Electron Proton Transfer Research Articles

A molecular mediator for reductive concerted proton-electron transfers via electrocatalysis.

Electrocatalytic approaches to the activation of unsaturated substrates via reductive concerted proton-electron transfer (CPET) must overcome competing, often kinetically dominant hydrogen evolution. We introduce the design of a molecular mediator for electrochemically triggered reductive CPET through the synthetic integration of a Brønsted acid and a redox mediator. Cathodic reduction at the cobaltocenium redox mediator substantially weakens the homolytic nitrogen-hydrogen bond strength of a Brønsted acidic anilinium tethered to one of the cyclopentadienyl rings. The electrochemically generated molecular mediator is demonstrated to transform a model substrate, acetophenone, to its corresponding neutral α-radical via a rate-determining CPET.

Excited State Molecular Dynamics of Photoinduced Proton-Coupled Electron Transfer in Anthracene-Phenol-Pyridine Triads.

Photoinduced proton-coupled electron transfer (PCET) in anthracene-phenol-pyridine triads exhibits inverted region behavior, where the more thermodynamically favorable process is slower. The long-lived transient charge-separated state (CSS) associated with electron transfer from phenol to anthracene and inverted region behavior were only observed experimentally for certain triads. Herein, excited state molecular dynamics simulations were performed on four different triads to simulate the nonequilibrium dynamics following photoexcitation to the locally excited state (LES) of anthracene. These simulations identified two distinct PCET pathways: the triads exhibiting inverted region behavior transitioned from the LES to the CSS, whereas the other triads transitioned to a local electron-proton transfer (LEPT) state within phenol and pyridine. The simulations suggest that PCET to the LEPT state is slower than PCET to the CSS and provides an alternative relaxation pathway. The mechanistic pathways, as well as the time scales of the electron and proton transfers, can be controlled by tuning the substituents.

PH dependence of CO2 electroreduction selectivity over size-selected Au nanoparticles

Electrocatalytic CO2 reduction to liquid fuels and chemical feedstock, powered by renewable electricity, is an important approach for storing renewable energy and closing carbon cycle. Here, we study the pH effect on CO2 electroreduction selectivity over size-selected Au nanoparticles (NPs) in citrate buffer solutions with pH 3.7–6.0. A maximum CO Faradaic efficiency of 96.3% is achieved over Au NPs at pH 6.0, and synthesis gas with tunable H2/CO ratios (0.04–22) can be produced in the studied pH range. By careful kinetic analysis, the observed pH dependence reveals a most likely mixed mechanism of concerted proton–electron transfer and sequential electron–proton transfer in the initial step of CO2 activation. The pH-dependent selectivity changes over Au NPs can be rationalized by the difference in the pH dependence of CO2 electroreduction and concurrent hydrogen evolution reaction. This work provides new guidelines into controlling activity and selectivity of CO2 electroreduction via efficiently tuning electrolyte pH. Selectivity of CO2 electroreduction over size-selected Au nanoparticles is tuned via electrolyte pH.

Catalytic conversion of fructose to 1,3-dihydroxyacetone under mild conditions

A novel zwitterionic catalyst containing imidazole, carboxyl and amino functional groups was synthesized to catalyze the retro-aldol condensation of fructose. The catalyst displayed efficiently catalytic activity for the conversion of fructose to 1,3-dihydroxyacetone (DHA). The yield of DHA and selectivity of DHA achieved 27.9% and 46.5% after reaction 2 h, respectively, at pH 9.5, 85 °C. A possible catalytic mechanism was suggested. The charged functional groups on the catalyst exhibited synergistic effect and played role in electron induction and proton transfer, which leaded to a good selectivity of DHA in the conversion of fructose under mild conditions.

