bluuebunny/crossref_metadata_2025 huggingface dataset:
Extended version of the van der Waals capillarity theory
Authors: Baidakov V. G. , Boltachev G. Sh.
Published: October 2004

An extended version of the van der Waals capillarity theory describing the liguid-vapor interface in the temperature range from the triple to the critical point is suggested. A model functional of thermodynamic potential for a two-phase Lennard-Jones system taking into account the effect of the highest degree terms of gradient expansion has been constructed. The identity of the thermodynamic and the mechanical definition of Tolman’s length has been proved in the framework of the adopted form of functional. The properties of nuclei of the liquid and the vapor phase are described. The paper determines: the work of formation of a nucleus, density profiles, size dependences of the surface tension, and the parameter δ in the Gibbs-Tolman-Koenig-Buff equation.
Characterizing the later 3d cyanides: The submillimeter spectrum of CoCN(X 3Φi)
Authors: Sheridan P. M. , Flory M. A. , Ziurys L. M.
Published: October 2004

The pure rotational spectrum of the CoCN radical has been recorded in the frequency range 350–500 GHz using direct absorption techniques. This study is the first spectroscopic observation of this molecule by any experimental technique. Spectra of Co 13CN have been measured as well. These data indicate that this species is linear in its ground electronic state and has the cyanide, as opposed to the isocyanide, geometry. The ground state term has been assigned as Φ3i, based on the measurement of three spin components (Ω=4, 3, and 2) and in analogy to other isovalent cobalt-bearing species. Hyperfine splittings resulting from the Co59 nuclear spin of I=7/2 were observed in every transition, each of which exhibited an octet pattern. For the lowest energy spin component, Ω=4, vibrational satellite features were also identified arising from the first quantum of the Co-C (v1=1) stretch and the v2=1 and v2=2 quanta of the bending mode, which were split by Renner-Teller interactions. The ground state measurements of CoCN were analyzed with a case aβ Hamiltonian, establishing rotational, fine structure, and hyperfine parameters. The vibrational and Co 13CN spectra for the Ω=4 component were fit as well. An r0 structure was also calculated, providing estimates of the Co-C and C-N bond distances, based on the Ω=4 transitions. CoCN is the fourth molecule in the 3d transition metal series to exhibit the linear cyanide structure, along with the Zn, Cu, and Ni analogs. The preference for this geometry, as opposed to the isocyanide form, may indicate a greater degree of covalent bonding in these species.
Potential energy surface, bound states, and rotational inelastic cross sections of the He-CH4 system: A theoretical investigation
Authors: Calderoni G. , Cargnoni F. , Martinazzo R. , Raimondi M.
Published: October 2004

We determined two potential energy surfaces (PES) for the He-CH4 system by means of MP4 and Valence Bond (VB) calculations. The MP4 potential is similar to the one commonly adopted for this system [U. Buck, K. H. Kohl, A. Kolhase, M. Faubel, and U. Staemmler, Mol. Phys. 55, 1255 (1985)], while the VB PES is slightly more attractive. To evaluate the reliability of these potentials, we investigated the scattering properties by performing close coupling calculations, and concluded that: (i) the available experimental data do not permit the ranking among the PES considered; (ii) some theoretical predictions differ considerably from the experimental data, and these discrepancies cannot entirely be ascribed to the inaccuracy of the ab initio calculations; (iii) the scattering properties at low energy might discriminate between the MP4 and VB potentials.
Effect of branching and confinement on star-branched polymeric systems
Authors: Maury-Evertsz Johnny R. , Estévez L. Antonio , López Gustavo E.
Published: October 2004

The effect of confinement, number of branches (functionality), and size of the molecules on various properties as a function of temperature of star-branched polymers confined between two walls was studied using Monte Carlo simulations with the parallel tempering technique. The coil-to-globule transition and the liquidlike to solidlike transition, similar to those observed for linear chains, were characterized in all systems by changes in the heat capacity, internal energy, and radius of gyration. The transitions were also characterized by the most probable isomeric structure at a given temperature. The radius of gyration of the star polymers was smaller than the values of linear chains when the number of arms f increased. For star chains with more than f=5 arms the values of the radius of gyration, and therefore the size of the molecules, were similar for every condition of confinement studied, especially at higher temperatures. As confinement was increased, the difference in the radius of gyration of linear chains and star polymers became even larger. The coil-to-globule transition temperatures shifted to higher temperatures as the size of the chains and the number of arms in a molecule were increased. Effects of confinement were higher on the properties of the system at the smallest separations (less than twice the monomer diameter), where the coil-to-globule transition shifted to lower temperatures. The liquidlike to solidlike transition was present at almost the same temperature for different conditions of confinement, chain size, and number of arms. The behavior of the systems for separations between the walls greater than five bead diameters was similar to the behavior in the unconfined case. Hence, no considerable effect of confinement was found above this separation.
A theoretical study of the excited states of CrH: Potential energies, transition moments, and lifetimes
Authors: Ghigo Giovanni , Roos Björn O. , Stancil P. C. , Weck P. F.
Published: October 2004

