bluuebunny/crossref_metadata_2025 huggingface dataset:
Electromodulation of fluorescence in a crystalline organic photoconductor (thionaphthenindole)
Authors: Kalinowski J. , Stampor W. , Di Marco P. G.
Published: July 2002

The influence of electric fields on the fluorescence of polycrystalline layers of thionaphthenindole has been studied by an electromodulation technique that allows internal electric fields to be accounted for the effect. The in-phase first harmonic (1ω) and out-of-phase second harmonic (2ω) of the fluorescence response to a modulating (sinusoidal) electric field of frequency ω reveal different electric-field behavior, the 2ω response providing nearly internal field-free signal suitable for verification of theoretical models. Fluorescence quenching with this signal has been observed and attributed to electric-field modulation of the probability of charge separation within an excited state as a precursor. It was found that fluorescence quenching data could not be well explained using theoretical models formulated by Onsager, Poole–Frenkel, and Noolandi–Hong–Popovic for the charge separation via delocalized charge-transfer excitons. The macrotrap model which reconciles both the experimental data and their physical interpretation is proposed. The model attributes the fluorescence quenching to carrier photogeneration by the field-assisted thermal dissociation of a trapped charge-transfer exciton which has a higher located molecular singlet S1 state as a precursor. An analysis of 1ω fluorescence signals provides information on the internal fields evaluated on ≂105 V/cm and attributed to spontaneous polarization effects in ordered crystalline layers of this polar compound, in accordance with conclusions drawn previously from electroabsorption data.
Re-entrant appearance of phases in a relaxed Langmuir monolayer of tetracosanoic acid as determined by x-ray scattering
Authors: Schwartz D. K. , Schlossman M. L. , Pershan P. S.
Published: July 2002

The structure of the fully relaxed phases of a Langmuir monolayer of tetracosanoic acid is determined by x-ray diffraction and reflection along an isotherm at ∼20.5 °C. Isotherms taken by allowing the surface pressure to stabilize between incremental compressions are seen to be qualitatively different from the constant-rate nonrelaxed isotherms typically seen in the literature. At low densities the monolayer consists of an inhomogeneous film of islands of a crystalline (or hexatic) phase with molecular tilt ordering that is analogous to that of the smectic I liquid crystal. Small amounts of impurities (∼0.5% of the monolayer) account for the change in surface pressure with area in this region. Upon compression to the point that the free space between islands becomes negligible the film appears homogeneous. On further compression the time required for full relaxation becomes long (i.e., ∼ hours), the tilt angle of the molecular axis decreases and the x-ray unit cell is compressed. Including this homogeneous I phase the phase sequence observed by diffraction upon compression is I-U-I-U, where U refers to an untilted orthorhombic phase. The outer two phases of this sequence are pure phases which form homogeneous monolayers, but the inner two are inhomogeneous phases each coexisting with an amorphous phase that does not have an observable diffraction signal. At the boundaries demarcating the I and U phases, a phase whose tilt ordering is analogous to that of a smectic F phase is seen to coexist. The preceding phase sequence is sensitive to the degree of relaxation permitted the monolayer after an incremental compression. In particular, if the monolayer is not allowed to relax completely after each compression, the untilted U phase may never appear. The U↔I transition is shown to be reversible for a relaxed monolayer.
Thermodynamic and stochastic theory for nonequilibrium systems with multiple reactive intermediates: The concept and role of excess work
Authors: Ross John , Hunt Katharine L. C. , Hunt Paul M.
Published: July 2002

We continue our development of a global thermodynamic and stochastic theory of open chemical systems far from equilibrium with an analysis of a broad class of isothermal, multicomponent reaction mechanisms with multiple steady states, studied under the assumption of local equilibrium. We generalize species-specific affinities of reaction intermediates, obtained in prior work for nonautocatalytic reaction mechanisms, to autocatalytic kinetics and define with these affinities an excess free energy differential Fφ. The quantity Fφ is the difference between the work required to reverse a spontaneous concentration change and the work available when the same concentration change is imposed on a system in a reference steady state. The integral of Fφ is in general not a state function; in contrast, the function φdet obtained by integrating Fφ along deterministic kinetic trajectories is a state function, as well as an identifiable term in the time-integrated dissipation. Unlike the total integrated dissipation, φdet remains finite during the infinite duration of the system’s relaxation to a steady state and hence φdet can be used to characterize that process. The variational relation δφ≥0 is shown to be a necessary and sufficient thermodynamic criterion for a stable steady state in terms of the excess work of displacement of the intermediates and φdet is a Liapunov function in the domain of attraction of such steady states. Based on these results and earlier work with nonautocatalytic and equilibrating systems, we hypothesize that the stationary distribution of the master equation may be obtained in the form Ps=𝒩 exp(−φdet/kT) and provide an analytical argument for this form for macroscopic systems. This generalizes the Einstein fluctuation formula to multivariable systems with multiple steady states, far from equilibrium. In the following article, the utility of the approximation to Ps for systems with single or multiple intermediates, and single or multiple steady states is shown by comparison with numerical solutions of the master equation.
Thermochromism in the insulating forms of polyaniline: Role of ring-torsional conformation
Authors: Masters J. G. , Ginder J. M. , MacDiarmid A. G. , Epstein A. J.
Published: July 2002

