The crystal structure of hydroxyzine dihydrochloride has been solved and refined using synchrotron X-ray powder diffraction data, and optimized using density functional techniques. Hydroxyzine dihydrochloride crystallizes in space group P21 (#4) with a = 11.48735(10), b = 7.41792(7), c = 14.99234(15) Å, β = 110.4383(10)°, V = 1197.107(13) Å3, and Z = 2. The hydroxyl-containing side chain of the cation is disordered over two conformations, with ~70/30% occupancy. The crystal structure consists of alternating polar (which includes the cation-anion interactions and hydrogen bonds) and nonpolar layers parallel to the ab-plane. The crystal structure is dominated by hydrogen bonds. Each of the protonated nitrogen atoms forms a very strong hydrogen bond to one of the chloride anions. The hydroxyl group forms a strong hydrogen bond to one of the chloride anions in both conformations, and there are subtle differences in the hydrogen bonding patterns between the conformations. The powder pattern is included in the Powder Diffraction File™ as entry 00-066-1603.
