Photocatalytic single electron transfer reactions on TiO2 semiconductor

The use of inorganic semiconductor particles such as titanium dioxide(TiO_2) has received relatively less attention in organic chemistry, although semiconductor particles have been widely used as a single electron transfer photocatalyst in waterpurification, air-cleaning, and self-cleaning. In recent years, the photocatalysis on semiconductor particles has become an active area of research even in organic chemistry, since the heterogeneous semiconductor photocatalysis leads to the unique redox organic reactions. In an early stage, the semiconductor photocatalysis was applied to the oxidation of organic molecules.Semiconductor particles have also the potential to induce the reductive chemical transformations in the absence of oxygen(O_2),by using the suitable sacrificial hole scavenger. In this review, we summarize the representative examples of the reductive and oxidative organic reactions using semiconductor particles and the recent applications to the stereoselective reactions.     

Electron transfer NO 2 + +NO→N02+NO+ in aromatic nitration

A simple model for computing the electron transfer rate constant of a cross-reaction has been proposed in the framework of semiclassical theory and employed to investigate the electron transfer system NO ₂ ⁺ /NO. The encounter complex of electron transfer NO ₂ ⁺ +NO→N0₂+NO⁺ has been optimized at the level of UHF/6-31G. In the construction of diabatic potential energy surfaces the linear coordinate was used and the kinetic quantities, such as the activation energies and the electron transfer matrix elements, have been obtained. For comparison, the related selfexchange reactlon systems NO ₂ ⁺ /NO₂ and NO⁺/NO were kinetically investigated. The calculated activation energies for the electron transfer reactions of systems NO ₂ ⁺ /NO, NO ₂ ⁺ /NO₂, and NO⁺/NO are 81.4, 128.8, and 39.8 kJ.mol^-1, respectively. With the solvent effect taken into account, the contribution of solvent reorganization to the activation energy has been estimated according to the geometric parameters of the transition states. The obtained rate constants show that the activity of NO ₂ ⁺ as an oxidizing reagent in the aromatic nitration will be greatly decreased due to a high activation barrier contributed mainly from the change of bond angle ONO.     

Collision complexes in dissociative single electron transfer between Ne2+ and N2

Experiments involving the coincident detection of the two monocationic products (Ne+ and N+) from the dissociative electron transfer reaction between Ne2+ and N2 at 7.8 eV collision energy allow the nascent velocity vectors of the ionic and neutral (N) products to be determined. Examination of the correlations between these vectors shows that one pathway to the products involves the dissociation of a transitory collision complex (N2Ne2+).     

Gas-phase oxidation reactions of Ta2+: synthesis and properties of TaO(2+) and TaO2(2+)

Gas-phase reactions of Ta(2+) and TaO(2+) with oxidants, including thermodynamically facile O-atom donor N(2)O and ineffective donor CO, as well as intermediate donors C(2)H(4)O (ethylene oxide), H(2)O, O(2), CO(2), NO, and CH(2)O, were studied by Fourier transform ion cyclotron resonance mass spectrometry. All oxidants reacted with Ta(2+) by electron transfer yielding Ta(+), in accord with the high second ionization energy of Ta (ca. 16 eV). TaO(2+) was also produced with N(2)O, H(2)O, O(2), and CO(2), oxidants with ionization energies above 12 eV; CO reacted only by electron transfer. The following charge separation products were also observed: TaN(+) and TaO(+) with N(2)O; and TaO(+) with O(2), CO(2), and CH(2)O. TaOH(2+), formed with H(2)O, reacted with a second H(2)O by proton transfer. TaO(2+) abstracted an electron from N(2)O, H(2)O, O(2), CO(2), and CO. Oxidation of TaO(2+) by N(2)O was also observed to produce TaO(2)(2+); on the basis of density functional theory (DFT) results, this species is a dioxide, {O-Ta-O}(2+). TaO(2)(2+) reacted by electron transfer with N(2)O, CO(2), and CO to give TaO(2)(+). Additionally, it was found that TaO(2)(2+) oxidizes CO to CO(2) and that it acts as a catalyst in the oxidation of CO by N(2)O. TaO(2)(2+) also activates H(2) to form TaO(2)H(2+). On the basis of the rates of electron transfer from N(2)O, CO(2), and CO to Ta(2+), TaO(2+), and TaO(2)(2+), the following estimates were made for the second ionization energies of Ta, TaO, and TaO(2): IE[Ta(+)] = 15.8 ± 0.3 eV, IE[TaO(+)] = 16.0 ± 0.5 eV, and IE[TaO(2)(+)] = 16.9 ± 0.4 eV. These IEs, together with recently reported bond dissociation energies, D[Ta(+)-O] and D[OTa(+)-O], result in the following bond energies: D[Ta(2+)-O] = 657 ± 58 kJ mol(-1) and D[OTa(2+)-O] = 500 ± 63 kJ mol(-1), the first of which is in good agreement with the value obtained by DFT.     

