Theoretical Analysis
of the Fragmentation of (CO)<sub>5</sub>: A Symmetry-Allowed Highly
Exothermic Reaction that Follows
a Stepwise Pathway
- Publication date
- Publisher
Abstract
B3LYP
and CCSD(T) calculations, using an aug-cc-pVTZ basis set,
have been carried out on the fragmentation of 1,2,3,4,5-cyclopentanepentone,
(CO)<sub>5</sub>, to five molecules of CO. Although this reaction
is calculated to be highly exothermic and is allowed to be concerted
by the Woodward–Hoffmann rules, our calculations find that
the <i>D</i><sub>5<i>h</i></sub> energy maximum
is a multidimensional hilltop on the potential energy surface. This <i>D</i><sub>5<i>h</i></sub> hilltop is 16–20
kcal/mol higher in energy than a <i>C</i><sub>2</sub> transition
structure for the endothermic cleavage of (CO)<sub>5</sub> to (CO)<sub>4</sub> + CO and 11–15 kcal/mol higher than a <i>C</i><sub>s</sub> transition structure for the loss of two CO molecules.
The reasons for the very high energy of the <i>D</i><sub>5<i>h</i></sub> hilltop are discussed, and the geometries
of the two lower energy transition structures are rationalized on
the basis of mixing of the e<sub>2</sub>′ HOMO and the a<sub>2</sub>″ LUMO of the hilltop