Photocatalytic hydrogen evolution from carbon-neutral oxalate with 2-phenyl-4-(1-naphthyl)quinolinium ion and metal nanoparticles

Photocatalytic hydrogen evolution has been made possible by using oxalate as a carbon-neutral electron source, metal nanoparticles as hydrogen-evolution catalysts and the 2-phenyl-4-(1-naphthyl)quinolinium ion (QuPh(+)-NA), which forms the long-lived electron-transfer state upon photoexcitation, as a photocatalyst. The hydrogen evolution was conducted in a deaerated mixed solution of an aqueous buffer and acetonitrile (MeCN) [1:1 (v/v)] by photoirradiation (λ > 340 nm). The gas evolved during the photocatalytic reaction contained H(2) and CO(2) in a molar ratio of 1:2, indicating that oxalate acts as a two-electron donor. The hydrogen yield based on the amount of oxalate reached more than 80% under pH conditions higher than 6. Ni and Ru nanoparticles as well as Pt nanoparticles act as efficient hydrogen-evolution catalysts in the photocatalytic hydrogen evolution. The photocatalyst for hydrogen evolution can be used several times without significant deactivation of the catalytic activity. Nanosecond laser flash photolysis measurements have revealed that electron transfer from oxalate to the photogenerated QuPh˙-NA˙(+), which forms a π-dimer radical cation with QuPh(+)-NA [(QuPh˙-NA˙(+))(QuPh(+)-NA)], occurs followed by subsequent electron transfer from QuPh˙-NA to the hydrogen-evolution catalyst in the photocatalytic hydrogen evolution. Oxalate acts as an efficient electron source under a wide range of reaction conditions.     

Viologen-modified platinum clusters acting as an efficient catalyst in photocatalytic hydrogen evolution

A highly efficient photocatalytic system for hydrogen evolution with dihydronicotinamide coenzyme (NADH) as a sacrificial agent in an aqueous solution has been constructed by using water-soluble platinum clusters functionalized with methyl viologen-alkanethiol (MVA2+) and a simple electron-donor dyad, 9-mesityl-10-methylacridinium ion (Acr+-Mes), which is capable of fast photoinduced electron transfer but extremely slow back electron transfer. The mean diameter of the platinum core was determined as R(CORE) = 1.9 nm with a standard deviation sigma = 0.5 nm by transmission electron microscopy (TEM). As a result, the hydrogen-evolution rate of the photocatalytic system with MVA2+-modified platinum clusters (MVA2+-PtC) is 10 times faster than the photocatalytic system with the mixture of the same amount of MVA2+ and platinum clusters as that of MVA2+-PtC under otherwise the same experimental conditions. The radical cation of NADH has been successfully detected by laser flash photolysis experiments. The decay of the absorbance due to NAD*, produced by the deprotonation from NADH*+, coincides with the appearance of the absorption band due to Acr*-Mes. This indicates electron transfer from NAD* to Acr+-Mes to give Acr*-Mes, which undergoes the electron-transfer reduction of MVA2+-PtC, leading to the efficient hydrogen evolution.     

Efficient photocatalytic hydrogen evolution without an electron mediator using a simple electron donor-acceptor dyad

A highly efficient photocatalytic hydrogen evolution system without an electron mediator such as methyl viologen (MV(2+)) has been constructed using 9-mesityl-10-methylacridinium ion (Acr(+)-Mes), poly(N-vinyl-2-pyrrolidone)-protected platinum nanoclusters (Pt-PVP) and NADH (beta-nicotinamide adenine dinucleotide, reduced form) as the photocatalyst, hydrogen evolution catalyst and electron donor, respectively. The photocatalyst (Acr(+)-Mes) undergoes photoinduced electron transfer (ET) from the Mes moiety to the singlet excited state of the Acr(+) moiety to produce an extremely long-lived ET state, which is capable of oxidizing NADH and reducing Pt-PVP, leading to efficient hydrogen evolution. The hydrogen evolution efficiency is 300 times higher than that in the presence of MV(2+) because of the much faster reduction rate of Pt-PVP by Acr(*)-Mes compared with that by MV(*+). When the electron donor (NADH) is replaced by ethanol in the presence of an alcohol dehydrogenase (ADH), NADH is regenerated during the photocatalytic hydrogen evolution.     

