bluuebunny/crossref_metadata_2025 huggingface dataset:
Electron nuclear dynamics for a zig–zag chain of nitrogen atoms
Authors: Pohl Anna , Calais Jean-Louis
Published: July 2002

We study the nitrogen zig–zag chain with two atoms per unit cell within the electron nuclear dynamics (END) formalism. This amounts to an approximate solution of the time-dependent Schrödinger equation for all the particles in the system. In the present approximation the nuclei are treated classically. The time dependence of the electronic motion is brought in through time-dependent linear combinations of fixed Bloch sums. This implies that the immediate mutual interaction between electronic and nuclear motion is taken into account. We investigate in particular the long-range terms of the interaction so as to arrive at convergent lattice sums. Before going to the general case when electronic and nuclear motion is coupled, we investigate the special cases of END traditional lattice dynamics and the random phase approximation (RPA) for the electrons.
Flory exponent of the chain of the expanding polyion gel
Authors: Sasaki Shigeo , Ojima Hiroaki , Yataki Kumi , Maeda Hiroshi
Published: July 2002

A theory to describe the swelling behavior of polyion gels is presented on the basis of de Gennes theory for the conformational entropy of chains under traction and ideal Donnan approximation combined with the counterion condensation of polyelectrolytes for the osmotic expansion force. The Flory exponent ν is regarded as a parameter in theory to describe the effect of the intrachain interaction on the fractal nature of the chain conformation. The volume of poly(acrylic acid) gel was measured as a function of the ionization degree, the salt concentration, and the polymerization degree of chains between crosslinks. The analysis of the expansion behavior showed that the ν values varied from about 0.8 to about 0.6 for the gels having long chains when the salt concentration changed from 10 to 100 mM, while for the gels consisting of short chains the exponent was kept to about 0.8, irrespective of the salt concentration change. The results indicate that the Flory exponent of the expanding polyion gel increases with the strength of the electrostatic interaction between ionized groups. The volume of a copolymer gel of fully neutralized maleic acid and styrene consisting of the short chains was also measured as a function of the salt concentration. The analysis of the expansion behavior gave ν=0.71 in the salt concentration range between 3 and 320 mM in contrast with a value of 0.8 for the poly(acrylic acid) gel under similar conditions. This result suggests that the hydrophobic interaction between the phenyl groups reduces the ν value.
Extraction, through filter-diagonalization, of general quantum eigenvalues or classical normal mode frequencies from a small number of residues or a short-time segment of a signal. I. Theory and application to a quantum-dynamics model
Authors: Wall Michael R. , Neuhauser Daniel
Published: July 2002

In a previous paper we developed a method, Filter-Diagonalization, for extracting eigenvalues and eigenstates of a given operator at any desired energy range. In essence, the method eliminates correlation between distant eigenstates through a short-time filter while correlations between closely lying states are eliminated by diagonalization. Here we extend Filter-Diagonalization. When used to extract eigenvalues for a given operator H, we show that all eigenvalue information is directly extracted from a short segment of the correlation function C(t)=(ψ(0)‖e−iHt‖ψ(0)), or alternately from a small number of residues (ψ(0)‖Rn(H)‖ψ(0)), where ψ(0) is a random initial function and Rn(H) is any desired polynomial expansion in H. The implications of this feature are twofold. First, in contrast to the previous version the wave packet needs only to be propagated once (to prepare C(t)), and eigenstates at all desired energy windows can then be extracted with negligible extra computation time (and negligible storage requirements). In a simulation presented here, accurate eigenvalues are extracted using propagation times which are only a 0.0041 fraction of the ‘‘natural’’ time, i.e., the time by which the relative phase of the two closest eigenstates reaches 2π. The second and more important feature is that the method is automatically suitable for extracting eigenvalues (or normal modes) using a short-time segment of any signal C(t) which is a sum of (unknown) Fourier components (C(t)=∑ndne−iεnt) regardless of its origin. In addition to its use for determining eigenvalues of known operators, the method may also be utilized to extract normal modes from classical-dynamics simulations, eigenstates from real-time Quantum Monte-Carlo studies, frequencies from experimental optical or electrical signals, or be utilized in any other circumstance where a correlation function or general signal is only known for short times (or expensive to generate at long times).
Molecular and electronic structure of TiH2
Authors: Kudo Takako , Gordon Mark S.
Published: July 2002

Ab initio electronic structure calculations using correlated wave functions have been performed to investigate the structure and energetics of TiH2 in its low-lying electronic states. Several triplet states have been found to lie very close to each other in energy (within 5 kcal/mol) and nearly 1 eV below the lowest singlet state. The lowest quintet states appear to be considerably higher in energy. The ground state of TiH2 is found to be bent 3B1 in C2v symmetry, with the 3A1 state lying only 1 kcal/mol higher in energy. The lowest singlet state, 1A1, is found to be slightly bent, but with a very flat potential energy surface. The Ti–H bond in all TiH2 electronic states is predicted to be strongly polarized Ti+H−. The use of state-averaged multiconfigurational self-consistent field wave functions is essential to obtain a consistent picture of all electronic states of interest.
Comparative study of krypton adsorption on graphite precovered with methanol and ethanol
Authors: Morishige K. , Hirakawa T.
Published: July 2002

