Promote Proton Transfer Research Articles

Metal/Oxide Heterojunction Boosts Fuel Cell Cathode Reaction at Low Temperatures

AbstractAmong several types of low‐temperature solid oxide fuel cells, hydrogen‐permeable metal‐supported fuel cells (HMFCs) are devices that can achieve outputs of approximately 1.0 W cm−2 at 400 °C. This work clarifies the mechanism for promoting the cathode reaction on proton‐conducting ceramics at such low temperatures. Combined numerical and electrochemical analyses demonstrate that blocking minor oxide ion conduction at metal/oxide heterojunctions promotes proton transfer at the cathode/electrolyte interfaces, thereby enhancing the turnover frequency of the cathode reaction at the triple‐phase boundary. The electrolyte membrane in HMFCs is forced to gain extra protons to compensate for the charge of oxide ions that accumulate because of the blocking, resulting in an increment of the proton concentration gradients near the cathode/electrolyte interfaces so as to eject the excess amount of proton. The interfacial proton concentration gradient increases and thus the cathode polarization resistance of HMFCs decrease with the cell bias. An HMFC with a highly oxygen‐deficient BaZr0.5Sc0.5O3−δ electrolyte accumulates a large amount of oxide ions, thereby developing large concentration gradients. Thus, it achieves a cathode reaction resistance of 0.54 Ω cm2 at 400 °C with conventional cathode materials, La0.6Sr0.4Co0.2Fe0.8O3−δ. These findings demonstrate that HMFCs can efficiently utilize overpotential.

Surface Sulfate Ion on CdS Catalyst Enhances Syngas Generation from Biopolyols.

Photocatalytic biomass conversion represents an ideal way of generating syngas because of the sustainable use of biomass carbon and solar energy. However, the lack of efficient electron-proton transfer limits its efficiency. We here report an unprecedented method to simultaneously increase both the electron and proton transfer by creating surface sulfate ions on the CdS catalyst ([SO4]/CdS). Surface sulfate ion [SO4] is bifunctional, serving as the proton acceptor to promote proton transfer, and increasing the oxidation potential of the valence band to enhance electron transfer. [SO4]/CdS produces a syngas mixture from glycerol without CO2. Compared with pristine CdS, [SO4]/CdS exhibits 9-fold higher CO generation rate (0.31 mmol g-1 h-1) and 4-fold higher H2 generation (0.05 mmol g-1 h-1). A wide range of sugars, such as glucose, fructose, maltose, sucrose, xylose, lactose, insulin, and starch, were facilely converted into syngas. This study reports the pivotal effect of surface sulfate ion on electron-proton transfer in photocatalysis and provides a facile method for increasing photocatalytic efficiency.

Barrierless HONO and HOS(O)2-NO2 Formation via NH3-Promoted Oxidation of SO2 by NO2.

In the troposphere, the knowledge about nitrous acid (HONO) sources is incomplete. The missing source of sulfate and fine particles cannot be explained during haze events. Air quality models cannot predict high levels of secondary fine-particle pollution. Despite extensive studies, one challenging issue in atmospheric chemistry is identifying the source of HONO. Here, we present direct ab initio molecular dynamics simulation evidence and typical air pollution events of the formation of gaseous HONO, nitrogen dioxide/hydrogen sulfite (HOS(O)2-NO2 or NO2-HSO3) from nitrogen dioxide (NO2), sulfur dioxide (SO2), water (H2O), and ammonia (NH3) molecules in a proportion of 2:1:3:3. The reactions show a new mechanism for the formation of HONO and NO2-HSO3 in the troposphere, especially when the concentration of NO2, SO2, H2O, and NH3 is high (e.g., 2:1:3:3 or higher) in the air. Contrary to the proportion NO2, SO2, H2O, and NH3 equaling to 1:1:3:1 and 1:1:3:2, the proportion (2:1:3:3) enables barrierless reactions and weak interactions between molecules via the formation of HONO, NO2-HSO3, and NH3/H2O. In addition, field observations are carried out, and the measured data are summarized. Correlation analysis supported the conversion of NO2 to HONO during observational studies. The weak interactions promote proton transfer, resulting in the generation of HONO, NO2-HSO3, and NH3/H2O pairs.

