Radiation-induced radicals in lithium formate monohydrate (LiHCO2·H2O). EPR and ENDOR studies of X-irradiated crystal and polycrystalline samples

Abstract

Single crystals and polycrystalline samples of lithium formate monohydrate (HCO₂Li·H₂O) were X-irradiated at 295 K and studied using X-band EPR, ENDOR, and ENDOR-induced EPR (EIE) spectroscopy at 200 or 295 K. Two different radical species were observed. The overall dominating species is the ˙CO₂⁻ radical trapped in the crystal matrix at an orientation not very different from that of the parent CO₂ fragment in the unirradiated matrix. The g- and ¹³C hyperfine coupling tensors of the ˙CO₂⁻ radical were determined. The large linewidth (about 1.5 mT) of the polycrystalline EPR spectrum is due to extensive hyperfine couplings with lithium ions and protons in the environment. Four lithium couplings and four proton couplings associated with the ˙CO₂⁻ radical were measured, and all couplings were assigned to specific matrix nuclei. The spectra yield evidence for a second radical in low relative abundance. One small lithium hyperfine interaction detected was ascribed to this radical. Spectral simulations of the EPR and ENDOR spectra support the conclusions made.