Table 1.

The calculated (B3LYP/def2-TZVPP) relative enthalpies and Gibbs free energies^[a] [kcal mol⁻¹] for the catalytic cycle of oxygen activation of different spin states.

	Δ H	Δ G
septet O2 adduct	3.1	12.6[b]
7TS1	11.2	26.4
7(FeO)2+	–45.8	–40.0
quintet O2 adduct	3.9	14.6[b]
5TS1	12.0	28.5
quintet FeII-peroxosuccinate	–42.3	–32.3
5(FeO)2+	–61.8	–56.7
triplet O2 adduct	2.6	12.2[b]
triplet bicyclic intermediate	14.5	31.0

^[a]All energies reported here are relative enthalpies and Gibbs free energies computed with respect to the five-coordinate Fe^II complex and free triplet O₂.

^[b]The addition of O₂ to the iron center of TauD was predicted to be endergonic by approximately 13 kcal mol⁻¹, which is mainly an entropic effect. As shown in the quantum mechanical/molecular mechanical (QM/MM) calculations on the O₂ binding to hemerythrin, van der Waals and electrostatic contribution from the protein environment amount to –10 kcalmol⁻¹ to the free energy.^[29] Thus, if one takes these effects into account, the O₂ binding reaction in TauD is only slightly endergonic or isoenergetic given the error of the employed methodology.