bluuebunny/crossref_metadata_2025 huggingface dataset:
Free energies of solvation with quantum mechanical interaction energies from classical mechanical simulations
Authors: Wood Robert H. , Yezdimer Eric M. , Sakane Shinichi , Barriocanal Jose A. , Doren Douglas J.
Published: July 2002

A free energy perturbation technique is described in which configurations from a classical simulation (molecular dynamics or Monte Carlo) with empirical solute–solvent interactions are used to calculate free energies with quantum mechanically derived solute–solvent interactions. This approach is much less costly than simulations with forces derived from quantum mechanics at each time step, since it only requires quantum energies to be calculated at classically determined configurations. The method is not limited to free energies of solvation, and can potentially be applied to calculations of activation energies and other condensed phase chemical transformations. As a test, this method was used to calculate the free energy of hydration of water at ambient conditions. With a good classical model the method gives accurate results with only 50 quantum calculations. The method is self-correcting in the sense that it can be used to recognize a bad classical model, and improved classical models can be derived by a least-squares fitting to the quantum energies. As a result, this method also provides novel information about the comparative strengths and weaknesses of classical solute models.
Theoretical study of the dipole-bound anion (HPPH3)−
Authors: Skurski Piotr , Gutowski Maciej , Simons Jack
Published: July 2002

The possibility of electron binding to the HPPH3 and H2PPH2 tautomers of diphosphine was studied at the coupled cluster level of theory with single, double, and noniterative triple excitations. The HPPH3 tautomer, with a dipole moment of 3.7 D, binds an electron by 333 cm−1, whereas the H2PPH2 tautomer forms neither a dipole- nor valence-bound anionic state. It is suggested that the HPPH3 tautomer, which is kinetically stable but thermodynamically unstable relative to H2PPH2, may be formed by photodetachment from the P2H4− species examined in this work. An unusual aspect of the (HPPH3)− anion is that electron correlation contributes 82% to the electronic stability and effects beyond the fourth order of the Mo/ller–Plesset perturbation theory contribute 55%.
Molecular bottle brushes in thin films: An off-lattice Monte Carlo study
Authors: Saariaho Mika , Ikkala Olli , ten Brinke Gerrit
Published: July 2002

The effect of intramolecular excluded volume interactions on the conformations of isolated comb-shaped copolymers—“molecular bottle brushes”—in athermal solvents confined between two hard impenetrable parallel plates is studied by off-lattice Monte Carlo simulations. Equilibrium conformational properties of molecules consisting of a freely jointed backbone of 100 hard spheres (beads), where 50 equally flexible side chains of lengths M up to 30 beads are connected, are determined as a function of the width of the slit, including the pure two-dimensional (2D) case. Besides general properties such as the size and the shape of the chains and the orientation of the chains with respect to the confining plates, the persistence length of the bottle-brush backbone λ is considered in particular. It is demonstrated that due to the side chains, the backbone is, in fact, effectively confined between two soft plates, i.e., the side chains act like softening springs between the backbone and plates. The most striking result is that, unlike the three-dimensional (3D) situation, in the 2D case the ratio between λ and the diameter D, λ/D, of the bottle brush starts to increase as a function of the side chain length for M⩾10. For 3D, an increase of λ/D (the essential parameter for lyotropic behavior) as a function of M is predicted by theory but has so far not been observed numerically for the regime studied (i.e., M⩽30). These results suggest that due to the weaker excluded volume effect in 3D, a possible upturn of λ/D may only be observed for longer side chains than studied so far.
Effective elastic moduli of a composite containing rigid spheres at nondilute concentrations: A multiple scattering approach
Authors: Mondescu Radu P. , Muthukumar M.
Published: July 2002

Based on the multiple scattering technique [K. F. Freed and M. Muthukumar, J. Chem. Phys. 69, 2657 (1978); 68, 2088 (1978); M. Muthukumar and K. H. Freed, J. Chem. Phys. 70, 5875 (1979)] previously applied to the study of suspensions of spheres and polymers, we propose an approach to the computation of the effective elastic properties of a composite material containing rigid, mono-sized, randomly dispersed, spherical particles. Our method incorporates the many-body, long-range elastic interactions among inclusions. The effective medium equations are constructed and numerically solved self-consistently. We have calculated the effective shear μ′ and Young E′ moduli, as well as the effective Poisson ratio σ′, as functions of the particle volume fraction Φ and of the Poisson ratio σ of the continuous phase. Comparisons with two sets of experimental data—glass beads in a polymer matrix and tungsten carbide particles in a cobalt matrix (Wc/Co)—and to a previous theoretical solution, are also presented. Our model can predict the effective Poisson ratio of the Wc/Co system for Φ⩽1 and for the glass/polymer system for Φ⩽0.5. In particular, the present work describes accurately composites with a high volume fraction of inclusions, where a percolation transition occurs. Very good agreement with the experimental data are obtained for E′ and μ′ when Φ⩽0.4, for both systems.
Electron transfer transitions among many electronic states coupled to multidimensional diffusive nuclear modes: A supramolecular triad system
Authors: Okada A. , Bandyopadhyay T. , Tachiya M.
Published: July 2002