Kinetics of the reactions Br + NO2 + M and I + NO2+ M

The reactions Br + NO₂ + M → BrNO₂ + M (1) and I + NO₂ + M → INO₂ + M (2) have been studied at low pressure (0.6-2.2 torr) at room temperature and with helium as the third body by the discharge-flow technique with EPR and mass spectrometric analysis of the species. The following third order rate constants were found k₁₍₀₎ = (3.7 ± 0.7) × 10^?31 and k₂₍₀₎ = (0.95 ± 0.35) × 10^?31 (units are cm⁶ molecule^?2 s^?1). The secondary reactions X + XNO₂ → X₂ + NO₂ (X = Br, I) have been studied by mass spectrometry and their rate constants have been estimated from product analysis and computer modeling.     

Theoretical study on the Co~(2+)OH_2/Co~(3+)OH_2 electron transfer reactivity

This paper presents a contact distance dependence analysis scheme and an ab initio calculation application for the electron transfer (ET) reactivity of Co2+OH2/Co3+OH2 reacting pair. The applicability of these schemes and the corresponding models has been discussed. The contact distance (Rcoco) dependence of the relevant quantities has been analyzed. The results indicate that the activation energy from the accurate PES method agrees well with that from the anharmonic potential method, and they are obviously better than that from the harmonic potential method. The pair distribution function varies from 10~(-2) to 10~(-5) along with Rcoco changing from 1.20 to 0.35 nm. The coupling matrix element exponentially decays along with the increase of Rcoco, and the effective electronic coupling requires Rcoco smaller than 0.75 nm. In the range from 0.50 to 0.75 nm for Rcoco, the corresponding electronic transmission coefficient falls within 1.0-10~(-6). The local ET rate also exponentially decays along with the incre     

Fischer carbene complexes: beautiful playgrounds to study single electron transfer (SET) reactions

The knowledge of the reactivity of Fischer carbene complexes in electron transfer processes is still in the early stage of development, but interesting advances are foreseeable in this young branch of metal-carbene chemistry. Although these compounds have a dual reactivity (which makes them good substrates for oxidation and reduction processes), their behavior towards chemical electron transfer (ET) reagents was unknown until very recently. This article covers the progress accomplished in the reactivity of these compounds towards chemical ET reagents (C(8)K or SmI(2)), as well as the use of nonconventional sources of electrons, such as electrospray ionization (ESI) to induce ET processes. Special emphasis will be made on the effect of the structure of the starting carbene in the outcome of the reaction and in discussing the different mechanisms proposed.     

Collisional energy transfer in the reactions I + NO + M → INO + M and I + NO2 + M → INO2 + M

The low-pressure recombination rate constants of the reactions I + NO + M → INO + M (with 14 different M) and I + NO₂ + M → INO₂ + M (with 26 different M) have been measured at 330°K by laser flash photolysis. The collision efficiencies β_c are analyzed and compared with other thermal activation systems. Whereas β_c increases in one reaction with an increasing number of atoms in M, practically no such effect is found when, for the same M, different reactions with varying complexities of the reacting molecules are considered.     

Electronic factors in (4 + 2)-cycloaddition reactions of phosphaalkenes

Reactions involving the (4 + 2)-cycloaddition of phosphaalkenes to 1,3-butadiene have been characterized in the framework of the frontier-orbital approach. MNDO and nonempirical calculations in the 4-31G basis have shown that most of these reactions take place under normal electronic conditions. The -electronic stabilization energies of the transition states of the reactions have been evaluated. The influence of substituents on the relative rate and regioselectivity of the reactions of phosphaalkenes with derivatives of 1,3-butadiene has been considered.Institute of Organic Chemistry, Academy of Sciences of the Ukrainian SSR, Kiev. Scientific-Research Institute of Technical Physics, Far-Eastern University. Vladivostok. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 27, No. 4, pp. 462–466, July–August, 1991. Original article submitted October 12, 1990.     

Theory of fast multiple bond-switching reactions: NO + NH2

A theoretical treatment is given for fast, multiple bond-switching reactions, such as NO + NH₂ → N₂ + H₂O. These reactions are characterized by all or most of the bonds being broken. The collision complex involved (whether long or short lived) is shown to be extremely anharmonic. Consideration of the master equation describing the competing processes of complex formation, internal rearrangement and collisional deactivation yields easily applied sufficient conditions for the recombination rate coefficient being independent of pressure.     

Double electron transfer reactions of CO22+ ions

Time-of-flight techniques have been used to measure fast neutral CO₂ products from double electron transfer reactions of CO₂²⁺ ions with 4.0–7.0 keV impact energies. Double electron transfer cross sections have been determined to be in the range of (1.1–12.5) × 10^?16 cm² for reactions of CO₂²⁺ ions with CO₂, CO, N₂, Ar and O₂.     

