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The photochemistry of HCo(C0)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 UV photons from the 'A1 ground state (preferably around 229 nm) to the 'E 3ds -o* excited state; (ii) from this excited state, dissociation to the primary products H + Co(C0)4 in the '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(C0)d in the 'E state on a longer time scale; (iii) intersystem crossing (ISC) from the vibrationally relaxed HCo(C0)4 ('E) molecule either to the 3A1 o -o* excited state or to the 3E 3ds -o* excited state; (iv) ultrafast dissociation into dominant product channelsH + Co(C0)4 (10 fs from the 3A1 state) or HCo(C0)3 + CO ('100 fs from the 3E state); (v) intramolecular vibrational energy redistribution (IVR) of the remaining fraction of nondissociative HCo(C0)4 in the 3E 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 low-temperature matrices experiments, and it does predict important details of the absorption spectra, product distribution, and femtochemistry which may be tested experimentally.

The traditional Polanyi rules for control of bimolecular reactions by selective investment of energy, e.g. preferentially translational, not vibrational energy for early-downhill reactions on attractive potential energy surfaces, are extended to ultrafast unimolecular reactions. Specifically, we consider photodissociations of the metal-hydrogen bond of HCo(C0)4( lE), occurring on a time scale of approximately 20 fs, much faster than competing intramolecular vibrational energy redistribution (IVR). Here the reaction path toward the products H + C O ( C O )~ is hindered by a barrier located in the H + Co(CO)4 exit valley of the potential energy surface.In order to overcome this barrier and, therefore, to increase reactivity, vibrational energy should be invested selectively into the bond to be broken, i.e. [H-Co], not into complementary "spectator" modes, e.g. [CO-Co].The required energetic preparation of reactants may be achieved by selective IR + UV two-photon excitations or by alternative techniques including frequency-selective UV single-photon photodissociation. The Polanyi rules for unimolecular reactions are demonstrated by fast Fourier transform (FFT) propagations of representative wave packets.

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