bluuebunny/crossref_metadata_2025 huggingface dataset:
Vibrational deactivation of surface OH chemisorbed on SiO2: Solvent effects
Authors: Heilweil E. J. , Casassa M. P. , Cavanagh R. R. , Stephenson J. C.
Published: July 2002

Picosecond infrared transmission spectroscopy was used to directly measure the vibrational energy relaxation time T1 of hydroxyl groups chemisorbed on the surface of colloidal silica (SiO2). T1 was obtained for OH(νstretch=1) in the strongly bound ‘‘isolated sites’’ of fumed silica particles in vacuum and dispersed in several liquids at T=293 K. At the SiO2/vacuum interface, T1=204±20 ps. When the SiO2 particles are surrounded by solvents, the relaxation time of the surface OH(v=1) groups decreases: for the liquids CCl4, CF2Br2, CH2Cl2, and C6H6, T1(ps)=159±16, 140±30, 102±20, and 87±30, respectively. T1 does not depend on the size of the SiO2 particles for the range 70 Å≤ diameter ≤150 Å, or on the surface OH coverage up to an average density of 4 OH/100 Å2. Significant amounts of physisorbed water (5 H2O/100 Å2) decreased T1 for the isolated OH(v=1) to T1=56±10 ps. For comparison to the surface hydroxyls, the vibrational deactivation time for OH(v=1) groups in the bulk of fused silica (OH/SiO2≊130 ppm by weight) was determined to be T1=109±11 ps. These observations are discussed in terms of the possible mechanisms of vibrational energy flow in these systems. The observed T1 values demonstrate that the spectral linewidths (e.g., IR and Raman) observed for these surface vibrations are too large (by factors of 200–2000) to be caused solely by T1 uncertainty broadening. The slow transfer of vibrational energy between surface and lattice vibrations may have important implications for surface chemistry.
A water dimer potential based on a
b i
n
i
t
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o calculations using Morokuma component analyses
Authors: Singh U. C. , Kollman Peter A.
Published: July 2002

We present a new water dimer potential. It is based on carrying out Morokuma component analyses at the ab initio level on 229 water dimer geometries and fitting the energy components to analytical functional forms. To these are then added a dispersion contribution, based on the analytical dispersion energy expression of Douketis et al. (Ref. 1). This potential gives a satisfactory description of the water dimer, with minimum R(O ⋅ ⋅ ⋅ O)=2.96 Å, ΔE=−5.2 kcal/mol, and minimum energy θ angle between H-bond donor and acceptor axis of 55°, in good agreement with experimental values of 2.98±0.04 Å, −5.4±0.2 kcal/mol, and 60±10°, respectively. The second virial coefficient B is also calculated in reasonable agreement with experiment, e.g., at 423 K, B (calculated) =−352, compared to B (experimental)=−332 cc/mol.
Dissociation of NH3 to NH2+H
Authors: McCarthy M. I. , Rosmus P. , Werner H.-J. , Botschwina P. , Vaida V.
Published: July 2002

Potential energy, dipole moment, and electronic transition moment surfaces for the lowest dissociative pathways of the singlet X̃ and Ã states of NH3 yielding NH2 (X̃ 2B1,Ã 2A1) +H(2S) products have been calculated using complete active space MCSCF ab initio wave functions. The Ã state dissociation proceeds via a minimum barrier at the following planar geometry: αHNH =113°, rNH =1.042 Å (in the NH2 fragment), and RNH =1.323 Å (in the dissociation coordinate). The barrier height is calculated to be 3226 cm−1 with an expected accuracy of about 300 cm−1. The barrier height increases with increasing out-of-plane angle. Close to the barrier there are strong variations of the shapes of the dipole moment and transition moment surfaces. The minimum energy path through the X̃–Ã conical intersection follows planar geometries. Along this pathway the angle αHNH decreases, but the distance rNH in the NH2 fragment hardly changes. The crossing distance RcNH of the X̃ and Ã states in planar structures depends strongly on αHNH and varies from about 1.68 Å (60°) to infinity (180°). The photodissociation process NH3(Ã) →NH2(X̃ 2B1) +H(2S) is discussed on the basis of the calculated potential energy surfaces.
Ultraviolet photoemission and thermal desorption studies of the chemisorption and decomposition of methanol on potassium-dosed Rh(111)
Authors: Solymosi Frigyes , Berkó András , Tarnóczi Tibor I.
Published: July 2002

The interaction of methanol with potassium-dosed (up to monolayer) Rh(111) surface has been studied by means of thermal desorption, ultraviolet photoemission (He ii) spectroscopy and work function measurements. Preadsorbed potassium significantly influenced the adsorption, desorption, and stability of CH3OH on this Rh surface, but the pathway of the dissociation of CH3OH and its surface decomposition were not altered. An increase (6–10 kJ/mol) in the binding energy of chemisorbed CH3OH was found even in the low potassium coverage range, θK=0.05–0.26. At higher potassium coverages (θK=0.36, monolayer), where K exhibits mainly metallic character, the relative amount of irreversibly adsorbed methanol was greatly increased. In this case methoxy species was detected by He ii photoemission studies: its complete decomposition occurred at around 492–505 K. The stabilization of the methoxy species was reflected in the high temperature for the evolution of H2(TP=515 K), too. In the interpretation of the data a direct chemical interaction between methanol and potassium and the formation of a stable potassium methoxide is proposed.
Counterion condensation in micellar and colloidal solutions
Authors: Ramanathan G. V.
Published: July 2002

