Anomalously enhanced ion transport and uptake in functionalized angstrom-scale two-dimensional channels.

Emulating angstrom-scale dynamics of the highly selective biological ion channels is a challenging task. Recent work on angstrom-scale artificial channels has expanded our understanding of ion transport and uptake mechanisms under confinement. However, the role of chemical environment in such channels is still not well understood. Here, we report the anomalously enhanced transport and uptake of ions under confined MoS2-based channels that are ~five angstroms in size. The ion uptake preference in the MoS2-based channels can be changed by the selection of surface functional groups and ion uptake sequence due to the interplay between kinetic and thermodynamic factors that depend on whether the ions are mixed or not prior to uptake. Our work offers a holistic picture of ion transport in 2D confinement and highlights ion interplay in this regime.

Advanced Materials for Energy-Water Systems: The Central Role of Water/Solid Interfaces in Adsorption, Reactivity, and Transport.

The structure, chemistry, and charge of interfaces between materials and aqueous fluids play a central role in determining properties and performance of numerous water systems. Sensors, membranes, sorbents, and heterogeneous catalysts almost uniformly rely on specific interactions between their surfaces and components dissolved or suspended in the water-and often the water molecules themselves-to detect and mitigate contaminants. Deleterious processes in these systems such as fouling, scaling (inorganic deposits), and corrosion are also governed by interfacial phenomena. Despite the importance of these interfaces, much remains to be learned about their multiscale interactions. Developing a deeper understanding of the molecular- and mesoscale phenomena at water/solid interfaces will be essential to driving innovation to address grand challenges in supplying sufficient fit-for-purpose water in the future. In this Review, we examine the current state of knowledge surrounding adsorption, reactivity, and transport in several key classes of water/solid interfaces, drawing on a synergistic combination of theory, simulation, and experiments, and provide an outlook for prioritizing strategic research directions.

Harnessing Intermolecular Interactions to Promote Long-Lived Photoinduced Charge Separation from Copper Phenanthroline Chromophores.

Facilitating photoinduced electron transfer (PET) while minimizing rapid charge-recombination processes to produce a long-lived charge-separated (CS) state represents a primary challenge associated with achieving efficient solar fuel production. Natural photosynthetic systems employ intermolecular interactions to arrange the electron-transfer relay in reaction centers and promote a directional flow of electrons. This work explores a similar tactic through the synthesis and ground- and excited-state characterization of two Cu(I)bis(phenanthroline) chromophores with homoleptic and heteroleptic coordination geometries and which are functionalized with negatively charged sulfonate groups. The addition of sulfonate groups enables solubility in pure water, and it also induces assembly with the dicationic electron acceptor methyl viologen (MV2+) via bimolecular, dynamic electrostatic interactions. The effect of the sulfonate groups on the ground- and excited-state properties was evaluated by comparison with the unsulfonated analogues in 1:1 acetonitrile/water. The excited-state lifetimes for all sulfonated complexes are similar to what we expect from previous literature, with the exception of the sulfonated heteroleptic complex whose metal-to-ligand charge-transfer (MLCT) lifetime in water has two components that are fit to 10 and 77 ns. For the sulfonated complexes, we detected reduced MV+• in both solvent environments following MLCT excitation, but control measurements in 1:1 acetonitrile/water with the unsulfonated analogues showed no PET to MV2+, indicating that electrostatically driven supramolecular assemblies of the sulfonated complexes with MV2+ facilitate the observed PET. Additionally, the strength of the intermolecular interactions driving the formation of these assemblies changes drastically with the solvent environment. In 1:1 acetonitrile/water, PET occurred from both sulfonated complexes with quantum yields (ΦET) of 2-3% but increased to a remarkable 98% for the sulfonated heteroleptic complex with a 3 µs CS-state lifetime in water.

Bimetallic Copper/Ruthenium/Osmium Complexes: Observation of Conformational Differences Between the Solution Phase and Solid State by Atomic Pair Distribution Function Analysis.

