Identification of Transient Radical Anions (LiClO4)n– (n = 1–3) in THF Solutions: Experimental and Theoretical Investigation on Electron Localization in Oligomers

Effect of temperature on the complexation of triscarbonatouranyl(VI) with calcium and magnesium in NaCl aqueous solution

The complex formation of triscarbonatouranyl(VI) UO₂(CO₃)₃^4- with the alkaline earth metal ions Mg²⁺ and Ca²⁺ in 0.10 mol kg_w^-1 NaCl was studied at variable temperatures: 5-30 °C for Mg²⁺ and 10-50 °C for Ca²⁺. Under appropriate conditions, the ternary complexes (M_nUO₂(CO₃)₃^(4-2n)- with n = 1 for Mg, n = {1; 2} for Ca) were identified by time-resolved laser-induced luminescence spectrometry. Their pure spectral components at 50 °C for Ca_nUO₂(CO₃)₃^(4-2n)- and 30 °C for MgUO₂(CO₃)₃^2- were recovered by multivariate curve resolution alternating least-squares analysis. Approximation models were tested to fit the experimental data-the equilibrium constants of complexation measured at different temperatures-and deduce the thermodynamic functions, i.e., enthalpy, entropy, and heat capacity. The weak influence of temperature on complexation constants induces large uncertainties in terms of thermodynamic functions. Assuming the enthalpy is constant with temperature using the Van't Hoff equation, the first stepwise complexation of UO₂(CO₃)₃^4- by Ca²⁺ is estimated to be slightly endothermic, with , while the second stepwise complexation of CaUO₂(CO₃)₃^2- by Ca²⁺ with is slightly exothermic, . In contrast to Ca²⁺, the complexation of UO₂(CO₃)₃^4- by Mg²⁺ is slightly exothermic, with . These values are not significantly different from zero inasmuch as the uncertainties are important due to a weak dependence of log₁₀ K° values. The entropic character of the complexation is verified as for the first stepwise complexation of UO₂(CO₃)₃^4- by Ca²⁺, for the second stepwise complexation of CaUO₂(CO₃)₃^2- by Ca²⁺, and for the complexation of UO₂(CO₃)₃^4- by Mg²⁺. The energetics of complexation and sensitivity analysis of the model estimates with temperature are discussed. The uranium speciation in the case of the safety of nuclear waste management, using the present thermodynamic functions, provides support to the assessment of underground nuclear waste repositories.

Radiolytic Oxidation of Two Inverse Dipeptides, Methionine-Valine and Valine-Methionine: A Joint Experimental and Computational Study

The two inverse peptides methionine-valine (Met-Val) and valine-methionine (Val-Met) are investigated in an oxidative radiolysis process in water. The OH radical yields products with very different absorption spectra and concentration effects: Met-Val yields one main product with a band at about 400 nm and other products at higher energies; there is no concentration effect. Val-Met yields at least three products, with a striking concentration effect. Molecular simulations are performed with a combination of the Monte Carlo, density functional theory, and reaction field methods. The simulation of the possible transients enables an interpretation of the radiolysis: (1) Met-Val undergoes an H atom uptake leaving mainly a neutral radical with a 2-center-3-electron (2c-3e) SN bond, which cannot dimerize. Other radicals are present at higher energies. (2) Val-Met undergoes mainly an electron uptake leaving a cation monomer with a (2c-3e) SO bond and a cation dimer with a (2c-3e) SS bond. At higher energies, neutral radicals are possible. This cation monomer can transfer a proton toward a neutral peptide, leaving a neutral radical.

