Structural, Thermodynamic, and Kinetic Aspects of the Polymorphism of Trimethylthiourea: The Influence of Kinetics on the Transformations between Polymorphs

Trimethylthiourea crystallizes in two different polymorphic modifications. Polymorph I crystallizes in the monoclinic space group P21/c with Z = 8, whereas polymorph II crystallizes in the alternative setting of the same space group, P21/n with Z = 4. Both polymorphs form chains of molecules linked by hydrogen bonds N–H···SC via glide planes, with translational repeats after four molecules (the two independent molecules alternate) or two molecules, respectively. Interplanar angles between molecules in the chain differ appreciably between I and II, and for I, one hydrogen bond is very nonplanar with respect to the N2CS acceptor plane. Solvent-mediated conversion experiments prove that polymorph II is the thermodynamically stable polymorph at room temperature, where I is metastable, and that I can be obtained by solidification of the melt. On heating, I transforms slowly into II with no detectable transfer of energy, and on further heating, melting of this polymorph is observed. DSC experiments reveal that I exhibits the higher melting point and the lower heat of fusion, and therefore, the polymorphs are related by enantiotropy, with I being stable at higher temperatures. Isothermic DSC experiments prove that the thermodynamic transition point is between 70 and 80 °C, in agreement with the value calculated from the melting enthalpy and the melting point of both polymorphs. Further experiments reveal that at very low heating rates II initially melts and that I crystallizes from the liquid and melts on further heating. This process can only be observed if crystals of polymorph I are present during melting of II, in order to induce crystallization of polymorph I.