Abstract
DSC measurements showed that 1,3-dimethyluracil crystals prepared by vacuum sublimation or recrystallization from various solutions were in most cases in a stable phase (cr2) at 300 K. When heated, the stable crystals did not transform to the high temperature stable phase (cr1). The superheated crystals melted at 394 K; this liquid then underwent a monotropic transformation to phase cri, which finally melted at 398 K. A melt-crystallized sample (phase cr1) did not readily transform into phase cr2 on cooling, whereas crystals in phase cr1 formed by the above-mentioned monotropic transformation from the liquid state transformed readily into phase cr2 at 380 K on cooling. DSC measurements on malonamide crystals indicated the occurrence of a high and a low temperature stable phase (cr1 and cr2, respectively) and a metastable phase (cr3) formed by melt-crystallization. Phase cri supercooled on cooling. Schematical Gibbs energy and enthalpy diagrams are given for both compounds.
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