bluuebunny/crossref_metadata_2025 huggingface dataset:
State-resolved collisional energy transfer in highly excited NO2. I. Cross sections and propensities for J, K, and mJ changing collisions
Authors: Abel Bernd , Lange Norbert , Reiche Florian , Troe Jürgen
Published: July 2002

State-resolved experiments probing the dynamics in NO2#–NO2 collisions at high internal energies (17 500<E<18 000 cm−1) are reported. A sequential optical double resonance technique with sensitive laser-induced fluorescence detection has been employed for the assignment of states of NO2 in the energy range between 17 500 and 18 000 cm−1, a spectral region where the optically “bright” B22 state is strongly coupled to high lying (“dark”) states of the A12 ground state and other electronic states. Subsequently, the decay of population and polarization following rotationally inelastic and elastic collisions has been probed directly using a time- and polarization-resolved optical double resonance technique. Total depopulation rates have been determined to be about 2–3 times above the Lennard-Jones estimate. The thermally averaged state-to-state cross sections have been derived from a master equation analysis of the kinetic traces. The rate constants have been scaled by angular momentum scaling expressions based upon the infinite order sudden approximation which were modified to account for dynamical restrictions on angular momentum and polarization transfer. Pure rotational energy transfer within a vibrational state turned out to be fast and dominating the collision dynamics, whereas rovibrational energy transfer was slower and proceeded with a lower efficiency. In addition, interesting propensity pattern for angular momentum and polarization transfer have been found. The individual state-to-state rate constants clearly indicated that rotational energy transfer in highly excited mixed (chaotic) states is still governed by pronounced propensities in J, K, and mJ changing collisions. Here mj is the projection of J on a space fixed axis, which is defined by the laser, and K is the projection of J on the body-fixed symmetry axis of the molecule. In particular, we have found a propensity for small changes of mJ in elastic and inelastic collisions, in accord with recently suggested theoretical models. Interestingly, we also found a considerably lower probability for ΔK changes in these collisions. The propensities found for ΔmJ and ΔK are discussed within the framework of dynamic (kinematic) collision models. The observed cross sections, their overall scaling behavior, as well as estimations of the Massey parameter are consistent with collisions following mostly a direct mechanism for rotational energy transfer rather than a complex forming mechanism.
Study of the OH and OD radicals with photoelectron spectroscopy using synchrotron radiation
Authors: Barr Jonathan D. , De Fanis Alberto , Dyke John M. , Gamblin Stuart D. , Hooper Nicole , Morris Alan , Stranges Stefano , West John B. , Wright Timothy G.
Published: July 2002

Photoionization of the OH and OD radicals, produced from the H+NO2 and D+NO2 reactions, has been studied in the gas phase in the photon energy region 13.0–17.0 eV using constant ionic state (CIS) and photoelectron spectroscopy (PES) employing synchrotron radiation. Structure in the CIS spectra, recorded for the first and second photoelectron bands, has been assigned to excitation to (a 1Δ,3d) and (A 3Π,3d) Rydberg states. A comparison of vibrationally specific OH and OD CIS spectra, and photoelectron spectra recorded at resonant wavelengths, has allowed a more complete assignment of structure observed in earlier photoionization mass spectrometric measurements. These assignments have been supported by the results of Franck–Condon calculations. The CIS spectra have been shown to be dominated by structure arising from excitation from the outermost valence molecular orbitals of OH [the nonbonding 1π(O 2p) orbital and the bonding 3σ orbital] to O nd Rydberg orbitals. Photoelectron spectra recorded for the first bands of OH and OD at resonant photon energies have allowed more extensive vibrational structure to be obtained than has previously been recorded by PES experiments performed with inert gas discharge photon sources.
A multilevel blocking approach to the sign problem in real-time quantum Monte Carlo simulations
Authors: Mak C. H. , Egger R.
Published: July 2002

We propose a novel approach toward the general solution of the sign problem in real-time path–integral simulations. Using a recursive multilevel blocking strategy, this method circumvents the sign problem by synthesizing the phase cancellations arising from interfering quantum paths on multiple timescales. A number of numerical examples with one or a few explicit degrees of freedom illustrate the usefulness of the method.
Reorganization energies for charge transfer reactions in binary mixtures of dipolar hard sphere solvents: A Monte Carlo study
Authors: Denk C. , Morillo M. , Sánchez-Burgos F. , Sánchez Antonio
Published: July 2002

We study the behavior of the reorganization energy for simple charge transfer reactions in mixtures of dipolar hard sphere fluids by Monte Carlo simulation. The static dielectric constants of the solvents are also obtained from the simulation. They are used as input in the reorganization energy expressions provided by the Marcus theory and the mean spherical approximation. Thus, a comparison between the values obtained from the theoretical expressions and our simulation results is possible. The dependence of the reorganization energy with the mixture composition and the influence of preferential solvation effects is also discussed.
Diffusive feed of reactants and Hopf bifurcations in an oscillatory reaction-diffusion model
Authors: von Haeften B. , Izús G. G.
Published: July 2002

We study an oscillatory chemical model (the “Brusselator”) with the aim of analyzing the effect of a controlled diffusive feed of reactants in the appearance of chemical oscillations. The reflectivities of the boundary, which adjust the external fluxes, act as control parameters capable to alter the attractive basin of the thermodynamic branch, leading to oscillatory behavior.
Optimal packing of polydisperse hard-sphere fluids
Authors: Zhang Junfang , Blaak Ronald , Trizac Emmanuel , Cuesta José A. , Frenkel Daan
Published: July 2002