A theory for the electron transfer (ET) transition rates among many electronic states taking into account multidimensional solvation dynamics is developed. An expression for the frequency-dependent ET rate is derived. The detailed balance principle for the frequency-dependent ET rate is proved for the first time. The theory is applied to ET which involves more than two electronic states, including the supramolecular triad system. The ET rate which depends on solvation timescale in a nonmonotonic way is found in the case where the transfer of an electron occurs from a donor to an acceptor states through an intermediate state, which is not fully equilibrated during the transfer. We explored the physical condition under which such a mechanism prevails. For this purpose useful approximations for the Green’s function of the Smoluchowski equation in the frequency domain are developed.
Thermodynamic analysis of the solvent effect on tautomerization of acetylacetone: An ab initio approach
Authors: Ishida Tateki , Hirata Fumio , Kato Shigeki
Published: July 2002

The keto-enol tautomerism of acetylacetone in solution is studied with the reference interaction site model self-consistent-field (RISM-SCF) method. We choose three solvents, H2O, dimethyl sulfoxide (DMSO), and carbon tetrachloride (CCl4), representing, respectively, protic polar, aprotic polar and nonpolar solvents. The analysis is made taking account of the solute electronic as well as geometrical change of the tautomers due to solvent effect. In addition, the electronic correlation energy of solute molecule and solute vibrational energies are considered. The free energy differences are analyzed by decomposing them into the enthalpy and entropy terms. The theory reproduces the total free energy determined by the experiment fairly well. We also find that, as solvent polarities increase, the keto tautomer shows the drastic geometric change in order to make its dipole moment larger and that the geometric change of the keto tautomer is enthalpically driven in H2O and entropically in DMSO. It is made clear that these depend on the solvent property—protic or aprotic.
M-resolved Zeeman spectra of rotational lines in the 9U band of CS2, and splittings of the 3A2(3Δu) state
Authors: Katô Hajime , Taroura Yoshikazu , Kasahara Shunji , Baba Masaaki
Published: July 2002

Excitation spectra and the change with an external magnetic field of the 9U band, which is assigned as the RB2(0,12,0)←X 1Σg+(0,0,0) transition of carbon disulfide (CS2), are measured with sub-Doppler resolution. Zeeman splittings into 2J+1 lines are observed both in the P(J+1) and R(J−1) lines, and the Zeeman splittings are attributed to the ones of the excited levels. The spacings between adjacent M levels are observed to be larger in low energy components than in high energy components. From the analysis, it is shown that the B2 level of the 3A2(3Δu) state lies about 48 cm−1 lower than the A1 and B1 levels and that the +M components lie in a higher energy region than the −M components. The M=0 component is observed to shift to lower energy as the magnetic field increases. This is shown to be originating from the Zeeman interaction with selection rules ΔJ=±1 in addition to the one of ΔJ=0.
Intermittency of activated events in single molecules: The reaction diffusion description
Authors: Wang Jin , Wolynes Peter
Published: July 2002

We generalize the reaction diffusion approach in order to calculate the higher moments of the distribution of the population which characterize intermittency and can be derived by statistical analysis of single molecule experiments. We show the way the usual Poisson statistics is asymptotically approached with increasing barrier height for a reaction dependent on diffusing up an activation free energy barrier to a critical configuration.
The rotational and vibrational dynamics of argon–methane. II. Experiment and comparison with theory
Authors: Miller Roger E. , Heijmen Tino G. A. , Wormer Paul E. S. , van der Avoird Ad , Moszynski Robert
Published: July 2002

Presented here is a detailed comparison between the experimental near infrared spectrum of argon–methane and the results of a theoretical calculation based upon the methods described in a companion paper, hereafter referred to as paper I [T. G. A. Heijmen et al., J. Chem. Phys. 110, 5639 (1999), preceding paper]. Many of the bands in the spectrum are easily assigned directly from this comparison. The spectrum is shown to be highly sensitive to the anisotropy of the argon–methane potential surface and the agreement with the ab initio spectrum, although not quantitative, is very good. The predissociation linewidths observed in the experimental spectra are found to be strongly dependent upon the symmetry of the excited state. Symmetry considerations place restrictions on the final rotational states that can be accessed, possibly explaining the differences in the lifetimes.
Orientations of linear nonpolar molecules in a regular chain
Authors: Klymenko V. E. , Rozenbaum V. M.
Published: July 2002

Energetically preferable orientational states for linear nonpolar molecules are determined which result from the competition of quadrupole–quadrupole and Van der Waals dipole–dipole interactions. It is shown that with the decreasing intermolecular distance a and hence with the increasing Van der Waals contribution to the total energy, the system concerned successively passes through the following orientational phases: alternating longitudinal and transverse molecular orientations relative to the chain axis (phase I), planar arrangement of molecules inclined to the chain axis at some angle which tends to diminish with decrease in a (phase II), molecules aligned parallel to the chain axis (phase III). In contrast to the continuous transition between phases II and III, the nature of the transition I→II depends on the radius of the interactions taken into account. In the approximation of short-range interactions, the phase transition represents a jump, whereas long-range interactions, if included, lead the system to change continuously from inhomogeneous to homogeneous structure in a narrow range of intermolecular distances. The applicability of the results obtained in the description of orientational structures of adsorbed nonpolar molecules is discussed.
Effects of field on temperature-induced segregation and folding of polymer chains
Authors: Foo Grace M. , Pandey R. B.
Published: July 2002

