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

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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 P2 1/ 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 ν as(OHO) and γ(OHO) vibrations of the short hydrogen bonds. Linear correlations, δ exp = a + b σ calc, between the experimental 1H and 13C NMR chemical shifts ( δ exp) of 34DCMP in D 2O and the calculated magnetic isotropic shielding constants ( σ calc) for optimized molecules ( 4– 5) are reported.