bluuebunny/crossref_metadata_2025 huggingface dataset:
Exotic transitions of random heteropolymers interacting with solid surfaces
Authors: Gutman Lorin , Chakraborty Arup K.
Published: July 2002

In many applications, it is desirable to control interfacial properties by employing adsorbed polymer layers. In this work, we study the adsorption of random heteropolymers and find a rich surface phase diagram that suggest ways in which the properties of the adsorbed layers can be controlled rather precisely by manipulating the physical conditions. Specifically, we present a comprehensive field-theoretic analysis of the surface phase diagram of a solution of random heteropolymers interacting with a chemically homogeneous solid surface, and find many surface transitions that may be exploited in applications. The different types of polymer segments interact with the solid surface in arbitrarily different ways. Our analysis, wherein a replica method is employed to average over the quenched sequence fluctuations, allows us to obtain the surface free energy functionals that show that our problem partially resembles a semi-infinite Ising spin system. Thus, akin to the Ising system, the phase diagram exhibits exotic surface transitions. In the infinitely dilute limit we find four ‘‘massless’’ transition lines: the ordinary (OT), the surface (ST), the extraordinary (ET), and the special (SPT) transition. At finite bulk solution concentration, we find two transitions; viz. the OT and the adsorption–depletion (ADT) transitions. The nature of the critical points that reside on the transition lines are analyzed, and the physical meaning of each of the surface transitions is elucidated. Our results are related to experiments and it is shown that the interesting behavior that random heteropolymers exhibit near surfaces is due to the quenched sequence fluctuations.
Treatment of the outlying charge in continuum solvation models
Authors: Klamt Andreas , Jonas Volker
Published: July 2002

Continuum solvation models have proven to yield very valuable information about solvation effects, if cavities close to the van der Waals surface of the solutes are used for the calculation of the screening charges. Unfortunately, such cavity size implies that a small but significant portion of the solute electron density reaches out of the cavity. This outlying charge causes serious problems in the context of the dielectric continuum treatment of the solvent. The present paper presents a critical consideration of the origin, the magnitude, and the different strategies for treatment of this problem. Finally, a novel ansatz using an additional, outer cavity is presented which provides rather accurate correction of the corresponding error.
How does basis set superposition error change the potential surfaces for hydrogen-bonded dimers?
Authors: Simon Sílvia , Duran Miquel , Dannenberg J. J.
Published: July 2002

We describe a simple method to automate the geometric optimization of molecular orbital calculations of supermolecules on potential surfaces that are corrected for basis set superposition error using the counterpoise (CP) method. This method is applied to the H-bonding complexes HF/HCN, HF/H2O, and HCCH/H2O using the 6-31G(d,p) and D95++(d,p) basis sets at both the Hartree–Fock and second-order Mo/ller–Plesset levels. We report the interaction energies, geometries, and vibrational frequencies of these complexes on the CP-optimized surfaces; and compare them with similar values calculated using traditional methods, including the (more traditional) single point CP correction. Upon optimization on the CP-corrected surface, the interaction energies become more negative (before vibrational corrections) and the H-bonding stretching vibrations decrease in all cases. The extent of the effects vary from extremely small to quite large depending on the complex and the calculational method. The relative magnitudes of the vibrational corrections cannot be predicted from the H-bond stretching frequencies alone.
Orbital hardness matrix and Fukui indices, their direct self-consistent-field calculations, and a derivation of localized Kohn–Sham orbitals
Authors: Liu Guang-Hua
Published: July 2002

Formulas governing fixed orbital hardnesses and their relation to the hardness kernel are derived. It is shown how the orbital hardness matrix and its inverse matrix, the orbital softness matrix, may thus be directly calculated, and then the total chemical hardness, softness, and electronegativity of a molecular species. These quantities are calculated for the molecule HCN, using Dirac exchange and von Barth–Hedin correlation in the local density form of Kohn–Sham theory. The result complies with the frontier orbital theory. As quantitative indicators of orbital reactivity, the frontier orbital softness and Fukui indices generally have larger values than inner electron orbitals. The relation of orbital hardness matrix elements to the two-electron orbital integrals in a typical molecular orbital calculation is discussed, and it is demonstrated that diagonalization of the orbital hardness matrix leads to orbitals more localized than conventional Kohn–Sham orbitals.
Internal coordinate Hamiltonian model for Fermi resonances and local modes in methane
Authors: Halonen Lauri
Published: July 2002

A vibrational model which is based on a Hamiltonian expressed in terms of curvilinear internal coordinates is applied to the overtone spectrum of methane, CH4. Symmetrized internal coordinates and their conjugate momenta are used as the bending variables. The stretching part of the Hamiltonian is expressed in an unsymmetrized form. Both the kinetic operator and the potential energy function are expanded as Taylor series around the equilibrium configuration. Symmetrized local mode basis functions for the stretches and symmetrized two- and three-dimensional harmonic oscillator basis functions in the Cartesian representations for bending degrees of freedom are used. Only resonance couplings are taken into account. Apart from some standard diagonal contributions harmonic oscillator matrix elements have been employed. This results in a simple block diagonal Hamiltonian model. The nonlinear least squares method is used to optimize model parameters for 12CH4. Observed vibrational term values up to 6050 cm−1 are included as data. Potential energy parameters obtained from the Hamiltonian parameters agree well with a previously published anharmonic force field calculation. A unitary transformation between internal coordinate and normal coordinate representations is found to provide simple interpretations for the standard normal mode theory based spectroscopic parameters.
The orientation of acetate on a TiO2(110) surface
Authors: Guo Q. , Cocks I. , Williams E. M.
Published: July 2002