Segregation of polymer chains and their conformations are studied as a function of temperature and field in two dimensions using a hybrid (discrete-to-continuum) simulation method. Temperature-induced segregation occurs on decreasing the temperature, i.e., from T=5 (desegregation) to T=1 (segregation) followed by chains folding (T⩽0.5) in the absence of field. The amplitude of folding increases on reducing the temperature. While the presence of field induces orientational ordering and anisotropy in the mass segregation of chains, it leads to desegregation as the field is increased above a characteristic value (Ec). Radius of gyration exhibits conformational crossover from extended (SAW) (high T) to collapse (low T) in the absence of field, but anisotropy sets in its spread in the presence of field. Variation of the radial distribution is consistent with these observations.
Direct ab initio calculation of ground-state electronic energies and densities for atoms and molecules through a time-dependent single hydrodynamical equation
Authors: Dey Bijoy Kr. , Deb B. M.
Published: July 2002

By using an imaginary-time evolution technique, coupled with the minimization of an expectation value, ground-state electron densities and energies have been directly calculated for six atomic and molecular systems (He, Be++, Ne, H2, HeH+, He2++), from a single time-dependent (TD) quantum fluid dynamical equation of motion whose real-time solution yields the TD electron density. For all the systems, a local Wigner-type correlation functional has been employed. For Ne, a local exchange functional is used while, for all the other systems, the exchange energy is calculated exactly. The static (ground-state) results are of beyond-Hartree–Fock quality for all the species.
Calculations of the phonon dispersion curves of C2H2, OC(ND2)2, and Na2CO3 from generic force field
Authors: Parlinski K. , Chapuis G.
Published: July 2002

The phonon dispersion curves of cubic acetylene, tetragonal urea, and monoclinic sodium carbonate are calculated from the generic force field using the direct method. The force constants of the dynamical matrix are determined from the forces induced by the displacement of an atom in the simulated supercell. The obtained dispersion curves are displayed along many high-symmetry lines of the Brillouin zone. For sodium carbonate, a soft mode has been found. The dispersion curves of urea fit quite well to the measured phonons. The results show that the force field approach leads to reasonable description of the lattice dynamics of crystals.
Upper and lower bounds for the kinetics of hard-particle adsorption
Authors: Poland Douglas
Published: July 2002

We compare upper and lower bounds for the rate of the reversible, cooperative adsorption of hard particles from a reservoir at constant activity to a lattice surface where the only cooperative effect is excluded volume. The adsorption rate is proportional to the density of groups of unoccupied lattice sites: holes, large enough to accommodate a particle. The bounds on the rate of adsorption are then bounds on the density of holes. The upper bound for particles that are infinitely mobile on the surface is obtained from the equilibrium Mayer activity series for the pressure, while the lower bound is obtained from the extensive exact series calculated by Gan and Wang [J. Chem. Phys. 108, 3010 (1998)] for the case of irreversible random sequential adsorption where the particles are immobile once adsorbed. In all cases the bounds coincide at low densities. For the one-dimensional lattice with nearest-neighbor exclusion (where the bounds are known exactly) the upper and lower bounds are very close for all densities below the limit of random close packing as they are for the adsorption of hard dimers on two-dimensional lattices. Thus in these cases equilibrium statistical mechanics can give useful information about the kinetics of cooperative processes.
Liquid-state theory of anisotropic flexible polymer fluids
Authors: Pickett Galen T. , Schweizer Kenneth S.
Published: July 2002

We extend the liquid-state theory of polymer fluids to include anisotropy as a key feature. The formalism is quite general. In determining the structure of anisotropic polymer fluids, it yields thermodynamic information as well. Our first application is to describe the nematic phases of flexible polymers, and to demonstrate a novel lyotropic transition.
Photogeneration of atomic hydrogen in rare gas matrices
Authors: Eloranta Jussi , Vaskonen Kari , Kunttu Henrik
Published: July 2002

Photodissociation of HCl and HBr upon excitation on their repulsive A 1Π states is studied in low-temperature Ar, Kr, and Xe matrices at photon energies of 5.0 and 6.4 eV. The dissociation is followed by fluorescence spectroscopy and electron paramagnetic resonance. In Ar matrix dissociation can be considered as a local event with simple first-order kinetics and 100% conversion efficiency of the precursor into isolated hydrogen atoms. In Kr matrix the conversion efficiency varies from 18% in 1:500 matrix to 100% in 1:8000 matrix. In Xe matrix the obtained H atom number density is extremely low and prevents detailed analysis of the photogeneration dynamics. The observed behavior is ascribed to long-range dissociation followed by efficient bimolecular reactive loss channels, and thus supports the previous findings by LaBrake, Ryan, and Weitz [J. Chem. Phys. 102, 4112 (1995)]. Molecular dynamics simulations based on a simplified model for dissociation are carried out. The initial 2.6 eV excess kinetic energy of the excited H atom is relaxed as local heating in Ar matrix, whereas in Kr and Xe matrices the excess energy is directed to long-range mobility with flight distances up to 40 Å.
Kinetic mechanism for condensation from supersaturated vapors that incorporate high levels of dimers
Authors: Bauer S. H. , Zhang Yi-Xue , Wilcox C. F.
Published: July 2002

