False asymmetry, pseudosymmetry, disorder, polymorphism and atomic displacement parameters

Two similar sugars, with chemical formulas differing only by the presence of a methyl group connected to the molecule backbones in different positions, crystallize in the monoclinic P21 space group giving rise to Z’ = 2 structures. They both bear an azide side chain which is the principal responsible for the lack of a higher symmetry for one compound only. We analyzed their most relevant features by means of X-ray single crystal diffraction coupled with a quantitative estimation of their potential tendency to crystallize in a different space group with higher symmetry. The latter tendency of the most promising of the two compounds is commented in the light of the anisotropic behaviour of the atomic displacement parameters.