Nuclear Resonance Vibrational Spectroscopy Applied to [Fe(OEP)(NO)]: The Vibrational Assignments of Five-Coordinate Ferrous Heme−Nitrosyls and Implications for Electronic Structure

Abstract

This study presents Nuclear Resonance Vibrational Spectroscopy (NRVS) data on the fivecoordinate (5C) ferrous heme nitrosyl complex [Fe(OEP)(NO)] (1, OEP 2− = octaethylporphyrinato dianion) and the corresponding 15 N 18 O labeled complex. The obtained spectra identify two isotope sensitive features at 522 and 388 cm −1 , which shift to 508 and 381 cm −1 , respectively, upon isotope labeling. These features are assigned to the Fe-NO stretch ν(Fe-NO) and the in-plane Fe-N-O bending mode δ ip (Fe-N-O), the latter has been unambiguously assigned for the first time for 1. The obtained NRVS data were simulated using our quantum chemistry centered normal coordinate analysis (QCC-NCA). Since complex 1 can potentially exist in 12 different conformations involving the FeNO and peripheral ethyl orientations, extended DFT calculations and QCC-NCA simulations were performed to determine how these conformations affect the NRVS properties of [Fe(OEP)NO]. These results show that the properties and force constants of the FeNO unit are hardly affected by the conformational changes involving the ethyl substituents. On the other hand, the NRVS-active porphyrin-based vibrations around 340 -360, 300 -320, and 250 -270 cm −1 are sensitive to the conformational changes. The spectroscopic changes observed in these regions are due to selective mechanical couplings of one component of E u -type (in ideal D 4h symmetry) porphyrin-based vibrations with the in-plane Fe-N-O bending mode. This leads to the observed variations in Fe(OEP) core mode energies and NRVS intensities without affecting the properties of the FeNO unit. The QCC-NCA simulated NRVS spectra of 1 show excellent agreement with experiment, and indicate that conformer F is likely present in the samples of this complex investigated here. The observed porphyrin-based vibrations in the NRVS spectra of 1 are also assigned based on the QCC-NCA results. The obtained force constants of the Fe-NO and N-O bonds are 2.83 -2.94 (based on the DFT functional applied) and about 12.15 mdyn/Å, respectively. The electronic structures of 5C ferrous heme nitrosyls in different model complexes are then analyzed, and variations in their properties based on different porphyrin substituents are explained. Finally, the shortcomings of different DFT functionals in describing the axial FeNO subunit in heme nitrosyls are elucidated.

Quantum-chemical study of the Fe(NO)2 fragment in the cation of mononuclear nitrosyl iron complex [Fe(SC(NH2)2)2(NO)2]Сl·H2O

“…Thus, regardless of the common opinion [42], hybrid functionals appear to be the best for description of complex I geometry. It should be noted that such results have been recently observed for other mononuclear nitrosyl iron complexes [43].…”

Nuclear Resonance Vibrational Spectra of Five-Coordinate Imidazole-Ligated Iron(II) Porphyrinates

“…DFT calculations for [Fe(OEP)(NO)] suggest that peripheral group conformations can have significant effects on the lower frequency regions as well. 78 …”

Definition of the intermediates and mechanism of the anticancer drug bleomycin using nuclear resonance vibrational spectroscopy and related methods

“…This study found the heterolytic cleavage pathway for the ðBLMÞFe III ─ H 2 O 2 complex to be thermoneutral and therefore feasible. Because ABLM is inaccessible to vibrational characterization by resonance Raman (rR) spectroscopy due to photodecay (16), we employed nuclear resonance vibrational spectroscopy (NRVS) (17)(18)(19)(20) to obtain vibrational information and define the axial ligands of Fe III BLM and ABLM, thus distinguishing between possible reaction mechanisms.…”