Coarse-Grained Models of Aqueous Solutions of Polyelectrolytes: Significance of Explicit Charges.

The structure of polyelectrolytes is highly sensitive to small changes in interactions between their monomers. In particular, interactions mediated by counterions play a significant role and are affected by both specific molecular effects and generic concentration effects. The ability of coarse-grained models to reproduce the structural properties of an atomic model is thus a challenging task. Our present study compares the ability of different kinds of coarse-grained models: (i) to reproduce the structure of an atomistic model of a polyelectrolyte (the sodium polyacrylate) and (ii) to reproduce the variations of this structure with the number of monomers and with the concentration of different species. We show that adequate scalings of the gyration radius of the polymer Rg with the number of monomers N and with the box size Lbox are only obtained, first, if the monomer charges and the counterions are explicitly described and, second, if an attractive Lennard-Jones contribution is added to the interaction between distant monomers. Also, we show that implicit ion models are relevant only to the high electrostatic screening regime.

Chemical Reactions on Metal-supported Hexagonal Boron Nitride Investigated with Density Functional Theory.

Nanotechnology, in order to become ultimately efficient, requires achieving the construction of elemental structures at the atomistic precision. One way toward this goal is using templated surfaces as support for directed synthesis. The nanomesh, a single-atom thick layer of hexagonal boron nitride on Rh(111) [Corso et al., Science 2004, 303, 217], has appeared as one candidate that provides a periodically corrugated structure on the nanometre scale. We present density functional theory studies where we investigate various properties of the nanomesh, ranging from intrinsic defects to covalent functionalisation with hydroxyl radicals. Further we study selective dehalogenation of an organic molecule (I6-CHP) on the nanomesh. This molecule adsorbs at particular sites of the template and has been activated for C-C coupling in recent experiments via dissociation of its halogen ligands. In all cases we find explicit templating effects, either in full agreement with experimental studies or predicting novel phenomena. The studies are evidence for the predictive power of modern electronic structure simulations and the insight that can be gained when used together with experiments on complex chemical structures.