The pressure-volume isotherm and x-ray diffraction measurements for the coadsorption systems of krypton with methanol and ethanol on graphite have been carried out at 77 K. On graphite krypton and methanol are miscible while krypton and ethanol are immiscible. The two-dimensional solid solution of krypton and methanol takes an incommensurate triangular structure with an intermolecular distance ∼4.10 Å and a molar composition Kr/CH3OH ∼0.6. Krypton adsorbed on top of the preadsorbed ethanol monolayer forms a triangular monolayer, incommensurate with the structure of the underlying monolayer.
The ring and superoxide isomers of SO2
Authors: Kellogg Charles B. , Schaefer Henry F.
Published: July 2002

The superoxide isomer of sulfur dioxide (Fig. 1) was first proposed by Myerson, Taylor, and Hanst in 1957 [J. Chem. Phys. 26, 1309 (1957)] as a possible intermediate in the combustion of CS2, COS, and H2S as well as a possible source of some troublesome ultraviolet absorptions in the spectra associated with those combustion processes. Subsequent experimental work on SO2 has also referred to the hypothesized asymmetric structure. Single reference post Hartree–Fock methods, including configuration interaction with single and double excitations (CISD), coupled cluster singles and doubles (CCSD), and coupled cluster singles and doubles with perturbative triples [CCSD(T)], as well as multireference configuration interaction (MRCISD) performed with CI natural orbitals (CINOs) have been employed in the interest of characterizing the relative energies of the open, ring and superoxide isomers of SO2. The largest basis used was a triple-ζ plus double polarization function set with f-type functions appended to each atom [TZ2P(f)]. The ring and superoxide isomers are predicted to lie approximately 111 and 104 kcal mol−1, respectively, above the open isomer ground state. Based upon these energy separations, it is predicted that neither the ring nor superoxide isomers are responsible for the troublesome UV absorption continuum, as postulated by Myerson et al. Moreover, neither the ring nor the superoxide structure is the source of the spectroscopic features very recently observed below 100 kcal mol−1 by Dai’s group.
Molecular dynamics in confined monomolecular layers. A field-cycling nuclear magnetic resonance relaxometry study of liquids in porous glass
Authors: Stapf Siegfried , Kimmich Rainer
Published: July 2002

Liquids filled in porous media such as porous glass do not freeze at the bulk freezing temperatures. Two phases must be distinguished. A one to at most two monolayer thick film adsorbed on the inner surfaces does not freeze at all, whereas free liquid within the pores freezes at reduced temperatures relative to the bulk values as predicted by the Gibbs/Thompson equation. The fraction of non-freezing liquid can be evaluated from the reduction factor of the low-frequency spin-lattice relaxation time upon freezing of the free liquid. A method for the determination of the pore size may be established on this basis. Water and tetradecane, i.e., a polar and a nonpolar adsorbate, filled in porous glass have been studied with the aid of field-cycling nuclear magnetic resonance (NMR) relaxometry. Above the freezing range the frequency dependences of the spin-lattice relaxation time T1 of the two liquids strongly deviate from each other owing to the different adsorption properties. On the other hand, with frozen samples the same frequency dependence of the liquid phase, that is essentially T1∝ν0.67, was found with both adsorbates. This proves that a nonpolar liquid confined to a thin layer on a polar surface underlies an equivalent relaxation mechanism as a strongly adsorbed polar liquid. As the dominating process, reorientations mediated by translational displacements along the curved and rugged surface are considered.
Solutions of the linearized Poisson–Boltzmann equation through the use of random walk simulation method
Authors: Ettelaie Rammile
Published: July 2002

It is shown that the random walk simulation technique, through the choice of a suitable set of rules, can be adapted to the problem of solving the linearized Poisson–Boltzmann equation, for general cases involving constant surface potentials. In order to validate the technique, the results of the method are compared to the exact solutions for a number of analytically tractable problems. Excellent agreements between the two sets of results are found in all cases. Further examples of the application of the method are given by considering systems involving nonideal colloidal particles. In particular, the solutions of the linearized Poisson–Boltzmann equation for simple models of porous colloids, as well as particles with nonuniform surface potentials, have been obtained. The implication of these preliminary results for the electrostatic interactions between such nonideal colloidal particles are briefly discussed.
Semiclassical approach to the kinetic theory of gases. I. Semiclassical BBGKY equation
Authors: Imam-Rahayu Susanto
Published: July 2002

By expanding both the interaction operator and the distribution function (in a quantum mechanical Liouville equation) in power series of a small quantum parameter λ, semiclassical BBGKY equation have been derived. A general method for its solution have been developed, yielding the various order of quantum corrections to the solutions of the classical BBGKY equation. Using the classical streaming operators in its formulations, these solutions can be interpreted in terms of motions along well-defined classical trajectories.
The effect of molecular association and tube dilation on the rotational viscosity and rotational diffusion in nematic liquid crystals
Authors: Raviol A. , Stille W. , Strobl G.
Published: July 2002

