At a temperature of 20 K the crystal structures of two paracetamol polymorphs (monoclinic, form I, SPG P2 1 /n and orthorhombic, form II, SPG Pbcа) 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.

The experimental electron density of paracetamol metastable polymorph (orthorhombic, Pbca) was derived from highresolution X-ray diffraction at 100 K. The multipole model was used to refine electron density and for experimental analysis of the features of the critical points for orthorhombic modification. Geometrical H-bonds features were obtained from spherical and multipole models. The current study provides topological H-bonds analysis in orthorhombic paracetamol compared with stable monoclinic paracetamol polymorphs in normal conditions