bluuebunny/crossref_metadata_2025 huggingface dataset:
Monte Carlo simulations of hydrophobic weak polyelectrolytes: Titration properties and pH-induced structural transitions for polymers containing weak electrolytes
Authors: Sassi Alexander P. , Beltrán Sagrario , Hooper Herbert H. , Blanch Harvey W. , Prausnitz John , Siegel Ronald A.
Published: July 2002

Monte Carlo simulation has been used to study titration and configurational properties of an isolated hydrophobic polymer containing weakly-ionizable groups. Using a cubic lattice, simulations were performed in the grand canonical ensemble to include the effect of the local charge environment on the ionization of weak electrolyte segments. Properties were studied as a function of polymer hydrophobicity, fraction of ionizable segments, solution ionic strength, and pH. The polymer segments experienced three types of competing interaction: excluded volume, attractive nearest-neighbor forces which account for the net balance of segment–segment, segment–solvent, and solvent–solvent interactions, and long-range electrostatic forces between ionized segments, calculated with a screened Debye–Hückel potential. Simulations show that the model chain expands with chain ionization, which depends on solution pH. As the chain becomes increasingly charged, the ionization process becomes more difficult because of rising local charge density around the ionizable segments. The effect of rising local charge density increases for larger fractions of ionizable groups, with increased hydrophobicity and with low ionic strength.
Methyl group dynamics in the crystalline alanine dipeptide: A combined computer simulation and inelastic neutron scattering analysis
Authors: Kneller G. R. , Doster W. , Settles M. , Cusack S. , Smith J. C.
Published: July 2002

The dynamics of the methyl groups in the crystalline alanine dipeptide is examined over a wide range of temperatures using elastic and inelastic neutron scattering experiments and molecular dynamics (MD) simulations of the full crystal. Neutron scattering spectra are calculated from the simulations and directly compared to the experimental profiles. The N-ter and C-ter methyl groups in the molecule have low rotational barriers, i.e., ≲1 kcal/mol. They undergo rotations that are activated on time scales faster than ≊1 ns at temperatures as low as 50–100 K and are on the picosecond time scale at 150 and 300 K. At 300 K the rotational motion becomes strongly diffusive in the simulation. In contrast, the side-chain methyls possess a significant intramolecular intrinsic torsional barrier, ≊3 kcal/mol. As a result, their dynamics consists of librations and rare jumps between wells. The simulations are further analyzed to characterize in detail the motions giving rise to the calculated scattering. Using a quaternion-based method the simulated methyl dynamics is decomposed into rigid-body rotational and translational components. The decomposed motions and their contributions to the calculated neutron profiles are examined.
Pattern selection in controlled reaction–diffusion systems
Authors: Middya Usuf , Graham Michael D. , Luss Dan , Sheintuch Moshe
Published: July 2002

When a chemical reaction is carried out on a catalytic ribbon, the spatial average temperature of which is kept constant by electrical heating, spatiotemporal temperature patterns form when the uniform steady state is unstable at the set temperature. Numerical simulations reveal periodic and aperiodic patterns of moving pulses, ‘‘breathing’’ pulses, or stationary and oscillatory fronts. The transitions between some of these patterns are intricate and proceed via global bifurcations. Bifurcation maps of parameter regions leading to specific patterns are used to gain insight into pattern formation and organization of these parameter regions. The relations among the dynamics of the uncontrolled system, the ribbon length, and the selected pattern are discussed. Similar patterns are expected to evolve in other reaction–diffusion systems subject to control of space-averaged properties.
Normal mode analysis of liquid CS2: Velocity correlation functions and self-diffusion constants
Authors: Moore P. , Keyes T.
Published: July 2002

Normal mode analysis (NMA) is applied in a molecular-dynamics simulation of liquid CS2, modeled with a potential including internal degrees of freedom. The entire supercooled liquid range, from the glass transition at 100 K to melting at 165 K, and the normal liquid from 165 to 293 K, are studied at P=1 atm. The normal modes of the liquid are classified as translation parallel (trans-∥) and perpendicular (trans-⊥) to the molecular axis, rotation, symmetric stretch, antisymmetric stretch, and bend. The configuration-averaged density of states, 〈ρ(ω)〉, with both stable and unstable modes, is correspondingly decomposed into separate contributions 〈ργ(ω)〉, with γ=trans-∥, etc. The trans-∥, trans-⊥, and rotational velocity correlation functions, and diffusion constants Dγ, are shown to be calculable from the same NMA techniques previously developed for atoms, so long as the appropriate 〈ργ(ω)〉 is used. Agreement between NMA theory and simulation is extremely good for the trans-⊥ velocity correlation function and for the diffusion constants in the lower temperature range, is good for the trans-∥ velocity correlation, and is fair for the rotational velocity correlation. Anharmonicities within wells of the many-body potential are seen to be more important in CS2 than in atomic liquids. At higher temperatures the rotational unstable modes, 〈ρurot(ω)〉, show a double-peak structure. It is proposed that the separate contributions of anharmonicity and barrier crossing are causing the two peaks, and a possible connection, respectively, with the separate β and α relaxation processes, observed in supercooled liquids, is suggested. Several other aspects of liquid-state NMA, including connections with spectroscopic measurements, are briefly considered.
Self-organized spiral and circular waves in premixed gas flames
Authors: Pearlman Howard G. , Ronney Paul D.
Published: July 2002

