Reactions of excess electrons with organic molecules in glassy matrices. Medium effect on stabilization of radical anions

Complete text of publication follows. Attachment of excess electrons to organic molecules is one of the key primary events in radiation chemistry and radiobiology. The effect of electron scavenging by carbonyl compounds is well known for several decades. Meanwhile, most of these compounds have no significant gas-phase electron affinity (EA). Thus, the formation of radical anions in condensed phases should by often controlled by intermolecular interactions. Here we present the results of experimental and ab initio study of excess electron attachment to a series of monofunctional and bifunctional compounds upon their irradiation in low-temperature matrices with various polarity and polarizability. The electron capture was monitored by suppression of the yield of trapped electrons and formation of radical anions as revealed by EPR and optical spectroscopy. It was found that molecules with EA > 0.5 eV (e.g., diacetyl) gave rather stable radical anions, independent of the matrix used. Meanwhile, the compounds with nearly zero (or negative) EA show dramatic matrix effect. In particular, acetone and diacetonyl radical anions were not observed in hydrocarbon glasses, although the yield of trapped electron is suppressed markedly. This result may be explained by reversible electron capture. Turning to slightly more polar ether matrices leads to the matrix-induced stabilization of diacetonyl radical anion. The quantum-chemical calculations at the MP2 level provide strong support to this concept. They reveal that coordination of 4 to 6 solvent molecules may increase excess electron binding energy from nearly zero to more than 0.5 eV. An alternative is self-stabilization due to formation of dimers, which probably occurs for acetone at higher concentrations. The interplay between these effects is determined by solute concentration, intermolecular interactions and conformation (in particular, for bifunctional species). Preliminary results show that such consideration may be applicable to a wide class of molecules with borderline EA values, including those presenting interest for biology and molecular electronics. This work was supported by the Russian Foundation for Basic Research (grant 09-03-00848a).