A third possibility based on the formation of the Fe(CO), fragment in a singlet state is a singlet-singlet emission through an internal conversion from the blA, state to the a'Al state at dissociation, followed with an allowed oxidative addition of H2 along the alA, potential energy curve to regenerate H,Fe(CO), in its ground state. In this last case chemiluminescence should be observed for the photoelimination of Hz in the gas phase.
ConclusionPotential energy curves corresponding to the elimination of molecular hydrogen have been computed for the ground and excited states of H,Fe(CO), from CASSCF and CCI calculations. From the shape of these potential energy curves a mechanism has been proposed for the photochemistry of H,Fe(CO), at 254 nm, which may be summarized as follows: the molecule first excited through allowed transitions to the states a'Bz d -u*" and blAl d -u *~ may evolve toward H2 elimination either along the b'Al potential energy curve leading to the products Hz and Fe(CO),, the latter in the excited state b'A,, or along the a3Bz curve leading beyond a small energy barrier to the products in their ground state. It appeared from these calculations that the a3B2 curve in its dissociative part corresponds to the excitation of an electron from a M O that is H-H antiboding and M-H bonding to a MO that is H-H bonding and M-H antibonding leading to the elimination of H2. We have identified the photoactive excited states responsible for the photoreactivity of H2Fe(C0)4. Chemiluminescence has been predicted for the photoelimination of H2 in the gas phase. We have been able to get a better understanding of the mechanism for the reverse oxidative addition observed experimentally.Improving the accuracy of the calculations could be achieved by (i) carrying out a CASSCF calculation for each excited state, (ii) optimizing the geometrical parameters at the CASSCF level, and (iii) introducing the spin-orbit coupling operator in the CCI calculations. The possibility of an alternative primary pathway, namely the loss of a carbonyl ligand, is not excluded, even though this reaction has not been reported in low-temperature matrix experiments. A complete understanding of the photochemistry of HzFe(C0)4 would require the calculation of the potential energy curves corresponding to the loss of a carbonyl ligand in order to corroborate this hypothesis. This work is actually in progress. Pseudopotential calculations including relativistic effects would be necessary in order to compare the behavior upon irradiation of various dihydride complexes with a different metal center (Os, Rh, Ir) and to get a better understanding of their photoreactivity.
Acknowledgment. The calculations have been carried out onthe CRAY-2 computer of the CCVR (Palaiseau, France) through a grant of computer time from the Conseil Scientifique du Centre de Calcul Vectoriel pour la Recherche.The room temperature ESR spectra of the radicals in four kinds of plasma-irradiated methacrylic polymers (acrylic resins), poly(methacry1ic acid) (PMAA), its methyl ...