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.