Charge transport in poly-imidazole membranes: a fresh appraisal of the Grotthuss mechanism

A detailed theoretical investigation of the charge transport mechanism in poly(4-vinyl-imidazole) (P4VI), the parent polymer of a series of N-heterocyclic-based membranes used as an electrolyte in proton exchange membrane fuel cells, is presented. In particular, Density Functional Theory (DFT) results obtained for small model systems (protonated imidazole dimers and trimers) suggest that the commonly accepted conduction mechanism, based on a sequential proton transfer between imidazole moieties, could be impeded by the geometrical constraints imposed by the polymeric backbone. Indeed only one kind of proton transfer reaction is energetically allowed between adjacent imidazoles, so that a rotation of the protonated imidazole is required for a second proton transfer. Molecular dynamics simulations on a larger model (15 oligomers with an excess proton) show that the rotation of the imidazole carrying the excess proton is a soft large amplitude motion. These results allow us to propose a new proton conduction mechanism in P4VI, where a frustrated rotation of the protonated imidazole before each proton transfer reaction represents the rate-limiting step. Furthermore, in contrast with the Grotthuss proton transport mechanism in water, our results indicate that here it is the same proton which could be successively transferred. From a chemical point of view, these new insights into the mechanism are relevant for a rational design of modified azole-based systems for Proton Exchange Membrane Fuel Cells.     

Nanodiamonds in sugar rings: an experimental and theoretical investigation of cyclodextrin-nanodiamond inclusion complexes

We report on the noncovalent interactions of nanodiamond carboxylic acids derived from adamantane, diamantane, and triamantane with β- and γ-cyclodextrins. The water solubility of the nanodiamonds was increased by attaching an aromatic dicarboxylic acid via peptide coupling. Isothermal titration calorimetry experiments were performed to determine the thermodynamic parameters (K(a), ΔH, ΔG and ΔS) for the host-guest inclusion. The stoichiometry of the complexes is invariably 1:1. It was found that K(a), ΔG and ΔH of inclusion increase for larger nanodiamonds. ΔS is generally positive, in particular for the largest nanodiamonds. β-Cyclodextrin binds all nanodiamonds, γ-cyclodextrin clearly prefers the most bulky nanodiamonds. The interaction of 9-triamantane carboxylic acid shows one of the strongest complexation constants towards γ-cyclodextrin ever reported, K(a) = 5.0 × 10(5) M(-1). In order to gain some insight into the possible structural basis of these inclusion complexes we performed density functional calculations at the B97-D3/def2-TZVPP level of theory.     

Precipitation-driven self-sorting of imines

Judicious choice of precipitation conditions can lead to self-sorting of equilibrating mixtures of aromatic aldehydes and substituted anilines into a handful of imine products. The selectivity of this process is caused by the solubility differences among possible imines in the EtOH-H(2)O solvent mixtures used in precipitation.     

Nickel(II) 3,4;9,10-perylenediimide bis-phosphonate pentahydrate: a metal-organic ferromagnetic dye

The new metal-organic compound nickel(II) 3,4;9,10-perylenediimide bis-phosphonate pentahydrate, i.e. Ni(2)[(PDI-BP)(H(2)O)(2)]·3H(2)O (1), has been synthesized and its structural and magnetic properties have been studied. Reaction of 3,4;9,10-perylenediimide bis-phosphonate (PDI-BP, hereafter) ligand and nickel chloride in water resulted in the precipitation of a red and poorly crystalline solid (1). As the solid shows a poor crystalline organization of aggregates, the energy dispersive X-ray diffraction analysis (EDXD) technique has been used to obtain short-range order structural information of the single nanoaggregates by radial distribution function analysis. The overall structure of the compound is characterized by layers containing perylene planes shifted in the direction perpendicular to the stacking axes in such a way that only the outer rings overlap. The edges of the perylene planes are connected to the phosphonate groups through an imido group. The oxygen atoms of the [-PO(3)](2-) group and those of the water molecules are bonded to the nickel ions resulting in a [NiO(6)] octahedral coordination sphere. The Ni-O bond lengths are 0.21 ± 0.08 nm and the Ni-O-Ni angles of aligned moieties are 95 ± 2°. The oxygen atoms of the water molecules and the nickel atoms are nearly planar and almost perpendicular to the perylene planes forming chains of edge-sharing octahedra. The magnetic properties of (1) show the presence of intrachain ferromagnetic Ni-Ni interactions and a long-range ferromagnetic order below 21 K with a canting angle and with a spin glasslike behavior due to disorder in the inorganic layer. Hysteresis cycles show a coercive field of ca. 272 mT at 2 K that decreases as the temperature is increased and vanishes at ca. 20 K.     

Consistent and transferable coarse-grained model for semidilute polymer solutions in good solvent

We present a coarse-grained model for linear polymers with a tunable number of effective atoms (blobs) per chain interacting by intra- and intermolecular potentials obtained at zero density. We show how this model is able to accurately reproduce the universal properties of the underlying solution of athermal linear chains at various levels of coarse-graining and in a range of chain densities which can be widened by increasing the spatial resolution of the multiblob representation, i.e., the number of blobs per chain. The present model is unique in its ability to quantitatively predict thermodynamic and large scale structural properties of polymer solutions deep in the semidilute regime with a very limited computational effort, overcoming most of the problems related to the simulations of semidilute polymer solutions in good solvent conditions.     

Strong π-delocalization and substitution effect on electronic properties of dithienylpyrrole-containing bipyridine ligands and corresponding ruthenium complexes

The first dithienylpyrrole (DTP)-based bipyridine ligands has been prepared and coordinated with ruthenium to give the corresponding homoleptic complexes. Bipyridine was bound at pyrrole (DTP(1)) or thiophene (DTP(2)) ring. A strong bathochromic effect was obtained by switching from pyrrole to thiophene for ligands and complexes. Interestingly the DTP(2) series offered a wide absorption window from UV to visible domain with an almost constant absorbance. These effects are due to a larger extent of delocalization as supported by DFT calculations and photophysical measurements.     

The 2000 Å absorption system of the benzene single crystal in polarized light

The 2000 Å absorption system in the benzene single crystal is investigated at liquid nitrogen temperature. The analysis of the polarized spectra recorded along b and c in (100) plates appears to be not in favour of the assignment of the π* └π system to an upper state of B _1u symmetry, and it is not in contrast with the hypothesis of a ¹ E _2g upper state.

The first band in the system, located at 46 565 cm^-1, and totally polarized along c, is discussed in terms either of an out-of-plane false origin of the ¹ E _2g └¹ A _1g system, or of a new transition lying between the first two well known π-systems of benzene, possibly of π* └σ type.     

Use of anisotropic atom-atom potential functions in lattice-dynamical calculations for solid nitrogen

For solid nitrogen, a set of ‘anisotropic’ atom-atom semi-empirical potential functions, which are easily derived from the usual ‘6-exp’ or ‘12-6’, eliminates the difficulties encountered in demonstrating the stability of the γ-phase in lattice-dynamical calculations according to the Born-von Karman procedure.

Negative eigenvalues of the dynamical matrices in certain regions of the Brillouin zone disappear, the agreement with experimental data improves and the α-γ transition is foreseen.