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 approximately 110 K, the structure is Pbca, a approximately 11.3, b approximately 20.6, c approximately 10 A, Z = 8. Below 110 K, the c axis is doubled and the space group becomes Pb2(1)a, 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 N(2) 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', but the present studies suggest it to be dependent on the history of the particular specimen as well.