Configuration of CO⁻₂ radicals in γ-irradiated A-type carbonated apatites: Theory and experimental EPR and ENDOR studies

Electron nuclear double resonance (ENDOR) spectroscopy and electronic structure calculations were combined in order to study the local environment of CO⁻₂ radicals in A-type carbonated apatite. At temperatures lower than 20 K, the ENDOR spectra are composed of hyperfine lines corresponding to the interaction of the CO⁻₂ unpaired electron with groups of ¹H and ³¹P nuclei located in the radical neighborhood. Hyperfine coupling constants, nuclear orientation in relation to the radical g-tensor axes, and distance between the electron and nuclear spins were estimated using the “molecular orientation-selection” principle and assuming purely dipolar anisotropic hyperfine interactions. It was verified that the CO⁻₂ radicals are not located on OH⁻ sites as is frequently suggested in the literature, but lie between two oxygen planes (z = 0.426) and z = 0.574). The radical’s O-O direction is tilted relative to the apatite hexagonal c axis. The vacancy inferred on the nearest OH⁻ site confirms the CO²⁻₃ → 2OH⁻ substitution mechanism. The determination of atomic positions and molecular orientation from ENDOR spectra relies upon assumptions about localization of electron spin density. Self-consistent field electronic structure calculations can provide the necessary check on these assumptions, and at the same time reveal details of bonding interactions which go beyond simple ionic models. The chemical environment induced by CO₃ and CO⁻₂ in the OH channel is studied by calculating the electronic structure of embedded clusters employing the first-principles self-consistent discrete variational method based on density-functional theory. Mulliken atomic-orbital populations, densities of states, magnetic moments, and charge and spin-density maps are obtained in order to corroborate the location of the CO⁻₂ radical inside the OH channel with that implied by experiment.