bluuebunny/crossref_metadata_2025 huggingface dataset:
Microwave measurements of cobalt and nitrogen quadrupole coupling in Co(CO)3NO
Authors: Kukolich S. G. , Roehrig M. A. , Haubrich S. T. , Shea J. A.
Published: July 2002

J=2→3, 3→4, 4→5, and 5→6 transitions in the oblate symmetric top molecule cobalt tricarbonyl nitrosyl were measured using a Flygare–Balle type pulsed beam microwave spectrometer. K=0 and K=3 transitions were observed for J=3→4 and 4→5. Hyperfine structure due to 59Co and 14N nuclear quadrupole coupling interactions was well resolved. The measured quadrupole coupling strengths are eQqcc (59Co)=35.14(30) MHz and eQqcc (14N)=−1.59(10). Measured rotation and distortion constants are B0=1042.1590(4) MHz and Dj =0.17(8) kHz. The measured B value is 4% smaller than the B value calculated from electron diffraction data. Spin–rotation and a quadrupole distortion term were also obtained for 59Co.
Probing icy surfaces with the dangling-OH-mode absorption: Large ice clusters and microporous amorphous ice
Authors: Rowland Brad , Fisher Mark , Devlin J. Paul
Published: July 2002

Infrared absorption bands that have previously been assigned to vibrations of the dangling bonds (db) of water molecules at the surfaces of crystalline ice clusters and the micropores of amorphous ice have been investigated to determine their usefulness in probing molecular-level events at icy surfaces. This study has established that the db bands are sensitive to adsorption of gases at the cluster or micropore surfaces, and that reversible adsorption/desorption at the db sites is readily monitored spectroscopically. Consequently, energetics for the interactions with adsorbents such as H2 and N2 are potentially measurable. It has also been demonstrated, for both clusters and the micropores, that surface HOD molecules give unique db band positions, and that the intensities of the db bands are indicative of a strong preference of the surface HOD molecules to engage in deuterium bonding to the subsurface molecules. The unique positions of the HOD db bands also signals a potential for using isotopic-exchange data to monitor point-defect activity at icy surfaces.
The effect of reagent excitation on the dynamics of the reaction O(1D2)+H2→OH(X 2Π)+H
Authors: Berg P. A. , Sloan J. J. , Kuntz P. J.
Published: July 2002

The effect of H2 translational, rotational, and vibrational excitation on the dynamics of the O(1D2)/H2 reaction are explored in a semiclassical trajectory study involving both of the energetically accessible potential energy surfaces of the system. Landau–Zener probabilities determine surface hopping. At low reagent excitation, the deep H2O potential minimum dominates the dynamics, causing the reagents to reorient towards a H–O–H (insertion) configuration and form the H2O intermediate, irrespective of the initial approach geometry. High vibrational excitation enhances the probability for transitions onto the excited state potential during the interaction. Reactions which sample the excited state potential have fundamentally different dynamics from those which remain on the lower state. For reactions involving H2(v=4), the OH product has a bimodal vibrational distribution, peaking in OH(v′=2) and OH(v′=9). The lower peak is due to reactions which access the excited state potential; the higher peak results from those which remain on the lower state for the entire interaction. High translational excitation shortens the interaction time and reduces the effect of the potential minimum to reorient the reagents. Rotational excitation also reduces the effect of the potential minimum by causing the system to rotate out of the insertion geometry before entering the potential minimum.
Dynamics of polymer solutions and melts. Reptation predictions and scaling of relaxation times
Authors: Paul Wolfgang , Binder Kurt , Heermann Dieter W. , Kremer Kurt
Published: July 2002

The bond fluctuation model on the simple cubic lattice is studied by Monte Carlo simulations on a multitransputer array, for polymer volume fractions φ in the range 0.025≤φ≤0.500 and chain lengths N in the range 20≤N≤200. Extensive data are presented on the dynamics of monomer displacements, center-of-gravity displacements, and relaxation times. This study is complementary to previous work, in which the crossover scaling properties of the chain linear dimensions, structure factor, and self-diffusion constant were tested for the same athermal model. The simulation technique takes both excluded volume interactions and entanglement constraints into account, but ignores hydrodynamic forces. Our results describe the crossover from Rouse behavior of swollen chains (τ∼N1+2ν, ν being the exponent describing the radius R of the chains, R∼Nν ) to reptation, τ∼N3. Since the excluded volume screening length is found to be smaller than the tube diameter by a factor of about 3, the rescaled times Wτ/N1+2ν decrease first as a function of the scaled chain length Ñ∼Nφ1/(3ν−1), before they increase due to the onset of reptation. Additional evidence for reptative behavior is found by identifying the several successive crossovers in the time-dependent displacements predicted by de Gennes.
Electronic spectra of jet-cooled 3-chlorotropolone. Proton tunneling in the S1 state
Authors: Tsuji Takeshi , Sekiya Hiroshi , Nishimura Yukio , Mori Akira , Takeshita Hitoshi
Published: July 2002