We consider the effect of intermolecular interactions on the optimal size-distribution of N hard spheres that occupy a fixed total volume. When we minimize the free-energy of this system, within the Percus–Yevick approximation, we find that no solution exists beyond a quite low threshold (η≈0.260). Monte Carlo simulations reveal that beyond this density, the size-distribution becomes bimodal. Such distributions cannot be reproduced within the Percus–Yevick approximation. We present a theoretical argument that supports the occurrence of a nonmonotonic size-distribution and emphasize the importance of finite size effects.
Third virial coefficient of argon
Authors: Mas Eric M. , Lotrich Victor F. , Szalewicz Krzysztof
Published: July 2002

The classical third virial coefficient of argon has been computed for temperatures between 113.15 K and 1223.15 K using an ab initio three-body potential. The results agree with the virial coefficients extracted directly from experimental data to within about 5% for temperatures between 220 K and 420 K, theory almost touching the experimental error bars. For lower temperatures the discrepancies become larger, but results from different experiments are also significantly apart. Our calculated values agreed very well with those of Dymond and Alder [J. Chem. Phys. 54, 3472 (1971)] determined by fitting experimental data with theoretical constraints. The equation of state obtained using the computed second and third virial coefficients reproduced experimental PV data over a wide range of pressure.
Stimulated Raman excitation of the ortho-H2–OH entrance channel complex
Authors: Wheeler Martyn D. , Todd Michael W. , Anderson David T. , Lester Marsha I.
Published: July 2002

The fundamental H2 vibration of the ortho-H2–OH complex in its ground electronic state has been observed at 4151.49±0.03 cm−1 via stimulated Raman excitation. The stimulated Raman transition has been identified with an ultraviolet probe laser using two different detection schemes. The transition was detected as a depletion in the H2–OH laser-induced fluorescence signal in the OH A 2Σ+–X 2Π (1,0) spectral region and through the appearance of OH A 2Σ+–X 2Π (0,1) laser-induced fluorescence following vibrational predissociation of the complex. Vibrational predissociation is found to proceed via a near-resonant pathway that transfers one quantum of vibrational excitation from H2 to OH. The remaining ∼529 cm−1 of available energy is distributed over excited rotational states of OH (v=1) and the lowest rotational level of ortho-H2 (v=1), with the balance flowing into translational recoil. The lifetime of vibrationally activated ortho-H2–OH (νH2=1) is determined to be less than 7 ns, the temporal resolution of the lasers, by monitoring the time evolution of the OH products. The results are compared with previous infrared studies of OH vibrational activation in ortho-H2–OH as well as full collision studies of the reaction of vibrationally excited H2 with OH.
Infrared spectra of the C2O4+ cation and C2O4− anion isolated in solid neon
Authors: Zhou Mingfei , Andrews Lester
Published: July 2002

Laser ablation of transition metal targets with concurrent 4 K codeposition of CO2/Ne mixtures produces metal independent infrared absorptions at 1658.2 and 1421.6 cm−1 due to CO2− and CO2+. Additional metal independent absorptions at 2130.8 and 1274.4 cm−1, and at 1852.4, 1189.2, and 679.2 cm−1 increase on annealing to 8 K. Isotopic substitution shows that both band sets involve two equivalent CO2 subunits. Based on density functional theory calculations of structures and vibrational frequencies, the 2130.8 and 1274.4 cm−1 bands are assigned to the C2O4+ cation with C2h symmetry, and the 1852.4, 1189.2, and 679.2 cm−1 bands are assigned to the C2O4− anion with D2d symmetry. Evidence is also obtained for (CO2−)(CO2)x(x=1,2) anion complexes.
Molecular dynamics simulations in the grand canonical ensemble: Formulation of a bias potential for umbrella sampling
Authors: Shroll Robert M. , Smith David E.
Published: July 2002

An extended Hamiltonian technique for performing grand canonical ensemble molecular dynamics simulations has been reformulated to include umbrella sampling, thus improving the efficiency of particle creation and annihilation processes. This was accomplished through incorporation of a bias potential in the Hamiltonian that modifies the free energy contour between integer particle number states. The extended Hamiltonian includes a continuous particle number variable that is the sum of the integer particle number and a coupling parameter, the latter being used in a scaling function for a fractional molecule’s interactions with the rest of the system. Equations of motion for the coupling parameter, derived from the Hamiltonian, integrate to yield density fluctuations at constant chemical potential. This new method may be adapted to a wide range of potential scaling functions. The technique was applied to calculations of extended simple point charge water density versus chemical potential, using both linear and nonlinear scaling. For each scaling function, a bias potential was constructed using a thermodynamic integration technique. Grand canonical ensemble simulations then yielded results in agreement with independent calculations.
Single-molecule fluorescence autocorrelation experiments on pentacene: The dependence of intersystem crossing on isotopic composition
Authors: Brouwer A. C. J. , Köhler J. , van Oijen A. M. , Groenen E. J. J. , Schmidt J.
Published: July 2002