Mechanism of (SCN)2·- Formation and Decay in Neutral and Basic KSCN Solution under Irradiation from a Pico- to Microsecond Range

The detailed mechanism of the reaction between SCN^- and the OH^· radical and the formation of the dimer radical (SCN)₂^·- are studied by picosecond pulse radiolysis. First, concentrated SCN^- solutions are used to observe directly the formation and decay of SCNOH^·- in neutral and basic solutions. Then, the spectro-kinetic data, constituting a large matrix of data of the absorbance at different times and different wavelengths, obtained by pulse radiolysis measurements with a streak camera, in neutral and basic SCN^- solutions, are analyzed simultaneously. Data analysis allowed us to deduce the absorption spectra of different radicals with their extinction coefficient and also to determine the rate constants of different reactions involved in the formation and decay of (SCN)₂^·-. Molecular simulations of the absorption spectra of the different species were also performed. The absorption spectrum of the radical SCN^· is determined and is found to be different than that reported previously. It does not present a Gaussian shape centered at 330 nm; the absorption around 310 and 380 nm is not negligible. In addition, in a solution at pH 13, it is found that the (SCN)₂^·- radical is paired with an alkaline cation, inducing a blueshift of the absorption band compared to the free (SCN)₂^·-. Finally, the presence of K⁺ cations catalyzes the disproportionation reaction of (SCN)₂^·- and affects the kinetics.

Picosecond Pulse Radiolysis Study on the Radiation-Induced Reactions in Neat Tributyl Phosphate

The ultrafast radiolytic behavior of tributyl phosphate, TBP, has been investigated using 7 ps electron pulses with 7 MeV kinetic energy, from which two key species have been observed and characterized: the TBP solvated electron (e_TBP^-) and the TBP triplet excited state TBP* (³a) or its fragmentation products. The e_TBP^- exhibits a broad absorption band in the visible and near-infrared (NIR) spectrum, with a maximum beyond our 1500 nm detection limit. Nitromethane was used to scavenge e_TBP^- to confirm its absorption spectrum and to determine its associated rate coefficient, 1.0 × 10¹⁰ M^-1 s^-1. The electron's molar extinction coefficients were found by an isosbestic method using biphenyl as a solvated electron scavenger. The time-dependent radiolytic yield of e_TBP^- was also determined directly from 7 ps to 7 ns and compared with those in water, tetrahydrofuran, and diethyl carbonate. In less than 10 ns, the decay is not due to the reaction with other solvent molecules and is instead predominantly due to the reactions with cations issued from the proton transfer by the TBP radical cation (TBP^•+). In addition to e_TBP^-, another absorption band, stable up to 7 ns, was identified in the visible range. This has been attributed mainly to the TBP triplet excited state, TBP*(³a), by a combination of molecular modeling methodologies. Interestingly, we did not observe any absorption band in the visible nor in the NIR range arising from TBP^•+. Calculations suggest that TBP^•+ undergoes rapid proton transfer to yield a UV-absorbing species, TBP(-H⁺). Experimental results and supporting molecular simulations provide detailed identification of the earliest species yielded from the radiolysis of neat TBP.

Effect of the solvation state of electron in dissociative electron attachment reaction in aqueous solutions

It is generally considered that the pre-solvated electron and the solvated electron reacting with a solute yield the same product. Silver cyanide complex, Ag(CN)₂^-, is used as a simple probe to demonstrate unambiguously the existence of a different reduction mechanism for pre-hydrated electrons. Using systematic multichannel transient absorption measurements at different solute concentrations from millimolar to decimolar, global data analysis and theoretical calculations, we present the dissociative electron attachment on Ag(CN)₂^-. The short-lived silver complex, Ag⁰(CN)₂^2-, formed by hydrated electron with nanosecond pulse radiolysis, can be observed at room temperature. However, at higher temperatures only the free silver atom, Ag⁰, is detected, suggesting that Ag⁰(CN)₂^2- dissociation is fast. Surprisingly, pulse radiolysis measurements on Ag(CN)₂^- reduction, performed by a 7 ps electron pulse at room temperature, show clearly that a new reduced form of silver complex, AgCN^-, is produced within the pulse. This species, absorbing at 560 nm, is not formed by the hydrated electron but exclusively by its precursor. DFT calculations show that the different reactivity of the hydrated and pre-hydrated electrons can be due to the formation of different electronic states of Ag⁰(CN)₂^2-: the prehydrated electron can form an excited state of this complex, which mainly dissociates into Ag⁰CN^- + CN^-.

Reaction mechanisms in swelling clays under ionizing radiation: influence of the water amount and of the nature of the clay mineral

Picosecond pulse radiolysis experiments performed on natural swelling clays evidence a fast trapping of electrons in the layers of the material.