Role of ligand bending in the photodissociation of O2 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 unphotolyzable fraction at low temperature. For CO-heme, the angle-sensitive states lie above the photoaccessed Q state and do not participate in photodissociation.