A diffusive-thermal high Lewis number (Le) gas-phase oscillator has been observed in premixed flames using a lean mixture of butane and oxygen diluted with helium (Le≊3.0). This reactive-diffusive system exhibits both propagating radial pulsations and rotating spiral waves perhaps analogous to those observed in other excitable media such as the Belousov–Zhabotinsky reaction.
High-accuracy measurement of vibrational Raman bands of ozone at 266 and 270 nm excitations
Authors: Chang Bor-Yu , Kung Chung-Yi , Kittrell Carter , Hsiao Chih-Wei , Johnson Bruce R. , Glogover Stella G. , Kinsey James L.
Published: July 2002

Resonance Raman spectra of ozone at two excitation wavelengths (266 and 270 nm) have been measured up to 11 000 cm−1. Band origins have been measured to high accuracy and determined to within a few cm−1. Several bands beyond the dissociation limit have been observed. All prominent bands observed in this work fit to a two-oscillator Darling–Dennison model. An analytical two-dimensional potential energy surface has been constructed based on the new experimental data.
Ab initio molecular orbital study of the molecular and electronic structure of FeCH+2 and of the reaction mechanism of FeCH+2+H2
Authors: Musaev Djamaladdin G. , Morokuma Keiji
Published: July 2002

Both complete active space self-consistent field (CASSCF) and internally contracted single and double configuration interaction from the CASSCF reference functions (MR-SDCI-CASSCF) methods have been used to study the molecular and electronic structures of FeCH+2, as well as the mechanism for the reaction FeCH+2+H2. Three quartet electronic states of FeCH+2 are very low-lying; the ground state is a nearly degenerate pair (4B2 and 4B1), with the 4A2 state only 1–2 kcal/mol higher in energy. The lowest doublet states are about 25–35 kcal/mol higher. The binding energy of FeCH+2(4B2), relative to the dissociation limit Fe+(6D,s1d6)+CH2(3B1), is estimated to be 68 kcal/mol. In the first step of reaction FeCH+2+H2, the reactants yield an ion-molecule complex, (H2)FeCH+2, stabilized by about 6 kcal/mol. Subsequently, the H–H bond is activated, leading to a four-center transition state with an energy barrier of about 24 and 14 kcal/mol for the ground 4A″ and excited 2A′ states of the reactants. This step is calculated to be the rate-determining for the entire reaction, and therefore the doublet excited state of FeCH+2 is expected to be more reactive than the ground state because of the lower barrier. The resultant HFeCH+3 complex is predicted to be a minimum at the CASSCF level, but at the highest MR-SDCI-CASSCF level this minimum disappears. Thus, following the H–H bond cleavage, an FeCH+4 ion-molecule complex is directly formed.

The ground state of the FeCH+4 complex is the sextet 6A2 state, which is stable relative to the ground state dissociation Fe+(6D,s1d6)+CH4 by 16 kcal/mol. The entire FeCH+2 hydrogenolysis reaction FeCH+2(4B2)+H2→Fe+(6D,s1d6)+CH4 (1) is estimated to be exothermic by 29 kcal/mol. The channels leading to formation of FeH++CH3 (2) and FeCH+3+H (3) are thermodynamically almost neutral. The reverse reaction Fe++CH4 for 6D, 4F or 2G states of Fe+ gives only one product, the ion-molecule complex FeCH+4 at moderate temperatures. An increase in the available kinetic energy for 6D and 4F states would make it possible to form dissociation products, FeH++CH3 and FeCH+3+H. Though the channel leading to FeCH+2+H2 is thermodynamically as easy as (2) and (3), a large barrier prevents it from taking place.
Further observations on the nitrogen orange afterglow
Authors: Piper Lawrence G.
Published: July 2002

We have extended earlier observations on the nitrogen orange afterglow that results from the excitation of N2(B 3Πg,v′=1–12) in the energy transfer reaction between N2(A 3Σ+u) and N2(X,v≥4). Spectral observations out to 1550 nm show that N2(B,v′=0) accounts for about 38% of the total N2(B) excitation. This makes the rate coefficient for N2(B) excitation in the energy-transfer reaction between N2(A) and N2(X,v≥4) equal to (4±2)×10−11 cm3 molecule−1 s−1. Experiments involving 14N2(A) and isotopically labeled 15N2(X,v) show 15N2(B) is the principal product. This demonstrates that the mechanism involves electronic energy transfer from the N2(A) to the N2(X,v). The vibrational distributions of N2(B,v≥4) are qualitatively similar whether 15N2(v) or 14N2(v) is excited although the magnitude of 15N2(B,v≥4) excitation is about 20% larger. These distributions can be characterized roughly as a 5200 K Boltzmann distribution. In contrast, the vibronic levels of 14N2(B,v=0–2) are substantially more excited than are those of 15N2(B,v=0–2). Interestingly, the overall excitation rates for both 14N2(X,v) and 15N2(X,v) are the same to within 20%. Adding 14N2(X) to the mixture of N2(A) with 15N2(X,v) results in quenching of 15N2(B) and the concomitant excitation of 14N2(B). The rate coefficient for this electronic energy exchange reaction is (8±2)×10−11 cm3 molecule−1 s−1, about 2.5 times greater than the rate coefficient for N2(B) removal by N2.
On the extent of spin contamination in open-shell coupled-cluster wave functions
Authors: Stanton John F.
Published: July 2002

