Abstract:The valence excited states of ferric and ferrous hexacyanide ions in aqueous solution were mapped with resonant inelastic X-ray scattering (RIXS) at the Fe L2,3-and N K-edges. Probing of both the central Fe and the ligand N atoms enabled identification of the metal-and ligand-centered excited states, as well as ligandto-metal and metal-to-ligand charge transfer excited states. Ab initio calculations utilizing the RASPT2 method was used to simulate the Fe L2,3-edge RIXS spectra and enabled quantification of the covalency of both occupied and empty orbitals of π and σ symmetry. We find that π back-donation in the ferric complex is smaller compared to the ferrous complex. This is evidenced by the relative amount of Fe 3d character in the nominally 2π CN -molecular orbital of 7% and 9% in ferric and ferrous hexacyanide, respectively. Utilizing the direct sensitivity of Fe L3-edge RIXS to the Fe 3d character in the occupied molecular orbitals we also find that the donation interactions are dominated by σ-bonding. The latter is found to be stronger in the ferric complex with a Fe 3d contribution to the nominally 5σ CN -molecular orbitals of 29% compared to 20% in the ferrous complex. These results are consistent with the notion that a higher charge at the central metal atom increases donation and decreases back-donation.

PostprintThis is the accepted version of a paper published in Journal of Chemical Physics. This paper has been peer-reviewed but does not include the final publisher proof-corrections or journal pagination.Citation for the original published paper (version of record):Pinjari, R V., Delcey, M G., Guo, M., Odelius, M., Lundberg, M. (2014) Restricted active space calculations of L-edge X-ray absorption spectra: From molecular orbitals to multiplet states. The metal L-edge (2p → 3d) X-ray absorption spectra are affected by a number of different interactions; electron-electron repulsion, spin-orbit coupling, and charge transfer between metal and ligands, which makes the simulation of spectra challenging. The core restricted active space (RAS) method is an accurate and flexible approach that can be used to calculate X-ray spectra of a wide range of medium-sized systems without any symmetry constraints. Here, the applicability of the method is tested in detail by simulating three ferric (3d 5 ) model systems with well-known electronic structure viz. atomic Fe 3+ , high-spin [FeCl 6 ] 3-with ligand donor bonding, and low-spin [Fe(CN) 6 ] 3-that also has metal backbonding. For these systems, the performance of the core RAS method, which does not require any system-dependent parameters, is comparable to that of the commonly used semi-empirical charge-transfer multiplet model. It handles orbitally degenerate ground states, accurately describes metal-ligand interactions, and includes both single and multiple excitations. The results are sensitive to the choice of orbitals in the active space and this sensitivity can be used to assign spectral features. A method has also been developed to analyze the calculated X-ray spectra using a chemically intuitive molecular orbital picture. Journal of Chemical

PostprintThis is the accepted version of a paper published in Journal of Computational Chemistry. This paper has been peer-reviewed but does not include the final publisher proof-corrections or journal pagination. Citation for the original published paper (version of record):Pinjari, R V., Delcey, M G., Guo, M., Odelius, M., Lundberg, M. (2016) Cost and sensitivity of restricted active-space calculations of metal L-edge X-ray absorption spectra. Abstract The restricted active space (RAS) approach can accurately simulate metal L-edge X-ray absorption spectra of first-row transition metal complexes without the use of any fitting parameters. These characteristics provide a unique capability to identify unknown chemical species and to analyze their electronic structure. To find the best balance between cost and accuracy, the sensitivity of the simulated spectra with respect to the method variables have been tested for two models, [FeCl 6 ] 3-and [Fe(CN) 6 ] 3-. For these systems the reference calculations give deviations, compared to experiment, of 1 eV in peak positions, 30% for the relative intensity of major peaks, and 50% for minor peaks. Compared to these deviations, the simulated spectra are sensitive to the number of final states, the inclusion of dynamical correlation and the ionization potential-electron affinity (IPEA) shift, in addition to the selection of the active space. The spectra are less sensitive to the quality of the basis set and even a double-ζ basis gives reasonable results. Inclusion of dynamical correlation through second-order perturbation theory can be done efficiently using the state-specific formalism without correlating the core orbitals. Although these observations are not directly transferable to other systems, they can, together with a cost analysis, aid in the design of RAS models and help extend the use of this powerful approach to a wider range of transitionmetal systems. Journal of Computational Keywords:transition metals, X-ray absorption spectroscopy, multiconfigurational wavefunction, spin-orbit coupling, charge transfer TABLE OF CONTENTSWith an appropriate choice of active space, basis set and computational procedure the restricted active space approach can be used to simulate metal L-edge X-ray absorption spectra with reasonable accuracy and computational cost. The sensitivity of the simulated results with respect to geometrical changes opens up for analysis of dynamical processes.2