Ab initio calculations of low-lying electronic states of CrH are presented, including potential energies, dipole and transition dipole moment (TDM) functions, and radiative lifetimes for X 6Σ+, A 6Σ+, 3 6Σ+, 1 6Π, 2 6Π, 3 6Π, and Δ6. Calculation of dynamic correlation effects was performed using the multistate complete active space second-order perturbation method, based on state-averaged complete active space self-consistent-field reference wave functions obtained with seven active electrons in an active space of 16 molecular orbitals. A relativistic atomic natural orbital-type basis set from the MOLCAS library was used for Cr. Good agreement is found between the current calculations and experiment for the lowest two Σ6 + states, the only states for which spectroscopic data are available. Potential curves for the 3 6Σ+ and 2 6Π states are complicated by avoided crossings with higher states of the same symmetry, thus resulting in double-well structures for these two states. The measured bandhead T0=27 181 cm−1, previously assigned to a Π6←X 6Σ+ transition, is close to our value of T0=28 434 cm−1 for the 2 6Π state. We tentatively assign the ultraviolet band found experimentally at 30 386 cm−1 to the 3 6Π←X 6Σ+ transition for which the computed value is 29 660 cm−1. The A 6Σ+←X 6Σ+ TDM and A 6Σ+ lifetimes are found to be in reasonable agreement with previous calculations.
Effect of the Perdew–Zunger self-interaction correction on the thermochemical performance of approximate density functionals
Authors: Vydrov Oleg A. , Scuseria Gustavo E.
Published: October 2004

The Perdew–Zunger self-interaction-corrected density functional theory (SIC-DFT) was implemented self-consistently using a quasi-Newton direct minimization method. We calculated SIC-DFT energies for a number of atoms and molecules using various approximate density functionals, including hybrids. Self-interaction errors (SIE) of these functionals were compared and analyzed in terms of contributions from valence and core orbitals. We also calculated enthalpies of formation of the standard G2-1 set of 55 molecules and found that self-interaction-correction (SIC) improves agreement with experiment only for the LSDA functional, while all other functionals show worse performance upon introducing SIC. This is the first systematic study of the effect of SIC on thermochemical properties. We found no direct connection between the magnitude of the SIE contained in a functional and its performance for thermochemistry. Approximate functionals with large self-interaction errors can accurately reproduce enthalpies of formation. Our results do not support the popular belief that a smaller SIE of hybrid functionals is the main reason for their higher accuracy.
Perturbations in the pure rotational spectrum of CoCl (X 3Φi): A submillimeter study
Authors: Flory M. A. , Halfen D. T. , Ziurys L. M.
Published: October 2004

The millimeter/submillimeter-wave spectrum of the CoCl radical (X 3Φi) has been recorded using direct absorption techniques in the frequency range 340–510 GHz. This work is the first pure rotational study of this molecule. The radical was created by the reaction of Cl2 with cobalt vapor. Rotational transitions arising from the Ω=4, 3, and 2 spin-orbit components of Co35Cl have been measured, all of which exhibit hyperfine splittings due to the Co59 nucleus (I=7/2). Transitions arising from the Co37Cl species were also recorded, as well as those originating in the v=1, 2, 3, and 4 vibrational states of both isotopomers. The spin-orbit pattern exhibited by the molecule is unusual, with the Ω=3 component significantly shifted relative to the other spin components. In addition, the regular octet hyperfine splittings become distorted above a certain J value for the Ω=3 transitions only. These effects suggest that the molecule is highly perturbed in its ground state, most likely a result of second-order spin-orbit mixing with a nearby isoconfigurational Φ13 state. The complete data set for Co35Cl and Co37Cl were fit successfully with a case (a) Hamiltonian but required a large negative spin-spin constant of λ=−7196 GHz and higher order centrifugal distortion corrections to the rotational, spin-orbit, spin-spin, and hyperfine terms. The value of the spin-spin constant suggests that the Ω=3 component is shifted to higher energy and lies near the Ω=2 sublevel. The hyperfine parameters are consistent with a δ3π3 electron configuration and indicate that CoCl is more covalent than CoF.
Molecular origin of demixing, prior to crystallization, of atactic polypropylene/isotactic polypropylene blends upon cooling from the melt
Authors: Choi Phillip , Mattice Wayne L.
Published: October 2004

An amorphous 50/50 atactic polypropylene (aPP)/isotactic polypropylene (iPP) mixture at 125 °C was simulated using a second nearest neighbor diamond lattice and a three states rotational isometric state model. The result suggests that at the liquidlike density that corresponds to the atmospheric pressure, aPP prefers to interact with other aPP chains rather than with iPP chains. The result is consistent with the inference of Keith and Padden [J. Appl. Phys. 35, 1286 (1964)] that aPP and iPP will tend to separate from one another in their melt at 125 °C, before the onset of crystallization of iPP. The tendency for immiscibility of the amorphous aPP/iPP blend is likely attributed to the presence of short syndiotactic sequences in the aPP chains adopting all-trans conformations. The attractive intermolecular interaction of pairs of such subchains at 125 °C promotes the separation of aPP from iPP. This interaction is weakened at higher temperature, where aPP and iPP become miscible. The result also shows that miscibility of the blend increases with increasing pressure. However, the origin of the pressure effect is not clear.
Effect of confinement on coil-globule transition
Authors: Mishra P. K. , Kumar Sanjay
Published: October 2004

