Photodissociation of (CO)+2: Theoretical studies of ground 2B u and excited 2B g potential energy surfaces

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Abstract
We have calculated ab initio ground and excited state potential energy surfaces for (CO)+2 and its constituent monomers at correlated, extended basis levels, in order to elucidate the anomalous photodissociation dynamics of this dimer cation. We obtained 160 points (UMP2//6‐31G*level) on the ground 2Bu surface and 144 points (54‐term MCSCF//6‐31G level) on the excited 2Bg surface for stretch–bend interactions in symmetric trans planar (C2h) geometries. The calculated surfaces reveal remarkable low‐energy ‘‘valleys’’ connecting regions of dissimilar electronic structure, leading to a surprisingly short CC approach distance, nonrigid geometry, and complex vibrational dynamics. Our results suggest that the experimental bond dissociation energy of Ng and co‐workers significantly understates the true value. Natural bond orbital (NBO) analysis is used to relate the structure of (CO)+2 to that of CO, CO+ monomers, and to construct simple orbital diagrams that account for the principal topological features of the potential surfaces. From these studies there emerges a satisfactory qualitative interpretation of the observed (CO)+2 photodissociation spectrum, and of the electronic factors that distinguish this species from isoelectronic (N2)+2 and other (XY)+2 gas phase dimer cations.