The kinetic molecular mechanism (KMM) for the onset of avalanche condensation from supersaturated vapors, described in this report, is more general than the one we used previously. While the expressions for the rate constants needed to compute the steady-state populations are the same, additional steps were introduced to take into account cluster growth via accretions with dimers. In the present version, the magnitudes for the stepwise enthalpy decrements are assigned using all the “best” available experimental and computed values, so that the program requires less arbitrary adjustable parameters. The derived values for the entropy decrements upon successive accretions present a basis for estimating the total entropy of clusters (size n), and thus impose constraints on any model for dynamics within clusters. For high ambient pressures an option is provided for correcting the condensation rate constants due to restricted diffusion of accreting units (monomers or dimers) into the reaction shell of the growing clusters. Published data for condensation flux levels in supersaturated methanol are reanalyzed to illustrate the use of the newly developed code.
The reaction ensemble method for the computer simulation of chemical and phase equilibria. II. The Br2+Cl2+BrCl system
Authors: Lı́sal Martin , Nezbeda Ivo , Smith William R.
Published: July 2002

The reaction ensemble Monte Carlo (REMC) method [W. R. Smith and B. Třı́ska, J. Chem. Phys. 100, 3019 (1994)] is used to study combined reaction and vapor–liquid equilibrium of the Br2+Cl2+BrCl system. The substances are modeled as nonpolar and dipolar two-site Lennard-Jones molecules with Lorentz–Berthelot mixing rules for unlike atoms. No parameters were fitted to any mixture properties in our calculations. The simulated data are compared with experimental results, and with previous simulation data for the mixture obtained by an indirect semigrand ensemble approach. The REMC method efficiently calculates the complete phase compositions, whereas only a limited subset is available experimentally. The agreement of the simulations with experiment is good. In the course of this work, we used the Gibbs ensemble Monte Carlo method (which may be regarded as a special case of the REMC method) to calculate the vapor–liquid equilibrium properties of pure BrCl; since this compound is chemically unstable, such data is experimentally inaccessible.
The unimolecular dissociation of HCO. IV. Variational calculation of Siegert states
Authors: Keller Hans-Martin , Schinke Reinhard
Published: July 2002

We present a new variational method for calculating complex resonance (Siegert) states in unimolecular dissociation reactions. The approach is based on the log-derivative version of the Kohn variational principle. The basic matrix equations can be formulated in terms of scattering-wave boundary conditions yielding the resonance states as homogeneous solutions for singular energies. Thus, the resonance positions and widths can be calculated directly without employing an artificial absorbing potential. This method is applied to the photodissociation of HCO and DCO and the results are compared to the resonance parameters obtained from absorption-type spectra.
Rotational dynamics of pyrrolopyrrole derivatives in alcohols: Does solute–solvent hydrogen bonding really hinder molecular rotation?
Authors: Dutt G. B. , Srivatsavoy V. J. P. , Sapre A. V.
Published: July 2002

Rotational reorientation times of two structurally similar nonpolar molecules, 2,5dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and 1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP) have been measured in n-alcohols using steady-state fluorescence depolarization technique. While both DMDPP and DPP contain two C=O groups, the latter also has two NH groups. As these groups are known to form hydrogen bonds with alcohol solvents, the present work is aimed at finding out whether or not such solute–solvent hydrogen bonding is effecting the rotation of the probe molecules. The rotational dynamics of DMDPP is explained reasonably well by the Stokes–Einstein–Debye (SED) hydrodynamic theory with slip boundary condition. It is also found that the hydrogen bonding between the two C=O groups of the probe and the solvent molecules is not influencing the rotation of DMDPP. However, the reorientation times of DPP are found to be longer by a factor of 2.2 to 3.3 compared to that of DMDPP, and followed a super-stick behavior which has been observed for the first time for a nonpolar solute molecule. This is due to the strong hydrogen bonding between the two NH groups of the probe, and the alcoholic solvent molecules.
Asymmetric growth in micelles containing oil
Authors: Nelson Peter H. , Hatton T. Alan , Rutledge Gregory C.
Published: July 2002

The effect of oil on the equilibrium microstructure of a dilute micellar solution is investigated using lattice Monte Carlo techniques. Dramatic growth of the self-assembled micelles into elongated worms is observed as oil is solubilized within the oily tail region of the micelles. These results confirm a microscopic explanation for experimentally observed changes in the phase behavior of surfactant solutions as oil is added. In these simulations, a two-box simulation technique is used to guarantee that the micellar system is stable with respect to emulsification failure (the appearance of a bulk oil phase).
Heterogeneous nucleation of a monomer gas on a growing gas-phase polymer
Authors: Seok Chaok , Oxtoby David W.
Published: July 2002

We present a new theory for the nucleation of gas-phase monomers in a supersaturated vapor on a growing short-chain polymer. Reiss and co-workers have shown that cloud chambers can be extraordinarily sensitive detectors of single molecules of particular size in such experiments, giving valuable information about rate constants for gas-phase polymerization. We have used a density functional approach to calculate the free energy and heterogeneous nucleation rate of liquid drops of monomer on oligomer chains of varying length. We show that the free energy is not a simple monotonic function of chain length as assumed in simple classical theories.
Small-angle scattering of interacting particles. III. D2O-C12E5 mixtures and microemulsions with n-octane
Authors: Brunner-Popela J. , Mittelbach R. , Strey R. , Schubert K.-V. , Kaler E. W. , Glatter O.
Published: July 2002

