Neutron diffraction of p-hydroxyacetanilide (Paracetamol): Libration or disorder of the methyl group at 100 K

A single-crystal pulsed neutron diffraction study of p-hydroxyacetanilide (the analgesic Paracetamol, C8H9NO2) was carried out at 100 K. The structure, in good agreement with previous X-ray determination, is monoclinic, space group P2(1)/a, a = 12.763(6), b = 9.250(4), c = 7.082(2) Angstrom, beta = 115.52(3)degrees, D-x = 1.34 g cm(-3), Z = 4. The present neutron data allowed full anisotropic refinement of all atoms including the hydrogens, yielding an agreement factor wR(F) = 0.063, for 2882 unique reflections and 181 refined parameters. Large, very anisotropic thermal ellipsoids are found on the methyl group hydrogen atoms; librational analysis was carried out along with attempts to model possible disorder associated with these large thermal vibrations. (C) 1997 Elsevier Science B.V.