Orbital interactions and chemical reactivity of metal particles and metal surfaces

A review is presented with 101 refs. on chem. bonding to metal surfaces and small metal particles demonstrating the power of symmetry concepts to predict changes in chem. bonding. Ab-initio calcns. of chemisorption to small particles, as well as semiempirical extended Hueckel calcns. applied to the study of the reactivity of metal slabs are reviewed. On small metal particles, classical notions of electron promotion and hybridization are found to apply. The surroundings of a metal atom (ligands in complexes, other metal atoms at surfaces), affect bonding and reactivity through the prehybridization they induce. A factor specific for large particles and surfaces is the required localization of electrons on the atoms involved in the metal surface bond. At the surface, the bond energy is found to relate to the grou8p orbital local d. of states at the Fermi level. The use of this concept is extensively discussed and illustrated for chemisorption of CO and dissocn. of NO on metal surfaces. A discussion is given of the current decompn. schemes of bond energies and related concepts (exchange (Pauli)-repulsion, polarization, charge transfer). The role of non-orthogonality of fragment orbitals and of kinetic and potential energy for Pauli repulsion and (orbital) polarization is analyzed. Numerous examples are discussed to demonstrate the impact of those concepts on chem. bonding theor

Chemical bonding in zeolites

A review is given with 28 refs. on the current status of chem. bonding of zeolites and AlPO's. Short range covalent bonding dominates, the tetrahedra have to be considered relatively rigid and the Si-O-Si bond angle flexible. Differences in energy of the SiO2 or AlPO4 polymorphs are small. The relative stability depends on a proper accounting of the small changes in electrostatic energy. The deprotonation energy is also mainly detd. by short range covalent interactions. These are only properly accounted for when full lattice relaxation is included in the calcns. Isomorphous substitution effects are also dominated by changes in covalent interaction energ

Experimental pressure-temperature phase diagram of boron: resolving the long-standing enigma

Boron, discovered as an element in 1808 and produced in pure form in 1909, has still remained the last elemental material, having stable natural isotopes, with the ground state crystal phase to be unknown. It has been a subject of long-standing controversy, if α-B or β-B is the thermodynamically stable phase at ambient pressure and temperature. In the present work this enigma has been resolved based on the α-B-to- β-B phase boundary line which we experimentally established in the pressure interval of ∼4 GPa to 8 GPa and linearly extrapolated down to ambient pressure. In a series of high pressure high temperature experiments we synthesised single crystals of the three boron phases (α-B, β-B, and γ-B) and provided evidence of higher thermodynamic stability of α-B. Our work opens a way for reproducible synthesis of α-boron, an optically transparent direct band gap semiconductor with very high hardness, thermal and chemical stability