A combination of director reorientation experiments with dielectric relaxation spectroscopy was used to study the relation between the rotational viscosity γ1 and the rotational diffusion constants in nematic compounds with different association tendency. Increased values for γ1 were found for compounds showing strong association. This is explained by increased rotational friction of the associates due to the necessary translational motion of the molecules within these groups. For nonassociating compounds, γ1 was found to be described quantitatively by Marrucci’s theory, when, in addition, the tube dilation effect described by Doi for the rotational diffusion in an orientationally ordered environment is taken into account.
Photodissociation of the HCO+ ion. I. Two-dimensional calculations through the I 1Π state
Authors: Koch A. , van Hemert M. C. , van Dishoeck Ewine F.
Published: July 2002

The HCO+ ion plays an important role in the chemistry of interstellar space and in combustion flames. The 1 1Π state is the only dissociative state below the hydrogen ionization potential, to which transitions are dipole allowed. Ab initio MRD-CI calculations have been performed for the two-dimensional potential energy surfaces of the ground state and the 1 1Π state of HCO+ as functions of the C–H and C–O bond distances, keeping the ion in the linear configuration. The 1 1Π state is interesting because of an avoided crossing with the 2 1Π state. The potential energy surfaces and geometry dependent dipole transition moments have been employed in two-dimensional photodissociation dynamics calculations. Total and vibrationally resolved partial cross sections are calculated as functions of the excitation energy. The potential barrier arising from the avoided crossing leads to vibrational resonances in the cross sections, which correspond to levels of the v3 (C–O) stretching motion in the excited state. The structure of the cross sections reveals the competition between resonant and direct photodissociation. The difference between the total cross section derived from the Fourier transform of the autocorrelation function and from the sum of the partial cross sections of the CO++H dissociation channel indicates the existence of a second photodissociation channel leading to CH++O. The total and partial cross sections of this channel have been computed in an independent calculation; its contribution to the integrated total cross section amounts to about 4%.
Structural and electronic properties of C78 isomers
Authors: Niles J. C. , Wang X. Q.
Published: July 2002

The structural and electronic properties of the five isolated-pentagon fullerene isomers of C78 were studied employing various quantum-chemistry methods ranging from the self-consistent Hartree–Fock method to the local-density-functional theory. Our results reveal that the energy ordering difference for the Hartree–Fock and local density functional calculations is intrinsic to the underlying approximation scheme, independent of the basis set used. Furthermore, the local-density function calculation results yield the same ordering for the total energy and the gap between highest occupied molecular orbital and lowest unoccupied molecular orbital for various basis sets and exchange functions, as well as a correlation between the averaged pentagon bond length and the gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital.
Diffusion in inhomogeneous polymer membranes
Authors: Kasargod Sameer S. , Adib Farhad , Neogi P.
Published: July 2002

The dual mode sorption solubility isotherms assume, and in instances Zimm–Lundberg analysis of the solubilities show, that glassy polymers are heterogeneous and that the distribution of the solute in the polymer is also inhomogeneous. Under some conditions, the heterogeneities cannot be represented as holes. A mathematical model describing diffusion in inhomogeneous polymer membranes is presented using Cahn and Hilliard’s gradient theory. The fractional mass uptake is found to be proportional to the fourth root of time rather than the square root, predicted by Fickian diffusion. This type of diffusion is classified as pseudo-Fickian. The model is compared with one experimental result available. A negative value of the persistence factor is obtained and the results are interpreted.
Stark spectroscopy of CdSe nanocrystallites: The significance of transition linewidths
Authors: Sacra A. , Norris D. J. , Murray C. B. , Bawendi M. G.
Published: July 2002

We use Stark spectroscopy to examine the nature of the excited states of CdSe nanocrystallites. The Stark spectra we obtain are in the small coupling limit in which the changes induced by the electric field to the absorption spectrum are small compared to the transition linewidths. Within this limit, we theoretically examine the dependence of the line shape of Stark difference spectra on the linewidth of the transitions involved. For systems such as CdSe nanocrystallites, which have overlapping transitions coupled by the electric field, we find that the usual association of derivatives of absorption features with dipole moments and polarizabilities is problematic. We show that the Stark absorption spectrum of the CdSe nanocrystallites can be explained by polarizable and delocalized nonpolar excited states.
Sheared liquids in the nanoscale range
Authors: Urbakh M. , Daikhin L. , Klafter J.
Published: July 2002

In this paper, we study two characteristic properties of thin confined liquids under shear: the induced velocity profile in the liquid at low shear rates, and the shear-dependent thinning of the effective viscosity. Our approach is based on the coupling between a time-dependent Ginzburg–Landau equation for a local order parameter and a local velocity field. Special attention is given to the role of the lateral nonuniformity of the liquid–wall interactions in determining these properties. We derive the Brinkman equation for the velocity profile and obtain a power low dependence of the viscosity on shear rate in the thinning regime, ηeff∼γ−α with 2/3≤α≤1.
Ultraviolet photoelectron spectroscopy of molybdenum and molybdenum monoxide anions
Authors: Gunion Robert F. , Dixon-Warren St. John , Lineberger W. Carl , Morse Michael D.
Published: July 2002