The spin purity of states described by the coupled-cluster singles and doubles (CCSD) approximation based on unrestricted and restricted open-shell Hartree–Fock (UHF and ROHF) determinants is investigated. By contracting matrix elements of the S2 operator with reduced one- and two-particle density matrices, the extent of spin contamination in ROHF-CCSD and UHF-CCSD solutions is quantitatively characterized. Results are presented for a representative set of molecules in doublet electronic states with UHF values of S2 that range from 0.76 to 1.18. In these examples, ROHF-CCSD and UHF-CCSD expectation values of S2 are similar, with both less than 0.015 above the nominally exact value of 3/4. This finding illustrates the characteristic insensitivity of CC methods with respect to the choice of orbitals. Finally, the negligible differences between UHF-CCSD values of S2 and those corresponding to a spin eigenfunction suggests that modest reference state spin contamination does not represent a serious problem when this method is applied to chemical problems.
Speed dependence of rotational relaxation induced by foreign gas collisions: Studies on CH3F by millimeter wave coherent transients
Authors: Rohart François , Mäder Heinrich , Nicolaisen Hans-Werner
Published: July 2002

The dependence of rotational relaxation rates on the speed of absorbing molecules has been studied by millimeter wave coherent transients for the J,K=1,1–2,1 rotational transition of methyl fluoride (CH3F). A new phenomenological model used to describe such a speed dependence has been introduced. It leads to a quite simple analytical expression for time-domain transient signals, the Fourier transform of which corresponds to the frequency-domain line shape (speed-dependent Voigt profile). The investigations were carried out on mixtures of CH3F with He, Ar, Xe, H2, D2, N2, and O2, yielding parameters which characterize the speed dependence of the observed decay rates and its pressure and temperature dependence. Special emphasis was given to the key role of the mass ratio of collision partners which clearly allowed the relation of the observed nonexponential decay behavior to collisional effects. However, the observations cannot be explained exclusively with consideration of speed-dependent rates, but must also be discussed with reference to velocity-changing collisions. The observed temperature dependence of the rates may allow discrimination between these two different collisional effects which lead to departures from Voigt profile line shapes.
Ab initio calculations on monohalogenophosphanes PH2X (X=F,Cl,Br,I), and experimental detection and characterization of PH2F and PH2Cl by high resolution infrared spectroscopy
Authors: Beckers H. , Bürger H. , Kuna R. , Paplewski M. , Thiel W.
Published: July 2002

The harmonic and anharmonic force fields of the title compounds have been calculated at the ab initio self-consistent-field level using effective core potentials and polarized double-zeta basis sets. Additional calculations for PH2F employ larger basis sets and include electron correlation. Many rovibrational constants are predicted theoretically. The infrared spectra generated from the ab initio data have guided the experimental identification of PH2F and PH2Cl in the gas phase. High resolution Fourier transform infrared spectra of these unstable molecules have been recorded for the first time. Rotational analyses for several bands are reported which provide accurate ground state constants and a precise characterization of a number of vibrationally excited states. The accuracy of the ab initio predictions for PH2F and PH2Cl is evaluated by comparisons with these experimental data.
Boundary tension: From wetting transition to prewetting critical point
Authors: Perković S. , Blokhuis E. M. , Tessler E. , Widom B.
Published: July 2002

We develop a mean-field model free energy which we use in a van der Waals-like theory to study the prewetting transition in a system of two fluid phases when an incipient third phase may wet the interface between them. The line of prewetting transitions in the phase diagram is determined from the bulk wetting transition to the prewetting critical point. As the prewetting critical point is approached, the two coexisting surface phases become more and more alike, and they become identical at the prewetting critical point. The values of the boundary tension of the one-dimensional boundary formed by the edge-on meeting of two coexisting surface phases are calculated exactly (numerically) in a range between the wetting transition and the prewetting critical point. The data points obtained are extrapolated to a finite and positive boundary tension at the wetting transition and to a zero boundary tension at the prewetting critical point. These results are consistent with related earlier work. After scaling the dimensionless boundary tensions with appropriate force units, we determine that their values range from 0 at the prewetting critical point to 𝒪(10−12) N close to the wetting transition. These orders of magnitude compare well with recent experimental results.
Application of integral equation theories to predict the structure, thermodynamics, and phase behavior of water
Authors: Lue Leo , Blankschtein Daniel
Published: July 2002

