Tri-methyl-pyrazole: a simple heterocycle reflecting Kitaigorodskii's packing principle.

The five-membered heterocycle 1,3,5-trimethyl-1 H -pyrazole, C 6 H 10 N 2 ( 1 ) crystallizes in space group Pnma with all non-hydrogen atoms of the mol-ecule on the crystallographic mirror plane. This arrangement has been recognized as favorable with respect to space filling by Kitaigorodskii and Wilson, pioneers in the field of crystal packing; Pnma represents a particularly rare space group for residues exclusively in a general position. Neighboring mol-ecules in 1 inter-act via non-classical C-H⋯N bonds in the plane and C-H⋯π contacts between adjacent layers. In Pnma , crystallographic inversion relates dipolar mol-ecules located on successive mirror planes and results in their head-to-tail arrangement. The inter-layer distance in the [010] direction is closely related to the van der Waals radii of C and N.