Binary and ternary mixtures of C12E5, n-octane, and D2O were examined by neutron scattering experiments. The scattering data are evaluated using the generalized indirect Fourier transformation, GIFT, to determine the particle shape and structure in real space. This recently developed technique allows for the simultaneous determination of the form factor and the structure factor with a minimum of a priori information. All microemulsion data, both on the water-rich side and on the oil-rich side, can be evaluated using the whole range of scattering data to determine the form factor and the structure factor in one procedure. By example of the binary water-surfactant mixture known to contain elongated particles, we were able to show the great advantage in using the GIFT instead of other techniques. Suppression of the interaction effects during the evaluation procedure by eliminating data at low angles leads to incorrect results because elongated particles appear to be globular. The GIFT, however, allows us to distinguish between intra- and interparticle scattering contributions so that the correct elongated shape of the particles is recovered from the scattering data. Therefore, the GIFT can be regarded as the method of choice to determine size, shape, and internal structure of colloidal particles with high accuracy in the size range of 1 to 100 nanometers up to volume fractions of about 0.3.
Vibrational dephasing of an anharmonic solute strongly coupled to solvent
Authors: Williams Ryan B. , Loring Roger F.
Published: July 2002

We present new results for an old model: an anharmonic solute linearly coupled to a harmonic bath. A comparison of numerical simulations of the classical mechanical absorption spectrum to the results of conventional perturbation theory and the instantaneous normal mode (INM) approximation shows that the INM performs poorly in the low-friction regime, but yields reasonable results in the high-friction regime, in which perturbation theory is inappropriate. An analytical theory for the line shape is formulated to describe the regime of intermediate friction, in which neither perturbation theory nor the INM approximation works well.
Photodissociation of HCl adsorbed on the surface of an Ar12 cluster: Nonadiabatic molecular dynamics simulations
Authors: Niv Masha Y. , Krylov Anna I. , Gerber R. Benny , Buck Udo
Published: July 2002

The photodissociation of HCl adsorbed on the surface of an Ar12 cluster is studied by semiclassical molecular dynamics simulations, using a surface-hopping approach for the nonadiabatic transitions. The DIM method is used to construct the 12 potential energy surfaces that are involved, and the nonadiabatic couplings. The results are compared with previous studies on HCl embedded inside Ar clusters and on the triatomic Ar–HCl cluster. The main findings are the following: (1) There is a yield of about 1% for recombination onto the ground electronic state of HCl, roughly the same as for HCl embedded inside Ar12. (2) Photodissociation lifetimes much longer than for Ar–HCl are found. (3) The kinetic energy distribution of the H atom shows large energy transfer to the cluster, greater than in the case of HCl in the embedded geometry in (Ar)12HCl. (4) An interesting mechanism leads to the formation of some fraction of very “hot” Cl atoms. (5) About 10% of the Cl is left trapped in (Ar)mCl clusters. (6) The branching ratio P1/2:P3/2 for the Cl atoms that leave the cluster shows electronic cooling compared to the isolated HCl molecule case. The results throw light on the role of local geometry in photodissociation/recombination processes, and in particular on the mechanisms pertinent in the case of surface-adsorbed species. The nature of the results, showing strong cage effects at the surface geometries is to a large extent a consequence of the encapsulation of the H atom, obtained for the structure of the (Ar)12HCl cluster.
Comparison of structural properties of different polymer network types as obtained by computer simulation
Authors: Lay Stefan , Sommer Jens-Uwe , Blumen Alexander
Published: July 2002

Using large-scale Monte Carlo simulations we study the properties of randomly cross-linked ordered AB diblock copolymer melts as a function of the A-B repulsion parameter. We compare the results to those for cross-linked homopolymer melts and for disordered diamondlike networks. We analyze the structure of the resulting networks using measures we borrowed from the theory of fractals. These are the chemical dimension obtained by counting the number of neighbors in a shell for a prescribed chemical distance (counted along the bonds) and a dimension which characterizes the relation between the masses of network clusters and their radius of gyration in 3D. Modified diamondlike topologies can only partly reproduce the structural properties of randomly cross-linked chains, which show a large crossover region with dimensions smaller than 3. A decrease in the A-B repulsion leads to the loss of order. Cross linking can only partly stabilize the ordered structures; in a large region of cross-link densities above the gel point the removal of the interaction still leads to structural disorder.
A Monte Carlo simulation of polymer/polymer interface in the presence of block copolymer. I. Effects of the chain length of block copolymer and interaction energy
Authors: Kim Seung Hyun , Jo Won Ho
Published: July 2002