Single pentacene molecules containing C13 or H1 in a pentacene-d14 doped p-terphenyl crystal have been studied by fluorescence autocorrelation. The triplet dynamics has been analyzed and a systematic dependence of the S1→T1 intersystem crossing rate on isotopic composition was found. This variation is discussed in terms of a modulation of the near resonance of the first excited singlet state S1 and vibrational levels of an intermediating triplet state T3 which results from the distinct isotope dependence of the zero-point energy of both electronic states.
On the role of hydrodynamic interactions in block copolymer microphase separation
Authors: Groot Robert D. , Madden Timothy J. , Tildesley Dominic J.
Published: July 2002

A melt of linear diblock copolymers (AnBm) can form a diverse range of microphase separated structures. The detailed morphology of the microstructure depends on the length of the polymer blocks An and Bm and their mutual solubility. In this paper, the role of hydrodynamic forces in microphase formation is studied. The microphase separation of block copolymer melts is simulated using two continuum methods: dissipative particle dynamics (DPD) and Brownian dynamics (BD). Although both methods produce the correct equilibrium distribution of polymer chains, the BD simulation does not include hydrodynamic interactions, whereas the DPD method correctly simulates the (compressible) Navier Stokes behavior of the melt. To quantify the mesophase structure, we introduce a new order parameter that goes beyond the usual local segregation parameter and is sensitive to the morphology of the system. In the DPD simulation, a melt of asymmetric block copolymers rapidly evolves towards the hexagonal structure that is predicted by mean-field theory, and that is observed in experiments. In contrast, the BD simulation remains in a metastable state consisting of interconnected tubes, and fails to reach equilibrium on a reasonable time scale. This demonstrates that the hydrodynamic forces play a critical part in the kinetics of microphase separation into the hexagonal phase. For symmetric block copolymers, hydrodynamics appears not to be crucial for the evolution. Consequently, the lamellar phase forms an order of magnitude faster than the hexagonal phase does, and thus it would be reasonable to infer a higher viscosity for the hexagonal phase than for the lamellar phase. The simulations suggest that the underlying cause of this difference is that the hexagonal phase forms via a metastable gyroid-like structure, and therefore forms via a nucleation-and-growth mechanism, whereas the lamellar phase is formed via spinodal decomposition.
Chiral symmetry breaking. II. Synthesis in cooperative systems
Authors: Bersuker Gennadi , Pekker Michael
Published: July 2002

As was suggested in Part I [J. Chem. Phys. 110, 10907 (1999), preceding paper], under certain conditions, in particular, when the system of molecules satisfies the conditions of cooperativity, the intermolecular interaction may result in enantiomeric excess in the solutions with chiral molecules, as well as with chiral–prochiral molecules. In the present paper, synthesis and conversion of chiral compounds is discussed within the microscopic approach. Kinetic description for the order–disorder phase transitions is proposed and applied to the system of conformational enantiomers. It is shown that the phase transition occurs in the second order with respect to the probability of the isomer conversion. Enantiomeric excess of the product is shown to depend on autocatalytic activity and intermolecular coupling of enantiomers, and on some characteristics of the solution as well. Based on the kinetic description, nonlinear effects in enantiomeric excess that resulted from the enantiomeric coupling of the chiral catalyst, as well as the transition of the solution of flexible chiral molecules to a new phase consisting of identical isomers, are considered.
Simulation of kinetic oscillations in surface reactions on reconstructing surfaces
Authors: Kortlüke O. , Kuzovkov V. N. , von Niessen W.
Published: July 2002

A recently introduced lattice gas model [Kuzovkov et al., J. Chem. Phys. 108, 5571 (1998)] is used for the microscopic description of surface reactions on reconstructing surfaces. The model can easily be adapted to different surface reaction systems, e.g., the CO+O2, CO+NO, NO+H2, and NO+NH3 reactions. In addition many reconstructing single crystal surfaces such as Pt(100), Pt(110), Rh(100), and Rh(110) can be simulated because only those properties of the different surface phases are considered which carry the essential physics. Changes in the coordination number are neglected for the different surface phases. In the present study the CO oxidation on Pt single crystal catalysts is investigated for illustration. The model takes CO diffusion and surface reconstruction into account. Very interesting phenomena are observed besides the kinetic oscillations in the particle densities: formation of mesoscopic patterns and synchronization. Only few parameters, as there are the CO gas phase concentration y, the CO diffusion constant D, the CO desorption constant k, and the surface phase propagation velocity V are sufficient to obtain a realistic model showing regular [Pt(110)] and irregular [Pt(100)] oscillations as well as the presence and absence of spatiotemporal patterns, respectively, as observed in experiments. In addition the model gives the critical CO coverage for the surface reconstruction as a result of the model. The small number of parameters renders it possible to study the whole parameter regime and to associate the observed phenomena with these parameters.
A comparison of techniques for predicting higher order correlation effects: Diatomic dissociation energies
Authors: Feller David
Published: July 2002

