ESR Studies of Low-Spin Fe (III) Heme Complexes with Hydrogen-Bonded Axial Ligands. Deuterium Isotope Effect on the Principal g values

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The deuterium isotope effect on ESR g values has been determined for the first time for low-spin Fe (III) heme complexes with hydrogen-bonded axial ligands. The ESR observations have been made on frozen solutions of Fe (TPP) (OMe-LOMe) _2^-in CH_2Cl_2-MeOL,where TPP is tetraphenylporphyrin dianion and L=H or D.The principal g avlues (g_X=1.9134, g_Y=2.1654, g_Z=2.4949 for L=H and g_X=1.9146, g_Y=2.1643, g_Z=2.4917 for L=D) are analyzed based on the low-symmetry crystal field model to show that both the d orbital energy splittings and the energies to the excited Kramers doublets are increased from the complex with L=H to the one with L=D by 10-30cm^<-1>. On the other hand, a quantum mechanical calculation using double-minimum potentials for O-L-o vibration has shown that the d orbital energy splittings as well as the excitation energies for L=D become greater in comparison with those for L=H in the case of a weaker L-bond in the electronic ground state than in the excited state, and vice versa. It is thus demonstrated that the L-bonding in Fe (TPP) (OMe-LOMe) _2^- is stronger in the excited states than in the ground state. Qualitatively, this finding is explained in terms of an increased electron density at the iron-bound oxygen atoms on electronic excitation. Such a change in the electron density is consistent with the prediction from the electronic structure generally accepted for low-spin Fe (III) heme complexes.