Probing Intermolecular Hydrogen Bonding in Sibenadet Hydrochloride Polymorphs by High-Resolution 1H Double-Quantum Solid-State NMR Spectroscopy

Molecular packing in two polymorphs of sibenadet hydrochloride (AR-C68397AA, Viozan™) is investigated using a combined experimental 1H double-quantum (DQ) solid-state magic-angle spinning nuclear magnetic resonance and computational (gauge including projected augmented wave calculation of chemical shifts) approach. For Form I, NH-NH and NH-OH 1H DQ peaks are observed corresponding to nearest distances of 2.62 and 2.87 Å, respectively, for the intermolecular hydrogen-bonding arrangement in the single-crystal X-ray diffraction structure. The same 1H DQ peaks at the same 1H chemical shifts are observed for Form II, for which there is no single-crystal diffraction structure, indicating the same intermolecular hydrogen-bonding arrangement of the benzothiazolone moieties as in Form I. 1H DQ build-up (as a function of the DQ recoupling time) curves are presented for the resolved NH-NH and NH-OH DQ peaks for the two polymorphs. For Form I, the ratio of the maximum intensity for the NH-OH and NH-NH DQ peaks is in excellent agreement with the ratio of the summed squares of the H-H dipolar couplings, as determined using H-H distances from the crystal structure up to 4 Å. Small differences in the 1H DQ build-up behaviour for the two polymorphs are attributed to differences in the longer-range NH-OH distances associated with different inter-layer arrangements. © 2012 Wiley Periodicals, Inc. and the American Pharmacists Association J Pharm Sci 101:1821-1830, 2012