The adsorption of acetic acid on a TiO2(110) surface has been studied using electron stimulated desorption ion angular distribution (ESDIAD) and low energy electron diffraction (LEED). The acetate intermediates arising from the dissociative adsorption of acetic acid form an ordered (2×1) overlayer at saturation coverage. The H+ ESD ion angular distributions can be resolved into two contributions: Those ions desorbing from hydrogen atoms bonded at the oxide substrate, and those ions desorbed via the rupture of the C–H bonds of the acetate. The geometry of the ESDIAD pattern led us to propose that the acetates are bridge bonded with the five-fold coordinated Ti 4+ ions, with their molecular plane perpendicular to the surface. Decomposition of acetate at room temperature occurs under electron beam irradiation, resulting in the desorption of CH2CO and CH3/CH4.
Tracing the phase boundaries of hard spherocylinders
Authors: Bolhuis Peter , Frenkel Daan
Published: July 2002

We have mapped out the complete phase diagram of hard spherocylinders as a function of the shape anisotropy L/D. Special computational techniques were required to locate phase transitions in the limit L/D→∞ and in the close-packing limit for L/D→0. The phase boundaries of five different phases were established: the isotropic fluid, the liquid crystalline smectic A and nematic phases, the orientationally ordered solids—in AAA and ABC stacking—and the plastic or rotator solid. The rotator phase is unstable for L/D⩾0.35 and the AAA crystal becomes unstable for lengths smaller than L/D≈7. The triple points isotropic-smectic-A-solid and isotropic-nematic-smectic-A are estimated to occur at L/D=3.1 and L/D=3.7, respectively. For the low L/D region, a modified version of the Gibbs–Duhem integration method was used to calculate the isotropic-solid coexistence curves. This method was also applied to the I-N transition for L/D>10. For large L/D the simulation results approach the predictions of the Onsager theory. In the limit L/D→∞ simulations were performed by application of a scaling technique. The nematic-smectic-A transition for L/D→∞ appears to be continuous. As the nematic-smectic-A transition is certainly of first order nature for L/D⩽5, the tri-critical point is presumably located between L/D=5 and L/D=∞. In the small L/D region, the plastic solid to aligned solid transition is first order. Using a mapping of the dense spherocylinder system on a lattice model, the initial slope of the coexistence curve could even be computed in the close-packing limit.
The conformation of hard-sphere polymers in hard-sphere solution calculated by single-chain simulation in a many-body solvent influence functional
Authors: Grayce Christopher J.
Published: July 2002

A fully many-body solvent influence functional for hard-sphere polymer chains in hard-sphere solvents is derived from scaled particle theory. Monte Carlo simulation of single chains in this “solvation potential” is used to calculate conformational properties of 3mers, 4mers and 10mers dissolved in 1mers. Comparison is made to the exact results of Monte Carlo simulation of the whole polymer+solvent fluid, and with the predictions of pairwise-additive integral-equation-based solvation potentials. The greater accuracy of the many-body potential suggests that at least in these systems solvent influence on polymer conformation is a strongly many-body phenomenon.
Symmetric diblock copolymers under large amplitude oscillatory shear flow: Dual frequency experiments
Authors: Zhang Yuanming , Wiesner Ulrich
Published: July 2002

Conceptually new experiments are described in which the effects of the superposition of two large amplitude oscillatory shear components on the morphology of a lamellar PS-b-PI diblock copolymer of low molecular weight close to TODT are investigated. First results of such dual frequency experiments are presented for the coupling of two shear components from the lower and intermediate frequency regimes I and II of the shear viscosity leading to parallel and perpendicular orientation behavior, respectively, in single frequency experiments. The employed frequencies were separated by more than two orders of magnitude, rendering possible coupling terms unimportant. Using two-dimensional small-angle x-ray scattering (2D-SAXS) measurements, it is shown that for large rim strain values the perpendicular orientation mechanism completely governs the behavior. In experiments where the applied strain for shear component II leading to perpendicular orientation is significantly reduced with respect to that of component I leading to parallel orientation, a characteristic flip from parallel to perpendicular orientation is observed along the radius of the sample disk. It is further demonstrated that the position of the flipping point along the radius can be altered by varying the amplitude of deformation component II. In these experiments regions of parallel as well as perpendicular orientation exhibiting high order parameters can be achieved. The results are interpreted using concepts for the orientation mechanisms developed in earlier single frequency studies, thereby corroborating the underlying ideas. Furthermore, the results of the dual frequency experiments supply valuable information concerning the competition between different orientation mechanisms not directly obtained from single frequency experiments.
Anomalous clustering and equation-of-state behavior as the adhesive-disk limit is approached
Authors: Bors̆tnik B. , Jesudason C. G. , Stell G.
Published: July 2002

