Temperature dependence of the crystal structure of 3,5-di-tert-butylpyrazole from 10 K

A single crystal of 3,5-di-tert-butylpyrazole has been the subject of structure determinations at 10, 100, 120 and 299 K. Above 110 K, the structure is Pbca, a 11.3, b 20.6, c 10 Å, Z = 8. Below 110 K, the c axis is doubled and the space group becomes Pb21a, Z = 16. Here the asymmetric unit is comprised of four molecules, arranged as a pair of overlapping hydrogen-bonded dimers, all components being ordered. In each case one methyl group of each t-butyl group lies coplanar with its parent pyrazole ring and, within each quasi-inversion-related dimer, one molecule has both of these methyl groups directed away from the N2 component `cis', while in the other, one methyl group is toward (`trans'). Above 110 K the four molecules collapse into one, with one t-butyl group (corresponding to an overlaid `cis/trans' pair of the low-temperature form) modelled as disordered. The disorder has been previously described as `temperature dependent' (Deacon et al., 2001), but the present studies suggest it to be dependent on the history of the particular specimen as well.