An ab initio study
of the interaction of O
2
, the most
abundant radical and oxidant species in the atmosphere, with a Cu
5
cluster, a new generation atomic metal catalyst, is presented.
The open-shell nature of the reactant species is properly accounted
for by using the multireference perturbation theory, allowing the
experimentally confirmed resistivity of Cu
5
clusters toward
oxidation to be investigated. Approximate reaction pathways for the
transition from physisorption to chemisorption are calculated for
the interaction of O
2
with quasi-iso-energetic trapezoidal
planar and trigonal bipyramidal structures. Within the multireference
approach, the transition barrier for O
2
activation can
be interpreted as an avoided crossing between adiabatic states (neutral
and ionic), which provides new insights into the charge-transfer process
and gives better estimates for this hard to localize and therefore
often neglected first intermediate state. For Cu
5
arranged
in a bipyramidal structure, the O–O bond cleavage is confirmed
as the rate-determining step. However, for planar Cu
5
,
the high energy barrier for O
2
activation, related to a
very pronounced avoided crossing when going from physisorption to
chemisorption, determines the reactivity in this case.