Hydrogen Bonds NH···N in Compressed Benzimidazole Polymorphs

Two phase transitions in compressed benzimidazole polymorphs reveal a remarkable interplay of the H-site in NH···N hydrogen-bonded aggregates and the crystal structure. The ambient-pressure polar polymorph α, space group Pna21, at p1 = 0.26 GPa transforms into centrosymmetric phase β, space group Pccn, and above p2 = 2.26 GPa into another centrosymmetric polymorph γ, space group Pbca. Single crystals of forms α, β, and γ have been in situ grown in isothermal and isochoric conditions in a diamond-anvil cell, and their structures were determined by X-ray diffraction. Both at p1 and p2 the H-bond distance N···N increases in the higher-pressure phase. However, the H-atom always assumes the site for which the H···N distance in the homoconjugated NH···N bond is shorter.