The 351 nm photoelectron spectra of Mo− and MoO− have been measured. The electron affinity of atomic molybdenum is 0.748(2) eV and that of molybdenum monoxide is 1.290(6) eV. The term energies of several MoO electronic states not previously observed are obtained and compared with ab initio predictions. The ground state of MoO is confirmed to have 5Π symmetry and the term energy of the 3Π excited state, 10 179(20) cm−1, closely matches calculations. The ground state of MoO− is a 4Π state with a vibrational frequency of 810(40) cm−1. The first excited state of molybdenum monoxide is tentatively assigned as a 3Δ state with T0=621(50) cm−1. At least one state, possibly a 5Σ− state, lies 8000(500) cm−1 above the ground state, and a 5Σ+ state is observed at 11 590(60) cm−1 above the ground state. The separations of spin–orbit levels for the MoO X 5Π, 3Π, and 3Δ states are 169(30), 410(20), and −720(20) cm−1, respectively. The vibrational frequencies of the 3Π and 3Δ states are found to be 600(20) and 1000(20) cm−1, respectively. These observations give new insight into the Mo–O bond.
Far-infrared spectrum of N2 and N2-noble gas mixtures near 80 K
Authors: Wishnow E. H. , Gush H. P. , Ozier I.
Published: July 2002

The far-infrared absorption spectra of gaseous nitrogen, and mixtures of nitrogen with the foreign gases argon and neon, have been measured at pressures near 1 atm and at temperatures near 78 and 89 K. Spectra were obtained over the wave number range 20–100 cm−1 using a Fourier transform spectrometer and a multiple reflection absorption cell of 52 m path length. They show a broad continuum associated with the pure rotational collision-induced S branch of the N2 molecule plus structure attributed to transitions in dimers, not previously observed in this spectral region. In the case of N2–Ar, there are strong similarities with the fundamental vibrational band under similar conditions of pressure and temperature as reported by McKellar [J. Chem. Phys. 88, 4190 (1988)]. The integrated absorption coefficient has been evaluated for the N2–N2 rotational band; at 78 K it equals 3.1×10−31 cm5 sec−1, a factor of 2 greater than typical values at temperatures above 100 K.
Simulations at conducting interfaces: Boundary conditions for electrodes and electrolytes
Authors: Perram J. W. , Ratner Mark A.
Published: July 2002

The purpose of this paper is to derive a set of boundary conditions appropriate for simulating a wide variety of electrochemical and biophysical systems which are not necessarily electrically neutral. This is done by using the method of Green’s function, a generalization of the method of images, for solving the Poisson equation to provide boundary conditions for simulating electrochemical reactions involving ions confined between metal electrodes as well as ions in membranes enclosed between regions of aqueous electrolytes.
Comment on ‘‘Entropy catastrophe and configurational entropies in supercooled and superheated regimes’’ [J. Chem. Phys. 101, 7037 (1994)]
Authors: Debenedetti Pablo G. , Atakan Murat M. , Speedy Robin J.
Published: July 2002

In a recent article [K. Kishore and H. K. Shobha, J. Chem. Phys. 101, 7037 (1994)], Kishore and Shobha proposed the extension of the concept of a Kauzmann paradox to the vapor–liquid transition. We show that there is no Kauzmann paradox associated with the vapor–liquid transition.
Accounting for electron–electron and electron–lattice effects in conjugated chains and rings
Authors: Rossi Giuseppe , Schneider William F.
Published: July 2002

Minimum total energy calculations are reported for π-conjugated hydrocarbons including neutral (ground, 1 1Bu, 2 1Ag) and doped (1+ and 2+) chains and rings with up to eight carbon atoms. Two models are considered; first, a semiempirical π-electron Hamiltonian that includes both electron–electron (Hubbard) and electron–lattice (Longuet-Higgins–Salem) interactions, and second, an accurate ab initio complete-active-space self-consistent-field (CASSCF) treatment that includes the π-electron correlation effects most important in determining the bond geometries. The results of the ab initio calculations can be used to estimate the phenomenological parameters entering the semiempirical Hamiltonian and thus to obtain quantitative predictions of bond geometries from the semiempirical treatment. The two models yield qualitatively the same results for the bond geometries in all states considered, and the changes in bond geometry following excitation from ground to doped or excited states find natural interpretation in terms of short-chain limiting behaviors of soliton and polaron distortions familiar for longer chains. Further, the absolute values and sensitivities of the phenomenological parameters of the semiempirical model to various fitting schemes provide an indication of the different roles played by electron–lattice and electron–electron interactions in determining the properties of these systems. While electron–lattice interactions are found to be the most important factor in determining bond geometries, particularly in the ground and doped states, electron–electron interactions play an important and subtle role in determining the bond geometries and relative energetic orderings of the excited states.
Dye laser excitation studies of the Ã 2Π(100)/(020)–X̃ 2Σ+(020)/(000) bands of CaOD: Analysis of the Ã 2Π(100)∼(020) Fermi resonance
Authors: Li Mingguang , Coxon John A.
Published: July 2002

