Determination of the Complete Set of Iron Normal Modes in the Heme Model Compound FeIII(OEP)Cl from Nuclear Resonance Vibrational Spectroscopic Data

The vibrational spectrum of 57 Fe in chloro iron octaethylporphyrin, Fe(OEP)Cl, has been calculated by normal-mode analysis refined to absorption data from nuclear resonance vibrational spectroscopy. This technique directly measures the amplitudes and frequencies for all modes that have significant iron participation, providing rigorous constraints to the best-fit values for the force constants. The calculated normal modes reveal the importance of Fe displacements perpendicular to the heme plane for both the lowest frequency modes and the ligand modes. The actual normal modes of Fe(OEP)Cl are not well described by single modes of the core porphyrin; instead they are hybrids of multiple core modes and ethyl and chlorine displacements.