bluuebunny/crossref_metadata_2025 huggingface dataset:
Variational transition state theory: Application to a symmetric exchange reaction in water
Authors: Gershinsky Gidon , Pollak Eli
Published: July 2002

Variational transition state theory (VTST) is applied for the first time to a chemical reaction in a liquid. The theory provides accurate estimates of reaction rates and leads to well defined microscopic friction functions. The structure of the optimized planar dividing surface provides insight into the range of solute–solvent interactions for which there is an appreciable effect on the reaction dynamics. The VTST method also allows for separation of the frictional effects of solvent translation, rotation, and stretch modes. The numerical cost is less than an analogous molecular dynamics reactive flux computation and the insight gained is greater.
Spatial coupling in heterogeneous catalysis
Authors: Yamamoto S. Y. , Surko C. M. , Maple M. B.
Published: July 2002

Spatial coupling mechanisms are studied in the heterogeneous catalytic oxidation of carbon monoxide over platinum at atmospheric pressure under oscillatory conditions. Experiments are conducted in a continuous flow reactor, and the reaction rate is monitored using both infrared imaging and thermocouples. The catalysts are in the form of platinum annular thin films on washer-shaped quartz substrates, and they provide highly repeatable oscillatory behavior. Oscillations are typically spatially synchronized with the entire catalyst ‘‘flashing’’ on and off uniformly. Spatial coupling is investigated by introducing various barriers which split the annular ring in half. Infrared images show that coupling through the gas phase dominates coupling via the diffusion of CO on the surface or heat diffusion through the substrate. The introduction of a localized heat perturbation to the catalyst surface does not induce a transition in the reaction rate. Thus, it is likely that the primary mode of communication is through the gas-phase diffusion of reactants.
Dynamically disordered hopping, glass transition, and polymer electrolytes
Authors: Lonergan Mark C. , Nitzan Abraham , Ratner Mark A. , Shriver D. F.
Published: July 2002

The central parameter of the dynamically disordered hopping (DDH) model, the renewal time, is correlated with the characteristic time constant of the glass transition relaxation in polymer–salt complexes. With this identification, the frequency-dependent permittivity of these materials can be quite adequately described. In particular, experimental evidence for a high-frequency relaxation predicted by the DDH model is presented. This relaxation corresponds to the polarization of ions in their local percolation clusters as they wait for a renewal event to occur. In light of information on the renewal time, the direct current properties of polymer–salt complexes are used to calculate the size of these local clusters. These calculations suggest that the motion of an ion in the absence of renewal (polymer segmental motion) corresponds to displacements within its local coordination ‘‘cage’’ rather than hopping between several available coordination sites.
Oxidation of H on Rh(111): H2O product velocity and angular distributions
Authors: Gibson K. D. , Colonell J. I. , Sibener S. J.
Published: July 2002

The translational energy distribution for the H2O product from the reaction of H2 and O2 on Rh(111) was measured as a function of surface temperature at two different oxygen coverages. The results are well represented by Maxwell–Boltzmann velocity distributions significantly cooler than the surface temperature. For [O]=0.2 monolayers (ML), the product H2O is slightly faster than for [O]=0.1 ML. The energy distributions are very close to those observed for the trapped and desorbed molecules when scattering low energy H2O molecular beams from the Rh(111). We also measured the angular dependence of the energy and intensity of the product H2O at Ts=650 K. The velocity distribution of the H2O product is independent of final angle, and the relative intensities are cosine distributed.
Quantum calculations of reaction probabilities for HO + CO→ H + CO2 and bound states of HOCO
Authors: Zhang Dong H. , Zhang John Z. H.
Published: July 2002

A time-dependent (TD) quantum wavepacket calculation of reaction probabilities is reported for the reaction HO + CO → H + CO2 for total angular momentum J=0. The dynamics calculation employs the potential-averaged five-dimensional model (PA5D) and is made possible by using a normalized angular quadrature scheme to minimize the requirement for computer memory. Reaction probabilities are obtained from the ground state as well as rotationally excited state in either one of the reactant diatoms. Strong resonances are found in the present study and calculated reaction probabilities are dominated by many narrow and overlapping resonances. These features are in qualitative agreement with several lower dimensional quantum dynamics studies. However, quantitative comparison of the present result with previously reported quantum calculations, including a recent planar four-dimensional (4D) calculation of Goldfield et al., shows that our calculated reaction probabilities are much smaller than those found in reduced dimensionality calculations. We also found reaction probability to be more sensitive to the rotational motion of CO than of HO. In addition to reaction probabilities, the bound state calculation for the stable intermediate complex HOCO has also been carried out and energies of several low-lying vibrational states are obtained. The potential energy surface (PES) of Schatz–Fitzcharles–Harding (SFH) is used in all the calculates presented in this paper.
Intermolecular vibrations of phenol⋅(H2O)3 and d1-phenol⋅(D2O)3 in the S and S1 states
Authors: Bürgi Thomas , Schütz Martin , Leutwyler Samuel
Published: July 2002

