Excited Uranyl Research Articles

Photouranium-Catalyzed C-F Activation Hydroxylation via Water Splitting.

The C-F bond is the strongest covalent single bond (126 kcal/mol) in carbon-centered bonds, in which the highest electronegativity of fluorine (χ = 4) gives rise to the shortest bond length (1.38 Å) and the smallest van der Waals radius (rw = 1.47 Å), resulting in enormous challenges for activation and transformation. Herein, C-F conversion was realized via photouranium-catalyzed hydroxylation of unactivated aryl fluorides using water as a hydroxyl source to deliver multifunctional phenols under ambient conditions. The activation featured cascade sequences of single electron transfer (SET)/hydrogen atom transfer (HAT)/oxygen atom transfer (OAT), highly integrated from the excited uranyl cation. The *UO22+ prompted water splitting under mild photoexcitation, caging the active oxygen in a peroxo-bridged manner for the critical OAT process and releasing hydrogen via the HAT process.

The effect of ancillary ligands on hydrocarbon C-H bond functionalization by uranyl photocatalysts.

The aqueous uranyl dication has long been known to facilitate the UV light-induced decomposition of aqueous VOCs (volatile organic compounds), via the long-lived highly efficient, uranyl excited state. The lower-energy visible light excited uranyl ion is also able to cleave unactivated hydrocarbon C-H bonds, yet the development of this reactivity into controlled and catalytic C-H bond functionalization is still in its infancy, with almost all studies still focused on uranyl nitrate as the precatalyst. Here, hydrocarbon-soluble uranyl nitrate and chloride complexes supported by substituted phenanthroline (Ph2phen) ligands are compared to each other, and to the parent salts, as photocatalysts for the functionalization of cyclooctane by H atom abstraction. Analysis of the absorption and emission spectra, and emission lifetimes of Ph2phen-coordinated uranyl complexes demonstrate the utility of the ligand in light absorption in the photocatalysis, which is related to the energy and kinetic decay profile of the uranyl photoexcited state. Density functional theory computational analysis of the C-H activation steps in the reaction show how a set of dispersion forces between the hydrocarbon substrate and the Ph2phen ligand provide control over the H atom abstraction, and provide predictions of selectivity of H atom abstraction by the uranyl oxo of the ring C-H over the ethyl C-H in an ethylcyclohexane substrate.

Excited-State Chemistry: Photocatalytic Methanol Oxidation by Uranyl@Zeolite through Oxygen-Centered Radicals.

We have elucidated the complex reaction network of partial methanol oxidation, H3COH + O2 → H2CO + H2O2, at a visible-light-activated actinide photocatalyst. The reaction inertness of C-H bonds and O═O diradicals at ambient conditions is overcome through catalysis by photoexcited uranyl units (*UO22+) anchored on a mesoporous silicate. The electronic ground- and excited-state energy hypersurfaces are investigated with quasirelativistic density-functional and ab initio correlated wave function approaches. Our study suggests that the molecular cluster can react on the excited energy surface due to the longevity of excited uranyl, typical for f-element compounds. The theoretically predicted energy profiles, chemical intermediates, related radicals, and product species are consistent with various experimental findings. The uranyl excitation opens various reaction pathways for the oxidation of volatile organic compounds (VOCs) by "hole-driven hydrogen transfer" (HDHT) through several exothermic steps over low activation barriers toward environmentally clean or chemically interesting products. Quantum-chemical modeling reveals the high efficiency of the uranyl photocatalysis and directs the way to further understanding and improvement of VOC degradation, chemical synthesis, and biologic photochemical interactions between uranyl and the environment.

Deactivation of lowest excited state of uranyl in the presence of acetate: A DFT exploration

Abstract Interaction of uranyl with acetate is studied by density functional theory (DFT) calculations. Particular attention is given to the interaction of excited uranyl with acetate to explain the quenching in aqueous solution and the photodecomposition of acetate in the presence of uranyl. The structures of uranyl with the acetate ion in monodentate and bidentante coordinations are calculated in the ground and in the lowest lying excited triplet state. Uranyl-acetate is found to be stable in both coordinations in the lowest lying triplet state. Uranyl can also form a number of stable structures with acetate involving apical oxygen in the triplet state. The photoreduction of U(VI) associated with hydrogen abstraction from ligand is observed in the case of uranyl coordinated by two molecules of acetic acid in monodentate and apical modes. The stability of the [UO(OH)(H 2 O) 4 ] 2+ species is also examined in the ground state. It is found that this structure is stable. These species may play a key role in the dynamic quenching of uranyl. The structures of [UO(OH)(H 2 O) 4 ] 2+ coordinated with acetate in monodentate and bidentate modes are also investigated. The nonradiative deactivation of the uranyl excited state in the presence of acetate with photodecomposition of acetate could not be demonstrated, taking into account the lowest lying triplet state of excited uranyl.