The temperature-dependent optical spectra of the major insulating forms of polyaniline—the fully reduced leucoemeraldine base, half-oxidized emeraldine base (EB), and fully oxidized pernigraniline base (PNB)—and of two oligomeric model compounds for polyaniline are reported. All three forms of the polymer possess a ∼3.8 eV optical absorption assigned to a transition from the valence band to a narrow π* conduction band; in all cases this transition shifts to higher energy and weakens with increasing temperature, T. The oxidized forms also exhibit transitions at ∼2 eV; in EB, this absorption feature has previously been assigned to a ‘‘molecular exciton’’ and is found to increase in energy with increasing T. In PNB, the observed ∼2.3 eV transition is correlated with the expectation of a Peierls dimerization gap in this half-filled band system; in contrast to the ∼2 eV transition in EB, this optical transition energy decreases with increasing T in PNB. These observations are for the most part understood by a simple model which accounts for the steric repulsion between the constituent phenyl rings of the polymer chain. The dependence of the steric potential on ring angle is anharmonic, giving rise to thermal expansion of the average phenyl ring-torsion angles and a concomitant reduction in the bandwidth and average energy of the valence band; the observed thermochromic behavior is generally consistent with these changes in electronic structure. The potential role of other conformational changes—rod-to-coil transitions, order–disorder transitions, bond-length order changes, and structural isomerizations—is discussed.
Investigation of viscosity and heat conduction effects on the evolution of a transient picosecond photoacoustic grating
Authors: Sun T. , Morais J. , Diebold G. J. , Zimmt M. B.
Published: July 2002

The production of acoustic waves by the absorption of radiation, known as the photoacoustic effect, is described by two coupled differential equations for pressure and temperature. Here, solutions to the coupled equations are given for sinusoidal deposition of heat in space as is typically generated using two picosecond laser beams to form a transient grating. The solutions to the coupled equations, along with the equation of state of the fluid, gives the pressure, temperature, and density of the fluid following rapid deposition of heat. The time dependence of the grating diffraction efficiency is readily calculated from the modulation of the density and temperature. For small values of the viscosity and heat conduction parameters, closed form expressions for the state variables are given which describe thermal and acoustic modes of wave motion in the fluid. The method is applied for computation of the state variables using a frequency-dependent viscosity. Experiments with ethylene glycol are reported where the viscosity and sound speed are measured as functions of temperature.
Generalized Huzinaga building-block equations for nonorthogonal electronic groups: Relation to the Adams–Gilbert theory
Authors: Francisco E. , Martín Pendás A. , Adams W. H.
Published: July 2002

We show in this article that the Huzinaga building-block equations for many-electron systems, derived under the hypothesis of strong orthogonality among the system groups, can be also deduced as a particular case of the Adams–Gilbert formalism, in which intergroup overlapping is recognized from the outset. This viewpoint is more realistic because the strong orthogonality cannot be fully achieved in most practical cases due to basis-set truncation. Incomplete orthogonality leads to nonzero expectation values of the projection operators appearing in the Fock Hamiltonian (projection energy). This fact raises some conceptual problems since the projection energy has been used to interpret several important physical effects in recent investigations. The present work clarifies several practical aspects relative to the incomplete fulfillment of the strong-orthogonality hypothesis, in particular (a) the analytical relationship between the projection energy and the intergroup overlap energy involving weakly overlapping groups; (b) the required values for the projection constants in cases of incomplete orthogonality; and (c) how the effects of basis-set truncation can be analyzed unambiguously.
The energy and structure of Bjerrum defects in ice Ih determined with an additive and a nonadditive potential
Authors: Hassan Razi , Campbell Edwin S.
Published: July 2002

The energy and the structure of the Bjerrum D defect have been determined with both an additive and a nonadditive potential and those of the L defect with the additive potential. The results showed that: the relaxations of the two molecules joined formally by the defect bond and their other six nearest neighbors were energetically and structurally significant for both D and L defects; the interactions of the sites permitted to relax with their surrounding neighbors should be included explicitly, since these interactions determine the orientations and positions of the former; the nonadditive component significantly altered both the energy and structure of the D defect.
Molecular hyperpolarizabilities
Authors: Sekino Hideo , Bartlett Rodney J.
Published: July 2002

We report a systematic study of the first and second hyperpolarizabilities of several small molecules at a consistent level of theory and basis sets. Coupled cluster (CC) methods for correlation, analytical high-order time dependent Hartree–Fock (TDHF) theory for dispersion effects, and polarizability-consistent basis sets give agreement to about 10% between the calculated hyperpolarizabilities and the gas phase experiments for the nonpolar molecules, H2, N2, CO2, and C2H4, and effectively nonpolar CO. Results for the polar molecules FH, H2O, NH3, and H2S are improved by adding lone-pair basis functions. For H2O and NH3 there is good (∼10%) agreement with experiment. However, a ∼20% difference between experiment and theory for the FH molecule persists; this difference is discussed in some detail.
X-ray absorption near edge structure (XANES) of Cu(II)–ATP and related compounds in solution: Quantitative determination of the distortion of the Cu site
Authors: Palladino L. , Della Longa S. , Reale A. , Belli M. , Scafati A. , Onori G. , Santucci A.
Published: July 2002

