An influence of intra- and intermolecular hydrogen bonding on crystal structure stability and density: dance of amino and nitro groups around the pyrazole ring

Hydrogen bond plays an important role in chemistry and chemical technology. Plenty of works are related to investigation of weak, moderate and strong hydrogen bonds and their influence on the structure and properties. In this work two pairs of isomers of dinitro-N-aminopyrazole and dinitro-diaminopyrazole which were synthesized and characterized earlier are considered as model compounds to study both intra- and intermolecular hydrogen bonding. The choice of isomers is related to the fact that having the same composition, such isomeric molecules differ in structure, and this difference and its influence on the crystal packing can be directly investigated. Using Δ OED densification approach, the role of each molecular unit (NH 2 and NO 2 groups and pyrazole ring) in a molecule itself and in crystal structure was investigated. This allowed us to conclude that while intermolecular hydrogen bonding is merely one type of intermolecular interaction among others, it is the intramolecular H-bond between amino and nitro groups that is the key interaction which allows molecules to actively participate in intermolecular bonding that, in turn, leads to close crystal packing thereby increasing crystal density, melting point and detonation properties.