Structure solution and refinement of tetracaine hydrochloride from X-ray powder diffraction data

Abstract

The previously unknown crystal structure of the local anaesthetic tetracaine hydrochloride (C₁₅H₂₅N₂O₂⁺Cl⁻) has been solved from synchrotron X-ray powder diffraction data using a direct-space global optimisation method in which the position and conformation of a structural model is adjusted. The C₁₅H₂₅N₂O₂⁺ moiety and the chloride counterion were treated as independent units during the structure solution process. This complex structure has been solved despite the nine torsional degrees of freedom in the cation. A restrained Rietveld refinement of the initial solution gives an excellent fit to the data, with R_wp = 0.0956 and R_F² = 0.1638, and reveals the presence of a significant degree of preferred crystallite orientation in the sample. The crystal structure shows efficient packing involving parallel sheets made up of molecules lying parallel to one another, head-to-tail, in chains. Hydrogen bonds are observed between adjacent molecules and an extensive pattern of N⁺–Cl⁻ contacts is apparent.