The CaOD Ã 2Π(100)/(020)–X̃ 2Σ+(020)/(000) bands have been rotationally analyzed via high resolution laser excitation. All measured line positions have been included in a global matrix deperturbation that takes account of the Renner–Teller, spin–orbit, and Fermi resonance interactions occurring in the Ã(100)(020) 2Π vibronic manifold. The corresponding bands of CaOH were studied previously; in the present work, two new CaOH subbands, Ã(020)κ 2Π–X̃(020), were recorded, and the complete data set for CaOH has been refitted using the improved model reported in this paper. The Fermi resonance parameter for CaOD has been determined as ‖W1‖=5.2707(22) cm−1; for CaOH, the newly determined value, ‖W1‖=10.3256(5) cm−1 is very close to that determined originally. The (100)∼(020) Fermi interaction in the X̃ 2Σ+ state has also been investigated for both isotopomers. The vibrational dependence of the Renner–Teller parameter εω2 has been characterized, yielding values of the anharmonic quartic parameter, ĝ4=−0.1002(3) and −0.0666(5) cm−1 for CaOH and CaOD, respectively. The ‘‘harmonic’’ Renner–Teller parameters are thus deduced as εω2=−35.6622(19) and −26.5605(31) cm−1 for CaOH and CaOD, respectively. The equilibrium bond lengths, molecular force constants and Coriolis coupling constants for both the Ã and X̃ states have been evaluated.
Molecular dynamics simulations of the rheology of normal decane, hexadecane, and tetracosane
Authors: Cui S. T. , Gupta S. A. , Cummings P. T. , Cochran H. D.
Published: July 2002

Extensive nonequilibrium molecular dynamics simulations have been carried out for liquid decane, hexadecane, and tetracosane at densities corresponding to atmospheric pressure and near ambient temperatures. The strain-rate-dependent viscosity has been obtained for strain rates ranging over several orders of magnitude. At high strain rate, the viscosities for all alkanes studied here have similar values and exhibit similar power-law shear-thinning behavior with a slope between about −0.40 and −0.33. Accompanying this shear thinning is the onset of orientational order and the alignment of the alkane molecules with the flow direction. The alignment angle tends to 45° at very low strain rate and is significantly smaller at high strain rate. This suggests that the chains substantially align in the flow direction and that the dominant motion at high strain rate is the sliding of the chains parallel to the flow. At low strain rate, the shear viscosity shows a transition to Newtonian behavior. The Newtonian viscosity can be obtained from the plateau value of the shear viscosity at the lowest strain rates calculated from the nonequilibrium molecular dynamics simulation (NEMD). This is demonstrated by comparing the viscosity of decane obtained by extrapolating the NEMD simulation with an independent calculation using the standard Green–Kubo method. The transition from the non-Newtonian regime to the Newtonian regime is also correlated with the disappearance of orientational order and with the longest relaxation time of the liquid alkanes simulated.
Electron stimulated desorption from PF3 adsorbed on Pt. II. Negative ions
Authors: Akbulut M. , Madey T. E. , Parenteau L. , Sanche L.
Published: July 2002

We have studied electron stimulated desorption (ESD) of negative ions from PF3 molecules adsorbed on a Pt substrate over a wide electron energy range (0–175 eV). ESD from adsorbed PF3 gives rise to several negative ion fragments: F− (predominantly), F−2, P−, and PF−. The F− yield produced in the electron energy range 0–15 eV proceeds via dissociative electron attachment (DEA); the F− yield exhibits a peak around 11.5 eV with an onset around 7.5 eV. At electron energies above ∼15 eV, F− ions are produced via dipolar dissociation (DD). We have found that the F− ions produced from a 1 ML PF3/Pt surface via a DEA process with 11.5 eV electron impact desorb with a peak kinetic energy of ∼0.7 eV, while the F− ions generated via DD by 175 eV electron impact desorb with a peak kinetic energy of ∼1.2 eV. The F−2 yield curve also shows a peak at ∼11.5 eV; the onset of the F−2 yield from adsorbed PF3 is ∼9 eV. The F−2 yield in the electron range 9–15 eV is initiated via DEA. The P− signal from PF3 adsorbed on Pt has an onset at ∼16 eV. We have identified some possible DEA and DD processes leading to desorption of negative ions from adsorbed PF3. We suggest that Rydberg core-excited (1-hole, 2-electron) transient anion states of PF3, formed by capture of low energy electrons, dissociate to produce the F− and F−2 ions for E(e)<15 eV.
Is acetylene radical anion with a trans–bent form observed in matrix experiment? An ab initio study
Authors: Ha Tae-Kyu , Suter Hans Ulrich , Nguyen Minh Tho
Published: July 2002

Based on extensive ab initio calculations of the hyperfine coupling constants of both hydrogen and carbon nuclei of three C2H−2 anions, it is confirmed that an acetylene radical anion has effectively been produced radiolytically in low temperature alkane matrices and it possesses a trans-bent form. The results are derived from different molecular orbital configuration interaction calculations (CIS, CISD, QCISD and MRD-CI) as well as density functional theory (DFT) employing extended basis sets. Both popular DFT methods (BLYP and B3LYP) show some difficulties in calculating the isotropic coupling constants of the carbon nuclei.
Calculation of nuclear magnetic resonance order parameters in proteins by normal mode analysis. II. Contribution from localized high frequency motions
Authors: Sunada Shinji , Go Nobuhiro
Published: July 2002