We analyze the predictive capabilities of the site–site Ornstein–Zernike equation and the Chandler–Silbey–Ladanyi equations for various potential models of water. Specifically, we solve (i) the site–site Ornstein–Zernike equation with the hypernetted-chain closure, and (ii) the Chandler–Silbey–Ladanyi equations with the hypernetted-chain closure as well as with the zeroth-order bridge functions, and compare their predictions of the structure, thermodynamics, and phase behavior of water with those obtained from computer simulations and experimental measurements. The predictions of the various site–site pair correlation functions of water for both integral equations are comparable. However, the Chandler–Silbey–Ladanyi equations seem to better predict the structure of the fluid beyond the first coordination shell. In addition, the Chandler–Silbey–Ladanyi equations provide better estimates of the thermodynamic properties of water as compared to those of the site–site Ornstein–Zernike equation, when the results are compared with those of computer simulations. We also predict the liquid–vapor coexistence curve and the vapor pressure of water using both integral equations. The Chandler–Silbey–Ladanyi equations predict higher densities of the coexisting liquid and vapor branches as compared to those predicted by the site–site Ornstein–Zernike equation. The predictions of the Chandler–Silbey–Ladanyi equations for the liquid branch are found to be in better agreement with the computer simulations data, while the site–site Ornstein–Zernike equation is found to work better for the vapor branch. The vapor pressure predictions of the site–site Ornstein–Zernike equation are found to be in good agreement with the experimental values, while the Chandler–Silbey–Ladanyi equations are found to give slightly higher predictions of the vapor pressure.
Coherent phase control of the photoionization of H2S
Authors: Kleiman Valeria D. , Zhu Langchi , Li Xiaonong , Gordon Robert J.
Published: July 2002

Coherent phase control was demonstrated for a bound-to-continuum transition of a polyatomic molecule. Three UV photons of frequency ω1 and one vacuum ultraviolet (VUV) photon of frequency ω3=3ω1 simultaneously excited H2S above its ionization threshold. The parent ion, H2S+, and fragment ions, HS+ and S+, produced by absorption of additional photons, were observed. All three ion signals were modulated as the phase difference between the light fields was varied.
Temperature dependence of diffusion of radical intermediates probed by the transient grating method
Authors: Okamoto Koichi , Terazima Masahide , Hirota Noboru
Published: July 2002

Diffusion processes of intermediate radicals created by the photoinduced hydrogen abstraction reactions of ketones, quinones, and N-hetero aromatic molecules in ethanol and 2-propanol are studied at various temperatures by using the transient grating (TG) method. The temperature dependences of the translational diffusion coefficients (D’s) of both the radicals and the parent molecules can be expressed by the Arrhenius relationship. The activation energies (ED) for diffusion of the radicals are larger than those of the parent molecules and the difference in ED depends on the molecular size. The different ED is explained in terms of the molecular volume dependence of ED; that is, larger molecular volumes of the radicals could be the cause of the larger ED. The larger apparent molecular volumes of the radicals are consistent with a model of microscopic aggregation of the surrounding molecules around the radical.
Kinetics of transformation for anisotropic particles including shielding effects
Authors: Birnie Dunbar P. , Weinberg Michael C.
Published: July 2002

A derivation is presented for X(t), the transformed fraction as a function of time, in a phase transformation process which produces randomly oriented anisotropic particles. The key novel element in the development and results is the inclusion of blocking (shielding) effects. The derivation is performed for a 2D system in which symmetrical particles of arbitrary shape are formed. In addition, approximate calculations are performed for X(t) for the case of elliptical particles in order to estimate the influence of growth rate anisotropy upon the magnitude of the blocking effects.
Optical bistability of small heterogeneous clusters
Authors: Neuendorf R. , Quinten M. , Kreibig U.
Published: July 2002

We report about the observation of intrinsic optical bistability of nanometer-size spherical CdS particles coated with silver. The particles were chemically prepared as isolated CdS clusters in aqueous suspensions and coated with silver by chemical reduction from added AgNO3. They exhibit intrinsic optical bistability due to high third order susceptibility in the CdS core particle. The effect was enhanced by the silver shell which yields strong enhancement of the local field at the frequencies of the plasmon resonances. The shell thickness was properly adjusted to obtain maximum plasmon resonance at the wavelength 514.5 nm, thus enabling measurements with a 2 W-argon ion laser. The obtained results show a strong dependence on the shell thickness. The third order susceptibility χ(3) of the CdS particle material was determined to χ(3)=(5.1±2.3)×10−11 (m/V)2 in agreement with theoretical work on the optical bistability of heterogeneous semiconductor systems.
Determination of first hyperpolarizability of nonlinear optical chromophores by second harmonic scattering using an external reference
Authors: Pauley M. A. , Guan H.-W. , Wang C. H. , Jen Alex K.-Y.
Published: July 2002

First hyperpolarizabilities (β) of two tricyanovinylthiophene nonlinear optical chromophores were determined using second harmonic, hyper-Rayleigh, scattering. The use of an external standard in the determinations is compared with the previous use of an internal standard. The first hyperpolarizability of the well-known chromophore 4 dimethylamino 4′ nitrostilbene (DANS) was also determined using the external standard method and its value is compared to those in the literature. A new method of analyzing the hyper-Rayleigh scattering signal by recording a histogram of the scattered energy is reported. This method is shown to give more reliable results in a shorter period of time than the usual static gate method. The histogram function provides additional information in the form of the histogram width which is shown to be an indication of the mean-square concentration fluctuations of the chromophores in solution. The effects of molecular interactions on the concentration dependence of the hyper-Rayleigh scattering signal is discussed. It is shown that depending on the concentration range, the β value of para-nitroaniline, used as the external standard, can agree with two different values reported in the literature. Results on the depolarization ratio of the hyper-Rayleigh scattered light from one of the tricyanovinylthiophene chromophores are presented. At low concentration the depolarization ratio agrees with the theoretically predicted value based on C2v symmetry. However, at higher concentration the measured depolarization ratio increases indicating molecular interactions.
Discrete step model of helix-coil kinetics: Distribution of fluctuation times
Authors: Poland Douglas
Published: July 2002

