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.