746 (C2oH 12)6 + PF6 EPSTEIN, A. J., ROMMELMANN, H., ABKOW1TZ, M. & GIBSON, H. W. (1981). Mol. Cryst. Liq. Cryst. 77, 81-96. GUPTA, R. R. (1986). Landolt-B6rnstein, New Series 11/16, edited by K.-H. HELLWEGE & A. M. HELLWEGE, pp. 176--177, 436-437. Berlin: Springer. KELLER, H. J., NOTHE, D., PRITZKOW, H., WEHE, D., WERNER, M., KOCH, P. & SCHWEITZER,D. (1980). Mol. Cryst. Liq. Cryst. 62, 181-200. KIVELSON, S. (1982). Phys. Rev. B, 25, 3798-3821. RAIBLE, CH., GMEINER, J., WINTER, H., DORMANN,E., STENGER- SMITH, J. D. & ENKELMANN, V. (1993). Synth. Met. 59, 71-80. RIESS, W., BROTTING, W. & SCHWOERER,M. (1993). Synth. Met. 55-57, 2664-2669. SHELDRICK, G. M. (1995). SHELX92. An Integrated System for Solving, Refining and Displaying Crystal Structures from Dif- fraction Data. J. Appl. Cryst. In preparation. Acta Cryst. (1994). BS0, 746--762 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. I. Static Aspects (Crystallography and Thermodynamics) BY ANTONIO L. LLAMAS-SAIZ, CONCEPCI6N FOCES-FOCES AND FI~LIX H. CANO Departamento de Cristalograffa, Instituto de Qufmica-Ffsica 'Rocasolano', CSIC, Serrano 119, E-28006 Madrid, Spain PILAR JIMI~MEZ AND JOSI~ LAYNEZ Instituto de Qulmica-Ffsica 'Rocasolano', CSIC, Serrano 199, E-28006 Madrid, Spain WIM MEUTERMANS AND Josl~ ELGUERO Instituto de Qufmica Mddica, CSIC, Juan de la Cierva 3, E-28006 Madrid, Spain AND HANS-HEINRICH LIMBACH AND FRANCISCO AGUILAR-PARRILLA Institut ffir Organische Chemie, Fachbereich Chemie, Freie Universitdt Berlin, Takustrasse 3, D-14195 Berlin, Germany (Received 31 January 1994; accepted 19 April 1994) Abstract 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. More- over, 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 hydro- gen H(I)/H(2) disorder. Crystal structure diagrams and intermolecular contacts were analysed for either © 1994 International Union of Crystallography Printed in Great Britain - all rights reserved pyrazole derivatives with and without nitro sub- stituents. 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 sub- stituents 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 pKa's, and second, between some packing descriptors and the sublimation enthalpies. Introduction We are in the process of carrying out a systematic study of the properties of NH-pyrazoles in the solid state by X-ray crystallography (Llamas-Saiz, Foces- Acta Crvstallographica Section B ISSN 0108-7681 ©1994