The electronic spectra have been measured for jet-cooled 3-chlorotropolone in the S1–S0 region to investigate the effect of an asymmetric substitution on proton tunneling in tropolone. Both the 0++ and 0−− transitions have been identified in the fluorescence excitation spectrum, while no 0−+ or 0+− transition has been detected. This suggests that the double minimum potential energy function is almost symmetric for both S1 and S0. The tunneling doublet separations for the origin bands of 3-chlorotropolone and its OD derivative have been measured to be 23 and 3 cm−1, respectively. These values are very similar to the corresponding values of 19 and 2 cm−1 for tropolone and its OD derivative, indicating that the substitution of a chlorine atom influences only a little on the strength of the intramolecular hydrogen bond.
Photoionization mass spectrometric study of N2H2 and N2H3: N–H, N=N bond energies and proton affinity of N2
Authors: Ruscic B. , Berkowitz J.
Published: July 2002

The photoion yield curves of N2H+2, N2H+, and H+2 from N2H2 have been studied by photoionization mass spectrometry. The adiabatic ionization potential of N2H2 determined (9.589 ± 0.007 eV) is in excellent agreement with that obtained by photoelectron spectroscopy (PES). The appearance potential of N2H+ (10.954 ± 0.019 eV at 0 K) is in good agreement with earlier electron impact measurements. The appearance potential of H+2 (<13.526 ±0.03; probably ≤13.403 ±0.036 eV, at 0 K) leads to ΔH0f0 (N2H2) >43.8 ±0.7; probably ≥46.6 ±0.8 kcal/mol. Consequently, proton affinity (PA) (N2) <121.8 ±0.8; probably ≤119.0 ±0.9 kcal/mol, which is in good agreement with results from proton transfer experiments and with most ab initio calculations. The N■N ‘‘double bond’’ energy determined from N2H2→2NH is <126.6 ±1.0; probably ≤123.8 ±1.1 kcal/mol. The adiabatic ionization potential of N2H3 is found to be 7.61±0.01 eV. This observation leads to D0 (H2NNH–H)=80.8±0.3 kcal/mol, and D0(HNNH–H)≂43.8±1.1 kcal/mol.
A Monte Carlo study of surface diffusion coefficients in the presence of adsorbate–adsorbate interactions. III. Repulsive nearest-neighbor and attractive next-nearest-neighbor interactions
Authors: Uebing C. , Gomer R.
Published: July 2002

Tracer, jump, and chemical diffusion coefficients D*, Dj, and D, respectively, were determined on a 50×50 square array of fixed sites by the fluctuation and Kubo–Green methods for the case of nearest-neighbor repulsions and numerically equal next-nearest-neighbor attractions. Both methods give the same D values except at θ=0.5 and T/Tc<0.9, where c(2×2) dominates and simulations are extremely sensitive to very small changes in the small amount of disorder. Activation energies are, in all cases, controlled by next-nearest-neighbor attraction, i.e., are positive relative to the noninteracting case and peak at θ=0.5. However, D has a maximum at θ=0.5 because of a deep minimum in 〈(δN)2〉/〈N〉. D* and Dj are very similar to each other and have a minimum at θ=0.5 and low T, because of the maxima in activation energies at this coverage, caused by the necessity to deform the c(2×2) structure.
The implementation of the multireference coupled-cluster method based on the single-reference formalism
Authors: Oliphant Nevin , Adamowicz Ludwik
Published: July 2002

A generalized version of the multireference coupled-cluster method using a single-reference formalism, which we presented in an earlier paper, has been implemented. Any number of determinants, that differ from the formal reference determinant by single or double excitations, can now be included in the reference space. In the present implementation, the single and double excitations from the secondary reference determinants have been truncated to include only those that correspond to triple excitations from the formal reference determinant. Calculations are done on a few model systems, LiH, BH, and H2O, at equilibrium and stretched geometries. Comparisons are made with full configuration interaction (CI) treatment for the single bond stretch in LiH and BH, and the results are quite promising. For the water molecule, comparisons are made with the results obtained with the coupled cluster method truncated at triple excitations (CCSDT), as well as with the full CI results. While the multireference method did not do as well for the simultaneous two-bond stretch in H2O as it did for the single bond cases, it did at least as well as the CCSDT at representing the points on the full CI potential curve.
A new diabatic representation of the coupled potential energy surfaces for Na(3p 2P)+H2→Na(3s 2S)+H2 or NaH+H
Authors: Halvick Philippe , Truhlar Donald G.
Published: July 2002

We present a new diabatic representation of the coupled potential energy surfaces for Na(3p 2P)+H2→Na(3s 2S)+H2 or NaH+H. The new representation is designed to yield, upon diagonalization, realistic values for the two lowest energy 2A′ adiabatic states at both asymptotes of the chemical reaction as well as near the conical intersection in the three-body interaction region. It is economical to evaluate and portable. It is suitable for dynamics calculations on both the quenching process and the electronically nonadiabatic chemical reaction.
Characterization of the cylindrical cavities of Anopore and Nuclepore membranes
Authors: Crawford G. P. , Steele L. M. , Ondris-Crawford R. , Iannacchione G. S. , Yeager C. J. , Doane J. W. , Finotello D.
Published: July 2002

The microscopic properties of the inner surface of the cavities of Anopore and Nuclepore membranes are investigated with scanning electron microscopy (SEM), nitrogen adsorption isotherms, and deuterium nuclear magnetic resonance (2H-NMR). Useful information about the cavity orientation and morphology, and internal surface area is obtained. Analysis of SEM photographs yields estimates of the surface area and the porosity of these membranes which complements the adsorption results. The orientation of liquid crystals permeated in the cavities of Anopore and Nuclepore membranes is probed with the 2H-NMR technique. It is found that the orientation of the liquid crystal molecules is governed by the confining volume and surface conditions. The 2H-NMR spectral line shape of the confined liquid crystal also provides information on the substrate morphology and roughness that is consistent with SEM and adsorption experiments.
Development of nonadditive intermolecular potentials using molecular dynamics: Solvation of Li+ and F− ions in polarizable water
Authors: Dang Liem X.
Published: July 2002

