THE (1)H AND (13)C N.M.R. RULES FOR THE ASSIGNMENT OF 1,3- AND 1,5-DISUBSTITUTED PYRAZOLES: A REVISION

Article abstract of DOI:10.1016/S0040-4020(01)86908-8

The H-5 signal of N-substituted pyrazoles is not 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 (13)C n.m.r. spectrum is recommended as an alternative method.