Electron Transfer Processes at Aryl‐Modified Glassy Carbon Electrode

The effect of a layer of electrochemically grafted 4‐diazo‐N,N‐diethylaniline (DEA) groups on the electron transfer kinetics of redox systems, displaying fast and slow heterogeneous electron transfer rate constants at edge and basal planes of carbon, was investigated. The properties of the modified electrode were characterized by cyclic voltammetry using four different inorganic redox systems (Fe(CN) , Co(phen) , Ru(NH₃) , and IrCl in acidic, neutral, and basic media. Two distinct blocking behaviors and electrostatic effects were observed. More precisely, a strong blocking effect of the grafted layer on Fe(CN) and Co(phen) was found, whereas Ru(NH₃) and IrCl showed to be rather unaffected by the presence of the DEA grafted layer.     

Steered molecular dynamics simulation of elastic behavior of adsorbed single polyethylene chains

We investigate the dynamics of the detachment of single polyethylene (PE) chains from a strongly adsorbing surface in vacuum using a united atom model. Various statistical properties, including the mean‐square end‐to‐end distance 〈R²〉, the mean‐square radii of gyration , , the shape factor , the torsion angle distribution, the average surface adsorption energy , the average total energy , and the average force , are analyzed. The relationship between the average force and the pulling velocity v shows two distinctive regions: a weakly dependence region at Å/ps and a strongly dependence region at Å/ps. Remarkably, the PE chain manifests force hysteresis under sequential stretching and releasing. These investigations may provide some insights into the elastic behavior of adsorbed polymer chains. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2322–2332, 2007     

Formation of Diverse Mesophases Templated by a Diprotic Anionic Surfactant

The synthesis system for mesophase formation, using the diprotic anionic surfactant N‐myristoyl‐L ‐glutamic acid (C₁₄GluA) as the structure‐directing agent (SDA) and N‐trimethoxylsilylpropyl‐N,N,N‐trimethylammonium chloride (TMAPS) as the co‐structure‐directing agent (CSDA), has been investigated and a full‐scaled synthesis‐field diagram is presented. In this system we have obtained mesophases including three‐dimensional (3D) micellar cubic Fm m, Pm n, Fd m, micellar tetragonal P4₂/mnm, two‐dimensional (2D) hexagonal p6mm and bicontinuous cubic Pn m, by varying the C₁₄GluA/NaOH/TMAPS composition ratios. From the diagram it can be concluded that the mesophase formation is affected to a high degree by the organic/inorganic‐interface curvature and the mesocage–mesocage electrostatic interaction. Bicontinuous cubic and 2D‐hexagonal phases were found in the low organic/inorganic‐interface curvature zones, whereas micellar cubic and tetragonal mesophases were found in the high organic/inorganic‐interface curvature zones. Formation of cubic Fm m and tetragonal P4₂/mnm was favoured in highly alkaline zones with strong mesocage–mesocage interactions, and formation of cubic Pm n and Fd m was favoured with moderate mesocage–mesocage interactions in the less alkaline zones of the diagram.     

Hydrogen isotope effects on covalent and noncovalent interactions: The case of protonated rare gas clusters

We investigate hydrogen isotope and nuclear quantum effects on geometries and binding energies of small protonated rare gas clusters (Rg $_n$ X $^ +$ , Rg = He,Ne,Ar, X = H,D,T, and $n$ = 1–3) with the any particle molecular orbital (APMO) MP2 level of theory (APMO/MP2). To gain insight on the impact of nuclear quantum effects on the different interactions present in the Rg $_n$ X $^ +$ systems, we propose an APMO/MP2 energy decomposition analysis scheme. For RgH $^ +$ ions, isotopic substitution leads to an increase in the stability of the complex, because polarization and charge transfer contributions increase with the mass of the hydrogen. In the case of Rg $_2$ H $^ +$ complexes, isotopic substitution results in a shortening and weakening of the rare gas‐hydrogen ion bond. For Rg $_3$ X $^ +$ complexes, the isotope effects on the rare gas binding energy are almost negligible. Nevertheless, our results reveal that subtle changes in the charge distribution of the Rg $_2$ X $^ +$ core induced by an isotopic substitution have an impact on the geometry of the Rg $_3$ X $^ +$ complex. © 2012 Wiley Periodicals, Inc.     

