IR and NMR spectroscopic study of substituted N-phenylphthalimides as model compounds of aromatic polyimides

Abstract To establish the nature of the electron interaction of the imide cycle with the aromatic fragments attached to it a number of substituted N-phenylphthalimides and polypyromellitimides have been investigated by IR and NMR spectroscopy. The imide cycle of the phthalimides is characterized by a high degree of coupling of the unshared electron pair of the nitrogen atom with the carbonyl groups. Transfer of the electron effects through the imide cycle is virtually absent. The frequency of the vibrations of the group of N-phenylphthalimides and polypyromellitimides rises with increase in the donor capacity of the N-attached aromatic fragment, which is explained by the redistribution of the σ electron density in the imide cycle. The character of the absorption of the N-phenylphthalimides in the region 1700–1800 cm −1 is due to Fermi resonance ν (C=O) of the imide cycle with the overtone 2ν (Ph-CO).