We have studied the Cu binding site in Cu(II)–ATP, Cu(II)–tripolyphosphate, and Cu(II)–D-ribose complexes in aqueous solution at several pH values ranging from 3 to 12 by means of high-resolution x-ray spectroscopy. We have measured the spectra in the x-ray absorption near edge structure (XANES) region to probe the tetragonal distortion of the Cu site in these compounds, as indicated by optical d–d spectroscopy. The spectrophotometric studies proposed that Cu(II) bind to the oxygen atoms of the phosphate groups of ATP with a distorted octahedral symmetry at low pH, while at high pH, the metal atom would be bound to the hydroxyl groups of the ribose moiety with an almost planar symmetry. We have compared the measured XANES spectra with multiple scattering calculations, by using as a structural model a simple tetragonal CuO6 cluster with fixed equatorial distance (1.95 Å) and variable axial distance (1.95–2.7 Å). The multiple scattering calculations fit well the experimental data of Cu–ATP in solution, and allow us to solve the different contribution in the XANES spectrum from axial and equatorial distances. The best agreement is obtained with values of the axial distance of 2.2 Å at low pH, and 2.7 Å at high pH. Our result indicates that an increasing percentage of highly distorted (square-plane) Cu site is present at high pH, and confirm the d–d spectroscopic model.
The nuclear magnetic shielding as a function of internuclear separation
Authors: Jameson Cynthia J. , de Dios Angel C.
Published: July 2002

Ab initio calculations of nuclear magnetic shielding surfaces for 23Na in the NaH molecule, 39Ar in ArNe, 21Ne in NeHe, and 39Ar in Ar...NaH are carried out over a wide range of internuclear separations, using a local origin method (LORG) which damps out the long-range errors due to incomplete basis sets. The R dependence of the intermolecular shielding in the attractive region of the potential in these systems and in Ar...Ar and Ar...Na+ are consistent with the long-range limiting forms associated with the shielding hyperpolarizability in conjunction with a mean square electric field approximation. The Cl and F shieldings over the range of nuclear displacements spanning the classical turning points of the ground vibrational states of ClF and F2 are found to be remarkably superposable upon scaling by the factors 〈a03/r3〉⋅Re. This holds as well for ClH compared with FH. The shielding of 23Na and 7Li in NaH and LiH molecules are almost superposable. These and the scaling of the intermolecular shielding in rare gas pairs indicate some general similarities of shielding surfaces. The systematic variation in the signs and magnitudes of the first derivative of X nuclear shielding at the equilibrium geometry in XHn molecules of the first and second row of the Periodic Table are shown to be consistent with a general shape for the shielding function σ(R), which we have found in rare gas pairs and for 23Na in NaH.
Donnan phenomena in membranes with charge due to ion adsorption. Effects of the interaction between adsorbed charged groups
Authors: Mafé Salvador , Manzanares José A. , Reiss Howard
Published: July 2002

A physical model for the modified Donnan phenomenon associated with ion adsorption on localized membrane sites is presented. This model accounts for the dependence of the concentration of adsorbed ions on electrolyte concentration and pH as it is influenced by the electrostatic interaction between adsorbed ions. The equilibrium thermodynamic concepts employed are based on the Donnan formalism for the ion equilibria between membrane and solution, and the Bragg–Williams approximation for an adsorption isotherm that incorported interaction between adsorbed ions. Our results include the concentration of charged groups in the membrane, the pH of the membrane phase solution, and the Donnan potential as functions of the pH and the electrolyte concentration of the external solution for different degrees of the electrostatic interaction. These magnitudes are of considerable interest in biopolymers, membranes, and conducting polymers.
Mathematical double points according to the simplified–perturbed-hard-chain theory
Authors: van Pelt A. , Peters C. J. , de Swaan Arons J. , Meijer P. H. E.
Published: July 2002

Critical curves of a binary fluid mixture are usually plotted as a function of pressure and temperature (p,T projection), temperature and composition (T,x projection) or as a function of the reduced densities of the two components (y1,y2 projection). In these three ways of representation, we will show the structure of the critical curves around a mathematical double point. Moreover, it will be shown that mathematical double points can conveniently be divided into two groups: the (meta)stable and unstable mathematical double points. To date, the (meta)stable mathematical double points have not been investigated with a pressure-explicit equation of state. In this paper, it will be shown how the two types of mathematical double points can be calculated. We will digress on the thermodynamic conditions to be obeyed for the calculation of the two types of mathematical double points.
Nonlinear optical line shapes of disordered molecular aggregates: Motional narrowing and the effect of intersite correlations
Authors: Knoester Jasper
Published: July 2002

