Photostereochemistry and Photoaquation Reactions of [Cr(tn)3]3+: Theoretical Studies Show the Importance of Reduced Coordination Conical Intersection Geometries

We have performed TD-DFT and CASSCF calculations to understand the spectroscopy and reactive photochemistry of the [Cr(tn)(3)](3+) complex. Our results show that, after population of a quartet ligand field excited state, the system relaxes by dissociation of a Cr-N bond to reach a quasi-trigonal bipyramid five-coordinate species that is a conical intersection connecting the excited and ground quartet manifolds. Nonadiabatic relaxation through these leads to square pyramidal structures that can coordinate water and account for the observed monoaquated photoproducts. Such features are also present on the potential energy surfaces of these photoproducts and account for the range of experimentally observed photostereoisomers of the photoaquation reactions.