Mechanistic borderline between one-step hydrogen transfer and sequential transfers of electron and proton in reactions of NADH analogues with triplet excited states of tetrazines and Ru(bpy)(3)2+

Efficient energy transfer from Ru(bpy)(3)(2+) (bpy = 2,2'-bipyridine, denotes the excited state) to 3,6-disubstituted tetrazines [R(2)Tz: R = Ph (Ph(2)Tz), 2-chlorophenyl [(ClPh)(2)Tz], 2-pyridyl (Py(2)Tz)] occurs to yield the triplet excited states of tetrazines ((3)R(2)Tz(*)), which have longer lifetimes and higher oxidizing ability as compared with those of Ru(bpy)(3)(2+). The dynamics of hydrogen-transfer reactions from NADH (dihydronicotinamide adenine dinucleotide) analogues has been examined in detail using (3)R(2)Tz(*) by laser flash photolysis measurements. Whether formal hydrogen transfer from NADH analogues to (3)R(2)Tz(*) proceeds via a one-step process or sequential electron and proton transfer processes is changed by a subtle difference in the electron donor ability and the deprotonation reactivity of the radical cations of NADH analogues as well as the electron-acceptor ability of (3)R(2)Tz(*) and the protonation reactivity of R(2)Tz(*)(-). In the case of (3)Ph(2)Tz(*), which is a weaker electron acceptor than the other tetrazine derivatives [(ClPh)(2)Tz; Py(2)Tz], direct one-step hydrogen transfer occurs from 10-methyl-9,10-dihydroacridine (AcrH(2)) to (3)Ph(2)Tz(*) without formation of the radical cation (AcrH(2)(*)(+)). The rate constant of the direct hydrogen transfer from AcrH(2) to (3)Ph(2)Tz(*) is larger than that expected from the Gibbs energy relation for the rate constants of electron transfer from various electron donors to (3)Ph(2)Tz(*), exhibiting the primary deuterium kinetic isotope effect. On the other hand, hydrogen transfer from 9-isopropyl-10-methyl-9,10-dihydroacridine (AcrHPr(i)) and 1-benzyl-1,4-dihydronicotinamide (BNAH) to (3)R(2)Tz(*) occurs via sequential electron and proton transfer processes, when both the radical cations and deprotonated radicals of NADH analogues are detected by the laser flash photolysis measurements.     

Antioxidant activity of trans-resveratrol toward hydroxyl and hydroperoxyl radicals: a quantum chemical and computational kinetics study

In this work, we have carried out a systematic study of the antioxidant activity of trans-resveratrol toward hydroxyl ((?)OH) and hydroperoxyl ((?)OOH) radicals in aqueous simulated media using density functional quantum chemistry and computational kinetics methods. All possible mechanisms have been considered: hydrogen atom transfer (HAT), proton-coupled electron transfer (PCET), sequential electron proton transfer (SEPT), and radical adduct formation (RAF). Rate constants have been calculated using conventional transition state theory in conjunction with the Collins-Kimball theory. Branching ratios for the different paths contributing to the overall reaction, at 298 K, are reported. For the global reactivity of trans-resveratrol toward (?)OH radicals, in water at physiological pH, the main mechanism of reaction is proposed to be the sequential electron proton transfer (SEPT). However, we show that trans-resveratrol always reacts with (?)OH radicals at a rate that is diffusion-controlled, independent of the reaction pathway. This explains why trans-resveratrol is an excellent but very unselective (?)OH radical scavenger that provides antioxidant protection to the cell. Reaction between trans-resveratrol and the hydroperoxyl radical occurs only by phenolic hydrogen abstraction. The total rate coefficient is predicted to be 1.42 × 10(5) M(-1) s(-1), which is much smaller than the ones for reactions of trans-resveratrol with (?)OH radicals, but still important. Since the (?)OOH half-life time is several orders larger than the one of the (?)OH radical, it should contribute significantly to trans-resveratrol oxidation in aqueous biological media. Thus, trans-resveratrol may act as an efficient (?)OOH, and also presumably (?)OOR, radical scavenger.     

