An Updated View of Primary Ionization Processes in Polar Liquids.

According to picosecond radiolysis data, primary radical cations in irradiated carbonates are very rapidly deprotonated. At the same time, analysis of the radiation-induced fluorescence from carbonate solutions indicates the formation of solvent-related radical cationic species with a relatively long lifetime. We use quantum chemical methods to develop a model of carbonate ionization that reconciles these conflicting data. Using ethylene carbonate as an example and assuming that its molecules exist in solution as a collection of dimeric associates, we show that both processes are the result of the loss of an electron from such dimers. This demonstrates that the generally accepted conceptualization of a primary ionization event, based on the idea of the formation of a radical cation of an individual molecule of an irradiated substance, requires revision in the case of polar aprotic liquids that tend to form molecular associates.

Interaction of spin-correlated radical pair with a third radical: Combined effect of spin-exchange interaction and spin-selective reaction.

The reaction of electron transfer between two paramagnetic particles may be strongly dependent on the total spin state of the pair. Such dependence can be used to control electron transfer in a molecular medium via the control of the spin degrees of freedom. In this work, the spin-selective electron transfer has been studied in a three-spin system composed of a spin-correlated radical ion pair (RIP) and the nitroxide radical, TEMPONE. The RIPs were created in an n-hexane solution of tetramethylpiperidine (TMP) and para-terphenyl (p-TP) using X-rays. To monitor the spin evolution of the RIPs with a nanosecond time resolution, the method of time-resolved magnetic field effect in the RIP recombination fluorescence was applied. It was found that increasing the TEMPONE concentration increased the rate of both the radiation-induced fluorescence intensity decay and the paramagnetic relaxation of the spin-correlated RIP. For the three-spin system studied, we developed a theoretical model to calculate the singlet state population of the spin-correlated RIP that described both the spin-selective reaction and the spin-exchange interaction during an encounter between RIP partners and a third radical. It was found that the effect of the spin exchange could be neglected if the rate of the spin-selective reaction is high enough. Based on quantum chemical calculations and experiments, we found that there was a spin-selective distant electron transfer from p-TP radical anions to the TEMPONE radical. Another partner of the RIP, the radical cation formed from TMP, was only involved in the spin exchange interaction with TEMPONE radicals.