The structure of clusters of two-dimensional particles interacting with a hard-core square-well pair potential is analyzed in order to demonstrate how thermodynamic stability ceases to exist when the system approaches the Baxter’s sticky limit. It is shown that the dependence of the sizes and the radii of gyration of the clusters of two-dimensional sticky disks behave qualitatively differently when the number of particles in the cluster exceeds 6. Cluster sizes with n<7 exhibit smooth transition when the Baxter limit is approached while clusters with seven more disks experience an anomalous transition towards a state of maximal connectedness. The configuration integrals that are needed to describe clusters of seven particles are then used to demonstrate the way in which their contribution to the virial expansion of the equation of state causes it to become pathological. From the results, it is concluded that the system will show the anomalous thermodynamic behavior associated with the approach to instability only for well widths of the order of 10−4 of the hard core diameter or less. The status of using approximate adhesive-particle results as approximate square-well-particle results is then summarized. It is pointed out that there is a fundamental defect in all currently available methods of approximating square-well liquid-gas phase separation using adhesive-sphere results.
The band edge luminescence of surface modified CdSe nanocrystallites: Probing the luminescing state
Authors: Kuno M. , Lee J. K. , Dabbousi B. O. , Mikulec F. V. , Bawendi M. G.
Published: July 2002

We study the luminescence of surface modified CdSe nanocrystallites. There has been much speculation as to the origin of the band edge emission in these quantum confined structures. Because of their large surface to volume ratios it has been suggested that the emission originates from surface-related states. However, recent theory suggests that the band edge luminescence arises from an optically inactive fine structure state or “dark” exciton. To address this issue we modify the surface of CdSe nanocrystallites with a variety of organic and inorganic ligands. We then monitor the effect changing the surface has on the energetics of the band edge luminescence through photoluminescence and fluorescence line narrowing experiments. Our results are compared with theoretical predictions for the nonresonant and resonant luminescence. We find good agreement between experiment and theory for CdSe nanocrystallites passivated with trioctylphosphine oxide, ZnS, 4-picoline, 4-(trifluoromethyl)thiophenol, and tris(2-ethylhexyl)phosphate. The lack of dependence of our data on surface modification is consistent with a dark exciton description of the band edge luminescence.
Spectroscopy and ultrafast dynamics of the 2A1 state of Z-hexatriene in gas phase
Authors: Fu\S W. , Schikarski T. , Schmid W. E. , Trushin S. A. , Hering P. , Kompa K. L.
Published: July 2002

As in longer polyenes, the strong 1A1→1B2 band in the UV spectrum of 1,3Z,5-hexatriene (Z-hexatriene) has a weak precursor, the 1A1→2A1 transition. It was measured in this work by a conventional spectrometer. Whereas the wave number of the 0–0 transition is by 5400 cm−1 lower than that of the 1B2 origin, the vibrational contour indicates that the vertical transitions of thetwo bands nearly coincide. From the fast decay of the rotational anisotropy in the time-resolved measurements we conclude that this band is perpendicularly polarized. We measured the lifetime of the 2A1 state after pumping it directly by 250 fs pulses and probing the excited molecules by ionizing it by delayed pulses. The lifetimes decreased from several ps to 730 fs, when the excess energy was increased from near 0 to 4000 cm−1 and more. From the temperature dependence we infer a barrier of about 170 cm−1 (2 kJ/mol).
Geometries and energy separations of electronic states of AsCl2, AsBr2, AsCl2+, and AsBr2+
Authors: Latifzadeh-Masoudipour Lida , Balasubramanian K.
Published: July 2002

Spectroscopic properties of the low-lying 2B1, 2A1, 2A2, 4A2, 2B1(II), 4B1, 2B2(I), 2B2(II) electronic states of AsCl2 and AsBr2 have been studied with the complete active space self-consistent field (CASSCF) followed by multireference singles and doubles configuration interaction (MRSDCI) methods. In addition, the three low-lying electronic states of AsCl2+ and AsBr2+, (1A1,3B1,1B1) and the ground states of AsCl and AsBr were investigated. The bond dissociation energies of AsCl2, AsBr2, AsCl, and AsBr have been computed at the CASSCF/MRSDCI and CASSCF/second order configuration interaction (SOCI) levels as De(ClAs–Cl)=2.56 eV, De(BrAs–Br)=1.96 eV, De(As–Cl)=2.68 eV, and De(As–Br)=2.26 eV. The adiabatic ionization energies obtained in this work are 8.40 eV for AsCl2 and 8.33 eV for AsBr2. All these species have been found to possess bent geometries at their ground electronic states.
Theoretical study of geometrical and electronic structures of various thiophene-based tricyclic polymers
Authors: Hong Sung Y. , Song Jung M.
Published: July 2002

