Harnessing ultrasound-derived hydroxyl radicals for the selective oxidation of aldehyde functions.

Ultrasonic irradiation holds potential for the selective oxidation of non-volatile organic substrates in the aqueous phase by harnessing hydroxyl radicals as chemical initiators. Here, a mechanistic description of hydroxyl radical-initiated glyoxal oxidation is constructed by gleaning insights from photolysis and radiation chemistry to explain the yields and kinetic trends for oxidation products. The mechanistic description and kinetic measurements reported herein reveal that increasing the formation rate of hydroxyl radicals by changing the ultrasound frequency increases both the rates of glyoxal consumption and the selectivity towards C2 acid products over those from C-C cleavage. Glyoxal consumption also occurs more rapidly and with greater selectivity towards C2 acids under acidic conditions, which favor the protonation of carboxylate intermediates into their less reactive acidic forms. Leveraging such pH and frequency effects is crucial to mitigating product degradation by secondary reactions with hydroxyl radicals and oxidation products (specifically hydrogen peroxide and superoxide). These findings demonstrate the potential of ultrasound as a driver for the selective oxidation of aldehyde functions to carboxylic acids, offering a sustainable route for valorizing biomass-derived platform molecules.

Quasi-Free Electron-Mediated Radiation Sensitization by C5-Halopyrimidines.

Substitution of the thymidine moiety in DNA by C5-substituted halogenated thymidine analogues causes significant augmentation of radiation damage in living cells. However, the molecular pathway involved in such radiosensitization process has not been clearly elucidated to date in solution at room temperature. So far, low-energy electrons (LEEs; 0-20 eV) under vacuum condition and solvated electrons (esol-) in solution are shown to produce the σ-type C5-centered pyrimidine base radical through dissociative electron attachment involving carbon-halogen bond breakage. Formation of this σ-type radical and its subsequent reactions are proposed to cause cellular radiosensitization. Here, we report time-resolved measurements at room temperature, showing that a radiation-produced quasi-free electron (eqf-) in solution promptly breaks the C5-halogen bond in halopyrimidines forming the σ-type C5 radical via an excited transient anion radical. These results demonstrate the importance of ultrafast reactions of eqf-, which are extremely important in chemistry, physics, and biology, including tumor radiochemotherapy.

Real-Time Observation of Solvation Dynamics of Electron in Actinide Extraction Binary Solutions of Water and n-Tributyl Phosphate.

The excess electron in solution is a highly reactive radical involved in various radiation-induced reactions. Its solvation state critically determines the subsequent pathway and rate of transfer. For instance, water plays a dominating role in the electron-induced dealkylation of n-tributyl phosphate in actinide extraction processing. However, the underlying electron solvation processes in such systems are lacking. Herein, we directly observed the solvation dynamics of electrons in H-bonded water and n-tributyl phosphate (TBP) binary solutions with a mole fraction of water (Xw) varying from 0.05 to 0.51 under ambient conditions. Following the evolution of the absorption spectrum of trapped electrons (not fully solvated) with picosecond resolution, we show that electrons statistically distributed would undergo preferential solvation within water molecules extracted in TBP. We determine the time scale of excess electron full solvation from the deconvoluted transient absorption-kinetical data. The process of solvent reorganization accelerates by increasing the water molar fraction, and the rate of this process is 2 orders of magnitude slower compared to bulk water. We assigned the solvation process to hydrogen network reorientation induced by a negative charge of the excess electron that strongly depends on the local water environment. Our findings suggest that water significantly stabilizes the electron in a deeper potential than the pure TBP case. In its new state, the electron is likely to inhibit the dealkylation of extractants in actinide separation.