We theoretically investigate nonlinear optical line shapes of linear molecular aggregates with Gaussian disorder in the molecular transition frequencies. A perturbative treatment in the disorder is used, within which the joint stochastic distribution function of the frequencies of all multiexciton states of an aggregate can be determined analytically. It is shown that motional narrowing, which is characteristic for the linear absorption spectra of aggregates, also occurs for nonlinear line shapes. An important aspect of our disorder model is that it allows for general correlations between the transition frequencies of molecules within one aggregate, thereby interpolating between continuous energy disorder and a segment or kink model. The general theory is applicable for nonlinearities of any order. Specific applications are discussed for linear absorption, nonlinear absorption, and two-color pump–probe spectra. Our theory suggests that pump–probe experiments provide a novel and very promising approach to obtain microscopic information on aggregate systems; in particular, this technique can be used to determine both the magnitude of the molecular disorder and its degree of intersite correlation within aggregates.
Experimental and theoretical studies of the F+H2 transition state region via photoelectron spectroscopy of FH−2
Authors: Bradforth Stephen E. , Arnold Don W. , Neumark Daniel M. , Manolopoulos David E.
Published: July 2002

The transition state region of the F+H2 reaction is studied by photoelectron spectroscopy of FH2−. The photoelectron spectra consist of overlapping electronic bands with different angular distributions. The ground state band shows partially resolved features which differ depending on whether the anion is made from normal or para hydrogen. This dependence on the anion nuclear spin statistics implies that these features are due to progressions in bending levels of the neutral FH2 complex. In order to confirm this, and to determine the sensitivity of the photoelectron spectrum to the bend potential near the F+H2 transition state, three-dimensional simulations of the FH2− photoelectron spectrum were performed assuming various potential energy surfaces for the F+H2 reaction. We found that the London–Eyring–Polanyi–Sato surface proposed by Takayanagi and Sato gave better agreement than either the T5a or 5SEC surfaces. From the higher energy band, we can extract information on the F+H2 excited electronic states, and several approximate simulations on model surfaces for these states are presented.
Exchange interactions, charge delocalization, and spin relaxation in a mixed-valence di-iron complex studied by Mössbauer spectroscopy
Authors: Ding X.-Q. , Bill E. , Trautwein A. X. , Winkler H. , Kostikas A. , Papaefthymiou V. , Simopoulos A. , Beardwood P. , Gibson J. F.
Published: July 2002

Exchange interactions and charge transfer in the Fe2+Fe3+ pair of the mixed valence [Fe2S2(dimethylmethanebisbenzimidazolate)2]3− trianion have been studied by analysis of Mössbauer spectra in the temperature range of 1.5–180 K and in applied fields of 10 mT, 0.35 T, and 6.2 T. The low-temperature spectra reveal a ground state with total spin St=1/2 and hyperfine parameters intermediate between values for a Fe2+Fe3+ localized mixed-valence pair and a fully delocalized system where the two iron atoms are equivalent. A consistent set of hyperfine parameters has been derived by fitting the spectra with a stochastic relaxation model taking into account spin relaxation in the St=1/2 state and electron hopping between the iron ions. An interpretation of the values of the hyperfine parameters has been given by solving a spin Hamiltonian, which includes antiferromagnetic and double exchange in an asymmetric Fe2+Fe3+ pair and which allows partial electron delocalization. Using the value a2=0.8 for the delocalization coefficient and an estimate of Δ=105 cm−1 for the difference between the St=1/2 and the first excited St=3/2 state we have derived limits for the exchange-coupling constant J, the double-exchange parameter B, and the energy difference EA−EB arising from the two possible configurations FeA2+FeB3+ and FeA3+FeB2+, i.e., 70 cm−1≲J≲300 cm−1, 0<‖B‖≲395 cm−1, and 0<‖EA−EB‖≲590 cm−1.
A Brownian dynamics algorithm for calculating the hydrodynamic friction and the electrostatic capacitance of an arbitrarily shaped object
Authors: Zhou Huan-Xiang , Szabo Attila , Douglas Jack F. , Hubbard Joseph B.
Published: July 2002

An algorithm originally devised for calculating the diffusion-controlled reaction rate toward an arbitrarily shaped object is adapted to calculate the scalar translational hydrodynamic friction and the electrostatic capacitance of the object. In this algorithm Brownian particles are launched from a spherical surface enclosing the object. Each particle is propagated until it either hits the enclosed object or crosses the starting surface. In the latter case the particle is allowed to escape to infinity with an analytically known probability. If the particle does not escape to infinity, it is put back on the starting surface with the correct distribution density and the process is repeated. The scalar friction or capacitance of the ‘‘probed’’ object is proportional to the fraction of particles that hit the object. This algorithm is illustrated on a dumbbell made of two equal-size spheres, a cube, and a phantom spherical shell having random distributed beads embedded in its surface.
The S(1A
g)–S1(1B2u) vibronic transition in benzene: An ab initio study
Authors: Orlandi Giorgio , Palmieri Paolo , Tarroni Riccardo , Zerbetto Francesco , Zgierski Marek Z.
Published: July 2002

