetylene has been measured by laser-induced fluorescence in a supersonic jet. CCH fragments in the X̃ 2Σ+ state are vibrationally hot, but rotationally cold. Predominant CCH fragments were observed at levels of the X̃ state with large mixing of Ã-state character, particularly levels near the potential minimum of Ã 2Π. This indicates that a nonadiabatic transition near the exit channels plays an important role in the 193.3 nm photodissociation of acetylene. Some, but not all, of the K=1 levels have distinctively bimodal rotational distributions. The relative vibrational energy distributions obtained from this work were used to simulate the translational energy distribution of the hydrogen atom by Balko, Zhang, and Lee [J. Chem. Phys. 94, 7958 (1991)] to extract the population distribution of CCH. It is thus determined that the majority of CCH radicals are formed in the ground electronic state (X̃). Less than half of the CCH population was detected at K=1 levels, and the rest was distributed among K=0, 2, and 3 stacks. The bond energy of HCC–H is estimated as 131.5±0.5 kcal/mol from the vibronic energy of the most populated CCH fragments determined in this work and the translational energy of the recoiled hydrogen atom reported previously by Balko, Zhang, and Lee and Segall, Wen, Lavi, Singer, and Wittig [J. Phys. Chem. 95, 8078 (1991)].
Density-matrix–electronic-oscillator representation of optical spectroscopy of semiconductor nanocrystals
Authors: Yokojima S. , Meier T. , Mukamel S.
Published: July 2002

The optical response of CdSe semiconductor nanocrystals is investigated using the reduced single-electron density matrix in real space, calculated by means of the time-dependent Hartree–Fock technique. The spectroscopic signatures of exciton confinement are analyzed using the frequency-dependent electronic coherence matrix (off-diagonal density-matrix elements). The effects of Hartree and the Fock (exchange) type Coulomb interactions on the exciton binding energy are discussed. The latter result in almost dark excitons situated energetically below the main transition. Off-diagonal Coulomb matrix elements lead to larger exciton binding energies compared with previous calculations, and result in a better agreement of the size dependence of the lowest optical transition with experiment.
Ab initio configuration interaction calculations of the predissociation of rovibrational levels of the C 3Πg and d 1Πg 3sσ Rydberg states of the oxygen molecule
Authors: Li Yan , Petsalakis Ioannis D. , Liebermann Heinz-Peter , Hirsch Gerhard , Buenker Robert J.
Published: July 2002

Ab initio configuration interaction calculations have been carried out for seven low-lying  3,1Πg states of the oxygen molecule. Three different types of nonadiabatic couplings have been considered: spin-orbit, radial, and rotational. The complex scaling method has been employed to compute rovibrational level locations and predissociation linewidths with a basis of 200 Hermite polynomials for each of 13 different Ω electronic states. The calculations correctly predict that the v=2 level has the narrowest linewidth for the O216CΠg3 state, while v=4 is narrowest for O218. Marked variations in the linewidths of the different Ω components of the C state are explained by the fact that the π*→3sσ Rydberg and σ→π* valence Πg3 states have different occupations of the π* orbital, causing opposite orderings of their respective Ω levels. Rotational coupling is found to be important for high J values of the C state. The d 1Πg 3sσ state shows even more unusual effects by virtue of the fact that there is a sharply avoided crossing between the corresponding Rydberg diabatic state with a bound σ→π* 1Πg valence state. The calculations find irregular spacings in the d-state vibrational manifold, wide variations in linewidth for different v,J levels, and a large change in the rotational constant in successive vibrational levels, all of which effects have been earlier demonstrated in experimental work. Satellite lines are indicated for both the v=2 and 3 levels as a result of the interaction with the bound Πg1 valence state, whereby experimental verification exists only for v=2. The v=3 state has not yet been successfully identified due to the broadness of the d-X spectrum in the energy range of interest. The observed temperature dependence of the linewidths of the two features near the expected location of the v=2 level can also be understood on the basis of these calculations. Finally, the change in the predissociation mechanism for the d state from spin-orbit to radial as v changes from 0 to 2 which has been deduced experimentally is also verified in the present theoretical treatment.
Centroid path integral molecular dynamics simulation of lithium para-hydrogen clusters
Authors: Kinugawa Kenichi , Moore Preston B. , Klein Michael L.
Published: July 2002