Efficient catalytic interconversion between NADH and NAD+ accompanied by generation and consumption of hydrogen with a water-soluble iridium complex at ambient pressure and temperature

Regioselective hydrogenation of the oxidized form of β-nicotinamide adenine dinucleotide (NAD(+)) to the reduced form (NADH) with hydrogen (H(2)) has successfully been achieved in the presence of a catalytic amount of a [C,N] cyclometalated organoiridium complex [Ir(III)(Cp*)(4-(1H-pyrazol-1-yl-κN(2))benzoic acid-κC(3))(H(2)O)](2) SO(4) [1](2)·SO(4) under an atmospheric pressure of H(2) at room temperature in weakly basic water. The structure of the corresponding benzoate complex Ir(III)(Cp*)(4-(1H-pyrazol-1-yl-κN(2))-benzoate-κC(3))(H(2)O) 2 has been revealed by X-ray single-crystal structure analysis. The corresponding iridium hydride complex formed under an atmospheric pressure of H(2) undergoes the 1,4-selective hydrogenation of NAD(+) to form 1,4-NADH. On the other hand, in weakly acidic water the complex 1 was found to catalyze the hydrogen evolution from NADH to produce NAD(+) without photoirradiation at room temperature. NAD(+) exhibited an inhibitory behavior in both catalytic hydrogenation of NAD(+) with H(2) and H(2) evolution from NADH due to the binding of NAD(+) to the catalyst. The overall catalytic mechanism of interconversion between NADH and NAD(+) accompanied by generation and consumption of H(2) was revealed on the basis of the kinetic analysis and detection of the catalytic intermediates.     

Size- and shape-dependent activity of metal nanoparticles as hydrogen-evolution catalysts: mechanistic insights into photocatalytic hydrogen evolution

The catalytic activity of Pt nanoparticles (PtNPs) with different sizes and shapes was investigated in a photocatalytic hydrogen‐evolution system composed of the 9‐mesityl‐10‐methylacridinium ion (Acr⁺–Mes: photocatalyst) and dihydronicotinamide adenine dinucleotide (NADH: electron donor), based on rates of hydrogen evolution and electron transfer from one‐electron‐reduced species of Acr⁺–Mes (Acr^.–Mes) to PtNPs. Cubic PtNPs with a diameter of (6.3±0.6) nm exhibited the maximum catalytic activity. The observed hydrogen‐evolution rate was virtually the same as the rate of electron transfer from Acr^.–Mes to PtNPs. The rate constant of electron transfer (k_et) increased linearly with increasing proton concentration. When H⁺ was replaced by D⁺, the inverse kinetic isotope effect was observed for the electron‐transfer rate constant (k_et(H)/k_et(D)=0.47). The linear dependence of k_et on proton concentration together with the observed inverse kinetic isotope effect suggests that proton‐coupled electron transfer from Acr^.–Mes to PtNPs to form the Pt? H bond is the rate‐determining step for catalytic hydrogen evolution. When FeNPs were used instead of PtNPs, hydrogen evolution was also observed, although the hydrogen‐evolution efficiency was significantly lower than that of PtNPs because of the much slower electron transfer from Acr^.–Mes to FeNPs.     

