Juan Carlos Paniagua

Controlling the spin of metal atoms adsorbed on oxide surfaces: Ni on regular and defective sites of the MgO(001) surface

We have analyzed the relative energy of nonmagnetic and magnetic low-lying electronic states of Ni atoms adsorbed on regular and defective sites of the MgO(001) surface. To this end cluster and periodic surface models are used within density functional theory. For Ni atoms adsorbed on oxygen vacancies at low coverage, the interaction energy between the metal and the support is much larger than on regular sites. Strong bonding results in a diamagnetic adsorbed species and the energy required to reach the high-spin state increases. Moreover, a correlation appears between the low-spin to high-spin energy difference and the interaction energy hypothesizing that it is possible to prepare the surface to tune the high-spin to low-spin energy difference. Magnetic properties of adsorbed thin films obtained upon increasing coverage are more difficult to interpret. This is because the metallic bond is readily formed and dominates over the effect of the atoms directly bound to the vacancy.

Distortive properties of σ- and π-electrons and aromaticity: a semiempirical localized molecular orbital approach

Abstract This paper analyzes the changes experienced by the valence localized molecular orbitals (LMOs) of benzene, singlet cyclobutadiene and (E)-1,3,5-hexatriene — with respect to both energy and degree of delocalization — under the effect of several geometrical distortions of the carbon-carbon frame. The analysis shows that, at the AM1 level of theory, there is a unique symmetrizing tendency of the π-LMOs of benzene, which are at the same time maximally delocalized and energetically most stable for a totally symmetric geometry. The π-LMOs of both cyclobutadiene and hexatriene show opposite behavior, increasing their energies when subjected to bond length equalizing distortions and decreasing them when the carbon-carbon bond length alternation of the system is enhanced. The distortive properties of the π-electrons, when analyzed in terms of the corresponding AM1-LMOs, thus appear to correlate well with the aromatic character of the molecule.

Topological localized molecular orbitals

Intrinsic and external π-orbital localization procedures which rely only on molecular topology are proposed and discussed. Localized molecular orbitals obtained by application of these procedures are referred to as ‘topological localized molecular orbitals”.

Chemisorption of atomic hydrogen on rhodium: An ab initio cluster-model approach☆

Abstract Interaction of atomic hydrogen with cluster models simulating the Rh(111) surface has been studied at the ab initio SCF level using relativistic non-empirical pseudopotentials and moderately large basis sets. From the bonding parameters, it is seen that chemisorption on the threefold open position is five times more favoured than on the threefold eclipsed one, and the bonding picture is rather different for both active sites. Since two different kinds of adsorbed hydrogen atoms should exist according to various experimental techniques, the present work adds further evidence to the existence of these two types of adsorption and reports some structural parameters which can help to a more detailed characterization in the near future.

Comment on: Deuterium nuclear fusion at room temperature: A pertinent inequality on barrier penetration

Gerald Rosen (Ref. 1) estimates the fusion rate for deuterium molecules trapped in a metallic lattice by applying WKB barrier penetration formula. This comment proves that Rosen’s formula is incorrect. (AIP)

A pairing theorem for localized molecular orbitals

Abstract The pairing theorem, holding for canonical Huckel orbitals of alternant hydrocarbons, is shown to be extensible to any set of non-canonical molecular orbitals and, in particular, for a localized set. This allows to obtain antibonding counterparts of the occupied localized orbitals without having to repeat the localization procedure for the virtual set. Some particularities arising for charged systems are discussed, and the results obtained for naphthalene are shown as an example.