The H-5 signal of N-substituted pyrazoles shifted to low field values on increasing solvent polarity when a nitrogen lone pair lies very close to the measured proton (H-bond). Typical examples of this “abnormal” behaviour are provided by C,N'-linked, pyrazole-containing heterocyclic dimers. On the contrary, models selected in the 1,1'-bipyrazole and 2,2'biindazole series (N,N'-linkage) showed normal shifts, due to their non-coplanar conformation around the N-N bond. For the assignment of disubstituted pyrazoles, a proton-coupled 13C n.m.r. spectrum is reconmended as an alternative method.

Authors are grateful to Comisión Asesora de Investigación Científica y Técnica (CAICYT) for financial support.