Polysulfide Chemistry in Sodium–Sulfur Batteries and Related Systems— A Computational Study by G3X(MP2) and PCM Calculations

The sodium–sulfur (NAS) battery is a candidate for energy storage and load leveling in power systems, by using the reversible reduction of elemental sulfur by sodium metal to give a liquid mixture of polysulfides (Na₂S_n) at approximately 320 °C. We investigated a large number of reactions possibly occurring in such sodium polysulfide melts by using density functional calculations at the G3X(MP2)/B3LYP/6‐31+G(2df,p) level of theory including polarizable continuum model (PCM) corrections for two polarizable phases, to obtain geometric and, for the first time, thermodynamic data for the liquid sodium–sulfur system. Novel reaction sequences for the electrochemical reduction of elemental sulfur are proposed on the basis of their Gibbs reaction energies. We suggest that the primary reduction product of S₈ is the radical anion ${{\rm S}{{{{\bullet}}- \hfill \atop 8\hfill}}}$ , which decomposes at the operating temperature of NAS batteries exergonically to the radicals ${{\rm S}{{{{\bullet}}- \hfill \atop 2\hfill}}}$ and ${{\rm S}{{{{\bullet}}- \hfill \atop 3\hfill}}}$ together with the neutral species S₆ and S₅, respectively. In addition, ${{\rm S}{{{{\bullet}}- \hfill \atop 8\hfill}}}$ is predicted to disproportionate exergonically to S₈ and ${{\rm S}{{2- \hfill \atop 8\hfill}}}$ followed by the dissociation of the latter into two ${{\rm S}{{{{\bullet}}- \hfill \atop 4\hfill}}}$ radical ions. By recombination reactions of these radicals various polysulfide dianions can in principle be formed. However, polysulfide dianions larger than ${{\rm S}{{2- \hfill \atop 4\hfill}}}$ are thermally unstable at 320 °C and smaller dianions as well as radical monoanions dominate in Na₂S_n (n=2–5) melts instead. The reverse reactions are predicted to take place when the NAS battery is charged. We show that ion pairs of the types ${{\rm NaS}{{{{\bullet}}\hfill \atop 2\hfill}}}$ , ${{\rm NaS}{{- \hfill \atop n\hfill}}}$ , and Na₂S_n can be expected at least for n=2 and 3 in NAS batteries, but are unlikely in aqueous sodium polysulfide except at high concentrations. The structures of such radicals and anions with up to nine sulfur atoms are reported, because they are predicted to play a key role in the electrochemical reduction process. A large number of isomerization, disproportionation, and sulfurization reactions of polysulfide mono‐ and dianions have been investigated in the gas phase and in a polarizable continuum, and numerous reaction enthalpies as well as Gibbs energies are reported.     

Experimental and Theoretical Study of the Broadening and Shifting of N2H+ Rotational Lines by Helium

Pressure broadening and pressure shift of N₂H⁺ rotational lines perturbed by collisions with He are studied for the first time using experiment and theory. Results are reported from measurements at 88 K for the rotational transitions ${j = 3 \leftarrow 2}$ , ${4 \leftarrow 3}$ , ${5 \leftarrow 4}$ and ${6 \leftarrow 5}$ with frequencies ranging from 0.28 to 0.56 THz. The agreement between experiment and theoretical data derived from close coupling calculations confirms the reliability of a theoretical framework used for state‐to‐state transition rates of interest in the interpretation of spectroscopic data from interstellar molecular clouds. The influence of hyperfine effects on shifts and widths of the rotational lines is discussed in detail. Although in principle possible, experiment and theoretical considerations lead to the conclusion that hyperfine effects only play a minor role.     

