Theoretical Analysis of the Fragmentation of (CO)₅: A Symmetry-Allowed Highly Exothermic Reaction that Follows a Stepwise Pathway

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)₅, 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>_5<i>h</i> energy maximum is a multidimensional hilltop on the potential energy surface. This <i>D</i>_5<i>h</i> hilltop is 16–20 kcal/mol higher in energy than a <i>C</i>₂ transition structure for the endothermic cleavage of (CO)₅ to (CO)₄ + CO and 11–15 kcal/mol higher than a <i>C</i>_s transition structure for the loss of two CO molecules. The reasons for the very high energy of the <i>D</i>_5<i>h</i> hilltop are discussed, and the geometries of the two lower energy transition structures are rationalized on the basis of mixing of the e₂′ HOMO and the a₂″ LUMO of the hilltop