Improved hydrodeoxygenation of bio-oil model compounds with polymethylhydrosiloxane by Brønsted acidic zeolites

Bio-oil, one of significant renewable energy, was blocked from its direct application by thermodynamic instability due to the high oxygen content, so its deoxygenation needs urgent solution. Avoiding the disadvantages of the traditional catalytic method, such as harsh reaction conditions, potential explosion risk and environmental pollution, we utilize solid-acidic zeolites as low-load Pd-based catalyst supports instead of corrosive acid additives, with polymethylhydrosiloxane (PMHS) as the hydrogen-supplying agent, to achieve efficient hydrodeoxygenation of bio-oil models (carbonyl compounds) under mild conditions. The reaction conditions such as Si/Al ratio of HZSM-5 zeolite, temperature, solvent, and the type of Pd salts precursor are optimized. In particular, we have found that polar protic solvents improve catalytic efficiency by promoting proton transfer in the reaction. In an open-to-air, 97.9% ethylbenzene yield can be obtained for acetophenone conversion under mild conditions (0.5 wt% Pd/HZSM-5(18), 65 °C, 3 h, n-butanol as solvent), which is more efficient and environmental friendly than currently reports. Meanwhile, hydrogenation–dehydration mechanism was proposed, and the Brønsted acid in HZSM-5 promotes the dehydration of the alcohol (rate-limiting step) by efficiently accelerating the removal of hydroxyl groups and the proton transfer of the reaction. Furthermore, the catalytic scheme exhibits the excellent stability (reusable seven times) and versatility. The potential of a green catalytic technology using with PMHS opens attractive opportunities for bio-oil upgrading.

Contributions of alanine and serine to sulfuric acid-based homogeneous nucleation

New particle formation (NPF) is the main source of atmospheric aerosols, and amino acids have been detected as the important components of atmospheric particulate matter. However, the role played by amino acids with different functional groups in the initial events of nucleation remains unclear. In this study, the interactions of alanine (Ala) and serine (Ser), which differ structurally by one hydroxyl group, with sulfuric acid were studied at the M06-2X/6-311 + G (d, p) theory level. Structural and thermochemical analysis results show that in small clusters, the introduction of the Ser's hydroxyl groups leads to the synergistic effect, which promotes proton transfer and improves the stability of the clusters. With the increase of cluster's size, the synergistic effect gradually reduces, the introduction of hydroxyl groups hinders the formation of hydrogen bonds between amino and carboxyl groups of Ser molecules with sulfuric acid molecules, resulting in the steric effect, reducing the stability of the clusters. Atmospheric relevance analysis results show that the formation rates of the clusters are positively correlated with the concentrations of sulfuric acid and amino acid, but negatively correlated with temperature. The temperature dependence of (SA)m (Ser)m is lower than that of (SA)n (Ala)n (n, m = 1–2, n = m), indicating for small clusters, the synergistic effect improves the stability of the clusters and plays an important role in the formation rates. The formation rates of (SA)m (Ser)m are slightly lower than that of (SA)n (Ala)n (n, m = 3–4), indicating for large clusters, the steric effect reduces the stability of the clusters and the concentrations of amino acid and sulfuric acid, temperature have greater influence on the formation rates.

Performance Evaluation of Intermediate Temperature Proton-Conducting Soec with BaZr0.6Ce0.2Y0.2O3-δ Electrolyte

A solid oxide steam electrolysis cell (SOEC) has attracted much attention for the production of a clean H2 from water by using renewable electricity. Activation energy of proton conduction in proton conducting perovskite oxides BaZr x Ce1-x-y Y y O3-δ (BZCY) is much lower than that of yttria stabilized zirconia which is practically used for the high temperature SOECs [1]. Hence the cells using the proton conductors (H+-SOEC) are expected to operate at lower temperatures than that of YSZ-base SOEC. In general, the solid solutions, i.e. BaCe1−x−y Zr x M y O3-δ (M = Y, Yb etc.) tend to be more tolerant to CO2 and steam with increasing Zr contents [2], and thus the Zr-rich side BaCe1−x−y Zr x M y O3-δ are recently attractive as a practical electrolyte for H+-SOECs. However, the poor sinterability and relatively large grain-boundary resistances retards the application of Zr-rich phase to device application.In this study, we successfully fabricated the thin film electrolysis cells using Zr-rich side BaZr0.6Ce0.2Y0.2O3−δ (BZCY622) electrolytes. The cathode-supported cells were fabricated by a solid-state reactive sintering method assisted by zinc nitrate sintering additives. The cell comprises of a porous Ni-BZCY622 cermet cathode support, dense electrolyte film with a 20 mm thickness and La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) porous anode. The cells with a 15 nm-thick interlayer of La0.5Sr0.5CoO3- δ (LSC) was also fabricated by sputtering deposition of LSC nanofilms on the surface of the electrolyte films before printing LSCF anodes. The cell with and without LSC yielded electrolysis currents of 420 and 200 mA cm-2, respectively, at 1.5 V cell bias at 600 ºC. EIS measurements confirmed that both polarization and ohmic resistances decreased with a use of LSC interlayer, which implies that LSC layer promotes proton transfer at the anode/electrolyte interface and thus reduce the proton transfer resistance and promote anode reactions. Y. Guo, R. Ran and Z. Shao, Int. J. Hydrogen Energy, 35, 5611, (2010). P. Sawant, S. Varma, B. N. Wani and S. R. Bharadwaj, Int. J. Hydrogen Energy, 37, 3848, (2012).

