Bridging Proton Research Articles

Main-Group Metal-Nonmetal Dynamic Proton Bridges Enhance Ammonia Electrosynthesis.

The electrochemical nitrogen reduction reaction (eNRR) is a crucial process for the sustainable production of ammonia (NH3) for energy and agriculture applications. However, the reaction's efficiency is highly dependent on the activation of the inert N≡N bond, which is hindered by the electron back-donation to the π* orbitals of the N≡N bond, resulting in low eNRR capacity. Herein, we report a main-group metal-nonmetal (O-In-S) eNRR catalyst featuring a dynamic proton bridge, with In-S serving as the polarization pair and O functioning as the dynamic electron pool. In situ spectroscopic analysis and theoretical calculations reveal that the In-S polarization pair acts as asymmetric dual-sites, polarizing the N≡N bond by concurrently back-donating electrons to both the πx* and πy* orbitals of N2, thereby overcoming the significant band gap limitations, while inhibiting the competitive hydrogen evolution reaction. Meanwhile, the O dynamic electron pool acts as a "repository" for electron storage and donation to the In-S polarization pair. As a result, the O-In-S dynamic proton bridge exhibits exceptional NH3 yield rates and Faradaic efficiencies (FEs) across a wide potential window of 0.3 V, with an optimal NH3 yield rate of 80.07±4.25 μg h-1 mg-1 and an FE of 38.01±2.02 %, outperforming most previously reported catalysts.

Main‐Group Metal‐Nonmetal Dynamic Proton Bridges Enhance Ammonia Electrosynthesis

The electrochemical nitrogen reduction reaction is a crucial process for the sustainable production of ammonia for energy and agriculture applications. However, the reaction’s efficiency is highly dependent on the activation of the inert N≡N bond, which is hindered by the electron back‐donation to the π* orbitals of the N≡N bond, resulting in low eNRR capacity. Herein, we report a main‐group metal‐non‐metal (O‐In‐S) eNRR catalyst featuring a dynamic proton bridge, with In‐S serving as the polarization pair and O functioning as the dynamic electron pool. In‐situ spectroscopic analysis and theoretical calculations reveal that the In‐S polarization pair acts as asymmetric dual‐sites, polarizing the N≡N bond by concurrently back‐donating electrons to both the πx* and πy* orbitals of N2, thereby overcoming the significant band gap limitations, while inhibiting the competitive hydrogen evolution reaction. Meanwhile, the O dynamic electron pool acts as a “repository” for electron storage and donation to the In‐S polarization pair. As a result, the O‐In‐S dynamic proton bridge exhibits exceptional NH3 yield rates and Faradaic efficiencies (FEs) across a wide potential window of 0.3 V, with an optimal NH3 yield of 80.07 ± 4.25 μg h‐1 mg‐1 and an FE of 38.01 ± 2.02%, outperforming most previously reported catalysts.

Very Strong Hydrogen Bond in Nitrophthalic Cocrystals.

This work presents the studies of a very strong hydrogen bond (VSHB) in biologically active phthalic acids. Research on VSHB comes topical due to its participation in many biological processes. The studies cover the modelling of intermolecular interactions and phthalic acids with 2,4,6-collidine and N,N-dimethyl-4-pyridinamine complexes with aim to obtain a VSHB. The four synthesized complexes were studied by experimental X-ray, IR, and Raman methods, as well as theoretical Car-Parrinello Molecular Dynamics (CP-MD) and Density Functional Theory (DFT) simulations. By variation of the steric repulsion and basicity of the complex' components, a very short intramolecular hydrogen bond was achieved. The potential energy curves calculated by the DFT method were characterized by a low barrier (0.7 and 0.9 kcal/mol) on proton transfer in the OHN intermolecular hydrogen bond for 3-nitrophthalic acid with either 2,4,6-collidine or N,N-dimethyl-4-pyridinamine cocrystals. Moreover, the CP-MD simulations exposed very strong bridging proton dynamics in the intermolecular hydrogen bonds. The accomplished crystallographic and spectroscopic studies indicate that the OHO intramolecular hydrogen bond in 4-nitrophthalic cocrystals is VSHB. The influence of a strong steric effect on the geometry of the studied cocrystals and the stretching vibration bands of the carboxyl and carboxylate groups was elaborated.

