Azapentalenes. XLIV.1H and13C-NMR study of mesoionic pyrazolo[1,2-a]pyrazoles

The 1H and 13C chemical shifts as well as the 1H–1H and 1H–13C coupling constants of sixteen pyrazolo[1,2-a]pyrazole derivatives have been measured. The most relevant features are discussed using resonance forms and simple additive models. AM1 semi-empirical calculations have been carried out to provide a rationale for the NMR results.