A method is outlined for the computer simulation of the cooperative kinetics required to construct the distribution function for time intervals between fluctuations in conformational states in macromolecules. Using the helix-coil transition in polyamino acids as an example, we develop a Monte Carlo cellular automata approximation of the kinetics of this system in discrete time. This approximation is tested against a number of exact solutions for homopolymers and is then used to calculate moments of the distribution function for the time intervals between switches in conformational state at a given site (e.g., given a switch from coil to helix at zero time, how long will it take before the state switches back). The maximum-entropy method is used to construct the very broad distribution function from the moments. In heteropolymers the diffusion of helix-coil boundaries is reduced, helix being more localized on strong helix-forming residues. We investigate the effect of a specific sequence of amino acid residues on conformational fluctuations by using the known σ and s values for the naturally occurring amino acids to simulate the kinetics of helix formation (limiting the range of cooperativity to the α-helix) in sperm whale myoglobin, giving the time evolution to the equilibrium probability profile in this system.
Time-dependent coupled cluster approach to multimode vibronic dynamics
Authors: Latha G. Sree , Prasad M. Durga
Published: July 2002

The time-dependent coupled cluster method is used to calculate the dynamics on coupled surfaces. The time-dependent self-consistent-field solution of the initial doorway state is used as the reference state. Autocorrelation functions and spectra of two model systems are presented. It is found that the spurious recurrences in the self-consistent-field autocorrelation functions are eliminated in the coupled cluster approach and the spectral features are correctly reproduced at T=T1+T2 level of approximation.
Asymptotic solution and numerical simulation of homogeneous condensation in expansion cloud chambers
Authors: Delale C. F. , Muitjens M. J. E. H. , van Dongen M. E. H.
Published: July 2002

Asymptotic solution of homogeneous condensation in expansion cloud chambers in different droplet growth regimes is presented. In particular an exactly solvable droplet growth model ranging between the Hertz–Knudsen and continuum droplet growth laws is introduced. The distinct condensation zones in each droplet growth regime are identified by the asymptotic solution of the condensation rate equation and the results are compared with those of direct numerical simulations using the classical nucleation theory. Excellent qualitative agreement is reached despite some minor quantitative differences in some of the condensation zones arising from the nature of the asymptotic solution in these zones.
Ab initio based effective Hamiltonians for long-range electron transfer: Hartree–Fock analysis
Authors: Kurnikov Igor V. , Beratan David N.
Published: July 2002

An ab initio electronic structure method is developed to describe electron transfer in large systems. The method is based on a molecular fragment effective Hamiltonian approach. The strategy pieces together results of ab initio quantum chemistry calculations on overlapping molecular segments in order to build an effective Hamiltonian that describes the long-range electronic interactions. This is accomplished by constructing fragment effective Hamiltonians that properly describe the electronic propagation characteristics of each fragment (computed at the ab initio Hartree–Fock level in an appropriate basis set). The fragment effective Hamiltonian is projected onto the valence orbital space of each fragment, and a relatively well-localized set of effective interactions is obtained. Combining these projected fragment Hamiltonians allows the construction of a valence effective Hamiltonian for the entire system. We find that the fragment Hamiltonian matrices constructed in this way are transferable between donor–acceptor systems with homologous electron-transfer bridges. The overall strategy of fragmentation and construction of valence effective Hamiltonians could enable ab initio quality computations of long-range tunneling interactions in macromolecules. We demonstrate the use of the method in a series of electron-transfer model systems of modest size.
Conventional transition state theory/Rice–Ramsperger–Kassel–Marcus theory calculations of thermal termolecular rate coefficients for H(D)+O2+M
Authors: Duchovic Ronald J. , Pettigrew J. David , Welling Beth , Shipchandler Taha
Published: July 2002

Several ab initio studies have focused on the minimum energy path region of the hydroperoxyl potential energy surface (PES) [J. Chem. Phys. 88, 6273 (1988)] and the saddle point region for H-atom exchange via a T-shaped HO2 complex [J. Chem. Phys. 91, 2373 (1989)]. Further, the results of additional calculations [J. Chem. Phys. 94, 7068 (1991)] have been reported, which, when combined with the earlier studies, provide a global description (but not an analytic representation) of the PES for this reaction. In this work, information at the stationary points of the ab initio PES is used within the framework of conventional Transition State Theory (TST)/RRKM. Theory to compute estimates of the thermal termolecular rate coefficients for the reaction between the H(D) atom and O2 in the presence of two different bath gases, argon and nitrogen, as a function of pressure and temperature. These calculations span a pressure range from 1.0 Torr to the high-pressure limit and a temperature range from 298.15 to 6000.0 K. Conventional TST/RRKM Theory was utilized within the framework of two models: an equilibrium model employing the strong collision assumption (model I), [R. G. Gilbert and S. C. Smith, Theory of Unimolecular and Recombination Reactions (Blackwell, Oxford, 1990), as implemented in the UNIMOL program suite]; and a steady-state model that includes chemical activation (model II), using the collisional energy transfer approximation proposed by J. Troe [J. Chem. Phys. 66, 4745, 4758 (1977); 97, 288 (1992)]. In this work we first summarize the pressure-dependent fall-off curves (calculated with model I) and the high-pressure limit rate coefficients (calculated with models I and II) over the entire temperature range, and then focus on the fall-off behavior for temperatures between 298.15 and 2000.0 K. Direct comparisons are made between the experimentally determined termolecular rate coefficients (either from direct measurements or based on recommended pressure/temperature-dependent expressions) and the estimates of these rate coefficients calculated in this work as a function of pressure at 298.15 and 500.0 K. In the fall-off region, we find better agreement between the theoretical and experimental values at low pressures than at pressures approaching the high-pressure limit. Significant deviations are observed between theory and experiment as the high-pressure limit is approached. The disagreement at 298.15 K is greater for N2 than for Ar.
Oscillatory surface reaction kinetics due to coupling of bistability and diffusion limitations
Authors: Kulginov D. , Zhdanov V. P. , Kasemo B.
Published: July 2002