Anagostic Axial Interactions Inhibit Cross‐Coupling Catalytic Activity in Square Planar Pyridinophane Nickel Complexes

AbstractHerein, we report for the first time the use of the nitrogen‐based bidentate molecule [2.2]pyridinophane (N2) as a ligand for metal complexes. Additionally, its improved synthesis allows for electronic modification of the pyridine rings to access the new para‐dimethylamino‐[2.2]pyridinophane ligand (p‐NMe2N2). These ligands bind nickel in an analogous fashion to other pyridinophane ligands, completing the series of tetra‐, tri‐, and bidentate pyridinophane‐nickel complexes. The new compounds exhibit geometrically enforced C−H anagostic interactions between the ethylene bridge protons and the nickel center that are not present in other pyridinophane systems. These ethylene bridge groups also act as an unusual form of steric encumbrance, enforcing square planar geometries in ligand fields that would otherwise adopt tetrahedral structures. In addition, these anagostic interactions inhibit the catalytic performance in Csp3–Csp3 Kumada cross coupling reactions relative to other common bidentate N‐ligand platforms, possibly by preventing the formation of the 5‐coordinate oxidative addition intermediates.

Chemistry of (η6-C6Me6)Ru(Cl)2(P(OMe)3) with AgOTf in different solvents, the high affinity for traces of water and structural characterization of mono-, di- and trinuclear derivatives

An extensive study of the reactions of (η6-C6Me6)Ru(Cl)2(P(OMe)3) (1) with AgOTf in three different deuterated and non-deuterated solvents: chloroform, acetone and nitromethane is described. A series of compounds with the moiety (η6-C6Me6)Ru(P(OMe)3) is isolated and characterized, along with their crystalline structures. A simple associative step assisted by coordination of the silver cation to two molecules of 1 through the Cl-Ru-Cl bonds affords [{(η6-C6Me6)Ru(Cl)2(P(OMe)3)}2(μ-Ag)](OTf) (2), loss of AgCl from 2 gives 1 and (η6-C6Me6)RuCl(OTf)(P(OMe)3) (3), meanwhile 3 dimerizes to [(η6-C6Me6)Ru(μ-Cl)(P(OMe)3)]2(OTf)2 (4) which in presence of P(OMe)3 gives [(η6-C6Me6)RuCl(P(OMe)3)2](OTf) (11OTf). A comparative study of the influence of the counterions OTf− and BF4− was carried out with the analogue [(η6-C6Me6)RuCl(P(OMe)3)2](BF4) (11BF4), as well as the corresponding demethylation of the coordinated trimethylphosphite ligands by the classical Michaelis-Arbuzov reactions. The resulting “quasi-chelate” (η6-C6Me6)RuCl(P(O)(OMe)2)(P(OH)(OMe)2) (17) where a bridging proton in the phosphinito-phosphinous acid ligands is replaced by BF2 affords (η6-C6Me6)RuCl({P(OMe)2(O)}2BF2) (18) which contains the chelating structure [Me2POBF2OPMe2]. The chemistry in solution at room temperature of 1 with AgOTf shows non-selective reactions, as can be determined by multinuclear NMR spectroscopy (1H, 13C{1H}, 31P, 19F). The mixture of products shows a great preference to coordinate water, even when it is in traces, as observed for the formation of [(η6-C6Me6)RuCl(H2O)(P(OMe)3)](OTf) (5), [(η6-C6Me6)Ru(H2O)(OTf)(P(OMe)3)]OTf (6) and [(η6-C6Me6)Ru(H2O)2(P(OMe)3)](OTf)2 (7), being the latter 7 the “sink” product. Also, in presence of anhydrous AgOTf compound 1 can be transformed into the highly hygroscopic (η6-C6Me6)Ru(OTf)2(P(OMe)3) (8) which readily replaces triflate ligands by trace water. Structural characterization by single crystal X-ray diffraction of trinuclear compound 2, dinuclear 4 and mononuclear 7, 11X (X = OTf, BF4), 18 and nitrile derivatives [(η6-C6Me6)Ru(NCR)2(P(OMe)3)](OTf)2 (R = Et, 9; Me,10) is discussed.