On the 4JHH long‐range coupling in 2‐bromocyclohexanone: conformational insights

2‐Bromocyclohexanone is a model compound in which a ⁴J_{H2, H6} coupling constant is observed, whereas the corresponding ⁴J_{H2, H4} is absent. The observed long‐range coupling is not only a result of the known W‐type coupling, in the axial conformation, but also because of the less usual diaxial spin–spin coupling in the equatorial conformer. The carbonyl group plays a determining role in describing the coupling pathway, as concluded by natural bond orbital (NBO) analysis; although the and interactions in the axial conformer contribute for transmitting the spin information associated with the W‐type coupling, the strong and hyperconjugations in the equatorial conformer define an enhanced coupling pathway for ⁴J_{H2, H6}, despite the inhibition of this coupling because of interaction and the large carbonyl angle. These findings provide the experimental evidence that orbital interactions contribute for the conformational isomerism of 2‐bromocyclohexanone. Copyright © 2008 John Wiley & Sons, Ltd.     

1H, 13C and 15N nuclear magnetic resonance coordination shifts in Au(III), Pd(II) and Pt(II) chloride complexes with phenylpyridines

¹H, ¹³C and ¹⁵N nuclear magnetic resonance studies of gold(III), palladium(II) and platinum(II) chloride complexes with phenylpyridines (PPY: 4‐phenylpyridine, 4ppy; 3‐phenylpyridine, 3ppy; and 2‐phenylpyridine, 2ppy) having the general formulae [Au(PPY)Cl₃], trans‐/cis‐[Pd(PPY)₂Cl₂] and trans‐/cis‐[Pt(PPY)₂Cl₂] were performed and the respective chemical shifts (δ, δ and δ) reported. ¹H, ¹³C and ¹⁵N coordination shifts (i.e. differences between chemical shifts of the same atom in the complex and ligand molecules: , , ) were discussed in relation to the type of the central atom (Au(III), Pd(II) and Pt(II)), geometry (trans‐/cis‐) and the position of a phenyl group in the pyridine ring system. Copyright © 2009 John Wiley & Sons, Ltd.     

The effect of water content on the ultimate properties of rubbery nanocomposite gels

An investigation was conducted into the effects of water content (R) on the ultimate tensile properties of nanocomposite hydrogels (NC gels) based on poly(N‐isopropylacrylamide)/clay networks. Rubbery NC gels with low clay contents (<NC10) exhibited unique changes in their stress–strain curves, depending on the R. At high R, where PNIPA chains are fully hydrated, NC gels retained their rubbery tensile properties, whereas they changed to exhibit plastic‐like deformations with decreasing R. Consequently, for a series of NC gels with different R, a failure envelope was obtained by connecting the rupture points in the stress–strain curves. Here, the counterclockwise movement was observed as either the R decreased or the strain rate increased. This seemed to be analogous to that of a conventional elastomer (e.g., SBR), although the mechanisms are different in the two cases. From the R and C_clay dependences of the ultimate properties, three critical values of R were defined, where R showed a maximum strain at break, a steep increase in initial modulus, and onset of brittle fracture. Compared with NC gels, OR gels (chemically crosslinked hydrogels) showed similar but very small changes in their stress–strain curves on altering R, whereas LR (viscous PNIPA solution) showed a monotonic decrease (increase) in ε_b (E_i) with decreasing R. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2328–2340, 2009     

Synthesis and Characterization of Stereo Multiblock Poly(lactic acid)s with Different Block Lengths by Melt Polycondensation of Poly(L‐lactic acid)/Poly(D‐lactic acid) Blends