Excited-state behavior and photoionization of 1,8-acridinedione dyes in micelles--comparison with NADH oxidation

The photophysics and photochemistry of 1,8-acridinedione dyes, which are analogues of reduced nicotinamide adenine dinucleotide (NADH), are studied in anionic and cationic micelles. Acridinedione dyes (ADDs) are solubilized in micelles at the micelle-water interface and are in equilibrium between the aqueous and micellar phase. The binding of the ADDs with micelles is attributed to hydrophobic interactions and the binding constants are determined with steady-state and time-resolved techniques. Nanosecond laser flash photolysis studies are carried out in aqueous, anionic, and cationic micellar solutions. The ADD undergoes photoionization in the excited state to give a solvated electron. The solvated electron reacts with the ADD to give an anion radical. In anionic micelles, the yield of the solvated electron increases because of the efficient separation of the cation radical and the electron. Cation radicals derived from the photooxidation of ADDs are involved in keto-enol tautomerization. Under acidic conditions, an enol radical cation of the acridinedione dye is formed from the keto form of the cation radical by intramolecular hydrogen atom transfer. In cationic micelles, due to electrostatic attraction, the electron cannot escape from the micelle and recombination of the cation radical and the electron results in the formation of a triplet state. For the first time, a solvated electron is observed in the laser flash photolysis of ADDs in anionic micelles. The photoionization of ADDs depends on the excitation wavelength and is biphotonic at 355 nm and monophotonic at 248 nm. From the results with this NADH model compound, the sequential electron-proton-electron transfer oxidation of NADH is confirmed and the nature of the intermediates involved in the oxidation is unraveled; these intermediates are found to depend on the pH value of the medium.     

Sequential electron-transfer and proton-transfer pathways in hydride-transfer reactions from dihydronicotinamide adenine dinucleotide analogues to non-heme oxoiron(IV) complexes and p-chloranil. Detection of radical cations of NADH analogues in acid-promoted hydride-transfer reactions

Hydride transfer from dihydronicotinamide adenine dinucleotide (NADH) analogues, such as 10-methyl-9,10-dihydroacridine (AcrH 2) and its derivatives, 1-benzyl-1,4-dihydronicotinamide (BNAH), and their deuterated compounds, to non-heme oxoiron(IV) complexes such as [(L)Fe (IV)(O)] (2+) (L = N4Py, Bn-TPEN, and TMC) occurs to yield the corresponding NAD (+) analogues and non-heme iron(II) complexes in acetonitrile. Hydride transfer from the NADH analogues to p-chloranil (Cl 4Q) also occurs to produce the corresponding NAD (+) analogues and the hydroquinone anion (Cl 4QH (-)). The logarithms of the observed second-order rate constants (log k H) of hydride transfer from NADH analogues to non-heme oxoiron(IV) complexes are linearly correlated with those of hydride transfer from the same series of NADH analogues to Cl 4Q, including similar kinetic deuterium isotope effects. The log k H values of hydride transfer from NADH analogues to non-heme oxoiron(IV) complexes are also linearly correlated with those of deprotonation of the radical cations of NADH analogues. Such linear correlations indicate that overall hydride-transfer reactions of NADH analogues to both non-heme oxoiron(IV) complexes and Cl 4Q occur via electron transfer from NADH analogues to the oxoiron(IV) complexes, followed by rate-limiting deprotonation from the radical cations of NADH analogues and subsequent rapid electron transfer from the deprotonated radicals to the Fe(III) complexes to yield the corresponding NAD (+) analogues and the Fe(II) complexes. The electron-transfer pathway was accelerated by the presence of perchloric acid, and the resulting radical cations of NADH analogues were detected by electron spin resonance spectroscopy and UV-vis spectrophotometry in the acid-promoted hydride-transfer reactions from NADH analogues to non-heme oxoiron(IV) complexes. This result provides the first direct evidence that a hydride transfer from NADH analogues to non-heme oxoiron(IV) complexes proceeds via an electron-transfer pathway.     

