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

Abstract 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 O H⋯O hydrogen bonds of 2.486(2) A 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 ν as (OHO) and γ(OHO) vibrations of the strong hydrogen bond. In the Raman spectrum the broad absorption is absent. Linear correlations, δ exp  =  a  +  b σ calc between the experimental 1 H and 13 C NMR chemical shifts ( δ exp ) of the investigated inner salt in D 2 O and the calculated magnetic isotropic shielding constants ( σ calc ) for the optimized monomer ( 4a ) solvated in water are reported. The p K a value for 1 of 2.31 ± 0.02 was determined by the potentiometric titration.