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 νsNO2 vibration is correlated with the type of interaction in which the NO2 group is involved. Analysis of the νsNO2 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).