Considering as an example the 2A+B2→2AB reaction occurring on the surface of a catalyst, we demonstrate that heterogeneous reactions may exhibit oscillatory behavior at isothermal conditions if the bistability of the nonoscillatory surface kinetics is combined with diffusion limitations in the gas phase. Exploring this general phenomenon for a spherical catalyst, we derive necessary conditions for its observation, estimate analytically the period of oscillations, and study numerically in detail both transient and sustained oscillatory regimes. In addition, we show that one can obtain damped oscillations in the regions where the necessary conditions for sustained oscillatory kinetics are not fulfilled (in this case the period of the oscillations monotonically increases with increasing time, and then the oscillations disappear). The requirements on experimental systems to observe this phenomenon are also discussed.
The glass transition dynamics of polymer micronetwork colloids. A mode coupling analysis
Authors: Bartsch E. , Frenz V. , Baschnagel J. , Schärtl W. , Sillescu H.
Published: July 2002

We studied the glass transition dynamics of polystyrene micronetwork colloids with an average cross-link density of 1:50 (inverse number of monomer units between cross-links) and a hydrodynamic radius of about 100 nm by dynamic light scattering. Special emphasis was put on extracting correct intermediate scattering functions in a system that might be termed as partially nonergodic. By using a charge-coupled device camera as a detector and averaging the intensity autocorrelation functions of 50 simultaneously monitored speckles the duration of the experiment could be significantly reduced as compared to the conventional “brute force’’ ensemble averaging. Despite some striking similarities to the behavior of hard sphere colloids the glass transition scenario in our system differs in several respects when analyzing the dynamics in the glass transition regime within the framework of mode coupling theory. Besides the existence of structural relaxation processes above φc we find indications that additional dynamic processes modify the β relaxation in the glassy phase. Our findings cannot be explained by the occurrence of hopping processes, but are rationalized via an increase of the particle compressibility and the surface friction on decreasing the cross-link density from its hard sphere limit.
Three pulse stimulated photon echo experiments as a probe of polar solvation dynamics: Utility of harmonic bath models
Authors: Passino Sean A. , Nagasawa Yutaka , Fleming Graham R.
Published: July 2002

The three pulse photon echo peak shift technique was used to study solvation dynamics in acetonitrile (297 K), methanol (297 and 323 K), and ethylene glycol (297 and 397 K) utilizing the tricarbocyanine laser dye, IR144, as a probe. The spectral density, ρ(ω), governing the solute-solvent interaction was obtained for each solvent and temperature through numerical fitting of the three pulse photon echo peak shift relaxation using finite temporal-duration optical fields. An ultrafast three pulse photon echo peak shift relaxation, ascribed to the inertial component, was nearly identical for ethylene glycol at 297 and 397 K; this indicates the spectral density is essentially temperature independent from 10 to 250 cm−1 over this temperature range. Conversely, the low-frequency spectral density (0–10 cm−1) obtained from three pulse photon echo peak shift relaxation of ethylene glycol at 297 and 397 K showed a strong temperature dependence which cannot be predicted using harmonic bath models. We calculated spectral densities for ethylene glycol, acetonitrile, and methanol using the simple dielectric continuum model and the dynamical mean spherical approximation, using where possible, the relative permittivity constants calculated from experimental far-infrared absorption data and dielectric dispersion data. Additionally, we calculated spectral densities in terms of the extended reference interaction site model for methanol and acetonitrile. These calculated spectral densities describe our experimental methanol and acetonitrile photon echo better than all other solvation model spectral densities. Our results give insight into the domain of applicability of the harmonic model of liquid dynamics.
Low energy electron total scattering cross section for the production of CO within condensed methanol
Authors: Lepage M. , Michaud M. , Sanche L.
Published: July 2002