We report a combined spectroscopic and theoretical investigation of the intermolecular vibrations of supersonic jet-cooled phenol⋅(H2O)3 and d1-phenol⋅(D2O)3 in the S0 and S1 electronic states. Two-color resonant two-photon ionization combined with time-of-flight mass spectrometry and dispersed fluorescence emission spectroscopy provided mass-selective vibronic spectra of both isotopomers in both electronic states. In the S0 state, eleven low-frequency intermolecular modes were observed for phenol⋅(H2O)3, and seven for the D isotopomer. For the S1 state, several intermolecular vibrational excitations were observed in addition to those previously reported. Ab initio calculations of the cyclic homodromic isomer of phenol⋅(H2O)3 were performed at the Hartree–Fock level. Calculations for the eight possible conformers differing in the position of the ‘‘free’’ O–H bonds with respect to the almost planar H-bonded ring predict that the ‘‘up–down–up–down’’ conformer is differentially most stable. The calculated structure, rotational constants, normal-mode eigenvectors, and harmonic frequencies are reported. Combination of theory and experiment allowed an analysis and interpretation of the experimental S0 state vibrational frequencies and isotope shifts.
Modeling localized electron pair correlation energies
Authors: Rauhut Guntram , Boughton James W. , Pulay Peter
Published: July 2002

Localized pair correlation energies of closed shell organic molecules have been modeled by a backpropagation neural network approach. The contributions of Boys localized pairs to the total correlation energy are predicted using geometrical and simple electronic information of the participating orbitals at the ab initio Hartree–Fock level. We have concentrated on modeling the numerous distant (dispersionlike) orbital interactions. Localized pair correlation energies have been calculated at the Mo/ller–Plesset second order level using the 6-31G(d,p) basis for 65 organic molecules, containing 9458 pair energies. A neural network was trained to reproduce these pair energies from readily accessible properties of the localized orbitals. The correlation energies calculated by the net-based formula compare well with conventional MP2 results for 10 test molecules.
The structure of Nb3O and Nb3O+ determined by pulsed field ionization–zero electron kinetic energy photoelectron spectroscopy and density functional theory
Authors: Yang Dong-Sheng , Zgierski Marek Z. , Rayner David M. , Hackett Peter A. , Martinez Ana , Salahub Dennis R. , Roy Pierre-Nicholas , Carrington Tucker
Published: July 2002

The geometrical structures of the ground states of triniobium monoxide, Nb3O, and its cation, Nb3O+, have been determined by an experimental and theoretical study. Vibrationally resolved photoelectron spectra of an Nb3O cluster beam were obtained at 100 and 300 K using the pulsed field ionization-zero electron kinetic energy technique. The spectra were simulated by calculating multidimensional Franck–Condon factors using the geometries and harmonic vibrational frequencies obtained from density functional theory for the minimum energy structures of the ion and neutral molecule. The rather remarkable agreement between the experiment and the simulated spectra establishes that Nb3O and Nb3O+ have planar C2v structures with the oxygen atom bridging two niobium atoms. These are the most complex transition metal cluster structures to date to be characterized by gas phase spectroscopic techniques.
Vibrational energy transfer in shock-heated norbornene
Authors: Barker John R. , King Keith D.
Published: July 2002

Recently, Kiefer et al. [J. H. Kiefer, S. S. Kumaran, and S. Sundaram, J. Chem. Phys. 99, 3531 (1993)] studied shock-heated norbornene (NB) in krypton bath gas using the laser-schlieren technique and observed vibrational relaxation, unimolecular dissociation (to 1,3-cyclopentadiene and ethylene), and dissociation incubation times. Other workers have obtained an extensive body of high-pressure limit unimolecular reaction rate data at lower temperatures using conventional static and flow reactors. In the present work, we have developed a vibrational energy transfer-unimolecular reaction model based on steady-state RRKM calculations and time-dependent master equation calculations to satisfactorily describe all of the NB data (incubation times, vibrational relaxation times, and unimolecular rate coefficients). The results cover the temperature range from ∼300 to 1500 K and the excitation energy range from ∼1 000 to 18 000 cm−1. Three different models (based on the exponential step-size distribution) for the average downward energy transferred per collision, 〈ΔE〉down were investigated. The experimental data are too limited to enable the identification of a preferred model and it was not possible to determine whether the average 〈ΔE〉down is temperature dependent. However, all three 〈ΔE〉down models depend linearly on vibrational energy and it is concluded that standard unimolecular reaction rate codes must be revised to include energy-dependent microcanonical energy transfer parameters. The choice of energy transfer model affects the deduced reaction critical energy by more than 2 kcal mol−1, however, which shows the importance of energy transfer in determining thermochemistry from unimolecular reaction fall-off data. It is shown that a single set of Arrhenius parameters gives a good fit of all the low temperature data and the shock-tube data extrapolated to the high pressure limit, obviating the need to invoke a change in reaction mechanism from concerted to diradical for high temperature conditions. Some possible future experiments are suggested.
First-principles molecular-dynamics simulation of liquid CsPb
Authors: de Wijs G. A. , Pastore G. , Selloni A. , van der Lugt W.
Published: July 2002