Nuclear magnetic resonance (NMR) order parameter S̃ 2 for spin pairs in protein is an important quantity for deducing protein static and dynamic three-dimensional structures from NMR experimental data. In the previous paper it has been shown that contribution from low frequency motions to the order parameter can be calculated reliably from normal mode analysis carried out in dihedral angle space for spin pairs for which a number of intervening dihedral angles (NIVDA) is 10 or less. However, it has been known that for spin pairs with NIVDA=0, high frequency motions also affect the value of the order parameter. In this paper we show that the effect of high frequency motions can be accounted for by a correction term to be added to the value obtained from normal mode analysis in dihedral angle space. A factor of the correction term determined by high frequency motions can be calculated by the usual normal mode analysis for a small peptide fragment which has the same local conformation as in protein.
The direction of electric dipole moment of ICl A 3Π1 state through the analysis of the Stark modulation spectroscopy
Authors: Takei Taka-aki , Watanabe Akiyoshi , Amako Yoshito
Published: July 2002

Recently, two works were reported that the permanent electric dipole moment of the ICl A state has the same direction as that of the X state, through the analysis of electric and magnetic pendular spectra and through the direct analysis of the hyperfine structures under the Stark effect, respectively. We determined clearly and sensitively the direction of the dipole moment in the A (v=27) state of I35Cl, through the line shape analysis of the Stark modulation spectra (SMS). While the result was the same as those of previous two works, the analysis of the line shape of the SMS was so useful to determine the relative direction of the dipole moment of the upper electronic state to that in the lower state, whichever of the first-order or the second-order Stark effect the state may show.
Kinetic phase transitions and reactive windows in reactions of monomers on two-dimensional lattices
Authors: Cortés Joaquı́n , Puschmann Heinrich , Valencia Eliana
Published: July 2002

Some conceptual considerations are made and Monte Carlo simulation studies are carried out to analyze a series of catalytic reactions of two and three monomers on a square lattice of sites. Two aspects are considered: The increase in the system’s degrees of freedom, leading to the formation of reactive sites that allow a change in the character of one of the kinetic phase transitions from the first order to a second order transition, and the classification and reactivity of the new system class.
Thermodynamic properties of aqueous solutions: Nonsymmetric sticky electrolytes with overlap between ions in the mean-spherical approximation
Authors: Hu Ying , Jiang Jian-wen , Liu Hong-lai , Prausnitz John M.
Published: July 2002

Based on a sticky-electrolyte model, the Ornstein–Zernike integral equation is solved for nonsymmetric electrolytes with stickiness between ions at various distances equal to or less than the collision diameter. The hypernetted chain or Percus–Yevick approximation is used for the closure inside the hard core, while the mean-spherical approximation for electrostatic interactions is used for the closure outside the hard core. Expressions for correlation functions and thermodynamic properties in term of the sticky parameters are derived. Numerical results are presented for various cases.
A classical time-frequency theory of transient absorption spectroscopy
Authors: Che Jianwei , Zhang Weimin , Yan YiJing
Published: July 2002

A method of quasiclassical dynamics simulation of transient pump-probe spectra is developed based on an exact time-frequency interference picture for the nonlinear spectroscopy in condensed phases. The present approach allows a proper treatment of the effect of light coherence and interference on material dynamics, and is applicable to fields with arbitrary time scales and profiles. Both the sequential and the coherent contributions to the spectral signal are considered. The proposed numerical approach is suitable to simulate the experimental spectra of molecular systems with many degrees of freedom. A numerical demonstration is presented for the transient laser induced fluorescence (LIF) of I2 molecule at 320 K excited by a pair of pulses.
Structure and energetics of CF3Cl−, CF3Br−, and CF3I− radical anions
Authors: Roszak S. , Koski W. S. , Kaufman J. J. , Balasubramanian K.
Published: July 2002

Theoretical studies for CF3X− (X=Cl, Br, I) anions and its parent molecules are presented. Equilibrium structures of molecular anions were found to be significantly different from those of the neutral molecules since the electron attachment elongates the C–X bond. The complete basis set extrapolation method for the Mo/ller–Plesset second-order perturbation theory has been applied for computing vertical and adiabatic molecular electron affinities, dissociation energies of anions, and neutral molecules, and the enthalpies of dissociative electron attachment reactions.
Photodesorption of Na atoms from rough Na surfaces
Authors: Balzer F. , Gerlach R. , Manson J. R. , Rubahn H.-G.
Published: July 2002

We investigate the desorption of Na atoms from large Na clusters deposited on dielectric surfaces. High-resolution translational energy distributions of the desorbing atoms are determined by three independent methods, two-photon laser-induced fluorescence, as well as single-photon and resonance-enhanced two-photon ionization techniques. Upon variation of surface temperature and for different substrates (mica vs lithium fluoride) clear non-Maxwellian time-of-flight distributions are observed with a cos2 θ angular dependence and most probable kinetic energies below that expected of atoms desorbing from a surface at thermal equilibrium. The half-width of the time-of-flight distribution decreases with increasing surface temperature. A quantitative description of the energy and angular distributions is presented in terms of a model which assumes that following the initial surface plasmon excitation neutral atoms are scattered by surface vibrations. Recent experiments providing time constants for the decay of the optical excitations in the clusters support this model. The excellent agreement between experiment and theory indicates the importance of both absorption of the laser photons via direct excitation of surface plasmons as well as energy transfer with the substrate during the desorption process.
The effect of reagent translational energy on the dynamics of the reaction O(3P)+CS(X 1Σ+)→CO(X 1Σ+)+S(3P)
Authors: Summerfield Dean , Costen Matthew L. , Ritchie Grant A. D. , Hancock Gus , Hancock Thomas W. R. , Orr-Ewing Andrew J.
Published: July 2002