First-principles study of the oxygen reduction reaction on the boron-doped C9N4 metal-free catalyst

Replacing precious Pt-based catalysts with cheap metal-free catalysts would make the hydrogen economy viable. Herein, based on first-principles calculations, we have systemically investigated the oxygen reduction reaction (ORR) under the acid environment catalyzed by the B-doped C9N4 monolayer, a nodal-line semimetal. It is found that the doped B atom energetically prefers to replace the inner C atom with an exothermicity of 0.89 eV, and the resulting B-doped C9N4 retains the metallic conductivity beneficial for the charge transfer during the electrocatalytic process. The ORR prefers to adopt efficient four-electron pathways by firstly forming two energetically almost degenerate OOH species (OOHα and OOHβ). For further hydrogenation reactions, the formations of OH + OH and H2O + OH with similar energy barriers of about 1.0 eV are the rate-determining steps for OOHα and OOHβ configurations, respectively. Moreover, for both ORR pathways the potential-determining step is the last electron-proton pair transfer reaction with an overpotential of 0.98 V. Electronic structure analysis shows that due to the small electronegativity of the B atom, O2 can be effectively activated by charge transfer and further orbital hybridization, which initiates the ORR. Present study suggests that the metal-free B-doped C9N4 monolayer has great potential application for ORR.

Base-Directed Photoredox Activation of C-H Bonds by PCET.

Photoredox catalysis using proton-coupled electron transfer (PCET) has emerged as a powerful method for bond transformations. We previously employed traditional chemical oxidants to achieve multiple-site concerted proton-electron transfer (MS-CPET) activation of a C-H bond in a proof-of-concept fluorenyl-benzoate substrate. As described here, photoredox oxidation of the fluorenyl-benzoate follows the same rate constant vs driving force trend determined for thermal MS-CPET. Analogous photoredox catalysis enables C-H activation and H/D exchange in a number of additional substrates with favorably positioned bases. Mechanistic studies support our hypothesis that MS-CPET is a viable pathway for bond activation for substrates in which the C-H bond is weak, while stepwise carboxylate oxidation and hydrogen atom transfer likely predominate for stronger C-H bonds.

Visualizing the protons in a metalloenzyme electron proton transfer pathway

In redox metalloenzymes, the process of electron transfer often involves the concerted movement of a proton. These processes are referred to as proton-coupled electron transfer, and they underpin a wide variety of biological processes, including respiration, energy conversion, photosynthesis, and metalloenzyme catalysis. The mechanisms of proton delivery are incompletely understood, in part due to an absence of information on exact proton locations and hydrogen bonding structures in a bona fide metalloenzyme proton pathway. Here, we present a 2.1-Å neutron crystal structure of the complex formed between a redox metalloenzyme (ascorbate peroxidase) and its reducing substrate (ascorbate). In the neutron structure of the complex, the protonation states of the electron/proton donor (ascorbate) and all of the residues involved in the electron/proton transfer pathway are directly observed. This information sheds light on possible proton movements during heme-catalyzed oxygen activation, as well as on ascorbate oxidation.

Natural acridones and coumarins as free radical scavengers: Mechanistic and kinetic studies

In this work, we combined density functional theory (i.e., the M05-2X/6–311++G(d,p)//M05-2X/6–31+G(d) level) and statistical rate models to investigate free-radical scavenging activities of natural compounds found in Paramignya trimera root: four coumarins (i.e., paratrimerins E, paratrimerins F, pandanusin A, xanthyletin) and three acridones (i.e., citrusinine-I, 5-hydroxynoracronycin, paratrimerin C). Reaction mechanisms, including hydrogen atom transfer (HAT), single electron transfer (SET), proton transfer (PT) and radical adduct formation (RAF), were studied thermodynamically and kinetically in different environments: gas-phase, water and pentyl ethanoate (PEA). 5-hydroxynoracronycin is found as the most potential antioxidant with the HAT as the most dominant mechanism in the considered media.

The C–H bond activation by non-heme oxidant [(N4Py)FeIV(O)]2+ with external electric field

The C–H bond activation by non-heme iron-oxo species is one of the most interesting fundamental steps in organic oxidation reactions. This work focuses on the C–H bond activation by an external electric field (EEF) in a non-heme complex. The hydrogen atom abstraction from 2,5-(MeO)2C6H3CH2OH by [(N4Py)FeIV(O)]2+ utilizing an EEF was studied using density functional theory. Our analysis suggests that the mechanism of this process can be characterized as a mixture of hydrogen atom transfer and concerted-asynchronous electron–proton transfer that can be modulated by the EEF. Our study demonstrates an EEF that decreases the reaction barrier by increasing the electron donor orbital energy, which facilitates electron transfer. The reaction finally becomes barrierless when the EEF is greater than 0.0045au. The finding from this work demonstrates the exciting possibility of tuning the reaction mechanism using the EEF.

