Supramolecular structure of 1H-pyrazoles in the solid state: a crystallographic and ab initio study

The secondary structure of 1H-unsubstituted pyrazole derivatives bearing on ly one hydrogen donor group and one or more acceptor groups has been analyz ed in terms of some descriptors representing the substituents at C3 and C5. The substituent at C4 appears to affect mainly the tertiary or quaternary structure of these compounds. The proposed semiquantitative model, which ex plains most hydrogen-bonded motifs as a combination of the effects of subst ituents at C3 and C5, has also been examined as a function of the steric an d polarizability effects of these substituents represented by molar refract ivity. The model also applies to other five-membered rings (1,2,4-triazoles , 1,2,4-diazaphospholes and 1,2,4-diazaarsoles). Furthermore, ab initio cal culations at RHF/6-31G* have been performed to discover the relative stabil ity of three of the four hydrogen-bond patterns displayed by several symmet rical pyrazoles (dimers, trimers, tetramers). The fourth motif, catemers, h as only been discussed geometrically.