Picolinic and isonicotinic acids: a Fourier transform microwave spectroscopy study.

The rotational spectra of laser ablated picolinic and isonicotinic acids have been studied using broadband chirped pulse (CP-FTMW) and narrowband molecular beam (MB-FTMW) Fourier transform microwave spectroscopies. Two conformers of picolinic acid, s-cis-I and s-cis-II, and one conformer of isonicotinic acid have been identified through the analysis of their rotational spectra. The values of the inertial defect and the quadrupole coupling constants obtained for the most stable s-cis-I conformer of picolinic acid, evidence the formation of an O-H center dot center dot center dot N hydrogen bond between the acid group and the endocyclic N atom. The stabilization provided by this hydrogen bond compensates the destabilization energy due to the adoption of a -COOH trans configuration in this conformer. Its rs structure has been derived from the rotational spectra of several C-13, N-15, and O-18 species observed in their natural abundances. Mesomeric effects have been revealed by comparing the experimental values of the N-14 nuclear quadrupole coupling constants in the isomeric series of picolinic, isonicotinic, and nicotinic acids.