AROMATIC PROPELLENES .2. STUDY OF CONFORMATIONAL ISOMERISM OF HEXA(PYRAZOL-1-YL)BENZENE - X-RAY CRYSTALLOGRAPHY AND SEMIEMPIRICAL CALCULATIONS

The molecular and crystal structures of two crystalline forms of hexa( pyrazol-1-yl)benzene were determined by x-ray analysis. They correspon d to two conformational polymorphs: form I is obtained in acetic acid and form II in ethanol or dichloromethane. The crystal packing of both conformers is different; however, that of form I is analogous to that of hexa(3,5-dimethylpyrazol-1-yl)benzene, having similar cell dimensi ons and space groups R-3. No significant interactions except the van d er Waals interactions were observed. Semiempirical calculations at the AM1 and SAM1 levels, exploring all possible conformations of the pyra zole rings, reveal that the most stable conformation presents the pyra zole rings with the N(2) alternating between both sides of the phenyl plane as it occurs in the solid state, crystalline form I (conformatio n 8h). The computed minimum energy for conformer 7a, which is related to crystal form II, presents a different sequence of pyrazole arrangem ents [N(2) up or down] and is only 1.6-2.0 kcal mol(-1) less stable th an the previous one in both parametrizations. The SAM1 method yields p yrazole moieties more perpendicular to the benzene ring than the AM1 o ne.