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Rare-earth induced layered structures on the Si(111) surface are investigated by a combined approach consisting of ab initio thermodynamics, electron and x-ray diffraction experiments, angle-resolved photoelectron spectroscopy, and scanning tunneling microscopy. Our density functional theory calculations predict the occurrence of structures with different periodicity, depending on the rare-earth availability. Microscopic structural models are assigned to the different silicide phases on the basis of stability criteria. The thermodynamically stable theoretical models are then employed to interpret the experimental results. The agreement between the simulated and measured scanning tunneling microscopy images validates the proposed structural models. The electronic properties of the surfaces are discussed on the basis of the calculated electronic band structure and photoelectron spectroscopy data.

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Stereochemistry, products, and driving forces of the "first and second Cinchona rearrangement" have been investigated and a unified theory is presented. The first cage expansion affords [3.2.2]azabicyclic alpha-amino ether and is formulated via a configurationally stable bridgehead iminium ion and quasiequatorial nucleophilic attack. The second cage expansion affords beta-functionalized [3.2.2]azabicycles. In this case a nonclassical nitrogen-bridged cation is postulated to account for retention of configuration and potential reversibility of the cage expansion. The second rearrangement is favored for the so-called cinch bases (6'-R = H) in trifluoroethanol. Stereoelectronic factors, electron demand at C9, ground state conformation, and solvent type are crucial in all cases. A two-step protocol for preparing 9-epi-configured Cinchona alkaloids from 9-nat precursors is described.