Trimethylpyrazole: a simple heterocycle reflecting Kitaigorodskii's packing principle

Abstract

The five-membered heterocycle 1,3,5-trimethyl-1H-pyrazole, C6H10N2 (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.