Network-Selectivity, Magnetism, and Proton Conduction of 2-D and 3-D Metal-Organic Frameworks of the Constituents {P(CH2OH)4}+/MII (MnII, FeII, or CoII)/[CrIII(ox)3]3.

Two types of metal-organic frameworks (MOFs), including {P(CH2OH)4}6[MnII3{CrIII(ox)3}4]·10H2O·2EtOH (1) and {P(CH2OH)4}[MIICrIII(ox)3]·H2O (M = Fe (2), and Co (3)), are selectively formed by a reaction of {P(CH2OH)4}Br, MCl2, and (NH4)3[Cr(ox)3]. 1 has the 3-D anionic framework, [Mn3{Cr(ox)3}4]n6n-, consisting of the Mn4Cr4 octagonal and Mn6Cr6 dodecagonal channels, in which {P(CH2OH)4}+ ions and the solvation molecules (H2O and EtOH) are accommodated. 2 and 3 have a 2-D network of oxalate-bridged bimetallic sheets with an intercalation of {P(CH2OH)4}+ ions. Magnetic studies revealed ferromagnetic ordering in all the MOFs, with TC = 5.6 K for 1, 10.6 K for 2, and 12.2 K for 3. The {P(CH2OH)4}+ ions in the MOFs promote proton transfer via the vehicle mechanism, which gives rise to a significant room-temperature conductivity of 1.03 × 10-4 S cm-1 at 70% RH for 1, 4.60 × 10-6 S cm-1 at 60% RH for 2, and 3.04 × 10-5 S cm-1 at 70% RH for 3.

Atmospheric implications of hydration on the formation of methanesulfonic acid and methylamine clusters: A theoretical study.

The effect of hydration on the formation mechanism of clusters consisting of methanesulfonic acid (MSA) and methylamine (MA) is investigated by quantum chemistry (Density Functional Theory, DFT) and kinetics simulation (Atmospheric Chemical Dynamic Code, ACDC) methods. The results showed that the process of hydration is favorable from the thermodynamic point of view, and the presence of water molecules can promote proton transfer significantly. Although MA has a significant influence on the formation rate of MSA-based clusters at the parts per trillion (ppt) levels, the effective nucleation of MSA-MA anhydrous clusters hardly seems to occur under common typical atmospheric conditions. The high concentrations of precursors ([MSA]>6×107 molecules·cm-3 and [MA]>1ppt or [MSA]>1×106 molecules·cm-3 and [MA]>100ppt) is necessary for the effective nucleation of the MSA-MA system. The formation rate of the MSA-MA system is enhanced significantly by hydration. The formation rate increases with the relative humidity (RH) and reached up to a factor of 2700at RH=40%. The formation mechanism of the hydrous system is different from the anhydrous system. The formation of (MSA)2 and (MSA)(MA) dimers is the rate-determining step of the anhydrous and hydrous systems, respectively. In addition, the growth pathway of clusters was complicated by low temperature and simplified by high humidity, respectively. In general, although humidity is a very favorable factor for the formation of the MSA-MA system, the involvement of other species (such as sulfuric acid) may be more effective to promote the nucleation of the MSA-MA system under typical atmospheric environment.