Asymptotic solutions are obtained to the Poisson–Boltzmann equation for large, highly charged spheres in an ionic solution. It is proved that as the size of the sphere is increased, keeping the surface charge density fixed, there is a critical value for the radius beyond which counterion condensation sets in. This critical radius is much larger than the Bjerrum length but small compared to the Debye length and depends on the ionic strength. An expression is derived for the effective charge. When the radius becomes much larger than the Debye length, it is shown that sufficiently many counterions condense in a shell of thickness small compared to the polyion radius to essentially neutralize the polyion charge.
On evaluating the reaction path Hamiltonian
Authors: Page Michael , McIver James W.
Published: July 2002

This paper examines a number of aspects of evaluating the reaction path Hamiltonian (RPH) of Miller, Handy, and Adams. The reaction path is represented as a Taylor series expansion of mass weighted Cartesian coordinates as a function of arc length. The second (path tangent) and third (path curvature) coefficients in the Taylor series are important in the RPH. General analytical formulas for all the coefficients as explicit functions of energy derivatives are derived. If the Taylor series is expanded about the saddle point, special limiting formulas for the coefficients are required. These are obtained using L′Hospital’s rule. In a local quadratic approximation (LQA) third and higher energy derivatives are ignored. Within this approximation all but the first two coefficients in the Taylor series expansion of the path are zero when the expansion point is the saddle point. At nonstationary points on the path the first three Taylor series coefficients are evaluated exactly within the LQA while the others have nonzero approximate values. The resulting LQA Taylor series can be summed exactly. This leads to a new method of stepping along the reaction path which is superior to the traditional Euler method and should be used whenever second energy derivatives are available. Extensions of this method which include third energy derivative information are also presented. Exact analytical formulas for the RPH coupling parameters are derived. These include

simplified formulas for the projection matrix and its derivative. At nonstationary points, the couplings of the transverse vibrations to the path depend only on first and second energy derivatives and hence are exactly calculated in the LQA. The remaining RPH parameters depend on third energy derivatives as well but have nonzero approximate values in the LQA. At the saddle point, all of the RPH parameters depend on third energy derivatives and they are zero when third derivatives are ignored. In general, when the complete set of RPH parameters are calculated, the same energy derivative information is required at the saddle point as at nonstationary points, namely the gradient, the force constants, and the components of the third derivatives along the path tangent. It is demonstrated that severe errors can occur when the RPH parameters are calculated at a point near the saddle point lying on the eigenvector corresponding to the negative eigenvalue of the force constant matrix at the saddle point. These errors occur even when the exact formulas are used and are due to slight deviations of this eigenvector from the exact reaction path. A remedy is described.
Rotationally inelastic rates for N2–N2 system from a scaling theoretical analysis of the stimulated Raman Q branch
Authors: Bonamy L. , Bonamy J. , Robert D. , Lavorel B. , Saint-Loup R. , Chaux R. , Santos J. , Berger H.
Published: July 2002

Self-broadened nitrogen isotropic Q(J) Raman linewidths have been inverted to obtain effective rotation–translation (R–T) state-to-state rate constants using the energy corrected sudden (ECS) formalism. These rate constants are discussed as a function of the rotational levels J and temperature T. Collisional Q(J) line shifts have been investigated by high-resolution inverse Raman spectroscopy (IRS) over a wide temperature range. Semiclassical calculations lead to a clear understanding of their J and T dependence. This exhaustive study of both diagonal and off-diagonal relaxation matrix elements has allowed us to calculate the collisionally narrowed Q branch at high pressure. New measurements of N2 Q branch at high pressure have been performed by IRS. The good agreement of ECS profiles with IRS data, for various pressures and temperatures, underlines the consistency of the present R–T ECS scaling analysis.
Inelastic neutron scattering analysis of low-frequency motions in proteins: Harmonic and damped harmonic models of bovine pancreatic tryspin inhibitor
Authors: Smith Jeremy , Cusack Stephen , Tidor Bruce , Karplus Martin
Published: July 2002