The equilibrium thermodynamic properties of a linear polymer chain confined to a space between two impenetrable walls (lines) at a distance D under various solvent conditions have been studied using series analysis and exact enumeration technique. We have calculated the end-to-end distance of polymer chain, which shows a nonmonotonic behavior with inter wall separation D. The density distribution profile shows a maxima at a particular value of (D=)D*. Around this D*, our results show that the collapse transition occurs at higher temperature as compared to its bulk value of 2d and 3d. The variation of θ-temperature with D shows a reentrance behavior. We also calculate the force of compression exerted by the walls (lines) on the polymer.
Molecular dynamics simulations of amphiphilic graft copolymer molecules at a water/air interface
Authors: Anderson Philip M. , Wilson Mark R.
Published: October 2004

Fully atomistic molecular dynamics simulations of amphiphilic graft copolymer molecules have been performed at a range of surface concentrations at a water/air interface. These simulations are compared to experimental results from a corresponding system over a similar range of surface concentrations. Neutron reflectivity data calculated from the simulation trajectories agrees well with experimentally acquired profiles. In particular, excellent agreement in neutron reflectivity is found for lower surface concentration simulations. A simulation of a poly(ethylene oxide) (PEO) chain in aqueous solution has also been performed. This simulation allows the conformational behavior of the free PEO chain and those tethered to the interface in the previous simulations to be compared.
Kinetics of electron-induced decomposition of CF2Cl2 coadsorbed with water (ice): A comparison with CCl4
Authors: Faradzhev N. S. , Perry C. C. , Kusmierek D. O. , Fairbrother D. H. , Madey T. E.
Published: October 2004

The kinetics of decomposition and subsequent chemistry of adsorbed CF2Cl2, activated by low-energy electron irradiation, have been examined and compared with CCl4. These molecules have been adsorbed alone and coadsorbed with water ice films of different thicknesses on metal surfaces (Ru; Au) at low temperatures (25 K; 100 K). The studies have been performed with temperature programmed desorption (TPD), reflection absorption infrared spectroscopy (RAIRS), and x-ray photoelectron spectroscopy (XPS). TPD data reveal the efficient decomposition of both halocarbon molecules under electron bombardment, which proceeds via dissociative electron attachment (DEA) of low-energy secondary electrons. The rates of CF2Cl2 and CCl4 dissociation increase in an H2O (D2O) environment (2–3×), but the increase is smaller than that reported in recent literature. The highest initial cross sections for halocarbon decomposition coadsorbed with H2O, using 180 eV incident electrons, are measured (using TPD) to be 1.0±0.2×10−15 cm2 for CF2Cl2 and 2.5±0.2×10−15 cm2 for CCl4. RAIRS and XPS studies confirm the decomposition of halocarbon molecules codeposited with water molecules, and provide insights into the irradiation products. Electron-induced generation of Cl− and F− anions in the halocarbon/water films and production of H3O+, CO2, and intermediate compounds COF2 (for CF2Cl2) and COCl2, C2Cl4 (for CCl4) under electron irradiation have been detected using XPS, TPD, and RAIRS. The products and the decomposition kinetics are similar to those observed in our recent experiments involving x-ray photons as the source of ionizing irradiation.
Molecular dynamics and theory for the contact values of the radial distribution functions of hard-disk fluid mixtures
Authors: Luding Stefan , Santos Andrés
Published: October 2004

We report molecular dynamics results for the contact values of the radial distribution functions of binary additive mixtures of hard disks. The simulation data are compared with theoretical predictions from expressions proposed by Jenkins and Mancini [J. Appl. Mech. 54, 27 (1987)] and Santos et al. [J. Chem. Phys. 117, 5785 (2002)]. Both theories agree quantitatively within a very small margin, which renders the former still a very useful and simple tool to work with. The latter (higher-order and self-consistent) theory provides a small qualitative correction for low densities and is superior especially in the high-density domain.
One- and two-photon absorption of three-coordinate compounds with different centers (B,Al,N) and a 2,2′-dipyridylnitrogen functional group
Authors: Liu Xiao-Juan , Feng Ji-Kang , Ren Ai-Min , Cheng Hong , Zhou Xin
Published: October 2004

A series of three-coordinate octupolar compounds with varied centers (boron, aluminum, and nitrogen), which exhibit very large effective two-photon absorption cross sections have been theoretically studied. The ground state geometries and electronic structures are obtained using the density functional theory with the B3LYP functional and 6-31G(d) basis set, and the results are comparable to the available experimental determinations. Based on the correct geometrical and electronic structures, the one- and two-photon absorptions are predicted by the ZINDO-SOS method. Among these compounds, the boron (B) and aluminum (Al) centers act as acceptors, while the nitrogen center acts as donor according to the net charge changes during the excitation. It is found that (i) the compounds with boron and aluminum centers show two large two-photon absorption peaks, while the molecule with nitrogen center show only one two-photon absorption peak; (ii) the cross sections of the molecules with B or Al as centers are larger than that of the molecule with nitrogen as center; furthermore, the two-photon absorption cross section of the molecule with Al center is larger than that of the molecule with B center, from this point of view, our theoretical prediction provides for the experiment a good new candidate with large two-photon absorption cross section for further research; (iii) lengthening the conjugation bridge by inserting a benzene ring on the organoborane compounds (forming the investigated molecule B-2) enhances the two-photon absorption cross section, and keeping good transparency at the same time.
Hydrogen adsorption on zigzag (8,0) boron nitride nanotubes
Authors: Wu Xiaojun , Yang Jinlong , Hou J. G. , Zhu Qingshi
Published: October 2004

