Ab-initio design of bulk materials assembled with silicon clusters

Tese de doutoramento, Física, Universidade de Lisboa, Faculdade de Ciências, 2011The fact that the founding papers of Density Functional Theory are among the most cited papers ever, testi es for the importance of Quantum Mechanics and its (often) counter intuitive features in characterizing many-particle systems at a nano and sub-nano scale. Density Functional Theory has enabled one to use the computer to predict quantitatively several of the properties of the aforementioned many-particle systems. The prediction of new materials, often exhibiting meta-stability, is one of its distinctive features. In this work we will discuss a new class of meta-materials which, being silicon based, exhibit properties which in no way resemble those of its main constituent. We investigate the feasibility of assembling the exceptionally stable isovalent X@Si16 (X=Ti, Zr and Hf) nanoparticles to form new bulk materials. We use rst principles density functional theory. Our results predict the formation of stable, wide band-gap materials crystallizing in HCP structures in which the cages bind weakly, similar to fullerite. The present study suggests new pathways through which endohedral cage clusters may constitute viable means toward the production of synthetic materials with pre-de ned physical and chemical properties. Within the same rst-principles framework we will investigate the vibrational modes and infrared spectra of the isovalent X@Si16 (X=Ti, Zr and Hf) nanoparticles. Our results predict the existence of high-intensity modes of low frequency. An estimate of the electron-phonon coupling strength is also provided based on a single-molecule method introduced recently. The large value of combined with predicted stability of bulk materials assembled with these nanoparticles suggest that these new materials, when appropriately doped, may exhibit high-temperature superconducting properties

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Pushing the limits of concertedness. A waltz of wandering carbocations.

Among the array of complex terpene-forming carbocation cyclization/rearrangement reactions, the so-called "triple shift" reactions are among the most unexpected. Such reactions involve the asynchronous combination of three 1,n-shifts into a concerted process, e.g., a 1,2-alkyl shift followed by a 1,3-hydride shift followed by a second 1,2-alkyl shift. This type of reaction so far has been proposed to occur during the biosynthesis of diterpenes and the sidechains of sterols. Here we describe efforts to push the limits of concertedness in this type of carbocation reaction by designing, and characterizing with quantum chemical computations, systems that could couple additional 1,n-shift events to a triple shift leading, in principle to quadruple, pentuple, etc. shifts. While our designs did not lead to clear-cut examples of quadruple, etc. shifts, they did lead to reactions with surprisingly flat energy surfaces where more than five chemical events connect reactants and plausible products. Ab initio molecular dynamics simulations demonstrate that the formal minima on these surfaces interchange on short timescales, both with each other and with additional unexpected structures, allowing us a glimpse into a very complex manifold that allows ready access to great structural diversity