Relation between Calculated Fermi Contact Spin Densities and Experimental Contact Shifts of Paramagnetic Ironporphyrins. A Novel Approach to the Electronic Structure of Intermediate-Spin Iron(II) Porphyrin(2001)

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Over the last few years, it has been found that, regarding the calculation of NMR chemical shifts, relatively simple and computationally inexpensive DFT approaches often give surprisingly accurate and stable results. To our knowledge, all previous works restrict their discussion to diamagnetic closed-shell systems except for one that is specific to paramagnetic iron-sulfur proteins.1 We will present a modern implementation for the calculation of the Fermi contact spin densities using DFT. The 1H and 13C nuclei correlate very well with contact shifts from experimental results for iron(III) porphyrin complexes with different spin states(high-spin Fe(TPP) Cl and low-spin [Fe(TPP) (Im) 2]+) . Four-coordinate iron(II) porphyrin complexes, with an intermediate-spin state, have been studied extensively by paramagnetic NMR spectroscopy. 2,3 Analyses of the contact shift confirmed that the nature of PFe p spin transfer mechanism is consistent with both 3A2g and 3Eg ground state electronic structures. Calculation of the Fermi contact spin densities revealed that a- and b-C are of opposite sign for these two states, and that the assignment of these two peaks in the corresponding 13C NMR spectra should be critical to the identification of the ground state.