Nonadditive intermolecular potentials for ion–H2O and ion–(H2O)2 complexes (ion=Li+ and F− ) were derived using molecular-dynamics methods. The successive H2O binding energies and structural properties of Li+(H2O)n and F−(H2O)n (n=3–6) clusters, including simulations of aqueous ionic solutions, were examined using these potential parameters. The results reproduce well-observed solvation enthalpies as well as structural properties for these ions. For n<5, water molecules distributed almost symmetrically around the Li+ ion, while water molecules preferred to cluster on the same side of the F− ion. These features are in agreement with earlier Monte Carlo, molecular dynamics, and molecular mechanics studies on similar systems. We studied the relationships between ionic clusters and bulk simulations by comparing structural properties for these simulations. In many cases, we found these properties were quite similar. These results provide valuable information in understanding the relationships between ionic clusters and solution chemistry.
Collisional excitation of metastable O(1D) atoms
Authors: Sun Yan , Dalgarno A.
Published: July 2002

The excitation of ground state O(3P) atoms into the metastable O(1D) state by collision with O(3P) atoms occurs predominantly via the lowest 3Πg states of O2. Experimental data on the decay products of the 3Πg Rydberg state are used in a scattering calculation to predict the cross sections and rate coefficients for the excitation and quenching of O(1D) atoms in collision with O(3P) atoms. At 1000 K, the quenching rate coefficient is 6.9×10−12 cm3 s−1. The excitation cross section reaches a maximum of 7.2×10−18 cm2 at a relative energy of 3.8 eV.
Density functional study of the bonding in small silicon clusters
Authors: Fournier René , Sinnott Susan B. , DePristo Andrew E.
Published: July 2002

We report the ground electronic state, equilibrium geometry, vibrational frequencies, and binding energy for various isomers of Sin(n = 2–8) obtained with the linear combination of atomic orbitals-density functional method. We used both a local density approximation approach and one with gradient corrections. Our local density approximation results concerning the relative stability of electronic states and isomers are in agreement with Hartree–Fock and Mo/ller–Plesset (MP2) calculations [K. Raghavachari and C. M. Rohlfing, J. Chem. Phys. 89, 2219 (1988)]. The binding energies calculated with the gradient corrected functional are in good agreement with experiment (Si2 and Si3) and with the best theoretical estimates. Our analysis of the bonding reveals two limiting modes of bonding and classes of silicon clusters. One class of clusters is characterized by relatively large s atomic populations and a large number of weak bonds, while the other class of clusters is characterized by relatively small s atomic populations and a small number of strong bonds.
Unstable modes in liquids density of states, potential energy, and heat capacity
Authors: Madan B. , Keyes T.
Published: July 2002

Theoretical expressions are presented, at various levels of approximation, for the density of states <ρu(ω)≳ of harmonic modes with imaginary frequencies in liquids; these are also referred to as unstable modes. The relation between the frequency integral of <ρu(ω)≳, which is the fraction of unstable modes fu and the barrier crossing Boltzmann factor for self-diffusion is clarified. A new relation between the potential energy of the supercooled liquid and fu is derived. The averaged barrier energy/particle for diffusion enters this relation as a parameter and our consequent estimates of the barrier are in good agreement with existing estimates.
Chemical softness and impurity segregation at grain boundaries
Authors: Dal Pino A. , Galván M. , Arias T. A. , Joannopoulos J. D.
Published: July 2002

We analyze the process of impurity segregation at grain boundaries as a chemical reaction between the impurity and the interface. From this point of view, we test the ability of the concepts of local softness and hardness to predict the most probable sites for impurity accumulation. As a test, an ab initio investigation of the ∑=5 tilt [310] grain boundary in germanium is presented and the energetics of the accumulation of dopant atoms in this interface are studied. Our results support the utility of an analysis in terms of softness for impurity segregation problems.
Temperature-enhanced electron detachment from C6F6− negative ions
Authors: Datskos P. G. , Christophorou L. G. , Carter J. G.
Published: July 2002

A method is described whereby photoelectrons generated by a short laser pulse at the cathode of a parallel-plate electrode arrangement are depleted by attachment to C6F6 molecules mixed with N2 in the interelectrode space as they drift to the anode under an externally applied electric field. The contribution of the initially produced (prompt) and the delayed (autodetached from C6F6−) electrons to the induced signal in the detector circuit is recorded as a function of time following the laser pulse and also as a function of gas number density, applied electric field, and gas temperature, T. Increases in T from ambient to 575 K enhance dramatically the autodetachment frequency, τd−1, for C6F6−. This heat-activated autodetachment correlates with the increase in the internal energy of the anion and has an activation energy of 0.477 eV. Electron attachment producing C6F6− initially increases slightly with increasing T below 500 K and subsequently decreases with further increases in T.
Cis-stilbene isomerization: Temperature dependence and the role of mechanical friction
Authors: Todd David C. , Fleming Graham R.
Published: July 2002