Electrophysical Properties of Methylammonium Alum Single Crystals: The Role of the Hydrogen Bond Network in the Formation of Temperature Anomalies

Pyroelectric, dielectric properties, and thermodepolarization currents of methylammonium alum crystallohydrate single crystals were studied in temperature range 120–200 K. Temperature dependences of these quantities demonstrated peaks at ∼143, ∼160, and 176.4 K (Curie point). The behavior, which is typical to many crystals with a network of hydrogen bonds with an O–H...O distance of ∼2.7 ± 0.1 ˚A, is considered from the point of view of a quantum chemical model of a hydrogen bond network containing water clusters in addition to individual H2O molecules. The movement of bridging protons in molecular clusters in electric fields is coherent and correlated, and all bridging protons of the system are involved in it. These displacements are associated with oscillations of the oxygen frame of the compression-tension type. The discreteness of the temperature peaks of the electrophysical properties is associated with the discreteness of the energy levels of the double-well potential, caused by the splitting of the excited levels.

Hydantoanabaenopeptins from Lake Kinneret Microcystis Bloom, Isolation, and Structure Elucidation of the Possible Intermediates in the Anabaenopeptins Biosynthesis.

Anabaenopeptins are common metabolites of cyanobacteria. In the course of reisolation of the known aeruginosins KT608A and KT608B for bioassay studies, we noticed the presence of some unknown anabaenopeptins in the extract of a Microcystis cell mass collected during the 2016 spring bloom event in Lake Kinneret, Israel. The 1H NMR spectra of some of these compounds presented a significant difference in the appearance of the ureido bridge protons, and their molecular masses did not match any one of the 152 known anabaenopeptins. Analyses of the 1D and 2D NMR, HRMS, and MS/MS spectra of the new compounds revealed their structures as the hydantoin derivatives of anabaenopeptins A, B, F, and 1[Dht]-anabaenopeptin A and oscillamide Y (1, 2, 3, 6, and 4, respectively) and a new anabaenopeptin, 1[Dht]-anabaenopeptin A (5). The known anabaenopeptins A, B, and F and oscillamide Y (7, 8, 9, and 10, respectively) were present in the extract as well. We propose that 1-4 and 6 are the possible missing intermediates in the previously proposed partial biosynthesis route to the anabaenopeptins. Compounds 1-6 were tested for inhibition of the serine proteases trypsin and chymotrypsin and found inactive at a final concentration of ca. 54 μM.

Isotope Effects on Ground and Excited States of Ethyl Cation, H+(C2H4)

The structure and spectra of ethyl cation, H+(C2H4), and its deuterated analogues are investigated using diffusion Monte Carlo (DMC). These calculations all show that the ground state wave function for H+(C2H4) is localized near the minimum energy configuration in which the excess proton is in a bridging configuration, although the amplitude of the vibrational motions of the bridging proton is large. Deuteration of the bridging proton reduces the amplitude of this motion, while deuteration of only the ethylenic hydrogen atoms in H+(C2D4) has little effect on the amplitude of the motion of the bridging proton. Excited states that are accessed by spectroscopically observed transitions in H+(C2H4) are calculated using fixed-node DMC. The calculated and measured frequencies for the states with one quantum of excitation in the ethylenic CH stretching vibrations show good agreement. We also explore the excited state with one quantum of excitation in the proton transfer vibration of the bridging proton and obtain a frequency of 616 cm-1 for H+(C2H4). This frequency increases to 629 cm-1 in H+(C2D4). Deuteration decreases this frequency to 491 and 495 cm-1 in D+(C2H4) and D+(C2D4), respectively. The effects of partial deuteration on the frequencies of the CH stretching vibrations, and the corresponding probability amplitudes are also explored. Finally, we report the vibrationally averaged rotational constants for the four isotopologues of ethyl cation considered in this study.