The chemical adsorption of H atoms on an (8,0) zigzag boron nitride nanotube is studied using the density functional theory with the supercell method. One to four H atoms per 32 B and 32 N are considered. The results show that H atoms prefer to adsorb on the top sites of adjacent B and N atoms to form an armchair chain along the tube axis. An even-odd oscillation behavior of the adsorption energy of H atoms on the tube is found, and the average adsorption energy of even H atoms is obviously bigger than that of odd H atoms. The results can be understood with the frontier orbital theory. Based on this adsorption behavior, several high-symmetric structures of H adsorbed boron nitride nanotubes with 50% and 100% coverages are studied. The pairs of lines’ pattern with 50% coverage has the biggest average adsorption energy per H2 among the chosen configurations, corresponding to ∼4 wt % hydrogen storage.
A first principles study on the solvation and structure of SO42−(H2O)n, n=6–12
Authors: Gao Bing , Liu Zhi-feng
Published: October 2004

The structures of hydrated sulfate clusters, SO42−(H2O)n with n=6–12, are obtained by density functional theory calculations. For SO42−(H2O)12, two structures with symmetric distribution of H2O molecules around the sulfate group are favored in energy. The structures for the smaller clusters, SO42−(H2O)n with n=6–11, are obtained by taking away one H2O molecule successively from the two symmetric SO42−(H2O)12 isomers. The hydrogen bonding between the sulfate O atoms and H2O molecules are strong. So are the hydrogen bonds among H2O molecules, which are facilitated by the structure of the polyatomic sulfate group. The solvation energy is quite large (often exceeding 15 kcal/mol). The patterns for structural and energy changes as the cluster size increases are very different from the well studied hydrated halide ions, although the competition between solute-solvent and solvent-solvent interactions is again an important factor. Ab initio molecular dynamics simulations also show “crowding” effects in the first solvation of SO42−(H2O)12 at raised temperature.
The Kawasaki identity and the Fluctuation Theorem
Authors: Carberry D. M. , Williams S. R. , Wang G. M. , Sevick E. M. , Evans Denis J.
Published: October 2004

In this paper we show that the Fluctuation Theorem of Evans and Searles [D. J. Evans, D. J. Searles, Phys. Rev. E 50, 1645 (1994)] implies that the Kawasaki function 〈exp(−Ωt)〉 is unity for all time t. We confirm this relationship using experimental data obtained using optical tweezers, and show that the Kawasaki function is a valuable diagnostic tool.
Thermodynamics and statistical mechanics of multilayer adsorption
Authors: Monleón Pradas Manuel , Salmerón Sánchez Manuel , Gallego Ferrer Gloria , Gómez Ribelles José Luis
Published: October 2004

The multilayer adsorption models of Brunauer-Emmett-Teller and Guggenheim-Anderson-de Boer are reconsidered. The relationship between the fitting parameters and the physical parameters of the equation is discussed. The preexponential factors of the parameters are shown to be in general far different from unity, contrary to a widespread use. A thermodynamical derivation illuminates the hypothesis on which the multilayer sorption equation is dependent and frees it from too restrictive hypothesis usually taken as necessary for its validity. Equations are derived for the number fraction of sorption sites occupied by different numbers of molecules. The Guggenheim-Anderson-de Boer equation is shown to imply incomplete occupation (jamming) of the first sorption layer at saturation.
Infrared spectroscopy of oxygen adsorbed on hydrogen covered Pt(111)
Authors: Gustafsson K. , Andersson S.
Published: October 2004

Using infrared reflection absorption spectroscopy we have investigated how preadsorbed hydrogen affects the adsorption of O2 on the Pt(111) surface at temperatures below the onset of the water formation reaction. On the fully hydrogen covered surface, ΘH=1, O2 physisorbs at temperatures below 45 K, the weakly dipole active internal stretch vibration is observed at 1548 cm−1. Unlike on the clean Pt(111) surface, this adsorption state does not act as a precursor for O2 chemisorption. The physisorbed molecules simply desorb above 45 K and no chemisorbed O2 state is populated directly from the gas phase in the temperature range 45–90 K. When the surface is approximately half covered, ΘH≈0.4, with preadsorbed hydrogen, O2 chemisorbs on the clean Pt(111) surface regions in the characteristic peroxolike and superoxolike states with vibration frequencies around 700 cm−1 and 870 cm−1. These values correspond to dense O2 islands which develop already at low O2 coverages. At this hydrogen coverage, we find that the initial sticking probability of chemisorbed O2 is drastically reduced at 90 K and the general uptake also proceeds slowly when compared with observations for the clean surface. We suggest that this is due to a change in the behavior of the physisorbed O2 precursor.
The mechanism of proton exchange: Guided ion beam studies of the reactions, H(H2O)n+ (n=1–4)+D2O and D(D2O)n+ (n=1–4)+H2O
Authors: Honma Kenji , Armentrout P. B.
Published: October 2004

