Wavepacket Motion via a Conical Intersection in the Photochemistry of Aqueous Transition-Metal Dianions

The photochemical reaction paths in aqueous PtBr6 2– and OsBr6 2– have been studied by femtosecond broad-band pump–probe spectroscopy supported by CASPT2 and DFT/TDDFT calculations. These paths lead to the separation of negative charges and propagate through distortions of nascent, penta-coordinated metal fragments caused by Jahn–Teller C 4v and D 3h conical intersections (CIs), respectively. Within 150 fs following 420 nm excitation of PtBr6 2–, the molecule undergoes internal conversion and intersystem crossing into the dissociative lowest triplet excited 3T1g state, loses a ligand, and relaxes via the C 4v CI to the nearly trigonal bipyramid 3PtBr5 – product in the triplet state. Direct 530 nm excitation of PtBr6 2– to 3T1g yields the same product. Oscillations observed in the bending and umbrella a1 modes of 3PtBr5 – arise from impulsive excitation of, respectively, one of the reaction coordinate modes, which is parallel to the gradient difference vector of the C 4v CI, and the “spectator” mode that preserves the electronic degeneracy.