Stereo multiblock PLAs with different block lengths are synthesized by melt polycondensation of low‐molecular‐weight poly(L ‐lactic acid)/poly(D ‐lactic acid) blends with a wide variety of $\overline {M} _{{\rm w}} $ in the range of 1.1–5.2 × 10³ g · mol^–1. The average block length (ν_av) of the stereo multiblock PLAs increases with increasing $\overline {M} _{{\rm w}} $ of the blend and with the reaction temperature, whereas $\overline {M} _{{\rm w}} $ and PDI of the stereo multiblock PLAs increases with increasing $\overline {M} _{{\rm w}} $ of the blend, the reaction time, and the temperature. Stereo multiblock PLAs with ν_av > 7 are crystallizable to form stereocomplex crystallites, and the crystallinity and melting temperature of the stereo multiblock PLAs increases with increasing ν_av and $\overline {M} _{{\rm w}} $ of the stereo multiblock PLAs.

     

Bound‐states of diatomic molecules in the Dirac equation with the q‐deformed Morse potential

We present the solutions of the ro‐vibrational motion of a diatomic molecule with a spatially dependent mass by solving the Dirac equation with position‐dependent mass for repulsive vector $V(r)$ and attractive scalar $S(r)$ q‐deformed Morse potential for any $\kappa$ value, within the framework of Pekeris approximation of the spin‐orbitcoupling term. The relativistic energy spectra are obtained using theNikiforov‐Uvarov method and the two‐component spinor wavefunctions are obtained in terms of the Laguerre polynomials. It is found that there exist only negative energy states for bound states, and the energy values for a fixed value of $n_r$ increase with decrease in $\kappa$ . © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Electric and magnetic dipole shielding constants for the ground state of the relativistic hydrogen‐like atom: Application of the Sturmian expansion of the generalized Dirac‐Coulomb Green function

The Sturmian expansion of the generalized Dirac‐Coulomb Green function (Szmytkowski, J Phys B, 1997, 30, 825; erratum 1997, 30, 2747) is exploited to derive closed‐form expressions for electric $(\sigma_{E})$ and magnetic $(\sigma_{M})$ dipole shielding constants for the ground state of the relativistic hydrogen‐like atom with a point‐like and spinless nucleus of charge Ze. It is found that $\sigma_{E}=Z^{-1}$ (as it should be) and where $\gamma_{1}=\sqrt{1-(Z\alpha)^{2}}$ (α is the fine‐structure constant). This expression for $\sigma_{M}$ agrees with earlier findings of several other authors, obtained with the use of other analytical techniques, and is elementary compared to an alternative one presented recently by Cheng et al. (J Chem Phys 2009, 130, 144102), which involves an infinite series of ratios of the Euler's gamma functions. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Understanding,Controlling and Programming Cooperativity in Self‐Assembled Polynuclear Complexes in Solution

Deviations from statistical binding, that is cooperativity, in self‐assembled polynuclear complexes partly result from intermetallic interactions ΔE^M,M, whose magnitudes in solution depend on a balance between electrostatic repulsion and solvation energies. These two factors have been reconciled in a simple point‐charge model, which suggests severe and counter‐intuitive deviations from predictions based solely on the Coulomb law when considering the variation of ΔE^M,M with metallic charge and intermetallic separation in linear polynuclear helicates. To demonstrate this intriguing behaviour, the ten microscopic interactions that define the thermodynamic formation constants of some twenty‐nine homometallic and heterometallic polynuclear triple‐stranded helicates obtained from the coordination of the segmental ligands L1 – L11 with Zn²⁺ (a spherical d‐block cation) and Lu³⁺ (a spherical 4f‐block cation), have been extracted by using the site binding model. As predicted, but in contrast with the simplistic coulombic approach, the apparent intramolecular intermetallic interactions in solution are found to be i) more repulsive at long distance ( > ), ii) of larger magnitude when Zn²⁺ replaces Lu³⁺ ( > ) and iii) attractive between two triply charged cations held at some specific distance ( <0). The consequences of these trends are discussed for the design of polynuclear complexes in solution.     