Formation of Oligo-Nuclear Carboxylate Nickel(II) Complexes with Nitrogen-Containing Ligands. Quantum-Chemical Simulation

Using the DFT method PBE0/6-31G(d, p) and taking into account the solvent, the formation of bi- and pentahedral Ni(II) complexes with carboxylate and N-containing ligands in the gas phase was studied, and ΔG° of their formation from simple compounds was estimated. The functional role of bridging groups and hydrogen bonds in the formation of polynuclear structures was revealed. A model of self-organization of penta-nuclear coordination compounds [NiII5(O2CR)8L2n(μ-OH)2]0 from binuclear complex [NiII2(O2CR)4Ln(μ-H2O)]0 was proposed, into the structure of which mononuclear NiII cations are embedding and promote proton transfer from bridging aqua ligands.

CO2 Capture via Crystalline Hydrogen-Bonded Bicarbonate Dimers

Summary Limiting global temperature rises is increasingly dependent on the development of energy-efficient carbon-capture methods. Here, we report a simple CO2-separation cycle using an aqueous bis(iminoguanidine) (BIG) sorbent that reacts with CO2 and crystallizes into an insoluble bicarbonate salt. X-ray diffraction analysis of the bicarbonate crystals revealed “anti-electrostatic” hydrogen-bonded (HCO3−)2 dimers, stabilized by guanidinium cations and water. Mild heating of the crystals releases the CO2 and regenerates the BIG sorbent quantitatively, thereby closing the CO2-separation cycle. Experimental and computational investigations support a CO2-release mechanism consisting of surface-initiated low-barrier proton transfer from guanidinium groups to bicarbonate anions with the formation of carbonic acid dimers, followed by CO2 and H2O release in the rate-limiting step, with a measured activation energy of 102 ± 12 kJ/mol. The minimum energy required for sorbent regeneration is 151.5 kJ/mol CO2, which is 24% lower than the regeneration energy of monoethanolamine, a benchmark industrial sorbent.

Two possible mechanisms of water-assisted proton transfer in the stack of adenine-thymine: Displacement polarization and oriented polarization

Proton transfer in DNA has attracted much attention because of its close relationship with charge transfer and DNA base damage. In the present study, the relationship between proton transfer and water polarization at a certain temperature in the radical cation stack of adenine-thymine is investigated by QM/MM molecular dynamics simulations. By analyzing the polarization of aqueous solution, it is found that at low temperature water molecules promote proton transfer by means of displacement polarization and oriented polarization, while at high temperature, water molecules promote proton transfer only by displacement polarization.

Membrane electrode assembly based on buckypaper with gradient distribution of platinum, proton conductor and electrode porosity

A three-dimensional catalyst layer (CL) with gradient distribution of key components based on double-layered buckypaper (BPCL) as cathode for proton exchange membrane fuel cells (PEMFCs) was fabricated. This novel structured BPCL was prepared by using multi-walled carbon nanotubes (MWCNTs) and carbon nanofibers (CNFs) as main framework structure and Pt/C catalyst as active sites. The gradient distribution of platinum (Pt) nanoparticles, proton conductor (Nafion ionomer) and electrode porosity along the thickness of CL were established to reduce Pt loading and facilitate the transportation of protons, reactants and products. Two critical parameters for fabricating membrane electrode assembly (MEA), Nafion content which influences the proton transfer process in CL and hot-pressing temperature which affects the combination situation of CL with electrolyte membrane, were further optimized to promote proton transfer efficiency and decrease the contact resistance between CL and membrane. By using this unique structure, a maximum power density of 536.8 mW cm−2 was achieved at cell temperature of 80 °C, inlet gas pressure of 100 kPa and relative humidity of 80% with an ultra-low Pt loading of 0.09 mgPt cm−2 in cathode. The generated cathodic Pt utilization is 0.168 gPt kW−1, reducing by 20.1% compared with the MEA based on traditional catalyst coated on membrane (CCM) process (0.21 gPt kW−1).

Enhancing the catalytic activity of a novel GH5 cellulase GtCel5 from Gloeophyllum trabeum CBS 900.73 by site-directed mutagenesis on loop 6

