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.