Spectroscopic and structural investigation of 2,5-dicarboxy-1-methylpyridinium inner salt

•The molecules are linked by OH⋯O hydrogen bonds into infinite chains.•The FTIR spectrum is dominated by a broad absorption in the 1500–800 cm−1 region.•13C chemical shifts of COOH and COO groups are distinguished by the HMBC spectrum.•In crystal the COO− group is at 2 position and the COOH group at 5 position.•pKa value of 2.31 ± 0.02 was determined by potentiometric titration.

The structure of 2,5-dicarboxy-1-methylpyridinium inner salt (1), has been studied by X-ray diffraction, B3LYP/6-311G(d,p) calculations, FTIR, Raman and NMR spectroscopy. The molecules are linked by short intermolecular and asymmetric OH⋯O hydrogen bonds of 2.486(2) Å between carboxyl and carboxylate groups of neighboring molecules into infinite chains. The hydrogen bonds in the molecules optimized by the B3LYP/6-311G(d,p) approach in trimer (2) and dimer (3) are slightly longer than in the crystal. The FTIR spectrum of the investigated inner salt is dominated by a broad and intense absorption in the 1500–800 cm−1 region attributed to the νas(OHO) and γ(OHO) vibrations of the strong hydrogen bond. In the Raman spectrum the broad absorption is absent. Linear correlations, δexp = a + b σcalc between the experimental 1H and 13C NMR chemical shifts (δexp) of the investigated inner salt in D2O and the calculated magnetic isotropic shielding constants (σcalc) for the optimized monomer (4a) solvated in water are reported. The pKa value for 1 of 2.31 ± 0.02 was determined by the potentiometric titration.

Graphical abstractThe molecules of 2,5-dicarboxy-1-methylpyridinium inner salt are linked by short OH⋯O hydrogen bonds into infinite chains. The FTIR spectrum is dominated by a broad absorption in the 1500–800 cm−1 region.Figure optionsDownload full-size imageDownload as PowerPoint slide