Application of RRKM theory to the reactions OH + NO2 + N2 → HONO2 + N2 (1) and ClO + NO2 + N2 → ClONO2 + N2 (2); a modified gorin model transition state

Rate constants as a function of both temperature and pressure were calculated for the title reactions using RRKM theory in conjunction with a modified Gorin transition state. The modification introduces a hindrance parameter which accounts for repulsive interactions between the rotating fragments. At the highest stratospheric pressures (～50 torr) and at stratospheric temperature (～220°K), the extent of “falloff” from first-order [N₂] dependence is ～70% for reaction (1) and ～35% for reaction (2).     

The formation of NO+ from the reaction of N2(2+) with O2

We have studied the potentially ionospherically significant reaction between N(2)2+ with O2 using position-sensitive coincidence spectroscopy. We observe both nondissociative and dissociative electron transfer reactions as well as two channels involving the formation of NO+. The NO+ product is formed together with either N+ and O in one bond-forming channel or O+ and N in the other bond-forming channel. Using the scattering diagrams derived from the coincidence data, it seems clear that both bond-forming reactions proceed via a collision complex [N2O2]2+. This collision complex then decays by loss of a neutral atom to form a daughter dication (NO2(2+) or N2O2+), which then decays by charge separation to yield the observed products.     

State-selective preparation of NO2+ and the effects of NO2+ vibrational mode on charge transfer with NO

Two color resonance-enhanced multiphoton ionization (REMPI) scheme of NO(2) through the E (2)Sigma(u)(+) (3psigma) Rydberg state was used to prepare NO(2)(+) in its ground and (100), (010), (02(0)0), (02(2)0), and (001) vibrational states. Photoelectron spectroscopy was used to verify >96% state selection purity, in good agreement with results of Bell et al. for a similar REMPI scheme. The effects of NO(2)(+) vibrational excitation on charge transfer with NO have been studied over the center-of-mass collision energy (E(col)) range from 0.07 to 2.15 eV. Charge transfer is strongly suppressed by collision energy at E(col) < approximately 0.25 eV but is independent of E(col) at higher energies. Mode-specific vibrational effects are observed for both the integral and differential cross-sections. The NO(2)(+) bending vibration strongly enhances charge transfer, with enhancement proportional to the bending quantum number, and is not dependent on the bending angular momentum. The enhancement results from increased charge transfer probability in large impact parameter collisions that lead to small deflection angles. The symmetric stretch also enhances reaction at low collision energies, albeit less efficiently than the bend. The asymmetric stretch has virtually no effect, despite being the highest-energy mode. A model is proposed to account for both the collision energy and the vibrational state dependence.     

Electronic reorganization triggered by electron transfer: The intervalence charge transfer of a Fe3+/Fe2+ bimetallic complex

The key role of the molecular orbitals in describing electron transfer processes is put in evidence for the intervalence charge transfer (IVCT) of a synthetic nonheme binuclear mixed‐valence Fe³⁺/Fe²⁺ compound. The electronic reorganization induced by the IVCT can be quantified by controlling the adaptation of the molecular orbitals to the charge transfer process. We evaluate the transition energy and its polarization effects on the molecular orbitals by means of ab initio calculations. The resulting energetic profile of the IVCT shows strong similarities to the Marcus' model, suggesting a response behaviour of the ensemble of electrons analogue to that of the solvent. We quantify the extent of the electronic reorganization induced by the IVCT process to be 11.74 eV, a very large effect that induces the crossing of states reducing the total energy of the transfer to 0.89 eV. © 2015 Wiley Periodicals, Inc.     

Evidence for single electron transfer in the reactions of organometallic compounds with 2,3,3-trimethyl-2-butyl peroxybenzoate

Utilization of a triptoxy radical probe in the reactions of organometallic compounds with 2,3,3-trimethyl-2-butyl peroxybenzoate indicates that the radical character of the reaction decreases in the order $\text{n}$-BuLi > $\text{n}$-BuMgCl > PhMgBr.     

Photochemical electron transfer reactions in alkylcobaloximes

It is shown that photolysis with visible light (λ > 420 nm) of any alkylcobaloxime procedes via a mechanism involving an initial electron transfer reaction from an equatorial ligand to the central metal to produce a cobalt(II) species which retains both original axial ligands. In a subsequent rearrangement of the equatorial ligand a hydrogen atom is ejected.     

Thermal [2+2] cycloaddition of morpholinoenamines with C60 via a single electron transfer

The thermal reaction of C(60) with five- and six-membered morpholinocycloalkenes in refluxing toluene exclusively gave the [2+2] cycloadducts in high yields. However, a seven-membered homologue sluggishly reacted with C(60) because of the increasing steric hindrance. This cycloaddition reaction is likely to proceed via a single electron transfer (SET), a radical-coupling, and subsequent ion cyclization rather than the prior proton transfer between the radical ions.