New organic ionic plastic crystals based on pyrrolidinium dication for a solid‐phase electrolyte

AbstractA series of 1,4‐bis(N‐alkylpyrrolidinium)butane salts combined with Br−, I−, PF6−, and Tf2N− anions were newly designed and synthesized. In nonpolar CDCl3 solution, the axial and equatorial proton peaks of the bis‐pyrrolidinium cation ring are distinguished in 1H NMR. Also, the strengths of ion paring could be estimated from the 1H NMR chemical shift changes of the bridge and pyrrolidinium proton peaks. 1,4‐Bis(N‐hepthylpyrrolidinium)butane I− is an organic ionic plastic crystal showing the lowest fusion entropy value (ΔSf = 3.4 J K·mol−1). Softer crystalline morphologies were found by polarized optical microscope images at plastic crystal phase of each sample and the results were corresponding to differential scanning calorimetry results. 1,4‐Bis(N‐ethylpyrrolidinium)butane Tf2N− showed the softest crystal phase. Ionic conductivity changes and electrochemical stabilities of the bis‐pyrrolidinium PF6− and Tf2N− salts were also measured, and they were stable up to 4.3 V (vs. Li/Li+) and expected to be applied as an electrolyte component.

Molecular mechanism of the transformation of oxidized lignin to N-substituted aromatics.

The cleavage of C-C bonds in oxidized lignin model compounds is a highly effective methodology for achieving lignin depolymerization, as well the generation of N-substituted aromatics. Here, density functional theory calculations were performed to understand the mechanism of the transformation of an oxidized lignin model compound (ligninox) and hydroxylamine hydrochloride to N-substituted aromatics. The reaction was proposed to proceed via an energetically viable mechanism featuring the initial production of HOAc acting as proton bridge. According to our calculations, Z-type oxime is the major intermediate of the reaction, with an energy barrier of 22.9 kcal mol-1, owing to the weak interactions between methoxy and oximino groups being stronger than that of E-type oxime. Additionally, the hydroxy addition is the rate-determining step, with an energy barrier of 27.0 kcal mol-1. Moreover, the huge net energy change of Beckmann and abnormal Beckmann rearrangements is the main overall thermodynamic driving force for producing N-substituted aromatics from oximes. The theoretical results have provided a clear picture of how ligninox transforms into N-substituted aromatics and are expected to provide valuable theoretical guidance for lignin depolymerization.

Evidence of anharmonicity in the vibrational spectrum of protonated ethylene

The vibrational spectrum of protonated ethylene, H+(C2H4), in the 2000 to 3500 cm−1 region is re-investigated. The spectrum is obtained by predissociation of the H+(C2H4)Ar2 complex following vibrational excitation, which leads to elimination of an argon atom. The equilibrium structure of this cation has the excess proton in a bridged position, where it undergoes large amplitude motions in the CH+C plane. These large amplitude vibrational motions result in features in the spectrum that cannot be assigned to Δv=1 transitions predicted by harmonic treatments. The origins of the anharmonicities that lead to these higher order transitions are explored. Based on vibrational perturbation theory and reduced dimensional calculations, transitions to states with two quanta in the HCH bending vibrations and the state with one quantum of excitation in the two vibrations of the bridging proton in the CH+C plane are found to carry significant intensity in this region of the spectrum.

Effect of pH on the sorption of dissolved organic carbon derived from six tree species in forest soils