Structure-antioxidant activity relationship of methoxy, phenolic hydroxyl, and carboxylic acid groups of phenolic acids

The antioxidant activities of 18 typical phenolic acids were investigated using 2, 2′-diphenyl-1-picrylhydrazyl (DPPH) and ferric ion reducing antioxidant power (FRAP) assays. Five thermodynamic parameters involving hydrogen atom transfer (HAT), single-electron transfer followed by proton transfer (SET-PT), and sequential proton-loss electron transfer (SPLET) mechanisms were calculated using density functional theory with the B3LYP/UB3LYP functional and 6–311++G (d, p) basis set and compared in the phenolic acids. Based on the same substituents on the benzene ring, -CH2COOH and -CH = CHCOOH can enhance the antioxidant activities of phenolic acids, compared with -COOH. Methoxyl (-OCH3) and phenolic hydroxyl (-OH) groups can also promote the antioxidant activities of phenolic acids. These results relate to the O-H bond dissociation enthalpy of the phenolic hydroxyl group in phenolic acids and the values of proton affinity and electron transfer enthalpy (ETE) involved in the electron donation ability of functional groups. In addition, we speculated that HAT, SET-PT, and SPLET mechanisms may occur in the DPPH reaction system. Whereas SPLET was the main reaction mechanism in the FRAP system, because, except for 4-hydroxyphenyl acid, the ETE values of the phenolic acids in water were consistent with the experimental results.

Electrochemical Oxidation of Organic Molecules at Lower Overpotential: Accessing Broader Functional Group Compatibility with Electron-Proton Transfer Mediators.

Electrochemical organic oxidation reactions are highly appealing because protons are often effective terminal electron acceptors, thereby avoiding undesirable stoichiometric oxidants. These reactions are often plagued by high overpotentials, however, that greatly limit their utility. Single-electron transfer (SET) from organic molecules generates high-energy radical-cations. Formation of such intermediates often requires electrode potentials far above the thermodynamic potentials of the reaction and frequently causes decomposition and/or side reactions of ancillary functional groups. In this Account, we show how electrocatalytic electron-proton transfer mediators (EPTMs) address this challenge. EPTMs bypass the formation of radical-cation intermediates by supporting mechanisms that operate at electrode potentials much lower (≥1 V) than those of analogous direct electrolysis reactions.The stable aminoxyl radical TEMPO (2,2,6,6-tetramethylpiperidine N-oxyl) is an effective mediator for electrochemical alcohol oxidation, and we have employed such processes for applications ranging from pharmaceutical synthesis to biomass conversion. A complementary electrochemical alcohol oxidation method employs a cooperative Cu/TEMPO mediator system that operates at 0.5 V lower electrode potential than the TEMPO-only mediated process. This difference, which arises from a different catalytic mechanism, rationalizes the broad functional group tolerance of Cu/TEMPO-based aerobic alcohol oxidation catalysts.Aminoxyl mediators address long-standing challenges in the "Shono oxidation," an important method for α-C-H oxidation of tertiary amides and carbamates. Shono oxidations are initiated by a high-potential SET step that limits their utility. Aminoxyl-mediated Shono-type oxidations have been developed that operate at much lower potentials and tolerate diverse functional groups. Analogous reactivity underlies α-C-H cyanation of secondary cyclic amines, a new method that enables efficient diversification of piperidine-based pharmaceutical building blocks and preparation of non-natural amino acids.Electrochemical oxidations of benzylic C-H bonds are commonly initiated by SET to generate arene radical cations, but such methods are again plagued by large overpotentials. Mediated electrolysis methods that promote hydrogen-atom-transfer (HAT) from benzylic C-H bonds to Fe-oxo species and phthalimide N-oxyl (PINO) support C-H oxygenation, iodination, and oxidative-coupling reactions. A complementary method merges photochemistry with electrochemistry to achieve amidation of C(sp3)-H bonds. This unique process operates at much lower overpotentials compatible with diverse functional groups.These results have broad implications for organic electrochemistry, highlighting the importance of "overpotential" considerations and the prospects for expanding synthetic utility by using mediators to bypass high-energy outer-sphere electron-transfer mechanisms. Principles demonstrated here for oxidation are equally relevant to electrochemical reductions.

Remote Nickel-Catalyzed Cross-Coupling Arylation via Proton-Coupled Electron Transfer-Enabled C-C Bond Cleavage.

