Photodissociation and Electronic Spectroscopy of Mn(H)(CO)3(H-DAB) (DAB = 1,4-Diaza-1,3-butadiene): Quantum Wave Packet Dynamics Based on ab Initio Potentials

The photodissociation dynamics of Mn(H)(CO) 3 (H-DAB) have been studied through wave packet propagations on CASSCF/MR-CCI potentials calculated for the electronic ground and low-lying excited states as a function of two coordinates q a and q b corresponding to the Mn−H bond homolysis and to the axial CO loss, respectively. The theoretical absorption spectrum is characterized by two bands, one intense peak centered at 476 nm (21 000 cm - 1 ) and one broad band centered at 285 nm (35 000 cm - 1 ). The visible band has been assigned to the low-lying metal-to-ligand charge-transfer (MLCT) states with a main contribution of the a 1 A‘ → c 1 A‘ transition corresponding to the 3d xz → π* DAB excitation. The second band calculated in the UV energy domain has been assigned to the d 1 A‘ (3d yz → 3d xy ) and e 1 A‘ (σ Mn - H → π* DAB ) states corresponding to a metal-centered (MC) and a σ-bond-to-ligand charge-transfer (SBLCT) state, respectively. Simulation of excited states dynamics upon visible irradiation by propagation of selected wave packets on the two-dimensional MLCT potentials coupled nonadiabatically indicates a probability of dissociation of the axial CO of 99% in 400 fs. Nonradiative transitions to the low-lying triplet states or homolysis of the metal−hydrogen bond via the dissociative 3 SBLCT state are not competitive with this ultrafast deactivation process. Simulation of the UV photochemistry points to a very low probability of dissociation in this energy region.