The impact of higher-order correlation effects on dissociation energies was measured for three diatomic molecules (HF, N2, and CO) using standard coupled cluster theory, including a perturbative treatment of triple excitations, as the baseline for comparison. Among the higher-order methods examined were two variations of coupled cluster theory [CCSDT and CCSD(TQ)] and two approximations to full configuration interaction. Basis sets were chosen from the correlation-consistent family of basis sets, with the largest being the aug-cc-pVQZ set. Polarized valence double zeta quality basis sets were found to yield corrections that differed substantially from larger basis set results. At the double zeta level, higher order corrections increased the binding energies, whereas calculations with triple and quadruple zeta basis sets gave the opposite effect. Although the absolute magnitude of the higher-order corrections was small for these diatomics, they were nonetheless significant in light of a target accuracy of ±1 kcal/mol. Among molecules composed of first-through-third period elements, such as those in the G2 and G2/97 collections, the contribution to D0 from higher order correlation effects could easily exceed 1 kcal/mol. If further studies corroborate the present findings, CCSDT should provide an effective method of determining the magnitude of the higher-order correction, at least when basis sets of triple zeta or better quality can be used. CCSD(TQ) often overestimated the higher order correction, sometimes exceeding the estimated full configuration interaction result by a factor of three.
Derivation of thermal equations of state for quantum systems using the quasi-Gaussian entropy theory
Authors: Apol M. E. F. , Amadei A. , Berendsen H. J. C. , Di Nola A.
Published: July 2002

In this article, the quasi-Gaussian entropy theory is derived for pure quantum systems, along the same lines as previously done for semiclassical systems. The crucial element for the evaluation of the Helmholtz free energy and its temperature dependence is the moment generating function of the discrete probability distribution of the quantum mechanical energy. This complicated moment generating function is modeled via two distributions: the discrete distribution of the energy-level order index and the continuous distribution of the energy gap. For both distributions the corresponding physical–mathematical restrictions and possible systematic generation are discussed. The classical limit of the present derivation is mentioned in connection with the previous semiclassical derivation of the quasi-Gaussian entropy theory. Several simple statistical states are derived, and it is shown that among them are the familiar Einstein model and the one-, two-, and three-dimensional Debye models. The various statistical states are applied to copper, α-alumina, and graphite. One of these states, the beta-diverging negative binomial state, is able to provide an accurate description of the heat capacity of both isotropic crystals, like copper, and anisotropic ones, like graphite, comparable to the general Tarasov equation.
Infrared spectra and density functional calculations of Cu(CO)1–4+, Cu(CO)1–3, and Cu(CO)1–3− in solid neon
Authors: Zhou Mingfei , Andrews Lester
Published: July 2002

Laser-ablated copper atoms, cations, and electrons react with CO molecules to give binary copper carbony neutral complexes as well as cation and anion complexes, which are isolated in solid neon and argon matrices. Based on isotopic substitution as well as density functional calculations of isotopic frequencies, absorptions at 2234.4, 2230.4, 2211.3, and 2202.1 cm−1 in neon are assigned to C–O stretching vibrations of the linear CuCO+ and Cu(CO)2+, trigonal planar Cu(CO)3+, and tetrahedral Cu(CO)4+ cations. The absorptions at 1746.2, 1793.9, and 1838.9 cm−1 in neon and at 1733.4, 1780.8, and 1829.7 cm−1 in argon are assigned to the linear CuCO− and Cu(CO)2−, and trigonal planar Cu(CO)3− anions, respectively. The solid neon observations of Cu(CO)1–3 are 20–9 cm−1 blue shifted from the argon matrix counterparts, which are in agreement with previous thermal copper atom matrix isolation studies. This work provides the first vibrational spectra of Cu(CO)1–4+ and Cu(CO)1–3−.
Solvation response of polar liquid mixtures: Water-dimethylsulfoxide
Authors: Laria Daniel , Skaf Munir S.
Published: July 2002

The solvation dynamics following the instantaneous creation of a positive or negative electronic charge in a previously neutral solute immersed in different water-dimethyl sulfoxide (DMSO) mixtures, spanning the entire composition range, is analyzed by molecular dynamics simulations. The solvation responses are strongly dependent on the sign of the solute charge, being considerably faster in the presence of cations for all mixtures considered. In terms of the composition dependence, the mixtures’ solvation response to the creation of the anion departs substantially from the pure solvents’, whereas for the cation, the mixtures’ responses are close to those exhibited by pure DMSO. In the case of anions, the mixture overall solvation time, defined as the time integral of the nonequilibrium response, can be as large as ten times the solvation time in pure DMSO, the slowest of the two cosolvents. The DMSO contribution to the mixtures’ solvation response may present an intriguing negative branch in the rotational-diffusion regime which persists for times much longer than the time scales typically found in other polar liquids and mixtures. This negative portion is nearly cancelled by an equally long-lasting positive contribution from water, resulting in a fast-decaying, total response curve which is typical of many polar liquid environments. This behavior is rationalized in terms of the time evolution of the first solvation shell around each type of solute.
Energy partitioning in two kinds of NO molecules generated from the reaction of O(1D) with N2O: Vibrational state distributions of “new” and “old” NO’s
Authors: Akagi Hiroshi , Fujimura Yo , Kajimoto Okitsugu
Published: July 2002

