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Two phase transitions in compressed benzimidazole polymorphsreveala remarkable interplay of the H-site in NH···N hydrogen-bondedaggregates and the crystal structure. The ambient-pressure polar polymorphα, space group Pna21, at p1= 0.26 GPa transforms into centrosymmetricphase β, space group Pccn, and above p2= 2.26 GPa into another centrosymmetric polymorphγ, space group Pbca. Single crystals of formsα, β, and γ have been in situgrownin isothermal and isochoric conditions in a diamond-anvil cell, andtheir structures were determined by X-ray diffraction. Both at p1and p2the H-bonddistance N···N increases in the higher-pressure phase.However, the H-atom always assumes the site for which the H···Ndistance in the homoconjugated NH···N bond is shorter. [ABSTRACT FROM AUTHOR]