The study of interactions with a halogen atom: influence of NH group insertion on the crystal structures of meta-bromonitrobenzene derivatives.

Halogen atoms in molecular crystals may be involved in various interactions, often playing a very important role in structure stabilization. By introducing electron-donating groups, such as NH, the electron density of the molecule is changed and thus interactions with the bromine substituent may alter. Herein, the crystal structures of meta-bromonitrobenzene and its NH-substituted derivatives are analyzed. In all four described structures, namely m-bromonitrobenzene [Charlton & Trotter (1963). Acta Cryst. 16, 313], 4-bromo-2-nitroaniline (CHBrNO, 1), 2-bromo-6-nitroaniline (2) and 2-bromo-4-nitroaniline [Arshad et al. (2009). Acta Cryst. E65, o480], the Br atom is engaged in different interactions (Br...π, Br...O, Br...Br and C-H...Br, respectively). The Hirshfeld surface analysis (HS) and Reduced Density Gradient NonCovalent Interaction (RDG NCI) plots are used to prove the relevance, directionality and stabilizing nature of these interactions. Their modifications have been associated with the position of the amino group in the molecular structure and its influence on charge distribution analyzed with electrostatic potential surfaces (EPS). The diversification of the interactions has been correlated with a σ-hole potential value that enables a switching of the Br-atom character from electrophilic to nucleophilic.