High-energy X-ray scattering and pair distribution function analysis (HEXS/PDF) is a powerful method to reveal the structure of materials lacking long-range order, but is underutilized for molecular complexes in solution. We demonstrate the application of HEXS/PDF with 0.26 Šresolution to uncover the solution structure of five bimetallic CuI /RuII /OsII complexes. HEXS/PDF of each complex in acetonitrile solution confirms the pairwise distances in the local coordination sphere of each metal center as well as the metal⋅⋅⋅metal distances separated by over 12 Å. The metal⋅⋅⋅metal distance detected in solution is compared with that from the crystal structure and molecular models to confirm that distortions to the metal bridging ligand are unique to the solid state. This work presents the first example of observing sub-Ångström conformational differences by direct comparison of solution phase and solid-state structures and shows the potential for HEXS/PDF in the determination of solution structure of single molecules.

Examination of abiotic cofactor assembly in photosynthetic biomimetics: site-specific stereoselectivity in the conjugation of a ruthenium(II) tris(bipyridine) photosensitizer to a multi-heme protein.

To understand design principles for assembling photosynthetic biohybrids that incorporate precisely-controlled sites for electron injection into redox enzyme cofactor arrays, we investigated the influence of chirality in assembly of the photosensitizer ruthenium(II)bis(2,2'-bipyridine)(4-bromomethyl-4'-methyl-2,2'-bipyridine), Ru(bpy)2(Br-bpy), when covalently conjugated to cysteine residues introduced by site-directed mutagenesis in the triheme periplasmic cytochrome A (PpcA) as a model biohybrid system. For two investigated conjugates that show ultrafast electron transfer, A23C-Ru and K29C-Ru, analysis by circular dichroism spectroscopy, CD, demonstrated site-specific chiral discrimination as a factor emerging from the close association between [Ru(bpy)3]2+ and heme cofactors. CD analysis showed the A23C-Ru and K29C-Ru conjugates to have distinct, but opposite, stereoselectivity for the Λ and Δ-Ru(bpy)2(Br-bpy) enantiomers, with enantiomeric excesses of 33.1% and 65.6%, respectively. In contrast, Ru(bpy)2(Br-bpy) conjugation to a protein site with high flexibility, represented by the E39C-Ru construct, exhibited a nearly negligible chiral selectivity, measured by an enantiomeric excess of 4.2% for the Λ enantiomer. Molecular dynamics simulations showed that site-specific stereoselectivity reflects steric constraints at the conjugating sites and that a high degree of chiral selectivity correlates to reduced structural disorder for [Ru(bpy)3]2+ in the linked assembly. This work identifies chiral discrimination as means to achieve site-specific, precise geometric positioning of introduced photosensitizers relative to the heme cofactors in manner that mimics the tuning of cofactors in photosynthesis.

Interprotein electron transfer biohybrid system for photocatalytic H2 production.

Worldwide there is a large research investment in developing solar fuel systems as clean and sustainable sources of energy. The fundamental mechanisms of natural photosynthesis can provide a source of inspiration for these studies. Photosynthetic reaction center (RC) proteins capture and convert light energy into chemical energy that is ultimately used to drive oxygenic water-splitting and carbon fixation. For the light energy to be used, the RC communicates with other donor/acceptor components via a sophisticated electron transfer scheme that includes electron transfer reactions between soluble and membrane bound proteins. Herein, we reengineer an inherent interprotein electron transfer pathway in a natural photosynthetic system to make it photocatalytic for aqueous H2 production. The native electron shuttle protein ferredoxin (Fd) is used as a scaffold for binding of a ruthenium photosensitizer and H2 catalytic function is imparted to its partner protein, ferredoxin-NADP+-reductase (FNR), by attachment of cobaloxime molecules. We find that this 2-protein biohybrid system produces H2 in aqueous solutions via light-induced interprotein electron transfer reactions (TON > 2500 H2/FNR), providing insight about using native protein-protein interactions as a method for fuel generation.

Electron Paramagnetic Resonance Characterization of the Triheme Cytochrome from Geobacter sulfurreducens.

Periplasmic cytochrome A (PpcA) is a representative of a broad class of multiheme cytochromes functioning as protein "nanowires" for storage and extracellular transfer of multiple electrons in the δ-proteobacterium Geobacter sulfurreducens. PpcA contains three bis-His coordinated hemes held in a spatial arrangement that is highly conserved among the multiheme cytochromes c3 and c7 families, carries low potential hemes, and is notable for having one of the lowest number of amino acids utilized to maintain a characteristic protein fold and site-specific heme function. Low temperature X-band electron paramagnetic resonance (EPR) spectroscopy has been used to characterize the electronic configuration of the Fe(III) and the ligation mode for each heme. The three sets of EPR signals are assigned to individual hemes in the three-dimensional crystal structure. The relative energy levels of the Fe(III) 3d orbitals for individual hemes were estimated from the principal g-values. The observed g-tensor anisotropy was used as a probe of electronic structure of each heme, and differences were determined by specifics of axial ligation. To ensure unambiguous assignment of highly anisotropic low-spin (HALS) signal to individual hemes, EPR analyses of iron atom electronic configurations have been supplemented with investigation of porphyrin macrocycles by one-dimensional 1H NMR chemical shift patterns for the methyl substituents. Within optimized geometry of hemes in PpcA, the magnetic interactions between hemes were found to be minimal, similar to the c3 family of tetraheme cytochromes.

