Anisotropic crystal structure distortion of the monoclinic polymorph of acetaminophen at high hydrostatic pressures

The anisotropy of structural distortion of the monoclinic polymorph of acet aminophen induced by hydrostatic pressure up to 4.0 GPa was studied by sing le-crystal X-ray diffraction in a Merrill-Bassett diamond anvil cell (DAC). The space group (P2(1)/n) and the general structural pattern remained unch anged with pressure. Despite the overall decrease in the molar volume with pressure, the structure expanded in particular crystallographic directions. One of the Linear cell parameters (c) passed through a minimum as the pres sure increased. The intramolecular bond lengths changed only slightly with pressure, but the changes in the dihedral and torsion angles were very larg e. The compressibility of the intermolecular hydrogen bonds NH ... O and OH ... O was measured. NH ... O bonds were shown to be slightly more compress ible than OH ... O bonds. The anisotropy of structural distortion was analy sed in detail in relation to the pressure-induced changes in the molecular conformations, to the compression of the hydrogen-bond network, and to the changes in the orientation of molecules with respect to each other in the p leated sheets in the structure. Dirichlet domains were calculated in order to analyse the relative shifts of the centroids of the hydrogen-bonded cycl es and of the centroids of the benzene rings with pressure.