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Accurate modeling of the X-ray absorption near-edge spectra (XANES) is required to unravel the local structure of metal sites in complex systems and their structural changes upon chemical or light stimuli. Two relevant examples are reported here concerning the following: (i) the effect of molecular adsorption on 3d metals hosted inside metal-organic frameworks and (ii) light induced dynamics of spin crossover in metal-organic complexes. In both cases, the amount of structural models for simulation can reach a hundred, depending on the number of structural parameters. Thus, the choice of an accurate but computationally demanding finite difference method for the ab initio X-ray absorption simulations severely restricts the range of molecular systems that can be analyzed by personal computers. Employing the FDMNES code [Phys. Rev. B, 2001, 63, 125120] we show that this problem can be handled if a proper diagonalization scheme is applied. Due to the use of dedicated solvers for sparse matrices, the calculation time was reduced by more than 1 order of magnitude compared to the standard Gaussian method, while the amount of required RAM was halved. Ni K-edge XANES simulations performed by the accelerated version of the code allowed analyzing the coordination geometry of CO and NO on the Ni active sites in CPO-27-Ni MOF. The Ni-CO configuration was found to be linear, while Ni-NO was bent by almost 90°. Modeling of the Fe K-edge XANES of photoexcited aqueous [Fe(bpy)3](2+) with a 100 ps delay we identified the Fe-N distance elongation and bipyridine rotation upon transition from the initial low-spin to the final high-spin state. Subsequently, the X-ray absorption spectrum for the intermediate triplet state with expected 100 fs lifetime was theoretically predicted.

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Liquid water plays a significant role, especially as a solvent in the fields of biology and biochemistry. Its strong impact on protein function and activity in the direct surrounding of proteins led to the coining of the term biological water. [1] Investigations of the hydration of polar and nonpolar protein sites, or of systems which can mimic this interaction, such as mixtures of water with solvents of different polarity, are of great interest. However, even for pure liquid water the complex nature of the hydrogen-bond (HB) network is still in the course of clarification. One open question in this context is whether in the dynamic bond-building and -breaking equilibrium of liquid water a continuum of almost tetrahedral bond configurations [2] or rather a number of distinct preferential species of broken and unbroken HB configurations exists.[3] X-ray absorption (XA) and X-ray emission (XE) spectroscopy, which reveal information about the local electronic structure, have been used to pursue this question. [3b, 4] In XA spectroscopy a core electron is excited to the unoccupied states of a molecule, and thus probes the corresponding local electronic structure. The oxygen K-edge XA spectrum of liquid water shows three main features: a pre-edge at 535 eV, a main edge around 537 eV and a postedge around 540 eV. Isotope-, temperature-, and phasedependent measurements on water in combination with theoretical simulations led to the proposal that these features are correlated to distinct HB conformations: the post-edge to tetrahedrally bonded water molecules and the pre-and main edges to conformations with broken HBs. [3,5] This interpretation indicated that a significant number of water molecules with only one strong donating and one strong accepting HB is present in the liquid phase, challenging the traditional tetrahedral model. This new interpretation has been questioned, and based on theoretical modeling it has been argued that XA spectra are rather insensitive to the HB network.[6] X-ray emission spectroscopy probes the occupied electronic states on detecting the energy distribution of the radiative decay of the core-hole state. The recent development of high-resolution XE spectrometers for liquid samples drew particular attention to the observation of the splitting of the sharpest peak in the spectrum associated with the lonepair orbital of the free water molecule.[4a,f,g] Tokushima et al. interpreted the double feature as further proof for the existence of two different structures, the tetrahedral and strongly distorted H-bonded species.[4g] Fuchs et al. assign the two distinct peaks to emission from species before and after core-hole-induced ultrafast dissociation.[4a] Experimental approaches to clarify the origin of the peak splitting included temperature-, isotope-, and state-of-aggregation-dependent measurements, as well as the study of the proposed dissociated species.To shed further light on this issue we present XA and XE spectra of the water molecule in a chemical environment in which the HB configura...

Functionalization of metal-organic frameworks with metal nanoparticles (NPs) is a promising way for producing advanced materials for catalytic applications. We present the synthesis and in situ characterization of palladium NPs encapsulated inside a functionalized UiO-67 metal-organic framework. The initial structure was synthesized with 10% of PdCl2bpydc moieties with grafted Pd ions replacing standard 4,4'-biphenyldicarboxylate linkers. This material exhibits the same high crystallinity and thermal stability of standard UiO-67. Formation of palladium NPs was initiated by sample activation in hydrogen and monitored by in situ X-ray powder diffraction and X-ray absorption spectroscopy (XAS). The reduction of PdII ions to Pd0 occurs above 200 °C in 6% H2/He flow. The formed palladium NPs have an average size of 2.1 nm as limited by the cavities of UiO-67 structure. The resulting material showed high activity towards ethylene hydrogenation. Under reaction conditions, palladium was found to form a carbide structure indicated by operando XAS, while formation of ethane was monitored by mass spectroscopy and infra-red spectroscopy.