Evaluation of the Relative Stability of Toropane Derivative Nitrate Polymorphs

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The determination of thermodynamic stability of 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 approach 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 by using the simultaneous X-ray diffractometry and differential scanning calorimetry (XRD-DSC) measurement. And the solubility of Form-I was lower than that of Form-II at and around ambient temperature. These data indicated that Form-I is the most stable form at and around ambient temperature and Form-II is the metastable form. However, transformation of Form-I into Form-II could not be observed directly by the kinetic approaches such as differential scanning calorimetry (DSC). Therefore, the transition temperature of Form-I into Form-II should be estimated by using thermodynamic approaches. The conventional technique and the microcalorimetric technique were carried out as thermodynamic approaches. The conventional technique means the vant Hoff plot of the temperature dependence TDN solubility. The microcalorimetric technique required heat of solution and only one point solubility data at the certain temperature for both polymorphs. The estimated transition temperature indicated 332 K by using the microcalorimetric technique. This temperature was almost the same by using conventional vant Hoff plot. These results mean the microcalorimetric technique is useful and is more convenient approach to estimate the thermodynamic stability.
2011-03-01

Evaluation of the Relative Stability of Toropane Derivative Nitrate Polymorphs

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