BackgroundCellulases of glycosyl hydrolase (GH) family 5 share a (β/α)8 TIM-barrel fold structure with eight βα loops surrounding the catalytic pocket. These loops exposed on the surface play a vital role in protein functions, primarily due to the interactions of some key amino acids with solvent and ligand molecules. It has been reported that motions of these loops facilitate substrate access and product release, and loops 6 and 7 located at the substrate entrance of the binding pocket promote proton transfer reaction at the catalytic site motions. However, the role of these flexible loops in catalysis of GH5 cellulase remains to be explored.ResultsIn the present study, an acidic, mesophilic GH5 cellulase (with optimal activity at pH 4.0 and 70 °C), GtCel5, was identified in Gloeophyllum trabeum CBS 900.73. The specific activities of GtCel5 toward CMC-Na, barley β-glucan, and lichenan were 1117 ± 43, 6257 ± 26 and 5318 ± 54 U/mg, respectively. Multiple sequence alignment indicates that one amino acid residue at position 233 on the loop 6 shows semi-conservativeness and might contribute to the great catalytic performance. Saturation mutagenesis at position 233 was then conducted to reveal the vital roles of this position in enzyme properties. In comparison to the wild type, variants N233A and N233G showed decreased optimal temperature (− 10 °C) but increased activities (27 and 70%) and catalytic efficiencies (kcat/Km; 45 and 52%), respectively. The similar roles of position 233 in catalytic performance were also verified in the other two GH5 homologs, TeEgl5A and PoCel5, by reverse mutation. Further molecular dynamics simulations suggested that the substitution of asparagine with alanine or glycine may introduce more hydrogen bonds, increase the flexibility of loop 6, enhance the interactions between enzyme and substrate, and thus improve the substrate affinity and catalytic efficiency.ConclusionThis study proposed a novel cellulase with potentials for industrial application. A specific position was identified to play key roles in cellulase–substrate interactions and enzyme catalysis. It is of great importance for understanding the binding mechanism of GH5 cellulases, and provides an effective strategy to improve the catalytic performance of cellulases.

Special feature of kinetics of ZcE isomerization of β-N-methylaminovinyl trifluoromethyl ketone in Ar matrix exposed to UV radiation and spontaneous E ⇌ Z isomerization of α-methyl-β-N-methylaminovinyl trifluoromethyl ketone

Although it is well known that reactivity of α,β-unsaturated enaminoketones is closely associated with spatial and electronic structure but until now little attention was devoted to quantitative investigation of interconversion of different stereoisomeric forms of enaminoketones. In present work we studied peculiarities of kinetics of Z⇌E isomerization of enaminoketone 4-(N-methylamino)-1,1,1-trifluorobut-3-en-2-one F3C-COCHCHNH(CH3) (1) in Ar-matrix exposed to UV-radiation (λ=340nm) with IR Fourier and 2D correlation spectroscopy and we found that Z-s-Z-s-trans isomer transforms primarily into two E-isomers, E-s-E-s-trans and E-s-Z-s-trans which further turn into the E-s-E-s-cis and E-s-Z-s-cis conformers all interconversion rate constants being comparable in magnitude. Along with this process long-term exposure to the UV-radiation results in proton transfer from nitrogen of methylamino group to carbonyl oxygen with simultaneous isomerization of ‘cyclic’ iminoenol form into ‘linear‘one. In solution of enaminoketone 4-(N-methylamino)-1,1,1-trifluoro-3-methylbut-3-en-2-one F3C-CO-C(CH3)CH-NH(CH3) (2) we observed reversed process, namely, spontaneous interconversion of the E-s-E-s-trans and E-s-Z-s-trans conformers into the Z-s-Z-trans isomer. It was found that rate constants of the dimeric forms of the E-s-E-s-trans and E-s-Z-s-trans conformers are higher than those of the monomers and are independent on total enaminoketone concentration. Addition of highly polar HMPA promotes proton transfer from nitrogen to oxygen in the Z-s-Z-s-trans isomer of 2 with subsequent isomerization into the linear imino-enol product but the rate constant of this transformation is ten-fold smaller than that for 1 in the Ar matrix exposed to UV radiation.

Enantioselective Tandem Cyclization of Alkyne-Tethered Indoles Using Cooperative Silver(I)/Chiral Phosphoric Acid Catalysis.

Reported is the enantioselective synthesis of tetracyclic indolines using silver(I)/chiral phosphoric acid catalysis. A variety of alkyne-tethered indoles are suitable for this process. Mechanistic studies suggest that the in situ generated silver(I) chiral phosphate activates both the alkyne and the indole nucleophile in the initial cyclization step through an intermolecular hydrogen bond and the phosphate anion promotes proton transfer. In addition, further modifications of the cyclization products enabled stereochemistry-function studies of a series of bioactive indolines.

