Low-temperature crystal structure of 4-chloro-1H-pyrazole

The structure of 4-chloro-1H-pyrazole, C3H3ClN2, at 170 K has orthorhombic (Pnma) symmetry and is isostructural to its bromo analogue. Data were collected at low temperature since 4-chloro-1H-pyrazole sublimes when subjected to the localized heat produced by X-rays. The structure displays intermolecular N-H center dot center dot center dot N hydrogen bonding and the packing features a trimeric molecular assembly bisected by a mirror plane (m normal to b) running through one chlorine atom, one carbon atom and one N-N bond. The asymmetric unit therefore consists of one and one-half 4-chloro-1H-pyrazole molecules. Thus, the N-H proton is crystallographically disordered over two positions of 50% occupancy each.