VIBRATIONAL STUDY AND PHASE-TRANSITION OF PYRIDINE OXALIC-ACID COMPLEXES

Raman and infrared spectra (4000-20 cm-1) of polycrystalline pyridine- oxalic acid complexes were investigated between 80 and 300 K and an as signment in term of group frequencies is given. The 2:2 and 2:3 compou nds are spectroscopically identified and the OH ... O and NH ... O hyd rogen bonds characterized. The spectroscopic results are consistent wi th crystallographic data. With the deuterated oxalic acid, only one 1: 1 complex is obtained and its crystalline structure is different. From spectroscopic analysis, it is found to be built of infinite chains of hydrogenoxalate ions with strong OD ... O hydrogen bonds (nuOD about 1300 cm - 1 and estimated R(O ... O) = 2.53 angstrom). The pyridinium moiety (C5H5ND+) forms ND ... O hydrogen bonds characterized by a nuND frequency of 2130 cm-1 and an N ... O distance of 2.75 angstrom. Furt her, in this compound, one second-order phase transition is observed a t 235 K and the structural modifications are discussed.