10.1021/ic902181e.s001
Nicolai Lehnert
Nicolai
Lehnert
Mary Grace I. Galinato
Mary
Grace I. Galinato
Florian Paulat
Florian
Paulat
George B. Richter-Addo
George B.
Richter-Addo
Wolfgang Sturhahn
Wolfgang
Sturhahn
Nan Xu
Nan
Xu
Jiyong Zhao
Jiyong
Zhao
Nuclear Resonance Vibrational Spectroscopy Applied to [Fe(OEP)(NO)]: The Vibrational Assignments of Five-Coordinate Ferrous Heme−Nitrosyls and Implications for Electronic Structure
American Chemical Society
2010
NRVS spectra
FeNO unit
force constants
Nuclear Resonance Vibrational Spectroscopy Applied
Nuclear Resonance Vibrational Spectroscopy
Electronic StructureThis study
DFT
15 N 18O
OEP
D 4h symmetry
2010-05-03 00:00:00
Journal contribution
https://acs.figshare.com/articles/journal_contribution/Nuclear_Resonance_Vibrational_Spectroscopy_Applied_to_Fe_OEP_NO_The_Vibrational_Assignments_of_Five_Coordinate_Ferrous_Heme_Nitrosyls_and_Implications_for_Electronic_Structure/2773006
This study presents Nuclear Resonance Vibrational Spectroscopy (NRVS) data on the five-coordinate (5C) ferrous heme−nitrosyl complex [Fe(OEP)(NO)] (<b>1</b>, OEP<sup>2−</sup> = octaethylporphyrinato dianion) and the corresponding <sup>15</sup>N<sup>18</sup>O labeled complex. The obtained spectra identify two isotope sensitive features at 522 and 388 cm<sup>−1</sup>, which shift to 508 and 381 cm<sup>−1</sup>, 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<sub>ip</sub>(Fe−N−O), the latter has been unambiguously assigned for the first time for <b>1</b>. The obtained NRVS data were simulated using our quantum chemistry centered normal coordinate analysis (QCC-NCA). Since complex <b>1</b> can potentially exist in 12 different conformations involving the FeNO and peripheral ethyl orientations, extended density functional theory (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<sup>−1</sup> are sensitive to the conformational changes. The spectroscopic changes observed in these regions are due to selective mechanical couplings of one component of E<sub>u</sub>-type (in ideal <i>D</i><sub>4<i>h</i></sub> 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 <b>1</b> 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 <b>1</b> 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.