Finite-length models of metallic and semiconducting carbon nanotubes (CNTs) based on Clar sextet theory of aromatic systems are proposed. For metallic CNTs, the electronic properties of finite-length models converge monotonically to the values expected for quasi-monodimensional metallic systems. For semiconducting CNTs, the use of finite-length models as proposed in this work leads to a fast convergence of the electronic properties to the values expected for the corresponding infinite-length nanotube.

We report a full quantum mechanical investigation, based on DFT calculations, on the unimolecular and bimolecular alkyne−vinylidene rearrangements in the prototype [Cl-Rh(Pi-Pr3)2(HC⋮CH)] complex, to solve the discrepancy between theory and recent experimental data and to provide a definitive answer concerning the largely debated molecularity issue of the 1,3-shift in d8 metal complexes. We calculate the intramolecular pathway to be kinetically favored over the intermolecular one by 15.0 kcal/mol, in agreement with recent crossover experiments. Comparison of our DFT calculations performed on the real systems with reduced models shows that a full quantum mechanical description of the investigated systems is mandatory for a correct description of their reactivity, owing to the relevant role played by the electron-donating phosphine ligands.

Throughout the years, a large number of paintings by Perugino have been investigated using the in situ non-destructive x-ray fluorescence (XRF) technique. Three anomalous characteristics were frequently identified, concerning a brown earth pigment containing zinc, the presence of manganese in some red lakes and the association of the presence of copper and tin with grey-blue regions. In an effort to understand these anomalies better, five micro-samples, taken from selected easel paintings by Perugino at the Galleria Nazionale dell'Umbria and a fresco at the Monastero di Sant'Agnese in Perugia, were studied by micro-Raman spectroscopy and scanning electron microscopy (SEM). Micro-Raman spectroscopy was a useful tool to investigate the molecular nature of such uncommon pigments and SEM provided useful information on the stratigraphy of the paint layers. The brown earth pigments, used by the master in the last period of his production, were definitely confirmed to be characterized by zinc impurities. The manganese contained in red lakes was shown to be due to ground alkaline glass, with a small quantity of manganese and iron, probably added to the oil binder in an attempt to speed up drying. Finally, in the sampled paintings, the grey-blue colour characterized by copper and tin was not present.

Density functional calculations have been performed on possible mechanisms for the hypothetic C−H bond cleavage process of benzene chemisorbed on the Si(100) surface, in order to shed light on the analogous process on larger polycyclic aromatic hydrocarbons. We first identified the minima on the potential energy surface for the benzene adsorption on Si(100) and for the breaking of two C−H bonds, with formation of two Si−H bonds, and then we analyzed possible pathways for the C−H bond cleavage, looking for the transition states connecting the adsorption configurations to the final products of C−H breaking. We identified two adsorbed configurations of benzene from which the breaking of two C−H bonds can be accessible, i.e., the 1,2 tilted di-σ bonded configuration on top of a single dimer (T) and the 1,4 di-σ bonded configuration where benzene bridges two dimer rows (BR). The kinetically most favorable reactive channel on the T configuration involves the abstraction of two hydrogen atoms on the sp3 carbon atoms by the silicon atoms of an adjacent dimer, with an energy barrier of 22.0 kcal mol-1. Although seemingly low, such an activation energy is not expected to be accessible at temperatures below the onset of benzene desorption from this configuration, which requires 15.9 kcal mol-1. The kinetically most favorable reactive channel on the BR configuration, which has not been experimentally detected for the benzene molecule, involves the rupture of one Si−C bond, passing through an energy barrier of 29.8 kcal mol-1, and ends with the formation of a Si−H bond and a vertical phenyl unit anchored on a silicon dimer.