The fluorescence decay time of cis-stilbene has been measured in a variety of solvents over a large temperature range. An isoviscosity Arrhenius plot in n-alkanes yields an activation energy of 386±29 cm−1. We interpret this result as an upper limit for the cis-stilbene to trans-stilbene barrier in nonpolar solvents. Isoviscosity plots in small alcohols are nonlinear, indicating complicated behavior in this solvent type. The excited state decay times in n-alcohols and n-alkane solvents correlate well with each other when plotted as a function of chain length, n. We infer from this plot that macroscopic viscosity is a poor measure of the friction felt by the isomerizing species when changing solvent, and that the potential energy surface is not substantially altered between n-alkanes and n-alcohols with n≥5. Decay times measured in 2-propanol at 490, 475, 453, and 440 nm emission differ by no more than 90 fs, indicating that there is little or no spectral evolution during the excited state lifetime. A short component in the fluorescence anisotropy decay of cis-stilbene and a decrease in the excited state lifetime due to deuterium substitution for the ethylenic hydrogens are observed in both polar and nonpolar solvents. Treating DHP formation as a thermally activated process which competes with the cis to trans isomerization, we determine a range of model dependent cis to trans activation energies in alkanes of −300 to +380 cm−1.
Rheological properties of suspensions of interacting rodlike FD-virus particles
Authors: Graf Christian , Kramer Hansgerd , Deggelmann Martin , Hagenbüchle Martin , Johner Christian , Martin Christoph , Weber Reinhart
Published: July 2002

Low shear (γ̇=1 s−1) and shear rate dependent (1 s−1<γ̇<100 s−1) viscosity measurements on aqueous suspensions of rodlike FD-virus particles (length=880 nm, diameter=9 nm) below and above the overlap concentration c* =1 particle/length3 are presented. Properties like intrinsic viscosity [η], the virus concentration and shear rate dependence of η are studied in deionized (‘‘saltfree’’) suspensions and in the presence of NaCl, where the Coulomb interaction between the particles is totally screened. In the latter case, [η] is in excellent agreement with theoretical predictions [A. R. Altenberger and J. S. Dahler, Macromolecules 18, 1700 (1985); R. M. Davis and W. B. Russel, Macromolecules 20, 518 (1987)]. As a function of the virus concentration, η follows certain power laws in c. The observed exponents depend here on the applied shear rate. In the low shear region, η(c) can be described by the well known Huggins behavior. An attempt to fit the data by the popular stretched exponential form failed. The variation of η with shear rate is compared with available theories [M. Doi and S. F. Edwards, The Theory of Polymer Dynamics (Clarendon, Oxford, 1986); A. R. Altenberger and J. S. Dahler, Macromolecules 18, 1700 (1985); J. S. Dahler, S. Fesciyan, and N. Xystris, Macromolecules 16, 1673 (1983)]. A theory of Hess [Z. Naturforsch. Teil A 35, 915 (1980)] allows us to evaluate the concentration dependent values of the rotational diffusion constant Drot from the η(γ̇) data which are found to be in very good agreement with the values of Drot, obtained by electric or magnetic birefringence [H. Kramer, M. Deggelmann, C. Graf, M. Hagenbüchle, C. Johner, and R. Weber, Macromolecules 25, 4325 (1992); J. F. Maguire and J. P. McTague, Phys. Rev. Lett. 45, 1891 (1980); H. Nakamura and K. Okano, Phys. Rev. Lett. 50, 186 (1983)]. For strong Coulomb interaction among the suspended viruses no adequate theory is available. Therefore, the data achieved under these conditions are interpreted in terms of the corresponding results of the non-Coulomb interacting samples.
Structure in liquid water: A study of spatial distribution functions
Authors: Svishchev I. M. , Kusalik P. G.
Published: July 2002

Despite the fact that an enormous literature has now accumulated on the structure in liquid water, the focus has been primarily limited to the average radial distributions of particles; local (atomic) pair-density maps which span both the radial and the angular coordinates of the separation vector have remained largely unexplored. In this work, we have obtained the spatial distribution functions gOO(r,Ω) and gOH(r,Ω) for liquid water and have applied them to an analysis of the equilibrium structure. Molecular dynamics simulations of SPC/E water have been carried out at temperatures of −10, 25, and 100 °C and the local liquid structure examined. It is found that the unfolded O...O distribution demonstrates, in addition to peaks consistent with a continuous tetrahedral network pattern, a distinct maximum in the local atomic pair density at ‘‘interstitial’’ separations of about 3.5 Å. This local maximum is lost in the spatially folded radial distribution function gOO(r) due to averaging over the entire angular space. By examining the peaks in gOO(r,Ω) due to nearest neighbors, we have shown that the tetrahedral network coordination number in liquid SPC/E water equals 4.0 and does not depend on temperature. The average number of molecules in additional nontetrahedral coordination, which is found to vary with temperature, has also been extracted, enabling us to establish full average coordination numbers of 4.8–5.0 in the temperature range of −10–100 °C. In addition, we have determined and analyzed statistical distributions for the pair energies and H-bond angles for different water fractions as identified from gOO(r,Ω) and gOH(r,Ω).
Energy landscapes and the collapse dynamics of homopolymers
Authors: Chan Hue Sun , Dill Ken A.
Published: July 2002