A theoretical study of a variety of tricyclic polymers [–(C8H2X2Y)n–] with two different types of bridging groups was performed, X=S and Y=CH2, SiH2, C=O, C=S, or C=CH2 for the fused bithiophene system and vice versa for the thieno-bicyclic system. These two types of the bridging groups are different from each other in that S favors the aromatic form of a cyclic polymer and the other groups prefer the quinonoid form. Geometrical structures of the polymers were obtained from semiempirical self-consistent-field (SCF) band calculations and the electronic properties from the modified extended Hückel band calculations. It is found that the ground-state geometrical structures of the tricyclic polymers are determined by the bridging groups in the outer rings. That is, the fused bithiophene system is aromatic in the ground state and the thieno-bicyclic system is quinonoid. The ground-state band gaps (which correspond to the absorption peaks of π–π* band transition) of the polymers were estimated to be in the range of 0.7–2.0 eV. The band gaps were analyzed in terms of the bond-length alternation along the conjugated carbon backbone, the C1–C4 interactions, and the electronic effect of the bridging groups. We also investigated the geometrical and electronic structures of polydicyanomethylene-cyclopenta-dicyclopentadiene (PDICNCY). Unlike the theoretical predictions of Toussaint and Bredas [Synth. Met. 69, 637 (1995)], PDICNCY in the ground state was estimated to be of the quinonoid form and to possess a large band gap (2.55 eV) comparable with the gap of polythiophene.
Alkaline earth metals (Ca, Sr) attached to liquid helium droplets: Inside or out?
Authors: Stienkemeier F. , Meier F. , Lutz H. O.
Published: July 2002

Superfluid helium droplets HeN at 0.4 K are doped with single alkaline earth atoms (Ca, Sr). The absorption spectra for excitation of the lowest singlet transitions display broad, blue-shifted peaks if compared to the atomic lines. The shift increases with droplet size increasing from N≈1000 up to N≈10000 but converges to only about one third of the bulk value.
Experimental determination of quantum state resolved differential cross sections for the hydrogen exchange reaction H+D2→HD+D
Authors: Schnieder L. , Seekamp-Rahn K. , Wrede E. , Welge K. H.
Published: July 2002

We have carried out a systematic crossed molecular beam study of the hydrogen exchange reaction in the H+D2→HD+D isotopic form at two collision energies: 0.53 and 1.28 eV. The Rydberg atom time-of-flight method was used to measure the D-atom product angle-velocity distribution. For the first time ro-vibrational quantum state resolved differential cross sections for the title reaction were measured, which can directly be compared to theoretical predictions at this detailed level. Experimental results are compared to theoretical predictions from both quasi classical and quantum mechanical calculations on different potential energy surfaces as well as to earlier experiments. A general good agreement is found for the converged quantum mechanical calculations with indications that the Boothroyd-Keogh-Martin-Peterson potential energy surface is better suited to describe the dynamics of the reaction. For the higher collision energy the quasi classical trajectory calculations reproduce the experimental data quite well, whereas they fail to describe the situation at the lower collision energy especially with respect to angular resolved differential cross sections.
Micellar aggregates near the isotropic-cubic liquid crystal phase transition
Authors: Castelletto Valeria , Itri Rosangela , Amaral Lia Q.
Published: July 2002

Micellar solutions of the palmitoyl-lyso-phosphatidyl choline (PLPC)/water system are studied by small angle x-ray scattering (SAXS) from the diluted isotropic (I) phase up to the concentrated I phase near the I→cubic (Q223) phase transition, for concentrations C in the range 5⩽C⩽37.5 wt % of PLPC. The particle distance distribution function p(r) is calculated for the less concentrated solution. A model for the product P(q)S(q), where P(q) is the micellar form factor and S(q) is the intermicellar interference function calculated using a hard sphere interaction potential, is fitted to the SAXS curves. It is found that PLPC micelles remain with a prolate ellipsoidal shape of constant anisometry ν=(1.80±0.05) in the whole concentration range. Such finding contrasts whit that found for systems that show I→hexagonal (H) phase transitions. In the latter, similar SAXS curves revealed that micelles grow to slightly higher anisometry (ν∼2.4). Results are analysed in terms of a recent model based on the micellar elastic bending energy, through the surfactant parameter p0=vpar/alc(vpar and lc are the volume and the critical length of the paraffin chain, a the polar head surface area). It is concluded that the formation of the Q223 phase requires that micelles remain spheroidal and avoid the spherocylindrical shape.
Molecular-dynamics simulation of moisture diffusion in polyethylene beyond 10 ns duration
Authors: Fukuda Mitsuhiro , Kuwajima Satoru
Published: July 2002

Molecular-dynamics (MD) simulation of the diffusion of water in amorphous polyethylene has been performed. In order to obtain information about the fundamental diffusion behavior of water, the constant particle number, constant pressure, and constant temperature (NPT) MD was employed for three model systems differing in the number of water molecules and methylene group in the polyethylene chains. Time duration beyond 10 ns showed that the water molecules came together forming water clusters of various sizes. Although the self-diffusion coefficient Dself estimated from the mean-square displacement curve scattered considerably in the order from 10−8–10−5 cm2/s, Dself for the water cluster is generally one order lower in magnitude than that of a single water. However, the linearity of the mean-square curve for the larger system which contained 4000 carbon atoms was very good, and Dself obtained for ten water molecules up to 11 ns duration showed 7.1×10−7 cm2/s which was in good agreement with the experimental value.
Lamellar biogels comprising fluid membranes with a newly synthesized class of polyethylene glycol-surfactants
Authors: Warriner Heidi E. , Davidson Patrick , Slack Nelle L. , Schellhorn Matthias , Eiselt Petra , Idziak Stefan H. J. , Schmidt Hans-Werner , Safinya Cyrus R.
Published: July 2002

