Pulse radiolytic reduction studies of 1,4,4a,8a-tetrahydro-endo–1,4-methano-naphtha-5,8-dione (THMND): effect of tertiary structure

Radiolytic reduction of a substituted 5,8-naptha dione (THMND), synthesized in our laboratory, has been investigated by pulse radiolysis and steady-state γ-radiolysis in pure aqueous, aqueous-formate and in aqueous-2-propanol-acetone mixed solvent systems. The rate constants of formation of the semi-dione radicals were approx. 109 dm3 mol-1 s-1 in aqueous-formate and aqueous-2-propanol-acetone mixed solvent. The semi-dione radicals decay by second order kinetics with rate constants (2k) of about 109 and 108 dm3 mol-1 s-1 in the above two solvents, respectively. The pK a value of the radical was found to be 5.0 in aqueous-formate solution and 5.8 in the aqueous-2-propanol-acetone mixed solvent. The one-electron reduction potential (E 1) value at pH 7, determined from the pulse-radiolysis experiment, was found to be –420 ± 20 mVvs. NHE at 298 K and was independent of solvent. Ab initio calculations on its one-electron reduction reaction suggest the formation of a radical, which is different from a semiquinone where the electron density is delocalised over the two oxygen atoms. Experimental absorption maxima of the radical in aqueous solution also agree very well with the ab initio calculated values. Steady-state γ-radiolysis of THMND produces the corresponding two-electron reduced species.