posted on 2021-11-18, 22:43authored byIvo B. Rietveld, Maria Barrio, René Ceolin, Josep-Lluis Tamarit
The
crystal structure of the low-temperature form II of butamben
has been solved in a P21/c space group very similar to that of form I. Form II possesses virtually
the same packing as that of the high-temperature form I, and the dimorphism
is mainly represented by a small discontinuous change in the size
of the unit cell and by a difference in the enthalpy. Because of the
small enthalpy difference between the two polymorphs of 375 J·mol–1, it will be difficult to predict the change in the
stability hierarchy by computer-aided methods. The pressure–temperature
phase diagram, constructed using volume and enthalpy differences between
the two phases at ordinary pressure, corresponds to a case of overall
enantiotropy, as the I–II and I–L equilibrium lines
diverge with increasing pressure. This conclusion is confirmed by
the experimental pressure–temperature phase diagram obtained
with differential thermal analysis measurements under pressure.