Where does the electron go? Electron distribution and reactivity of peptide cation radicals formed by electron transfer in the gas phase

We report the first detailed analysis at correlated levels of ab initio theory of experimentally studied peptide cations undergoing charge reduction by collisional electron transfer and competitive dissociations by loss of H atoms, ammonia, and N-C alpha bond cleavage in the gas phase. Doubly protonated Gly-Lys, (GK + 2H) (2+), and Lys-Lys, (KK + 2H) (2+), are each calculated to exist as two major conformers in the gas phase. Electron transfer to conformers with an extended lysine chain triggers highly exothermic dissociation by loss of ammonia from the Gly residue, which occurs from the ground ( X ) electronic state of the cation radical. Loss of Lys ammonium H atoms is predicted to occur from the first excited ( A ) state of the charge-reduced ions. The X and A states are nearly degenerate and show extensive delocalization of unpaired electron density over spatially remote groups. This delocalization indicates that the captured electron cannot be assigned to reduce a particular charged group in the peptide cation and that superposition of remote local Rydberg-like orbitals plays a critical role in affecting the cation-radical reactivity. Electron attachment to ion conformers with carboxyl-solvated Lys ammonium groups results in spontaneous isomerization by proton-coupled electron transfer to the carboxyl group forming dihydroxymethyl radical intermediates. This directs the peptide dissociation toward NC alpha bond cleavage that can proceed by multiple mechanisms involving reversible proton migrations in the reactants or ion-molecule complexes. The experimentally observed formations of Lys z (+*) fragments from (GK + 2H) (2+) and Lys c (+) fragments from (KK + 2H) (2+) correlate with the product thermochemistry but are independent of charge distribution in the transition states for NC alpha bond cleavage. This emphasizes the role of ion-molecule complexes in affecting the charge distribution between backbone fragments produced upon electron transfer or capture.     

吖啶和钴肟配合物协同光催化苯乙烯放氢二聚反应构筑1,2-二氢-1-芳基萘（英文）

芳基二氢萘类衍生物是许多生物活性的天然产物以及药物的常见结构单元,其合成一直都受到化学家们的关注.传统的1,2-二氢-1-芳基萘骨架化合物的构筑大都需要进行底物的预官能团化,在高温条件下进行,且产物的选择性较差,因此发展一种简单温和的制备方法很有必要.最近兴起的可见光催化因具有条件温和、环境友好等特点而成为了合成化学家的研究热点.近期研究发现,在可见光作用下利用吖啶光敏剂的强氧化能力,可以实现苯乙烯的加成.但此类反应需要当量的氧化剂或氢原子转移试剂,容易导致苯乙烯的二聚环合产物的进一步氧化或还原.我们在前期发展的"放氢交叉偶联"反应的基础上,利用吖啶光催化和钴肟催化的协同作用,实现了苯乙烯的放氢二聚反应,在室温下高效构筑了1,2-二氢-1-芳基萘骨架,反应条件温和,底物脱除的电子和质子在钴肟催化剂作用下以氢气的形式释放,反应具有中等及以上的收率.本文以苯乙烯为模型底物,吖啶为光敏剂,钴肟配合物为质子还原催化剂,在乙腈溶剂中,蓝色LED灯下光照24 h可以获得56%的产率,对于其它的光敏剂如fac-Ⅰr(ppy)3等则不能催化该反应.通过催化剂种类及用量筛选表明,7 mol%的Co(dmgH_2)pyCl配合物具有最好的反应效果,可以获得72%的收率.控制实验表明,光敏剂、钴肟催化剂和光照都是必须的.通过底物拓展我们发现,烷基、卤素等不同取代基的苯乙烯类化合物均可以获得较好的收率,不同苯乙烯之间也可以发生交叉反应.随后,我们进一步通过光谱和中间体捕获实验对反应机理进行了研究.自由基捕获实验说明反应过程可能涉及自由基历程;光谱淬灭实验表明苯乙烯和Co(dmgH_2)pyCl均可淬灭吖啶的发光,但苯乙烯淬灭吖啶的程度远大于Co(dmgH_2)pyCl淬灭吖啶的程度.在反应时苯乙烯的浓度远大于催化剂的溶度,因此,我们认为激发态吖啶首先与苯乙烯发生反应;可见光照射反应体系1 min后在440–500和550–650 nm处观察到明显的Co~Ⅱ和Co~Ⅰ的吸收峰.基于以上实验结果,我们提出了可能的催化循环:吖啶受光激发到达激发态后,首先与底物苯乙烯发生单电子转移生成苯乙烯正离子自由基和吖啶阴离子自由基Acr~·-Mes,Acr~·-Mes还原Co(dmgH_2)pyCl生成Co ~Ⅱ中间体,从而回到基态完成光催化循环.苯乙烯正离子自由基与另一分子苯乙烯加成环合,进而通过芳构化生成自由基中间体,再与Co Ⅱ作用生成目标产物1,2-二氢-1-芳基萘和Co~Ⅰ,Co~Ⅰ通过结合体系中的质子进而释放出氢气回到Co~ Ⅲ从而完成钴肟催化循环.     