The reaction of O(1D) with N2O produces two kinds of NO molecules, the “old” one which originally exists in N2O and the “new” one which includes the attacking O atom. Using the isotopically labeled reagent, we determined the vibrational state distributions of these NO’s (X 2Π; v=0–17) separately. To obtain the distributions, two types of experiments were performed with the laser-induced fluorescence (LIF) technique via the NO A←X and B←X transitions. First, the relative populations of NO molecules (the sum of the two kinds of NO’s) in v=0–11 levels were measured with unlabeled reagents. Then, isotopically labeled reaction, O18(D1)+N216O→N18O+N16O, was utilized to determine the relative ratio between the two kinds of NO’s in the vibrational levels of v=0–5 and 12–15. Combining the above results with previously determined vibrational state distribution of NO in high vibrational levels (v=11–17) [J. Chem. Soc., Faraday Trans. 94, 1575 (1998)], we were able to obtain a complete set of vibrational state distributions. It was found that the old NO dominantly populated in v=0 and 1 whereas the new NO extended its population toward higher vibrational levels (v=4–15). However, in high vibrational levels, the old NO still have a considerable population due to the rapid energy transfer to the old NO. The observed efficient energy transfer to the old NO is attributed to the absence of light atoms in the present reacting system. Compared with the system including hydrogen atoms, the state density and the momentum coupling among the vibrational modes are much larger and accelerate the energy redistribution in spite of the short lifetime.
Liquid-drop formalism and free-energy surfaces in binary homogeneous nucleation theory
Authors: Laaksonen Ari , McGraw Robert , Vehkamäki Hanna
Published: July 2002

Three different derivations of the classical binary nucleation theory are considered in detail. It is shown that the derivation originally presented by Wilemski [J. Chem. Phys. 80, 1370 (1984)] is consistent with more extensive derivations [Oxtoby and Kashchiev, J. Chem. Phys. 100, 7665 (1994)]; Debenedetti, Metastable Liquids: Concepts and Principles (Princeton University Press, Princeton, 1996) if and only if the assumption is made that the surface of tension of the binary nucleus coincides with the dividing surface specified by the surface condition ∑nsivli=0, where the nsi denote surface excess numbers of molecules of species i, and the v’s are partial molecular volumes. From this condition, it follows that (1) the surface tension is curvature independent and (2) that the nucleus volume is V=∑nlivli=∑givli, where the nli are the numbers of molecules in the uniform liquid phase of the droplet model encompassed by the surface of tension, and the gi are the total molecular occupation numbers contained by the nucleus. We show, furthermore, that the above surface condition leads to explicit formulas for the surface excess numbers nsi in the nucleus. Computations for the ethanol–water system show that the surface number for water molecules (ns,H2O) causes the negative occupation numbers (gH2O) obtained earlier using the classical nucleation theory. The unphysical behavior produced by the classical theory for surface active systems is thus a direct consequence of the assumption of curvature independence of surface tension. Based on the explicit formulas for nsi, we calculate the full free-energy surfaces for binary nucleation in the revised classical theory and compare these with the free-energy surfaces in the Doyle (unrevised classical) theory. Significant differences in nucleus size and composition are found between these models and they are related to surface excess density. It is shown that only for the revised classical theory is the nucleus composition consistent with the Gibbs dividing surface model.
Calculating excess chemical potentials using dynamic simulations in the fourth dimension
Authors: Pomès Régis , Eisenmesser Elan , Post Carol B. , Roux Benoı̂t
Published: July 2002

A general method for computing excess chemical potentials is presented. The excess chemical potential of a solute or ligand molecule is estimated from the potential of mean-force (PMF) calculated along a nonphysical fourth spatial dimension, w, into which the molecule is gradually inserted or from which it is gradually abstracted. According to this “4D-PMF” (four dimensional) scheme, the free energy difference between two limiting states defines the excess chemical potential: At w=±∞, the molecule is not interacting with the rest of the system, whereas at w=0, it is fully interacting. Use of a fourth dimension avoids the numerical instability in the equations of motion encountered upon growing or shrinking solute atoms in conventional free energy perturbation simulations performed in three dimensions, while benefiting from the efficient sampling of configurational space afforded by PMF calculations. The applicability and usefulness of the method are illustrated with calculations of the hydration free energy of simple Lennard-Jones (LJ) solutes, a water molecule, and camphor, using molecular dynamics simulations and umbrella sampling. Physical insight into the nature of the PMF profiles is gained from a continuum treatment of short- and long-range interactions. The short-range barrier for dissolution of a LJ solute in the added dimension provides an apparent surface tension of the solute. An approximation to the long-range behavior of the PMF profiles is made in terms of a continuum treatment of LJ dispersion and electrostatic interactions. Such an analysis saves the need for configurational sampling in the long-range limit of the fourth dimension. The 4D-PMF method of calculating excess chemical potentials should be useful for neutral solute and ligand molecules with a wide range of sizes, shapes, and polarities.
Fourier transform microwave rotational spectra of the Ne2–N2O and Ar2–N2O van der Waals trimers
Authors: Ngarı̃ Mwanı̃ki S. , Jäger Wolfgang
Published: July 2002

