Fourier transform EPR study and quantum chemical calculations of dihydrofuran radicals formed by triplet sensitized electron transfer in aqueous solution
The radicals formed by electron transfer from 2,3-dihydrofuran, 2-methyl-4,5-dihydrofuran and 2,3-dimethyl-4,5-dihydrofuran to the laser induced triplet state of anthraquinone-2,6-disulfonic acid were studied in aqueous solution by Fourier transform electron paramagnetic resonance (FT EPR) in the nanosecond time-scale. With 2,3-dimethyl-4,5-dihydrofuran and 2-methyl-4,5-dihydrofuran as electron donor the radical cations were observed directly, whereas with 2,3-dihydrofuran as electron donors the radicals observed are successor radicals of the radical cations. The assignment of the radical structures was done by simulation of the experimental EPR spectra and was supported by quantum chemical density functional theory (DFT) calculations. In the experiments with 2-methyl-4,5-dihydrofuran only one OH−-adduct could be determined although other so far unknown radicals contribute to the measured spectra. With 2,3-dihydrofuran two OH−-adduct radicals and the deprotonated radical cation were detected together with contributions from a dimer radical. The rate constants of the electron transfer to the anthraquinone-2,6-disulfonate triplet and of the formation of the successor radicals from the primary 2,3-dihydrofuran radical cation were determined, respectively. The hyperfine coupling constants calculated by quantum chemical DFT method agree quite well with the experimental one.