Correlation of the crystal structure and solid-state13C NMR spectrum of pyromellitic dianhydride

The interatomic distances for crystalline pyromellitic dianhydride (PMDA) were calculated from x-ray crystallographic data. The CP-MAS C-13 NMR spectrum of PMDA was found to be consistent with and accountable for in terms of these distances after literature assignment errors were corrected. An anomalous peak was observed in the differential scanning calorimetry thermogram of PMDA. The entropy change associated with this transition indicates that a two-fold change in symmetry occurs approximately 60-degrees-C below the melting point of PMDA. Attempts to observe this transition by CP-MAS C-13 NMR were unsuccessful.