Role of ligand bending in the photodissociation of O-2 vs CO-heme: A time-dependent density functional study.

Time-Dependent DFT calculations reveal a strong dependence of low-lying excited states on the Fe-O-O angle of a model O2-heme adduct, and ready access to a metastable side-on isomer. The predicted state crossings provide a pathway for internal conversion to the ground state, and account for the low-quantum yield for O2-dissociation, while the side-on isomer can account for the observation of an unphotolizable fraction at low temperature. For CO-heme, the angle-sensitive state lie above the photo-accessed Q states, and do not partecipate in photodissociation.