Free radical products in X-irradiated Rochelle salt: low temperature ENDOR and DFT studies

Single crystals of Rochelle salt, [−OOC–CHOH–CHOH–COO−, Na+, K+]·4H2O, X-irradiated at 10 K, have been examined using EPR, ENDOR and EIE spectroscopic techniques to characterize the radiation induced radicals stable at that temperature and their reactions upon warming. The one-electron gain product was observed and from the hyperfine interaction with a β-proton it was unambiguously centered at the C4 position of the tartrate moiety. An additional nearly isotropic hyperfine structure of about 21 MHz was tentatively assigned to interaction with a sodium ion exhibiting a close contact to O3 in the crystal. Evidence was obtained that the one-electron reduced radical had become protonated at one of the C4 bonded carboxyl oxygens, most probably O4. No evidence for the corresponding C1-centered reduction product was found. Two resonance lines (R2, A1) were shown by EIE to belong to a species formed by decarboxylation at C3, a secondary oxidation product. Two other resonance lines (K1, K2) were assigned to two varieties of another decarboxylation radical, centered at C2, distinguished by differences in the potassium ion coordination. Furthermore, one other resonance line (A2) was tentatively ascribed to a third decarboxylation radical, centered at the opposite end of the tartrate moiety. The precursor of these products, that is, the one-electron loss product, was not observed after X-irradiation at 10 K. Thermally induced free radical reactions followed by EPR in the temperature range of 12–119 K indicate that a water molecule or a hydroxyl ion is eliminated from the one-electron reduction product radical and that a C3-centered radical is formed. The reduction and oxidation reaction pathways of hydroxy acid derivatives are discussed. - Highlights: ► Crystalline Rochelle salt was X-irradiated at 10 K and studied using EMR techniques. ► Interpretation of present and previous data was aided by cluster DFT calculations. ► Regioselective reduction and non-selective oxidation apparently occur at 10 K. ► Thermally induced free radical reactions were followed to provide reaction mechanisms. ► A complete solid-state radical reaction scheme for (di)carboxylic acids is proposed.