What is the shape of the conformational free energy landscape for the collapse of a single homopolymer molecule? We model the dynamics of homopolymer collapse in a poor solvent to a state of high compactness using short self-avoiding walks on two-dimensional square lattices. The conformations are obtained by exhaustive enumeration. The time-evolution is modeled by an analytic transition matrix using Metropolis dynamics with two different move sets. A main finding is that the shape of the energy landscape depends strongly on the move set, suggesting the need to be cautious in interpreting kinetic sequences of events and shapes of landscapes in Monte Carlo polymer dynamics. For some of the collapse pathways, we observe two different rates: a fast collapse to nonoptimal compact states, then a much slower rearrangement to maximally compact conformations. This two-stage process resembles protein folding through compact intermediate states. But we note that the nature and existence of the kinetic traps strongly depends on the move set. In addition, we find that the ‘‘Levinthal paradox’’ argument for protein folding does not correctly predict the collapse time when applied to homopolymers.
Molecules in helium clusters: SF6HeN
Authors: Barnett R. N. , Whaley K. B.
Published: July 2002

Variational and diffusion Monte Carlo results are presented for the ground states of several SF6HeN clusters in the range N=1–499. The diffusion Monte Carlo computations are well converged, yielding an expected accuracy in the energy well under 1%. Computations are performed employing both an isotropic and an anisotropic He–SF6 interaction potential. Novel trial wave functions are used to describe both the shell structure of these clusters and the anisotropy arising from the potential. The ground state helium densities show the SF6 located at the cluster center, inducing a large degree of localization and a shell-like structure in the surrounding helium. Although the full potential causes a large degree of anisotropy in the helium density, general characteristics such as the energy and size are not greatly affected by the potential anisotropy. Finally, we compute spectral shifts for the ν3 SF6 vibration due to the instantaneous dipole–induced dipole mechanism and compare with recent experiments. We find a red shift which for N≤111 increases with N to a maximum value of 0.93 cm−1, with a width of 0.25 cm−1, at N=111.
Structural characterization of C60 and C70 fullerenes by small-angle neutron scattering
Authors: Affholter K. A. , Henderson S. J. , Wignall G. D. , Bunick G. J. , Haufler R. E. , Compton R. N.
Published: July 2002

Small-angle neutron scattering (SANS) is a proven tool for examining the structure and interactions of particles in solution, though the dimensions of carbon-cage molecules are close to the lower resolution limit of the technique. Deuterated solvents (toluene-d8 and benzene-d6) have virtually no scattering contrast with carbon, and the high incoherent cross section of protonated (hydrogen containing) solvents severely limits the path length of solutions by reducing the sample transmission. We have circumvented these difficulties by using CS2 as a solvent which has good contrast with carbon, and a low incoherent cross section which allows the use of long sample path lengths (up to ∼10 cm). In addition, CS2 has good solubility for fullerenes and these properties permit the measurement of the radii of gyration (Rg) of both C60 (Rg=3.82±0.05 Å) and C70 (Rg=4.13±0.05 Å). These dimensions are similar to those calculated from the atomic coordinates after allowing for a solvent exclusion volume. Close agreement between the measured absolute scattering cross sections and the values calculated from the particle and solvent scattering length densities serves as a cross check on the validity of this methodology. To our knowledge, this represents the first successful application of SANS for the characterization of fullerenes. SANS makes it possible to study the size and shapes of modified buckyballs, solute/solvent interactions, crystal growth from saturated solutions, and temperature dependent transitions in solutions.
Collective motions in liquids with a normal mode approach
Authors: Wu Ten-Ming , Loring Roger F.
Published: July 2002

We present a theory of collective dynamics in simple liquids within a harmonic approximation. We extend the normal mode approximation, which has previously been applied to single-particle properties, such as the velocity autocorrelation function, to the calculation of the longitudinal and transverse particle current autocorrelation functions. Within the harmonic approximation, these autocorrelation functions may be related to a configuration-averaged phonon Green’s function, which is a generalization of the conventional Green’s function for a perfect crystal. We show that the calculation of this Green’s function is equivalent to the evaluation of a propagator in a random walk problem, in which a walker with internal states hops among sites located at the particles of the fluid. We develop an approximate, self-consistent theory for this Green’s function, which is used to calculate the longitudinal current correlation function for a dense Lennard-Jones fluid. The results are compared to previous computer simulations of this correlation function.
Dissociations of the ethyne dication C2H2+2
Authors: Thissen R. , Delwiche J. , Robbe J. M. , Duflot D. , Flament J. P. , Eland J. H. D.
Published: July 2002

Dissociations of the ethyne dication following its production by photoionization in the photon energy range of 35–65 eV have been investigated by the photoelectron–ion–ion coincidence technique using both synchrotron radiation and laboratory light sources. New quantum mechanical calculations identify and locate the electronic states of the molecular dication in this energy range and show that the dissociation products are formed in their ground states by heterogeneous processes. Five reaction channels leading to three molecular fragments have been identified and are interpreted as sequential processes, several faster than fragment rotation and one possibly involving dissociation of CH+ to H+ with a lifetime of the order of 25 fs.
Liquid benzene confined between graphite surfaces. A constant pressure molecular dynamics study
Authors: Winkler Roland G. , Hentschke Reinhard
Published: July 2002