Inelastic neutron scattering spectra are calculated from harmonic and damped harmonic models of the internal dynamics of a small protein, the bovine pancreatic trypsin inhibitor (BPTI). Numerical Fourier transformation of the intermediate scattering function Fvibinc (q, t) is used to calculate the inelastic scattering. This permits the inclusion of multiphonon scattering and frictional damping effects. Although for a typical experimental configuration, the multiphonon contribution does not significantly alter the form of the scattering at frequencies below about 30 cm−1, it does have a significant effect on the scattering intensity at higher frequencies. Frictional damping is introduced into the harmonic model by assuming that each mode acts as an independent damped Langevin oscillator. With this model and the assumption that the lowest frequency modes are overdamped while the higher frequency modes are underdamped, improved agreement with the experimental BPTI powder results is obtained. The measured scattering from BPTI in solution shows increased intensity at frequencies below 50 cm−1 relative to the powder results. The solution scattering profile can be reproduced approximately by the addition of overdamped Langevin oscillator normal modes to the dynamic model in best agreement with the powder data. Several other aspects of neutron scattering from proteins are examined. Anisotropy in the harmonic resolution broadened scattering is demonstrated. Spectra calculated assuming classical equations of motion are shown to agree with those calculated with the full quantum-mechanical dynamical model. Translational diffusion broadening is found to be small compared to the instrumental resolution broadening for the range of scattering wave vectors of interest. The contribution of the coherent scattering to the measured intensity is calculated for the case of a partially hydrated protein. Under typical experimental conditions, the measured cross sections are dominated by the incoherent scattering and the self part of the coherent scattering, a result that justifies the comparison of experimental data with calculated incoherent scattering spectra.
The relationship of the virtual orbitals of self-consistent-field theory to temporary negative ions in electron scattering from molecules
Authors: Chen Donghai , Gallup G. A.
Published: July 2002

The theory given here shows how certain of the virtual orbitals of self-consistent field (SCF) theory provide a natural ingredient for constructing basis functions to study the shape resonances associated with temporary molecular ions. We give specific calculations of resonant behavior in H2, N2, ethylene, cyclopropene, and cyclobutene. In addition, we show how the theory can rationalize the relationship between Koopmans’ theorem values of the energy of a resonance and the experimental values.
Color effects in pressure-tuned hole-burned spectra
Authors: Gradl G. , Zollfrank J. , Breinl W. , Friedrich J.
Published: July 2002

We applied hydrostatic pressure to spectral holes burned into a resorufin doped ethanol/methanol glass. We found that the line shift is perfectly linear with pressure and showed a pronounced dependence on the burn frequency as predicted by theory [J. Chem. Phys. 90, 3274 (1989)]. We exploited the burn frequency dependence to determine the solvent shift of the dye probe and the compressibility of the alcohol glass used. On the other hand, the behavior of the hole width under pressure shows features not predicted by theory: The broadening is, like the line shift, dependent on the burn frequency within the inhomogeneous band, yet in a nonlinear fashion. We attribute the color effect in the pressure induced broadening of the hole to a breakdown of the Gaussian approximation.
Vibrational circular dichroism and electric-field shielding tensors: A new physical interpretation based on nonlocal susceptibility densities
Authors: Hunt Katharine L. C. , Harris Robert A.
Published: July 2002

Motion of nuclei within a molecule induces a magnetic moment me in the electronic charge distribution, giving a nonzero electronic contribution to the magnetic transition dipole that produces vibrational circular dichroism. In this paper, we develop a new susceptibility density theory for the induced magnetic moment. The theory is based on the response of the electrons to changes in the nuclear Coulomb field, due to shifts in nuclear positions. The electronic response to these changes depends on the same susceptibility densities that determine response to external fields. Our analysis suggests a new physical picture of vibrational circular dichroism. It yields an equation for the density of the induced electronic magnetic moment within a molecule; it also yields a new relation connecting the electric-field shielding at nucleus I of a molecule in an applied magnetic field of frequency ω to the derivative of me with respect to the velocity of nucleus I, regarded as a parameter in the electronic wave function. Within our theory, the derivative of me with respect to nuclear velocity separates into quantum-mechanical and classical components in close analogy with the Hellmann–Feynman theorem for forces on nuclei. In matrix-element form, results from our theory are identical to those obtained with nonadiabatic perturbation theory, to leading order. In general, the leading nonadiabatic corrections to electronic properties are determined directly by the electrons’ response to the changes in the nuclear Coulomb field, when the nuclei move.
Resonance charge transfer, transport cross sections, and collision integrals for N+(3P )–N(4S) and O+(4S)–O(3P ) interactions
Authors: Stallcop James R. , Partridge Harry , Levin Eugene
Published: July 2002

N+2 and O+2 potential energy curves have been constructed by combining measured data with the results from electronic structure calculations. These potential curves have been employed to determine accurate charge exchange cross sections, transport cross sections, and collision integrals for ground state N+ –N and O+ –O interactions. The cross sections have been calculated from a semiclassical approximation to the scattering using our computer code that fits a spline curve through the discrete potential data and incorporates the proper long-range behavior of the interaction forces. The charge exchange cross sections are slightly smaller than the values we reported previously using an asymptotic approximation and also agree well with the results of beam measurements at high energies. The collision integrals are tabulated for a broad range of temperatures 250–100 000 K and are intended to reduce the uncertainty in the values of the transport properties of nonequilibrium air, particularly at high temperatures.
Stretched exponential relaxation in polymer dynamics
Authors: Cherayil Binny J.
Published: July 2002

A model of connected longitudinal dipoles is proposed in partial explanation of stretched exponential decay profiles in relaxation phenomena involving polymers. The net dipole moment of the system lies along the end-to-end vector of the chain, which is initially constrained to have the value R. The constraint represents the effects of an applied perturbation. Projection operator techniques are then used to derive an exact equation for the evolution of R in the absence of the constraint. This equation is solved under a series of well-controlled approximations, and in the limit of long times R(t) is found to vary as a stretched exponential, with an exponent of 1/2. A memory term in the evolution equation for R is identified as the factor responsible for the observed decay.
Local density of states of aperiodic polymers using the localized orbitals from an ab initio elongation method
Authors: Aoki Yuriko , Imamura Akira
Published: July 2002