Electron density and its derivatives at the nucleus for spherically confined hydrogen atom

It is shown that the energy of a hydrogen‐like atom confined inside a spherical cavity of radius, R, and potential barrier, V₀, is quantitatively defined by the ratio . Here, the conventional spherical density (r) is scaled as η_l(r) = and the ratio of the second derivative η(r) to η_l(r) is evaluated at the nucleus. Numerical results of the ratios are presented for 1s, 2s, 2p, and 3d states at several values of V₀. For such states, the characteristic radii of confinement leading to the well‐defined values of energy are identified. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

A New Quantum Chemical Approach to the Magnetic Properties of Oligonuclear Transition‐Metal Complexes: Application to a Model for the Tetranuclear Manganese Cluster of Photosystem II

Broken‐symmetry DFT calculations on transition‐metal clusters with more than two centers allow the hyperfine coupling constants to be extracted. Application of the proposed theoretical scheme to a tetranuclear manganese complex that models the S₂ state of the oxygen‐evolving complex of photosystem II yields hyperfine parameters that can be directly compared with experimental data. The picture shows the metal–oxo core of the model and the following parameters; exchange coupling constant J_ij, the expectation value of the site‐spin operator , and the isotropic hyperfine coupling parameters.

     

New Nuclear‐Spin‐Induced Cotton–Mouton Effect in Fluids at High DC Magnetic Field

Based on Buckingham and Pople’s theory of magnetic double refraction, a theoretical expression is derived for a new Cotton–Mouton effect ${\phi _{{\rm{C}} - {\rm{M}}}^{(IB)} }$ in liquid induced by the crossed effect between the high dc magnetic field B₀ and the nuclear magnetic moment ${m_z^{(I)} }$ . It contains temperature‐independent and ‐dependent parts. The latter is proportional to the product between anisotropy of polarizability and the nuclear magnetic shielding tensor. For this new effect ${\phi _{{\rm{C}} - {\rm{M}}}^{(IB)} }$ , its order in magnitude for a molecule with large polarizability anisotropy is estimated to be comparable to the nuclear‐spin‐induced optical Faraday rotation (NSOFR). In the multipass approach, ${\phi _{{\rm{C}} - {\rm{M}}}^{(IB)} }$ can be eliminated by time‐reversal symmetry arguments, but NSOFR is enhanced.     

Guanidinium Alkynesulfonates with Single‐Layer Stacking Motif: Interlayer Hydrogen Bonding Between Sulfonate Anions Changes the Orientation of the Organosulfonate R Group from “Alternate Side” to “Same Side”

Hydrolyses of HC?CSO₃SiMe₃ ( 1 ) and CH₃C?CSO₃SiMe₃ ( 2 ) lead to the formation of acetylenic sulfonic acids HC?CSO₃H?2.33 H₂O ( 3 ) and CH₃C?CSO₃H?1.88 H₂O ( 4 ). These acids were reacted with guanidinium carbonate to yield [⁺C(NH₂)₃][HC?CSO₃^?] ( 5 ) and [⁺C(NH₂)₃][CH₃C?CSO₃^?] ( 6 ). Compounds 1 – 6 were characterized by spectroscopic methods, and the X‐ray crystal structures of the guanidinium salts were determined. The X‐ray results of 5 show that the guanidinium cations and organosulfonate anions associate into 1D ribbons through ${{\rm R}{{2\hfill \atop 2\hfill}}}$ (8) dimer interactions, whereas association of these ions in 6 is achieved through ${{\rm R}{{2\hfill \atop 2\hfill}}}$ (8) and ${{\rm R}{{1\hfill \atop 2\hfill}}}$ (6) interactions. The ribbons in 5 associate into 2D sheets through ${{\rm R}{{2\hfill \atop 2\hfill}}}$ (8) dimer interactions and ${{\rm R}{{3\hfill \atop 6\hfill}}}$ (12) rings, whereas those in 6 are connected through ${{\rm R}{{1\hfill \atop 2\hfill}}}$ (6) and ${{\rm R}{{2\hfill \atop 2\hfill}}}$ (8) dimer interactions and ${{\rm R}{{4\hfill \atop 6\hfill}}}$ (14) rings. Compound 6 exhibits a single‐layer stacking motif similar to that found in guanidinium alkane‐ and arenesulfonates, that is, the alkynyl groups alternate orientation from one ribbon to the next. The stacking motif in 5 is also single‐layer, but due to interlayer hydrogen bonding between sulfonate anions, the alkynyl groups of each sheet all point to the same side of the sheet.     