We derive expressions for the internal and external stress tensor for a molecular system which is periodic in two dimensions and confined by an external potential along the third dimension. Based on the external pressure, we propose an extended-phase-space constant pressure molecular dynamics simulation method. We apply this method to simulations of liquid benzene confined between two graphite surfaces. The intramolecular interactions are thereby adopted from the amber force field, whereas the liquid-surface interaction is described in terms of a static external potential acting on the benzene molecules. Our results test the method and provide a molecular picture of the surface induced structure of the liquid as well as its dynamical behavior near the surface. We find that the surface affects the structure of the liquid on a length scale significantly larger than the direct range of the liquid-surface potential. Additionally, we observe a fast exchange of benzene molecules between the first and second liquid layer adjacent to the surface.
n-propanol on graphite: The occurrence of two-dimensional smectic mesophase
Authors: Morishige Kunimitsu
Published: July 2002

The x-ray diffraction measurements of n-propanol adsorbed on graphite have been taken over the temperature range 90–250 K and the coverage range 1.34–4.17 molecule/nm2. The diffraction pattern clearly illustrates that a two-dimensional smectic liquid crystal is formed in a melting process of an anisotropic, incommensurate monolayer of n-propanol on graphite. In this phase a hydrogen-bonded head-to-head association of molecules is arranged in lamellae with their long axes on average normal to the lamella. The results are compared with the theory based on the unbinding of dislocations.
Microwave spectroscopy of hydroquinone: The rotational spectrum of the cis conformer
Authors: Caminati Walther , Melandri Sonia , Favero Laura B.
Published: July 2002

The microwave spectra of hydroquinone and of its O–D mono- and di-deuterated species have been investigated in the frequency range 8–40 GHz. Only the cis rotamer, which possesses a permanent dipole moment, has been observed. For this conformer the rotational and quartic centrifugal distortion constants have been obtained and the dipole moment has been measured. No doubling due to internal rotation between the two equivalent minima of the hydroxyl groups has been resolved.
Self-quenching of nitrobenzoxadiazole labeled phospholipids in lipid membranes
Authors: Brown R. Stephen , Brennan John D. , Krull Ulrich J.
Published: July 2002

The emission intensity, wavelength, and lifetime of the fluorophore nitrobenzoxadiazole dipalmitoylphosphatidylethanolamine (NBD-PE) are sensitive to the local environmental structure when this species is present as a component of an amphiphilic membrane. Alterations of the physical and electrostatic structure of a membrane can result in changes in the fluorescence signal owing to changes in the extent of self-quenching of the probe. To investigate self-quenching, NBD-PE was incorporated into monolayers and vesicles composed of Egg phosphatidylcholine at concentrations of 0.1 to 50 mol %. Monolayer samples were dipcast onto glass slides at a pressure of 35 mN m−1. Both the integrated intensity per fluorophore (quantum yield) from vesicles and dipcast monolayers, and the mean fluorescence lifetime from vesicles decreased as the concentration of fluorophore in the membranes was increased. At all concentrations studied the decay of NBD-PE fluorescence was fitted to two discrete exponentials, and both lifetime components were observed to change with concentration. The complexity of the fluorescence decay did not permit the use of standard theoretical models such as the Klafter–Blumen or Stern–Volmer equations which are normally employed to describe changes in fluorescence lifetime with changes in quencher concentration. Instead, a phenomenological approach was used to develop an empirical model of fluorescence self-quenching which could describe the observed alterations in the fluorescence lifetime and intensity. The model was based on a combination of Perrin quenching and Förster energy transfer. The fluorescence data was fit by a model wherein NBD-PE formed nonemissive trap sites with a critical radius of Rc=1.0±0.1 nm (Perrin quenching), with Förster energy transfer occurring to the trap sites with an R0 value of 2.55±0.10 nm as determined from spectral overlap integrals.
Anisotropic friction and excluded volume effects in freely jointed bead–rod polymer chain models
Authors: Xu Z. , Kim S. , de Pablo J. J.
Published: July 2002

We have studied the kinematics and rheological properties of undiluted polymers by modeling the fluids as a collection of interacting Kramers freely jointed bead–rod chains with anisotropic friction using Brownian dynamics simulations. The anisotropic friction depends on the instantaneous configuration of the system. The resulting stochastic differential equations have multiplicative noise. The correlation of the ‘‘Langevin’’ Brownian forces appearing in these equations turns out to be anisotropic as well. The excluded volume effect, often ignored in kinetic theory, plays an important role in the behavior of short polymer chains. The chain kinematics may be distorted in its absence. In this work, we also discuss the effects of two types of excluded volume (EV) forces (hard-sphere and soft-repulsive forces) on rheological properties of concentrated polymer solutions.
Rotational energy surfaces of molecules exhibiting internal rotation
Authors: Ortigoso Juan , Hougen Jon T.
Published: July 2002

