THE INFLUENCE OF THE NITRO-GROUP ON THE SOLID-STATE STRUCTURE OF 4-NITROPYRAZOLES - THE CASES OF PYRAZOLE, 3,5-DIMETHYLPYRAZOLE, 3,5-DI-TERT-BUTYLPYRAZOLE AND 3,5-DIPHENYLPYRAZOLE .1. STATIC ASPECTS (CRYSTALLOGRAPHY AND THERMODYNAMICS)

We have determined the enthalpies of sublimation of 3,5-di-tert-butylpyrazole (3), 3,5-diphenylpyrazole (4), 4-nitropyrazole (5), 3,5-dimethyl-4-nitropyrazole (6), 3,5-di-tert-butyl-4-nitropyrazole (7) and 3,5-diphenyl-4-nitropyrazole (8); those of pyrazole (1) and 3,5-dimethylpyrazole (2) were already known. The effect of the C-substituents (Me, Bu(t), Ph and NO2) on the enthalpies of sublimation of pyrazoles and benzenes are additive and linearly related. Moreover, we report the structure of three of these 4-nitropyrazole derivatives, (5), (7) and (8), which have been solved by X-ray crystallography; those of the remaining five compounds were already known. Except for (8), there appears to be an opening of the intramolecular angle at C(4) due to the presence of the nitro group, that, on the other hand, seems to have no correlation with the presence of the hydrogen H(1)/H(2) disorder. Crystal structure diagrams and intermolecular contacts were analysed for either pyrazole derivatives with and without nitro substituents. There appear to be two general modes of packing: the first is based upon a secondary structure of trimers in sheets, which distorts into helices; the second is made of dimers, which then pack into sheets. The nitro group seems to have no influence in the packing, which is more controlled by the substituents at C(3) and C(5). Although only partly successful, we have established some relationships between crystallographic results and thermodynamic properties. First, between the planarity or not of the 4-nitro group and the acid and basic pK(a)'s, and second, between some packing descriptors and the sublimation enthalpies.