PHOTODISSOCIATION DYNAMICS OF ORGANOMETALLIC COMPLEXES - MODEL SIMULATION FOR H+CO(CO)(4)[-HCO(CO)4-ASTERISK-]HCO(CO)(3)+CO

The photochemistry of HCo(CO)(4) has been studied through dynamical calculations based on ab initio potential energy surfaces for the metal-hydrogen bond homolysis and for the dissociation of the axial carbonyl ligand. The dynamics of the two competitive primary pathways are simulated by adiabatic motions of representative wave packets on the CASSCF/CCI potential energy surfaces corresponding to the lowest excited states by means of the fast Fourier transform (FFT) technique. The present study suggests the following sequential mechanism: (i) initial excitation of the molecule by W photons from the (1)A(1) ground state (preferably around 229 nm) to the (1)E 3d(delta) --> sigma* excited state; (ii) from this excited state, dissociation to the primary products H + Co(CO)(4) in the (1)E excited state on an ultrashort time scale (ca. 10 fs) competes with intramolecular vibrational energy redistribution (IVR) of the rest of the molecule HCo(CO)(4) in the (1)E state on a longer time scale; (iii) intersystem crossing (ISC) from the vibrationally relaxed HCo(CO)(4) (1E) molecule either to the (3)A(1) sigma --> sigma* excited state or to the (3)E 3d delta --> sigma* excited state; (iv) ultrafast dissociation into dominant product channels H + Co(CO)(4) (10 fs from the (3)A(1) state) or HCo(CO)(3) + CO (>100 fs from the 3E State); (v) intramolecular vibrational energy redistribution (IVR) of the remaining fraction of nondissociative HCo(CO)(4) in the (3)E state, with possible transition back to the ground state of the molecule. This sequential reaction mechanism (i-v) of the title reaction does account for some experimental results obtained by Sweany in tow-temperature matrices experiments, and it does predict important details of the absorption spectra, product distribution, and femtochemistry which may be tested experimentally.