A series of four polymer–surfactant macromolecules, each consisting of a double-chain hydrophobic moiety attached onto a monofunctional polyethylene glycol (PEG) polymer chain, were synthesized in order to study their effect upon the fluid lamellar liquid crystalline (Lα) phase of the dimyristoylphosphatidylcholine/pentanol/water system. The main finding of this study is that the addition of these compounds induces a new lamellar gel, called Lα,g. We have determined the phase diagrams as a function of PEG–surfactant concentration, cPEG, and weight fraction water, ΦW. All phase diagrams are qualitatively similar and show the existence of the gel. Unlike more common polymer physical gels, this gel can be induced either by increasing cPEG or by adding water at constant cPEG. In particular, less polymer is required for gelation as water concentration increases. Moreover, the gel phase is attained at concentrations of PEG–surfactant far below that required for classical polymer gels and is stable at temperatures comparable to the lower critical solution temperature of free PEG–water mixtures. Small angle x-ray experiments demonstrate the lamellar structure of the gel phase, while wide angle x-ray scattering experiments prove that the structure is Lα, not Lβ′ (a common chain-ordered phase which is also a gel). The rheological behavior of the Lα,g phase demonstrates the existence of three dimensional elastic properties. Polarized light microscopy of Lα,g samples reveals that the Lα,g is induced by a proliferation of defect structures, including whispy lines, spherulitic defects, and a nematiclike Schlieren texture. We propose a model of topological defects created by the aggregation of PEG–surfactant into highly curved regions within the membranes. This model accounts for both the inverse relationship between ΦW and cPEG observed along the gel transition line and the scaling dependence of the interlayer spacing at the gel transition with the PEG molecular weight. These Lα hydrogels could serve as the matrix for membrane-anchored peptides, proteins or other drug molecules, creating a “bioactive gel” with mechanical stability deriving from the polymer–lipid minority component.
Cooperativity or phase transition? Unfolding transition of DNA cationic surfactant complex
Authors: Mel’nikov Sergey M. , Sergeyev Vladimir G. , Yoshikawa Kenichi , Takahashi Hiroshi , Hatta Ichiro
Published: July 2002

We recently reported that single duplex DNA, with the size above the order of several tens kilobase pairs, undergoes a large discrete transition from an elongated coil into a collapsed globule with the addition of a cationic surfactant. In the present article, we describe the manner of the unfolding transition of compact long DNA, or globule DNA, complexed with cationic surfactants, cetyltrimethylammonium bromide (CTAB) and distearyldimethylammonium bromide (D18DAB), as is induced by the addition of sodium bromide. The conformational dynamics of individual single duplex T4DNA molecules was directly observed with the use of fluorescence microscopy. We found that on the level of individual DNAs, the salt-induced unfolding transition of the globules is largely discrete, or first-order phase transition for the both complexes with CTAB and D18DAB. On the other hand, for the ensemble average of the DNAs, the transition is discrete with CTAB but is continuous (sigmoidal) with D18DAB. The discreteness for the coil-globule transition in the ensemble of DNAs complexed with CTAB is attributed to the existence of the phase transition in whole over the bulk solution: the sphere-rod transition in surfactant micelles. On the other hand, for D18DAB such phase transition on the micelle structure in the bulk solution seems to be absent. In correspondence to such a large difference on the manner of the transition, x-ray diffraction analysis indicates marked difference on the structure of DNA complexes with CTAB and with D18DAB.
Infrared multiphoton dissociation of two perfluorobutenes
Authors: Longfellow Cheryl A. , Berrie Cindy L. , Suits Arthur G. , Lee Yuan T.
Published: July 2002

Photofragment translational spectroscopy was used to examine the infrared multiphoton dissociation of octafluoro-1-butene and octafluoro-2-butene. The predominant unimolecular reaction in octafluoro-1-butene at moderate laser fluences is cleavage of a carbon–carbon single bond to give the products CF3 and C3F5. The two other reactions that take place are CF2 elimination and the formation of equal weight fragments with the chemical composition C2F4; both reactions take place via a diradical intermediate. Dissociation of octafluoro-1-butene to the resonance stabilized perfluoroallyl radical is suggested to account for the favoring of simple bond rupture. These three reaction pathways were also observed in octafluoro-2-butene dissociation, however, the branching fraction is different than from octafluoro-1-butene. In octafluoro-2-butene all three channels occur with roughly equal probability. The reactions involving CF2 loss and C2F4 formation in octafluoro-2-butene are thought to proceed through the same diradical intermediate as in octafluoro-1-butene, necessitating a 1,2-fluorine migration.
A semiclassical model of the angular distribution of the photofragments of predissociating molecules
Authors: Liyanage Rohana , Gordon Robert J.
Published: July 2002

A semiclassical model is introduced for calculating the differential cross section for the dissociation of a molecule in a long-lived state. The model is based on the idea that the angular distribution is a “snapshot” of the excited state wave function, provided that exit channel effects may be neglected. Algebraic expressions are derived for one- and two-photon excitation. The model is compared with velocity-aligned Doppler spectra of Cl atoms produced by the predissociation of various states of HCl.
Dynamics of unbinding of polymers in a random medium
Authors: Bhattacharjee Somendra M. , Baumgärtner A.
Published: July 2002

