Solid-state structure of orphenadrine hydrochloride and conformational comparisons with diphenhydramine hydrochloride and nefopam hydrochloride.

The solid-state structure of (+-)-orphenadrine hydrochloride [(CH3)2NCH2CH2OCH(o-CH3C6H4)(Ph).HCl], a skeletal muscle relaxant drug, was determined by single-crystal X-ray diffraction analysis. Orphenadrine hydrochloride gave crystals belonging to the monoclinic P2(1)/n space group, and at low temperature (92 K), the following parameters were found: a = 6.923 (4), b = 7.508 (5), c = 33.22 (3) A, V = 1720 (3) A3, Z = 4, R(F) = 0.109, and Rw(F) = 0.131. Data were collected from poor crystalline material because of the low volume of the needle-shaped crystals (0.025 x 0.025 x 0.15 mm3). A molecular mechanics model was calculated by using an input structure based on atomic coordinates of the crystallographically determined molecular structure. The resulting molecular mechanics model and the structure determined by X-ray crystallography have the same molecular conformation. Whereas both solid-state (+-)-orphenadrine hydrochloride and diphenhydramine hydrochloride [(CH3)2NCH2CH2OCH(Ph)2.HCl] have synclinal N-C-C-O and antiperiplanar NC-C-O-CAr2 torsion angles, the former has a helical arrangement for Ar2CH, as expected, and the phenyl rings in the latter are disposed in a nonhelical, "open-book" arrangement.