Structural and theoretical analysis of 2-chloro-4-nitroaniline and 2-methyl-6-nitroaniline salts

The crystal structures of five new salts of 2-chloro-4-nitroaniline (2Cl4na) and 2-methyl-6-nitroaniline (2m6na) with inorganic acids, namely, 2-chloro-4-nitroanilinium bromide, C₆H₆ClN₂O₂⁺·Br^- (1), 2-chloro-4-nitroanilinium hydrogen sulfate, C₆H₆ClN₂O₂⁺·HSO₄^- (2), 2-methyl-6-nitroanilinium bromide, C₇H₉N₂O₂⁺·Br^- (3), 2-methyl-6-nitroanilinium triiodide, C₇H₉N₂O₂⁺·I₃^- (4), and 2-methyl-6-nitroanilinium hydrogen sulfate, C₇H₉N₂O₂⁺·HSO₄^- (5), were determined by single-crystal X-ray diffraction. Theoretical calculations of the relaxed potential energy surface (rPES) revealed that the energy barriers for the rotation of the nitro group for isolated H2Cl4na⁺ and H2m6na⁺ cations are 4.6 and 11.6 kcal mol^-1, respectively. The ammonium group and respective anions form hydrogen bonds which are the most important interactions and are arranged in zero- (in 3), one- (in 1 and 4) or two-dimensional (in 2 and 5) networks. Hydrogen-bonding patterns were analyzed by means of mathematical relationships between graph-set descriptors and compared with previously reported nitroaniline salts. Hirshfeld surface analysis indicates that the nitro group plays a dominant role among the weak interactions, i.e. C-H...O(NO₂), NO₂...π(Ar) and O(NO₂)...π(NO₂). The frequency of the ν_sNO₂ vibration is correlated with the type of interaction in which the NO₂ group is involved. Analysis of the ν_sNO₂ band observed in the IR and Raman spectra allowed an assessment of its shift in the sequence (H2m6na)I₃ (4) < (H2m6na)HSO₄ (5) < (H2m6na)Br (3) < (H2Cl4na)Br (1) < (H2Cl4na)HSO₄ (2).