An elongation method, which was already proposed to calculate efficiently the electronic states of aperiodic polymers, is developed to provide the local density of states. Elongation method is carried out by repeating the uniform localization of wave functions with inclusion of the interaction between the cluster and an attacking molecule. At first, we develop this method to the ab initio level. Next, we show that the local density of states (LDOS) is obtained efficiently following the elongation step without treating the whole system directly. The validity of our method is examined via application to a hydrogen molecule cluster by comparing the results with those by the conventional ab initio method for the whole system. This approach makes it possible to investigate the highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO), and their density of states on each site of a polymer chain, which will provide a novel approach to examine intra- and intermolecular reactivities within and between aperiodic polymers.
Slow spinodal decomposition in binary liquid mixtures of polymers. V. Molecular weight dependence in the later stage of unmixing
Authors: Takenaka Mikihito , Izumitani Tatsuo , Hashimoto Takeji
Published: July 2002

The unmixing process via spinodal decomposition of poly(styrene-ran-butadiene) (SBR)/polyisoprene (PI) mixtures was investigated by the time-resolved light-scattering method to see how it is affected by the temperature of phase separation T and by the molecular weight of PI with that of SBR fixed. Time changes in the wave number qm(t;T) and intensity Im(t;T) at the peak of the scattering function were followed to explore the coarsening process in the later stage of spinodal decomposition. For each mixture the observed changes at different T were reduced to a master curve when plotted against a reduced time τ. This finding is consistent with the scaling postulate by Langer et al. and Chou and Goldburg. The master curves for mixtures with different PI molecular weights were divergent, showing the phenomenon called the N branch. However, it was possible to bring them to a master curve when τ was multiplied by a shift factor depending on the average number of entanglements per chain. The dependence was weaker than that predicted theoretically by Onuki.
Cluster studies of La2CuO4: CuO6
Authors: Martin Richard L. , Hay P. Jeffrey
Published: July 2002

We present the results of ab initio calculations for a CuO6 cluster embedded in a background potential appropriate for La2CuO4. These calculations differ from prior work in that an improved embedding potential for the cluster is utilized which enforces orthogonality between the cluster electrons and the background point charges. This is accomplished through the use of effective core potentials to represent the point charges nearest the CuO6 cluster. The major qualitative change this brings about is a destabilization of the Cu4s level relative to the Cu3d orbital. The cluster results for n doping now give a 3d10 configuration for Cu as opposed to the 3d94s configuration observed in earlier work. In addition, much more extensive treatments of electron correlation have been examined using the technique of configuration interaction (CI). Results for n-doped, undoped, and p-doped clusters are presented. The ligand →metal charge transfer excitations, the crystal-field spectrum, and the photoemission spectrum are discussed. Special attention is given to the characterization of the wave function for the p-doped cluster. While the loss in electron density is almost entirely from the O2pσ orbital, in agreement with experiment, it is shown that the wave function is closer to the independent-particle ‘‘molecular orbital’’ limit than the ‘‘valence-bond’’ local singlet, and that a description of the cluster ion ground states in terms of the occupation of a single ‘‘effective’’ Cu 3d orbital is possible.
The HeCl2 potential: A combined scattering-spectroscopic study
Authors: Beneventi Laura , Casavecchia Piergiorgio , Volpi Gian Gualberto , Bieler Craig R. , Janda Kenneth C.
Published: July 2002

Total differential scattering angular distribution measurements for He scattering from Cl2 are reported. The scattering data are combined with previous excitation spectroscopy and vibrational predissociation product state distributions for He–Cl2 to determine the potential-energy surfaces for the ground X and excited B electronic states. The potentials are somewhat deeper, De=−38.2 cm−1 for the X state, than previously estimated. The X state potential minimum is for the ‘‘T’’ shaped configuration with a Rmin value of 3.55 Å. The potentials are quite successful at describing the scattering data, the rotationally resolved spectroscopy of the complex, and the product state distributions for vibrational predissociation. The B state potential underestimates the dependence of the rate of vibrational predissociation on the initial vibrational level.
Rescaled mean spherical approximation for a colloidal suspension confined in a planar slit
Authors: González-Mozuelos P.
Published: July 2002

In this paper an extension of the rescaled mean spherical approximation is presented, which describes the local concentration profile of a colloidal suspension of charged particles confined between two parallel highly repulsive walls. The predictions of this scheme are in good quantitative agreement with those of the hypernetted-chain approximation. It is found that, for a given set of values of the bulk parameters, the structure of the concentration profile only depends on the value of the minimum of the external potential.
Effective potential approximation to reaction–diffusion kinetics in nonhomogeneous media: Micellar systems
Authors: Barzykin A. V. , Tachiya M.
Published: July 2002

