Modeling the heme vibrational spectrum: Normal-mode analysis of nickel (II) etioporphyrin-I from resonance raman, ft-raman, and infrared spectra of multiple isotopomers

Abstract

Nearly complete vibrational assignments have been obtained for a heme model, nickel etioporphyrin-I (NiEPI), using variable-wavelength resonance Raman (RR), and FT-Raman (FT-R), as well as infrared (IR) spectroscopy, on a series of isotopomers labeled at positions in the skeleton (¹⁵N, β-¹³C, meso-d₄, ¹⁵N-meso-d₄) and in the peripheral substituents (methyl-d₁₂, ethyl-d₈, and ethyl-d₁₂). The vibrational bands are assigned to the porphyrin skeletal and substituent modes on the basis of the mode description scheme developed for nickel octaethylporphyrin (NiOEP) with the aid of a normal-mode analysis of NiEPI, explicitly including the peripheral substituents, i.e., the methyl and ethyl groups. The previously reported NiOEP force field was refined to account for the observed isotope shifts of NiEPI isotopomers. An important result is the requirement of relatively large, long-range force constants for methine bridge bonds on opposite sides of the porphyrin ring. These 1–8 and 1–9 interaction force constants are required to reproduce the frequencies and isotope shifts of six C_α-C_m stretching modes and especially to predict the relative order of the two highest-frequency E_u modes, v(C_α-C_m) (v₃₈, ∼ 1570 cm⁻¹) and v(C_β-C_β) (v₃₇, ∼ 1600 cm⁻¹). Most of the substituent (methyl and ethyl) vibrations are located in the RR and IR spectra. Strong RR enhancement of some substituent modes can be attributed to hyperconjugative interaction of the aliphatic groups with the porphyrin a_1u orbital, as well as vibrational mixing of substituent modes with the nearby skeletal modes. © 1995 John Wiley & Sons, Inc.