Structure of 3,4-dicarboxy-1-methylpyridinium inner salt studied by X-ray diffraction, DFT calculations, FTIR, Raman and NMR spectra

The structure of 3,4-dicarboxy-1-methylpyridinium inner salt (34DCMP) has been studied by X-ray diffraction, DFT calculations, FTIR, Raman and NMR spectra. The crystals are monoclinic, space group P21/c. The molecules are linked by a short intermolecular and asymmetric O-H⋯O hydrogen bonds of 2.459(7) Å between 4-carboxyl and 3-carboxylate groups of neighboring molecules into infinite chains along the crystal direction [0 1 0]. The hydrogen bonds in molecules optimized by the B3LYP/6-311G(d,p) approach in dimer (2) and trimer (3) are longer than in the crystal, but in monomers (4a-4b) they are shorter. The FTIR spectrum of 34DCMP is dominated by a broad and intense absorption in the 1500-400 cm−1 region. This broad absorption, absent in the Raman spectrum, is assigned to the νas(OHO) and γ(OHO) vibrations of the short hydrogen bonds. Linear correlations, δexp = a + b σcalc, between the experimental 1H and 13C NMR chemical shifts (δexp) of 34DCMP in D2O and the calculated magnetic isotropic shielding constants (σcalc) for optimized molecules (4-5) are reported.