The four e2g false origin bands and the a1g progression of the S0(1Ag)–S1(1B2u) transition in benzene are simulated ab initio with the recently introduced configuration interaction singles (CIS) with 6-31G orbitals. The ground and excited state CC and CH bond lengths are optimized and compared with the experiment; the CC bond elongation upon excitation is found to be slightly underestimated. The vibrational force fields are calculated at the stationary points of S0 and S1. The 1Ag force field is calculated at the Hartree–Fock level while the 1B2u force field is calculated at the CIS level of theory. The two force fields are scaled to fit the experimental frequencies and the normal mode rotation upon excitation, i.e., the Duschinsky matrix, is obtained. In agreement with previous empirical fitting of the S1(1B2u) vibrational frequencies, the Duschinsky matrix is found to be nearly diagonal with the exception of the b2u modes submatrix which shows a large amount of mixing. The mixing of the b2u modes is larger before scaling but is subsequently reduced after scaling. The normal modes and the optimized geometries are used to calculate the amount of displacement, upon excitation, of the equilibrium position of the totally symmetric modes. This displacement causes the Franck–Condon progression and is slightly underestimated by the calculation. The intensity of the four e2g false origins in the absorption spectrum of S1 is calculated and the Herzberg–Teller intensities of the four bands are found to be very close to the experiment. In particular, the relative intensity of the CCC bend (ν6) and CC stretch (ν8) bands is nicely reproduced. This result is discussed in light of similar calculations at the semiempirical level of theory. We conclude that CIS can be of great value for the unravelling of vibronic spectra of conjugated systems.
Fluctuation–dissipation relations for chemical systems far from equilibrium
Authors: Vlad Marcel O. , Ross John
Published: July 2002

A new fluctuation–dissipation relation is suggested for constant step, one intermediate chemical processes far from equilibrium. It establishes a relationship between the net reaction rate t̃(x), the probability diffusion coefficient D(x) in the composition space, and the species-specific affinity A(x): t̃(x)=2D(x)tanh[−A(x)/2kT], where x is the concentration of the active intermediate, k is Boltzmann’s constant, and T is the absolute temperature. The theory is valid for nonlinear fluctuations of arbitrary size. For macroscopic systems the fluctuation–dissipation relation may be viewed as a force-flux relationship. We distinguish four fluctuation–dissipation regimes which correspond to the decrease of the absolute value of the species-specific affinity. The passage from high ‖A(x)‖ to small ‖A(x)‖ corresponds to a crossover from a linear dependence of the species-specific dissipation rate φ̇(x) on ‖A(x‖)‖, φ̇(x)∼−‖A(x)‖, to a square one: φ̇(x)∼−A2(x). A main feature of the fluctuation–dissipation relation is its symmetry with respect to the contributions of the forward and backward chemical processes to fluctuation and relaxation. Two new physical interpretations of the probability diffusion coefficient are given: one corresponds to a measure of the strength of fluctuations at a steady state, and the other to a measure of the instability of a given fluctuation state. The dispersion of the number q of reaction events in a given time interval is given by a generalized Einstein relation: 〈Δq2〉=2VD(x)t, where V is the volume of the system. The diffusion coefficient D(x) is proportional to the reciprocal value of the mean age 〈τ(x)〉 of a fluctuation state characterized by the concentration x: D(x)=1/[2V〈τ(x)〉]. These interpretations are not related to the use of a Fokker–Planck approximation of the chemical Master Equation.
Triplet correlation functions for hard-spheres: Computer simulation results
Authors: Bildstein Bernhard , Kahl Gerhard
Published: July 2002

We present results for the triplet distribution function g(3)(r,s,t) of hard-spheres obtained in extensive molecular-dynamics simulations; the packing fractions we have investigated range from 0.15 to 0.45. The simulation data have been compared to results for g(3)(r,s,t) which we calculated via some recently proposed analytical and numerical methods; two of these methods are based on density-functional theory and the Wertheim–Thiele solution of the Percus–Yevick equation; another method, proposed by Barrat, Hansen, and Pastore uses a factorization ansatz for the pair direct correlation function and the last approximation is based on a formal density expansion of g(3)(r,s,t), truncated after second order. Furthermore we compared, simulation results to data obtained by the ‘‘source-particle method’’ (or PY3 method) proposed a few years ago by Attard. Attard’s method shows an extremely good agreement not only for general configurations, but in particular for particles at direct contact; this approximation has to be considered as the most reliable method available for the numerical determination of the triplet-structure of a simple liquid. Concerning the results of the other methods discrepancies with the simulation data are observed in particular near the contact and for very close triplet-configurations. Apart from Attard’s approximation the second order density expansion gives the best agreement. For less close configurations, i.e., if particles are separated by 1.5 to 2 hard-sphere diameters, the results of all the methods investigated practically coincide.
Binding energies, molecular structures, and vibrational frequencies of transition metal carbonyls using density functional theory with gradient corrections
Authors: Delley Bernard , Wrinn Michael , Lüthi Hans Peter
Published: July 2002

The performance of the density functional theory (DFT) methods with different gradient corrections as an approach for the computation of transition metal complexes has been evaluated. As a test, the structures, binding energies, and vibrational frequencies of a series of binary transition metal carbonyl complexes were calculated. Comparison with previous studies shows that the gradient correction significantly improves the performance of the DFT schemes, and that the results obtained generally match the quality of the data obtained from coupled cluster and pair functional methods.
The fast Fourier Poisson method for calculating Ewald sums
Authors: York Darrin , Yang Weitao
Published: July 2002