An approximation is introduced that enables one to describe nonhomogeneous reaction–diffusion kinetics via a one-dimensional backward equation for the survival probability of a pair of particles interacting with each other through the effective potential associated with the equilibrium distance distribution in the absence of reaction. Employing the effective potential and mean reaction time approximations, the kinetics of diffusion-controlled reactions in a finite volume are analyzed, a spherical micelle being taken as a typical example of the system. A general relationship between the pseudo-first-order rate constant and the spatial arrangement of reactants is obtained. Several spatial distributions of practical importance are considered and the corresponding rate constants are calculated and compared against the existing exact analytical solutions and the results of numerical random-walk simulations.
Time domain information from resonant Raman excitation profiles: A direct inversion by maximum entropy
Authors: Remacle F. , Levine R. D.
Published: July 2002

A direct method for the inversion of resonant Raman excitation profiles to the time domain is proposed. The inversion procedure is implemented within the maximum entropy (ME) formalism. The constraints used in the ME procedure are the values of the Fourier transform of the Raman excitation profile at a given set of values of times tr. It is shown that the ME functional form of the Raman cross section can be expressed in terms of a Raman amplitude, depending on the Lagrange multipliers and on the constraints. The Fourier transform of the Raman amplitude yields the time dependent cross correlation function. Another route to direct inversion, using a Fourier series expansion of the dispersion relation, is also considered. The analytical requirements that must be satisfied by the input excitation profile for a successful inversion to be possible are discussed. The optimum values of the times tr and of the Lagrange multipliers which determine the Raman amplitude are computed using a new algorithm (the min–max algorithm). The proposed ME numerical procedure is implemented for computed resonant Raman excitation profiles of the B̃ electronic state of the iodobenzene molecule and of a model anharmonic system. In addition, the analytical implications of the ME functional form of the excitation profile are discussed with special reference to the separation of time scales in the dynamics.
Stability ladder of various HC2N conformers and their excitation energies
Authors: Aoki Kozo , Ikuta Shigeru , Nomura Okio
Published: July 2002

The molecular structure of various stationary points of HC2N has been studied using the quadratic configuration interaction including single and double substitutions with triples contributions (QCISD(T)). A Huzinaga–Dunning double-zeta plus polarization (D95**) basis set was used. A stability ladder of these species is calculated using the configuration interaction including single and double substitutions (SDCI) with the Davidson correction (SDCI+Q). The general contraction scheme of the [5s3p2d1f(C and N)/3s2p1d(H)] atomic natural orbital basis set was chosen. The natural orbitals obtained in the preceding complete active space self-consistent field (CASSCF) calculations were applied to the open-shell SDCI. The triplet cyanomethylene is bent, and the barrier to linearity is 1.4 kcal/mol at the single-reference SDCI+Q level of theory in accord with the Schaefer and Roos’ values (0.8–1.0 kcal/mol). The nine-reference SDCI+Q calculation increases the energy separation by 0.7 kcal/mol. The thermal energy change between the two (1 and 2) including the calculated zero-point vibrational energies is 1.4 kcal/mol. The enthalpy barrier (ΔH≠) to linearize 1 to transition state 2 in a (hypothetical) reaction is predicted to be 0.0 kcal/mol using the single-reference SDCI+Q bent-linear energy separation. As a result, experiments might find no barrier to inversion of bent triplet 1. The most stable singlet species 6 (ring form of HC2N) lies 7.7 kcal/mol above the triplet bent cyanomethylene. Vertical excitation energies from the ground-state (3A″) of triplet bent cyanomethylene to 1A′ and 1A″ are studied using the CASSCF and SDCI methods; the SDCI+Q energies are 0.93 and 1.11 eV, respectively. These excitation energies are compared with those in the methylene and oxygen molecules having a qualitatively similar electronic structure to the present HCCN species, where two electrons occupy near-degenerate π-like (or degenerate π) highest occupied molecular orbitals (HOMO) forming a triplet ground-state. The excitation energies of 0.93 and 1.11 eV are close to the relative energies of the 3, 8, and 9 to the most stable triplet bent cyanomethylene.
Deuterium and carbon-13 kinetic isotope effects for the reaction of OH with CH4
Authors: Melissas Vasilios S. , Truhlar Donald G.
Published: July 2002

Interpolated variational transition state theory calculations with centrifugal-dominant, small-curvature tunneling coefficients have been carried out for the case of the deuterium kinetic isotope effect (KIE) in the reaction OH+12CD4→HDO+12CD3 and for the 13C KIE for the reaction OH+13CH4→H2O+13CH3. The interpolated variationally optimized generalized transition states predict notably different nontunneling KIEs than the conventional ones, and factorization analyses of the KIEs are presented to illustrate the origin of the differences. The zero-point energies at the variational transition states differ from those at the saddle point by up to 0.19 kcal/mol for the OH+12CD4 reaction and by up to 0.34 kcal/mol for the OH+13CH4 reaction. The incorporation of multidimensional tunneling effects partly cancels the effect of variational optimization of the transition state.
Parametrization of kinetic energy dependences of ion–polar molecule collision rate constants by trajectory calculations
Authors: Su Timothy
Published: July 2002

Kinetic energy dependences of ion–polar molecule capture rate constants have been calculated using the trajectory method at varies temperatures. An empirical equation was formulated to fit the calculated data. The parametrized equation gives rate constants within 5% error for re- lative kinetic energies ranging from thermal to several eV and temperatures ranging from 50 K to 1000 K.
Dynamic-mechanical and light scattering study of the glass transition of poly(vinylacetate) and a poly(vinylacetate) +poly(4-hydroxystyrene) blend
Authors: Luengo Gustavo , Ortega Francisco , Rubio Ramón G. , Rey Antonio , Prolongo Margarita G. , Masegosa Rosa M.
Published: July 2002

