Crystal structure of two paracetamol polymorphs at 20 K: A search for the “structure-property” relationship

At a temperature of 20 K the crystal structures of two paracetamol polymorphs (monoclinic, form I, SPG P 2 1 / n and orthorhombic, form II, SPG Pbca ) are solved by single crystal X-ray diffraction and a comparative analysis of the geometric characteristics of intra- and intermolecular interactions is performed. Polymorphous transformations were not observed until cooling to this temperature. It is shown that in form II hydrogen bonds remain longer than those in form I up to a temperature of 20 K, and the density of metastable form II remains higher than that of stable form I. At the same time, in form II, thermal parameters of nitrogen and oxygen atoms remain higher than those in form I. The features observed in the behavior of the heat capacity of both forms at temperatures below 100 K are not directly related to a change in the geometry of hydrogen bonds. The methyl group orientation determined from the electron density maps does not alter as compared to higher temperatures in both modifications. Thus, changes in the Raman spectra observed in the orthorhombic paracetamol form below 100 K are explained by the features of its dynamics rather than a change in the overall average orientation of the methyl group determined by X-ray diffraction.