We have studied the aging effect on the dynamics of unbinding of a double stranded directed polymer in a random medium. By using the Monte Carlo dynamics of a lattice model in two dimensions, for which disorder is known to be relevant, the unbinding dynamics is studied by allowing the bound polymer to relax in the random medium for a waiting time and then allowing the two strands to unbind. The subsequent dynamics is formulated in terms of the overlap of the two strands and also the overlap of each polymer with the configuration at the start of the unbinding process. The interrelations between the two and the nature of the dependence on the waiting time are studied.
Dwell time of nonadiabatic electron transfer reaction: Solvent dynamic effects
Authors: Yang Dah-Yen , Sheu Sheh-Yi
Published: July 2002

We study the tunneling behavior in a curve crossed parabolic free energy surface with friction via a dwell time method. The dwell time is defined as the difference of the mean first passage time between the delocalized and the localized nonadiabatic electron transfer reactions. The equation of motion of nonadiabatic electron transfer reaction in the condensed phase obeys a semiclassical Zusman equation [L. D. Zusman, Chem. Phys. 40, 295 (1980)]. By using the eigenfunction expansion method, the dwell time and the tunneling probability density at short time are solved numerically. It is shown that the dwell time of the nonadiabatic electron transfer reaction between two parabolic free energy surfaces depends on the delocalization width (or diabatic width) and the force difference at the crossing point. This dwell time is proportional to (longitudinal dielectric relaxation time)/(electronic coupling constant)2.
Microscopic theory of critical folding nuclei and reconfiguration activation barriers in folding proteins
Authors: Takada Shoji , Wolynes Peter G.
Published: July 2002

An explicit droplet calculation is developed to address two aspects of the folding kinetics of large proteins: the thermodynamic folding barrier and the reconfiguration rate. First, a nonspecific folding nucleus is described as the instanton or droplet solution of a free energy functional derived for a minimally frustrated polymer Hamiltonian of the Gō type. Second, a theory for the barriers for transitions between trapped misfolded states is developed using a replica approach extended to inhomogeneous cases near the glass transition temperature of a random heteropolymer. Replica instantons are computed and their shape described. These two factors are then combined to give a microscopic theory of the folding time.
Dissociation of singly and multiply charged fullerenes: Emission of C4, or sequential emission of C2?
Authors: Foltin M. , Echt O. , Scheier P. , Dünser B. , Wörgötter R. , Muigg D. , Matt S. , Märk T. D.
Published: July 2002

We have obtained direct mass spectrometric evidence that fullerene ions C60z+ (z=1, 2, or 3) and C58z+(z=1,2) undergo unimolecular dissociation by sequential emission of two C2 units, on a time scale of 10−5 s. Moreover, a comparison of experimental and theoretical breakdown graphs reveals that unimolecular formation of C56+ from the C60+ parent ion within a given observational time window is dominated by successive loss of C2; direct C4 loss does not contribute significantly. This conclusion is not affected by uncertainties in our knowledge of the energetics of C2 vs C4 loss.
Correlation between the intensities of vibrational overtone transitions and the repulsive branch of the molecular potential
Authors: Medvedev Emile S.
Published: July 2002

Energy levels εn and n←0 vibrational overtone transition intensities for a distorted Morse potential (DMP) and a linear dipole moment function are calculated, and then are treated as “observed” quantities. The values of (εn−ε0)/n vs n fall on a straight line very closely. A linear least-squares fit provides an effective anharmonicity parameter (xe)eff which is then used to construct an “effective” Morse potential (EMP). The EMP closely follows the DMP near equilibrium but declines in the far repulsive and attractive regions. The intensities calculated for the EMP systematically overestimate or underestimate the DMP intensities, depending on whether the repulsive branch of the EMP above the dissociation limit runs over or under that of the DMP, respectively, and the discrepancies rapidly increase with the overtone number. This effect of the repulsive branch is further investigated quantitatively by comparing the steepness of the repulsive potential β calculated directly from the potential curve with its value found from the rate of the intensities falloff with the overtone energy. It is shown that these two β values coincide with each other in a wide range of parameters, as predicted by the quasiclassical theory. A semiempirical potential for hydroxyl radical is tested for the occurrence of the effect of the repulsive branch. The EMP and RKR potentials are simulated, and the latter is fitted with a kth-order polynomial, k=6–12. In all cases the energy levels and lower overtone intensities are reproduced with a high precision, but the higher overtone intensities are accurate only when the repulsive branch of the fitting function is close enough to that of the original semiempirical potential. An interpolation/extrapolation procedure commonly used to represent the RKR potential is also discussed.
Low-field hopping among randomly-distributed sites with uncorrelated energetic disorder
Authors: Parris P. E.
Published: July 2002

The low-field mobility μ of a small concentration of charge carriers hopping among a random distribution of transport sites is studied, as a function of the mean interparticle spacing ρ and the temperature T, for model systems having different site-energy distribution functions. For a uniform density of states our calculations show that the mobility obeys empirical scaling laws similar to those found in the theory of variable-range hopping. For a binary distribution of site energies we observe a crossover as a function of site density between trap-limited conduction and trap-mediated conduction. For a Gaussian density of states our results confirm the quadratic inverse temperature dependence of lnμ found in Monte Carlo studies, although quantitative characterization of this dependence is found to depend sensitively on the degree of spatial disorder in ways that could impact the extraction of microscopic parameters from experimental data.
Exchange functionals with improved long-range behavior and adiabatic connection methods without adjustable parameters: The mPW and mPW1PW models
Authors: Adamo Carlo , Barone Vincenzo
Published: July 2002