The dynamics of poly(vinylacetate) (PVAc) and of a blend of PVAc and 20% in weight of poly(4-hydoxystyrene) have been studied in the glass transition region using dynamic-mechanical and photon-correlation spectroscopies. Both kinds of data show the overlapping of the main α relaxation with a faster one. The complex part of the elastic modulus shows the existence of a β′ sub-Tg transition overlapping with the α one in the high-frequency region. The light scattering data show some degree of overlapping of the α and β transitions. As a consequence, at least two Kohlraush–Williams–Watts terms are needed to describe the relaxation functions. The results show a good agreement with the predictions of the mode coupling theory except for the fact that the calculated critical temperature seems to be lower than the calorimetric glass transition temperature.
Statistical thermodynamics in the classical molecular dynamics ensemble. I. Fundamentals
Authors: Lustig Rolf
Published: July 2002

The statistical thermodynamics of a classical system composed of rigid molecules is considered in the molecular dynamics ensemble. Accepting Boltzmann’s S=kB ln W as the basic assumption of statistical mechanics, exact formalisms for two classical choices of W are derived. Since there are no restrictions on the order of thermodynamic derivatives, any measurable quantity is directly accessible in this ensemble. Explicit statistical analogs are given for the derivatives of the Helmholtz energy including an approximation for the chemical potential. Basic phase space functions are identified and their properties are explored. It is shown that the complete thermodynamics is governed by small perturbations of these functions from universal behavior.
Chemiluminescent reactions of group 2 (Ca, Sr, and Ba) elements with H2O2, t-BuOOH, HNO3, and NO2: Reactivities and product state distributions
Authors: Cheong B. S. , Parson J. M.
Published: July 2002

The chemiluminescent (CL) reactions of group 2 metal elements (Ca, Sr, and Ba) in both the ground and metastable states with various OH- and/or O-containing molecules have been studied in a beam–gas configuration. In reactions with OH-containing molecules, it was found that while the ground state atoms produce both the metal oxide and metal hydroxide in excited electronic states, the metastable atoms produce solely metal hydroxide. Also, spectral changes have been observed which can be attributed to changes in the reaction exoergicity or the collision geometry dictated by various scattering gases. Computer simulations of the observed CL spectra have afforded further insight into the potential energy surfaces which govern the reactions.
Determination of hydrogen bonding in water clusters through conductivity (dc) measurements of aqueous solutions of NaCl
Authors: Roberts J. A. , Zhang X. , Zheng Y.
Published: July 2002

Conductivity (dc) measurements over the temperature range 273≤T≤360 K of aqueous solutions of NaCl at various concentrations were made and used to establish transport properties of ions in solution. The dynamical properties of the electrolytes were used to establish the nature of the hydrogen bonding in these aqueous systems. Rate equations for ion formation and recombination were used to establish the temperature ranges in which hydrogen bonding dominated in forming polymeric species. Experimental data obtained on the aqueous systems demonstrate that over the range of temperatures 273≤T≤323 K the water system maintains a structure which appears to be independent of solute concentration. This is evidenced by the nearly constant slope in the Arrhenius plot of the data over that range. For the higher range of temperatures, 323≤T≤360 K, the slope of the Arrhenius plot varied more than one order of magnitude for the concentration levels studied. An average value of hydrogen bond energy of 2.8 kcal mol−1 was obtained from the Arrhenius plots for 273≤T≤323 K for samples of pure water. This value was taken for pure water as the slopes remained nearly constant in this range of temperatures for all aqueous solutions but varied systematically with ion concentration over the range of temperature 323≤T≤360 K. If the total range of temperature is chosen, the average value for EHB was found to be 2.5 kcal mol−1.
Effects of the magnetic part of the Breit term on the 2Π states of diatomic hydrides
Authors: Baeck Kyoung Koo , Lee Yoon Sup
Published: July 2002

All-electron relativistic self-consistent-field (RSCF) method based upon Dirac–Coulomb Hamiltonian is extended to include the magnetic part of the Breit interaction term in the SCF cycle. The explicit form of the Fock matrix for the magnetic part is derived and simplified within the restricted open-shell scheme. The effects of the magnetic part of the Breit term on spin–orbit splittings and dipole moments are investigated for 2Π states of diatomic hydrides AH (A=Be,B,C,N,O,F,Mg,Al,Si,P,S). Calculated spin–orbit splittings are improved significantly by the inclusion of the magnetic part and approach the experimental values to within 10 cm−1 even for the second-row diatomic hydrides. The magnitude of the effect of the magnetic part is similar for the molecules composed of atoms in the same group regardless of the nuclear charge implying that the magnetic part and probably the whole Breit term become less important for molecules with large spin–orbit interactions. With the present RSCF method, the effects of magnetic term on properties can be estimated from expectation values. The effects of the magnetic term on dipole moments of 2Π states of AH molecules are quite small.
Theory of two-dimensional Fourier transform electron spin resonance for ordered and viscous fluids
Authors: Lee Sanghyuk , Budil David E. , Freed Jack H.
Published: July 2002