A pulsed molecular beam cavity Fourier transform microwave spectrometer was used to measure pure rotational spectra of nine isotopomers of Ne2–N2O, and of three isotopomers of the Ar2–N2O van der Waals trimer. For Ne2–N2O, these are 20Ne20Ne–14N14N16O, 20Ne22Ne–14N14N16O, 22Ne22Ne–14N14N16O, 20Ne20Ne–15N14N16O, 20Ne22Ne–15N14N16O, 22Ne22Ne–15N14N16O, 20Ne20Ne–14N15N16O, 20Ne22Ne–14N15N16O, and 22Ne22Ne–14N15N16O. Those for Ar2–N2O are 40Ar40Ar–14N14N16O, 40Ar40Ar–15N14N16O, and 40Ar40Ar–14N15N16O. The spectra were measured in the frequency range between 3 and 18 GHz. Both a- and c-type transitions were measured for all Ne2–N2O isotopomers. In the case of the mixed, 20Ne22Ne containing, isotopomers a small b-dipole moment occurs and two b-type transitions were measured. In the spectra of Ar2–N2O only b- and c-type transitions were measured. Rotational and centrifugal distortion constants were determined for all the isotopomers of each complex. The spectral analyses show that Ne2–N2O is a highly asymmetric prolate rotor (κ=−0.158 for 20Ne20Ne–14N14N16O) while Ar2–N2O is a highly asymmetric oblate rotor (κ=0.285 for 40Ar40Ar–14N14N16O). Both trimers were found to have distorted tetrahedral structures with the rare gases tilted towards the O atom of the N2O subunit. Nuclear quadrupole hyperfine structures due to both terminal and central 14N nuclei were observed and analyzed to give the nuclear quadrupole coupling constants, χaa(1), χbb(1) and χaa(2), χbb(2). The resulting spectroscopic constants were utilized to derive ground state effective structures, ground state average structures, and partial substitution structures. Harmonic force field analyses were performed for each complex using the obtained quartic centrifugal distortion constants. The results of the spectroscopic analyses are discussed in the light of possible three-body nonadditive interactions.
Crossover behavior in dilute polymer solutions: Square-well chains
Authors: Lue L. , Kiselev S. B.
Published: July 2002

Monte Carlo simulations for model polymer chains composed of hard spheres with square-well attractions were performed to find a precise relation between the parameters of a crossover theory and the parameters of the square-well chain. For sufficiently large bond lengths, there was a direct relation between the effective parameters of the crossover theory and the second- and third-virial coefficients of the square-well atoms. For the systems under study, the theta temperature is close to the Boyle temperature of the disconnected monomers, is mainly dependent on the details of the monomer–monomer interactions, and is only weakly dependent on the bond length of the chain. Above the theta temperature, the crossover theory is in good agreement with the simulation data. Near the theta point, the main effect of tricritical corrections is a shift in the critical amplitudes. Other effects are extremely small. The simulation data are consistent with the tricritical crossover theory. However, more precise data are needed to quantitatively test the tricritical crossover theory.
Geminate charge pair recombination in sensitized photoconducting polymer
Authors: Abramavicius D. , Gulbinas V. , Ruseckas A. , Undzenas A. , Valkunas L.
Published: July 2002

Recombination kinetics of the geminate charge pairs, generated by photoexcitation of charge transfer (CT) complexes in sensitized poly-N-epoxypropylcarbazole, have been investigated by transient absorption and time-resolved fluorescence techniques and modeled by Monte Carlo method. The quantitative agreement of simulations with experimental data is obtained by assuming the distance between the thermalized charge pairs to be several times longer than the typical distance between electron donor and acceptor in CT complexes. Faster recombination is observed in the films with higher sensitizer concentrations and explained by assuming that electron acceptor molecules act as scattering centers for hole motion during thermalization, thus reducing the thermalization distance. The dynamical blue shift of the fluorescence band and comparison of hole density with fluorescence kinetics suggest that heterogeneity of CT complexes results in different radiative probabilities and in distribution of the initial charge pair separation distances.
Photodissociation dynamics of CH2BrCl studied using resonance enhanced multiphoton ionization (REMPI) with time-of-flight mass spectrometry
Authors: McGivern W. Sean , Li Runjun , Zou Peng , North Simon W.
Published: July 2002

The photodissociation dynamics of CH2BrCl have been studied using resonance-enhanced multiphoton ionization with time-of-flight mass spectrometry. Polarization dependent time-of-flight profiles were collected for a range of wavelengths from 248 to 268 nm, corresponding to the red wing of the absorption spectrum. Forward convolution fits to the data have provided translational energy distributions and anisotropy parameters over the entire wavelength range for both Br(2P3/2) and Br*(2P1/2). The average translational energies for the Br and Br* channels are 20 and 23 kcal/mol, respectively. The measured anisotropy parameters indicate that both channels arise preferentially from a parallel transition and that the relative contribution of this transition increases with decreasing wavelength. Nonadiabatic transitions appear to play a smaller role in CH2BrCl dissociation than in its monohalogenated analogues, specifically CH3Br. We suggest that this difference is the result of the intrinsic Cs symmetry and lower radial velocity of CH2BrCl, and it is discussed in terms of a one-dimensional Landau–Zener model. A C–Br bond dissociation energy of 67.5 kcal/mol in CH2BrCl was also calculated using ab initio methods at the MP2/cc-pVtz//MP2/cc-pVdz level.
Coexistence curve of polystyrene in methylcyclohexane. X. Two-phase coexistence curves for ternary solutions near the tricritical compositions
Authors: Nakata Mitsuo , Dobashi Toshiaki , Inakuma Yu-ichi , Yamamura Kiyotaka
Published: July 2002