Rotational energy surfaces [W. G. Harter and C. W. Patterson, J. Chem. Phys. 80, 4241 (1984)] for a molecule with internal rotation are constructed. The study is limited to torsional states at or below the top of the barrier to internal rotation, where the extra (torsional) degree of freedom can be eliminated by expanding eigenvalues of the torsion–K-rotation Hamiltonian as a Fourier series in the rotational degree of freedom. For acetaldehyde, considered as an example, this corresponds to considering vt=0, 1, and 2 (below the barrier) and vt=3 (just above the barrier). The rotational energy surfaces are characterized by locating their stationary points (maxima, minima, and saddles) and separatrices. Rather complicated catastrophe histories describing the creation and annihilation of pairs of stationary points as a function of J are found at moderate J for given torsional quantum number (vt) and symmetry species (A,E). Trajectories on the rotational energy surface which quantize the action are examined, and changes from rotational to vibrational trajectories caused by changes in the separatrix structure are found as a function of J for vt=2. The concept of a ‘‘best’’ quantization axis for the molecule-fixed component of the total angular momentum is examined from a classical point of view, and it is shown that labeling ambiguities encountered in the literature for torsion–rotation energy levels, calculated numerically in the rho-axis system, can be eliminated by reprojecting basis-set K values onto an axis passing through an appropriate stationary point on the rotational energy surface.
Second-harmonic generation from chiral surfaces
Authors: Kauranen Martti , Verbiest Thierry , Maki Jeffery J. , Persoons André
Published: July 2002

We present a theory of second-harmonic generation from chiral surfaces including contributions of electric and magnetic dipole transitions to the surface nonlinearity. The nonlinear polarization and magnetization of the surface as well as the second-harmonic fields that are radiated in the reflected and transmitted directions are expressed in terms of the six possible bilinear combinations of the components of the electric field of the fundamental beam. For the case in which the polarization of the fundamental beam is controlled by means of a quarter-wave plate between p-polarized linear and left- and right-hand circular, the second-harmonic fields can be expanded in terms of only three different functions of the rotation angle of the wave plate. The process exhibits nonlinear optical activity, i.e., it responds differently to the two circular polarizations of the fundamental beam if the phases of certain expansion coefficients are different. The theory is used to explain the results of a recent experiment and excellent agreement is found. The results suggest that in the experiment the largest components of the susceptibility tensors that include magnetic contributions were of the order of ∼10% of those of the electric dipole-allowed susceptibility tensor.
The Kirchhoff elastic rod, the nonlinear Schrödinger equation, and DNA supercoiling
Authors: Shi Yaoming , Hearst John E.
Published: July 2002

We have derived a time-independent, one-dimensional nonlinear Schrödinger equation for the stationary state configurations of supercoiled DNA. The effect of DNA self-contact has been included analytically. For the cases of non-self-contact and periodic boundary conditions, closed-form solutions have been obtained which describe the stationary state configurations of supercoiled DNA.
Large-scale microsegregation in polyacrylamide gels (spinodal gels)
Authors: Asnaghi D. , Giglio M. , Bossi A. , Righetti P. G.
Published: July 2002

Exceptionally large pore sizes have recently been reported for polyacrylamide gels grown in the presence of polyethylene glycol. We present small angle static light scattering measurements performed during the evolution of this peculiar gelation process. We give evidence that the large pores are generated by a microsegregation process caused by the competition between gelation and a phase separation of the polymers solution. The separation occurs via spinodal decomposition, and the size of the pores is determined by the stage at which the decomposition is stopped by the gelation process.
Interaction of hydrogen with nitrogen atoms chemisorbed on a Ru(0001) surface
Authors: Shi H. , Jacobi K. , Ertl G.
Published: July 2002

In order to investigate microscopic steps of ammonia synthesis on Ru surfaces, hydrogen adsorption on a Ru(0001) surface, precovered by atomic nitrogen, has been studied using high-resolution electron energy loss spectroscopy (HREELS) and thermal desorption spectroscopy (TDS). Hydrogen adsorption has been performed with the Ru sample at 90 and 300 K. At 90 K, the saturation coverage of hydrogen decreases with increasing N precoverage. The vibrational properties of H do not change much in the presence of N compared to those on the bare Ru(0001) surface exhibiting one single mode at 85 meV for small H coverages and losses at 99 and 141 meV at H saturation. Heating of the H+N coadsorbed layer from 90 to 300 K does not result in any observable N–H bond formation. After exposure of N/Ru(0001) to H2 at room temperature, however, NH3 and NH species are observed on the surface. NH3 is characterized by its symmetric bending mode δs at 145 meV. The reaction intermediate NH is stable up to 400 K and shows losses of ν(Ru-NH), δ(N–H), and ν(N–H) at 86, 166, and 410 meV, respectively. A barrier height of 93 kJ/mol is estimated for the NH3 synthesis reaction from N and H. In the presence of some coadsorbed Cs, NHx species are not observed at 300 K indicating a destabilization of NHx by Cs.
Photoelectron spectroscopy of FeO− and FeO−2: Observation of low-spin excited states of FeO and determination of the electron affinity of FeO2
Authors: Fan Jiawen , Wang Lai-Sheng
Published: July 2002

The photoelectron spectra of FeO− and FeO−2 are obtained at 3.49 eV photon energy. Transitions to the ground state (5Δ) and three low-lying excited states (5Σ+, 3Σ+, and 3Δ) of FeO are observed. The two low-spin excited states found at 6770 and 8310 cm−1 above the ground state, respectively, have not been observed before. The two Σ states, characteristic of detachment of a nonbonding electron from the FeO− anion, exhibit no vibrational progressions while a vibrational progression is observed for each of the two Δ states. The two high-spin states 5Δ and 5Σ+ are in agreement with a previous photoelectron study [P.C. Engelking and W. C. Lineberger, J. Chem. Phys. 66, 5054 (1977)]. The 3Δ state has a vibrational frequency of 800 (50) cm−1. The spectrum of FeO−2 only shows one major feature with little vibrational structure at this photon energy. The electron affinity of FeO2 is determined to be 2.358 (0.030) eV.
Measuring internal electric fields with atomic resolution
Authors: Kohler Bryan E. , Woehl Jörg C.
Published: July 2002

