31P and 1H powder ENDOR and molecular orbital study of a CO3 3? ion in X-irradiated carbonate containing hydroxyapatites

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Abstract
An X-irradiated synthetic carbonate-containing apatite powder is examined with EPR and ENDOR. At low microwave powers, the room-temperature EPR spectrum contains a major contribution of a signal with g values: gx= 2.0045, gy= 2.0034 and gz= 2.0014. In a related 13C-enriched sample, the radical was shown to exhibit a hyperfine interaction with one carbon nucleus. The 13C hyperfine tensor values are: Ax= 263 MHz, Ay= 263 MHz and Az= 423MHz. The radical is assigned to a CO3 3– molecular ion. It is demonstrated by means of CNDO/II and INDO calculations that by lowering the symmetry of the CO3 3– ion from C3v to Cs, an orthorhombic g tensor can be obtained. However, the deviation from axial symmetry for the 13C hyperfine tensor is so small that it is not measurable on a powder specimen. The thus-calculated spin-Hamiltonian parameters are in very good qualitative and quantitative agreement with the experimental ones, adding strong evidence for the assignment of the observed signal to a CO3 3– radical. At low temperatures, both 31P and 1H ENDOR spectra are recorded for different settings of the magnetic field (i.e. when the magnetic field is swept through the EPR CO3 3– spectrum). By a careful analysis of the ENDOR powder spectra using computer simulations based on the ‘orientation-selection’ principle, a detailed model for the CO3 3– ion could be proposed. In this way, it is established unambiguously that the CO3 3– ion substitutes for a phosphate group in the hydroxyapatite lattice, with a vacancy on the nearest hydroxy-group site. In addition, some deductions can be made about the substitution mechanism according to which the precursor of the CO3 3– radical (i.e. a carbonate ion) is incorporated into the apatitic lattice.

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