Magnetic susceptibility effects on 13C MAS NMR spectra of carbon materials and graphite

C-13 high-resolution solid-state nuclear magnetic resonance (NMR) was employed to study carbon materials prepared through the thermal decomposition of four different organic precursors (rice hulls, endocarp of babassu coconut, peat, and PVC). For heat treatment temperatures (HTTs) above about 600 degreesC, all materials presented C-13 NMR spectra composed of a unique resonance line associated with carbon atoms in aromatic planes. With increasing HTT, a continuous broadening of this resonance and a diamagnetic shift in its central frequency were verified for all samples. The evolution of the magnitude and anisotropy of the magnetic susceptibility of the heat-treated carbon samples with HTT explains well these findings. It is shown that these results are better understood when a comparison is made with the features of the C-13 NMR spectrum of polycrystalline graphite, for which the magnetic susceptibility effect is also present and is much more pronounced. (C) 2001 Academic Press.