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-butylp yrazole (3), 3,5-diphenylpyrazole (4), 4-nitropyrazole (5), 3,5-dimeth yl-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-dimethylpy razole (2) were already known. The effect of the C-substituents (Me, B u(t), Ph and NO2) on the enthalpies of sublimation of pyrazoles and be nzenes are additive and linearly related. Moreover, we report the stru cture of three of these 4-nitropyrazole derivatives, (5), (7) and (8), which have been solved by X-ray crystallography; those of the remaini ng 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 correla tion with the presence of the hydrogen H(1)/H(2) disorder. Crystal str ucture diagrams and intermolecular contacts were analysed for either p yrazole derivatives with and without nitro substituents. There appear to be two general modes of packing: the first is based upon a secondar y structure of trimers in sheets, which distorts into helices; the sec ond is made of dimers, which then pack into sheets. The nitro group se ems to have no influence in the packing, which is more controlled by t he substituents at C(3) and C(5). Although only partly successful, we have established some relationships between crystallographic results a nd thermodynamic properties. First, between the planarity or not of th e 4-nitro group and the acid and basic pK(a)'s, and second, between so me packing descriptors and the sublimation enthalpies.