Understanding structure, size, and charge effects for the H2 dissociation mechanism on planar gold clusters

Abstract

The H2 dissociation on several planar Aunq gold clusters, with n = 4,..., 10 and charges q = 0, ±1, has been studied in detail as a function of the nuclear configuration of the cluster and at different sites of attack. It is found that the formation of a well in the entrance channel is a necessary condition for the dissociation to occur. This well always appears in sites of Aunq where there is a defect in the electronic density with respect to that of the n neutral and isolated gold atoms or, in other words, where there is a positive charge due to the polarization of the electronic density associated to the electronic correlation. When H 2 attacks on linear sides of three atoms, on the middle gold atom, the reactivity is fully determined by this entrance well. On the contrary, when attacking corners there is a second step, in which a b2 antibonding orbital crosses the a1 HOMO orbital. The b2 orbital is strongly stabilized by an important bonding overlap between the H2 and the two neighboring gold atoms orbitals. For obtuse corners, with atoms of coordination 3, the stabilization due to this H-Au bonding overlap occurs at shorter distances than for acute angles, of coordination 2, simply because the neighboring gold atoms are structurally closer. Thus, the crossing occurs at shorter H-H distances for the obtuse angle, yielding lower dissociation barriers, while for the acute case the barrier is always high. The height of the barrier as a function of the charge is explained by the occupation of the frontier orbitals. For those cases in which the Aun- anion presents the entrance well, the stabilizing b2 orbital has typically the maximum occupation yielding the lower reaction barriers. The relaxation of the gold cluster in the reaction is analyzed by optimizing the total system at the stationary points. For the relaxed case, the MEP's obtained are nearly parallel to those obtained for the frozen gold cluster, which validates the main conclusions of this work. © 2010 American Chemical Society.