Comparison of hydrogen bonds and diverse weak interactions of the nitro group in 2-methyl-4-nitroanilinium nitrate, bisulfate and two hexafluoridosilicates: elementary graph-set approach

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).

Structural, theoretic and spectroscopic analysis of 2-methyl-5-nitroaniline salts with various inorganic acids

Crystal structures of six new salts of 2-methyl-5-nitroaniline with inorganic acids [(H2Me5NA)Br, (H2Me5NA)I, (H2Me5NA)NO₃, (H2Me5NA)Cl, (H2Me5NA)HSO₄ and (H2Me5NA)I₃·0.5H₂O] are determined by single-crystal X-ray diffraction. The most important hydrogen-bonding patterns are formed by the ammonio group and respective anions composing 1D or 2D networks. The patterns are analysed using the graph-set approach and mathematical interrelations between graph-set descriptors are shown for comparative purposes. Analysis of IR spectra enables the strength of hydrogen bonds in the crystals to be assessed. The frequency of N-H and O-H stretching vibrations and NH₃ group libration indicates that the strongest hydrogen bonds are present in (H2Me5NA)HSO₄, whereas the weakest ones occur in (H2Me5NA)I₃·0.5H₂O. Hirshfeld surface analysis reveals that apart from obvious N-H...anion hydrogen bonds, the molecules are also connected to each other by exclusive C-H...O_NO2 interactions. The opposite occurs in the crystal structure of 2-methyl-4-nitroaniline salts, where a variety of O_NO2...π(N)_NO2 non-hydrogen bonding interactions are observed.

Phase transition in polar 2-nitroanilinium nitrate. Graph-set approach of hydrogen bonding patterns and analysis of vibrational spectra

A phase transition in new compound 2-nitroanilinium nitrate, (H2NA)NO₃, was found. A symmetry lowering from orthorhombic, Pmn2₁, to monoclinic one, P2₁, at 249 K is observed. During the phase transition, the H2NA⁺ and nitrate ions displace from the mirror plane in high-temperature phase. As a result, hydrogen bonding network constructed by the ammonio group and NO₃^- anion is changed. Especially, a bifurcated hydrogen bond disappears. Mathematical operations using both elementary and hydrogen bond graph-set descriptors were used for the first time to describe a mechanism of phase transition. Changes in hydrogen bonding network were also studied by means of vibrational spectroscopy. Band shift associated with stretching and bending vibrations of the ammonio group indicates that the energy of intermolecular interactions rises along with temperature decrease. SHG response for the studied compound is higher than KDP, I_(H2NA)NO3 = 1.1·I_KDP, although a decay of the signal was observed due to instability of the sample.