Intermolecular interactions of proton transfer compounds: synthesis, crystal structure and Hirshfeld surface analysis

Three new proton transfer compounds, [2-ammonio-5-methylcarboxybenzene perchlorate (1), (C8H10NO2+center dot ClO4-), 4-(ammoniomethyl)carboxybenzene nitrate (2), (C8H10NO2+center dot NO3-), and 4-(ammoniomethyl)carboxybenzene perchlorate (3), (C8H10NO2+center dot ClO4-)], have been synthesized, their IR modes of vibrations have been assigned and their crystal structures studied by means of single-crystal X-ray diffraction. Their asymmetric units consist of one cation and one anion for both compounds (1) and (2). However, the crystal structure of compound (3) is based on a pair of cations and a pair of anions in its asymmetric unit. The three-dimensional Hirshfeld surface analysis and the two-dimensional fingerprint maps revealed that the three structures are dominated by H center dot center dot center dot O/O center dot center dot center dot H and H center dot center dot center dot H contacts. The strongest hydrogen-bonding interactions are associated with O-H center dot center dot center dot O and N-H center dot center dot center dot O constituting the highest fraction of approximately 50%, followed by those of the H center dot center dot center dot H type contributing 20%. Other close contacts are also present, including weak C center dot center dot center dot H/H center dot center dot center dot C contacts (with about 10%).