Electron spin resonance (ESR), electron nuclear double resonance (ENDOR) and general triple resonance of irradiated biocarbonates

Several irradiated biocarbonates were studied by magnetic resonance techniques. Seven paramagnetic species, attributed to CO2−, SO2− and SO3− were identified. Comparison between radiation induced defects in bioaragonites and aragonite single-crystals shows that isotropic and orhorhombic CO2− centers with broad line spectra are not produced in the latter samples. Vibrational and rotational properties of isotropic CO2− centers were studied from low temperature Q-band spectra. Vibrational frequency is determined from the 13CO2− hyperfine spectrum and yielded v=1.54 × 1013 s−1. The correlation time for isotropic CO2−, τc=1.2 × 10−11s (T=300 K), is typical of radicals rotating in liquids. ENDOR and General Triple spectroscopy show that orthorhombic CO2− centers are surrounded by water molecules located in the second nearest CO32− sites at 5.14, 5.35 and 6.02 Å. Water molecules replacing carbonates or as liquid inclusion of growth solution in local crystal imperfections may be responsible for the variety of orthorhombic and isotropic CO2− species, respectively.