Conjugation effects in a para-nitroaniline molecule and its structural non-rigidity

A thorough investigation of the spatial and electronic structures of p-nitroaniline using the precision X-ray diffraction analysis and quantum chemical calculations in the framework of the Atoms in Molecules theory revealed that the conjugation of functional groups with the benzene ring is small and amino group flattening in crystal compared to the gas phase is caused by the inductive effects. The structure non-rigidity of the nitro group was observed, which appears as its dynamic disordering confirming that the conjugation with the benzene ring is energetically unfavorable compared to the intrinsic conjugation effects. In case of the amino group, only the static temperature-independent disorder as a superposition of two non-planar configurations can be proposed according to the results of normal coordinate analysis by the simulation of mean-square amplitude matrixes using multi-temperature X-ray diffraction data.