Two different regimes of collision energy are used to explore the role of additional translational energy on the dynamics of the reaction O(3P)+CS(X 1Σ+)→CO(X 1Σ+)+S(3P). Product CO rotational quantum-state population distributions for CO(v′=12, 13, and 14) are used as an indicator of the reaction dynamics, and these rotational distributions are presented for reaction of thermal reagents (at 298 K) and for translationally hot oxygen atoms formed by the 355 nm photolysis of NO2. The experimental measurements are compared with the results of quasiclassical trajectory calculations performed on an empirical London–Eyring–Polanyi–Sato potential energy surface tailored to model the observed dynamics for thermal reagents. Efficient conversion of the extra translational energy into product rotation is seen for all vibrational levels studied. The data are found to fit a simple model in which the fraction of the extra kinetic energy which appears as product rotation varies linearly with kinetic energy, and becomes unity for the fastest oxygen atoms produced by photolysis. The experimental results are interpreted in terms of an increasingly bent transition state for the reaction at higher collision energies, with the possibility of reagent reorientation towards a more linear transition state as the kinetic energy is decreased.
Kohn-Sham orbital formulation of the chemical electronic responses, including the hardness
Authors: Senet P.
Published: July 2002

In a recent paper [J. Chem. Phys. 105, 6471 (1996)], nonlinear chemical responses of a system to a simultaneous change of its external potential and of its number of electrons have been formulated in terms of the ground-state electronic density for a given model of the Hohenberg-Kohn functional. In the present work, an exact one-electron formulation of all the chemical responses is derived in terms of the Kohn-Sham orbitals of the unperturbed system. The present formulation encompasses the band-structure formulation of the linear Fukui function derived recently [M.H. Cohen, M.V. Ganduglia-Pirovano, and J. Kudrnovský, J. Chem. Phys. 101, 8988 (1994)] and provides an exact orbital expression of the linear hardness. The latter is compared to the hardness matrix used in the construction of ab initio pseudo-potentials [M. Teter, Phys. Rev. B 48, 5031 (1993)]. In addition, the relation between the covalent radius of atoms and the linear and nonlinear hardnesses is discussed.
Transitions through fluctuating barrier: Role of asymmetry and memory
Authors: Fuliński A.
Published: July 2002

The kinetics of single transition A→B over a fluctuating barrier is considered. Fluctuations are modeled by dichotomous noise. The average first passage time (AFPT) tp* is defined as the time elapsed from the beginning of the process (system in the state A with probability 1) to the moment when the system attains for the first time the state B with the average probability equal to p. It is found that the non-Markovianity of the barrier fluctuations may introduce oscillations in the process of barrier crossing and in the effective reaction rate, and elongate the AFPTs. Especially, t0.95* may become infinite, even when t0.5* remains finite—the process of barrier crossing is reverted after some time. However, in some cases (strong asymmetry of barrier fluctuations, high AFPTs in the absence of fluctuations, together with long memory characteristic time of the non-Markovian part of the fluctuations) the effective reaction rates can be enhanced and AFPTs shortened in comparison with those for static barrier.
Theoretical interpretation of the electroabsorption spectra of polyacene crystals. I. Role of Frenkel states
Authors: Slawik Michał , Petelenz Piotr
Published: July 2002

The recently proposed two-dimensional model of the coupling between the Frenkel and charge transfer (CT) excitations of a molecular crystal, successfully applied for the calculation of the electroabsorption (EA) spectra of polyacenes [Petelenz et al., J. Chem. Phys. 105, 4427 (1996)], is applied to probe the influence of different intramolecular excited states on the EA signal in the CT energy range. The study highlights the preeminent role of Frenkel excitons that are energetically close to the CT manifold, even if they are inactive in conventional absorption spectroscopy. The high-energy intense excitons are found to have a lesser effect. The study suggests EA spectroscopy as a potential tool to detect and locate some of the absorption-inactive intramolecular excitations.
The effects of different substrates on the electron stimulated desorption dynamics of O− from physisorbed O2
Authors: Hedhili M. N. , Parenteau L. , Huels M. A. , Azria R. , Tronc M. , Sanche L.
Published: July 2002