A Monte Carlo simulation for the segregation behavior of symmetric diblock copolymer at the interface between immiscible homopolymers was performed on a simplified lattice model. The simulation results show that the segregation behavior of block copolymer is significantly dependent on the molecular parameters, such as the chain length Nc of block copolymer and the interaction energy εAB between A and B segments. In a weakly repulsive system with εAB=0.1, the block copolymers are more strongly segregated at the interface with increasing the chain length when the block length is shorter than and comparable to the homopolymer length. However, when the block length becomes longer than the homopolymer length, the concentration of copolymer segments at the interface rather decreases and its distribution becomes broadened. In a strongly repulsive system with εAB=0.5, the distribution of copolymer segments at the interface becomes broader as the chain length of block copolymer increases. This behavior is closely related to the stretching and orientation of block copolymer chains at the interface and to the extent of penetration of homopolymers into the copolymer layer. These simulation results are in excellent agreement with the numerical predictions by the self-consistent mean field theory.
Competitive fragmentation and electron loss kinetics of photoactivated silver cluster anions: Dissociation energies of Agn− (n=7–11)
Authors: Shi Yang , Spasov Vassil A. , Ervin Kent M.
Published: July 2002

The photodecomposition kinetics of silver cluster anions, Agn− (n=7–11), has been investigated. The time-resolved intensities of Agn− parent ions and Agn−1− and Agn−2− photofragment product ions are measured following excitation with visible laser radiation, 415–750 nm. The atom-loss and dimer-loss product yields and reactant cluster ion depletion are compared to elucidate the decomposition kinetics of photoexcited silver cluster anions, including electron loss channels. Both prompt, direct electron photodetachment and delayed, statistical electron emission are observed in competition with cluster fragmentation product channels for some clusters. Dissociation threshold energies are determined by fitting the measured time profiles for fragmentation products using a statistical unimolecular dissociation model. The photodissociation lifetime method for measuring cluster dissociation energies is compared with previous energy-resolved collision-induced dissociation experiments on silver cluster anions.
The electronic spectrum, molecular structure, and oscillatory fluorescence decay of jet-cooled germylidene (H2C=74Ge), the simplest unsaturated germylene
Authors: Hostutler David A. , Smith Tony C. , Li Haiyang , Clouthier Dennis J.
Published: July 2002

The electronic spectrum of germylidene (H2C=Ge), the simplest unsaturated germylene, has been observed for the first time. Jet-cooled H2CGe and D2CGe were produced by an electric discharge through tetramethylgermane diluted in argon at the exit of a supersonic expansion. High-resolution spectra of H2C74Ge and D2C74Ge, obtained from (CH3)474Ge prepared from isotopically enriched Ge74 metal, have been rotationally analyzed to yield the following r0 structures: r0″(CGe)=1.7908(2) Å, r0″(CH)=1.1022(5) Å, θ0″(HCH)=115.05(5)°, r0′(CGe)=1.914(4) Å, r0′(CH)=1.082(9) Å, and θ0′(HCH)=139.3(11)°. The 367–354 nm B̃1B2–X̃ 1A1 band system consists of prominent perpendicular bands involving the CGe stretching (ν3) and CH2 scissors (ν2) vibrations and a weaker series of vibronically induced parallel bands involving the CH2 rocking mode (ν6). Vibronic bands involving Δv=2 changes in ν6(b2) and ν4(b1) have also been assigned. The fluorescence decays of single rotational levels of the 000 band of H2C74Ge exhibit molecular quantum beats for about 70% of the levels surveyed. Density of states arguments reveal that most of the beats originate from interactions with high rovibronic levels of the ground state. In one case, hyperfine splittings in the Fourier transform of the beat pattern indicate an accidental coincidence with an excited triplet state level. The less frequent occurrence of quantum beats in germylidene compared to silylidene, where they are almost universal, can be attributed to the smaller density of ground state levels at the zero-point energy of the S2 state in the former.
The effect of counterpoise correction and relaxation energy term to the internal rotation barriers: Application to the BF3⋯NH3 and C2H4⋯SO2 dimers
Authors: Salvador Pedro , Duran Miquel
Published: July 2002

The relevance of the fragment relaxation energy term and the effect of the basis set superposition error on the geometry of the BF3⋯NH3 and C2H4⋯SO2 van der Waals dimers have been analyzed. Second-order Mo/ller–Plesset perturbation theory calculations with the d95(d,p) basis set have been used to calculate the counterpoise-corrected barrier height for the internal rotations. These barriers have been obtained by relocating the stationary points on the counterpoise-corrected potential energy surface of the processes involved. The fragment relaxation energy can have a large influence on both the intermolecular parameters and barrier height. The counterpoise correction has proved to be important for these systems.
Dynamical nucleation theory: Calculation of condensation rate constants for small water clusters
Authors: Kathmann Shawn M. , Schenter Gregory K. , Garrett Bruce C.
Published: July 2002

In previous work we began the description of a molecular theory of homogeneous vapor-to-liquid nucleation based on the kinetics of cluster formation and decomposition. In this work we focused on a new theoretical approach to calculating rate constants for evaporation of molecules from clusters. In the present work, we present a molecular theory for calculating condensation rate constants that are consistent with the evaporation rate constants. The new method, which uses variational transition state theory (VTST), provides an expression for the evaporation rate constant that is proportional to the derivative of the Helmholtz free energy for cluster formation with respect to the radius of the spherical volume constraining the cluster. Furthermore, the theory provides a physically justified procedure for selecting a unique value of the radius of the spherical volume for each i-molecule cluster. Since VTST obeys detailed balance, condensation rate constants can be obtained from the evaporation rate constants and the corresponding equilibrium constants. In the present work, we provide a theoretical approach to obtain the equilibrium constants that are consistent with the evaporation rate constants. Monte Carlo methods are presented for calculating the dependence of the Helmholtz free energy of cluster formation on the radius of the constraining volume, which are needed for the evaporation rate constants. In addition, Monte Carlo methods are presented for calculating the relative differences in Helmholtz free energies for clusters of different sizes, which are needed for the equilibrium constants and condensation rate constants. The volume dependent Helmholtz free energies for the water dimer up to the decamer are calculated at 243 K.
Nuclear magnetic resonance line shapes of methyl-like quantum rotors in low-temperature solids
Authors: Szymański S.
Published: July 2002

