Evaluation of the Relative Stability of Toropane Derivative Nitrate Polymorphs

Abstract

The determination of the thermodynamic stability of a polymorphic compound is very important. The toropane derivative compound nitrate salt (TDN) has three enantiotropically related polymorphs. To determine the relative stability of TDN polymorphs, kinetic and thermodynamic approaches were carried out and compared. The kinetically irreversible transformation from Form-I to Form-III, and the kinetically reversible transformation from Form-III to Form-II were observed using simultaneous X-ray diffractometry and differential scanning calorimetry (XRD-DSC) measurement. The solubility of Form-I was lower than that of Form-II at and around ambient temperature. These data indicate that Form-I is the most stable form at and around ambient temperature and Form-II is the metastable form. However, transformation from Form-I to Form-II could not be observed directly by the kinetic approaches such as differential scanning calorimetry (DSC). Therefore, the transition temperature from Form-I to Form-II should be estimated using thermodynamic approaches. The conventional technique and the microcalorimetric technique were carried out as thermodynamic approaches. The conventional technique means the van't Hoff plot of the temperature dependence TDN solubility. The microcalorimetric technique required the heat of solution and only one point solubility data at a certain temperature for both polymorphs. The estimated transition temperature was 332 K using the microcalorimetric technique. This temperature was almost the same as that using conventional van't Hoff plot. These results mean the microcalorimetric technique is a useful and a more convenient approach to estimate the thermodynamic stability.