Enantioselective Tandem Cyclization of Alkyne‐Tethered Indoles Using Cooperative Silver(I)/Chiral Phosphoric Acid Catalysis

AbstractReported is the enantioselective synthesis of tetracyclic indolines using silver(I)/chiral phosphoric acid catalysis. A variety of alkyne‐tethered indoles are suitable for this process. Mechanistic studies suggest that the in situ generated silver(I) chiral phosphate activates both the alkyne and the indole nucleophile in the initial cyclization step through an intermolecular hydrogen bond and the phosphate anion promotes proton transfer. In addition, further modifications of the cyclization products enabled stereochemistry–function studies of a series of bioactive indolines.

Proton conductive Pt-Co nanoparticles anchoring on citric acid functionalized graphene for efficient oxygen reduction reaction

Designing highly efficient electro-catalysts for the oxygen reduction reaction (ORR) has been regarded as a demanding task in the development of renewable energy sources. However, little attention has been paid on improving Pt-based catalysts by promoting proton transfer from the electrolyte solutions to the catalyst layer at the cathode. Herein, we design proton conductive Pt-Co alloy nanoparticles anchoring on citric acid functionalized graphene (Pt-Co/CA-G) catalysts for efficient ORR. The facile modification approach for graphene can introduce oxygenated functional groups on the graphene surface to promote proton transfer as well as keeping the high electron conductivity without destroying the graphene original structure. The electrochemical results show that the Pt-Co/CA-G catalyst exhibits more excellent ORR activity and stability than the commercial Pt/C catalyst, which can be attributed to its improved proton transfer ability. The fast proton transfer comes from the hydrogen-bonding networks formed by the interaction between the oxygenated functional groups and water molecules. This work provides not only a novel and simple approach to modify graphene but also an effective strategy to improve Pt-based catalysts for the ORR.

Construction of well interconnected metal-organic framework structure for effectively promoting proton conductivity of proton exchange membrane

To obtain proton exchange membrane (PEM) with high proton conductivities under both high humidity and anhydrous condition, interconnected UiO-66-NH2 was tethered onto graphene oxide (GO) surfaces, and then incorporated into Nafion matrix. Thanks to tethering effect of GO surfaces and interconnection among MOF grains, well interconnected metal-organic framework (MOF) structures (GO@UiO-66-NH2) were constructed. The structural advantage and mechanism of GO@UiO-66-NH2 in proton conduction were explored. It was found the synergistic effect between GO and UiO-66-NH2 with suitable particle size was especially important for promoting proton transfer. Such interconnected structure of MOF on GO also made acid/base pair pathways between –SO3H of Nafion and –NH2 of GO@UiO-66-NH2 more consecutive. This was favorable to proton conduction via both vehicle mechanism and Grotthuss mechanism. The proton conductivity of the as-prepared composite membrane reached up to 0.303S/cm under 90°C, 95% RH, and 3.403×10−3S/cm under anhydrous condition, which was about 1.57 and 1.88 times higher than that of the recast Nafion (0.118S/cm and 1.182×10−3S/cm), respectively. Furthermore, the composite membrane showed a reduced methanol permeability, which was attributed to the barrier effect of the two-dimensional GO and the trapping of methanol by UiO-66-NH2 pores.

Theoretical Insight into the Mechanisms and Regioselectivity of [4 + 3] and [4 + 1] Annulations of Enals with Azoalkenes Catalyzed by N-Heterocyclic Carbenes.

The reaction mechanisms on the NHC-catalyzed [4 + 3] and [4 + 1] annulations of enals with azoalkenes have been theoretically investigated with the aid of the density functional theory calculations. It is found that the additives play an important role in promoting proton transfer and dehydration. The impacts of catalysts (NHC-A and NHC-B) and substituent groups on the regioselectivity were rationalized. The origin of the regioselectivity involved in these reactions was probed by performing distortion-interaction analysis. For reaction A with NHC-A as the catalyst and Boc group as the substituent, the regioselectivity is predicted to be determined by the interaction energy of 3 with R2' in TS3A versus TS8A. For reaction B with NHC-B as the catalyst and Boc group as the substituent, the unstable boat-type conformation versus the stable chair-type conformation involved in the NHC-B moiety accounts for the regioselectivity. In reaction C with NHC-A as the catalyst and Ts group as the substituent, distortion energies of 3 and R2' in TS3C versus TS6C are found to be the major reason delivering regioselectivity even higher than that of reaction A.

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.