The real-time quantum dynamics of a series of lithium para-hydrogen clusters, Li(p-H2)n (n=13, 55, and 180), has been investigated at 2.5 and 4.0 K by means of normal mode centroid path integral molecular dynamics (NMCMD) simulation, following the methodology originally proposed by Cao and Voth [J. Chem. Phys. 101, 6168 (1994)]. The Li(p-H2)34 and neat (p-H2)34 clusters have also been simulated at 2.5 K to see the effect of doping of a Li atom on the cluster dynamics. We explicitly display both the microcanonical and the constant-temperature equations of motion for NMCMD simulations using the Nosé–Hoover chain thermostats and the reference system propagator algorithm (RESPA). In addition to the energetic and structural properties, the real-time semi-classical dynamics of the centroids of the Li atom and p-H2 molecules in the clusters has been explored to investigate the diffusive and vibrational properties. In general, quantization of the nuclear motion enhances the ease of melting and diffusion, and also causes the power spectra of the velocity autocorrelation functions of hydrogen molecules to be dramatically shifted to lower frequency than in the classical regime. The self-diffusion coefficient of p-H2 molecules in the Li(p-H2)13 cluster, which is melted completely at both 2.5 and 4.0 K, is comparable to the experimental value of bulk liquid p-H2 just above the melting temperature (14 K). No significant differences have been found for the structural properties, molecular diffusion, and the quantum vibrational spectra of p-H2 molecules between the neat (p-H2)34 and Li(p-H2)34 clusters at 2.5 K. The n=34, 55, and 180 clusters show a solidlike core and a tendency towards surface melting at 2.5 K, with diffusion restricted to the outer region. The core of the Li(p-H2)55 cluster exhibits slow liquidlike diffusion at 4.0 K, whereas that of the Li(p-H2)180 cluster remains solidlike at this temperature. The quantum power spectrum of the velocity autocorrelation function of the core molecules in the Li(p-H2)180 cluster reveals vibrational frequencies in good accord with experimental neutron and Raman data on the bulk solid. The power spectrum of the Li atom on the cluster surface is peaked at low frequency (<20 cm−1).
A multipole reaction-field model for gauge-origin independent magnetic properties of solvated molecules
Authors: Mikkelsen Kurt V. , Jo/rgensen Poul , Ruud Kenneth , Helgaker Trygve
Published: July 2002

We present the first model for calculating gauge-origin independent magnetic properties of solvated molecules using London atomic orbitals. The solvent is represented as a dielectric medium. We use London atomic orbitals in order to ensure gauge-origin independence and fast basis-set convergence. We present results for the magnetizability and the nuclear shielding constants of the two molecules H2O and CH4.
The quantum dynamics of interfacial hydrogen: Path integral maximum entropy calculation of adsorbate vibrational line shapes for the H/Ni(111) system
Authors: Kim Dongsup , Doll J. D. , Gubernatis J. E.
Published: July 2002

Vibrational line shapes for a hydrogen atom on an embedded atom model (EAM) of the Ni(111) surface are extracted from path integral Monte Carlo data. Maximum entropy methods are utilized to stabilize this inversion. Our results indicate that anharmonic effects are significant, particularly for vibrational motion parallel to the surface. Unlike their normal mode analogs, calculated quantum line shapes for the EAM potential predict the correct ordering of vibrational features corresponding to parallel and perpendicular adsorbate motion.
A multi-configurational time-dependent Hartree approach to the direct calculation of thermal rate constants
Authors: Matzkies Frank , Manthe Uwe
Published: July 2002

A method employing the multi-configurational time-dependent (MCTDH) approach for the direct calculation of the thermal rate constant is presented. It is based on the flux-position correlation function of Miller et al. [J. Chem. Phys. 79, 4889 (1983)]. Eigenvalues of the thermal flux operator F̂T=e−Ĥ/2kTF̂e−Ĥ/2kT are calculated employing an iterative diagonalization scheme suitable for the MCTDH approach. F̂T has only a few significant eigenvalues, which can be interpreted as contributions of the ground and excited vibrational states of the activated complex. The rate constant is calculated by propagating the eigenfunctions of F̂T in time. As an example, the H+H2 reaction is studied. Exact results for vanishing total angular momentum (J=0) are given. The total thermal rate constant is calculated by a J shifting scheme which takes the linear geometry of the transition state into account.
The solution of the Schrödinger equation by the (t,t) method using the wave operator procedure: Theory and application to molecular photodissociation with ultrashort laser pulses
Authors: Jolicard G. , Balakrishnan N.
Published: July 2002

