A short hydrogen bond investigation by polarized Raman spectra of Co2+ and Zn2+ salts of pyromellitic acid

Cobalt and zinc salts of 1,2,4,5-benzenetetracarboxylic acid (pyromellitic acid), [C6H2(COO)4H4], have been synthesized and investigate by polarized Raman spectroscopy. These compounds present short intramolecular hydrogen bonds (SHB) between adjacent carboxyl groups. Raman spectra indicate the presence of this interaction in both salts. Three specific vibrational of SHB modes have been investigated: O-H-O symmetric [νsym(OHO)] and asymmetric [νasym(OHO)] stretching modes and O-H stretching mode [ν(O-H)], which they were observed around 300, 850 and 2500 cm−1, respectively. In crystallographic point of view, the cobalt salt presents a symmetric SHB while the zinc salt presents an asymmetric SHB. In cobalt salt all three vibrational modes of O-H-O groups in polarized Raman spectra occur in Ag orientation although in zinc salts two of them are observed in Ag orientation and one in Bg. Spectra analysis indicate that νsym(OHO) mode is observed as Ag to cobalt salt and Bg to zinc salt. This mode occurs in a crowed spectral region and its identification was made by deconvolution techniques. Comparing spectra of the two salts, it is observed a small difference in relative intensity and wavenumber shift of νsym(OHO) (deviance of 43 cm−1) and ν(OH) (deviance of 21 cm−1) modes due probably to differences in O⋯O distance between salts and in orientation of pyromellitate anion in unit cell. The νasym(OHO) mode does not present significant wavenumber shift due difference in SHB. The ν(OH) band presents a great potential for hydrogen bond studies due to the fact that in its vibrational region (around 2500 cm−1) it is not observed other vibrational modes of these compounds.