ИСТИНА

Reactions of excess electrons with organic molecules play a very important role in radiation chemistry with a number of implications, including radiobiology, molecular electronics, and fabrication of nanostructures. Meanwhile, many compounds known as “electron scavengers” actually have virtually zero or negative gas-phase electron affinity (EA). Apparently the molecular interactions and solvation should play crucial role in reactions of such molecules with electrons and stabilization of the corresponding radical anions in condensed phase. However, this aspect was not studied in detail from experimental and theoretical points of view. In this work we present the results of experimental and computational studies of the excess electron capture by ketone molecules in low-temperature matrices of different polarity at 77 K. Excess electrons were generated by X-ray irradiation of solutions of acetone, diacetyl and diacetonyl in glassy diethyl ether, methylal, 2-MTHF, 3-methylpentane and MCH. Electron capture and stabilization of ketone radical anions was monitored by EPR and optical absorption spectroscopy. In all the cases, the yields of trapped electrons were found to be suppressed markedly in the presence of ketones. In the case of diacetyl with significant EA (≈ 0.7 eV), diacetyl radical anion was unambiguously identified in all the matrices at relatively low ketone concentration. The EPR spectrum of this species in agreement with known data. In the case of acetone (EA < 0) we failed to observe any definite sign of trapped radical anion at low ketone concentrations (< 1 mol %). This implies that, in contrast with some previous reports, monomeric acetone radical anion is not stabilized in low-polar media. However, increasing the acetone concentration up to 2 – 7.5 mol%, resulted in appearance of new optical absorption band (λmax ≈ 460 nm), which can be ascribed presumably to dimer or cluster acetone radical anions. Meanwhile, in the case of diacetonyl, radical-anion was not found in non-polar hydrocarbon matrices. However, in low-polar ether matrices (diethyl ether, 2-MTHF, methylal) we have observed a well-defined multiplet EPR signal and a broad absorption band (λmax ≈ 600 nm) in the UV/vis spectrum at low ketone concentration (≤ 1 mol %). These features were attributed to a previously unknown diacetonyl radical anion stabilized by the ether environment. To get a deeper insight into this effect, we have made extensive quantum-chemical calculations of cluster radical anions consisting of diacetonyl and dimethyl ether molecules at the MP2 level. These calculations, indeed, predict a stabilizing effect of ether: computed vertical detachment energy (VDE) increases from zero (or slightly negative) up to the value of ca. 1 eV for the cluster containing six ether molecules. The calculations revealed several stable conformers for clusters; meanwhile, the experimental EPR parameters show the best agreement with the hyperfine coupling constants computed for a symmetric (C2) conformer. Thus, the stabilization of diacetonyl radical anion can be understood in the frame of microsolvation (cluster) concept. In general, our findings clearly reveal the role of association and relatively weak matrix interactions in stabilization of ketone radical anions in condensed media, which implies an opportunity of fine tuning in reactions of excess electrons with molecules of slightly negative EA.