The role of carboxylate ligand orbitals in the breathing dynamics of a metal-organic framework by resonant X-ray emission spectroscopy.

Metal-organic frameworks (MOFs) exhibit structural flexibility induced by temperature and guest adsorption, as demonstrated in the structural breathing transition in certain MOFs between narrow-pore and large-pore phases. Soft modes were suggested to entropically drive such pore breathing through enhanced vibrational dynamics at high temperatures. In this work, oxygen K-edge resonant X-ray emission spectroscopy of the MIL-53(Al) MOF was performed to selectively probe the electronic perturbation accompanying pore breathing dynamics at the ligand carboxylate site for metal-ligand interaction. It was observed that the temperature-induced vibrational dynamics involves switching occupancy between antisymmetric and symmetric configurations of the carboxylate oxygen lone pair orbitals, through which electron density around carboxylate oxygen sites is redistributed and metal-ligand interactions are tuned. In turn, water adsorption involves an additional perturbation of π orbitals not observed in the structural change solely induced by temperature.

Infrared Spectra and Hydrogen-Bond Configurations of Water Molecules at the Interface of Water-Insoluble Polymers under Humidified Conditions.

Elucidating the state of interfacial water, especially the hydrogen-bond configurations, is considered to be key for a better understanding of the functions of polymers that are exhibited in the presence of water. Here, an analysis in this direction is conducted for two water-insoluble biocompatible polymers, poly(2-methoxyethyl acrylate) and cyclic(poly(2-methoxyethyl acrylate)), and a non-biocompatible polymer, poly(n-butyl acrylate), by measuring their IR spectra under humidified conditions and by carrying out theoretical calculations on model complex systems. It is found that the OH stretching bands of water are decomposed into four components, and while the higher-frequency components (with peaks at ∼3610 and ∼3540 cm-1) behave in parallel with the C═O and C-O-C stretching and CH deformation bands of the polymers, the lower-frequency components (with peaks at ∼3430 and ∼3260 cm-1) become pronounced to a greater extent with increasing humidity. From the theoretical calculations, it is shown that the OH stretching frequency that is distributed from ∼3650 to ∼3200 cm-1 is correlated to the hydrogen-bond configurations and is mainly controlled by the electric field that is sensed by the vibrating H atom. By combining these observed and calculated results, the configurations of water at the interface of the polymers are discussed.

Critical In-Plane Density of Polyelectrolyte Brush for the Ordered Hydrogen-Bonded Structure of Incorporated Water.

A polymer electrolyte brush is a reasonable platform to confine water molecules within a nanoscopic area to study their role in the function of interacting media because of their adjustable nanospace and charge by changing the in-plane density and side chains of the brush. Here, we demonstrate how the in-plane spacing of cationic polymer brush chains, poly[2-(methacryloyloxy)ethyltrimethylammonium chloride] (PMTAC), affects the hydrogen bond configuration of incorporated water using soft X-ray emission spectroscopy. At the critical in-plane density σ = 0.30 chains/nm2 of PMTAC, tetrahedrally coordinated water molecules started to melt into distorted or broken hydrogen-bonded configurations. Considering the charge on the quaternary ammonium cations, the electric field required to form a tetrahedrally coordinated hydrogen-bonded configuration was estimated as ∼500 kV cm-1 and is effective up to ∼1 nm from the surface of the polymer chain. These findings are useful for designing specific interface properties and the resultant surface function of polyelectrolyte-based materials.

Identification of Valence Electronic States Reflecting the Hydrogen Bonding in Liquid Ethanol.

The temperature-dependent X-ray emission spectra of liquid ethanol were calculated theoretically using a semi-classical approximation to the Kramers-Heisenberg formula, which includes the dynamical effects induced by a core-hole. Soft X-ray emission spectroscopic measurements were performed to discern the changes in the hydrogen bonding (h-bonding) structure of liquid ethanol using a temperature-controlled liquid cell at 241 and 313 K. The relative intensities of the peaks at approximately 526.5 and 527.1 eV varied with temperature, and the corresponding behavior was reproduced theoretically, although the variation with temperature in the calculated spectra were more enhanced than that in the experiment. The two peaks can be attributed to the 3a″ + 10a' mixed state and pure 3a″ state, respectively, depending on the behavior of the local h-bonding structure. The splitting of the 3a″ component occurred because of the h-bonding behavior of liquid ethanol. Furthermore, the size of the ethanol cluster decreased with an increase in temperature, mainly due to the breaking of the one-donor/one-acceptor type h-bonding. Our studies suggest that the electronic state of liquid ethanol reflects several types of h-bonding structures, and the ratios of these h-bonding types vary with temperature.

Hydration Mechanism in Blood-Compatible Polymers Undergoing Phase Separation.

