Quantum-Chemical Calculations in Studying the Conformation of γ-Irradiated Calcium Gluconate

Five types of radicals were considered in order to describe the experimental EPR spectra of polycrystalline calcium gluconate irradiated with gamma quanta. This set made it possible to correctly describe the X- and Q-band EPR spectra of CaGluc2·H2O with one set of anisotropic parameters HFI and g-factors. Because it is assumed that irradiation with gamma quanta leads to the rupture of the C-H bond; hydrogen atoms in the structure of the calcium gluconate molecule, bound directly to the carbon atom, were successively removed. Tensors of anisotropic HFIs and g-factors were calculated by the density functional theory (DFT) (unrestricted Kohn-Shem method). Meta-GGA functional (TPSS) and ccpVDZ basis were used. In this case, the EPR spectra parameters calculated in the ORCA program and determined by fitting in the EasySpin program differ insignificantly. The torsion angles obtained from the EPR data are consistent with the X-ray diffraction data [10]. This indicates the possibility of using this method for the conformational analysis of the studied compounds.