Study of natural diamonds by dynamic nuclear polarization-enhanced 13C nuclear magnetic resonance spectroscopy.

The results of a study of two types of natural-diamond crystals by dynamic 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 for electrons and 20.2 MHz for carbons. The power of the microwave source was about 30 W and the highest DNP enhancement factor came near to 10(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. From the hyperfine structure of the DNP enhancement as a function of frequency, four kinds of nitrogen-centered and one kind of carbon-centered free radicals could be identified in the I(b)-type diamond. The hyperfine structures of the DNP enhancement curve that originated from the anisotropic hyperfine interaction between electron and nuclei could be partially averaged out by MAS. The C-13 polarization time of DNP was rather long, i.e. 1500 s, and the spin-lattice relaxation time (without microwave irradiation) was about 300 s, which was somewhat shorter than anticipated. Discussions on these experimental results have been made in this report.