Formation of excited uranyl in oxidation of tetravalent uranium with oxygen in HClO4 aqueous solutions: IV. Enhancement of the chemiluminescent reaction in the presence of Fe2+

Experimental data that support the hypothesis on the determining role of •OH radicals in the emergence of luminescence during the oxidation of U(IV) with atmospheric oxygen in aqueous HClO4 solutions have been obtained using the H2O2-FeSO4 system as a source of OH radicals. It has been found that brighter chemiluminescence (CL) is observed in the presence of 10−5 mol/l Fe2+ in a 5 × 10−4 mol/l U(IV) solution in 0.1 mol/l HClO4 compared with the FeSO4-free solution. The CL yield in the presence of Fe2+ (ηCL = 3.9 × 10−8) is 2.8 times that in the solution without iron (ηCL = 1.4 × 10−8). These results can be regarded as a further piece of evidence for the idea that the elementary event of the formation of a CL emitter—electronically excited uranyl ion *(UO22+)—in radical chain U(IV) oxidation reactions is electron transfer from the uranoyl ion (UO2+) to the oxidant, the •OH radical. Thus, one of the main prerequisites for light emission during U(IV) oxidation reactions is a high generation efficiency of •OH radicals and their easy access to the uranoyl UO2+ ion.

EPR study of the production of oh radicals in aqueous solutions of uranium irradiated by ultraviolet light

The aim of the study was to establish whether hydroxyl radicals (?OH) were produced in UV-irradiated aqueous solutions of uranyl salts. The production of ?OH was studied in uranyl acetate and nitrate solutions by an EPR spin trap method over a wide pH range, with variation of the uranium concentrations. The production of ?OH in uranyl solutions irradiated with UV was unequivocally demonstrated for the first time using the EPR spin-trapping method. The production of ?OH can be connected to speciation of uranium species in aqueous solutions, showing a complex dependence on the solution pH. When compared with the results of radiative de-excitation of excited uranyl (+22*UO ) by the quenching of its fluorescence, the present results indicate that the generation of hydroxyl radicals plays a major role in the fluorescence decay of + 22 *UO . The role of the presence of carbonates and counter ions pertinent to environmental conditions in biological systems on the production of hydroxyl radicals was also assessed in an attempt to reveal the mechanism of +22*UO de-excitation. Various mechanisms, including ?OH production, are inferred but the main point is that the generation of ?OH in uranium containing solutions must be considered when assessing uranium toxicity.

Formation of excited uranyl ion in oxidation of U(IV) with xenon difluoride in aqueous solutions of sulfuric and deuterated sulfuric acids

Kinetic features of the chemiluminescence accompanying oxidation of U(IV) with xenon difluoride in solutions of 0.2 M H2SO4 in H2O and 0.2 M of D2SO4 in D2O at temperatures within 283–313 K and concentrations (M) 10−6 ≤ [U(IV)] ≤ 10−4 and 10−4 ≤ [XeF2] ≤ 10−3 were examined. The shape of the kinetic curve depends on the concentration of the reactants and the solution temperature. At [U(IV)] = 10−6 and [XeF2] = 10−4 M, the curve exhibits a maximum, more prominent at low temperatures. At relatively high concentration ([U(IV)] = 10−4 and [XeF2] = 10−3 M), self-acceleration of the reaction is observed: The mechanism of oxidation of U(IV) turns into a branched-chain mode owing to a higher concentration in the solution of radicals ·OH, HO 2 · , and XeF 2 · , whose additional amounts are yielded by hydrolytic reduction of XeF2. A kinetic isotope effect of the solvent k H/k D was revealed, which attains a maximum of 1.8 at 283 K. The activation energies of the oxidation of U(IV) with xenon difluoride in H2SO4 and D2SO4 solutions were estimated at E a,H = 12 and E a,D = 13 kcal mol−1, respectively. The occurrence of the isotope effect is an indirect evidence of participation in the reaction of the OH (or OD) groups. The rate of hydrolytic reduction of XeF2 in deuterated solvent (0.2 M D2SO4 in D2O) in its photochemical stage is several times lower, and the luminescence accompanying the reaction is by an order of magnitude smaller than that in 0.2 M H2SO4.

