Electronic Structure of Hemin in Solution Studied by Resonant X-ray Emission Spectroscopy and Electronic Structure Calculations

Abstract

Resonant inelastic X-ray scattering spectra at the iron L-edge from hemin in dimethyl sulfoxide liquid solution are reported. Our experiments, which are interpreted with the help of electronic structure calculations, support earlier assignments of hemin-solvent interactions, including the iron spin state and the role of the chloride ligand obtained from a total fluorescence yield study. The analysis of the explicit radiative relaxation channels of 2p core-level excited iron, explored in the present work, allows for a rather quantitative assignment of the orbitals involved in the excitation-deexcitation process of the core-excited hemin in solution. We specifically distinguish between contributions of partially and fully occupied valence orbitals to the broad X-ray emission band. In addition, our calculations reveal a detailed picture of the character of these orbitals.

Intermolecular bonding of hemin in solution and in solid state probed by N K-edge X-ray spectroscopies

“…[6] Molecular geometry optimization was performed using the B3LYP DFT method together with the def2-TZVP(-f) basis set. [16][17][18] The N contributions to both occupied and unoccupied valence MO are acquired from Löwdin population analysis based on the DFT calculations.…”

“…For example, iron protoporphyrin IX chloride (FePPIX-Cl, or hemin, Figure 1a) forms monomer species when dissolved in dimethyl sulfoxide (DMSO), while dimer structures are present in aqueous solution. [6][7][8][9] It is known that the electronic structure of hemin, especially the local electronic structure at the Fe center, which often serves as the major functional site, is altered due to dimerization. [7] Hemin oligomer species in various solvents have therefore been investigated previously by UV-Vis spectroscopy and X-ray absorption/emission spectroscopy (XAS/XES) at the Fe L-edge, [6][7][8][9] although the nature of the probed electronic transitions implied that the hemin intermolecular bonding interactions were probed rather indirectly.…”

“…These solutions contain hemin monomer and dimer species, respectively. [6,7] The high pH value (from NaOH) of the aqueous solution increases hemin solubility in water, but adds potential additional complexity due to formation of the carboxylate anionic form in basic solution, as shown in Figure 1c. However, because the carboxylate groups are not conjugated with the system of the porphyrin ring their influence on the N K-edge is expected to be minor, in contrast to the recently presented N K-edge spectra of para-aminobenzoic acid in solution, where the amine nitrogen felt the direct influence of the carboxylate group as both were part of the delocalised system of the aromatic ring.…”

Towards an ab initio theory for metal L-edge soft X-ray spectroscopy of molecular aggregates

“…Since even within the L 3 -band the lifetimes may vary, 30 the Γ n parameters in Eq. (7) for both monomer and dimer have been assumed to be 0.09 eV for states below 709.2 eV, 0.26 eV between 709.2 and 711.6 eV, 0.43 eV between 711.6 and 719.2 eV, and 0.61 eV above 719.2 eV as a best fit to the experimental data 18,19 . The hemin monomer has a sextet ground state, where the degeneracy of the six components is slightly lifted due to SOC and thus the states are split into three Kramers doublets.…”

“…Staying monomeric in polar solvents like ethanol or dimethyl sulfoxide (DMSO), it forms dimers in water solution 17 . The effect of aggregation was recently addressed by means of soft X-ray Fe L-edge absorption spectroscopy in transmission (XAS) and partial fluorescence (PFY) modes as well as by off-resonant X-ray emission (XES) and RIXS on the examples of DMSO and aqueous solutions 18,19 . The general shape of the spectra for both cases was quite similar, and the pronounced difference in broadenings for RIXS as well as a 1.3 eV energy shift in off-resonant XES was taken as an indication of aggregation.…”