Interactions involving intermediate water are crucial for the design of novel blood-compatible materials. Herein, we use a combination of atomic force microscopy, quartz crystal microbalance measurements, and soft X-ray emission spectroscopy to investigate the local hydrogen-bonded configuration of water on blood-compatible poly(2-methoxyethyl acrylate) and non-blood-compatible poly(n-butyl acrylate) grafted on a gold substrate. We find that the initially incorporated water induces polymer-dependent phase separation, facilitating further water uptake. For the blood-compatible polymer, tetrahedrally coordinated water coexists with water adsorbed on C═O groups in low-density regions of the grafted polymer surface, providing a scaffold for the formation of intermediate water. The amount of intermediate water is determined by the type of functional groups, local polymer configuration, and polymer morphology. Thus, blood compatibility is governed by the complex water/polymer interactions.

Hydrogen-Bonded Structure of Water in the Loop of Anchored Polyrotaxane Chain Controlled by Anchoring Density.

Hydrogen-bonded network of water surrounding polymers is expected to be one of the most relevant factors affecting biocompatibility, while the specific hydrogen-bonded structure of water responsible for biocompatibility is still under debate. Here we study the hydrogen-bonded structure of water in a loop-shaped poly(ethylene glycol) chain in a polyrotaxane using synchrotron soft X-ray emission spectroscopy. By changing the density of anchoring molecules, hydrogen-bonded structure of water confined in the poly(ethylene glycol) loop was identified. The XES profile of the confined water indicates the absence of the low energy lone-pair peak, probably because the limited space of the polymer loop entropically inhibits the formation of tetrahedrally coordinated water. The volume of the confined water can be changed by the anchoring density, which implies the ability to control the biocompatibility of loop-shaped polymers.

Ion Selectivity of Water Molecules in Subnanoporous Liquid-Crystalline Water-Treatment Membranes: A Structural Study of Hydrogen Bonding.

We demonstrate hydrogen-bonded structures of water in self-organized subnanoporous water treatment membranes obtained using synchrotron-based high-resolution soft X-ray emission spectroscopy. The ion selectivity of these water treatment membranes is usually understood by the size compatibility of nanochannels in the membrane with the Stokes radius of hydrated ions, or by electrostatic interaction between charges inside the nanochannels and such ions. However, based on a comparison between the hydrogen-bonded structures of water molecules in the nanochannels of the water treatment membrane and those surrounding the ions, we propose a definite contribution of structural consistency among the associated hydrogen-bonded water molecules to the ion selectivity. Our observation delivers a novel concept to the design of water treatment membranes where water molecules in the nanochannel can be regarded as a part of the material that controls the ion selectivity.

Elucidation of Structure-Activity Correlations in a Nickel Manganese Oxide Oxygen Evolution Reaction Catalyst by Operando Ni L-Edge X-ray Absorption Spectroscopy and 2p3d Resonant Inelastic X-ray Scattering.

Herein, we report the synthesis and electrochemical oxygen evolution experiments for a graphene-supported Ni3MnO4 catalyst. The changes that occur at the Ni active sites during the electrocatalyic oxygen evolution reaction (OER) were elucidated by a combination of operando Ni L-edge X-ray absorption spectroscopy (XAS) and Ni 2p3d resonant inelastic X-ray scattering (RIXS). These data are compared to reference measurements on NiO, ß-Ni(OH)2, ß-NiOOH, and γ-NiOOH. Through this comparative analysis, we are able to show that under alkaline conditions (0.1 M KOH), the oxides of the Ni3MnO4 catalyst are converted to hydroxides. At the onset of catalysis (1.47 V), the ß-Ni(OH)2-like phase is oxidized and converted to a dominantly γ-NiOOH phase. The present study thus challenges the notion that the ß-NiOOH phase is the active phase in OER and provides further evidence that the γ-NiOOH phase is catalytically active. The ability to use Ni L-edge XAS and 2p3d RIXS to provide a rational basis for structure-activity correlations is highlighted.

Measurements of ultrafast dissociation in resonant inelastic x-ray scattering of water.

There has been a discussion on the interpretation of the resonant inelastic x-ray scattering (RIXS) spectra of liquid water in terms of either different structural environments or that core hole dynamics can generate well-resolved dissociative spectral components. We have used RIXS with high resolution in the OH stretch vibration energy part, at extremely high overtones going toward the continuum of full OH bond breakage, to identify the amount of dissociative contributions in the valence band RIXS spectra at different excitation energies. We observe that at low excitation energies, corresponding to population of states with strongly antibonding character, the valence band RIXS spectra have a large contribution from a well-resolved dissociative feature. Instead, at higher excitations, this spectral component diminishes and becomes a weak structure on the high-energy side of one of the spectral peaks related to the 1b1 state from tetrahedral configurations. This result brings both interpretations to be essential for the understanding of RIXS spectra of liquid water.

Enhancement of the Hydrogen-Bonding Network of Water Confined in a Polyelectrolyte Brush.

Water existing in the vicinity of polyelectrolytes exhibits unique structural properties, which demonstrate key roles in chemistry, biology, and geoscience. In this study, X-ray absorption and emission spectroscopy was employed to observe the local hydrogen-bonding structure of water confined in a charged polyelectrolyte brush. Even at room temperature, a majority of the water molecules confined in the polyelectrolyte brush exhibited one type of hydrogen-bonding configuration: a slightly distorted, albeit ordered, configuration. The findings from this study provide new insight in terms of the correlation between the function and local structure of water at the interface of biological materials under physiological conditions.