Vibrational spectra of Na, K, Mn2+, Ni2+ and Zn2+ salts of 1,2,4,5-benzenetetracarboxylic (pyromellitic) acid-a short hydrogen bond evidence

Five salts of 1,2,4,5-benzenetetracarboxylic acid (pyromellitic acid), [C6H2(COO)4H4], have been synthesized and investigated by infrared and Raman spectroscopy and by single crystal X-ray diffraction methods: sodium salt [Na2(H2O)2][C6H2(COO)4H2], potassium salt [K(H2O)3][C6H2(COO)4H3] and transition metal salts [M(H2O)6][C6H2(COO)4H2], which M = Mn, Ni and Zn. Crystal structures of all five compounds show short intramolecular asymmetric hydrogen bonds (SHB) between adjacent carboxyl groups with O···O distance average of 2.40 Å. The Raman and infrared spectra reported indicate the presence of short hydrogen bonds in all salts, in agreement with the X-ray data. The O-H stretching mode [ν(OH)] had been observed at about 2500 cm−1. Deuterated analogues were synthesized and their Raman spectra show that νOH/νOD ratio average is about unit. The symmetric [νsym(O··H··O)] and asymmetric [νasym(O··H··O)] stretching modes have been attributed about 300 and 870 cm−1, respectively, in all salts, and for deuterated analogues, the ratio νOH/νOD to νsym(O··H··O, O··D··O) is close to unit like it occurs in ν(OH). The vibrational modes, mainly SHB modes, are tentatively assigned by molecular orbital ab initio calculations of pyromellitic acid and anions [C6H2(COO)4H3]− and [C6H2(COO)4H2]2−. Geometry optimizations showed a good agreement with experimental data. Frequency calculation confirms the assignment of specific vibrational modes. Ab initio calculations show that ν(C=O) and νsym(COO) are strongly coupled with in plane OH bending [δ(OH)]. In Raman spectra of deuterated analogues is observed a frequency shift of these bands.