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.