Reactions of protonated water clusters, H(H2O)n+ (n=1–4) with D2O and their “mirror” reactions, D(D2O)n+ (n=1–4) with H2O, are studied using guided-ion beam mass spectrometry. Absolute reaction cross sections are determined as a function of collision energy from thermal energy to over 10 eV. At low collision energies, we observe reactions in which H2O and D2O molecules are interchanged and reactions where H-D exchange has occurred. As the collision energy is increased, the H-D exchange products decrease and the water exchange products become dominant. At high collision energies, processes in which one or more water molecules are lost from the reactant ions become important, with simple collision-induced dissociation processes, i.e., those without H-D exchange, being dominant. Threshold energies of endothermic channels are measured and used to determine binding energies of the proton bound complexes, which are consistent with those determined by thermal equilibrium measurements and previous collision-induced dissociation studies. A kinetic scheme that relies only on the ratio of isomerization and dissociation rate constants successfully accounts for the kinetic energy dependence observed in the branching ratios for H-D and water exchange products in all systems. Rice–Ramsperger–Kassel–Marcus theory and ab initio calculations confirm the feasibility and establish the details of this kinetic model.
Spectroscopy and reactivity of size-selected Mg+-ammonia clusters
Authors: Lee James I. , Sperry David C. , Farrar James M.
Published: October 2004

Photodissociation spectra for mass-selected Mg+(NH3)n clusters for n=1 to 7 are reported over the photon energy range from 7000 to 38 500 cm−1. The singly solvated cluster, which dissociates primarily via a N–H bond cleavage, exhibits a resolved vibrational structure corresponding to two progressions in the intracluster Mg+–NH3 modes. The addition of the second, third, and fourth solvent molecules results in monotonic redshifts that appear to halt near 8500 cm−1, where a sharp feature in the electronic spectrum is correlated with the formation of a Mg+(NH3)4 complex with Td symmetry and the closing of the first solvation shell. The spectra for the clusters with 5 to 7 solvent molecules strongly resemble that for the tetramer, suggesting that these solvent molecules occupy a second solvation shell. The wavelength-dependent branching-ratio measurements show that increasing the photon energies generally result in the loss of additional solvent molecules but that enhancements for a specific solvent number loss may reveal special stability for the resultant fragments. The majority of the experimental evidence suggests that the decay of these clusters occurs via the internal conversion of the initially excited electronic states to the ground state, followed by dissociation. In the case of the monomer, the selective cleavage of a N–H bond in the solvent suggests that this internal-conversion process may populate regions of the ground-state surface in the vicinity of an insertion complex H–Mg+–NH2, whose existence is predicted by ab initio calculations.
Polarized absorption of quaterthiophene single crystals
Authors: Tavazzi S. , Borghesi A. , Laicini M. , Spearman P.
Published: October 2004

Polarized optical spectra of quaterthiophene single crystals are reported over a wide spectral range for different planes and angles of incidence corresponding to either transverse or mixed longitudinal/transverse wave propagation. From the absorbance and reflectance spectra the corresponding absorption coefficients are deduced. In the region of the first molecular transition, polarized exciton states of Frenkel origin are found to be responsible for a strong optical anisotropy and for the modification of the state of polarization during light propagation in the crystal. Transmission measurements with crossed polarizer and analyzer allow to clearly identify Frenkel replica of the principal transition.
Experimental isotherms of HCl on H2O ice under stratospheric conditions: Connections between bulk and interfacial thermodynamics
Authors: Henson B. F. , Wilson Kevin R. , Robinson J. M. , Noble C. A. , Casson J. L. , Worsnop D. R.
Published: October 2004

The adsorption of HCl on the surface of H2O ice has been measured at temperatures and pressures relevant to the upper troposphere and lower stratosphere. The measured HCl surface coverage is found to be at least 100 times lower than currently assumed in models of chlorine catalyzed ozone destruction in cold regions of the upper atmosphere. Measurements were conducted in a closed system by simultaneous application of surface spectroscopy and gas phase mass spectrometry to fully characterize vapor/solid equilibrium. Surface adsorption is clearly distinguished from bulk liquid or solid phases. From 180 to 200 K, submonolayer adsorption of HCl is well described by a Bragg-Williams modified Langmuir model which includes the dissociation of HCl into H+ and Cl− ions. Furthermore, adsorption is consistent with two distinct states on the ice substrate, one in which the ions only weakly adsorb on separate sites, and another where the ions adsorb as an H+-Cl− pair on a single site with adsorption energy comparable to the bulk trihydrate. The number of substrate H2O molecules per adsorption site is also consistent with the stoichiometry of bulk hydrates under these conditions. The ionic states exist in equilibrium, and the total adsorption energy is a function of the relative population of both states. These observations and model provide a quantitative connection between the thermodynamics of the bulk and interfacial phases of HCl/H2O, and represent a consistent physicochemical model of the equilibrium system.
High Tg photorefractive polymers: Influence of the chromophores’ β tensor
Authors: Acebal P. , Blaya S. , Carretero L.
Published: October 2004

In this paper we study the effect of the chromophores’ β tensor active components on the diffraction efficiency of a high Tg photorefractive polymer. In particular, we study the two simplest structures with nonvanishing dipole moment, the one-dimension push-pull systems, and the Λ-shaped chromophores. We have developed a model that relate the diffraction efficiency expression with experimental conditions and microscopic properties of the molecules used. Using this model we determine the optimum experimental conditions for both kinds of chromophores and the criteria for the design of chromophores with improved microscopic properties. The model was also used to evaluate the diffraction efficiency of the chromophore Disperse Red 1 (DR1) with a good agreement with experimental data present in bibliography, and of other chromophores selected with the criteria derived from the model, using quantum mechanical calculations to obtain the microscopic properties. Using the designed chromophores diffraction efficiencies more than one order of magnitude higher than that calculated for DR1 with the experimental conditions has been obtained in simulations. These chromophores also exhibit a low dependency of η on the electric field polarization in contrast to the DR1 or the low Tg photoreactive materials.
Thermoelectric transport properties in atomic scale conductors
Authors: Zheng Xiaohong , Zheng Wei , Wei Yadong , Zeng Zhi , Wang Jian
Published: October 2004

