Selective Catalytic Activation of Acetylene by a Neutral Gold Cluster of Experimentally Known Gas-Phase Geometry

The electronic and structural details for the acetylene selective catalytic activation by one of the few small gold clusters whose experimental gas-phase initial geometry in neutral charge state is known, the gold heptamer, are investigated. Doing an exhaustive search of the acetylene–gold heptamer ZORA scalar relativistic PW91/TZ2P configurational space, we determine the main, secondary, and also the unimportant structures relative to the catalytic activation. We found that the leading mechanism of activation consists in the tendency to the disappearance of one of the acetylene π bonds at the expense of the formation of C–Au strong interactions, by the predominant dative interaction of s/d-like gold heptamer molecular orbitals toward a π* orbital of acetylene. This results in adducts having very diminished energy barriers toward processes such as, for example, the selective hydrogenation reaction to ethylene. This activation would occur with a considerable change of the participant species geometries and, with the formation of a frontier single occupied molecular orbital, very localized at carbon atoms region.