Dissolved organic carbon (DOC) is an important component of the total carbon (C) pool in temperate forest soils. The ability of forest soils to sorb DOC is an essential feature in the context of ecosystem changes (such as climate change) and the resulting changes in the C cycle. The aim of this study was to investigate the ability of forest soils with different pH levels to sorb DOC from litter of different forest tree species. To accomplish this, we conducted two laboratory experiments in which the soil differed in the depth from which they were sampled (topsoil vs. subsoil) and the initial DOC content. We modified the soil pH to obtain a range covering most soils in temperate forests (approximately 3.5–8). Soil samples were leached with extracts of similar DOC content obtained from the litter of six main forest-forming tree species in temperate zone. Increased DOC sorption was observed at approximate soil pH levels of 3.5 and 7.5, with the highest sorption capacity occurring at low pH. The DOC sorption capacity was decreasing with consecutive leaching with litter extract. The capacity of DOC sorption also depended on the litter species. In addition, the initial concentration of C in soil had no significant relationship with sorption capacity. Litter species were related to the sorption of released DOC, which was probably linked to the Al content in the litter. The increased DOC sorption at low pH was likely because of dissolved organic matter protonation (saturation of functional groups with H+ ions resulting in proton bridging and van der Waals forces), while Ca2+ ions causing cation bridging, flocculation or non-columbic interactions were likely responsible for the increased sorption at high pH. Our research shed light on the significant effects of soil pH on the stabilization of DOC, as well as the important role and positive influence of Al on DOC sorption by forest soils. Our findings provide new, significant insight by presenting that litter-derived DOC sorption is dependent both on soil pH and litter chemical composition.

Helium Nanodroplet Infrared Action Spectroscopy of the Proton-Bound Dimer of Hydrogen Sulfate and Formate: Examining Nuclear Quantum Effects.

The proton-bound dimer of hydrogen sulfate and formate is an archetypal structure for ionic hydrogen-bonding complexes that contribute to biogenic aerosol nucleation. Of central importance for the structure and properties of this complex is the location of the bridging proton connecting the two conjugate base moieties. The potential energy surface for bridging proton translocation features two local minima, with the proton localized at either the formate or hydrogen sulfate moiety. However, electronic structure methods reveal a shallow potential energy surface governing proton translocation, with a barrier on the order of the zero-point energy. This shallow potential complicates structural assignment and necessitates a consideration of nuclear quantum effects. In this work, we probe the structure of this complex and its isotopologues, utilizing infrared (IR) action spectroscopy of ions captured in helium nanodroplets. The IR spectra indicate a structure in which a proton is shared between the hydrogen sulfate and formate moieties, HSO4–···H+···–OOCH. However, because of the nuclear quantum effects and vibrational anharmonicities associated with the shallow potential for proton translocation, the extent of proton displacement from the formate moiety remains unclear, requiring further experiments or more advanced theoretical treatments for additional insight.

Competition of Intra- and Intermolecular Forces in Anthraquinone and Its Selected Derivatives.

Intra- and intermolecular forces competition was investigated in the 9,10-anthraquinone (1) and its derivatives both in vacuo and in the crystalline phase. The 1,8-dihydroxy-9,10-anthraquinone (2) and 1,8-dinitro-4,5-dihydroxy-anthraquinone (3) contain Resonance-Assisted Hydrogen Bonds (RAHBs). The intramolecular hydrogen bonds properties were studied in the electronic ground and excited states employing Møller-Plesset second-order perturbation theory (MP2), Density Functional Theory (DFT) method in its classical formulation as well as its time-dependent extension (TD-DFT). The proton potential functions were obtained via scanning the OH distance and the dihedral angle related to the OH group rotation. The topological analysis was carried out on the basis of theories of Atoms in Molecules (AIM—molecular topology, properties of critical points, AIM charges) and Electron Localization Function (ELF—2D maps showing bonding patterns, calculation of electron populations in the hydrogen bonds). The Symmetry-Adapted Perturbation Theory (SAPT) was applied for the energy decomposition in the dimers. Finally, Car–Parrinello molecular dynamics (CPMD) simulations were performed to shed light onto bridge protons dynamics upon environmental influence. The vibrational features of the OH stretching were revealed using Fourier transformation of the autocorrelation function of atomic velocity. It was found that the presence of OH and NO substituents influenced the geometric and electronic structure of the anthraquinone moiety. The AIM and ELF analyses showed that the quantitative differences between hydrogen bonds properties could be neglected. The bridged protons are localized on the donor side in the electronic ground state, but the Excited-State Intramolecular Proton Transfer (ESIPT) was noticed as a result of the TD-DFT calculations. The hierarchy of interactions determined by SAPT method indicated that weak hydrogen bonds play modifying role in the organization of these crystal structures, but primary ordering factor is dispersion. The CPMD crystalline phase results indicated bridged proton-sharing in the compound 2.

