ИСТИНА

Aliphatic carbonyl compounds are widely used as electron scavengers in radiation chemistry. Furthermore, the aliphatic carbonyl fragments are supposed to be responsible for electron scavenging by biomolecules, which may have important implications for radiobiology. Meanwhile, the gas-phase electron affinity (EA) of these compounds is often close to zero, or even negative. In order to get better understanding of the factors controlling stabilization of such radical anions in condensed media, we have studied the attachment of excess electrons to simple diketones in low-polar glassy matrices at 77 K. Excess electrons were generated by X-ray irradiation of dilute solutions of diketones in glassy matrices of 3-methylpentane, methylcyclohexane, diethyl ether, 2-MTHF and methylal. The production of radical anions was monitored by EPR and optical spectroscopy. Irradiation of neat glassy solvents resulted in formation of trapped electrons characterized by narrow singlet signals in the EPR spectra. Addition of diketones (0.5 – 1 mol %) led to remarkable suppression of these signals. In the case of diacetyl (EA ≈ 0.7 eV), the radical anion is stabilized in all the studied matrices. The EPR spectrum of this species (septet with a splitting of ca. 0.5-0.6 mT) is in agreement with the calculated data. In the case of diacetonyl, stabilization of radical anion was not found in nonpolar hydrocarbon matrices (3-methylpentane, methylcyclohexane). Meanwhile, a multiplet EPR signal with a splitting of 0.7-0.8 mT was observed after irradiation at 77 K in glassy ether matrices. This signal correlates with appearance of a characteristic broad absorption band (λmax ~ 620 nm) and it can be attributed to diacetonyl radical anion. The spectral assignment assumes magnetic non-equivalence of the two methyl groups in this radical anion. Quantum-chemical calculations (at the MP2 level) with explicit inclusion of solvent molecules were used to interpret the experimental results. The excess electron may be localized on one carbonyl group or delocalized over both of them, as revealed by theory. The computed vertical ionization potential of diacetonyl radical anion is close to zero for the isolated species, but reaches substantial values (> 0.5 eV) as 4 to 6 ether molecules coordinate the anion. Thus, the intermolecular interactions are crucial for the stabilization of the carbonyl radical anions produced from molecules with nearly zero EA. The work was supported by Russian Foundation for Basic Research (project no. 09-03-00848a).