Metal-ligand covalency of iron complexes from high-resolution resonant inelastic X-ray scattering

Feasibility of Valence-to-Core X-ray Emission Spectroscopy for Tracking Transient Species

X-ray spectroscopies, when combined in laser-pump, X-ray-probe measurement schemes, can be powerful tools for tracking the electronic and geometric structural changes that occur during the course of a photoinitiated chemical reaction. X-ray absorption spectroscopy (XAS) is considered an established technique for such measurements, and X-ray emission spectroscopy (XES) of the strongest core-to-core emission lines (Kα and Kβ) is now being utilized. Flux demanding valence-to-core XES promises to be an important addition to the time-resolved spectroscopic toolkit. In this paper we present measurements and density functional theory calculations on laser-excited, solution-phase ferrocyanide that demonstrate the feasibility of valence-to-core XES for time-resolved experiments. We discuss technical improvements that will make valence-to-core XES a practical pump-probe technique.

Characterizing the Intermediates Compound I and II in the Cytochrome P450 Catalytic Cycle with Nonlinear X-ray Spectroscopy: A Simulation Study

Cytochrome P450 enzymes are an important family of biocatalysts that oxidize chemically inert CH bonds. There are many unresolved questions regarding the catalytic reaction intermediates, in particular P450 Compound I (Cpd-I) and II (Cpd-II). By using simple molecular models, we simulate various X-ray spectroscopy signals, including X-ray absorption near-edge structure (XANES), resonant inelastic X-ray scattering (RIXS), and stimulated X-ray Raman spectroscopy (SXRS) of the low- and high-spin states of Cpd-I and II. Characteristic peak patterns are presented and connected to the corresponding electronic structures. These X-ray spectroscopy techniques are complementary to more conventional infrared and optical spectroscopy and they help to elucidate the evolving electronic structures of transient species along the reaction path.

Photon-in photon-out hard X-ray spectroscopy at the Linac Coherent Light Source

X-ray free-electron lasers (FELs) have opened unprecedented possibilities to study the structure and dynamics of matter at an atomic level and ultra-fast timescale. Many of the techniques routinely used at storage ring facilities are being adapted for experiments conducted at FELs. In order to take full advantage of these new sources several challenges have to be overcome. They are related to the very different source characteristics and its resulting impact on sample delivery, X-ray optics, X-ray detection and data acquisition. Here it is described how photon-in photon-out hard X-ray spectroscopy techniques can be applied to study the electronic structure and its dynamics of transition metal systems with ultra-bright and ultra-short FEL X-ray pulses. In particular, some of the experimental details that are different compared with synchrotron-based setups are discussed and illustrated by recent measurements performed at the Linac Coherent Light Source.

Dynamic structure elucidation of chemical reactivity by laser pulses and X-ray probes

Visualising chemical reactions by X-ray methods is a tantalising prospect. New light sources provide the prospect for studying atomic, electronic and energy transfers accompanying chemical change by X-ray spectroscopy and inelastic scattering. Here we assess how this adventure can illuminate inorganic and catalytic chemistry. In particular X-ray inelastic scattering provides a means of exploiting X-ray free electron lasers, as a parallel to laser Raman spectroscopy.

Resonant inelastic X-ray scattering on ferrous and ferric bis-imidazole porphyrin and cytochrome c: nature and role of the axial methionine-Fe bond

Axial Cu-S(Met) bonds in electron transfer (ET) active sites are generally found to lower their reduction potentials. An axial S(Met) bond is also present in cytochrome c (cyt c) and is generally thought to increase the reduction potential. The highly covalent nature of the porphyrin environment in heme proteins precludes using many spectroscopic approaches to directly study the Fe site to experimentally quantify this bond. Alternatively, L-edge X-ray absorption spectroscopy (XAS) enables one to directly focus on the 3d-orbitals in a highly covalent environment and has previously been successfully applied to porphyrin model complexes. However, this technique cannot be extended to metalloproteins in solution. Here, we use metal K-edge XAS to obtain L-edge like data through 1s2p resonance inelastic X-ray scattering (RIXS). It has been applied here to a bis-imidazole porphyrin model complex and cyt c. The RIXS data on the model complex are directly correlated to L-edge XAS data to develop the complementary nature of these two spectroscopic methods. Comparison between the bis-imidazole model complex and cyt c in ferrous and ferric oxidation states show quantitative differences that reflect differences in axial ligand covalency. The data reveal an increased covalency for the S(Met) relative to N(His) axial ligand and a higher degree of covalency for the ferric states relative to the ferrous states. These results are reproduced by DFT calculations, which are used to evaluate the thermodynamics of the Fe-S(Met) bond and its dependence on redox state. These results provide insight into a number of previous chemical and physical results on cyt c.

Recent advances on ultrafast X-ray spectroscopy in the chemical sciences

Molecular snapshots obtained by ultrafast X-ray spectroscopy reveal new insight into fundamental reaction mechanisms at single electron and atomic levels.