Molecular and Crystal Structures of N-Picryl-m-phenolidine and Investigation of Single Crystal Polarized Raman Spectra

Abstract

The molecular and crystal structures of N-picryl-m-phenolidine (or, 3-(2,4,6-trinitrophenylamino)phenol; TAP) were investigated by quantum chemical electronic structure calculations (including state-of-the-art fully-periodic DFT calculations for the crystal) and single crystal X-ray diffraction. The molecule of the compound has four low-energy conformers, which have calculated relative energies within 0.7 kJ mol–1. In all conformers, an N–H…O intramolecular hydrogen bond exists, which is established between the bridging amine moiety and one of the nitro groups of the picryl fragment, and an average angle between the two rings of ca. 55º. The crystals of the compound are monoclinic, P21/n space group, with a = 11.8098(3), b = 6.5706(2) and c = 16.5172(4) Å,  = 91.5640(10)º, Z = 4 and Z’ = 1, and is constituted by molecules assuming a geometry similar to that of the second most stable conformer of the isolated molecule. In the crystal, besides the N–H…O intramolecular hydrogen bond already present for the isolated molecule, a strong intermolecular H-bond exists where the OH group works as proton donor. The geometric constrains resulting from the establishment of this strong intermolecular H-bond is suggested to be the main factor precluding the existence of easily obtainable additional crystalline phases, contrarily to what has been found for similar molecules. Single crystal polarized Raman spectra of TAP and of the three polymorphs exhibiting different colors of its analogue N-picryl-m-toluidine (2,4,6-trinitro-N-(m-tolyl)aniline; TMA), where the phenol OH group is replaced by a methyl group, were also studied at light of fully periodic first principle calculations. Both TAP and TMA belong to the family of the prominent N-picryl-p-toluidine compound, which is well-known for showing color polymorphism, the obtained results thus being also a contribution for the available knowledge about this rare and interesting phenomenon.