Left, low-temperature MCD spectra of [(TMG3tren)Fe(O)]2+ (A), SyrB2 Fe(IV)=O (B), and α-O (C). Middle, geometric structures, including first shell atoms. All other atoms have been omitted for clarity. Right, corresponding DFT-calculated thermodynamics of HAA. Adapted from refs 87 and 29. Copyright 2016 American Chemical Society for ref 87 and Copyright 2016 Nature Publishing Group for ref 28. acalculated at the B3LYP+D2/def2-TZVP/COSMO(ε=4.0) level. bcalculated at the B3LYP+D2/def2-TZVP/COSMO(ε=4.0) level, in the absence of steric interference from a second-sphere Arg residue. cdetermined from experiment. dcalculated at the B3LYP+D3/6–311+G* level, using models reported in ref. 29.