The conventional Ewald expression for the electrostatic energy and forces is recast in a form that can be evaluated to high accuracy in order N log(N) steps using fast Fourier transforms. The fast Fourier Poisson method does not rely on interpolation approaches or Taylor/multipole expansions, and can be easily integrated with conventional molecular dynamics algorithms.
Ionic versus apolar behavior of the tetramethylammonium ion in water
Authors: Turner J. Z. , Soper A. K. , Finney J. L.
Published: July 2002

Neutron diffraction involving seven isotope substitution experiments is used to extract the methyl hydrogen to the water hydrogen partial structure factor in a 2.0 m aqueous solution of tetramethylammonium chloride, in order to investigate the orientation of water molecules around the TMA ion. The correlation function of the TMA centers with water molecule centers is obtained and shows that the average distance is about 4.7 Å, in agreement with the result found previously using a nitrogen isotope substitution. The technique of spherical harmonic reconstruction is used to obtain orientational correlation functions between TMA ions and water molecules. The results show that the TMA ion has a predominantly ‘‘apolar’’ character in water but that a range of water molecule orientations around the ion are possible.
Statistical mechanics of a Coulomb gas with finite size particles: A lattice field theory approach
Authors: Coalson Rob D. , Walsh Alan M. , Duncan Anthony , Ben-Tal Nir
Published: July 2002

A recently proposed lattice field theory approach to the statistical mechanics of a classical Coulomb gas [J. Chem. Phys. 97, 5653 (1992)] is generalized to treat gas particles of finite size. To do this, a repulsive Yukawa interaction between all pairs of gas particles is added to the usual pairwise Coulombic interactions of the gas particles with each other and also with an arbitrary collection of immobile charges. Such a model is directly relevant for understanding the energetics of systems composed of macroions in electrolytic solutions when the simple ions that comprise the electrolyte are sufficiently large. A field theoretic representation of the grand partition function for the modified Coulomb gas is derived. Two coupled three-dimensional scalar fields are involved. Physically, one is related to the electrostatic potential and the other to the Yukawa potential. The field theory expression, once discretized onto an appropriate lattice, can be evaluated via saddle point expansion. The zeroth order or mean field approximation is found to be analogous to the Poisson–Boltzmann equation in the simple (infinitesimal particle) Coulomb gas problem. Higher order corrections can be obtained via a loop expansion procedure. Successful numerical application is reported for systems consisting of two spherical, equally charged macroions immersed in an electrolytic solution. Imbuing the simple ions in the solution with finite size prevents the degree of polarization of the ion cloud which is found in the infinitesimal ion limit.
Fukui function from a gradient expansion formula, and estimate of hardness and covalent radius for an atom
Authors: Chattaraj Pratim K. , Cedillo Andrés , Parr Robert G.
Published: July 2002

The Fukui function for a neutral atom is expressed as its LDA approximation plus a one-parameter gradient correction, and the resultant formula is numerically tested. Expressing hardness as a density functional involving this Fukui function, global hardness values are determined for several atoms. Estimates also are made of the covalent radii of neutral atoms. Calculated Fukui functions exhibit characteristics similar to those reported in the literature. Calculated hardnesses compare favorably with experimental values, and predicted covalent radii are in agreement with existing theoretical values and experimental data. No information other than the electron densities of the neutral species enter in the calculations. An exact nuclear cusp condition on the Fukui function is derived.
Fluid wetting on molecularly rough surfaces
Authors: Tang James Z. , Harris Jonathan G.
Published: July 2002

We examine the effects of structural changes of solid surfaces on fluid wetting by performing molecular dynamics simulations on a model system. The system consists of a Lennard-Jones fluid confined between two identical solid surfaces under conditions that induce the formation of both a liquid and vapor region. The smooth surface is three layers of solid particles arranged in a perfect fcc lattice. We represent a molecularly rough surface by adding to the smooth surface a fourth layer consisting of less than a full monolayer of solid particles. This model corresponds to some physical situations where there is surface irregularity, such as surface defects due to vacancies or surface structures caused by adsorption of foreign particles onto a perfect surface. The fluid density profiles near the rough surfaces are different from those near the smooth surface and reflect the detailed molecular structure of the surfaces. More importantly, the changes in surface structure alter significantly the contact angle between the liquid–vapor interface and the solid surface, which is determined by measuring the interfacial tensions between liquid, vapor, and solid phases. The wetting transition on molecularly rough surfaces occurs at a higher value of the liquid–solid interaction strength than that for a smooth surface.
Investigation of the effect of reagent CN rotational excitation on the dynamics of the CN+O2 reaction
Authors: Wright Scott A. , Dagdigian Paul J.
Published: July 2002

The reaction of CN with O2 has been studied through a photolysis-probe laser experiment in a cell at a total pressure of 70 mTorr. Rotationally hot CN reagent was prepared by 193 nm photolysis of BrCN. NCO(X̃ 2Π) product in various vibronic levels was detected by laser fluorescence excitation in its Ã 2Σ+–X̃ 2Π band system at variable delays after the photolysis laser. In order to monitor the CN collisional relaxation which is taking place simultaneously with the reaction, we have also derived the CN rotational state distribution as a function of the photolysis-probe delay from laser fluorescence excitation spectra of the CN B 2Σ+–X 2Σ+ (0,0) band. From these observations, we deduce that rotationally hot CN reacts more slowly than thermalized CN. Moreover, reaction of the former yields NCO product with greater bending vibronic excitation. These results are compared with previous dynamical studies of this reaction, carried out with crossed beams and in cells.
Transport through random arrays of conductive cylinders dispersed in a conductive matrix
Authors: Tomadakis Manolis M. , Sotirchos Stratis V.
Published: July 2002

