Slow Proton Transfer Research Articles

Enhancing Oxygen Reduction Kinetics and Proton Transfer of La0.6Sr0.4Co0.2Fe0.8O3−δ Cathode through Pr2Ni0.5Co0.5O4−δ Impregnation for Protonic Ceramic Fuel Cells

AbstractSluggish reaction kinetics in oxygen reduction reaction (ORR) is one of the most important challenges to the development of protonic ceramic fuel cells (PCFCs). La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) exhibits high mixed ionic–electronic conductivity in traditional solid oxide fuel cells, but their slow proton transfer and ORR kinetics impedes practical applications. Herein, composite Pr2Ni0.5Co0.5O4−δ (PNC) particles composed of a perovskite PrNi0.5Co0.5O3−δ phase and a PrO2 phase are impregnated into a LSCF cathode to enhance the ORR activity and proton transfer. The polarization tested in a symmetric cell with PNC‐impregnated LSCF cathode is 0.06 Ω cm2 at 700 °C. The fuel cell with this impregnated cathode shows maximum power densities of 1857 mW cm−2 at 700 °C. Moreover, the impregnated cathode exhibits a low degradation rate in the durability test. This work not only provides a novel and practical approach to improving the performance of current cathode materials for PCFCs but also highlights the potential for enhancing the commercial viability of PCFC technology.

Accelerating Proton and Electron Transfer Enables Highly Active Fe─N─C Catalyst for Electrochemical CO2 Reduction

AbstractAtomically dispersed Fe─N─C catalysts display great potential for efficient CO production in the field of electrochemical CO2 reduction (ECR), but still suffer from unsatisfactory activity limited by the slow proton and electron transfer during the ECR process. Here, a superior Fe─N─C electrocatalyst is designed by anchoring the individual FeN4 sites and Fe nanoparticles onto highly conductive carbon nanotubes. The resultant catalyst displays a commendable CO partial current density of 16.01 mA cm−2 with a turnover frequency of 3519.6 h−1 at −0.65 V in an H‐type cell, and also exhibits CO selectivity > 90% under high current density over 120 mA cm−2 in a flow cell. This remarkable activity exceeds a host of previously reported Fe─N─C catalysts. The findings indicate that the carbon nanotube facilitates CO production due to its strong capability of electron transport and charge transfer. In situ spectroscopic analysis, controlled experiments, and theoretical calculations reveal that Fe nanoparticles effectively promote water dissociation and the subsequent protonation step, accelerate the formation of *COOH intermediate, and thus greatly enhance the ECR activity.

Electronic Structure Optimization and Proton-Transfer Enhancement on Titanium Oxide-Supported Copper Nanoparticles for Enhanced Nitrogen Recycling from Nitrate-Contaminated Water.

Electrocatalytic reduction of nitrate to NH3 (NO3RR) on Cu offers sustainable NH3 production and nitrogen recycling from nitrate-contaminated water. However, Cu affords limited NO3RR activity owing to its unfavorable electronic state and the slow proton transfer on its surface, especially in neutral/alkaline media. Furthermore, although a synchronous "NO3RR and NH3 collection" system has been developed for nitrogen recycling from nitrate-laden water, no system is designed for natural water that generally contains low-concentration nitrate. Herein, we demonstrate that depositing Cu nanoparticles on a TiO2 support enables the formation of electron-deficient Cuδ+ species (0 < δ ≤ 2), which are more active than Cu0 in NO3RR. Furthermore, TiO2-Cu coupling induces local electric-field enhancement that intensifies water adsorption/dissociation at the interface, accelerating proton transfer for NO3RR on Cu. With the dual enhancements, TiO2-Cu delivers an NH3-N selectivity of 90.5%, mass activity of 41.4 mg-N h gCu-1, specific activity of 377.8 mg-N h-1 m-2, and minimal Cu leaching (<25.4 μg L-1) when treating 22.5 mg L-1 of NO3--N at -0.40 V, outperforming most of the reported Cu-based catalysts. A sequential NO3RR and NH3 collection system based on TiO2-Cu was then proposed, which could recycle nitrogen from nitrate-contaminated water under a wide concentration window of 22.5-112.5 mg L-1 at a rate of 209-630 mgN m-2 h-1. We also demonstrated this system could collect 83.9% of nitrogen from NO3--N (19.3 mg L-1) in natural lake water.

Proton transfer kinetics of transition metal hydride complexes and implications for fuel-forming reactions.

