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

The photochemistry of HCo(CO)(4) has been studied through dynamical ca lculations 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 simu lated by adiabatic motions of representative wave packets on the CASSC F/CCI potential energy surfaces corresponding to the lowest excited st ates by means of the fast Fourier transform (FFT) technique. The prese nt study suggests the following sequential mechanism: (i) initial exci tation of the molecule by W photons from the (1)A(1) ground state (pre ferably 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 l onger time scale; (iii) intersystem crossing (ISC) from the vibrationa lly relaxed HCo(CO)(4) (1E) molecule either to the (3)A(1) sigma --> s igma 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 s tate, with possible transition back to the ground state of the molecul e. This sequential reaction mechanism (i-v) of the title reaction does account for some experimental results obtained by Sweany in tow-tempe rature matrices experiments, and it does predict important details of the absorption spectra, product distribution, and femtochemistry which may be tested experimentally.