Metal Bridging for Directing and Accelerating Electron Transfer as Exemplified by Harnessing the Reactivity of AIBN

A new strategy for tuning the electron transfer between radicals and enolates has been developed. This method elicits the innate reactivity of AIBN with a copper catalyst and enables a cascade reaction with cinnamic acids. Electron paramagnetic resonance studies and control experiments indicate that the redox‐active copper species not only activates the radical by coordination, but also serves as a bridge to bring the radical and nucleophile within close proximity to facilitate electron transfer. By exploiting possible combinations of redox‐active metals and radical entities with suitable coordinating functional groups, this strategy should contribute to the development of a broad range of radical‐based reactions.     

Photochemical charge separation within aromatic hydrazines and the effect of excited-state intervalence in dihydrazines

Photolysis into the longest wavelength absorption band of 2-tert-butyl-2,3-diazabicyclo[2.2.2]oct-3-yl hydrazine (Hy) substituted naphthalenes causes aryl group reduction electron transfer to give (+)Hy-Ar(-). Electrooptical absorption measurements characterize the charge separation properties from these bands. Emission studies demonstrate that the separation between absorption and emission maxima for symmetrically disubstituted compounds is smaller than that for monosubstituted compounds, which is attributed to excited-state intervalence. The excited-state diabatic surfaces may be described as a Hy(+)-NA(- )-Hy(0), Hy(0)-NA(-)-Hy(+) pair, for which electronic interaction produces a double minimum that qualitatively resembles that in the ground state of the disubstituted intervalence radical cations.     

ZIF-9（Co）衍生物磷化钴复合CeVO4协同高效产氢性能（英文）

众所周知,太阳能是一种清洁,可持续的能源.如何更有效地利用太阳能来解决人类面临的能源和环境问题已成为近几十年来科研工作者们的研究热点.半导体光催化技术被认为是人工光合作用的主要发现.光催化技术是解决日益严重的能源短缺和环境污染问题的有效途径,越来越受到人们的关注.氢作为理想的清洁能源,具有燃烧价值高,无污染的优点.光催化制氢技术的应用是最具发展性的制氢方法之一.因此,有效光催化剂的设计和开发显得十分重要.由于光催化析氢反应(HER)主要是半反应,因此必须引入牺牲试剂.同时,光敏剂的存在加速了光催化剂对光的吸收.在这种情况下研究光催化材料的结构和性质之间的关系至关重要,它能指导人们开发低成本,高稳定性,高活性的析氢光催化剂.本文首次成功地合成了以ZIF-9(Co-MOFs)作为前驱体的CoP纳米粒子,并通过简单的化学沉淀法制备了CeVO4光催化剂.深入研究了CoP,CeVO4及其复合催化剂的光催化制氢性能.发现CoP/CeVO4复合催化剂在染料敏化条件下表现出优异的光催化活性.当CoP和CeVO4结合质量比为1:1时,所得样品V1C1的复合光催化活性对于析氢最佳,在5 h内氢产生量达到444.6μmol.由于CeVO4和CoP偶联是一步完成.CeVO4牢固地粘附在CoP颗粒的表面上,形成“小点”到“大点”异质结.XRD,XPS,SEM,EDX和TEM的结果显示,CoP和CeVO4纳米颗粒的形成和复合物的结构.基于对Mott-Schottky曲线,UV-vis漫射光谱,光电流-时间曲线,Tafel曲线,奈奎斯特曲线,线性伏安曲线和稳态/瞬态荧光测量结果表明,CoP/CeVO4高效析氢的原因是CoP和CeVO4复合后存在肖特基势垒,导致能带发生弯曲,并且CoP与CeVO4之间异质结所形成的内建电场能加速电荷转移.此外,CoP和CeVO4之间独特的协同效应为彼此提供了新的析氢活性中心.提高了载流子分离效率,降低了光生载流子复合率.因此,CoP/CeVO4复合催化剂具有优异的光催化析氢活性.本文为过渡金属磷化物光催化剂的电子结构和载流子行为的调控提供了新的策略.     