We propose a method based on high-resolution electron-energy-loss spectroscopy to measure in situ the neutral fragmentation products arising from the impact of low energy electrons on thin solid films at low temperature. We show more particularly that the detection of electronic states from a dissociation product is a good alternative when the corresponding vibrational levels are obscured by those of the deposited film. In the case of thin methanol film condensed at 18 K, we find that low energy electrons can dissociate the CH3OH molecules into CO fragments that remain within the film. The production of CO fragments, clearly identified from its lowest electronic state a 3Π, is studied as a function of the electron dose, electron energy, and film thickness. The energy dependence of the CO production rate, which is also calibrated in terms of an electron total scattering cross section σp, is characterized by an energy threshold at 8 eV, a shoulder at about 11.5 eV, a broad maximum centered around 14 eV, and a rise above 19 eV. A value of σp≈4.2×10−18 cm2 is obtained at 14 eV. The shoulder and the broad maximum are specifically attributed to the ⋯(6a′)1(3sa′)2,2A′, ⋯(1a″)1(3sa′)2,2A″, and ⋯(5a′)1(3sa′)2,2A′ core excited electron resonances, which decay into their parent repulsive states. The rise above 19 eV is correlated to the lowest dissociative photoionization processes known to produce neutral CO in the gas phase.
Theory and simulation of vibrational effects on structural measurements by solid-state nuclear magnetic resonance
Authors: Ishii Yoshitaka , Terao Takehiko , Hayashi Soichi
Published: July 2002

Vibrational effects on structural parameters obtained by solid-state NMR are studied by theoretical calculations and molecular-dynamics simulations. The structural parameters treated contain internuclear distances between directly bonded or remote nuclei including nonproton pairs in a molecule and bond and dihedral angles. In addition to the intramolecular normal mode vibrations, the libration of the whole molecule is considered in the theory. It is shown that the molecular libration as well as the intramolecular vibrations reduce dipolar interactions, and consequently lengthen the internuclear distances obtained from the dipolar interactions (RNMR). In contrast, the internuclear distances obtained by single crystal x-ray or neutron diffraction (Rcor) are proved to be shortened by the molecular libration. Molecular-dynamics simulations for glycine molecules in the crystal at room temperature reveal that RNMR are 1%–4% longer than Rcor, confirming the theoretical results. It is also demonstrated that the effect of the molecular libration on distances between nonproton nuclei is dominant over that of the intramolecular vibrations. Especially for long distances, the molecular libration is shown to be an almost unique vibrational effect and to give differences of 1% to 2% between RNMR and Rcor. On the other hand, the theoretical calculations on the vibrational effects on bond and dihedral angles determined by correlating two dipolar tensors show very little angular deviations, and it is confirmed by molecular-dynamics simulations for glycine molecules.
Confined thin films of linear and branched alkanes
Authors: Dijkstra Marjolein
Published: July 2002

We report computer simulations in the Grand canonical ensemble of a system of several alkanes between two solid surfaces. We computed the solvation force exerted by the fluid on the plates. The solvation force for linear decane oscillates with distance with a periodicity close to the width of the molecules. The branched alkanes (2-methylundecane and 2-methylheptane) show a similar oscillatory behavior, however the oscillations are decreased and are shifted to the attractive regime. In addition, we computed the liquid-vapour equilibria by using Gibbs ensemble Monte-Carlo simulations of n-pentane confined in a slit of 9, 13, 17 Å. The critical temperature of the liquid-vapour coexistence shifts to lower temperatures upon confining. At a plate separation of 5 Å, no liquid-vapour equilibrium is found.
Dissociative electron attachment in nanoscale ice films: Temperature and morphology effects
Authors: Simpson W. C. , Sieger M. T. , Orlando T. M. , Parenteau L. , Nagesha K. , Sanche L.
Published: July 2002

The electron-stimulated desorption (ESD) of D− ions from condensed D2O films is investigated. Three low-energy peaks are observed which are identified as arising from excitation of B12, A12, and B22 dissociative electron attachment (DEA) resonances. A fourth, higher energy feature is also seen in the D− yield which is likely due to the formation of a transient anion state that dissociates and/or decays into a dissociative excited state. The energies and ion yields of the resonances vary with the temperature and morphology of the D2O film. Below 60 K, the work function of the ice films changes with temperature and the DEA resonances shift in energy. The D− ESD yield generally increases with temperature, but it deviates from this trend at temperatures corresponding to structural phase transitions in ice. The (2B1) D− temperature dependence is remarkably similar to that observed for the ESD of low-energy D+ ions from D2O ice, even though the two originate from different electronic excitations. These results are attributed to thermally induced changes in the hydrogen bonding network, which changes the lifetimes of the predissociative states that lead to ESD and which also allows for the reorientation of surface molecules.
Nature of the power-dependent ultrafast relaxation process of photoexcited charge carriers in II-VI semiconductor quantum dots: Effects of particle size, surface, and electronic structure
Authors: Roberti Trevor W. , Cherepy Nerine J. , Zhang Jin Z.
Published: July 2002

The power-dependent relaxation dynamics of photoexcited charge carriers in a number of II-VI semiconductor quantum dots have been studied using femtosecond laser spectroscopy. The dynamics are obtained via excitation of the quantum dots with high power 390 nm pulses of 150 fs duration, and probing of the photoexcited species by monitoring the change in absorption at 790 nm as a function of time. Particles with vastly differing surfaces, sizes, electronic structures, and solvents all show a fast 1.5–4 picosecond decay component which grows in with power, a 17 ps (CdSe) or 50 ps (CdS and Cd0.5Zn0.5S) decay component, and some transient absorption persisting beyond 600 ps. The power-dependent component for CdSe quantum dots in glass has a 1.5 ps decay time constant, while for the liquid dispersed CdS and Cd0.5Zn0.5S quantum dots it has 2–4 ps decay time constants. This variation in the time constant is due to its power dependence, the time constant decreases with increasing power. It is also shown that the power-dependent decay is only weakly dependent on surface, size, and electronic structure. With the assistance of a power-dependent nanosecond fluorescence study, we have assigned the power-dependent decay primarily to exciton–exciton annihilation. This decay mechanism becomes dominant under high intensity excitation where multiple photoexcited charge carriers are created in each quantum dot, leading to trap state saturation and an accumulation of band edge excitons. Auger recombination may also play an important role at very high excitation intensities.
Unstable modes in ionic melts
Authors: Ribeiro M. C. C. , Madden P. A.
Published: July 2002

