High-pressure recrystallisation-a route to new polymorphs and solvates of acetamide and parabanic acid

Although the application of high pressure to organic compounds may favour thermodynamically the adoption of a new polymorphic form, for compounds with high melting points there is often a substantial kinetic barrier to be overcome before the molecules can rearrange. Hence pressure-induced phase changes are often very slow or may not occur at all. We have overcome this problem by growing single crystals from solution at high pressure. Using this technique we have prepared and structurally characterised for the first time a new polymorph of acetamide and a sesquihydrate of parabanic acid. We also report the results of a high-pressure powder neutron diffraction study on parabanic acid-d2, which show that up to a pressure of 2.1 GPa direct compression does not induce a phase transition although several intermolecular interactions are substantially altered.