Cross-coupling reactions for carbon-carbon and carbon-heteroatom bond formation are of great importance in modern chemical synthesis. In addition to classical cross-couplings involving preformed or preactivated coupling partners, more recently breakthroughs have been made in the selective, direct coupling of abundant aliphatic carbon-hydrogen bonds using hydrogen atom transfer reactions in which the bond-dissociation energy is the thermodynamic driving force. The more challenging carbon-carbon bond activation is still rather underdeveloped due to the bond inertness. Herein, we report a mild and general strategy for the activation of a diverse set of readily available cyclic alcohols for the remote and site-specific arylation of ketones via the combination of photoredox-mediated multisite concerted proton-electron transfer (MS-PCET) and nickel catalysis. The current cross-coupling proceeds with the generation of an alkoxy radical utilizing bond-dissociation free energy (BDFE) as the thermodynamic driving force. Subsequently, the resulting remote carbon-centered radicals formed by C-C cleavage merge with the nickel catalytic cycle to create the challenging C(sp3)-C(sp2) bonds.

Toward Rational Understandings of α-C-H Functionalization: Energetic Studies of Representative Tertiary Amines.

SummaryFunctionalization of α-C–H bonds of tertiary amines to build various α-C–X bonds has become a mainstream in synthetic chemistry nowadays. However, due to lack of fundamental knowledge on α-C–H bond strength as an energetic guideline, rational exploration of new synthetic methodologies remains a far-reaching anticipation. Herein, we report a unique hydricity-based approach to establish the first integrated energetic scale covering both the homolytic and heterolytic energies of α-C–H bonds for 45 representative tertiary amines and their radical cations. As showcased from the studies on tetrahydroisoquinolines (THIQs) by virtue of their thermodynamic criteria, the feasibility and mechanisms of THIQ oxidation were deduced, which, indeed, were found to correspond well with experimental observations. This integrated scale provides a good example to relate bond energetics with mechanisms and thermodynamic reactivity of amine α-C–H functionalization and hence, may be referenced for analyzing similar structure-property problems for various substrates.

Concerted proton-electron transfer oxidation of phenols and hydrocarbons by a high-valent nickel complex.

The high-valent nickel(iii) complex Ni(pyalk)2 + (2) was prepared by oxidation of a nickel(ii) complex, Ni(pyalk)2 (1) (pyalk = 2-pyridyl-2-propanoate). 2 and derivatives were fully characterized by mass spectrometry and X-ray crystallography. Electron paramagnetic resonance spectroscopy and X-ray photoelectron spectroscopy confirm that the oxidation is metal-centered. 2 was found to react with a variety of phenolic and hydrocarbon substrates. A linear correlation between the measured rate constant and the substrate bond dissociation enthalpy (BDE) was found for both phenolic and hydrocarbon substrates. Large H/D kinetic isotope effects were also observed for both sets of substrates. These results suggest that 2 reacts through concerted proton-electron transfer (CPET). Analysis of measured thermodynamic parameters allows us to calculate a bond dissociation free energy (BDFE) of ∼91 kcal mol-1 for the O-H bond of the bound pyalk ligand. These findings may shed light onto CPET steps in oxidative catalysis and have implications for ligand design in catalytic systems.

Synthesis of a binary alloy nanoparticle catalyst with an immiscible combination of Rh and Cu assisted by hydrogen spillover on a TiO2 support.

This work demonstrated the use of TiO2 as a promising platform for the synthesis of non-equilibrium RhCu binary alloy nanoparticles (NPs). These metals are regarded as immiscible based on their phase diagram but form NPs with the aid of the significant hydrogen spillover on TiO2 with concurrent proton–electron transfer. The resulting RhCu/TiO2 exhibited 2.6 times higher catalytic activity than Rh/TiO2 during hydrogen production from the hydrolysis of ammonia borane (AB), due to a synergistic effect. Theoretical simulations showed a higher energy value for the adsorption of AB on the RhCu alloy and a lower activation energy for the rate determining N–B bond dissociation by the attack of H2O during AB hydrolysis compared to monometallic Rh. High-angle annular dark-field scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy confirmed the formation of RhCu alloy NPs with a mean diameter of 2.0 nm on the TiO2. H2-temperature programmed reduction and in situ X-ray absorption fine structure analyses at elevated temperature under H2 demonstrated that Rh3+ and Cu2+ precursors were simultaneously reduced only on the TiO2 support. This effect resulted from the improved and limited reducibility of Cu2+ and Rh3+, respectively. The rate of hydrogen spillover of TiO2 is faster as compared to γ-Al2O3 and MgO as evidenced by sequential H2/D2 exchanges during in situ Fourier transform infrared analyses. Density functional theory calculations also showed that the migration of H atoms on TiO2 proceeds with a lower energy barrier than that on Al2O3, and the reduction of Cu2+ species is facilitated by H spillover on the support rather than by direct reduction by H2. These results confirm the vital role of TiO2 in the formation of the alloy and may represent a new strategy for the synthesis of different non-equilibrium solid solution alloys.