Resolution of Electronic and Structural Factors Underlying Oxygen-Evolving Performance in Amorphous Cobalt Oxide Catalysts.

Non-noble-metal, thin-film oxides are widely investigated as promising catalysts for oxygen evolution reactions (OER). Amorphous cobalt oxide films electrochemically formed in the presence of borate (CoBi) and phosphate (CoPi) share a common cobaltate domain building block, but differ significantly in OER performance that derives from different electron-proton charge transport properties. Here, we use a combination of L edge synchrotron X-ray absorption (XAS), resonant X-ray emission (RXES), resonant inelastic X-ray scattering (RIXS), resonant Raman (RR) scattering, and high-energy X-ray pair distribution function (PDF) analyses that identify electronic and structural factors correlated to the charge transport differences for CoPi and CoBi. The analyses show that CoBi is composed primarily of cobalt in octahedral coordination, whereas CoPi contains approximately 17% tetrahedral Co(II), with the remainder in octahedral coordination. Oxygen-mediated 4 p-3 d hybridization through Co-O-Co bonding was detected by RXES and the intersite dd excitation was observed by RIXS in CoBi, but not in CoPi. RR shows that CoBi resembles a disordered layered LiCoO2-like structure, whereas CoPi is amorphous. Distinct domain models in the nanometer range for CoBi and CoPi have been proposed on the basis of the PDF analysis coupled to XAS data. The observed differences provide information on electronic and structural factors that enhance oxygen evolving catalysis performance.

Structural and Functional Characterization of a Short-Chain Flavodoxin Associated with a Noncanonical 1,2-Propanediol Utilization Bacterial Microcompartment.

Bacterial microcompartments (BMCs) are proteinaceous organelles that encapsulate enzymes involved in CO2 fixation (carboxysomes) or carbon catabolism (metabolosomes). Metabolosomes share a common core of enzymes and a distinct signature enzyme for substrate degradation that defines the function of the BMC (e.g., propanediol or ethanolamine utilization BMCs, or glycyl-radical enzyme microcompartments). Loci encoding metabolosomes also typically contain genes for proteins that support organelle function, such as regulation, transport of substrate, and cofactor (e.g., vitamin B12) synthesis and recycling. Flavoproteins are frequently among these ancillary gene products, suggesting that these redox active proteins play an undetermined function in many metabolosomes. Here, we report the first characterization of a BMC-associated flavodoxin (Fld1C), a small flavoprotein, derived from the noncanonical 1,2-propanediol utilization BMC locus (PDU1C) of Lactobacillus reuteri. The 2.0 Å X-ray structure of Fld1C displays the α/ß flavodoxin fold, which noncovalently binds a single flavin mononucleotide molecule. Fld1C is a short-chain flavodoxin with redox potentials of -240 ± 3 mV oxidized/semiquinone and -344 ± 1 mV semiquinone/hydroquinone versus the standard hydrogen electrode at pH 7.5. It can participate in an electron transfer reaction with a photoreductant to form a stable semiquinone species. Collectively, our structural and functional results suggest that PDU1C BMCs encapsulate Fld1C to store and transfer electrons for the reactivation and/or recycling of the B12 cofactor utilized by the signature enzyme.

Solution Structures of Highly Active Molecular Ir Water-Oxidation Catalysts from Density Functional Theory Combined with High-Energy X-ray Scattering and EXAFS Spectroscopy.

