Structure of 2,3-dicarboxy-1-methylpyridinium monohydrate studied by X-ray diffraction, DFT calculations, FTIR, Raman and NMR spectra

The structure of 2,3-dicarboxy-1-methylpyridinium monohydrate (1) has been studied by X-ray diffraction, DFT calculations and on the basis of FTIR, Raman and NMR spectra. The crystals are monoclinic, space group P21/n. The water molecule is linked by three intermolecular and asymmetric OH⋯O hydrogen bonds of 2.554(1), 2.715(1) and 2.753(2) Å between 3-carboxyl and 2-carboxylate groups of neighboring molecules of 2,3-dicarboxy-1-methylpyridinium inner salt. The different conformations of monohydrate molecules (1a, 2, 3), anhydrous molecules (4–6) in the gas phase and in water solutions (7, 8) have been optimized by the B3LYP/6-311++G(d,p) approach and the results have been compared with the X-ray data of 1. The correlations between the experimental 1H and 13C NMR chemical shifts (δexp) of 1 in D2O and the magnetic isotropic shielding constants (σcalc) calculated by the GIAO/B3LYP/6-311G++(d,p) approach, using the screening solvation model (COSMO), δexp = a + bσcalc, for optimized molecules 7 and 8 in water solutions are linear and correctly reproduce the experimental chemical shifts. The assignments of water vibrations in FTIR spectrum are consistent with those for other monohydrate crystals.

► 2,3-Dicarboxy-1-methylpyridinium inner salt crystallizes as a monohydrate.
► The COO− group is at 2 position and the COOH group at the 3 position.
► The water molecule is engaged in three H-bonds with three 2,3-dicarboxy-1-methylpyridinium inner salts.
► The lowest energy has the molecule optimized in water solution.