Quantum mechanical study and vibrational spectra of indazolium-3-carboxylate and its decarboxylation product, the N-heterocyclic carbene indazol-3-ylidene.

Indazolium-3-carboxylate is a molecule that can be found as the nucleus of several pseudo-cross-conjugated mesomeric betaines, such as the alkaloid nigellicine. From a chemical point of view, one of the more interesting properties of this class of molecules is the possibility of forming an N-heterocyclic carbene by thermal decarboxylation. In this paper we have studied the carbene generation by decarboxylation of 1,2-dimethyl indazolium-3-carboxylate, using vibrational (infrared and Raman) spectroscopy and quantum chemistry calculations. Normal mode analysis allowed us to analyse the changes in the stretching force constants upon decarboxylation and to establish spectroscopic-structure relationships. We also investigate the effect of 5-halogen (fluoro, chloro) substitution on the carbene generation. Decarboxylation energy profiles of the three derivatives were calculated. Crossing of the energy paths when going from the transition state to the final product were obtained. The theoretical tendency found for the activation energies agrees with that observed for the decarboxylation temperatures and for the calculated NICS values of the benzene moieties.