VIBRATIONAL-SPECTRA AND STRUCTURE OF SOME OXALIC-ACID SUBSTITUTED PYRIDINE (METHYL-COMPLEXES, AMINO-COMPLEXES, AND DIHALOGENO-COMPLEXES) - HYDROGEN-BOND FEATURES

The Raman and infrared spectra of some polycrystalline substituted pyr idine/oxalic acid complexes have been investigated and assignments in terms of group frequencies are given. Various hydrogen bonds (NH ... O , OH ... O, OH ... N) are distinguished and crystal structures are pro posed. For the stronger bases (methyl- or aminopyridines with pk(a) ap proximate to 6) proton transfer occurs. The 1/1 complex contains infin ite chains of hydrogen oxalate ions linked by strong OH ... O hydrogen bonds with nu OH between 2000 and 800 cm(-1). R(OH ... O) distances a re 2.47-2.62 Angstrom. The substituted pyridinium cations are linked t o the chain backbone by medium NH ... O hydrogen bonds with NH ... O l engths of 2.71-2.81 Angstrom A. The 3,5-dichloropyridine forms a 2/1 a dduct without proton transfer, in accordance with its pk(a) (0.6), and strong OH ... N hydrogen bonds occur (nu OH about 2000 cm(-1) and R(O H ...)N = 2.65 Angstrom). Finally, the 2,6-dihalogenopyridine derivati ves do not form complex with oxalic acid, presumably because of steric hindrance.