Proton transfer reactions involving transition metal hydride complexes are prevalent in a number of catalytic fuel-forming reactions, where the proton transfer kinetics to or from the metal center can have significant impacts on the efficiency, selectivity, and stability associated with the catalytic cycle. This review correlates the often slow proton transfer rate constants of transition metal hydride complexes to their electronic and structural descriptors and provides perspective on how to exploit these parameters to control proton transfer kinetics to and from the metal center. A toolbox of techniques for experimental determination of proton transfer rate constants is discussed, and case studies where proton transfer rate constant determination informs fuel-forming reactions are highlighted. Opportunities for extending proton transfer kinetic measurements to additional systems are presented, and the importance of synergizing the thermodynamics and kinetics of proton transfer involving transition metal hydride complexes is emphasized.

Boosting electrochemical nitrate-ammonia conversion via organic ligands-tuned proton transfer

Electrochemical nitrate (NO3-) reduction reaction (NO3-RR) offers an ideal route to harvest ammonia (NH3) under ambient conditions. Despite recent advances in Cu-based NO3-RR electrocatalysts, their synthesis heavily relies on the regulation of adsorption strength towards nitrogen-containing intermediates, and other important factors are ignored (i.e., the proton transfer rate). Here, we select Cu nanoparticles (NPs) as model catalysts to investigate whether and how the proton transfer rate impacts the NO3-RR kinetics. The results indicate that the proton transfer is involved in the rate-determining step (RDS) of NO3-RR, and the weak water dissociation ability of Cu leads to slow proton transfer rate and consequently sluggish NO3-RR kinetics. To this end, we enhance the water dissociation ability of Cu NPs by incorporating uncoordinated carboxylate ligands to enable rapid proton transfer, which in turn boosts the hydrogenation of key intermediates for reducing the overall energy barrier of NO3-RR. As a result, Cu NPs with the ligands display a maximum NH3 yield rate of 496.4 mmol h−1 gcat−1, outperforming counterpart without ligands. This work not only deepens our knowledge on the NO3-RR mechanism, but also offers new guidelines for the smart design of efficient electrocatalysts.

Kinetic Regularities of Slow Proton Transfer from β-Substituted Porphyrazines to Organic Bases

The regularities of intermolecular proton transfer from β-substituted porphyrazines to dimethylsulfoxide, cyclic and acyclic nitrogen-containing bases in inert solvents are considered. We found that the process rates are unusually low. We showed that the acidic properties of a porphyrazine macrocycle, the proton-acceptor ability of a base, and the dielectric constant of the environment influence the kinetic parameters of acid–base interaction. The structure and the stability of proton transfer complexes of porphyrazines are discussed.

Boosting Electrochemical Nitrate-Ammonia Conversion Via Organic Ligands-Tuned Proton Transfer

Electrochemical nitrate (NO3-) reduction reaction (NO3-RR) offers an ideal route to harvest ammonia (NH3) under ambient conditions. Despite recent advances in Cu-based NO3-RR electrocatalysts, their synthesis heavily relies on the regulation of adsorption strength towards nitrogen-containing intermediates, and other important factors are ignored (i.e., the proton transfer rate). Here, we select Cu nanoparticles (NPs) as model catalysts to investigate whether and how the proton transfer rate impacts the NO3-RR kinetics. The results indicate that the proton transfer is involved in the rate-determining step (RDS) of NO3-RR, and the weak water dissociation ability of Cu leads to slow proton transfer rate and consequently sluggish NO3-RR kinetics. To this end, we enhance the water dissociation ability of Cu NPs by incorporating uncoordinated carboxylate ligands to enable rapid proton transfer, which in turn boosts the hydrogenation of key intermediates for reducing the overall energy barrier of NO3-RR. As a result, Cu NPs with the ligands display a maximum NH3 yield rate of 496.4 mmol h-1 gcat-1, outperforming counterpart without ligands. This work not only deepens our knowledge on the NO3-RR mechanism, but also offers new guidelines for the smart design of efficient electrocatalysts.

Evaluation of the Synthetic Scope and the Reaction Pathways of Proton-Coupled Electron Transfer with Redox-Active Guanidines in C-H Activation Processes.

Proton‐coupled electron transfer (PCET) is currently intensively studied because of its importance in synthetic chemistry and biology. In recent years it was shown that redox‐active guanidines are capable PCET reagents for the selective oxidation of organic molecules. In this work, the scope of their PCET reactivity regarding reactions that involve C−H activation is explored and kinetic studies carried out to disclose the reaction mechanisms. Organic molecules with potential up to 1.2 V vs. ferrocenium/ferrocene are efficiently oxidized. Reactions are initiated by electron transfer, followed by slow proton transfer from an electron‐transfer equilibrium.