A comprehensive theory for interpreting two-dimensional Fourier transform (2D-FT) electron spin resonance (ESR) experiments that is based on the stochastic Liouville equation is presented. It encompasses the full range of motional rates from fast through very slow motions, and it also provides for microscopic as well as macroscopic molecular ordering. In these respects it is as sophisticated in its treatment of molecular dynamics as the theory currently employed for analyzing cw ESR spectra. The general properties of the pulse propagator superoperator, which describes the microwave pulses in Liouville space, are analyzed in terms of the coherence transfer pathways appropriate for COSY (correlation spectroscopy), SECSY (spin–echo correlation spectroscopy), and 2D-ELDOR (electron–electron double resonance) sequences wherein either the free-induction decay (FID) or echo decay is sampled. Important distinctions are made among the sources of inhomogeneous broadening, which include (a) incomplete spectral averaging in the slow-motional regime, (b) unresolved superhyperfine structure and related sources, and (c) microscopic molecular ordering but macroscopic disorder (MOMD). The differing effects these sources of inhomogeneous broadening have on the two mirror image coherence pathways observed in the dual quadrature 2D experiments, as well as on the auto vs crosspeaks of 2D-ELDOR, is described. The theory is applied to simulate experiments of nitroxide spin labels in complex fluids such as membrane vesicles, where the MOMD model applies and these distinctions are particularly relevant, in order to extract dynamic and ordering parameters.

The recovery of homogeneous linewidths from FID-based COSY experiments on complex fluids with significant inhomogeneous broadening is also described. The theory is applied to the ultraslow motional regime, and a simple method is developed to determine rotational rates from the broadening of the autopeaks of the 2D-ELDOR spectra as a function of the mixing time, which is due to the development of ‘‘motional crosspeaks.’’ The application of this method to recent experiments with nitroxide probes illustrates that rotational correlation times as slow as milliseconds may be measured. It is shown how 2D-ELDOR can be useful to distinguish between the cases of very slow motional (SM) rates with little or no ordering and of very high ordering (HO) but substantial motional rates even though the cw ESR spectra are virtually the same. The effects of motion and of microscopic ordering on the nuclear modulation patterns in 2D-FT-ESR are compared, and it is suggested that these effects could be utilized to further distinguish between SM and HO cases. Key aspects of the challenging computational problems are discussed, and algorithms are described which lead to significant reductions in computation time as needed to permit nonlinear least-squares fitting of the theory to experiments.
Atomic hydrogen-induced decomposition of chemisorbed formate at 100 K on the Ru(001) surface
Authors: Xie Jun , Mitchell William J. , Lyons Kevin J. , Weinberg W. Henry
Published: July 2002

Atomic hydrogen-induced C–O bond cleavage in chemisorbed formate on the Ru(001) surface has been investigated with high-resolution electron energy loss spectroscopy. The chemisorbed formate reacts at 100 K with gas-phase atomic hydrogen to produce chemisorbed formyls (η1-HCO and η2-HCO) and hydroxyls, and the hydroxyls react further with the impinging hydrogen to produce adsorbed water. The observation of these ‘‘direct’’ surface reactions provides strong support for a ‘‘coupled’’ reaction mechanism, postulated previously, in which hydrogen from C–H bond cleavage in one formate (which produces CO2) reacts with an adjacent formate to form HCO and OH, resulting in a CO to CO2 ratio of unity in the overall thermal decomposition reaction.
Electronic energy transfer in concentrated micellar solutions
Authors: Barzykin A. V. , Tachiya M.
Published: July 2002

A problem of electronic energy transfer among chromophores solubilized in a micellar solution is addressed theoretically with the emphasis on the intermicellar interactions. Using the formalism of characteristic functions, a general method is advanced for determining the distribution of distance between two molecules located in separate spherical micelles. Averaging the microscopic decay over the pair distance distribution function is the key step in relating the macroscopic observables to the underlying structure of the embedding space if the irreversible direct energy transfer between two chemically distinct species is considered. The problem of excitation transport among identical chromophores can also be formulated to a good approximation as a superposition of pairwise interactions. Analytical solutions to the distance distribution function as well as the corresponding configurationally averaged energy transfer observables are presented for several spatial arrangements of practical importance. Applications to concentrated micellar solutions and to clusters of micelles are discussed.
The reaction of CCl3 radicals and Cl atoms with hydrogen molecules
Authors: Xing G. , Huang X. , Bersohn R. , Tsukiyama K. , Katz B.
Published: July 2002

A small amount of H atoms are observed as products of the reaction of Cl atoms generated by photodissociating Cl2 at 351 nm in the presence of H2. A much larger H atom concentration is detected when a mixture of H2 and CCl4 is photodissociated at 193 nm with an average kinetic energy of 6.5±0.5 kcal/mol. The reactant is shown to be the vibrationally hot CCl3 radical. In the reaction of CCl3 and Cl atoms with HD the atomic H/D product ratios are 1.47±0.16 and 0.59±0.11, respectively. The former result is in accord with the theory of Johnson et al. [J. Chem. Phys. 94, 2749 (1991)] that for kinematic reasons the D end of the HD molecule is more reactive unless the HD molecule is strongly rotating. The F and Cl atoms which give H/D ratios less than 1 when reacting with HD are special cases for which higher J states of HD are still reactive.
Time-dependent nucleation during reheating of a rapidly quenched glass
Authors: Shneidman V. A.
Published: July 2002