Dissipative dynamics of a tunneling, methyl-like rotor, whose spatial coordinate is weakly coupled to a thermal bath, are described using the reduced density matrix (RDM) approach. It is found that, owing to selection rules imposed on thermally induced transitions by the symmetrization postulate, there are two sorts of coherences between the rotor eigenstates that live long enough to be observed on the nuclear magnetic resonance (NMR) time scale. One comprises degenerate pairs of Kramers sublevels at sequential librational levels of the rotor. The other involves nearly degenerate pairs each of which engages one Kramers sublevel and the remaining sublevel, separated from the Kramers doublet by tunneling quantum. These are the coherences which are seen in the inelastic neutron scattering (INS) patterns of methyl-like rotors. From the RDM equation of motion, augumented with spin-dependent terms relevant in the presence of an external magnetic field, the NMR line shape equation is derived. With no loss of information it can be formulated in terms of only the spin degrees of freedom. Its dissipative part includes two rate constants that describe damping of the long-lived tunneling and Kramers coherences, respectively; coherent tunneling is represented in the Hamiltonian part by an apparent spin-spin coupling. These rate constants are the widths of the inelastic and quasielastic lines, respectively, in the INS spectra of methyl-like rotors; the apparent coupling constant is the shift of the inelastic line. This seems to be the first full exposition of the parallelism between INS and NMR images of tunneling rotors. Rationalization of previous findings involving a CD3 rotor was achieved by use of a simple model of rotor-bath couplings, combined with inferences from numerical simulations of NMR line shapes.
Measurement of the HD(v′=2,J′=3) product differential cross section for the H+D2 exchange reaction at 1.55±0.05 eV using the photoloc technique
Authors: Fernández-Alonso Félix , Bean Brian D. , Zare Richard N.
Published: July 2002

We describe a time-of-flight apparatus that uses core extraction to determine nascent product laboratory velocity distributions from which differential cross sections may be deduced. We emphasize the characterization of the instrument, the reaction conditions, and the calibration procedure. For this purpose, we have measured H-atom velocity distributions from HBr photolysis, as well as the H2(v′=4,J′=1) velocity distribution arising from the H+HBr reaction under quasi-monoenergetic collision conditions at 1.9 eV. Collisional energy spread and reagent internal state distributions were determined from the rotational and translational temperatures of the HBr photolytic precursor and the D2 diatomic reagent. The differential cross section for H+D2→HD(v′=2,J′=3)+D at 1.55±0.05 eV is presented and found to peak near 145°±10° with an approximate full width at half maximum (FWHM) of 40°.
Unexpectedly large O37ClO/O35ClO intensity ratios of the fluorescence from the low-energy vibrational levels of OClO (Ã 2A2)
Authors: Lim Goo-Il , Lim Soon-Mi , Kim Sang Kyu , Choi Young S.
Published: July 2002

The fluorescence intensity ratios of O37ClO/O35ClO excited to the Ã 2A2 state have been found to be much larger than that expected from the 37Cl/35Cl natural abundance ratio for several low-energy vibrational levels. For instance, the O37ClO/O35ClO ratios are 1.67 and >10 for the (0,0,0) and (1,1,0) bands, respectively, which are 5 and 30 times larger than those expected from the natural abundance ratio. These unusual isotope effects are observed in a narrow energy region within ∼1000 cm−1 above the electronic origin. It is likely that the isotopomer-selective, near-resonant spin-orbit coupling of the low-energy vibrational levels of the 2A2 state with those of the 2A1 state, which provides a decay channel for the 2A2 state via spin-orbit coupling, is responsible for the unusual isotope effect.
On the generalized Kramers problem with oscillatory memory friction
Authors: Reigada Ramon , Romero Aldo H. , Lindenberg Katja , Sancho José M.
Published: July 2002

The time-dependent transmission coefficient for the Kramers problem exhibits different behaviors in different parameter regimes. In the high friction regime it decays monotonically (“nonadiabatic”), and in the low friction regime it decays in an oscillatory fashion (“energy-diffusion limited”). The generalized Kramers problem with an exponential memory friction exhibits an additional oscillatory behavior in the high friction regime (“caging”). In this paper we consider an oscillatory memory kernel, which can be associated with a model in which the reaction coordinate is linearly coupled to a nonreactive coordinate, which is in turn coupled to a heat bath. We recover the nonadiabatic and energy-diffusion-limited behaviors of the transmission coefficient in appropriate parameter regimes, and find that caging is not observed with an oscillatory memory kernel. Most interestingly, we identify a new regime in which the time-dependent transmission coefficient decays via a series of rather sharp steps followed by plateaus (“stair-like”). We explain this regime and its dependence on the various parameters of the system.
Angular distributions of Ar reflected from molten metal surfaces
Authors: Muis A. , Manson J. R.
Published: July 2002