Many alkali–post-transition group IV alloy systems exhibit clearly defined equiatomic compounds together with a pronounced intermediate range ordering, indicated by a first sharp diffraction peak at ≊0.9 Å−1. These phenomena have been explained assuming that tetrahedral group IV anions, ‘‘Zintl’’ ions, survive in the liquid state. As a prototype system we considered liquid CsPb, for which several experimental results are available, and studied it by means of first-principles molecular-dynamics. Agreement with experiment is satisfactory, provided the 5s and 5p electrons of cesium are explicitly taken into account in the computation of the electronic valence charge density. In particular, our calculations reproduce the structure factor prepeak reasonably well. The local liquid structure however is quite complex. This can be described as a disordered network, which still has many features in common with the ‘‘Zintl’’ ion model. For instance, the average Pb-Pb coordination is close to 3, the value for perfect tetrahedra, but the coordination distribution of Pb around Pb shows a broad range of values. The calculated electronic density of states shows a minimum at the Fermi level indicating compound formation.
A two-color (1+1′)+1 multiphoton ionization study of CS2 in the 61 000–65 600 cm−1 energy region
Authors: Baker Jacob , Couris Stelios
Published: July 2002

The (1+1′)+1 resonance enhanced multiphoton ionization (REMPI) spectrum of jet-cooled CS2 has been recorded in the 61 000–65 600 cm−1 excitation energy range. Four prominent band groups are observed that can be assigned to Δν2=−1 and Δν2=1 sequences of the two-photon electronically forbidden 4p 1,3Δu←X̃ 1Σ+g transitions. Weak bands to higher energy appear to be associated with the 310, 230 and 110210 bands and corresponding sequence bands. The results show that the upper states are not 3d Rydberg states as has been previously supposed, and are consistent with a recent reinvestigation of the corresponding (3+1) REMPI spectrum. Further experimental information is obtained on the anomalous vibrational band structure of transitions to the 3Δu state. The 201 and 212 bands of the 4p 1Πu←X̃ 1Σ+g transition are also observed, but are much weaker, suggesting that vibronic interactions are less important in this state compared to the 4p 1,3Δu states.
Critical scaling behavior in the activated barrier crossing problem. II. Power-law potential
Authors: Singh Surjit , Robinson G. Wilse
Published: July 2002

Using the Pollak–Grabert–Hänggi (PGH) weak-coupling approximation, the activated barrier crossing (ABC) problem is studied with a general memory friction kernel and for a general power-law potential added to a parabolic barrier. We focus on the recently discovered critical behavior of the rate for large memory friction correlation times. All the relevant critical exponents in different regimes of the strength of the friction are determined, and explicit expressions for the scaling function are obtained. We verify that the universality of exponents and amplitudes is applicable for this model within the PGH approximation. The results are compared with the results for canonical variational transition state theory (CVTST) recently obtained by us.
Collisional enhancement of Rydberg lifetimes observed in vibrational wave packet experiments
Authors: Vrakking Marc J. J. , Fischer Ingo , Villeneuve D. M. , Stolow Albert
Published: July 2002

Evidence for lifetime enhancement of Rydberg states by interactions with surrounding ions is obtained from femtosecond pump–probe experiments on the vibrational wave packet dynamics of the I2(B)-state, in which either detection of I+2 ions or zero-kinetic energy (ZEKE) electrons was used. The use of the ZEKE technique leads to the appearance of new frequency components in the Fourier transform power spectra of pump–probe time delay scans, as well as the observation of enhanced peak-to-valley ratios in these scans. These observed frequency components do not correspond to any energy level spacings in the molecule; they correspond to a sum of two energy level spacings, suggesting a nonlinear detection mechanism in the ZEKE technique (i.e., interaction with ions). Additional evidence for our interpretation is presented through experiments in which additional ions were formed through nonresonant multiphoton ionization of I2 with a picosecond 266 nm laser, as well as experiments in which both the decay time of the ZEKE signal and the appearance of the pump–probe time delay scans as a function of the pulsed field time delay were studied. Theoretical wave packet calculations which support the conclusions are presented.
The role of correlated many chain motions in linear chain polymer melts
Authors: Herman Michael F.
Published: July 2002

A theory of correlated many chain motions is developed for linear chain polymer melts. The center of mass diffusion constant of the chains is shown to be inversely proportional to the number of monomers that have their motion correlated with the motion of any given monomer. This quantity is expressed as the integral over the normalized correlation function for the displacements of different monomers in the melt. Two limiting cases are considered for this correlation function in monodisperse melts. The loss of correlation in the motion of monomers occurs because of the relative motion of the chains. The first limiting case assumes that the relative chain motions are correlated over the entire range of the correlated many chain motion. This case leads to the largest single contribution to the diffusion constant in the long chain limit. The second limiting case employs the shortest possible correlation length for the relative displacements of the chains. Since this case allows for the greatest randomness in the motion of the chains, it is expected that the contribution from this type of correlated motion should dominate the chain diffusion constant. The diffusion constant based on the correlated motions in the melt is consistent with the experimentally observed chain length scaling of both the terminal time and the chain diffusion constant. The case of the diffusion of a single probe chain in a melt of host chains of different length is also discussed. 1995 American Institute of Physics.
Statistical properties of molecular spectra and molecular dynamics: Analysis of their correspondence in NO2 and C2H+4
Authors: Leitner David M. , Köppel H. , Cederbaum L. S.
Published: July 2002