Imaginary frequency instantaneous normal modes (INM) of a number of ionic melts are examined in the light of recent proposals which connect them to diffusion and to the strong/liquid character of the fluid. Separate INM density-of-states may be found for each ionic species. A linear relationship between the temperature dependence of the fraction of INMs which are unstable (fu) and that of the diffusion coefficient of each species has been found. The shapes of the single-mode potential energy curves of the fluid along INM eigenvectors are examined. The results do not support a general significance for “double-well” modes as the only ones involved in diffusion. An alternative explanation of how single or double well features develop on the single-mode potential curves is proposed. The shape of the unstable INM density-of-states reflects the character of the ionic motion: a crossover from |ω|exp(−Bω2) to |ω|exp(−Bω4) occurs as the ionic motion becomes increasingly hindered. Both behaviors, which have been reported previously for the Lennard-Jones atomic fluid at normal liquid and supercooled conditions, respectively, can occur simultaneously in a single melt for each ionic species.
Spectroscopy of the Ca+–acetylene π complex
Authors: France M. R. , Pullins S. H. , Duncan M. A.
Published: July 2002

Ca + – C 2 H 2 is produced in a molecular beam and studied with mass-selected photodissociation spectroscopy. A spectrum with sharp vibrational and rotational structure is measured near the Ca+ (2P←2S) transition. Ca+–C2H2 has a π-complex structure with a Ca+–C2H2 bond distance of r0″=2.80±0.20 Å, a vibrational frequency (Ca+–C2H2 stretch) of ΔG1/2″=169 cm−1, and a dissociation energy of D0″=18.6±5.0 kcal/mol. This is the first spectroscopic determination of these properties for an isolated metal cation π complex.
Electron impact dissociative ionization of ethane: Cross sections, appearance potentials, and dissociation pathways
Authors: Tian Cechan , Vidal C. R.
Published: July 2002

Cross sections of the electron impact ionization as well as the dissociative ionizations of ethane have been measured for electron energies from threshold to 600 eV. The complete collection of all the ionic fragments has been verified directly in the experiment. The results are thus believed to be reliable. The results for the products of Hn+(n=1–3) are obtained for the first time. The appearance potentials of the ionic products are also measured. The disagreement with respect to the appearance potentials between the present work and previous measurements is explained by the dipole-forbidden transitions in the electron impact excitation process. The starting channels for the ionic products are discussed based on the appearance potentials, the kinetic energy distributions of the ionic products, and the ejection of the electrons out of the different orbitals of the ethane molecule.
The chemistry of the superheavy elements. II. The stability of high oxidation states in group 11 elements: Relativistic coupled cluster calculations for the di-, tetra- and hexafluoro metallates of Cu, Ag, Au, and element 111
Authors: Seth Michael , Cooke Fiona , Schwerdtfeger Peter , Heully Jean-Louis , Pelissier Michel
Published: July 2002

The stability of the high oxidation states +3 and +5 in Group 11 fluorides is studied by relativistic Mo/ller–Plesset (MP) and coupled cluster methods. Higher metal oxidation states are stabilized by relativistic effects. As a result, the hexafluoro complex of the Group 11 element with nuclear charge 111 and oxidation state +5 is the most stable compared to the other congeners. The results also suggest that AgF6− is thermodynamically stable and, therefore, it might be feasable to synthesize this compound. For the copper fluorides we observe very large oscillations in the Mo/ller–Plesset series up to the fourth order. Nonrelativistic calculations lead to the expected trend in the metal–fluorine bond distances for the MF2− compounds, CuF2−<AgF2−<AuF2−<(111)F2−. However, relativistic effects change this trend to CuF2−<AuF2−<(111)F2−<AgF2−. Vibrational frequencies are predicted for all compounds. Where experimental data are available, they generally agree very well with our calculated results.
Dependence between transport properties of gas phase C60 at low density and interfullerene interaction
Authors: Luo You-Hua , Qiu Shu-Tian , Wang Guanghou
Published: July 2002

The viscosity and self-diffusion coefficients of C60 in the gas phase are reported, based upon a first-principles interaction potential. It is found that these transport coefficients increase monotonically with increasing temperature, but the calculated collision integrals, which are closely related to the transport coefficients, do not decrease monotonically with increasing temperature and exhibit a number of plateaus or the oscillating behavior. It is completely different from those of inert gases. This phenomenon should be ascribed to the interaction potential between C60 molecules. We confirm that the potential employed here does not behave like a Lennard-Jones (LJ) potential or Tang–Toennies (TT) potential and in turn a LJ-type or TT-type potential cannot describe exactly the interaction between C60 molecules.