Tricritical phase equilibrium properties were studied for the ternary solution of two polystyrene homologs in methylcyclohexane by numerical analysis of the generalized Flory–Huggins equation with an empirical interaction parameter and by experimental measurements. For critical solutions at the tricritical molecular weight ratio two-phase coexistence curves were calculated very near to the critical points as 10−7 in the reduced temperature. The shape of the coexistence curves changed markedly depending on the overall compositions of the solution. For the tricritical solution the coexistence curve was asymmetric due to an interference with the critical line but the asymptotic behavior near the tricritical point was not revealed by the present calculation. Two-phase coexistence curves were measured for ternary solutions with the molecular weight M1=1.87×104 and M2=41.2×104 which yielded the molecular weight ratio 22 close to the empirical tricritical value 23 found in literatures. The coexistence curves measured for solutions not far from the tricritical one exhibited the characteristic features calculated for solutions near the tricritical one.
Viscosity dependence and solvent effects in the photoisomerization of cis-stilbene: Insight from a molecular dynamics study with an ab initio potential-energy function
Authors: Berweger Christian D. , van Gunsteren Wilfred F. , Müller-Plathe Florian
Published: July 2002

Molecular-dynamics simulations of the photoisomerization of cis-stilbene in supercritical argon were performed. The stilbene molecule is represented by ab initio quantum chemistry, while the solvent, the interaction with solvent, and the time evolution were described by classical mechanics. Reaction rate constants are estimated and their dependence on temperature, pressure, and viscosity are investigated. Agreement with available experimental data was obtained. Our simulations strongly suggest a minimum on the excited-state potential-energy surface at a gauche conformation which is very rapidly reached after excitation, which leads to nonequilibrium barrier transitions. Specific solvent effects were identified. Implications on the current opinion on stilbene photoisomerization are discussed.
Response to “Comment on ‘A comparison of the efficiency of Fourier- and discrete time-path integral Monte Carlo’ ” [J. Chem. Phys. 111, 7685 (1999)]
Authors: Chakravarty C. , Gordillo M. C. , Ceperley D. M.
Published: July 2002

The motivation, ground rules, and analysis of the systematic error for the comparison of the various path integral Monte Carlo methods are reviewed.
Further dynamical studies of the dissociation and elimination reactions of methyl nitrite
Authors: Martı́nez-Núñez Emilio , Vázquez Saulo A.
Published: July 2002

Classical trajectory calculations were performed to investigate the effects of molecular rotation, deuterium substitution, and the possibility of mode-specific effects in the two unimolecular channels that initiate the thermal decomposition of methyl nitrite (MeONO): O–N bond dissociation giving CH3O and NO and concerted elimination to produce CH2O and HNO. The calculations were carried out at a total energy of 140 kcal/mol, at which a microcanonical ensemble of excited molecules is maintained throughout the decomposition. Total and individual rate coefficients were evaluated under several types of initial sampling conditions: microcanonical (i.e., random) distribution of vibrational energy, selective excitation of normal modes, and various angular momentum orientations. Comparisons of the results obtained from random initial conditions and normal mode excitations show that there is significant enhancement of the decomposition rates for excitations of several vibrational modes (apparent non-RRKM behavior). The calculations predict rapid energy exchange among modes 465 (ONO bend), 715 (CO stretch), and 931 (O–N stretch) as well as strong coupling between modes 246 (CONO torsion) and 1670 (N=O stretch). The vibrational state distributions for the nascent NO species computed under excitations of modes 246 and 1670 are much broader than that obtained under random initial conditions. This gives further evidence for incomplete relaxation of vibrational energy on the time scale of reaction. Molecular rotation enhances the decomposition rates significantly. More specifically, exciting the symmetric top axis promotes elimination, while exciting either of the remaining two axes promotes dissociation. The presence of two-dimensional rotors at the dissociation transition state may explain the inverse isotope effect found in our previous classical trajectory calculations [J. Chem. Phys. 109, 8907 (1998)]. Finally, the importance of anharmonicity in the unimolecular density of states was estimated by fits of modified RRK schemes to our previously reported microcanonical rate coefficients.
Theory of nonadiabatic electron transfer at electrode/liquid interfaces: Role of quantum effects
Authors: Tanaka Shigenori , Hsu Chao-Ping
Published: July 2002

The electron transfer rate constant at electrode/liquid interfaces is theoretically described on the basis of the Anderson–Newns–Schmickler model. A compact formula for the rate constant is derived in the nonadiabatic limit, which is expressed in terms of the spectral density of surrounding media, the density of states of electrons in the electrode, and the weighted electronic coupling constant between the electrode and the redox couple in the liquid. The outer-sphere spectral density is then related to the experimentally accessible data on the frequency-dependent dielectric response functions of the solvent and the electrode with the aid of the dielectric continuum approximation. The derived formula provides a quantum-mechanical extension of the conventional nonadiabatic expression for the heterogeneous electron transfer reactions at electrode/liquid interfaces, taking into account the quantum effects associated with the high-frequency modes of both outer and inner spheres. On this basis, the quantum correction for the electron-transfer rate constant is numerically analyzed for some metal or semiconductor electrodes in contact with the Fe2+/3+ redox couple dissolved in water solvent at room temperature. In the case of zero energy gap, the quantum correction is found to be a factor of 4–5 for a typical configuration of the redox couple regardless of the species of electrode, while the rate constant itself is significantly affected by the dielectric property of the electrode. The energy gap dependence of the quantum correction is also discussed.
Extracting molecular axis alignment from photoelectron angular distributions
Authors: Reid Katharine L. , Underwood Jonathan G.
Published: July 2002