We present computer simulation results for the effective conductivities of random dispersions of cylinders of arbitrary conductivity relative to that of the matrix phase. The cylinders are randomly positioned in space with their axes parallel to a line, parallel to a plane, or without preferred orientation. Effective conductivities are computed from the mean square displacement of carriers traveling in the two-phase medium according to a discrete step-by-step random walk mechanism. The probability that a carrier colliding with the interface will be reflected back into its phase of travel or cross into the other phase is determined on the basis of the conductivities of the two phases meeting at the interface. The computed values of effective conductivity are compared with variational bounds and analytical approximations of the literature.
Semiempirical molecular orbital calculations with linear system size scaling
Authors: Dixon Steven L. , Merz Kenneth M.
Published: July 2002

Details are provided for the implementation of a density matrix divide-and-conquer approximation into the framework of molecular orbital theory on nonperiodic systems. Originally developed for density functional theory, the divide-and-conquer procedure is one of the most promising in a growing list of techniques that exhibit linear scaling with respect to the number of basis functions in the system. The key to linear scaling is the division of the electronic structure calculation into a series of calculations over a set of small, overlapping subsystems. A semiempirical molecular orbital program designed around the divide-and-conquer approach has been written and a number of tests are carried out on polyglycine structures in order to evaluate its performance. For the systems examined, linear scaling is indeed observed, and the accuracy of the calculations can be controlled quite readily by the manner in which the system is divided into its component subsystems. For very large structures, the expense associated with the computation of two-center interactions will ultimately dominate the calculation, and quadratic scaling will become apparent. Techniques to linearize this aspect of the calculation are investigated and discussed.
Computer simulations of chain conformations in dilute polymer solutions under shear flow
Authors: Brostow Witold , Drewniak Marta
Published: July 2002

Using the method of Brownian dynamics simulations, we have analyzed chain conformations and the presence of chain overlaps and entanglements in dilute polymer solutions. The results show that polymer + polymer interactions exist in static dilute solutions as well as in solutions under shear flow. The effect of solution concentration and the shear rate is examined. The fundamental problem of existence of chain overlaps in dilute solutions is related to the drag reduction phenomenon and is discussed.
Response of a ring-down cavity to an arbitrary excitation
Authors: Hodges Joseph T. , Looney J. Patrick , van Zee Roger D.
Published: July 2002

An eigenmode analysis of the response of an empty ring-down cavity to an arbitrary laser excitation is presented. By explicitly taking into account both the mode structure of the ring-down cavity and the spectral content of the laser pulse, it is found that the complicated ring-down signals commonly observed in the laboratory can be interpreted in terms of cavity mode beating. Some conclusions drawn from this analysis are verified experimentally by measurements of the time and frequency response of empty cavities. These observations provide clear evidence for the existence of longitudinal and transverse mode structures in ring-down cavities.
Fragment rotational distributions from the dissociation of NeBr2: Experimental and classical trajectory studies
Authors: Nejad-Sattari Mehdi , Stephenson Thomas A.
Published: July 2002

The Br2 fragment rotational distributions that result from the vibrational predissociation of NeBr2 in the B electronic state have been measured for several initial vibrational levels. In each case, the rotational distributions extend to the effective energetic limit determined by the amount of energy available (Eavl) for disposal into the fragment rotational and translational degrees of freedom. Analysis of the data allows refinement of the NeBr2 dissociation energy; we find that D0=70.0±1.1 cm−1 for the X electronic state, v=0. Both Δv=−1 and −2 dissociation events have been examined. For dissociation pathways with approximately the same value of Eavl the Δv=−2 pathways are observed to have a higher fraction of the fragment energy in rotational excitation. The overall shape of the Δv=−1 distributions are insensitive to the value of Eavl, suggesting that a Franck–Condon model for the dissociation may have some validity, though quantitative quantum mechanical calculations demonstrate that this model does not reproduce the large degree of fragment rotational excitation. Two classical models for the dissociation also fail to reproduce the extent of fragment rotational distribution. This result is discussed in light of previous experimental and theoretical investigations, focusing on the apparent agreement of classical models with the IBr fragment rotational distributions that result from the dissociation of NeIBr.
A high resolution energy-selected kinetic energy release study of the process SF6+hν→SF5++F+e−: Heat of formation of SF5+
Authors: Evans M. , Ng C. Y. , Hsu C.-W. , Heimann P.
Published: July 2002

Using the newly constructed photoelectron-photoion coincidence apparatus associated with the chemical dynamics beamline at the advanced light source, we have performed a high resolution energy-selected kinetic energy release measurement for the dissociative photoionization process SF6+hν→SF5++F+e−. After taking into account the center-of-mass kinetic energy release, the thermochemical threshold for this process is determined to be 14.11±0.08 eV. This value yields 18.5±1.9 and −202.9±2.2 kcal/mol for the heats of formation at 0 K for SF5+ and SF5, respectively.
Distributions and averages of electron density parameters: Explaining the effects of gradient corrections
Authors: Zupan Ales̆ , Burke Kieron , Ernzerhof Matthias , Perdew John P.
Published: July 2002

