Investigation of a naphthalene pitch by high-resolution solid-state dynamic nuclear polarization.

The possibility of applying the dynamic nuclear polarization (DNP) technique to a study of char is explored with a naphthalene-derived pitch. It is shown that a C-13 DNP enhancement factor of about 10(2) is obtained when the polarization is directly transferred from the unpaired electrons to the C-13 nuclei. An undistorted spectrum with an enhancement factor of 8 is obtained by the DNP cross-polarization magic-angle spinning nuclear magnetic resonance (DNP-CP-MAS NMR) method. With such a high increase in S/N, it is possible to measure the C-13 polarization time (T-p) and the spin-lattice relaxation time (T-1) of the system in a reasonable experimental time. The resultant values are T-p = 19 s and T-1 = 38 s, respectively. Based on the DNP enhancement as a function of the microwave frequency, it is found that the predominant DNP mechanism in the pitch is the solid-state effect.