Starting from an analysis of the low-density and large gradient regions which dominate van der Waals interactions, we propose a modification of the exchange functional introduced by Perdew and Wang, which significantly enlarges its field of applications. This is obtained without increasing the number of adjustable parameters and retaining all the asymptotic and scaling properties of the original model. Coupling the new exchange functional to the correlation functional also proposed by Perdew and Wang leads to the mPWPW model, which represents the most accurate generalized gradient approximation available until now. We next introduce an adiabatic connection method in which the ratio between exact and density functional exchange is determined a priori from purely theoretical considerations and no further parameters are present. The resulting mPW1PW model allows to obtain remarkable results both for covalent and noncovalent interactions in a quite satisfactory theoretical framework encompassing the free electron gas limit and most of the known scaling conditions. The new functionals have been coded with their derivatives in the Gaussian series of programs, thus allowing fully self-consistent computations of energy and properties together with analytical evaluation of first and second geometry derivatives.
Quantum mechanical and semiclassical studies of N+N2 collisions and their application to thermalization of fast N atoms
Authors: Balakrishnan N. , Kharchenko V. , Dalgarno A.
Published: July 2002

Energy loss of fast N(4S) atoms in a bath gas of N2 molecules is investigated taking into account elastic and inelastic collisions. Quantum mechanical calculations using a vibrationally close-coupled rotationally sudden approach are performed to obtain the elastic scattering cross sections. Inelastic cross sections involving ro-vibrational transitions of the molecules are determined from a quantum-classical approach in which the vibrational motion of the molecule is treated by the time-dependent quantum mechanical method and the remaining degrees of freedom described by classical mechanics. The computed angular and energy resolved cross sections are used to construct the Boltzmann kernel for energy relaxation of fast N(4S) atoms from which the parameters governing the thermalization are readily extracted.
The spectrum of arsenic hydride: An ab initio configuration interaction study employing a relativistic effective core potential
Authors: Alekseyev Aleksey B. , Liebermann Heinz-Peter , Hirsch Gerhard , Buenker Robert J.
Published: July 2002

An ab initio configuration interaction (CI) study including the spin–orbit interaction is carried out for numerous valence and Rydberg states of the AsH radical by employing a relativistic effective core potential for the arsenic atom. The computed spectroscopic constants are in good agreement with corresponding experimental data, with a tendency toward a slight overestimation of bond lengths (by 0.01–0.02 Å) and Te values (by 450–550 cm−1) for the lowest singlet states. Measured spin–orbit splittings for the X 3Σ− and A 3Π multiplets are also accurately reproduced in the present calculations and the Ω=0−, 1, and 2 components of the latter state are shown to be strongly predissociated due to spin–orbit interaction with the corresponding components of the repulsive Σ-5 state. Dipole moments μ(v=0) for the lowest-lying X 3Σ−, a 1Δ, and b 1Σ+ states, all arising from the ⋯σ2π2 electronic configuration, are computed to have small (e.g., 0.1266 D for X1 3Σ0+−) and nearly equal positive values (As−H+ polarity). This finding is used to explain why the partial radiative lifetime for the parallel b–X1 transition (τp=44 ms) is much longer than that (τp=0.95 ms) of the perpendicular b–X2. The lifetime of the a 1Δ state is calculated to be 97 ms, significantly longer than that of the b 1Σ+ state, while the A 3Π substates have much shorter lifetimes (⩽1 μs) for radiative decay to the X 3Σ− ground state. A number of other bound states and avoided crossings are indicated in the calculations which may be of relevance in future experimental studies of this system.
X-ray scattering study of amorphous formamide
Authors: Nasr Salah , Bosio Louis
Published: July 2002

Noncrystalline solid formamide can be prepared by vapor deposition onto substrates kept at 77 K. A structural investigation of the so-obtained samples using x-ray scattering indicates that there are, on average, four hydrogen-bonds per molecule. The amorphous structure can be described by a short-range order which resembles the one in the crystalline solid.
A nonequilibrium simulation method for calculating tracer diffusion coefficients of small solutes in n-alkane liquids and polymers
Authors: van der Vegt N. F. A. , Briels W. J. , Wessling M. , Strathmann H.
Published: July 2002

The tracer diffusion coefficients of methane in n-alkane liquids of increasing chain length were calculated by measuring the friction from short time nonequilibrium molecular dynamics simulations. The frictional constant was calculated from the exponentially decaying distance between two methane tracer molecules connected by a harmonic spring. The diffusion constants calculated with this method are in good agreement with those obtained from mean square displacement calculations. Unfortunately, both methods required the same computational input even for large values of n. The tracer diffusion coefficient is found to obey an algebraic law D∼n−α for n⩽100, where n is the chain length of the solvent. The value of α=1.1 is in perfect agreement with experimental tracer diffusivities determined with the Taylor dispersion method, the value being mainly determined by the tracer’s size. For larger solvent chain lengths n>100 (tracer diffusion in polymer melts) a second regime of much slower decay is found. The tracer diffusion coefficient converges to a constant value in correspondence with the converging melt density.
Correction of repulsive potential energy surface for photodissociation of H2O in the Ã state
Authors: Wang Dunyou , Zhang John Z. H.
Published: July 2002

