31P and 1H powder ENDOR and molecular orbital study of a CO33– ion in X-irradiated carbonate containing hydroxyapatites

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: g_x= 2.0045, g_y= 2.0034 and g_z= 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: A_x= 263 MHz, A_y= 263 MHz and A_z= 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 C_3v to C_s, 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.