The thermoelectric transport properties in atomic scale conductors consisting of a Si atom connected by two electrodes are investigated. It is found that both the electrical current and the heat current have two contributions, one from the voltage and the other from the temperature gradient. The quantities such as the Seebeck thermopower and the thermal conductance that characterize the thermoelectric transport properties of the tunnel atomic junction are studied quantitatively with a first-principles technique within the framework of Landauer–Büttiker formalism in the linear response regime. A finite thermopower only exists in a very narrow range where the energy derivative of the transmission function is nonzero. The thermopower anomaly is observed in the tunneling regime in this device but this does not violate the thermodynamic law with respect to the heat current.
Ionization and solvation of HCl adsorbed on the D2O-ice surface
Authors: Kondo M. , Kawanowa H. , Gotoh Y. , Souda R.
Published: October 2004

The interaction of HCl with the D2O-ice surface has been investigated in the temperature range 15–200 K by utilizing time-of-flight secondary ion mass spectroscopy, temperature-programmed desorption, and x-ray photoelectron spectroscopy. The intensities of sputtered H+(D2O) and Cl− ions (the H+ ions) are increased (decreased) markedly above 40 K due to the hydrogen bond formation between the HCl and D2O molecules. The HCl molecules which form ionic hydrates undergo H/D exchange at 110–140 K and a considerable fraction of them dissolves into the bulk above 140 K. The neutral hydrates of HCl should coexist as evidenced by the desorption of HCl above 170 K. They are incorporated completely in the D2O layer up to 140 K. The HCl molecules embedded in the thick D2O layer dissolve into the bulk, and the ionic hydrate tends to segregate to the surface above 150 K.
A molecular interpretation of vitreous boron oxide dynamics
Authors: Fullerton Susan K. , Maranas Janna K.
Published: October 2004

The mobility of vitreous boron oxide is studied by molecular dynamics simulation. A polarization model that incorporates induced dipoles arising both from charges and from other induced dipoles on atoms with nonzero polarizability is used to simulate boron oxide glass at various temperatures above the glass transition temperature. Particle mobility is investigated through the calculation of the self-intermediate scattering function and the mean-squared displacement. The calculations clearly reveal a two-step relaxation with a plateau at intermediate times for all investigated temperatures. With respect to atomic species, boron atoms are less mobile than oxygen atoms at all temperatures within the plateau region. Through analyzing particle trajectories, it is revealed that BO3 groups move as one unit and follow each other in a stringlike manner. Three connected BO3 groups comprise a six-membered boroxol ring, which is shown to move in a collective manner, requiring the simultaneous movement of all ring atoms. The boroxol ring is observed to be confined, or caged, during the plateau region, and jumps to a new location at longer times. This observation is linked to the concept of strong versus fragile glass formers and the potential energy landscape. In addition to the caging feature, an overshoot or dip occurs in the plateau regions of the mean-squared displacement and self-intermediate scattering functions respectively. These features are followed by a ringing pattern, previously associated with finite size effects in other strong glass formers, which persist for the duration of the plateau region. Both features are shown to be consistent with the bending of atomic “cages” from the plane of the boroxol ring, and arise due to the displacement of atoms from local minimum energy configurations.
Cluster size effects in core excitons of 1s-excited nitrogen
Authors: Flesch R. , Kosugi N. , Bradeanu I. L. , Neville J. J. , Rühl E.
Published: October 2004

Cluster size effects in core excitons below the N 1s ionization energy of nitrogen clusters are reported in the energy regime 405–410 eV. These results are compared to the molecular Rydberg states as well as the corresponding bulk excitons of condensed nitrogen. The experimental results are assigned using ab initio calculations. It is found that the lowest excitons (N 1s→3sσ and N 1s→3pπ) are blueshifted relative to the molecular Rydberg transitions, whereas others (N 1s→3dπ and N 1s→4pπ) show a redshift. Results from ab initio calculations on (N2)13 clearly indicate that the molecular orientation within a cluster is critical to the spectral shift, where bulk sites as well as inner- and outer-surface sites are characterized by different inner-shell absorption energies. These results are compared to the experimental spectra as well as previous work on site-selectively excited atomic van der Waals clusters, providing an improved spectral assignment of core exciton states in weakly bound molecular clusters and the corresponding condensed phase.
Potential energy curves of diatomic molecular ions from high-resolution photoelectron spectra. II. The first six electronic states of Xe2+
Authors: Rupper P. , Zehnder O. , Merkt F.
Published: October 2004