Recent experimental measurements of the angular distributions of monoenergetic beams of Ar atoms, with incident energies of up to 1 eV scattered from a molten In surface, are compared with calculated intensities. The data are described by classical scattering theory and the agreement indicates that the shapes of the lobes are dominated by single collisions with the surface, and the interaction potential has a smooth repulsive barrier similar to that commonly used to describe rare-gas scattering from crystalline metals. The attractive adsorption well of the interaction potential is considered, and is shown to have significant effects on the angular distributions at low incident energy. The dependence of the angular distributions on surface temperature is also well described.
New way of controlling molecular processes by time-dependent external fields
Authors: Teranishi Yoshiaki , Nakamura Hiroki
Published: July 2002

A new idea of controlling molecular processes by time-dependent external fields is proposed. Molecular processes in external fields are considered to be composed of a sequence of time-dependent nonadiabatic transitions in which the external fields play a role of adiabatic parameters. Unit final transition probability can be achieved with the use of the interference effects among various paths created by nonadiabatic transitions. The basic idea is to sweep the external field periodically at each avoided crossing and to control the transition there completely as we desire. This idea is quite general, and can hold whatever the external field is. Various control schemes can be proposed corresponding to the various types of time-dependent nonadiabatic transitions. The methods of π-pulse and chirped laser pulse with the adiabatic rapid passage may be considered as special cases of the present idea. As an example, a one-dimensional model of the laser-induced ring-puckering isomerization of trimethylenimine is considered, and comparative studies on the effectiveness and the stability of the various control schemes proposed in this paper are made together with presentation of numerical examples.
The effect of attractions on the structure of fused sphere chains confined between surfaces
Authors: Patra Chandra N. , Yethiraj Arun , Curro John G.
Published: July 2002

The effect of attractive interactions on the behavior of polymers between surfaces is studied using Monte Carlo simulations. The molecules are modeled as fused sphere freely rotating chains with fixed bond lengths and bond angles; wall–fluid and fluid–fluid site–site interaction potentials are of the hard sphere plus Yukawa form. For athermal chains the density at the surface (relative to the bulk) is depleted at low densities and enhanced at high densities. The introduction of a fluid–fluid attraction causes a reduction of site density at the surface, and an introduction of a wall–fluid attraction causes an enhancement of site density at the surface, compared to when these interactions are absent. When the wall–fluid and fluid–fluid attractions are of comparable strength, however, the depletion mechanism due to the fluid–fluid attraction dominates. The center of mass profiles show the same trends as the site density profiles. Near the surface, the parallel and the perpendicular components of chain dimensions are different, which is explained in terms of a reorientation of chains.
Rotational-resolved pulsed field ionization photoelectron study of NO+(a 3Σ+,v+=0–16) in the energy range of 15.6–18.2 eV
Authors: Jarvis G. K. , Song Y. , Ng C. Y.
Published: July 2002

We have obtained rotationally resolved pulsed field ionization photoelectron (PFI-PE) spectra of NO in the energy range of 15.6–18.2 eV, covering ionization transitions of NO+(a 3Σ+,v+=0–16,J+)←NO(X 2Π3/2,1/2,v″=0,J″). The PFI-PE bands for NO+(a 3Σ+,v+=1–5,7–10,12–14,16) obtained in this experiment represent the first rotationally resolved spectroscopic data for these states. The simulation of these PFI-PE bands provides accurate molecular constants for NO+(a 3Σ+,v+=0–5,7–10,12–14,16), including ionization energies, vibrational constants (ωe+=1295.2±0.1 cm−1, ωe+χe+=15.198±0.002 cm−1), and rotational constants (Be+=1.3501±0.0070 cm−1, αe+=0.0206±0.0001 cm−1). As observed in the PFI-PE study of NO+(X 1Σ+), this experiment reveals a generally increasing trend for the maximum ΔJ value and intensities of higher ΔJ branches as v+ (or bond distance) for NO+(a 3Σ+) is increased. This observation can be taken as strong support of the electron-molecular-ion-core scattering model for angular momentum and energy exchanges in the threshold photoionization of NO.
Lateral interactions in coadsorbate layers: Vibrational frequency shifts
Authors: Wang R. L. C. , Kreuzer H. J. , Jakob P. , Menzel D.
Published: July 2002

The effect of coadsorbed argon, hydrogen, and oxygen on the internal vibration of CO on Ru(001) has been studied by infrared absorption spectroscopy in order to disentangle electrostatic and chemical frequency shifts. Ar is expected to lead only to the former, H only to the latter, and O to a combination. In all cases, intermolecular interactions among CO molecules are avoided by working at very low CO coverages (0.01–0.03 ML). Interestingly, the observed frequency shifts are discrete rather than continuous which is attributed to a local interaction. Density functional calculations for suitable clusters have been used to model the frequency shifts, arriving at good agreement with experiment. Analysis of these theoretical results is then used to quantify the contributions of electrostatic fields and of chemical effects on these shifts. It is shown that, despite very different signatures of the various coadsorbate species, the observed C–O frequency shifts are largely of electrostatic origin, provided one uses the electrostatic field generated by the coadsorbate and not an effective constant field.