1-Carboxyethylpyridinium-4-carboxylate inner salt studied by X-ray diffraction, DFT calculations, vibrational and NMR spectroscopy

The structure of 1-carboxyethylpyridinium-4-carboxylate inner salt (1) has been studied by X-ray diffraction, B3LYP/6-31G(d,p) calculations, FTIR, Raman and NMR spectroscopic techniques. The crystals are monoclinic, space group P21/n. In 1 the acidic proton residing on the carboxyethyl group forms a strong and asymmetric OH⋯O hydrogen bond of length 2.533(2) Å with the ring COO group of the adjacent molecule, forming a chain in a direction of the c axis. Structures of five linear most stable complexes have been optimized by the B3LYP/6-31G(d,p) level of theory and the results have been compared with the X-ray data. The FT-IR spectrum shows a broad and intense absorption in the 2600–1000 cm−1 region, typical of such short hydrogen bonds. In the Raman spectrum the broad absorption is absent. The measured bands in FTIR and Raman spectra are in terms of the calculated vibrations. Linear correlations between the experimental 13C and 1H NMR chemical shifts (δexp) in D2O and the GIAO/B3LYP/6-31G(d,p) calculated magnetic isotropic shielding tensors (σcal), δexp = a + b σcal, are analyzed.