Correspondence between the spectral statistics and classical dynamics of NO2 and C2H+4 is investigated. Particular focus is given to the role of the lower of the two coupled adiabatic surfaces on the dynamics and level statistics, which is found to yield the main contribution to the level statistics of NO2 and C2H+4. The specific features of the potential surfaces characterizing each molecule that contribute to their spectral and dynamical properties are identified. Classical dynamics simulations run on the lower adiabatic surface correspond closely with results of the statistical analysis of the energy levels on this surface: The correspondence sought between the phase space fraction that is chaotic, qcl, and parameter q determined by the level statistics, is confirmed in our analysis. The classical analysis moreover reveals qualitatively distinct trends in the relation between qcl and the total energy of each molecule, the cause of which is investigated in detail. Classical simulations are also carried out on the coupled-surface potential using the Meyer–Miller analog Hamiltonian, from which comparisons with level statistics of the corresponding Hamiltonian are drawn. The effect of surface coupling is to enhance q by about 10% for NO2 and by about 30% for the less chaotic C2H+4.
The coordination of Ni2+ in aqueous solution at elevated temperature and pressure
Authors: Howell Ian , Neilson G. W.
Published: July 2002

The first order isotopic difference method of neutron diffraction was used to investigate the coordination of Ni2+ in a 1.2 m NiSO4 heavy water solution at 300 °C and 1 kbar. The results show that the ambient hydration structure is completely destroyed under these extreme conditions, and suggests that the structure is completely dominated by cation–anion interactions.
High resolution spectroscopy of the He79Br2 van der Waals molecule: An experimental and theoretical study
Authors: Jahn Douglas G. , Barney William S. , Cabalo Jerry , Clement Simon G. , Rohrbacher Andreas , Slotterback Timothy J. , Williams Jason , Janda Kenneth C. , Halberstadt Nadine
Published: July 2002

The structure, dissociation dynamics, and intermolecular potential energy surfaces of the He79Br2 van der Waals molecule have been studied using high resolution, two color, pump–probe laser induced fluorescence spectroscopy and three dimensional quantum mechanical calculations. A conical nozzle produces higher centerline cluster densities than a standard nozzle, and allows data collection further downstream from the nozzle. This yields improved signal to noise ratios and lower Doppler widths. He79Br2 is found to have a T-shaped average geometry with He to Br2 center-of-mass distances of 3.98 Å and 4.11 Å for the X and B states, respectively, somewhat longer than previously reported. Spectra were also obtained for excitation to excited bending levels of the van der Waals coordinate. However, these spectra have yet to be rotationally assigned. Vibrational predissociation line widths for the B state of He79Br2 have been measured for three new vibrational levels and range from 0.036 cm−1 for B, v′=8 to 0.062 cm−1 for B, v′=12.

These values are somewhat larger than was expected based on previous HeBr2 linewidth measurements for higher vibrational levels. Forms for the potential energy surface that have previously been used to simulate the spectra of HeCl2 have been applied to the HeBr2 data. For the HeBr2 X state, two potentials are tested. First, a slightly anisotropic, one center Morse–Spline–van der Waals potential with angle dependent parameters is used. Second a much more anisotropic potential obtained from ab initio calculations is tested. The more anisotropic potential produces a significantly better fit to the data. The B state potential is constructed using Morse atom–atom potentials for the short range part of the He–Br interaction. This simple potential is sufficient to fit the main excitation band, but does not yield a fit to spectra involving vibrationally excited van der Waals modes.
The reaction of manganese clusters and manganese cluster carbides with hydrogen: The Mn–CH3 bond energy
Authors: Parks E. K. , Nieman G. C. , Riley S. J.
Published: July 2002

Manganese clusters are formed by laser vaporization in an inert gas condensation source cooled to −160 °C. A pure manganese target is used as well as a manganese target containing 2 mol % carbon. The clusters are reacted with hydrogen both in the region of cluster growth and in a flow-tube reactor (FTR) downstream of the cluster source. The reactions, both with hydrogen atoms in the cluster growth region and dissociative chemisorption of H2 in the FTR, result in hydrogen atoms bound to the clusters, except that for Mn15 and smaller clusters the H atoms are unstable against H2 desorption. Above Mn15 stable hydrides are formed, but the rate of reaction in the FTR varies considerably with cluster size. This abrupt change in the ability to bind hydrogen may reflect a significant change in the character of the bonding within the cluster, perhaps from van der Waals to metallic. MnnC clusters readily react with hydrogen for n≳6 to form MnnCH2. Further reactivity generally follows the pattern of the bare clusters, suggesting that the two hydrogen atoms on MnnCH2 are bound to the carbon. Under certain conditions a large intensity of MnnCH3 species (n≥1) is seen. From these results and earlier reports of reactions of Mn with various hydrocarbons, an energy of 1.21±0.09 eV is determined for the Mn–CH3 bond.
Observation of the broadening and shift of the frustrated translation vibrational mode of CO on Cu(001) by high resolution helium atom scattering
Authors: Graham Andrew , Hofmann Frank , Toennies J. Peter
Published: July 2002

The lifetime broadening of the frustrated translation (T) mode parallel to the surface of isolated CO molecules [≲6% of a c(2×2) coverage] chemisorbed on Cu(001) has been studied with high resolution inelastic helium atom scattering as a function of the surface temperature and the parallel momentum transfer. The T excitation peak shows a temperature dependent peak shift and a broadening. The latter is attributed to a quartic anharmonic potential contribution described by a spectroscopic constant χe=−0.0085±0.0008. The extrapolated TS=0 K peak broadening is then γ0=85±5 μeV, corresponding to a vibrational lifetime τ=8±1 ps. We attribute this lifetime to substrate phonon mediated damping and electronic damping.
Effect of coupling between frustrated translation and libration on the nonthermal desorption of physisorbed CO: Three-dimensional quantum calculations
Authors: Dzegilenko Fedor N. , Herbst Eric
Published: July 2002