Metal-free oxygenation of cyclohexane with oxygen catalyzed by 9-mesityl-10-methylacridinium and hydrogen chloride under visible light irradiation

Photooxygenation of cyclohexane by O(2) occurs efficiently under visible-light irradiation of an O(2)-saturated acetonitrile solution containing 9-mesityl-10-methylacridinium ions (Acr(+)-Mes) and HCl to yield cyclohexanone, cyclohexanol and hydrogen peroxide. The photocatalytic reaction is initiated by electron transfer from Cl(-) to the mesitylene radical cation moiety.     

Photoinduced electron transfer and geminate recombination for photoexcited acceptors in a pure donor solvent

Photoinduced electron transfer and geminate recombination are studied for the systems rhodamine 3B (R3B(+)) and rhodamine 6G (R6G(+)), which are cations, in neat neutral N,N-dimethylaniline (DMA). Following photoexcitation of R3B(+) or R6G(+) (abbreviated as R(+)), an electron is transferred from DMA to give the neutral radical R and the cation DMA(+). Because the DMA hole acceptor is the neat solvent, the forward transfer rate is very large, approximately 5x10(12) s(-1). The forward transfer is followed by geminate recombination, which displays a long-lived component suggesting several percent of the radicals escape geminate recombination. Spectrally resolved pump-probe experiments are used in which the probe is a "white" light continuum, and the full time-dependent spectrum is recorded with a spectrometer/charge-coupled device. Observations of stimulated emission (excited state decay-forward electron transfer), the R neutral radical spectrum, and the DMA(+) radical cation spectrum as well as the ground-state bleach recovery (geminate recombination) make it possible to unambiguously follow the electron transfer kinetics. Theoretical modeling shows that the long-lived component can be explained without invoking hole hopping or spin-forbidden transitions.     

Elementary steps of enzymatic oxidation of nifedipine catalyzed by horseradish peroxidase

The elementary steps of the enzymatic oxidation of nifedipine (NF) catalyzed by horseradish peroxidase (HRP) have been described based on analysis of kinetic magnetic field effects (MFEs). It has been shown that the first step of the catalytic cycle is single electron transfer resulting in formation of NF*(+) radical cation and ferroperoxidase (Per(2+)). As a result, comparison with an earlier studied oxidation reaction of NADH catalyzed by HRP evidenced that the enzymatic oxidations of two substrates-native, NADH, and its synthetic analogue, NF-catalyzed by HRP in the absence of H(2)O(2) follow identical mechanisms.     

Direct detection of radical cations of NADH analogues

The radical cation of an NADH analogue (BNAH: 1-benzyl-1,4-dihydronicotinamide) has been successfully detected as the transient absorption and ESR spectra in the thermal electron transfer from BNAH to Fe(bpy)33+ (bpy = 2,2'-bipyridine). The ESR spectra of the radical cations of BNAH and the dideuterated compound (BNAH-4,4'-d2) indicate that the observed radical cation is the keto form rather than the enol form in the tautomerization. The deprotonation rate and the kinetic isotope effects of the keto form of BNAH*+ were determined from the kinetic analysis of the electron-transfer reactions.     

Sustainable radical reduction through catalytic hydrogen atom transfer

A system with coupled catalytic cycles is described that allows radical reduction by hydrogen atom abstraction from rhodium hydrides. These intermediates are generated from H2 activation by Wilkinson's catalyst. Radical generation is carried out by titanocene-catalyzed electron transfer to epoxides.