Voronoi-Dirichlet tesselation as a tool for investigation of polymorphism in molecular crystals with CwHxNyOz composition and photochromic properties.

The non-bonded interactions in five sets of polymorph substances with photochromic properties have been investigated within the Voronoi-Dirichlet approach. Twenty compounds with the general formula C(w)H(x)N(y)O(z) were analyzed. Among ten possible types of non-bonded interactions at least five types are observed in the crystal structures of compounds under discussion. For all the structures the majority of interactions involve H atoms, namely London forces (H...H and H...C) and hydrogen bonds (H...O and H...N). A conformational polymorph was stated to be characterized by a unique set of inter- and intramolecular non-bonded interactions. It was quantitatively demonstrated that molecules in the same conformation can pack in a different way, and, vice versa, the change in conformation of a molecule does not prevent a substance from realising the same set of intermolecular contacts. In accordance with the data obtained for 2,4-dinitrobenzylpyridine derivatives, only conformational polymorphs with an intramolecular N...N interaction between a nitro group and a pyridine are photochromic.