Three-dimensional time-dependent quantum calculations have been used to study the nonthermal desorption of CO physisorbed on a rigid NaCl(100) surface. The three strongly coupled degrees of freedom are CO physisorption, libration, and translation along the surface. The wave packet is expanded in a discrete variable representation basis and is propagated in time using the Chebyshev expansion algorithm. Nonthermal amounts of energy have been put into both the CO librational and translational modes at t=0, while the physisorption mode was placed in the ground state. The probability of desorption and the corresponding unimolecular rate coefficients have been computed for a variety of initial states with different translational and librational quantum numbers. The results are compared with our previous work, where molecular translation was not considered. We find that the inclusion of the latter degree of freedom significantly lowers the librational desorption threshold, though some librational energy still needs to be present at t=0 for desorption to take place. The inclusion of molecular translation also causes the appearance of desorption from previously ‘‘dark’’ odd librational states and significantly accelerates desorption from ‘‘bright’’ even librational states. We have also observed the translational ‘‘saturation’’ effect at fixed values of the librational quantum number; namely, the desorption probability becomes insensitive to further increase in translational excitation starting with the translational quantum number=3. Librational motion has been shown to play a mediating role in energy transfer between the translational and physisorption modes.
Nonphotochemical hole burning in hyperquenched glassy films of water: A pronounced deuteration effect
Authors: Kim W.-H. , Reinot T. , Hayes J. M. , Small G. J.
Published: July 2002

The effects of deuteration on hole burning of aluminum phthalocyanine tetrasulfonate (APT) in glassy films of water is reported. Deuteration has no effect on the zero phonon hole width of the APT electronic transition, but a large effect on the hole burning kinetics. These effects are discussed in terms of the two level systems of glassy water.
Electronic spectroscopy and quenching dynamics of OH–H2/D2 pre-reactive complexes
Authors: Loomis Richard A. , Schwartz Rebecca L. , Lester Marsha I.
Published: July 2002

Binary complexes of OH X 2Π and H2/D2 have been stabilized in the entrance valley to the hydrogen abstraction reaction and identified in the OH A 2Σ+–X 2Π 0–0 spectra region. Nearly all of the intermolecular vibrational levels supported by the OH A 2Σ+ (v′=0)+H2/D2 potential energy surface have been observed in fluorescence depletion experiments. Rapid electronic quenching precludes the observation of OH–H2/D2 prepared in these levels by laser-induced fluorescence. A sharp onset of laser-induced fluorescence occurs at the OH A 2Σ+ (v′=0)+H2/D2 dissociation limit. The binding energies for OH–H2/D2 in the ground state correlating with OH X 2Π (v″=0)+H2/D2 have been determined to be 54 cm−1 and more than 66 cm−1, respectively. The OH A 2Σ+ (v′=0)+H2/D2 excited state is found to be at least 577 cm−1 (H2) and 639 cm−1 (D2) more strongly bound than the ground state. The positions of observed features are compared with the corresponding intermolecular levels observed by laser-induced fluorescence in the OH A–X 1–0 region as well as theoretical predictions of the transition energies based on ab initio potentials for the ground and excited electronic states. The OH–H2/D2 intermolecular levels correlating with OH A 2Σ+ (v′=0)+H2/D2 have lifetimes of 3.2–4.5 ps, deduced from homogeneous linewidths, due to quenching and/or chemical reaction.
Weighted-density approximation and its application to classical fluids
Authors: Kim Soon-Chul , Suh Soong-Hyuck
Published: July 2002

A simple weighted-density approximation based on both local average and bulk densities is proposed. The weighting function is constructed to agree with that of the hybrid weighted-density approximation (HWDA) proposed by Leidl and Wagner [J. Chem. Phys. 98, 4142 (1993)] for the homogeneous fluid; it has the advantage of being simpler to apply. The new approximation is applied to predict the homogeneous and inhomogeneous properties of classical fluids. For the homogeneous classical fluids, the new approximation generates the same accurate higher-order direct correlation functions as those of the HWDA. For the properties of inhomogeneous classical fluids such as the density profiles of hard-sphere and Lennard-Jones fluids restricted in spherical cages, the results are in good agreement with the computer simulations, and comparable with those of the HWDA. Through these calculations, the density-functional perturbation theory based on the second-order perturbation theory of the uniform liquid has also been examined.
A hybrid quantum mechanical force field molecular dynamics simulation of liquid methanol: Vibrational frequency shifts as a probe of the quantum mechanical/molecular mechanical coupling
Authors: Wang Jian , Boyd Russell J. , Laaksonen Aatto
Published: July 2002