If the temperature of a glass is continuously increased nucleation can start only after the nascent nuclei are able to escape the advancing critical size. In case the starting temperature T° exceeds the temperature of the maximum nucleation rate, corrections to the steady-state values of the number of nucleated particles become dramatic. Analytical estimations are confirmed with high accuracy by numerical solutions of the time-dependent Becker–Döring equation.
Adiabatic and nonadiabatic outersphere electron transfer reactions in methanol: Effects of the ultrafast solvent polarization modes
Authors: Roy Srabani , Bagchi Biman
Published: July 2002

Recent studies have demonstrated that the solvation dynamics in common dipolar liquids like water and acetonitrile is dominated by an initial ultrafast Gaussian component which seems to account for about 60%–70% of the total energy relaxation. Methanol, on the other hand, exhibits a rather different behavior with a much smaller amplitude of the initial Gaussian component and the relaxation is primarily caused by a much slower exponential decay. In the present study, we have investigated the role of these solvent modes on both adiabatic and nonadiabatic outersphere electron transfer reactions in methanol. It is found that the rate of the adiabatic barrier crossing is greatly enhanced due to the ultrafast solvation. For nonadiabatic reactions, the relative importance of the solvent dynamic modes increases enormously compared to the situation when only the slow, overdamped modes are included. Another important conclusion is that because of the dominance of the inertial modes, the rate of electron transfer reaction is almost independent of the longitudinal relaxation time, τL, of the solvent. The results of the present study are compared with those obtained earlier by us for water and acetonitrile to elucidate the underlying difference in the high frequency polar response of these liquids.
Rectifying organic junctions of molecular assemblies based on perylene ion salts
Authors: Hamm S. , Wachtel H.
Published: July 2002

Using a solid-state diffusion technique, conducting perylene radical ion salt films have been prepared with alkali ions (Na+ and K+) and antimony hexachloride (SbCl6−) as counter ions. The successful preparation of separate radical anion salt films and cation salt films, respectively, is proved by in situ absorption spectroscopy, conductivity measurements, and electron spin resonance. In contrast to comparatively weakly doped inorganic semiconductors, the ion salts have a high concentration of charge carriers of roughly one charge per molecular unit entailing correlation of the carriers. The sandwich structures consisting of both the cation salt layer and the anion salt layer, form an electrical junction comparable to the PIN-junctions (p-doped, intrinsic, n-doped) known from inorganic diodes. The rectifying ratio amounts up to 105:1 at ±0.5 V and the breakdown voltage in the reverse sense is −20 V at 300 K.
Dynamical properties of polymer comblike structures: Phantom network approximation
Authors: Schulz Michael , Reineker Peter , Möller Martin
Published: July 2002

The relaxation spectrum at long times has been calculated for comblike micronetworks of Gaussian chains with definite length L, number of subchains N, and generation order g. The calculation is based on the well-known phantom network approximation; thus excluded volume and entanglements effects are not considered. The longest relaxation time is in the present model of the order Ng and diverges for networks of infinite size. This model gives a reasonable phantom description of a rubberlike network with a large number of dangling ends.
A novel wave packet description of electron transfer and dissociation in molecule/surface reactive scattering
Authors: Qian Jiwen , Jacobs Dennis C. , Tannor David J.
Published: July 2002

Recently reported molecular ion/surface scattering experiments [J. Chem. Phys. 100, 6791 (1994)] demonstrate that the translational and vibrational energies of incident NO+ each have a distinct influence on molecular dissociation for the NO+/GaAs(110) system. The detailed analysis of possible mechanisms suggest that the initial translational, vibrational, and rotational energies, as well as the molecule’s orientation and point of impact at the surface are important in determining the dissociation probability. In this paper, a wave packet simulation of NO+/GaAs(110) scattering is performed. The emphasis in the study is on the branching ratio for production of NO, NO−, O, and O− as a function of vibrational and translational energy of the incident NO+. A novel procedure for treating the coupling of a molecular state with a band of substrate electronic states is combined with a mixed classical-quantal treatment of the nuclear motion. The simulations suggest that electron transfer and collision induced dissociation (CID) cooperate and form a vibrational coherence through which the initial vibrational state strongly influences the dissociation dynamics.
Experimental determination of the Compton profile of C60 through binary encounter electron spectroscopy
Authors: DePaola B. D. , Parameswaran R. , Walch B. P. , Troike M. D. , Richard P. , Puska M. J. , Nieminen R. M.
Published: July 2002

The method of 0° electron spectroscopy was used to study binary encounter electrons resulting from hard collisions between 1.5 MeV/u C6+ ions and the electrons in a C60 vapor target. The Compton profile of C60 was then extracted from the electron spectra using an impulse approximation treatment. The experimental results are in excellent agreement with theoretical Compton profiles of C60. The C60 Compton profile is compared with that of atomic carbon, as well as those for graphite and diamond.