Study of natural diamonds by dynamic nuclear polarization-enhanced C 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 ¹³C nuclear magnetic resonance (NMR) are reported. The home-built DNP magic-angle spinning (MAS) ¹³C 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 10³. It was shown that in the MAS spectra the ¹³C 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-centred and one kind of carbon-centred 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 ¹³C 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.