An Enolate‐Structure‐Enabled Anionic Cascade Cyclization Reaction: Easy Access to Complex Scaffolds with Contiguous Six‐, Five‐, and Four‐Membered Rings

AbstractCatalyst‐free addition of ketone enolate to non‐activated multiple C−C bonds involves non‐complementary reaction partners and typically requires super‐basic conditions. On the other hand, highly aggregated or solvated enolates are not reactive enough to undergo direct addition to alkenes or alkynes. Herein, we report a new anionic cascade reaction for one‐step assembly of intriguing molecular scaffolds possessing contiguous six‐, five‐, and four‐membered rings, representing a formal [2+2] enol–allene cycloaddition. Reaction proceeds under very mild conditions and with excellent diastereoselectivity. Deeper mechanistic and computational studies revealed unusually slow proton transfer phenomenon in cyclic ketone intermediate and explained peculiar stereochemical outcome.

An Enolate-Structure-Enabled Anionic Cascade Cyclization Reaction: Easy Access to Complex Scaffolds with Contiguous Six-, Five-, and Four-Membered Rings.

Catalyst-free addition of ketone enolate to non-activated multiple C-C bonds involves non-complementary reaction partners and typically requires super-basic conditions. On the other hand, highly aggregated or solvated enolates are not reactive enough to undergo direct addition to alkenes or alkynes. Herein, we report a new anionic cascade reaction for one-step assembly of intriguing molecular scaffolds possessing contiguous six-, five-, and four-membered rings, representing a formal [2+2] enol-allene cycloaddition. Reaction proceeds under very mild conditions and with excellent diastereoselectivity. Deeper mechanistic and computational studies revealed unusually slow proton transfer phenomenon in cyclic ketone intermediate and explained peculiar stereochemical outcome.

Slow Proton Transfer in Nanoconfined Water.

The transport of protons in nanoconfined environments, such as in nanochannels of biological or artificial proton conductive membranes, is essential to chemistry, biology, and nanotechnology. In water, proton diffusion occurs by hopping of protons between water molecules. This process involves the rearrangement of many hydrogen bonds and as such can be strongly affected by nanoconfinement. We study the vibrational and structural dynamics of hydrated protons in water nanodroplets stabilized by a cationic surfactant using polarization-resolved femtosecond infrared transient absorption spectroscopy. We determine the time scale of proton hopping in the center of the water nanodroplets from the dynamics of the anisotropy of the transient absorption signals. We find that in small nanodroplets with a diameter <4 nm, proton hopping is more than 10 times slower than in bulk water. Even in relatively large nanodroplets with a diameter of ∼7 nm, we find that the rate of proton hopping is slowed by ∼4 times compared with bulk water.

Deprotection of N-Boc Groups under Continuous-Flow High-Temperature Conditions.

The scope of thermolytic, N-Boc deprotection was studied on 26 compounds from the Pfizer compound library, representing a diverse set of structural moieties. Among these compounds, 12 substrates resulted in clean (≥95% product) deprotection, and an additional three compounds gave ≥90% product. The thermal de-Boc conditions were found to be compatible with a large number of functional groups. A combination of computational modeling, statistical analysis, and kinetic model fitting was used to support an initial, slow, and concerted proton transfer with release of isobutylene, followed by a rapid decarboxylation. A strong correlation was found to exist between the electrophilicity of the N-Boc carbonyl group and the reaction rate.

The effect of protic solvents on the excited state proton transfer of 3-hydroxyflavone: A TD-DFT static and molecular dynamics study

The effect of intermolecular hydrogen bonding played by protic solvents (ammonia, methanol and water) on the excited state proton transfer (ESPT) reaction of 3-hydroxyflavone (3HF) was theoretically investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT). The formation of intermolecular hydrogen bond induced by protic solvents indicates that the intramolecular hydrogen bond may be interrupted in favor of a complex causing low quantum yield of keto emission and exhibiting dual emission (both enol and keto) in experiment. The strengthening of intermolecular hydrogen bond in the S1 state has been confirmed by the red-shift of IR vibrational spectra and shorter bond distances involving proton transfer (PT) process in comparison with those of the S0 state. From potential energy curves (PECs) of PT coordinate, PT process is likely to proceed in S1 state and PT in 3HF(NH3) occurs more easily than those of 3HF(CH3OH) and 3HF(H2O) due to its lower barrier. Moreover, on-the-fly dynamics simulations of all complexes were carried out to provide the detailed information on the PT mechanism. The dynamic results show that ESPT process of 3HF with protic solvent takes place through intermolecular hydrogen bond with slower PT time (259, 117 and 104fs for 3HF(NH3), 3HF(CH3OH) and 3HF(H2O), respectively) than that of 3HF (76fs) via intramolecular hydrogen bond. Furthermore, the ultrafast PT time is found to be nicely correlated with polarity of solvent and PT probability is also anti-correlated with PT barrier.