The pulsed-field-ionization zero-kinetic-energy photoelectron spectrum of Xe2 has been measured between 90 000 and 109 000 cm−1 following single-photon excitation from the ground neutral state. Transitions to five of the six low-lying electronic states of Xe2+ could be observed. Whereas extensive vibrational progressions were observed for the X0g+→I(1/2u), I(3/2g), and II(1/2u) photoelectron transitions, only the lowest vibrational levels of the I(3/2u) and II(1/2g) states could be detected. Unambiguous assignments of the vibrational quantum numbers were derived from the analysis of the isotopic shifts of the vibrational bands and of the intensity distribution and from the modeling of the potential energy curves. Analytical potential energy curves of spectroscopic accuracy (i.e., ∼1 meV) were determined for all six low-lying electronic states using a global model, which includes the first (charge-induced dipole, ∝1/R4) member of the long-range interaction series and treats the spin-orbit interaction explicitely. The assumption of an R-independent spin-orbit coupling constant was tested and found to be an excellent approximation.
Effect of torsinA on membrane proteins reveals a loss of function and a dominant-negative phenotype of the dystonia-associated ΔE-torsinA mutant
Authors: Torres Gonzalo E. , Sweeney Ava L. , Beaulieu Jean-Martin , Shashidharan Pullani , Caron Marc G.
Published: October 2004

Most cases of early-onset torsion dystonia (EOTD) are caused by a deletion of one glutamic acid in the carboxyl terminus of a protein named torsinA. The mutation causes the protein to aggregate in perinuclear inclusions as opposed to the endoplasmic reticulum localization of the wild-type protein. Although there is increasing evidence that dysfunction of the dopamine system is implicated in the development of EOTD, the biological function of torsinA and its relation to dopaminergic neurotransmission has remained unexplored. Here, we show that torsinA can regulate the cellular trafficking of the dopamine transporter, as well as other polytopic membrane-bound proteins, including G protein-coupled receptors, transporters, and ion channels. This effect was prevented by mutating the ATP-binding site in torsinA. The dystonia-associated torsinA deletion mutant (ΔE-torsinA) did not have any effect on the cell surface distribution of polytopic membrane-associated proteins, suggesting that the mutation linked with EOTD results in a loss of function. However, a mutation in the ATP-binding site in ΔE-torsinA reversed the aggregate phenotype associated with the mutant. Moreover, the deletion mutant acts as a dominant-negative of wild-type torsinA through a mechanism presumably involving association of wild-type and mutant torsinA. Taken together, our results provide evidence for a functional role for torsinA and a loss of function and a dominant-negative phenotype of the ΔE-torsinA mutation. These properties may contribute to the autosomal dominant nature of the condition.
AtTPK4, an
Arabidopsis
tandem-pore K
+
channel, poised to control the pollen membrane voltage in a pH- and Ca
2+
-dependent manner
Authors: Becker D. , Geiger D. , Dunkel M. , Roller A. , Bertl A. , Latz A. , Carpaneto A. , Dietrich P. , Roelfsema M. R. G. , Voelker C. , Schmidt D. , Mueller-Roeber B. , Czempinski K. , Hedrich R.
Published: October 2004

The
Arabidopsis
tandem-pore K
+
(TPK) channels displaying four transmembrane domains and two pore regions share structural homologies with their animal counterparts of the KCNK family. In contrast to the
Shaker
-like
Arabidopsis
channels (six transmembrane domains/one pore region), the functional properties and the biological role of plant TPK channels have not been elucidated yet. Here, we show that AtTPK4 (KCO4) localizes to the plasma membrane and is predominantly expressed in pollen. AtTPK4 (KCO4) resembles the electrical properties of a voltage-independent K
+
channel after expression in
Xenopus
oocytes and yeast. Hyperpolarizing as well as depolarizing membrane voltages elicited instantaneous K
+
currents, which were blocked by extracellular calcium and cytoplasmic protons. Functional complementation assays using a K
+
transport-deficient yeast confirmed the biophysical and pharmacological properties of the AtTPK4 channel. The features of AtTPK4 point toward a role in potassium homeostasis and membrane voltage control of the growing pollen tube. Thus, AtTPK4 represents a member of plant tandem-pore-K
+
channels, resembling the characteristics of its animal counterparts as well as plant-specific features with respect to modulation of channel activity by acidosis and calcium.
Facile isolation and the characterization of human retinal stem cells
Authors: Coles Brenda L. K. , Angénieux Brigitte , Inoue Tomoyuki , Del Rio-Tsonis Katia , Spence Jason R. , McInnes Roderick R. , Arsenijevic Yvan , van der Kooy Derek
Published: October 2004

This study identifies and characterizes retinal stem cells (RSCs) in early postnatal to seventh-decade human eyes. Different subregions of human eyes were dissociated and cultured by using a clonal sphere-forming assay. The stem cells were derived only from the pars plicata and pars plana of the retinal ciliary margin, at a frequency of ≈1:500. To test for long-term self-renewal, both the sphere assay and monolayer passaging were used. By using the single sphere passaging assay, primary spheres were dissociated and replated, and individual spheres demonstrated 100% self-renewal, with single spheres giving rise to one or more new spheres in each subsequent passage. The clonal retinal spheres were plated under differentiation conditions to assay the differentiation potential of their progeny. The spheres were produced all of the different retinal cell types, demonstrating multipotentiality. Therefore, the human eye contains a small population of cells (≈10,000 cells per eye) that have retinal stem-cell characteristics (proliferation, self-renewal, and multipotentiality). To test the
in vivo
potential of the stem cells and their progeny, we transplanted dissociated human retinal sphere cells, containing both stem cells and progenitors, into the eyes of postnatal day 1 NOD/SCID mice and embryonic chick eyes. The progeny of the RSCs were able to survive, migrate, integrate, and differentiate into the neural retina, especially as photoreceptors. Their facile isolation, integration, and differentiation suggest that human RSCs eventually may be valuable in treating human retinal diseases.
Self/nonself recognition in ascidian fertilization: Vitelline coat protein HrVC70 is a candidate allorecognition molecule
Authors: Sawada Hitoshi , Tanaka Etsuko , Ban Susumu , Yamasaki Chiho , Fujino Junko , Ooura Kazuto , Abe Yukichi , Matsumoto Ken-ichi , Yokosawa Hideyoshi
Published: October 2004

