“…Estimation of the gap DE O 2 ð 1 D g / 3 S À g Þ for isolated O 2 itself is problematic (24): the simple system has limitations and difficulties because both O 2 and O 2 -complexes are open-shell systems. Methods such as MP2, CCSD, and MRCI predict the singlet-triplet energy gap of the isolated oxygen to be from 0.9 eV to 1.16 eV (24,25) while the experimental estimations give 0.99 eV of this value (26). Electron paramagnetic resonance data show that in the b 6 f complex, the heme b n can bind the O 2 molecule via one of the pyrrol rings (27) fixing, thus, the O 2 molecule in the hydrophobic region between hemes b n and c n (28,29).…”