A hybrid quantum mechanical molecular dynamics method is used to study liquid methanol at room temperature and normal density. Frequencies of the twelve vibrational modes are calculated from the simulation data at the ab initio Hartree–Fock/3-21G(d,p) level. Good overall agreement is found between the experimental and calculated frequencies. Three different, successive levels of quantum mechanical/molecular mechanical (QM/MM) coupling schemes are investigated using gas-to-liquid vibrational frequency shifts as a probe. The results suggest, somewhat surprisingly, that the method with the weakest QM/MM coupling gives the best overall agreement between the experimental and simulated results for vibrational frequency shifts. The most elaborate coupling scheme overestimates the shifts towards the red direction due to overestimation of the attractive interactions between quantum mechanical and molecular mechanical molecules, while it is found to be most successful in describing the O–H stretch. The effects of the solvent on the geometrical parameters of methanol are investigated in detail.
Direct subsurface absorption of hydrogen on Pd(111)
Authors: Lo/vvik Ole Martin , Olsen Roar Aspesæter
Published: July 2002

We summarize and discuss some of the available experimental and theoretical data important for understanding the role played by subsurface sites in dissociative chemisorption calculations for the H2/Pd(111) system. Then we use a semi-empirical potential energy surface (PES) to model the interaction of a H2 molecule impinging on a Pd(111) surface. The London–Eyring–Polanyi–Sato (LEPS) construction has been extended to make direct subsurface absorption possible. A two-dimensional wave packet calculation is used to find qualitative trends in the direct subsurface absorption and to reveal the time scales involved. We suggest that a partial in-plane relaxation occurs for the slowest incoming particles, thus resulting in a higher direct subsurface absorption probability for low energies.
A coherent transition model for photodesorption from metals: NH3/Cu
Authors: Liu Li , Guo Hua , Seideman Tamar
Published: July 2002

A new time-dependent quantum mechanical scheme is proposed to study the uv photodesorption dynamics of ammonia from Cu(111). The desorption is considered here as the result of substrate-mediated electronic excitation and subsequent de-excitation. The transition from a short-lived excited electronic state to the ground state is described in a coherent manner. The dynamics of the system is represented by nuclear wave packets on two quasicoupled potential energy surfaces with two degrees of freedom (the desorption and N–H3 inversion modes). The desorbed molecules are found to have significant vibrational excitation and their translational energy distributions are highly structured due to the dominance of a predesorption mechanism. The desorption yield and the isotope effect are found to depend sensitively on the excited state lifetime. The results are compared with previous wave packet models and with experiments.
Matrix isolation study of the interaction of excited neon atoms with BCl3: Infrared spectra of BCl+3, BCl+2, and BCl−3
Authors: Jacox Marilyn E. , Irikura Karl K. , Thompson Warren E.
Published: July 2002

When a Ne:BCl3 sample is codeposited at approximately 5 K with a beam of neon atoms that have been excited in a microwave discharge, the infrared spectrum of the resulting solid deposit shows a weak to moderately intense absorption of BCl2 and more prominent absorptions which are assigned to the ν3 fundamentals of BCl+3 (D3h) and of linear, centrosymmetric BCl+2. The boron- and chlorine-isotopic structure of the spectrum is consistent with both of these assignments. Ab initio calculations support the BCl+2 assignment. An absorption is also tentatively assigned to ν3(e) of BCl−3. Ab initio calculations for BCl−3 are consistent with that assignment. The processes which occur when the solid deposit is exposed to visible and ultraviolet radiation are considered.
Reactions of Zn(4s4p 3P1) and Cd(5s5p 3P1) with SiH4
Authors: Wang J.-H. , Umemoto H. , Leung Allen W. K. , Breckenridge W. H.
Published: July 2002

The first nsnp 3P1 excited states of Cd and Zn atoms are shown to readily activate Si–H bonds in SiH4. The nascent quantum state distributions of the CdH(v;N) and ZnH(v;N) products in the reactions of Cd(3P1) and Zn(3P1) with SiH4 have been determined using the laser pump–probe technique. The results are discussed within the context of our current knowledge about the chemical interactions of valence M(nsnp 3P) excited states with Si–H, H–H, and C–H bonds, where M=Mg, Zn, Cd, and Hg. It is proposed that the high reactivity of M(nsnp 3P1) states with H–H and Si–H bonds compared to C–H bonds is simply due to the lack of steric hindrance in the localized, side-on, M(npπ)–XH(σ*) donor–acceptor molecular orbital interactions, since the Si–H bond-length in SiH4 is ∼1.5 Å compared to C–H bond lengths of ∼1.1 Å.
Orientational order in a liquid crystalline mixture studied by molecular dynamics simulation and NMR
Authors: Sandström Dick , Komolkin Andrei V. , Maliniak Arnold
Published: July 2002

We have studied the orientational order in a liquid crystalline mixture consisting of benzene and 4-n-pentyl-4′-cyanobiphenyl (5CB) employing molecular dynamics simulation and NMR spectroscopy. The temperature-dependent order parameters obtained from the NMR experiments were used to determine the average benzene–5CB and 5CB–5CB interaction parameters. It was found, using mean field theory, that the benzene–5CB interaction is ∼45% of that between the solvent particles. This analysis is based on a cascade of approximations. The validity of some of these assumptions was tested in the computer simulation. Various pair correlation functions were also calculated.
The GaOH–HGaO potential energy hypersurface and the necessity of correlating the 3d electrons
Authors: Richards Claude A. , Yamaguchi Yukio , Kim Seung-Joon , Schaefer Henry F.
Published: July 2002