We analyze the electron densities n(r) of atoms, molecules, solids, and surfaces. The distributions of values of the Seitz radius rs=(3/4πn)1/3 and the reduced density gradient s=|∇n|/(2(3π2)1/3n4/3) in an electron density indicate which ranges of these variables are significant for physical processes. We also define energy-weighted averages of these variables, 〈rs〉 and 〈s〉, from which local spin density (LSD) and generalized gradient approximation (GGA) exchange-correlation energies may be estimated. The changes in these averages upon rearrangement of the nuclei (atomization of molecules or solids, stretching of bond lengths or lattice parameters, change of crystal structure, etc.) are used to explain why GGA corrects LSD in the way it does. A thermodynamic-like inequality (essentially d〈s〉/〈s〉>d〈rs〉/2〈rs〉) determines whether the gradient corrections drive a process forward. We use this analysis to explain why gradient corrections usually stretch bonds (but not for example H–H bonds), reduce atomization and surface energies, and raise energy barriers to formation at transition states.
Rate constants for deactivation of N2(A) v=2–7 by O, O2, and NO
Authors: De Benedictis S. , Dilecce G.
Published: July 2002

Rate constants for N2(A,v) quenching by O, for levels v=2–7, by O2 for levels v=3–7, and by NO for levels v=2–4, have been measured in this work. This is the first data set for the quenching by O of vibrational levels v>3. The results of this work are based on the measurement by laser induced fluorescence (LIF) of N2(A,v) decay in a rf pulsed postdischarge, supported by LIF measurements of NO density. O atom density is deduced by N2(A,v=0,1) decay using the known rate constants of N2(A,v=0,1) quenching by O, O2, and NO. Finally, from appropriate scaling of LIF results for the various v levels, N2(A,v) vibrational distributions are deduced, showing a quite low vibrational excitation of the triplet metastable, characterized by an average Boltzmann vibrational temperature of the order of 2000–2500 K with some superimposed structures.
A study on the structure of water in an aqueous solution by the solvent effect on a volume phase transition of N-isopropylacrylamide gel and low-frequency Raman spectroscopy
Authors: Suzuki Yoshiharu , Suzuki Naofumi , Takasu Yuichi , Nishio Izumi
Published: July 2002

We introduce a new technique to investigate the effect of residual substances on the hydrogen-bonded network structure of water molecules. We have characterized the effect of very low concentrations of several alcohol and phenol compounds on the structure of water from observations of their effect on the volume phase transition of N-isopropylacrylamide (NIPA) gels. The alteration of the volume phase transition was observed as a remarkable drop of the transition temperature associated with the change in entropy of the gel fluid. In addition, we have investigated the effect of these same compounds on water structure using low-frequency Raman spectroscopy, the results verify quantitatively the increase in entropy of the gel fluid with the addition of the residual substances. We found the effect of phenols on the structure of water clusters was remarkably large compared to the effect of alcohols. We demonstrate there is a strong correlation between the measurements of the drop in transition temperature of the volume phase transition and the results obtained from low-frequency Raman spectroscopy. We conclude that the collapse mechanism of the NIPA gel is induced by the disruption of the tetrahedral structure of water.
The effect of attractions on the structure and thermodynamics of model polymer blends
Authors: Tillman Patrick A. , Rottach Dana R. , McCoy John D. , Plimpton Steven J. , Curro John G.
Published: July 2002

Molecular dynamics (MD) simulations were carried out on a blend of 50 unit, pearl-necklace chains with the object of examining the effect of intermolecular attractions on the blend structure and thermodynamics. Repulsive interactions were imposed between intermolecular sites so that the effective hard core diameter of B type chains was 1.2 times that of the A type chains. Simulations were performed for a series of runs in which the attractive interactions were systematically varied from zero to the full Lennard-Jones value. Examination of the radial distribution functions revealed that the attractive interactions had very little effect on the blend structure up to the point of phase separation. Furthermore, we found that perturbation theory accurately predicted the changes in pressure associated with the variations in the attractive strengths.
The semiclassical calculation of nonadiabatic tunneling rates
Authors: Schwieters Charles D. , Voth Gregory A.
Published: July 2002

In this paper semiclassical low-temperature rate theory is extended to treat nonadiabatic transitions which are typically important in electron transfer reactions. This theory is appropriate for arbitrary coupling strength between electronic states. As in adiabatic semiclassical tunneling theory, it is found that the leading order contribution to the tunneling rate is due to periodic orbits which exist in the barrier region of configuration space between reactant and product. In the current case, these orbits move on effective potentials generated from upside-down (nuclear) potentials of the coupled electronic states. A stable method of finding these mixed quantum/classical “trajectories” is developed using a Newton–Raphson method. Examples employing model systems demonstrate that the current nonadiabatic theory well-reproduces the known adiabatic and Golden Rule limits and that the theory can indeed be applied to systems with more than one degree of freedom.