Why do RuO2 electrodes catalyze electrochemical CO2 reduction to methanol rather than methane or perhaps neither of those?

The electrochemical CO2 reduction reaction (CO2RR) on RuO2 and RuO2-based electrodes has been shown experimentally to produce high yields of methanol, formic acid and/or hydrogen while methane formation is not detected. This CO2RR selectivity on RuO2 is in stark contrast to copper metal electrodes that produce methane and hydrogen in the highest yields whereas methanol is only formed in trace amounts. Density functional theory calculations on RuO2(110) where only adsorption free energies of intermediate species are considered, i.e. solvent effects and energy barriers are not included, predict however, that the overpotential and the potential limiting step for both methanol and methane are the same. In this work, we use both ab initio molecular dynamics simulations at room temperature and total energy calculations to improve the model system and methodology by including both explicit solvation effects and calculations of proton–electron transfer energy barriers to elucidate the reaction mechanism towards several CO2RR products: methanol, methane, formic acid, CO and methanediol, as well as for the competing H2 evolution. We observe a significant difference in energy barriers towards methane and methanol, where a substantially larger energy barrier is calculated towards methane formation than towards methanol formation, explaining why methanol has been detected experimentally but not methane. Furthermore, the calculations show why RuO2 also catalyzes the CO2RR towards formic acid and not CO(g) and methanediol, in agreement with experimental results. However, our calculations predict RuO2 to be much more selective towards H2 formation than for the CO2RR at any applied potential. Only when a large overpotential of around −1 V is applied, can both formic acid and methanol be evolved, but low faradaic efficiency is predicted because of the more facile H2 formation.

Concerted proton-electron transfer reactions of manganese-hydroxo and manganese-oxo complexes.

The enzymes manganese superoxide dismutase and manganese lipoxygenase use MnIII-hydroxo centres to mediate proton-coupled electron transfer (PCET) reactions with substrate. As manganese is earth-abundant and inexpensive, manganese catalysts are of interest for synthetic applications. Recent years have seen exciting reports of enantioselective C-H bond oxidation by Mn catalysts supported by aminopyridyl ligands. Such catalysts offer economic and environmentally-friendly alternatives to conventional reagents and catalysts. Mechanistic studies of synthetic catalysts highlight the role of Mn-oxo motifs in attacking substrate C-H bonds, presumably by a concerted proton-electron transfer (CPET) step. (CPET is a sub-class of PCET, where the proton and electron are transferred in the same step.) Knowledge of geometric and electronic influences for CPET reactions of Mn-hydroxo and Mn-oxo adducts enhances our understanding of biological and synthetic manganese centers and informs the design of new catalysts. In this Feature article, we describe kinetic, spectroscopic, and computational studies of MnIII-hydroxo and MnIV-oxo complexes that provide insight into the basis for the CPET reactivity of these species. Systematic perturbations of the ligand environment around MnIII-hydroxo and MnIV-oxo motifs permit elucidation of structure-activity relationships. For MnIII-hydroxo centers, electron-deficient ligands enhance oxidative reactivity. However, ligand perturbations have competing consequences, as changes in the MnIII/II potential, which represents the electron-transfer component for CPET, is offset by compensating changes in the pKa of the MnII-aqua product, which represents the proton-transfer component for CPET. For MnIV-oxo systems, a multi-state reactivity model inspired the development of significantly more reactive complexes. Weakened equatorial donation to the MnIV-oxo unit results in large rate enhancements for C-H bond oxidation and oxygen-atom transfer reactions. These results demonstrate that the local coordination environment can be rationally changed to enhance reactivity of MnIII-hydroxo and MnIV-oxo adducts.