The solution structures of highly active Ir water-oxidation catalysts are elucidated by combining density functional theory, high-energy X-ray scattering (HEXS), and extended X-ray absorption fine structure (EXAFS) spectroscopy. We find that the catalysts are Ir dimers with mono-µ-O cores and terminal anionic ligands, generated in situ through partial oxidation of a common catalyst precursor. The proposed structures are supported by (1)H and (17)O NMR, EPR, resonance Raman and UV-vis spectra, electrophoresis, etc. Our findings are particularly valuable to understand the mechanism of water oxidation by highly reactive Ir catalysts. Importantly, our DFT-EXAFS-HEXS methodology provides a new in situ technique for characterization of active species in catalytic systems.

Butterfly Deformation Modes in a Photoexcited Pyrazolate-Bridged Pt Complex Measured by Time-Resolved X-Ray Scattering in Solution.

Pyrazolate-bridged dinuclear Pt(II) complexes represent a series of molecules with tunable absorption and emission properties that can be directly modulated by structural factors, such as the Pt-Pt distance. However, direct experimental information regarding the structure of the emissive triplet excited state has remained scarce. Using time-resolved wide-angle X-ray scattering (WAXS), the excited triplet state molecular structure of [Pt(ppy)(µ-t-Bu2pz)]2 (ppy = 2-phenylpyridine; t-Bu2pz = 3,5-di-tert-butylpyrazolate), complex 1, was obtained in a dilute (0.5 mM) toluene solution utilizing the monochromatic X-ray pulses at Beamline 11IDD of the Advanced Photon Source. The excited-state structural analysis of 1 was performed based on the results from both transient WAXS measurements and density functional theory calculations to shed light on the primary structural changes in its triplet metal-metal-to-ligand charge-transfer (MMLCT) state, in particular, the Pt-Pt distance and ligand rotation. We found a pronounced Pt-Pt distance contraction accompanied by rotational motions of ppy ligands toward one another in the MMLCT state of 1. Our results suggest that the contraction is larger than what has previously been reported, but they are in good agreement with recent theoretical efforts and suggest the ppy moieties as targets for rational synthesis aimed at tuning the excited-state structure and properties.

Cofactor-specific photochemical function resolved by ultrafast spectroscopy in photosynthetic reaction center crystals.

High-resolution mapping of cofactor-specific photochemistry in photosynthetic reaction centers (RCs) from Rhodobacter sphaeroides was achieved by polarization selective ultrafast spectroscopy in single crystals at cryogenic temperature. By exploiting the fixed orientation of cofactors within crystals, we isolated a single transition within the multicofactor manifold, and elucidated the site-specific photochemical functions of the cofactors associated with the symmetry-related active A and inactive B branches. Transient spectra associated with the initial excited states were found to involve a set of cofactors that differ depending upon whether the monomeric bacteriochlorophylls, BChl(A), BChl(B), or the special pair bacteriochlorophyll dimer, P, was chosen for excitation. Proceeding from these initial excited states, characteristic photochemical functions were resolved. Specifically, our measurements provide direct evidence for an alternative charge separation pathway initiated by excitation of BChl(A) that does not involve P*. Conversely, the initial excited state produced by excitation of BChl(B) was found to decay by energy transfer to P. A clear sequential kinetic resolution of BChl(A) and the A-side bacteriopheophytin, BPh(A), in the electron transfer proceeding from P* was achieved. These experiments demonstrate the opportunity to resolve photochemical function of individual cofactors within the multicofactor RC complexes using single crystal spectroscopy.

Multimerization of solution-state proteins by tetrakis(4-sulfonatophenyl)porphyrin.

Surface binding and interactions of anionic porphyins bound to cationic proteins have been studied for nearly three decades and are relevant as models for protein surface molecular recognition and photoinitiated electron transfer. However, interpretation of data in nearly all reports explicitly or implicitly assumed interaction of porphyrin with monodisperse proteins in solutions. In this report, using small-angle X-ray scattering with solution phase samples, we demonstrate that horse heart cytochrome (cyt) c, triheme cytochrome c7 PpcA from Geobacter sulfurreducens, and hen egg lysozyme multimerize in the presence of zinc tetrakis(4-sulfonatophenyl)porphyrin (ZnTPPS). Multimerization of cyt c showed a pH dependence with a stronger apparent binding affinity under alkaline conditions and was weakened in the presence of a high salt concentration. Ferric-cyt c formed complexes larger than those formed by ferro-cyt c. Free base TPPS and FeTPPS facilitated formation of complexes larger than those of ZnTPPS. No increase in protein aggregation state for cationic proteins was observed in the presence of cationic porphyrins. All-atom molecular dynamics simulations of cyt c and PpcA with free base TPPS corroborated X-ray scattering results and revealed a mechanism by which the tetrasubstituted charged porphyrins serve as bridging ligands nucleating multimerization of the complementarily charged protein. The final aggregation products suggest that multimerization involves a combination of electrostatic and hydrophobic interactions. The results demonstrate an overlooked complexity in the design of multifunctional ligands for protein surface recognition.

