Crystal Structure Explains Crystal Habit for the Antiviral Drug Rimantadine Hydrochloride

The crystal structure of the antiviral drug rimantadine hydrochloride, C₁₂H₂₂N⁺ Cl⁻, has been elucidated by a single-crystal X-ray structure analysis. The structure consists of 1-(1-adamantyl)ethanamine (rimantadinium) cations and chloride anions. The Cl⁻ anions link the rimantadinium cations via N–H···Cl hydrogen bonds into infinite rectangular chord-like structural units with charged groups in the inner channel and aliphatic groups on the surface, and oriented along the unit cell c axis. In contrast to strong electrostatic and hydrogen bonding inner interactions the chords in the crystal are held together by weak van der Waals forces only. A two-fold symmetry axis passes through the center of the chord. By indexing of the crystal faces it has been shown that the maximal dimension of the needle-like crystals coincides with the direction of the unit cell c axis. These structural features explain the crystal habit and the anisotropy of the mechanical properties of rimantadine hydrochloride crystals observed upon slicing and cleavage.