Theoretical understanding of the electrochemical reaction barrier: a kinetic study of CO2 reduction reaction on copper electrodes.

The electrochemical reduction of CO2 is a promising route for converting intermittent renewable energy into storable fuels and useful chemical products. A theoretical investigation of the reaction mechanism and kinetics is beneficial for understanding the electrocatalytic activity and selectivity. In this report, a kinetic model based on Marcus theory is developed to compute the potential-dependent reaction barrier of the elementary concerted proton-electron transfer steps of electrochemical CO2 reduction reactions, different from the previous hydrogen atom transfer model. It is found that the onset potentials and rate-determining steps for CO and CH4 formation are determined by the first and third concerted proton-electron transfer steps C1 and C3. The influence of binding energy, electrode potential, and reorganization energy on the computed reaction barriers of the C1 and C3 reactions is discussed. In general, the calculated reaction barrier shows a quadratic relationship with the applied electrode potential. Specifically, the reaction barrier is merely determined by the reorganization energy at equilibrium potential. The present kinetic model is applied to compare the electrocatalytic activities in the electrochemical reduction of CO2 on various copper crystal surfaces. Among the four studied copper single-crystal surfaces, Cu(211) exhibits the best electrocatalytic activity for CO formation and CH4 formation due to its low onset potential and overpotential.

Substituent Effects on Photochemistry of Anthracene-Phenol-Pyridine Triads Revealed by Multireference Calculations.

Inverted region behavior for concerted proton-coupled electron transfer (PCET) was recently demonstrated for biomimetic anthracene-phenol-pyridine molecular triads. Photoexcitation of the anthracene to a locally excited state (LES) is followed by concerted electron transfer from the phenol to the anthracene and proton transfer from the phenol to the pyridine, forming a relatively long-lived charge separated state (CSS). The long-lived CSS and the inverted region behavior associated with the decay from the CSS to the ground state through charge recombination were experimentally observed only for triads with certain substituents on the anthracene and the pyridine. To explain this distinction, we computed the proton potential energy curves in four substituted triads using the complete active space self-consistent-field method and multireference perturbation theory, including solvent effects with a dielectric continuum model. The calculations revealed a local electron-proton transfer (LEPT) state, in which both the electron and proton transfer from the phenol to the pyridine. When the LEPT state is lower in energy than the CSS, it may provide an alternative pathway for fast decay from the LES to the ground state and thereby preclude detection of the CSS and the inverted region behavior. These calculations predict that substituents stabilizing negative charge on the pyridine and destabilizing negative charge on the anthracene will favor the LEPT pathway, while substituents with the reverse effects will favor the CSS pathway, which could exhibit inverted region behavior. These insights about the stabilization of energy-storing charge-separated states have implications for designing and controlling PCET reactions in artificial photosynthetic systems and other energy conversion processes.

DETERMINATION OF THE MECHANISM OF PHENOMENAXY OF BENZOPHENONES WITH PHENOLS WITH THE HELP OF CNP EFFECTS

The study of elementary acts of charge transfer occurring in conjugated systems under photoirradiation conditions is the most important task of chemical physics. Such studies allow us to obtain new ideas about the directions of generation of radical and ion-radical structures and to form approaches to the transformation of photoenergy into other types of energy. The currently developed general theoretical framework for proton-electron transfer reactions (PETR) is applicable to a wide range of experimentally studied reactions in solutions, proteins and electrochemistry, which play a crucial role in various chemical and biological processes, including photosynthesis, various enzyme reactions and energy devices. Currently, there is a methodology for simulating mixed quantum and classical molecular dynamics for PETR reactions; moreover, non-adiabatic dynamics methods have been developed for modeling ultrafast nonequilibrium dynamics of photoinduced PETR reactions for model systems. This paper discusses the results of the study of elementary stages and a detailed analysis of the internal mechanisms of stable generation of radio emission in photochemical reactions using the effects of chemical polarization of nuclei using the example of phototransformations of benzophenones with various electron donors. According to the results of studies conducted in a modified sensor of a multi-core NMR spectrometer with simultaneous recording of the spectrum, detailed mechanisms and elementary acts of photoreduction of benzophenones with triethylamine, N, N-diethyl-p-toluidine and N, N-dimethylaniline, as well as phenols and aniline, were established. It was found that the benzophenone photoreduced in two stages (first electron transfer, then proton) or one (transfer of a hydrogen atom only).