Simulations of iron K pre-edge X-ray absorption spectra using the core restricted active space method.Physical Chemistry, http://dx.doi.org/10.1039/c5cp07487hAccess to the published version may require subscription. N.B. When citing this work, cite the original published paper. Permanent link to this version:http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-243571Simulations of iron K pre-edge X-ray absorption spectra using the restricted active space method † Meiyuan Guo, a Lasse Kragh Sørensen, a Mickaël G. Delcey, a,b Rahul V. Pinjari, a,c and Marcus Lundberg * aThe intensities and relative energies of metal K pre-edge features are sensitive to both geometric and electronic structure. With the possibility to collect high-resolution spectral data it important to find theoretical methods that include all important spectral effects: ligand-field splitting, multiplet structures, 3d-4p orbital hybridization, and charge-transfer excitations. Here the restricted active space (RAS) method is used for the first time to calculate metal K pre-edge spectra of open-shell systems, and its performance is tested against six iron complexes: [FeCl 6 ] n-, [FeCl 4 ] n-, and [Fe(CN) 6 ] n-in ferrous and ferric oxidation states. The method gives good descriptions of the spectral shapes for all six systems. The mean absolute deviation for the relative energies of different peaks is only 0.1 eV. For the two systems that lack centrosymmetry [FeCl 4 ] 2-/1-, the ratios between dipole and quadrupole intensity contributions are reproduced with an error of 10%, which leads to good descriptions of the integrated pre-edge intensities. To gain further chemical insight, the origins of the pre-edge features have been analyzed with a chemically intuitive molecular orbital picture that serves as a bridge between the spectra and the electronic structures. The RAS method can thus be used to predict and rationalize the effects of changes in both oxidation state and ligand environment in a number of hard X-ray studies of small and medium-sized molecular systems.

Electronic structure and molecular electrostatic potential (MESP) in ferrocene (FC), cucurbit[n]urils (CB[n]) with n = 5-8, and their host-guest complexes are obtained within the framework of density functional theory. MESP topography that is employed to gauge the dimensions of the CB[n] cavity estimates that the cavity height increases from 7.25 to 7.70 A along CB[n] homologue series, whereas the diameter of the CB[8] (8.57 A) cavity is larger than twice that of CB[5] (3.91 A). MESP investigations reveal deeper minima near ureido oxygens in CB[5] along with large electron-rich regions at its portal. A lateral interaction of the guest FC with hydrophilic exterior of the CB[n] portal and its encapsulation within hydrophobic cavity of the host are analyzed. The present calculations suggest that CB[5] does not yield stable complexes in either case. FC interacts laterally with CB[6], and inclusion of the guest occurs, both parallel as well as perpendicular to the CB[n] axis, in the cavity of higher homologue. Self-consistent reaction field studies indicate that, in the presence of water as a solvent, encapsulation of FC in parallel fashion is favored within CB[7] and CB[8] cavities. NMR chemical shifts (delta(H)) of CB[n] protons remain practically unchanged with an increase in the cavity size; however, they are influenced significantly by water. The spectra thus obtained in aqueous solution agree with those observed experimentally. The delta(H) values in FC-CB[n] complexes indicate deshielding of FC protons directed toward portals, while those pointing toward nitrogens exhibit up-shifts in the spectra.