Francesca Costanzo

Physisorption, Diffusion, and Chemisorption Pathways of H2 Molecule on Graphene and on (2,2) Carbon Nanotube by First Principles Calculations.

We investigate the interaction of the H2 molecule with a graphene layer and with a small-radius carbon nanotube using ab initio density functional methods. H2 can interact with carbon materials like graphene, graphite, and nanotubes either through physisorption or chemisorption. The physisorption mechanism involves the binding of the hydrogen molecule on the material as a result of weak van der Waals forces, while the chemisorption mechanism involves the dissociation of the hydrogen molecule and the ensuing reaction of both hydrogen atoms with the unsatured C-C bonds to form C-H bonds. In our calculations, we take into account van der Waals interactions using a recently developed method based on the concept of maximally localized Wannier functions. We explore several adsorption sites and orientations of the hydrogen molecule relative to the carbon surface and compute the associated binding energies and adsorption potentials. The most stable physisorbed state on graphene is found to be the hollow site in the center of a carbon hexagon, with a binding energy of -48 meV, in good agreement with experimental results. The analysis of diffusion pathways between different physisorbed states on graphene shows that molecular hydrogen can easily diffuse at room temperature from one configuration to another, which are separated by energy barriers as small as 10 meV. We also compute the potential energy surfaces for the dissociative chemisorption of H2 on highly symmetric sites of graphene, the lowest activation barrier found being 2.67 eV. Much weaker adsorption characterizes instead the physisorption interaction of the H2 molecule with the small radius (2,2) CNT. The barriers for H2 dissociation on the nanotube external surface are significantly lowered with respect to the graphene case, showing the remarkable effect of the substrate curvature in promoting hydrogen dissociation.

On the Polarity of Buckminsterfullerene with a Water Molecule Inside

Since the recent achievement of Kurotobi and Murata to capture a water molecule in a C(60) fullerene (Science 2011, 333, 613), there has been a debate about the properties of this H(2)O@C(60) complex. In particular, the polarity of the complex, which is thought to be underlying the easy separation of H(2)O@C(60) from the empty fullerene by HPLC, was calculated and found to be almost equal to that of an isolated water molecule. Here we present our detailed analysis of the charge distribution of the water-encapsulated C(60) complex, which shows that the polarity of the complex is, with 0.5 ± 0.1 D, indeed substantial, but significantly smaller than that of H(2)O. This may have important implications for the aim to design water-soluble and biocompatible fullerenes.

The Thermally-Induced Bulk Polymerization of Hexachlorocyclotriphosphazene to Polydichlorophosphazene by First-Principles Simulations

In this paper we report first principles calculations using the Car-Parrinello molecular dynamics approach in order to clarify the mechanism of the ring-opening polymerization of hexachlorocyclotriphosphazene, N3P3Cl6 to polydichlorophosphazene (NPCl2)n. In particular, we highlight the key role played by the cyclophosphazene phosphorus cation N3P3Cl5+ generated from the heterolytic cleavage of the P–Cl bond as the main component in the polymerization process. Moreover, the generation of the maximally localized Wannier functions, together with the Mulliken population analysis, allows us to clarify some basic issues of the investigated reaction. The activation energy, mainly due to the formation of phosphazenium cation, is found to be in good agreement with experiments. Structural and electronic analysis of the molecules involved is also reported.

Adsorption of Toluene on Si(100) from First Principles

ion from the methyl group opens up new opportunity for σ bonding of the methylene (CH 2) group because of different energy involved in the C-H bond clevage re spectively 85 and 110Kcal/mol. Note that this configuration is obtained by breaking one CH bond of the methyl group since this requires a lower energy (85Kcal/mol) than breaki ng a C-H bond in the aromatic ring (110Kcal/mol).This also agrees with the experimental results found out from Leung which indicate that benzene desorbs molecularly, at difference from tolue ne that dissociates upon adsorption. Among the undissociated forms, the “Tight bridge” (TiB) a nd “twisted bridge” configurations are the lowest energy configurations, formed by tet ra-σ-bonded structure and characterized by the presence of one C-C double bond (see Fig 1). Like in the case of benzene, the TwB is similar to TiB configur ation but is slightly higher in energy (see Table 1). In these configurations, the going in of energy due to the format ion of 4 bonds, balances the lost of the aromaticity of the ring after reaction with silicon surfa ce. Our calculations show also that there are, at somewhat higher energies, three different “butterfly” s tructures, characterized by two C-Si 576 Silicon Carbide and Related Materials 2002