The Role of Hydrogen Bonding in Aromatic Molecular Crystals at High Pressures*

Noncovalent interactions wield remarkable control in molecular crystals, which can alter properties such as impact sensitivity and chemical reactivity. The presence of hydrogen bonding has been shown to stabilize organic molecular crystals against pressure-dependent polymorphism and amorphization. These effects can be seen in melamine (C₃H₆N₆; 1,3,5-triazine-2,4,6-triamine) and resorcinol (C₆H₆O₂; 1,3-dihydroxybenzene), which are used industrially to produce laminates, adhesives, and flame retardants. Melamine is chemically and structurally similar to many energetic materials, including TATB and RDX. In the crystalline state melamine forms corrugated sheets of individual molecules linked by extensive intra- and inter-plane N-H hydrogen bonds. In comparison, resorcinol has less hydrogen bonding ability as only two hydroxyl groups form intermolecular links. At high pressure, melamine experiences a symmetry change from monoclinic to triclinic above approximately 36 GPa in helium. Conversely, resorcinol remains crystalline to only approximately 6 GPa. Examining the high-pressure intermolecular interactions in these compounds may allow their improved utilization as chemical feedstocks and analogues for related energetic compounds.