Excited-State Proton Transfer on the Surface of a Therapeutic Protein, Protamine.

Proton transfer reactions on biosurfaces play an important role in a myriad of biological processes. Herein, the excited-state proton transfer reaction of 8-hydroxypyrene-1,3,6-trisulfonate (HPTS) has been investigated in the presence of an important therapeutic protein, Protamine (PrS), using ground-state absorption, steady-state, and detailed time-resolved emission measurements. HPTS forms a 1:1 complex with Protamine with a high association constant of 2.6 × 104 M-1. The binding of HPTS with Protamine leads to a significant modulation in the ground-state prototropic equilibrium causing a downward shift of 1.1 unit in the acidity constant (pKa). In contrast to a large number of reports of slow proton transfer of HPTS on biosurfaces, interestingly, HPTS registers a faster proton transfer event in the presence of Protamine as compared to that of even the bulk aqueous buffer medium. Furthermore, the dimensionality of the proton diffusion process is also significantly reduced on the surface of Protamine that is in contrast to the behavior of HPTS in the bulk aqueous buffer medium, where the proton diffusion process is three-dimensional. The effect of ionic strength on the binding of HPTS toward PrS suggests a predominant role of electrostatic interaction between anionic HPTS and cationic Protamine, which is further supported by molecular docking simulations which predict that the most preferable binding site for HPTS on the surface of Protamine is surrounded by multiple cationic arginine residues.

Fast and slow excited-state intramolecular proton transfer in 3-hydroxychromone: a two-state story?

The photodynamics of 3-hydroxychromone in its first-excited singlet electronic state (bright state of ππ* character) is investigated with special emphasis given to two types of reaction pathways: the excited-state intramolecular-proton-transfer coordinate and the hydrogen-torsion coordinate linking the excited cis and trans isomers. A newly-found conical intersection with the second-excited singlet electronic state (dark state of nπ* character) is suspected to be, to some extent, the reason for the slower rate constant. This hypothesis based on quantum-chemistry calculations is supported by quantum-dynamics simulations in full dimensionality. They show significant transfer of electronic population and provide consistently a vibronic interpretation for the forbidden band in the UV absorption spectrum.

Proton-Coupled Electron Transfer in a Hydrogen-Bonded Charge-Transfer Complex.

A proton-coupled electron transfer (PCET) reaction in a hydrogen-bonded charge-transfer (CT) complex of 4-([2,2'-bipyridin]-4-yl)phenol (bpy-phenol) with a F- ion has been investigated by ultrafast time-resolved transient absorption spectroscopy. The phenolic receptor molecule, bpy-phenol, binds to the F- ion through a hydrogen bond and senses the F- ion via the Stokes-shifted CT band. Upon photoexcitation, CT from the phenol residue to the bpy residue promotes proton transfer from the phenol radical cation (ArOH•+) to the fluoride ion at ultrafast time scales of <150 fs (instrument response function limited) and 3 ps, separately. The fast and slow proton-transfer times are linked to two different types of hydrogen-bonding networks between the phenol residue and fluoride ion. Crystalline water in the fluoride salt hydrates mediates the proton-transfer reaction. This work demonstrates the participation of a hydrogen-bonded water bridge within a PCET reaction in a water-restricted environment.

Environment-sensitive quinolone demonstrating long-lived fluorescence and unusually slow excited-state intramolecular proton transfer kinetics

A new small fluorescent dye based on 3-hydroxybenzo[g]quinolone, a benzo-analogue of Pseudomonas quinolone signal species, has been synthesized. The dye demonstrates interesting optical properties, with absorption in the visible region, two band emission due to an excited-state intramolecular proton transfer (ESIPT) reaction and high fluorescence quantum yield in both protic and aprotic media. Time-resolved fluorescence spectroscopy shows that the ESIPT reaction time is unusually long (up to 8 ns), indicating that both forward and backward ESIPT reactions are very slow in comparison to other 3-hydroxyquinolones. In spite of these slow rate constants, the ESIPT reaction was found to show a reversible character as a result of the very long lifetimes of both N* and T* forms (up to 16 ns). The ESIPT reaction rate is mainly controlled by the hydrogen bond donor ability in protic solvents and the polarity in aprotic solvents. Using large unilamellar vesicles and giant unilamellar vesicles of different lipid compositions, the probe was shown to preferentially label liquid disordered phases.