A new method for studying intense field photodissociation of molecules using ultrashort laser pulses is presented. The method is based on a generalized Floquet approach making use of the (t,t) formalism and the wave operator theory. The usefulness of the scheme for ultrafast photodissociation is illustrated by studying H2+ multiphoton dissociation with moderately intense and very short laser pulses.
Far-infrared absorption spectra of water, ammonia, and chloroform calculated from instantaneous normal mode theory
Authors: Kindt J. T. , Schmuttenmaer C. A.
Published: July 2002

Instantaneous normal mode (INM) theory was used to calculate absolute far-infrared absorption spectra of water, ammonia, and chloroform. Three procedures for weighting the INM density of states to yield absorption intensities were tested against spectra based on dipole time correlation functions generated from molecular dynamics (MD) simulations. Weighting method I, which utilizes only the rotational character of a mode in determining its contribution to absorption, performed slightly better than method II, a more exact treatment which incorporates the extent to which a mode is IR-active. Method III, which includes the contributions of induced dipoles, was successful in describing the influence of induced dipoles on the far-infrared spectra of the model liquids. The contribution to absorption of unstable modes with imaginary frequencies was found to be significant at low frequencies, and was treated by a simple approximation. Agreement between INM theory and the MD analysis was quite good for chloroform and ammonia, but less so for water. Agreement with experimental data in the range of 4–100 cm−1 was generally poor, but no worse than that for the MD calculations, primarily reflecting the simple intermolecular potentials used rather than the computational method itself.
An effective transition state for a complex cluster isomerization process: Comparison between anharmonic and harmonic models for Mg+Ar12
Authors: Fanourgakis G. S. , Farantos S. C. , Parneix P. , Bréchignac Ph.
Published: July 2002

The isomerization reaction between two discernible categories of structures of Mg+Ar12 with the magnesium cation inside the argon cluster (solvated), and on the surface of the argon cluster (non-solvated), discovered recently, is studied by computing the density of states with microcanonical and canonical molecular dynamics techniques. Thermodynamical properties and equilibrium constants are then evaluated after establishing the validity of a single effective transition state. A complex kinetic model based on the transition state theory and the harmonic approximation is also developed to calculate thermodynamical properties, and isomerization rate and equilibrium constants. This model includes tens of intermediate minima and transition states located on the potential energy surface of the cluster. The general good agreement between the two models is encouraging for studying complex isomerization processes in atomic clusters with simplified theories.
Semiclassical model of Λ-doublet states in diatomic molecules
Authors: Kovalenko Laurie J. , Delos John B.
Published: July 2002

An intuitive picture of Λ-doubling in diatomic molecules is presented using a semiclassical theory. A common view of Λ-doubling as arising from electrons “lagging” behind the rotating internuclear axis is shown to be misleading; rather, the eigenfunctions are symmetric about the molecular axes and can be expressed as a superposition of pure nonrotating orbitals and travelling waves. These results are shown to be consistent with a full quantum treatment. We also examine, for the first time, time-dependent states, by monitoring expectation values of electronic- and nuclear-angular momenta. For low rotation frequency, the expectation value of the electronic-angular momentum locks onto the rotating internuclear axis, while for high rotation frequency it locks onto the space-fixed total-angular momentum axis. At intermediate frequencies is a complicated behavior.
Electron nuclear dynamics of proton collisions with methane at 30 eV
Authors: Jacquemin D. , Morales J. A. , Deumens E. , Öhrn Y.
Published: July 2002

The reactive collisions of protons with methane molecules at 30 eV in the laboratory frame are studied with the electron nuclear dynamics (END). The results from this theoretical approach, which does not invoke the Born–Oppenheimer approximation and does not impose any constraints on the nuclear dynamics, are compared to the results from time-of-flight measurements. Total differential cross sections and integral cross sections as well as fragmentation ratios and energy loss spectra are discussed.
Study of spatial pattern formation during the NO+H2/Rh(111) reaction by means of mathematical modeling
Authors: Makeev A. G. , Janssen N. M. H. , Cobden P. D. , Slinko M. M. , Nieuwenhuys B. E.
Published: July 2002

Recent investigations with the photoemission electron microscope showed the formation of spatial patterns (target patterns, spiral waves, disordered patterns) during the NO+H2 reaction over a Rh(111) single crystal surface. A five-variable mathematical model of the reaction-diffusion type has been developed to describe the experimental observations. A simplified version of this model was originally designed to explain the complex temporal behavior (e.g., oscillatory) found for the NO+H2 reaction on Rh(111). The simulation results successfully reproduce the main experimental findings and explain the underlying reasons for spatial pattern formation. In addition, the numerical studies predict a variety of self-organization phenomena which should be experimentally verified.