Photochemistry of organometallics: Quantum chemistry and photodissociation dynamics

The photodissociation dynamics of three model systems in the lowest electronically excited states, HCo(CO)(4), H2Fe(CO)(4) and HMn(CO)(3)(alpha-diimine), each representative of a class of transition metal complexes, are reported for the following elementary processes: (i) ultrafast (within ca. 20 to 100 fs) direct dissociation on a single totally repulsive potential; (ii) ultrafast (within ca. 20 fs) dissociation on three kinetically coupled potentials; (iii) fast (within ca. 10 ps) indirect dissociation via tunneling through a potential energy barrier; (iv) indirect dissociation via fast (within ca. 10 ps) spin-orbit coupling induced intersystem crossing. By extension of the strategy applied with success to small di- and triatomic molecules to multidimensional transition metal complexes, the dynamics are simulated using a time-dependent wave packet propagation technique on ab initio CASSCF/CCI potential energy surfaces. The nature of the photoactive excited states is determined without ambiguity, as well as the time scales, the branching ratio of the primary reactions and some important features of the absorption spectra. An example of mode selective control of a unimolecular dissociation by a two-photon laser strategy is reported for HCo(CO)(4). (C) 1997 Elsevier Science S.A.