Kinetics of interconversion of sulphamethoxydiazine crystal forms

Abstract

A quantitative Nujol mull technique is described for the determination of sulphamethoxydiazine crystal forms in mixtures, so that the rates of their interconversion may be determined. The technique is applied to the transformations taking place in the more thermo-dynamically active as well as biologically available crystal form (Form II). Kinetic parameters indicate a highly temperature dependent transformation (Ea ⋍ 100 kcal mol−1; 418·6 kJ mol−1) of Form II to the more stable Form I. Suspension in water results in a much faster transformation of Form II (Ea ⋍ 20 kcal mol−1; 83·7 kJ mol−1, t1/2 ⋍ 15 min) to the water-stable Form III. Water and water vapour are shown to be important factors in the transformation of Form II at room temperature.