Audrey L. Companion

A new diatomics-in-molecules study of Li3 and Li4

Abstract The diatomics-in-molecules method, with an improved triplet diatomic curve for Li 2 , is employed in a reexamination of the stability of Li 3 and Li 4 species. Results are compared to other theoretical and experimental values.

Theoretical studies of molecules on metal surfaces I. Site stability and vibrational frequencies of CO on Ni(111), Ni(110) and Ni(110)

Abstract Abundant free access to an IBM Supercomputer has facilitated the thousands of calculations necessary to optimize the geometries of arrays of CO molecules chemisorbed on large clusters simulating surfaces of Ni metal, to compute directly from the potential energy surfaces the CO and NiCO vibrational frequencies, and to examine the conditions under which interactions between the molecules alter these frequencies. In this paper we report results for one and two CO molecules on terminal, bridge and hollow sites on the (111), (110) and (100) faces of Ni. Terminal CO stretching frequencies on the three faces at low coverages do not differ much, nor do the bridge frequencies at normal NiNi distances (2.489 A); however a decidedly lower second frequency is predicted for the long bridge on the (110) face. For three-fold hollow sites on Ni(111), no difference is predicted between sites over Ni atoms and sites over holes. CO molecules occupying the four-fold sites on Ni(100) should have a frequency about 100 cm −1 lower than that observed for three-fold sites on Ni(111). We present strong theoretical evidence that a CO in the rectangular four-fold site on Ni(100) is really a terminal CO bonded to a second-layer atom and is characterized by a frequency only slightly lower than the on-top CO on this surface. Our computed binding energies agree with the trends of previous theoretical calculations, with on-top positions least stable, bridged sites intermediate, and hollow (three-fold or four-fold) sites most stable. However, it is proposed that the increased polarity of the CO molecules observed on going from one-top to bridge to hollow positions causes at high coverage preferential occupation of on-top sites due to intermolecular repulsions. Finally, in contradiction to some recent suggestions, we find that the stretching frequency of NiCO bonds increases as the frequency of the associated CO increases, in apparent agreement with experimental results.

An ehmo study of the interaction of CO molecules absorbed on a Ni(111) surface with neighboring CO molecules, H atoms and O atoms

Abstract The extended Huckel molecular orbital (EHMO) method has been employed to study the stability of clusters of CO, H and O on a Ni(111) surface as proposed by Marzouk, Arunkumar and Bradley [Surface Sci. 147 (1984) 477] from their recent NURS study of this system. A 25 atom nickel cluster is used to represent the metal surface, and 40 coabsorbed groups are examined. While some proposed interacting groups are found to be energetically stable, others are shown to be precursors to formation of new molecules on the surface. Stretching frequencies of the CO molecule in each of the 40 coabsorbed groups are estimated from calibration curves of computed bond orders and known axial stretching frequencies of several metal carbonyls. In most cases considered, our computed CO frequency shifts appear to be quite different from those suggested by Marzouk, Arunkumar and Bradley.

On the location of a hydrogen atom in body-centered cubic 3d transition metal lattices

Abstract Extended Huckel molecular orbital calculations were carried out for 22-atom clusters simulating the b.c.c. forms of vanadium, chromium, iron and cobalt. In each study the Hiickel parameter K was optimized to reproduce the observed cohesive energy of the isolated metal. Results indicate that in all cases a hydrogen atom implanted in the cluster is more stable in a site of distorted tetrahedral symmetry than in a site of distorted octahedral symmetry, and that a hydrogen atom in either site is stabilized by an adjacent lattice vacancy. An inverse relationship is observed between the computed binding energies of hydrogen atoms in both tetrahedral and octahedral sites and the d electron repulsive potential.

On the location of an absorbed hydrogen atom in aluminium metal

Abstract Extended Huckel molecular orbital calculations indicate that a hydrogen atom absorbed in aluminum metal is bound in an off-center tetrahedral interstitial site adjacent to a vacancy, thus confirming previous jellium-based theoretical predictions and results of nuclear channelling experiments. Our work further indicates that these are the only stable internal sites for hydrogen atoms in an aluminum lattice.

The interaction of hydrogen with point defects in aluminum metal

Abstract The extended Huckel molecular orbital approach previously applied to the study of the interaction of a hydrogen atom with a monovacancy in an aluminum lattice is used to examine the interaction of a single hydrogen atom with Frenkel defects and divacancies, the stability of a hydrogen molecule and multiple hydrogen atoms in defect structures, and the activation energies for diffusion of hydrogen through the lattice in the presence of vacancies or impurities. The relevance of these results to embrittlement and blistering processes is discussed.

A molecular orbital study of hydrogen atoms implanted in titanium metal clusters

Extended Huckel Molecular Orbital (EHMO) theory has been applied to the problem of hydrogen atom site location in titanium clusters Ti4, Ti5, Ti17 and Ti20, the latter simulating the hcp metal face (0001). In agreement with previous ab initio calculations, it is found that an external position for the hydrogen atom over a three-fold coordination site is favored over all internal positions in a perfect crystal. Among the internal positions, tetrahedral sites are favored over octahedral ones. The binding energies of internal sites appear to be enhanced by the presence of vacancies; indeed, the only internal site that is stable with respect to the surface is a tetrahedral hole adjacent to a vacancy. Density of states diagrams for Ti17 show a hydrogen induced band at approximately 6.5 eV below the Fermi level, in agreement with photoemission results.

On the cobalt (1120) surface as a template for hydrocarbon chain formation in Fischer—Tropsch synthesis

An explanation is proposed for the observations of Geerlings, Zonnevylle and de Groot [Surf. Sci. 241 (1991) 302, 315] that longer hydrocarbon chains grow on the zigzag grooved cobalt (1120) surface during Fischer-Tropsch reactions, while on stepped (1012) and smooth (0001) surfaces mainly C1 fragments are observed. Molecular orbital calculations show that the zigzag troughs on this surface may act as templates favoring CO dissociation and skeletal carbon chain formation, and that such events do not occur on smooth surfaces. Some speculations are offered on the nanotechnological design of “custom” templates for hydrocarbon synthesis.