Photoconversion and photocatalytic activity of uranyl in acetone solutions

We have studied the effect of irradiation on the uranyl nitrate — acetone and uranyl perchlorate — acetone systems. We have established that when the uranyl perchlorate — acetone system is irradiated, polymerization of the acetone occurs and the catalyst for the process is excited uranyl complexes. In the polymer, uranium is found in the form of nanoclusters of pentavalent and tetravalent uranium, formed as a result of photochemical reactions. Polymerization does not occur in the uranyl nitrate — acetone system. We consider possible factors responsible for the noted differences.

Formation of excited uranyl ion in oxidation of U(IV) with xenon trioxide in aqueous H2SO4 and HClO4 solutions: III. Kinetic mode of self-accelerated reaction

Self-acceleration was revealed for chemiluminescent radical-chain oxidation of U(IV) with xenon trioxide in aqueous HClO4 solutions in a reactor made of Teflon (instead of conventionally used glass). In oxidant excess, U(IV) oxidation with XeO3 follows the first-order kinetics only at low (under 10−6 M) concentrations of U(IV) in solution. Self-acceleration is observed at relatively high concentrations of the reactants, when the contribution of the heterogeneous decay of the radicals to chain termination substantially decreases and that of the degenerate branching reactions to the U(IV) oxidation mechanism increases. Oxidation of U(IV) with XeO3 demonstrates a critical phenomenon, uncommon for liquid-phase radical-chain processes: a minor increase in the concentrations of the reactants is responsible for transition from a steady-state mode to self-acceleration of the redox process.

Time-resolved luminescence studies in hydrogen uranyl phosphate intercalated with amines

Time-resolved luminescence decays of intercalated compounds of hydrogen uranyl phosphate (HUP) with p-toluidinium (HUPPT), benzylaminium (HUPBZ), α–methylbenzylaminium (HUPMBZ) and hydroxylaminium (HUPHAM) were studied. The prepared compounds belong to the tetragonal P4/ncc space group and showed 00 l reflections shifted to lower angles relative to HUP, indicating that the intercalation increases the c parameter of the unit cell. The luminescence decays of the compounds with 100% of intercalation ratio (HUPHAM and HUPBZ) were analyzed by Global Analysis, assuming Lianos’ stretched exponential as the model function, which can be applied to compounds with restricted geometry and mobile donor and quencher molecules. It was remarkable that the luminescence decays showed that the quenching of the emission of the uranyl ions by the intercalated protonated amines is not restricted by low dimensionality of the host uranyl phosphate, and that a diffusion mechanism occurs. Benzylaminium cation efficiently quenches the excited energy of the uranyl ions at close distance, but the long-range and long-lifetime quenching is hindered. A different situation is found in the case of the small hydroxylaminium cation, where the long distance diffusion of the species is fast, playing an important role in the quenching of the excited uranyl ions at longer times.

In situ FT-IR studies on mechanistics of heterogeneous photocatalytic oxidation of ethene over uranyl species anchored on MCM-41

The uranyl species encapsulated within the mesopores of siliceous MCM-41 serves as efficient heterogeneous photo-catalyst for the sunlight-assisted direct oxidation of ethene. The mode of oxidation is through abstraction of H-atom from ethene by the photolytically excited uranyl species and the consequent formation of peroxy species. The in situ IR spectroscopy results indicate that these peroxy species give rise to final products such as carbon dioxide and water on further oxidation via formation of formate-type transient species. Furthermore, the silanol groups of the host matrix help in immobilization of these peroxy species through hydrogen bonding and, at the same time, they participate in the subsequent oxidation reactions also.

Influence of Temperature on the Lifetime of Electronically Excited Uranyl Ion: III. Effect of Solvent Deuteration

The shape of the temperature dependences of the lifetime (τ) of electronically excited uranyl ion (UO22+)*, measured on warming from 77 K of UO2SO4 solutions in H2SO4 + H2O, differs from that measured in D2SO4 + D2O. The lifetime in glassy D2SO4 is shorter than in the polycrystalline sample, whereas in frozen H2SO4 solutions the pattern is opposite. The isotope effect is due both to different course of phase transitions in H2SO4 and D2SO4 solutions and to the fact that the probability of nonradiating deactivation of (UO22+)* adsorbed on the surface of D2SO4·4H2O crystal hydrate is appreciably lower than that of (UO22+)* adsorbed on the surface of H2SO4·4H2O and H2SO4·6.5H2O crystal hydrates. We suggest that the increase in τ due to adsorption appreciably exceeds the concentration quenching of excited uranyl ions in the course of crystallization of deuterated sulfuric acid.

