Tri­methyl­pyrazole: a simple heterocycle reflecting Kitaigorodskii’s packing principle

The five-membered heterocycle 1,3,5-trimethyl-1H-pyrazole, C(6)H(10)N(2) (1) crystallizes in space group Pnma with all non-hydrogen atoms of the mol­ecule 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 mol­ecules in 1 inter­act via non-classical C—H⋯N bonds in the plane and C—H⋯π contacts between adjacent layers. In Pnma, crystallographic inversion relates dipolar mol­ecules located on successive mirror planes and results in their head-to-tail arrangement. The inter­layer distance in the [010] direction is closely related to the van der Waals radii of C and N.