Cobaloxime-based artificial hydrogenases.

Cobaloximes are popular H2 evolution molecular catalysts but have so far mainly been studied in nonaqueous conditions. We show here that they are also valuable for the design of artificial hydrogenases for application in neutral aqueous solutions and report on the preparation of two well-defined biohybrid species via the binding of two cobaloxime moieties, {Co(dmgH)2} and {Co(dmgBF2)2} (dmgH2 = dimethylglyoxime), to apo Sperm-whale myoglobin (SwMb). All spectroscopic data confirm that the cobaloxime moieties are inserted within the binding pocket of the SwMb protein and are coordinated to a histidine residue in the axial position of the cobalt complex, resulting in thermodynamically stable complexes. Quantum chemical/molecular mechanical docking calculations indicated a coordination preference for His93 over the other histidine residue (His64) present in the vicinity. Interestingly, the redox activity of the cobalt centers is retained in both biohybrids, which provides them with the catalytic activity for H2 evolution in near-neutral aqueous conditions.

Domain structure for an amorphous iridium-oxide water-oxidation catalyst characterized by X-ray pair distribution function analysis.

The domain structure of an amorphous, "blue layer" iridium-oxide water-oxidation catalyst film (BL) electrodeposited from the soluble precursor complex, [Cp*Ir(H2O)3]SO4, was characterized by X-ray pair distribution function (PDF) analysis. The results show that the experimental PDF can be fit remarkably well using a single Ir5O22 cluster extracted from the rutile lattice. The model includes distortions that indicate the presence of Ir(µ-O)3Ir or distorted Ir(µ-O)2Ir substructures, and hence deviations from a rutile structure. The five Ir atom cluster is suggested to represent the population-averaged distribution of metal-oxo clusters in the film. BL is found to be distinguished from other amorphous film water-oxidation catalysts because of the remarkably small domain size and homogeneity. As such, the blue layer catalyst provides a model for investigating ligand-determined metal-oxide cluster assembly and catalyst mechanism.

Structure-function analyses of solar fuels catalysts using in situ X-ray scattering.

This tutorial review illustrates opportunities for the resolution of structure-function relationships to aid in the development of new materials for solar energy conversion using a combination of spectroscopy and catalysis measurements with X-ray scattering analyses to provide in situ structural characterization of solar fuels catalysts. As an example, the use of molecular cobaloxime catalysts in bimolecular and supramolecular photocatalysis schemes for proton reduction is briefly reviewed. These highlight the need to develop new modular, hierarchical, self-healing supramolecular architectures for solar fuels catalysis. Examples of the X-ray scattering structural analysis of amorphous materials in the context of photocatalytic function are discussed in detail.

Toward a quantitative description of solvation structure: a framework for differential solution scattering measurements.

Appreciating that the role of the solute-solvent and other outer-sphere interactions is essential for understanding chemistry and chemical dynamics in solution, experimental approaches are needed to address the structural consequences of these interactions, complementing condensed-matter simulations and coarse-grained theories. High-energy X-ray scattering (HEXS) combined with pair distribution function analysis presents the opportunity to probe these structures directly and to develop quantitative, atomistic models of molecular systems in situ in the solution phase. However, at concentrations relevant to solution-phase chemistry, the total scattering signal is dominated by the bulk solvent, prompting researchers to adopt a differential approach to eliminate this unwanted background. Though similar approaches are well established in quantitative structural studies of macromolecules in solution by small- and wide-angle X-ray scattering (SAXS/WAXS), analogous studies in the HEXS regime-where sub-ångström spatial resolution is achieved-remain underdeveloped, in part due to the lack of a rigorous theoretical description of the experiment. To address this, herein we develop a framework for differential solution scattering experiments conducted at high energies, which includes concepts of the solvent-excluded volume introduced to describe SAXS/WAXS data, as well as concepts from the time-resolved X-ray scattering community. Our theory is supported by numerical simulations and experiment and paves the way for establishing quantitative methods to determine the atomic structures of small molecules in solution with resolution approaching that of crystallography.