Formation of Excited Uranyl Ion in Oxidation of U(IV) with Xenon Trioxide in Aqueous Solutions of H2SO4and HClO4: I. Influence of Sulfate Anions

Kinetic and chemiluminescent parameters of U(IV) oxidation with xenon trioxide in 1 M H2SO4 were studied in relation to complexation of U(IV) with SO42- anions. The rate constant of oxidation of the sulfate complexes increases from ∼1 to 43 l mol-1 s-1 with increasing [Na2SO4] in the solution from 10-3 to 3×10-1 M. The equilibrium concentrations of aqua- and mono- and bisulfate complexes of U(IV) were calculated at different [Na2SO4] in the solution. The rate constant of reaction of U(SO4)2 with XeO3 [k = 72 l mol-1 s-1] is higher by an order of magnitude than the rate constant of oxidation of U(IV) monosulfate complex [k = 6.5 l mol-1 s-1]. The dependence of the lifetime of excited uranyl ions (τ) on the Na2SO4 concentration in 1 M HClO4 was measured. The chemiluminescence (CL) yield increases in the presence of sulfate anions owing to the growth of both the emission output of excited uranyl sulfate complexes and the yield of excitation of the CL emitter.

Photooxidation of cellulose acetate and cellobiose by the uranyl ion.

The photooxidation of cellulose acetate by uranyl nitrate in acetone solutions has been investigated. Studies of the effect of the polymer on the uranyl luminescence showed an initial increase in intensity, followed by quenching. This is interpreted in terms of competition between complexation of uranyl ions by the polymer and dynamic quenching. In the quenching region, Stern-Volmer kinetics are followed. Upon photolysis of the solution, a decrease in viscosity was observed, consistent with chain scission. However, there was no sign of formation of reduced uranium species, suggesting that they are reoxidised by molecular oxygen. Model studies were carried out with cellobiose and it was confirmed that the luminescence quenching involves both dynamic and static processes. Photolysis of aqueous solutions of cellobiose and uranyl nitrate or perchlorate led to formation of uranium(v) and a decrease in pH. Upon interruption of photolysis, uranium(v) was seen to disproportionate. Yields of reduced uranium species were higher in degassed than aerated solutions, consistent with their oxidation by molecular oxygen in the latter case. Organic radicals were detected by electron paramagnetic resonance spectroscopy upon photolysis of cellulose acetate saturated with uranyl nitrate. The mechanism of photooxidation is suggested to involve hydrogen atom abstraction from the substrate by excited uranyl ions.

Sensitized photo-oxidation of osazone by uranyl ions

Glucosazone quenches excited uranyl ions, which involves excited state charge transfer complex (exciplex) formation. The quenching behaviour shows pH dependence. At pH 2.00, glucosone has been characterized as the photoproduct and intermediacy of uranium(v) is proved spectrophotmetrically. A mechanism has been proposed.

Deactivation processes of the lowest excited state of [UO2(H2O)5]2+ in aqueous solution.

A detailed analysis of the photophysical behaviour of uranyl ion in aqueous solutions at room temperature is given using literature data, together with results of new experimental and theoretical studies to see whether the decay mechanism of the lowest excited state involves physical deactivation by energy transfer or a chemical process through hydrogen atom abstraction. Comparison of the radiative lifetimes determined from quantum yield and lifetime data with that obtained from the Einstein relationship strongly suggests that the emitting state is identical to that observed in the lowest energy absorption band. From study of the experimental rate and that calculated theoretically, from deuterium isotope effects and the activation energy for decay support is given to a deactivation mechanism of hydrogen abstraction involving water clusters to give uranium(v) and hydroxyl radicals. Support for hydroxyl radical formation comes from electron spin resonance spectra observed in the presence of the spin traps 5,5-dimethyl-1-pyrroline N-oxide and tert-butyl-N-phenylnitrone and from literature results on photoinduced uranyl oxygen exchange and photoconductivity. It has previously been suggested that the uranyl emission above pH 1.5 may involve an exciplex between excited uranyl ion and uranium(v). Evidence against this mechanism is given on the basis of quenching of uranyl luminescence by uranium(v), together with other kinetic reasoning. No overall photochemical reaction is observed on excitation of aqueous uranyl solutions, and it is suggested that this is mainly due to reoxidation of UO2+ by hydroxyl radicals in a radical pair. An alternative process involving oxidation by molecular oxygen is analysed experimentally and theoretically, and is suggested to be too slow to be a major reoxidation pathway.

