A two-dimensional 13C-NMR study of powdered and oriented mesophase pitches Original Research Article

A two-dimensional 13C-NMR chemical-shift-chemical-shift exchange experiment is described which permits the measurement of solid state 13C resonance frequencies at two different sample orientations. This technique provides a method for probing the principal values of multiple 13C chemical shift tensors in overlapping powder patterns as well as the orientational distribution function in partially oriented materials. Two mesophase pitches derived catalytically from naphthalene and methyl naphthalene have been examined. These two pitches have similar chemical shift tensors for the aromatic carbons indicating that they have similar carbon structural units. However, significant differences in the molecular structure of these two pitches are also noted. The aromatic rings in both oriented pitches exhibit axial ordering with the preferential orientation along the draw axis. The probability of finding the aromatic ring along the draw direction in the oriented mesophase pitch of methyl naphthalene is approximately three times greater than that in the oriented mesophase pitch of the naphthalene sample.