Spin probe mobility in relation to free volume and relaxation dynamics of glass‐formers: A series of spin probes in poly(isobutylene)

We present the dynamics of a series of three paramagnetic molecules of different volume, mass, and shape in amorphous glass‐forming polymer poly(isobutylene) (PIB) as investigated by means of electron spin resonance (ESR) technique. The reorientation behavior of spin probes is related to the ortho‐positronium (o‐Ps) annihilation in PIB from positron annihilation lifetime spectroscopy (PALS) and the extracted free volume information. It is also related to the dynamic data of PIB from broadband dielectric spectroscopy (BDS), neutron scattering (NS), and nuclear magnetic resonance (NMR) spectroscopy from literature. In the case of the smallest spin probe, 2,2,6,6‐tetramethyl‐1‐piperidinyloxy (TEMPO), a discontinuous course of the spectral parameter 2A_zz′ versus T dependence was observed and the subsequent phenomenological model‐free analyses of the spectral parameter, 2A_zz′ versus T, as well as of the correlation time, τ_c, versus 1/T plots provided the characteristic ESR temperatures ( , T_50G, ) and (T, T, T). These characteristic ESR temperatures were found to be consistent with the characteristic PALS temperatures: T, T = T from temperature dependences of the mean o‐Ps lifetime, τ₃, or the width of o‐Ps lifetime distribution, σ₃, respectively. In addition, the relationships between the spin probe size, V, and the free volume hole size distributions g_n(V_h) at the characteristic ESR temperatures indicate the significant influence of the free volume fluctuation at the crossover from slow to rapid regime as well as within the rapid motional regime. On the other hand, the two larger spin probes exhibit a rather continuous 2A_zz′–T plots with the respective T_50G's lying in the vicinity of T independently of their volume, mass and shape, suggesting the common origin of underlying process controlling this T_50G transition. Finally, these mutual PALS and ESR findings were compared with the known dynamic behavior of PIB which suggest that the dynamics of the TEMPO and the larger spin probes are related to free volume fluctuation associated with primary α ‐ and secondary β processes, respectively. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1058–1068, 2009     

High Accuracy of Karplus Equations for Relating Three‐Bond J Couplings to Protein Backbone Torsion Angles

³J_C′C′ and ³J_HNHα couplings are related to the intervening backbone torsion angle ${\varphi }$ by standard Karplus equations. Although these couplings are known to be affected by parameters other than ${\varphi }$ , including H‐bonding, valence angles and residue type, experimental results and quantum calculations indicate that the impact of these latter parameters is typically very small. The solution NMR structure of protein GB3, newly refined by using extensive sets of residual dipolar couplings, yields 50–60 % better Karplus equation agreement between ${\varphi }$ angles and experimental ³J_C′C′ and ³J_HNHα values than does the high‐resolution X‐ray structure. In intrinsically disordered proteins, ³J_C′C′ and ³J_HNHα couplings can be measured at even higher accuracy, and the impact of factors other than the intervening torsion angle on ³J will be smaller than in folded proteins, making these couplings exceptionally valuable reporters on the ensemble of ${\varphi }$ angles sampled by each residue.