Proton Bridging in Catalysis by and Inhibition of Serine Proteases of the Blood Cascade System.

Inquiries into the participation of short hydrogen bonds in stabilizing transition states and intermediate states in the thrombin, factor Xa, plasmin and activated protein C–catalyzed reactions revealed that specific binding of effectors at Sn, n = 1–4 and S’n, n = 1–3 and at remote exosites elicit complex patterns of hydrogen bonding and involve water networks. The methods employed that yielded these discoveries include; (1) kinetics, especially partial or full kinetic deuterium solvent isotope effects with short cognate substrates and also with the natural substrates, (2) kinetic and structural probes, particularly low-field high-resolution nuclear magnetic resonance (1H NMR), of mechanism-based inhibitors and substrate-mimic peptide inhibitors. Short hydrogen bonds form at the transition states of the catalytic reactions at the active site of the enzymes as they do with mechanism-based covalent inhibitors of thrombin. The emergence of short hydrogen bonds at the binding interface of effectors and thrombin at remote exosites has recently gained recognition. Herein, I describe our contribution, a confirmation of this discovery, by low-field 1H NMR. The principal conclusion of this review is that proton sharing at distances below the sum of van der Waals radii of the hydrogen and both donor and acceptor atoms contribute to the remarkable catalytic prowess of serine proteases of the blood clotting system and other enzymes that employ acid-base catalysis. Proton bridges also play a role in tight binding in proteins and at exosites, i.e., allosteric sites, of enzymes.

Sensitivity of 31 P NMR chemical shifts to hydrogen bond geometry and molecular conformation for complexes of phosphinic acids with pyridines.

The results of the quantum-chemical investigation of a series of hydrogen-bonded 1:1 acid-base complexes formed by model phosphinic acids, Me2 POOH, and PhHPOOH, are reported. A series of substituted pyridines (pKa range from 0.5 to 10) was chosen as proton acceptors. Gradual changes of isotropic 31 P nuclear magnetic resonance (NMR) chemical shift, δP, were correlated with the bridging proton position in the intermolecular OHN hydrogen bond, namely, r (OH) distance; the proposed correlation could easily be extended to other phosphinic acids as well. For complexes with pyridine and 2,4,6-trimethylpyridine, we have investigated in more detail several factors influencing the δP values: (1) the proton transfer within the OHN hydrogen bond; (2) the rotation of the pyridine ring around the hydrogen bond axis (associated with the formation/breakage of additional weak PO···H-C hydrogen bond); and (3) the rotation of the phenyl substituent in phenylphosphinic acid around the P-C axis. All these factors appeared to be of similar magnitude, thus masking their individual contributions that have to be independently estimated for a reliable spectral interpretation.

Dynamics of coordination of H3O+ and NH4+ in crown ether cavities.

Crown ethers stand out for their ability to form inclusion complexes with metal cations and positively charged molecular moieties. Hydronium and ammonium interact strongly with crown ethers and potentially modulate their ionophoric activity in protic solvents and physiological environments commonly involved in (bio)technological applications. In this work, Born-Oppenheimer molecular dynamics (BOMD) computations are employed to gain insights into the coordination arrangements of H3O+ and NH4+ in the complexes with the native crown ethers 15-crown-5 (15c5) and 18-crown-6 (18c6). Both cations display dynamic changes in coordination inside the cavities of the crown ethers. On the one hand, hydronium explores different coordination arrangements, through rotation around its C3 axis in the 15c5 complex, and through breathing motions, involving rapid inversions of the O atom along the C3 axis in the 18c6 complex. On the other hand, ammonium undergoes a facile rotation in three dimensional space, leading to frequent changes in the NH bonds involved in the coordination with the crown ether. The reduced host-guest symmetry matching of the 15c5 macrocycle enhances the reorientation dynamics and, in the case of H3O+, it promotes short H-bonding distances yielding events of proton transfer to the crown ether. The infrared vibrational spectra predicted by the BOMD computations within this dynamic framework reproduce with remarkable accuracy the action spectra of the isolated complexes obtained in previous infrared laser spectroscopy experiments. The experimentally observed band positions and broadening can then be rationalized in terms of orientational diffusion of the cations, changes in the coordinating H-bonding pairs sustaining the complex and eventual proton bridge formation.

