H₂ Dissociation in Nickel. A Cluster Model

Abstract

Dedicated t o Prof. Dr. A. SEECER on the occasion of his 60th birthday A cluster formed by 38 Ni atoms and two H atoms is studied by a semiempirical Hartree-Fock (HF) self-consistent-field (SCF) variant based on the INDO (intermediate neglect of differential overlap) approximation. The energy of the system is analyzed as a function of the spatial localization of t h e two H atoms. The energy minimum in the considered part of the singlet-type potential surface corresponds t o a spatial arrangement where the H atoms occupy nearest-neighbor oetahedral sites in the f.c.c. Ni-lattice (d(H-H) = 0.249 nm). This configuration is w l eV below anotlier local minimum which is associated to a "H2" molecule (d(H-H) = 0.085 nm) in the host system. The H atoms in the Ni-lattice can be characterized as overscreened protons. Ein Cluster von 38 Ni-Atomen und 2 H-Atomen wird mit einem semiempirischen Hartree-Fock (HE')-SCF-Verfahren auf der Basis einer INDO (intermediate neglect of differential overlap)-Naherung untersucht. Die Energie des Systems wird als Funktion der riiumlichen Lokalisierung der beiden H-Atome analysiert. Das Energieminimum in dem beriicksichtigten Teil der ,,Singlet"-Energie-Hyperflache wird fur eine Anordnung gefunden, in der die H-Atome auf benachbarten Okt~ederplatzen des kubiseh-flachenzentrierten Ni-Gitters lokalisiert sind (d(H-H) = 0,249 nm). Diese Anordnung ist zz 1 eV stebiler als ein zweites lokales Minimum, das einem ,,H,-Molekul" im Wirtsmetall entspricht (d(H-H) = 0,085 nm). Die H-Atome im Ni-Gitter verhalten sich wie "uberabgeschirmt.e" Protonen. l) Heisenbergstr. 1, D-7000 Stuttgart 80, FRG. 2, Petersenstr. 20, D-6100 Darmstadt, FRG. s, Permanent address: Physics Department, USACH, Santiago-2, Chile. 20 pliyniea (b) l44/l

Hepatocellular carcinoma with main portal vein tumor thrombus

“…As referred by Halet et al29 different possible mechanisms to stabilize H–H interaction in solids are possible, all of them inspired by the already mentioned σ‐back donation bonding. Vargas et al30 have investigated the H occupancy on interstitial sites of fcc Nickel in the diluted solution limit. They found that the double hydrogen occupation of an octahedral site leads to a metastable geometry, where the H–H distance is around 0.85 Å and appear 1 eV/H above the (stable) single occupation case.…”

“…The analysis of the density charge in the solid state exhibits the presence of a CP between hydrogen atoms. This particular H–H interaction cannot be rationalized in terms of the standard σ‐back donation mechanism24, 25 and neither does it correspond to a simple dimerization as it has been discussed in hydrogen diluted phases29, 30 or a dihydrogen bond scheme 23, 31, 33. The Westlake's criteria are not followed in this case owing to the particular low dimensionality of the H arrangement in the NiH 2 units.…”

An orbital analysis of hydrogen pairing in nonstoichiometric transition-metal hydrides

“…This secondary minimum is computed to lie only 1 eV above the lowest energy minimum, which corresponds to a spatial arrangement where the H atoms occupy different octahedral sites. 13 In this case, the small size of the cavity is a consequence of the short metal-metal separation. Analogous results obtained in calculations on the Li10H2 edge-sharing bioctahedral cluster might have the same origin.30 Although various theoretical studies of through-space H-H interaction in Pd point to repulsive interactions,14 when the two H atoms were forced to lie inside the same octahedral vacancy, an H-H equilibrium distance of ~0.9 A was calculated,14® a value close to the one found in the above-mentioned13 calculations on nickel hydride.…”