Orientational analysis of atomic pair correlations in nanocrystalline indium oxide thin films.

The application of grazing-incidence total X-ray scattering (GITXS) for pair distribution function (PDF) analysis using >50 keV X-rays from synchrotron light sources has created new opportunities for structural characterization of supported thin films with high resolution. Compared with grazing-incidence wide-angle X-ray scattering, which is only useful for highly ordered materials, GITXS/PDFs expand such analysis to largely disordered or nanostructured materials by examining the atomic pair correlations dependent on the direction relative to the surface of the supporting substrate. A characterization of nanocrystalline In2O3-derived thin films is presented here with in-plane-isotropic and out-of-plane-anisotropic orientational ordering of the atomic structure, each synthesized using different techniques. The atomic orientations of such films are known to vary based on the synthetic conditions. Here, an azimuthal orientational analysis of these films using GITXS with a single incident angle is shown to resolve the markedly different orientations of the atomic structures with respect to the planar support and the different degrees of long-range order, and hence, the terminal surface chemistries. It is anticipated that orientational analysis of GITXS/PDF data will offer opportunities to extend structural analyses of thin films by providing a means to qualitatively determine the major atomic orientation within nanocrystalline and, eventually, non-crystalline films.

Protein delivery of a Ni catalyst to photosystem I for light-driven hydrogen production.

The direct conversion of sunlight into fuel is a promising means for the production of storable renewable energy. Herein, we use Nature's specialized photosynthetic machinery found in the Photosystem I (PSI) protein to drive solar fuel production from a nickel diphosphine molecular catalyst. Upon exposure to visible light, a self-assembled PSI-[Ni(P2(Ph)N2(Ph))2](BF4)2 hybrid generates H2 at a rate 2 orders of magnitude greater than rates reported for photosensitizer/[Ni(P2(Ph)N2(Ph))2](BF4)2 systems. The protein environment enables photocatalysis at pH 6.3 in completely aqueous conditions. In addition, we have developed a strategy for incorporating the Ni molecular catalyst with the native acceptor protein of PSI, flavodoxin. Photocatalysis experiments with this modified flavodoxin demonstrate a new mechanism for biohybrid creation that involves protein-directed delivery of a molecular catalyst to the reducing side of Photosystem I for light-driven catalysis. This work further establishes strategies for constructing functional, inexpensive, earth-abundant solar fuel-producing PSI hybrids that use light to rapidly produce hydrogen directly from water.

Characterization of an amorphous iridium water-oxidation catalyst electrodeposited from organometallic precursors.

Upon electrochemical oxidation of the precursor complexes [Cp*Ir(H(2)O)(3)]SO(4) (1) or [(Cp*Ir)(2)(OH)(3)]OH (2) (Cp* = pentamethylcyclopentadienyl), a blue layer of amorphous iridium oxide containing a carbon admixture (BL) is deposited onto the anode. The solid-state, amorphous iridium oxide material that is formed from the molecular precursors is significantly more active for water-oxidation catalysis than crystalline IrO(2) and functions as a remarkably robust catalyst, capable of catalyzing water oxidation without deactivation or significant corrosion for at least 70 h. Elemental analysis reveals that BL contains carbon that is derived from the Cp* ligand (∼ 3% by mass after prolonged electrolysis). Because the electrodeposition of precursors 1 or 2 gives a highly active catalyst material, and electrochemical oxidation of other iridium complexes seems not to result in immediate conversion to iridium oxide materials, we investigate here the nature of the deposited material. The steps leading to the formation of BL and its structure have been investigated by a combination of spectroscopic and theoretical methods. IR spectroscopy shows that the carbon content of BL, while containing some C-H bonds intact at short times, is composed primarily of components with C═O fragments at longer times. X-ray absorption and X-ray absorption fine structure show that, on average, the six ligands to iridium in BL are likely oxygen atoms, consistent with formation of iridium oxide under the oxidizing conditions. High-energy X-ray scattering (HEXS) and pair distribution function (PDF) analysis (obtained ex situ on powder samples) show that BL is largely free of the molecular precursors and is composed of small, <7 Å, iridium oxide domains. Density functional theory (DFT) modeling of the X-ray data suggests a limited set of final components in BL; ketomalonate has been chosen as a model fragment because it gives a good fit to the HEXS-PDF data and is a potential decomposition product of Cp*.