Ascidians are hermaphrodites releasing sperm and eggs simultaneously, but many species are self-sterile because of a self/nonself-recognition system in spermegg interaction. Here, we show that a 70-kDa vitelline coat protein, HrVC70, consisting of 12 epidermal growth factor-like repeats, plays a key role in self/nonself recognition during ascidian fertilization. We discovered that the amount of HrVC70 of the self-sterile mature oocytes is markedly higher than that of the self-fertile immature oocytes and that the selfsterile mature oocytes become self-fertile by acid treatment, which is able to release the HrVC70 from isolated vitelline coats. In addition, fertilization is strongly inhibited by the pretreatment of sperm with HrVC70 from a different individual, but not from the same individual, and the number of nonself sperm bound to HrVC70-agarose was significantly higher than that of self-sperm. A sequence analysis of HrVC70 disclosed that several amino acid residues in a restricted region are substituted at an individual level, with no identical sequences among the 10 individuals tested. Furthermore, genomic DNA analysis revealed that the epidermal growth factor-like domains correspond to the exons, and each intron is highly conserved among even- and odd-numbered introns, suggesting that multiple gene duplications or amplification of this region might have taken place during evolution. It was also found that diversity in cDNA sequences is derived from genomic DNA polymorphism probably elicited by crossing over and specific nucleotide substitutions. These results indicate that HrVC70 is a candidate allogeneic recognition molecule in the gamete interaction of the ascidian
Halocynthia roretzi
.
Conformational activation of Ca
2+
entry by depolarization of skeletal myotubes
Authors: Cherednichenko Gennady , Hurne Alanna M. , Fessenden James D. , Lee Eun Hui , Allen Paul D. , Beam Kurt G. , Pessah Isaac N.
Published: October 2004

Store-operated Ca
2+
entry (SOCE) occurs in diverse cell types in response to depletion of Ca
2+
within the endoplasmic/sarcoplasmic reticulum and functions both to refill these stores and to shape cytoplasmic Ca
2+
transients. Here we report that in addition to conventional SOCE, skeletal myotubes display a physiological mechanism that we term excitation-coupled Ca
2+
entry (ECCE). ECCE is rapidly initiated by membrane depolarization. Like excitation-contraction coupling, ECCE is absent in both dyspedic myotubes that lack the skeletal muscle-type ryanodine receptor 1 and dysgenic myotubes that lack the dihydropyridine receptor (DHPR), and is independent of the DHPR
l
-type Ca
2+
current. Unlike classic SOCE, ECCE does not depend on sarcoplasmic reticulum Ca
2+
release. Indeed, ECCE produces a large Ca
2+
entry in response to physiological stimuli that do not produce substantial store depletion and depends on interactions among three different Ca
2+
channels: the DHPR, ryanodine receptor 1, and a Ca
2+
entry channel with properties corresponding to those of store-operated Ca
2+
channels. ECCE may provide a fundamental means to rapidly maintain Ca
2+
stores and control important aspects of Ca
2+
signaling in both muscle and nonmuscle cells.
Organization of a primitive memory: Olfaction
Authors: de Gennes P.-G.
Published: October 2004

I present a tentative, and schematic, picture of the olfaction storage system in early mammalians: (
i
) how the excitation trains from the olfactory bulb excite selectively
N
types of cells in the piriform cortex and (
ii
) how these cells transfer their information to a storage area. Simple size and connectivity requirements impose that each odor is stored in a very small cluster of ≈3 neurons. This surprising result holds even if the model parameters are strongly altered (e.g., if
N
= 10
3
rather than 10
2
).
Nodal protein processing and fibroblast growth factor 4 synergize to maintain a trophoblast stem cell microenvironment
Authors: Guzman-Ayala Marcela , Ben-Haim Nadav , Beck Séverine , Constam Daniel B.
Published: October 2004

Before implantation in the uterus, mammalian embryos set aside trophoblast stem cells that are maintained in the extraembryonic ectoderm (ExE) during gastrulation to generate the fetal portion of the placenta. Their proliferation depends on diffusible signals from neighboring cells in the epiblast, including fibroblast growth factor 4 (Fgf4). Here, we show that Fgf4 expression is induced by the transforming growth factor β-related protein Nodal. Together with Fgf4, Nodal also acts directly on neighboring ExE to sustain a microenvironment that inhibits precocious differentiation of trophoblast stem cells. Because the ExE itself produces the proteases Furin and PACE4 to activate Nodal, it represents the first example, to our knowledge, of a stem cell compartment that actively maintains its own microenvironment.