The ground state potential energy hypersurface of the GaOH–HGaO system has been investigated using high level ab initio molecular electronic structure theory. The geometries and physical properties of two equilibrium structures, one isomerization transition state and one inversion transition state were determined at the self-consistent field (SCF), configuration interaction with single and double excitations (CISD), coupled cluster with single and double excitations (CCSD), and CCSD with perturbative triple excitations [CCSD(T)] levels of theory with four sets of basis functions. It has been found that freezing the 3d electrons of the Ga atom in the correlation procedures is not appropriate for this system. For the energy difference ΔE (GaOH–HGaO) the freezing of the 3d electrons results in an error of 25 kcal/mol! The dipole moments, harmonic vibrational frequencies, and infrared (IR) intensities are predicted for the four stationary points. At the highest level of theory employed in this study, CCSD(T) using triple zeta plus double polarization with higher angular momentum and diffuse functions [TZ2P(f,d)+diff] basis set, the bent GaOH was found to be 41.9 kcal/mol more stable than the linear HGaO species; with the zero-point vibrational energy (ZPVE) correction, the energy separation becomes 40.4 kcal/mol. The classical barrier height for the exothermic isomerization (1,2 hydrogen shift) reaction HGaO→GaOH is determined to be 44.5 kcal/mol and the barrier height with the ZPVE correction 42.3 kcal/mol. The classical barrier to linearity for the bent GaOH molecule is determined to be 1.7 kcal/mol and the barrier height with the ZPVE correction to be 1.2 kcal/mol. The predicted dipole moments of GaOH and HGaO are 1.41 and 4.45 Debye, respectively. The effects of electron correlation reduce the dipole moment of HGaO by the sizable amount of 1.2 Debye. The two equilibrium species may be suitable for microwave spectroscopic investigation. Furthermore, they may also be detectable by IR techniques due to the relatively large intensities of their vibrational modes. The geometrical and energetic features are compared with those of the valence isoelectronic HXO–XOH systems, where X is a group IIIA atom and the HXO+–XOH+ systems, where X is a group IVA atom.
Shear induced ordering in simulations of colloidal suspensions: Oscillatory shear and computational artefacts
Authors: Butler Scott , Harrowell Peter
Published: July 2002

We report on the nature of the shear induced order observed in nonequilibrium Brownian dynamics simulations of particles interacting via a screened Coulomb potential. Under steady shear, the nature of the ordered phase differs depending on the temperature. Below the equilibrium melting temperature, the shear induced order takes the form of hexagonally packed strings aligned along the direction of flow. Above the melting temperature, the liquid organizes itself into unstructured layers whose normal lies parallel to the shear gradient. We find a significant and anisotropic system size dependence of the ordering transition under steady shear. The critical shear rate required for ordering increases with increasing length of the simulation cell along the direction of flow. No such size dependence is found in oscillatory shears whose amplitude is less than half the cell length. Our results suggest that the order found in simulations under steady shear is an artefact of pseudo-oscillations resulting from shearing through the periodic boundary conditions.
Photofragmentation of mass-selected ICl−(CO2)n cluster ions: Solvation effects on the structure and dynamics of the ionic chromophore
Authors: Nadal María E. , Kleiber Paul D. , Lineberger W. C.
Published: July 2002

Photofragmentation studies at 644 nm and 740 nm of ICl−(CO2)n cluster ions (n=0–8) have been carried out in a tandem time-of-flight mass spectrometer. Photodissociation of these cluster ions at a wavelength at which bare ICl− produces only I− results in the formation of three classes of fragment ions: I−, Cl−, and ICl− based clusters. The I− based clusters correspond to the direct photoproduct in which a Cl atom has escaped the cluster ion. The ICl− and Cl− based clusters are a result of a nonadiabatic electronic transition to the ground state mediated by the solvent. The relative importance of these photofragment channels strongly depends on the cluster ion size. An ICl− caged product is first observed for ICl−(CO2)2, increasing rapidly to a maximum at n≊6 and then decreasing. This caging efficiency is dramatically different from the I−2(CO2)n cluster ions where complete caging was observed for 16 solvent molecules. The Cl− photofragment channel increases smoothly for the cluster size range studied and becomes the dominant channel for n=8. The relative yields of the ICl− and Cl− based products reflect the extent to which solvation influences the photodissociation pathways of ICl−.
A multidimensional semiclassical approach for coupled-coordinate tunneling: Level splitting in α-methyl-β-hydroxyacrolein
Authors: Guo Yin , Thompson Donald L.
Published: July 2002

A multidimensional semiclassical method is developed for calculating energy-level splittings in polyatomic molecules due to multiple tunneling motions. The procedure can be easily implemented within standard trajectory simulations and thus allows for full-dimensional treatment of tunneling in large systems. The method is applied to α-methyl-β-hydroxyacrolein and the calculated ground-state splitting is in good agreement with the experimental value. This study further demonstrates that facile semiclassical approaches can be used to accurately treat tunneling in many-atom systems.
Observation of the second ordered phase of water on the MgO(100) surface: Low energy electron diffraction and helium atom scattering studies
Authors: Ferry D. , Glebov A. , Senz V. , Suzanne J. , Toennies J. P. , Weiss H.
Published: July 2002