The immobilisation and reactivity of Fe(CN)63\u2212/4\u2212 in an intrinsically microporous polyamine (PIM-EA-TB)

Protonation of the molecularly rigid polymer of intrinsic microporosity PIM-EA-TB can be coupled to immobilisation of Fe(CN)63−/4− (as well as immobilisation of Prussian blue) into 1–2 nm diameter channels. The resulting films provide redox-active coatings on glassy carbon electrodes. Uptake, transport, and retention of Fe(CN)63−/4− in the microporous polymer are strongly pH dependent requiring protonation of the PIM-EA-TB (pKA ≈ 4). Both Fe(CN)64− and Fe(CN)63− can be immobilised, but Fe(CN)64− appears to bind tighter to the polymer backbone presumably via bridging protons. Loss of Fe(CN)63−/4− by leaching into the aqueous solution phase becomes significant only at pH > 9 and is likely to be associated with hydroxide anions directly entering the microporous structure to combine with protons. This and the interaction of Fe(CN)63−/4− and protons within the molecularly rigid PIM-EA-TB host are suggested to be responsible for retention and relatively slow leaching processes. Electrocatalysis with immobilised Fe(CN)63−/4− is demonstrated for the oxidation of ascorbic acid.Graphical abstract

H/D Isotope Effects on 1H-NMR Chemical Shifts in Cyclic Heterodimers and Heterotrimers of Phosphinic and Phosphoric Acids.

Hydrogen-bonded heterocomplexes formed by POOH-containing acids (diphenylphosphoric 1, dimethylphosphoric 2, diphenylphosphinic 3, and dimethylphosphinic 4) are studied by the low-temperature (100 K) 1H-NMR and 31P-NMR using liquefied gases CDF3/CDF2Cl as a solvent. Formation of cyclic dimers and cyclic trimers consisting of molecules of two different acids is confirmed by the analysis of vicinal H/D isotope effects (changes in the bridging proton chemical shift, δH, after the deuteration of a neighboring H-bond). Acids 1 and 4 (or 1 and 3) form heterotrimers with very strong (short) H-bonds (δH ca. 17 ppm). While in the case of all heterotrimers the H-bonds are cyclically arranged head-to-tail, ···O=P–O–H···O=P–O–H···, and thus their cooperative coupling is expected, the signs of vicinal H/D isotope effects indicate an effective anticooperativity, presumably due to steric factors: when one of the H-bonds is elongated upon deuteration, the structure of the heterotrimer adjusts by shortening the neighboring hydrogen bonds. We also demonstrate the formation of cyclic tetramers: in the case of acids 1 and 4 the structure has alternating molecules of 1 and 4 in the cycle, while in case of acids 1 and 3 the cycle has two molecules of 1 followed by two molecules of 3.

Novel composite membrane based on zirconium phosphate-ionic liquids for high temperature PEM fuel cells

Composite membranes composed of zirconium phosphate (ZrP) and imidazolium-based ionic liquids (IL), supported on polytetrafluoroethylene (PTFE) were prepared and evaluated for their application in proton exchange membrane fuel cells (PEM) operating at 200 °C. The experimental results reported here demonstrate that the synthesized membrane has a high proton conductivity of 0.07 S cm−1, i.e, 70% of that reported for Nafion. Furthermore, the composite membranes possess a very high proton conductivity of 0.06 S cm−1 when processed at 200 °C under completely anhydrous conditions. Scanning electron microscopy (SEM) images indicate the formation of very small particles, with diameters in the range of 100–300 nm, within the confined pores of PTFE. Thermogravimetric analysis (TGA) reveals a maximum of 20% weight loss up to 500 °C for the synthesized membrane. The increase in proton conductivity is attributed to the creation of multiple proton conducting paths within the membrane matrix. The IL component is acting as a proton bridge. Therefore, these membranes have potential for use in PEM fuel cells operating at temperatures around 200 °C.