We report condensed phase measurements of kinetic energy (Ek) distributions of O−, produced by dissociative electron attachment (DEA) at 6 eV incident electron energy; they are obtained under identical experimental conditions from submonolayer quantities of O216 deposited on disordered multilayer substrates of O218, Ar, Kr, Xe, CH4, and C2H6, all condensed at 20 K on polycrystalline platinum (Pt). The results suggest that the desorption dynamics of O− DEA fragments is, in part, determined by large angle elastic scattering of O− prior to desorption, as well as the net image charge potential (Ep) induced in the condensed dielectric solid and the Pt metal. The measurements also indicate that, particularly at small Kr substrate thicknesses, the Ep may not necessarily be uniform across the surface, but may fluctuate due to surface roughness. Thus, in addition to energy losses in the substrate prior to, and during, DEA, these effects may influence the dissociation dynamics of the O2− resonance itself, as well as the desorption of the DEA O− fragment.
On the influence of electron mobilities on the yields of formation of positronium in liquids: New I3 measurements and Monte Carlo simulations
Authors: Billard Isabelle , Goulet Thomas , Jay-Gerin Jean-Paul , Bonnenfant André
Published: July 2002

We study the formation of positronium (Ps) at the end of the radiation tracks generated by the passage of energetic positrons in various liquids. We focus particularly on the possible influence of the thermal electron mobility μ(e−) on this process. To this end, we present a compilation of experimental data of the probability of ortho-Ps formation I3 and μ(e−) in 51 pure liquids at room temperature. We also present new measurements of I3 as a function of temperature for n-hexane, 2,2-dimethylbutane, tetramethylsilane, and tetramethylstannane from the melting point up to 294 K. Those results are compared to the variation of μ(e−) with temperature which can be found in the literature. Finally, we use Monte Carlo simulation techniques to perform a sensitivity study in which we evaluate the relative contributions of various physicochemical properties of the liquids (such as the mean thermalization distances of e+ and e−, their thermal mobilities, the dielectric constant, and the temperature) on the values of I3. A review of previous works on this question is also given.
Polydisperse solution of randomly branched homopolymers, inversion symmetry and critical and theta states
Authors: Gujrati P. D.
Published: July 2002

We discuss the phase behavior of a model of a binary mixture of randomly branched homopolymers in a solution. The monomer–solvent interaction is determined by a Boltzmann weight w. The theory has been presented recently and is obtained by approximating the underlying lattice by a Bethe lattice of the same coordination number q. Of special interest is the class of randomly branched polymers with inversion symmetry (see the text). This class includes linear polymers. The phase diagram for the special class of polymers is very simple. There is a line C of critical points in the dilute limit on which branched polymers become a critical object in a good solvent. This is an extension of the result due to de Gennes for linear chains in an athermal solution to the above class of branched polymers in any good solvent. The line C meets with another critical line C′ for phase separation in a poor solvent. We identify the theta point as a tricritical point as first suggested by de Gennes for linear chains only. The theta point appears only in the limit of infinite polymers such that the second virial coefficient A2 vanishes. We calculate various exponents and identify the order parameter. We point out a subtle difference between the theta state and the random walk state. However, the radius of gyration exponent does have its mean-field value of 1/2 in the theta state but only in d⩾3. There does not exist a tricritical point for randomly branched polymers without inversion symmetry.
A quantitative theory of linear chain polymer dynamics in the melt. IV. Comparison with experimental diffusion constant data
Authors: Panajotova Biliana V. , Herman Michael F.
Published: July 2002

The results of a recently presented theory for polymer dynamics are compared with experimental diffusion constant data for polyethylene, poly (ethylene oxide), and poly (dimethyl siloxane) polymer melts. The molecular weight dependence of the self-diffusion constants for these systems is investigated. The various constants that appear in the theory are optimized using theoretical analysis and published results from computer simulations. The only adjustable parameters for each system are the friction coefficients for lateral and reptative motions. It is found that the same ratio of the monomer friction coefficients for lateral and reptative motions can be employed for all three experimental systems and for the two computer simulation systems considered. This is reasonable, since all these systems have fairly simple monomer units without large side groups. This leaves the monomer friction coefficient for lateral motions as the only system-dependent adjustable parameter in these calculations. Excellent agreement is found between the predictions of the theory and the experimental data.
Photoinduced nucleation in supersaturated mercury vapor
Authors: Uchtmann H. , Dettmer R. , Baranovskii S. D. , Hensel F.
Published: July 2002

Photoinduced nucleation of mercury vapor is studied experimentally in an upward thermal diffusion cloud chamber. The rate of nucleation is greatly increased when the supersaturated vapor is illuminated with light absorbed by the vapor molecules. In analogy to ion-induced nucleation the photoinduced nucleation can be explained as a general effect for all condensing atoms or molecules forming long living electronically excited states.
A modification of the generalized Born theory for improved estimates of solvation energies and pK shifts
Authors: Jayaram B. , Liu Y. , Beveridge D. L.
Published: July 2002

We present herein an appraisal on the performance of the generalized Born (GB) model in estimating the solvation energies of small molecules and pKa shifts of dicarboxylic acids. The quality of the solvation energy results obtained with the GB model was exceedingly good as already reported in the literature but the pKa shift estimates fell short of expectations. Analysis of the problem on a simple prototype system revealed that with the GB model, the estimates of the two components, viz. the shielding and the self-energy terms, to be somewhat in error. These errors compensate each other in the calculation of solvation energies but affect the intramolecular interaction energies and hence pK shifts differently. We examine here the feasibility of introducing modifications to the GB model for a simultaneous evaluation of both solvation and intramolecular interaction energies.