Hydrogen bonding patterns and C-H...π interactions in the structure of the antiparkinsonian drug (R)-rasagiline mesylate determined using laboratory and synchrotron X-ray powder diffraction data.

The structure of (R)-rasagiline mesylate [(R)-RasH⁺·Mes^-], an active pharmaceutical ingredient used to treat Parkinson's disease, is presented. The structure was determined from laboratory and synchrotron powder diffraction data, refined using the Rietveld method, and validated and optimized using dispersion-corrected DFT calculations. The unit-cell parameters obtained in both experiments are in good agreement and the refinement with both datasets converged to good agreement factors. The final parameters obtained from laboratory data were a = 5.4905 (8), b = 6.536 (2), c = 38.953 (3) Å, V = 1398.0 (4) ų and from synchrotron powder data were a = 5.487530 (10) Å, b = 6.528939 (12) Å, c = 38.94313 (9) Å, V = 1395.245 (5) ų with Z = 4 and space group P2₁2₁2₁. Preferred orientation was properly accounted for using the synchrotron radiation data, leading to a March-Dollase parameter of 1.140 (1) instead of the 0.642 (1) value obtained from laboratory data. In the structure, (R)-RasH⁺ moieties form layers parallel to the ab plane connected by mesylate ions through N-H...O and C-H...O hydrogen bonds. These layers stack along the c axis and are further connected by C-H...π interactions. Hirshfeld surface analysis and fingerprint plot calculations indicate that the main interactions are: H...H (50.9%), H...C/C...H (27.1%) and H...O/O...H (21.1%).