STUDY OF NATURAL DIAMONDS BY DYNAMIC NUCLEAR POLARIZATION-ENHANCED C-13 NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPY

The results of a study of two types of natural-diamond crystals by dyn amic nuclear polarization (DNP)-enhanced high-resolution solid-state C -13 nuclear magnetic resonance (NMR) are reported. The home-built DNP magic-angle spinning (MAS) C-13 NMR spectrometer operates at 54 GHz fo r electrons and 20.2 MHz for carbons. The power of the microwave sourc e was about 30 W and the highest DNP enhancement factor came near to 1 0(3). It was shown that in the MAS spectra the C-13 NMR linewidths of the I(b)-type diamond were broader than those of I(aB3-type diamond. F rom the hyperfine structure of the DNP enhancement as a function of fr equency, four kinds of nitrogen-centered and one kind of carbon-center ed free radicals could be identified in the I(b)-type diamond. The hyp erfine structures of the DNP enhancement curve that originated from th e anisotropic hyperfine interaction between electron and nuclei could be partially averaged out by MAS. The C-13 polarization time of DNP wa s rather long, i.e. 1500 s, and the spin-lattice relaxation time (with out microwave irradiation) was about 300 s, which was somewhat shorter than anticipated. Discussions on these experimental results have been made in this report.