Edlira Suljoti

Nature of the Chemical Bond of Aqueous Fe2+ Probed by Soft X-ray Spectroscopies and ab Initio Calculations

Aqueous iron(II) chloride is studied by soft X-ray absorption, emission, and resonant inelastic Raman scattering techniques on the Fe L-edge and O K-edge using the liquid-jet technique. Soft X-ray spectroscopies allow in situ and atom-specific probing of the electronic structure of the aqueous complex and thus open the door for the investigation of chemical bonding and molecular orbital mixing. In this work, we combine theoretical ab initio restricted active space self-consistent field and local atomic multiplet calculations with experimental soft X-ray spectroscopic methods for a description of the local electronic structure of the aqueous ferrous ion complex. We demonstrate that the atomic iron valence final states dominate the resonant inelastic X-ray scattering spectra of the complex over the ligand-to-metal charge transfer transitions, which indicates a weak interaction of Fe(2+) ion with surrounding water molecules. Moreover, the oxygen K-edge also shows only minor changes due to the presence of Fe(2+) implying a small influence on the hydrogen-bond network of water.

Chemical bonding in aqueous ferrocyanide: experimental and theoretical X-ray spectroscopic study.

Resonant inelastic X-ray scattering (RIXS) and X-ray absorption (XA) experiments at the iron L- and nitrogen K-edge are combined with high-level first-principles restricted active space self-consistent field (RASSCF) calculations for a systematic investigation of the nature of the chemical bond in potassium ferrocyanide in aqueous solution. The atom- and site-specific RIXS excitations allow for direct observation of ligand-to-metal (Fe L-edge) and metal-to-ligand (N K-edge) charge-transfer bands and thereby evidence for strong σ-donation and π-backdonation. The effects are identified by comparing experimental and simulated spectra related to both the unoccupied and occupied molecular orbitals in solution.

Electronic structural insights into efficient MnOx catalysts

Soft X-ray absorption and resonant inelastic X-ray scattering at the Mn L-edge are established as tools for gaining electronic structural insights into water oxidation catalysis. The MnOx catalyst with the lowest d–d transitions, strongest charge transfer and a higher proportion of Mn3+ over Mn2+/4+ produces itinerant electrons that contribute to a higher catalytic activity.

DMSO-Water Clustering in Solution Observed in Soft X-ray Spectra.

The significant deviation from the ideality of dimethyl sulfoxide (DMSO)/water mixtures can be addressed based on the change of the local molecular orbitals of each solvent upon mixing. Oxygen K-edge absorption and emission spectra of DMSO/water solutions were measured using the liquid microjet technique. The spectra demonstrate that the hydrogen bond network in liquid water is already influenced at small DMSO concentrations, and at the molar fraction xDMSO = 0.43 we find strong evidence of DMSO-water clustering reflected by the influence on the occupied molecular orbitals.

Probing ion-specific effects on aqueous acetate solutions: Ion pairing versus water structure modifications

The effect of monovalent cations (Li+, K+, NH4+, Na+) on the water structure in aqueous chloride and acetate solutions was characterized by oxygen K-edge X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy, and resonant inelastic X-ray scattering (RIXS) of a liquid microjet. We show ion- and counterion dependent effects on the emission spectra of the oxygen K-edge, which we attribute to modifications of the hydrogen bond network of water. For acetates, ion pairing with carboxylates was also probed selectively by XAS and RIXS. We correlate our experimental results to speciation data and to the salting-out properties of the cations.

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

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.

Probing orbital symmetry in solution: polarization-dependent resonant inelastic soft x-ray scattering on liquid micro-jet

Polarization-dependent resonant inelastic x-ray scattering is demonstrated here for liquid acetonitrile, acetone and dimethyl sulfoxide, using the liquid micro-jet technique. Selective excitation to an unoccupied orbital with a specific symmetry at the K-edge x-ray absorption of liquid samples determines the polarization-dependent emission of the occupied states. Considering the well-defined unoccupied molecular orbital configuration and utilizing the results of ab initio molecular orbital calculations, the polarization-dependent anisotropy in resonant inelastic soft x-ray scattering is discussed in a membrane-free configuration.

Measurement of the predicted asymmetric closing behaviour of the band gap of silicon using x-ray absorption and emission spectroscopy

We experimentally observe the theoretically anticipated but so far unverified asymmetric band gap closing for the prototypical semiconductor silicon from room temperature up to the melting point. The shift of the valence band maximum contributes more than 60% to the band gap closing in comparison to the shift of the conduction band minimum. Since we determine the temperature-dependent band edge positions with x-ray absorption and x-ray emission spectroscopy, our analysis also includes electron–phonon coupling features and processes inherent to core-level spectroscopies, like the multi-electron screening response.

Chemical Speciation and Bond Lengths of Organic Solutes by Core-Level Spectroscopy: pH and Solvent Influence on p-Aminobenzoic Acid

Through X-ray absorption and emission spectroscopies, the chemical, electronic and structural properties of organic species in solution can be observed. Near-edge X-ray absorption fine structure (NEXAFS) and resonant inelastic X-ray scattering (RIXS) measurements at the nitrogen K-edge of para-aminobenzoic acid reveal both pH- and solvent-dependent variations in the ionisation potential (IP), 1s→π* resonances and HOMO–LUMO gap. These changes unequivocally identify the chemical species (neutral, cationic or anionic) present in solution. It is shown how this incisive chemical state sensitivity is further enhanced by the possibility of quantitative bond length determination, based on the analysis of chemical shifts in IPs and σ* shape resonances in the NEXAFS spectra. This provides experimental access to detecting even minor variations in the molecular structure of solutes in solution, thereby providing an avenue to examining computational predictions of solute properties and solute–solvent interactions.

Local electronic structure of aqueous zinc acetate: oxygen K-edge X-ray absorption and emission spectroscopy on micro-jets

Oxygen K-edge X-ray absorption, emission, and resonant inelastic X-ray scattering spectra were measured to site selectively gain insights into the electronic structure of aqueous zinc acetate solution. The character of the acetate ion and the influence of zinc and water on its local electronic structure are discussed.