Abstract
Free radicals ascribed to ascorbic acid (AscH2), vitamin C, in the solid matrix of synthetic calcium carbonate have been studied using electron spin resonance (ESR) after UV irradiation. A new ESR signal with g-factors of g|| =2.0024 and g⊥=2.0053 was found together with a broad singlet signal around g=2.005 and a doublet signal at g=2.0053 separated by 0.18 mT due to the ascorbyl radical (Asc•-). The molecular orbitals of Asc•- and two other types of ascorbyl radical (AscH•-) were calculated using the semi-empirical PM3 unrestricted Hartree–Fock (UHF) method, which indicated that the hyperfine splitting due to hydrogen bonded to one of the carbons in the pentagonal ring was dominant. The axial signal was ascribed to AscH•-, while the doublet signal was ascribed to Asc•- in CaCO3. Possible pharmaceutical and nutritional applications of embedding unstable active molecules into the crystalline lattice of CaCO3 and a new nondestructive method for determination of vitamin C contents are discussed because the vitamin C has higher thermal stability in the carbonate than that in aqueous solution.