Unusually slow intramolecular proton transfer dynamics of 4'-N,N-dimethylamino-3-hydroxyflavone in high n-alcohols: involvement of solvent relaxation.

Excited state intramolecular proton transfer (ESIPT) time-constants of 4'-N,N-dimethylamino-3-hydroxyflavone (DMA3HF) in high n-alcohols--1-butanol, 1-hexanol and 1-decanol--were measured to be 90 ps, 130 ps and 190 ps, respectively, which are unusually slow. At the same time, the solvation time-constants of the DMA3HF enol in the same set of solvents were measured as 100 ps, 150 ps and >300 ps, respectively. Thus, both the ESIPT and enol solvation time-constants in high n-alcohols increase monotonically with the alkyl chain-length of the solvent, although the increase is not strictly proportional. It appears that the H-bonding capacity of the solvent is the single major factor influencing both processes, causing them to become closely correlated. Solvation causes a drastic change in the solvent molecular configuration around the excited enol, E*, inducing the breakage of DMA3HF···solvent inter-molecular H-bonding, which in turn promotes ESIPT. Following previously reported theoretical work on ESIPT, a qualitative description of the S1 potential energy surface can be formulated, where the involvement of solvent relaxation with the ESIPT process is explained.

Configurationally-Coupled Protonation of Polyproline-7.

Structure and dynamics regulate protein function, but much less is known about how biomolecule-solvent interactions affect the structure-function relationship. Even less is known about the thermodynamics of biomolecule-solvent interactions and how such interactions influence conformational entropy. When transferred from propanol into 40:60 propanol:water under acidic conditions, a remarkably slow protonation reaction coupled with the conversion of the polyproline-I helix (PPI, having all cis-configured peptide bonds) into polyproline-II (PPII, all trans) helix is observed in this work. Kinetics and equilibrium measurements as a function of temperature allow determination of the thermochemistry and insight into how proton transfer is regulated in this system. For the proton-transfer process, PPI(+)(PrOH) + H3O(+) → PPII(2+)(PrOH/aq) + H2O, we determine ΔG = -20 ± 19 kJ·mol(-1), ΔH = -75 ± 14 kJ·mol(-1), and ΔS= -188 ± 48 J·mol(-1)·K(-1) for the overall reaction, and values of ΔG(⧧) = 91 ± 3 kJ·mol(-1), ΔH(⧧) = 84 ± 9 kJ·mol(-1), and ΔS(⧧) = -23 ± 31 J·mol(-1)·K(-1) for the transition state. For a minor process, PPI(+)(PrOH) → PPII(+)(PrOH/aq) without protonation, we determine ΔG = -9 ± 20 kJ·mol(-1), ΔH = 64 ± 14 kJ·mol(-1), and ΔS= 247 ± 50 J·mol(-1)·K(-1). This thermochemistry yields ΔG = -10 ± 29 kJ·mol(-1), ΔH = -139 ± 20 kJ·mol(-1), and ΔS= -435 ± 70 J·mol(-1)·K(-1) for PPII(+)(PrOH/aq) + H3O(+) → PPII(2+)(PrOH/aq) +H2O. The extraordinarily slow proton transfer appears to be an outcome of configurational coupling through a PPI-like transition state.

Slow proton transfer to coordinated carboxylates: studies on [Ni(O2CR){PhP(CH2CH2PPh2)2}]+ (R = Et or Ph)

The reactions between [Ni(O2CR)(triphos)]+ (R = Et or Ph, triphos = PhP(CH2CH2PPh2)2) and mixtures of lutH+ and lut (lut = 2,6-dimethylpyridine) have been studied in MeCN at 25.0 °C using stopped-flow spectrophotometry. The kinetics and spectroscopic changes indicate an equilibrium reaction, presumably involving protonation of an oxygen site (the only sites on the complex containing lone pairs of electrons). Proton transfer is slow and comparison of the kinetic data shows that the rates are insensitive to the R substituent. Using the kinetic data, the pKas of [Ni(HO2CR)(triphos)]2+ (pKa = 14.5) have been calculated showing that when coordinated to the {Ni(triphos)}2+ site, RCO2H is about 8 pKa units more acidic than the free acid. Comparison of the kinetic results on the reactions of [Ni(O2CR)(triphos)]+ with mixtures of lutH+ and lut and those of the analogous [Ni(S2CR)(triphos)]+ show that protonation at oxygen is at least 7.6 × 103 times faster than to sulfur, and the coordinated carboxylic acid is ca. 8 pKa units less acidic than the corresponding coordinated carboxydithioic acid.