This paper describes a scheme for analyzing the effects of external electric field on the excitation energy of a conjugated probe molecule that is faithful to the actual microscopic situation and treats all aspects of the problem consistently. Structure is obtained from a molecular mechanics simulation, internal and local fields are calculated by classical electrostatics, and shifts in excitation energy are calculated by incorporating site energies into a simple π-electron model. An example calculation demonstrates the ability of this procedure to determine microscopic electric fields and molecular charge distributions.
Computer simulation of vapor-liquid equilibria of linear dipolar fluids: Departures from the principle of corresponding states
Authors: Garzón Benito , Lago Santiago , Vega Carlos , Rull Luis F.
Published: July 2002

Liquid-vapor equilibrium of linear dipolar fluids has been determined by using the Gibbs ensemble simulation technique. Several elongations and values of the dipole moment were considered. Dipole moment increases the critical temperature and affects slightly the critical density and pressure. Compressibility factor at the critical point decreases as the dipole moment of the molecule increases. Dipole moment provokes deviations from the principle of corresponding states. It is shown that the temperature-density coexistence curve is broadened and that the slope of the vapor pressure curve increases with increasing dipole moment. We propose a new way of reducing the dipole moment so that the increase of the critical temperature becomes almost independent on the molecular elongation. We have also obtained the vapor-liquid equilibrium of models having both a dipole and a quadrupole moment. The obtained data were used to describe the behavior of some relatively complex fluids, namely, 1,1,1-trifluoroethane and 2,2,2-trifluoroethanol. Good agreement for coexistence densities and pressures was obtained. The results presented in this work for linear dipolar fluids along with previous work on linear quadrupolar fluids provide a very comprehensive view of the effect of polar forces on the vapor-liquid equilibrium of linear fluids.
Description of core-excitation spectra by the open-shell electron-attachment equation-of-motion coupled cluster method
Authors: Nooijen Marcel , Bartlett Rodney J.
Published: July 2002

The theoretical description of core-excitation spectra presents a difficult problem due to the large excitation energies involved, and the extensive relaxation effects that occur upon promotion of a core electron to a valence or Rydberg level. For this reason we follow a two-step procedure to evaluate core-excitation energies. We start from a coupled cluster singles-doubles (CCSD) description of the core ion to include the large relaxation effects, followed by adding an extra electron to the core-ionized state to obtain the various core-excited states of the neutral by using the open-shell electron attachment equation-of-motion coupled cluster method (EA-EOMCC). An important feature of the approach is that the term values, the core-excitation energies relative to the relevant core-ionization potential, are calculated directly and this allows us to achieve high accuracy. This work describes the extension of the EA-EOMCC method to open-shell reference states and we make applications to a number of molecular systems. The assignment of recently obtained high-resolution core-excitation spectra for acetylene and ethylene is discussed, and we compare our open-shell EA-EOMCC results to results obtained from closed-shell EA-EOMCC calculations based on the equivalent core ion corresponding to the core-excited molecular system. Special attention is paid to the singlet–triplet splitting for core-excited states, and we address the multireference character of core-ionized and core-excited states for molecules that contain symmetry-equivalent heavy nuclei, which relates to a persistent controversy in the literature concerning localized versus delocalized core holes.
The polarization contribution to the free energy of hydration
Authors: Orozco Modesto , Luque F. J. , Habibollahzadeh Darius , Gao Jiali
Published: July 2002

The magnitude of the polarization contribution to the free energy of hydration is analyzed using several approaches. Monte Carlo quantum mechanical-molecular mechanical (MC-QM/MM), self-consistent reaction field (SCRF), and classical free energy perturbation calculations were used to obtain independent estimates of the polarization contribution to the free energy of hydration. The results derived from these techniques are in reasonable agreement, which supports the reliability of the calculated polarization values. The polarization contribution to the free energy of hydration was partitioned into components to elucidate the nature of the polarization in the solvation process. Strategies to capture the polarization effects in force-field simulations are discussed.
A new approach to molecular classical optimal control: Application to the reaction HCN→HC+N
Authors: Botina Jair , Rabitz Herschel , Rahman Naseem
Published: July 2002

We present a new method for classical control theory of Hamiltonian systems. This approach is based on a special treatment of the adjoint or Lagrange multiplier equations of motion. The latter function is only asked to preserve the mean of the ensemble of molecular trajectories. In the present case only four such equations are involved greatly simplifying the field design process and making it faster and more stable. Good results are obtained for the selective photodissociation of HCN. The objective is to control the intramolecular reaction HCN→HC+N (i.e., break the stronger bond). Hamilton’s equations of motion are employed to model the HCN molecule, initially in its ground state. The control equations are integrated to obtain a high degree of selectivity in the unimolecular dissociation. The robustness of the results to changes in the initial conditions and pulse durations are investigated. An increase of the pulse duration beyond a certain point makes it more difficult to dissociate the N atom due to strong intramolecular coupling. The resultant pulse fields may serve as a basic indicator for future experimental selective dissociation of HCN→HC+N using high power lasers.