We present in this paper the application of the IPSVD (inverse perturbation via singular value decomposition) method to correct repulsive potential energy surfaces (PES) for half-scattering problems by directly inverting the experimental spectroscopic data. Specifically in the present model study, we start from the ab initio PES of Engel, Schinke, and Staemmler for the excited Ã state and use the IPSVD method to correct the PES by directly inverting the measured absorption spectrum. The corrected model PES can accurately reproduce the total absorption spectrum in photodissociation of H2O in the Ã state within the energy range of our study. Our model study shows great promise for future application of the IPSVD method to correct multidimensional repulsive potential energy surfaces for half- and full-scattering problems.
Response to “Comment on ‘Normal, single-file, and dual-mode diffusion of binary adsorbate mixtures in AlPO4-5’ ” [J. Chem. Phys. 109, 5691 (1998)]
Authors: Sholl David S. , Fichthorn Kristen A.
Published: July 2002

We present results from molecular dynamics simulations of single CF4 molecules diffusing in AlPO4-5 pores. The single particle diffusivities measured in these simulations are substantially smaller than the diffusivities that can be extrapolated from available experimental data. We also re-examine our previous smart Monte Carlo simulations of the dual-mode diffusion of Ne and CF4 in AlPO4-5. These results provide conclusive evidence that the presence of CF4 greatly reduces the diffusivity of Ne.
Hydration and transport properties of aqueous solutions of α-α-trehalose
Authors: Magazù S. , Maisano G. , Migliardo P. , Middendorf H. D. , Villari V.
Published: July 2002

We report measurements of the sound velocity diffusivity and viscosity of aqueous solutions of α-α-trehalose at temperatures between 20 and 85 °C for volume fractions from 0.041 to 0.705. The ultrasonic data show that the mixing process is not ideal and that the trehalose-water interaction strength, because of the saturation of H-bond sites, decreases with increasing concentration. Photon correlation spectroscopy reveals marked changes in the effective diffusion coefficient in the dilute region over a limited temperature range. These measurements, interpreted in conjunction with viscosity data on the basis of a model by Mooney and Padova, quantify the hydration properties of α-α-trehalose. The dependence on volume fraction and the sensitivity to temperature changes characterize the “fragility” of this disaccharide-water system.
A simple model to investigate the effects of non-Coulombic interactions on the structure of charged interfaces
Authors: Stafiej J. , di Caprio D. , Badiali J. P.
Published: July 2002

A simple field theoretical model is considered for an electrified interface in which the electrolyte solution is in contact with a charged hard wall. The model for the electrolyte includes Coulombic interactions, local ideal entropy, functional and specific nonlocal interactions. For the specific nonlocal interactions, the simplest form of the square gradient term is selected. The charged hard wall is characterized by an additional short range potential reduced to the form of a Dirac function. An exact relation, analogous to Maxwell relations in thermodynamics, is derived describing the effect of the wall specificity on the electric properties. In the mean field theory, the consistency equations for the ionic profiles are solved analytically in the limit of small charge densities and weak wall potentials. The electric double layer structure is discussed in terms of two characteristic lengths—Debye length for electrostatics and the characteristic length for the specific interaction. We also present and discuss charge and density profiles of ions calculated numerically by a differential equation solver. With the results thus obtained we find a behavior much different from the classical Gouy–Chapman theory.
Ab initioand semiempirical studies of the adsorption and dissociation of water on pure, defective, and doped MgO (001) surfaces
Authors: Almeida A. L. , Martins João B. L. , Taft C. A. , Longo E. , Lester W. A.
Published: July 2002

Ab initio and semiempirical calculations of large cluster models have been performed in order to study water adsorption and dissociation on pure, defective (vacancies) and doped (Li, Na, K, Ca, Fe) MgO (001) surfaces. The geometries of the adsorbed and dissociated molecules have been optimized preparatory to analysis of binding energies, stretching frequencies, charge transfers, preferential sites of interaction, and bond distances. We have used Mulliken, natural bond order, and electrostatic-derived atomic and overlap populations to analyze charge distributions in the clusters. We have also investigated transition structures, activation energies, energy gaps, HOMO, density of states, SCF orbital energies as well as the acid–base properties of our cluster model. Numerical results are compared, where possible, with experiment, interpreted in the framework of various analytical models, and correlated with site coordination numbers, corner and edge site preferential locations, and direction of charge transfer. A thorough charge analysis indicates substantial charge redistribution in the magnesium oxide crystal as a result of water adsorption and dissociation in pure, defective, and doped MgO crystals. The introduction of heavier impurities and vacancies could produce substantial changes in the physical and chemical properties of the catalyst and increase the binding and dissociation energies. Some of the largest changes originate from the introduction of vacancies. Two and three-dimensional potential energy surfaces are used to investigate activation energies of hydroxylation on the MgO surface. Stretching frequencies are correlated with magnesium and oxygen coordination numbers.