The interaction of water with MgO (100) single crystal surfaces cleaved in situ has been studied by low energy electron diffraction and helium atom scattering in the temperature range 80 K–230 K. At Tcrystal=100–180 K water forms a layer with a c(4×2) symmetry in good agreement with previous spot profile analysis of low energy electron diffraction experiments. Adsorption at Tcrystal=185–221 K leads to the formation of a new ordered phase. The results of the low energy electron diffraction and elastic helium atom scattering experiments show that this high-temperature phase has a (3×2) symmetry, and that the unit cell contains a glide plane. The isosteric heat of adsorption at half coverage Qst=85.3 kJ/mol has been determined from equilibrium adsorption isotherms measured between 210 and 221 K.
Solution of the Chandler–Silbey–Ladanyi equation for the multicomponent hard-sphere site–site molecular fluid: Percus–Yevick approximation
Authors: Kalyuzhnyi Yu. V. , Cummings P. T.
Published: July 2002

The analytical solution of the Chandler–Silbey–Ladanyi Percus–Yevick (CSL-PY) approximation for multicomponent molecular site–site fluids is presented. The molecules are modeled by a collection of an arbitrary number of hard-sphere sites of any size and geometrical arrangement, provided only that all sites are in contact and the bonding distance is equal to the contact distance between the sites of the molecule. Assuming an additional approximation for the intramolecular correlation between the molecular sites not bonded directly the solution is extended to the case of flexible molecules. A closed-form analytical expression for the compressibility equation of state is derived. In the case of the linear flexible chain model polymer system, this equation of state coincides with the equation of state derived earlier [Y. C. Chiew, Mol. Phys. 70, 129 (1990)]. Comparison of the theory with computer simulation results shows that predictions of the CSL-PY theory for the compressibility pressure of the star polymer system is fairly accurate, while the structural predictions for linear chain n-mers are quantitatively accurate only for the systems of dimers.
Hyperthermal H atom interactions with D/Si(100): Effects of incident H atom kinetic energy on the removal of adsorbed D
Authors: Buntin Steven A.
Published: July 2002

The interactions of H atoms having hyperthermal energies with a monodeuteride-terminated Si(100) surface are investigated. H atoms having mean kinetic energies of 1.0 and 2.9 eV are generated by 248 and 193 nm laser photolysis, respectively, of a pulsed, free-jet expansion of HI. Full characterization of the laser photolysis conditions allows the determination of the relative, as well as absolute, H atom exposures for these two kinetic energies. The depletion probability of adsorbed D per incident H atom is identical for species having incident kinetic energies of 1.0 and 2.9 eV and has an absolute value of 0.3±0.2.
An improved estimate of the heat of formation of FOOCl
Authors: Francisco J. S.
Published: July 2002

Ab initio calculation of the heat of formation of FOOCl is examined using four independent methods. Values calculated from these methods are all consistent, and a new value is predicted for FOOCl at 0 K as 22.5±2 kcal mol−1 from the isodesmic reaction.
Solvent-induced nonadiabatic transitions in iodine: An ultrafast pump–probe computational study
Authors: Ben-Nun M. , Levine R. D. , Fleming Graham R.
Published: July 2002

The solvent-induced electronic predissociation [B→a1g(3Π)] following an ultrafast X→B transition in molecular iodine is studied using a classical ensemble representation of Heisenberg’s equations of motion. An N electronic state quantum mechanical Hamiltonian is used to derive (coupled) equations of motion for the population (and the coherence) of the different electronic states as well as classicallike coupled equations for the nuclear dynamics (of both the molecule and the solvent) on each electronic state. The ultrafast excitation of the intermediate B state creates a coherent vibrational motion in this bound state. The localized nature of the solvent-induced B–a1g(3Π) coupling results in a steplike depletion of the excited B state population and hence in a bulletlike appearance of population on the dissociative a1g(3Π) state twice per vibrational period. The depletion of the B state population and the appearance of products on the a1g(3Π) state are discussed as a function of solvent density and polarizability. The magnitude of the nonadiabatic B–a1g(3Π) coupling depends both on the molecule–quencher separation and on the quencher’s polarizability. It is found that at all reduced densities the small Ar atom is the most effective quencher (when compared to either Kr and/or Xe). We attribute this unexpected trend to the local density of atoms around the solute molecule. For all the rare gas solvents the local density around the iodine molecule does not quite scale with the global one and there is an observed tendency for the solvent to cluster around the solute in a T-shaped configuration. It is this close-packed configuration that compensates for the smaller polarizability of the Ar atom and hence provides for a more effective quenching. These arguments are used to explain the experimental results which demonstrate that for a series of homologous alkanes the extent of predissociation scales with the length of the molecular chain although the global polarizability density remains roughly constant.
Direct mapping of vibrational-specific angular distributions of the polyatomic reaction product: CN+D2→DCN+D
Authors: Lai Lih-Huey , Wang Jeng-Han , Che Dock-Chil , Liu Kopin
Published: July 2002

A newly developed Doppler-selected time-of-flight (a 3D) method was applied to map out for the first time the vibrational-specific angular distributions of the DCN product from the title reaction. A tentative vibrational assignment was given for the observed structures. It was found that the nascent DCN is formed with substantial excitations in both C–D stretch and DCN bend modes. While the pure C–D stretch and those associated with low bend excitations are predominantly backward scattered, the highly excited DCN in both modes also exhibit a substantial forward scattered component in the angular distribution.