The role of molecular structure in the crystal polymorphism of local anesthetic drugs: Crystal polymorphism of local anesthetic drugs, part X

Purpose. This report is the resume of a comprehensive investigation on the solid-state properties and the crystal polymorphism of the structurally homogenous class of local anesthetic drugs. The goal is to explore the relationship between crystal polymorphism and molecular structural features. Methods. A salt form (mostly the hydrochloride) as well as the free base of 24 local anesthetics has been characterized by means of thermomicroscopy, differential scanning calorimetry, pycnometry, Fourier transform infrared, Fourier transform Raman, and solid-state NMR spectroscopies, as well as X-ray diffraction methods (single crystal, powder). Results. Based on the thermochemical data, the relative thermodynamic stabilities of the different crystal forms of each polymorphic system were evaluated and visualized as semiquantitative energy/temperature diagrams. Conclusion. This study is the first step in recognizing relationships between the structure and the solid-state properties within this limited group of active substances with common structural elements. The results clearly show that there are certain relationships, but, to understand the phenomenological behavior in more detail on a molecular level, more structural information must be collected and analyzed by computational methods.