We present a procedure that will enable the extraction of molecular axis alignment from evolving photoelectron angular distributions measured following (1+1) resonance-enhanced multiphoton ionization in which the first step prepares the initial alignment. This procedure is applicable to a picosecond time- and angle-resolved photoelectron spectroscopy experiment [such as the one presented by Reid et al., J. Chem. Phys. 111, 1438 (1999)], and relies on the radial dipole matrix elements for the ionization process having no time dependence on the time scale of the experiment. As an illustration, we present a model calculation of the evolution of photoelectron angular distributions expected following the ionization of a prepared pure rotational wavepacket.
Combined iterative diagonalization and statistical sampling in accurate reaction rate calculations: Rotational effects in O+HCl→OH+Cl
Authors: Matzkies Frank , Manthe Uwe
Published: July 2002

A modification of the statistical approach to the direct calculation of thermal rate constants [J. Chem. Phys 110, 88 (1998)] is presented. Within this approach, the trace in the flux-correlation function is evaluated by iterative diagonalization in the (strongly interacting) internal degrees of freedom and by a statistical sampling scheme in the (weekly interacting) rotational degrees of freedom. The wave packet dynamics is calculated employing the multiconfigurational time-dependent Hartree (MCTDH) approach. The O+HCl→OH+Cl reaction is studied. This mixed approach is found to be more efficient and accurate than the purely statistical approach. The accuracy of the J-shifting approximation, the modified J-shifting approximation, and the K-conserving approximation is investigated in detail between 200 and 700 K.
Gas–liquid nucleation in associating fluids
Authors: Talanquer V. , Oxtoby David W.
Published: July 2002

The density functional approach to nucleation is extended to the study of the condensation of associating fluids. We use Wertheim theory for associating liquids in a system of spherical interacting molecules with single association sites. Our calculations show that classical nucleation theory largely underestimates the height of the nucleation barrier in this kind of system, but most of that error can be corrected through the classical model of Katz, Saltsburg, and Reiss for nucleation in associated vapors.
Globular state of random copolymers with arbitrary amphiphilicity
Authors: Ganazzoli Fabio
Published: July 2002

In this paper we extend our recent approach to the collapse or random AB copolymers [F. Ganazzoli, J. Chem. Phys. 108, 9924 (1998)] to amphiphilic chains. These copolymers are formed by unlike units having varying degrees of solvophilicity and solvophobicity (or polarity and hydrophobicity if the solvent is water) and model the behavior of globular proteins. The equilibrium state is found by self-consistent minimization of the intramolecular free energy, which includes two-body interactions, either attractive or repulsive depending on the interacting units, three-body and screened repulsive interactions among all the units, and configurational entropy. Chain connectivity is accounted for throughout. We consider a fixed sequence of units along the chain, which does not give rise to any entropy of mixing. In keeping with our earlier results, we find that amphiphilic copolymers form monomolecular micelles with a compact core formed by the hydrophobic units and a looser outer corona with the polar units. Different metastable states are also present, either with a few isolated hydrophobic units in the outer region, or with two distinct globular cores. In the latter case, the coalescence of the cores is prevented by their polar periphery. A notable feature is that the multiplicity of metastable states, and sometimes also of the stable ones, is often significantly larger than in AB copolymers. Advantages and shortcomings of the procedure in connection also with other theoretical studies of protein folding are briefly discussed.
Pump–probe spectrum of molecular assemblies of arbitrary structure and dimension
Authors: Juzeliūnas Gediminas , Knoester Jasper
Published: July 2002

Using the hard-core boson approach, we study the pump-probe spectrum of molecular assemblies carrying Frenkel excitons of arbitrary structure and dimension. We present a rigorous justification of the hard-core boson approach by using the Agranovich–Toshich transformation from paulions to bosons. The resulting two-exciton Green function is used to derive a general expression of the assembly’s pump–probe spectrum. We show that this expression considerably simplifies for ordered systems occupying a lattice, where we allow for the occurrence of more than one equivalent molecule in the unit cell (Davydov components). Explicit semianalytical expressions are given for the pump–probe spectrum of linear chains with alternating dipoles, ring aggregates, chains with a herringbone structure, and monolayers. In the analysis of these expressions, we focus on the overall shape of the spectrum and on the effects of probe polarization. It is shown that relaxation during the pump–probe delay time may drastically affect the pump–probe spectrum.
Vibrational dynamics of hydrogen-bonded HCl-diethyl ether complexes
Authors: Giebels I. A. M. E. , van den Broek M. A. F. H. , Kropman M. F. , Bakker H. J.
Published: July 2002

We studied the relaxation of the H–Cl stretch vibration of hydrogen-bonded HCl-diethyl ether [HCl–(CH3CH2)2O] complexes at room temperature using femtosecond mid-infrared pump-probe spectroscopy. The lifetime of the H–Cl stretch vibration is determined to be 0.9±0.2 ps. The relaxation is found to occur via an intermediate state which causes a transient blue-shift of the H–Cl stretch frequency. This blue-shift indicates that the low-frequency (CH3CH2)2O⋅⋅⋅H–Cl hydrogen bond is the main accepting mode of the vibrational energy. The excited population of this hydrogen-bond mode decays with a time constant of 3.1±0.5 ps to the ground state.