On the kinetics and energetics of one-electron oxidation of 1,3,5-triazines.

One-electron oxidation of 1,3,5-triazines is observed with both excited uranyl ion (*UO2(2+)) and sulfate radical anion (SO4.-) in aqueous solution, but not with Tl2+, indicating that the standard reduction potentials E degree of 1,3,5-triazine radical cations are = 2.3 +/- 0.1 V vs. NHE, consistent with theoretical calculations; this suggests that if triazines inhibit electron transfer during photosynthesis, they would need to act on the reductive part of the electron transport chain.

Photosensitized degradation of dinitrosalicylic acid by uranyl ions

The photodegradation of dinitrosalicylic acid (DNS) by photoexcited uranyl ion was studied in aqueous solutions. The failure of DNS to degrade directly with light highlights the importance of the photoexcited uranyl ion in controlling the photochemical processes. Fluorescence quenching studies showed that an electron‐transfer process from the DNS to the excited uranyl ion is involved leading to the formation of UO2 +/DNS•+ radical pair complex as an initial step. Illumination of the UO2 2+/DNS solution in presence of oxygen results in mineralization of DNS. The results are explained on the basis of a catalytic cycle involving UO2 2+ ion and molecular oxygen that generates reactive superoxide O2 •− anion and its conjugate acid HO2 •. The efficiency of the photocatalytic cycle is enhanced markedly by addition of dilute NaOH solution.

Formation of Excited Uranyl in Oxidation of U(IV) with Oxygen in HClO4 Aqueous Solutions: I. Effect of pH on Kinetic and Chemiluminescence Parameters of the Reaction

Chemiluminescence accompanying U(IV) oxidation with atmospheric oxygen in 0.1-0.0004 M HClO4 was studied. It was found that the electronically excited uranyl ion, (UO2 2 +)*, is the luminescence emitter. The maximum on the kinetic curve is due to accumulation in the solution of UO2 + and H2O2, which are intermediates of U(IV) oxidation. The kinetic scheme of U4 + reaction with O2 suggests that uranyl excitation proceeds in the elementary stage of electron transfer from UO2 + to the oxidant, which is OH radical. With increasing pH from 1 to 3.4, the rate constant of the chemiluminescent stage (k) of the reaction increases almost 50 times and the luminescence efficiency (ηcl), 3 times. The effect of pH on the oxidation rate is due to the high reactivity of U4 + hydrolysis products UOH3+ and U(OH)22+ with respect to O2. The rate constant k of the reaction between U4+ and O2 and the chemiluminescence efficiency are the least among the other U(IV) chemiluminescence reactions: at 298 K and 0.1 M HClO4, k (l mol- 1 s-1) is equal to 3, 36, 40, and 108 and ηc l, to 7.4 × 10- 8, 8.1 × 10- 5, 1.6 × 10- 6, and 3 × 10- 7 for O2, XeO3, H2O2, and HSO5 - as oxidants, respec- tively. The activation energy of the chemiluminescent stage of U(IV) oxidation with oxygen in 0.001 M HClO4 E a is 90.5 kJ mol-1 within the 285-310 K range.

Formation of Excited Uranyl in Oxidation of U(IV) with Oxygen in HClO4 Aqueous Solutions: II. Catalytic Effect of UO22+

Strong effect of uranyl ion on the kinetic and chemiluminescence characteristics of U(IV) oxidation with oxygen in weakly acidic HClO4 solutions is found. For instance, in the presence of 8 × 10-4 M UO22+ the reaction rate constant, maximal intensity of the chemiluminescence, and chemiluminescence efficiency increase by factors of 50, 100, and 8, respectively. The catalytic effect of uranyl ion is probably caused by both formation of the complex UO22+·UO2+ (which decreases the rate of disproportionation of UO2+ participating in the chain propagation) and generation of UO2+ in the reaction of UO22+ with U(IV).