The thermal charge-transfer reduction of uranyl UO22+(VI) to UO2+(V) by various functionalized organic compounds, and evidence for possible spin-spin interactions between UO2+(V) and hydroxymethyl ([rad]CH2OH) radical and between UO2+(V) and diphenyl sulfide radical cation (Ph2S[rad]+)

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The linear uranyl UO22+(VI) cation (D∞h symmetry) exhibited strong and broad absorptions at 350–400 nm in anhydrous methanol and methanol-water mixtures in the UV-Vis spectra. The intensity of the absorptions (represented by absorbance at 375 nm) is directly proportional to molar concentrations of methanol and UO22+(VI), respectively. The linear relationships indicate formation of an electron-donor-acceptor (EDA) complex [UO22+, CH3OH]. The absorptions at 350–400 nm originate from the charge-transfer (single-electron transfer) from CH3OH (electron donor) to UO22+ (electron acceptor) within the [UO22+, CH3OH] complex. Electron paramagnetic resonance (EPR) studies of various mixtures of UO22+-CH3OH and UO22+-CH3OH-H2O have shown that the charge-transfer also took place slowly in the dark, resulting in thermal reduction of UO22+(VI) to UO2+(V) (singlet, g = 2.08) by CH3OH, and CH3OH was oxidized to the hydroxymethyl CH2OH radical (generating an axial signal). The charge-transfer oxidation-reduction reaction is believed to take place via the EDA [UO22+, CH3OH] complex. EPR studies suggested spin-spin coupling between UO2+(V) and CH2OH in anhydrous methanol, supporting the formation of a [UO2+, CH2OH] ion-radical pair. The EPR studies have also shown that UO22+(VI) was reduced to UO2+(V) thermally by other alcohols (ethanol, 2-propanol, and cyclohexanol), and by diphenyl sulfide (Ph2S), l-ascorbic acid (AA), and 2-methyl-5-(propan-2-yl)phenol (carvacrol, ArOH), respectively. Ph2S, AA, and ArOH were oxidized to the diphenyl sulfide Ph2S+ radical cation (singlet, g = 2.00), ascorbic acid AA radical (singlet, g = 2.00), and carvacrol ArO radical (singlet, g = 1.98), respectively. Both EPR and UV-Vis studies indicate that the reactions followed the ground-state charge-transfer mechanisms similar to that of the UO22+/methanol reaction. EPR evidence supported formation of the [UO2+, Ph2S+] ion-radical pair in the charge-transfer reaction of UO22+ and Ph2S and spin-spin interactions within the ion-radical pair. The sulfuric-acid-catalyzed isomerization of CH2OH to CH3O was found by EPR studies.