15N NMR spectroscopy of partially unsaturated pyrazoles

Partially saturated pyrazoles, namely 1-(4-substituted)phenyl-3-methoxycarbonyl-5-ethoxycarbonyl-4,5-dihydropyrazoles, were submitted to extensive 15N NMR spectroscopic analyses, performed in natural abundance. Nitrogen chemical shifts were measured by means of INEPT and HMBC experiments, while long range proton–nitrogen scalar coupling values were taken through J-HMBC experiments. A linear plot between nitrogen chemical shifts and Hammett σp was observed, enabling us to relate quantitatively the observed chemical shifts to the electronic features of the substituent in the 1-position of the 4,5-dihydropyrazole ring.