Aromatic propellenes. Part 2. Study of conformational isomerism of hexa(pyrazol-1-yl)benzene: X-ray crystallography and semiempirical calculations

✍ Scribed by Concepción Foces-Foces; Antonio L. Llamas-Saiz; Consuelo Escolástico; Rosa María Claramunt; José Elguero

• Publisher: John Wiley and Sons
• Year: 1996
• Tongue: English
• Weight: 526 KB
• Volume: 9
• Category: Article
• ISSN: 0894-3230
• DOI: 10.1002/(sici)1099-1395(199603)9:3<137::aid-poc767>3.0.co;2-y

✦ Synopsis

The molecular and crystal structures of two crystalline forms of hexa(pyrazo1-1-yl)benzene were determined by x-ray analysis. They correspond to two conformational polymorphs: form I is obtained in acetic acid and form I1 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-l-yl)benzene, having similar cell dimensions and space groups R-3. No significant interactions except the van der Waals interactions were observed. Semiempirical calculations at the AM1 and SAMl levels, exploring all possible conformations of the pyrazole rings, reveal that the most stable conformation presents the pyrazole rings with the N(2) alternating between both sides of the phenyl plane as it occurs in the solid state, crystalline form I (conformation 8h). The computed minimum energy for conformer 7a, which is related to crystal form 11, presents a different sequence of pyrazole arrangements "(2) up or down] and is only 1.6-2.0 kcalmol-' less stable than the previous one in both parametrizations. The SAMl method yields pyrazole moieties more perpendicular to the benzene ring than the AM1 one.