Crystal Structure of Polymorph II and the Pressure–Temperature Phase Diagram of the Dimorphic Anesthetic Butamben
Résumé
The crystal structure of the low-temperature form II of butamben has been solved in a P2 1 /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.