Abstract
The Raman and infrared spectra of some polycrystalline substituted pyridine/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 proposed. For the stronger bases (methyl- or aminopyridines with pk a ≈ 6) proton transfer occurs. The 1/1 complex contains infinite chains of hydrogen oxalate ions linked by strong OH⋯O hydrogen bonds with vOH between 2000 and 800 cm −1. R OH⋯O distances are 2.47–2.62 Å). The substituted pyridinium cations are linked to the chain backbone by medium NH⋯O hydrogen bonds with NH⋯O lengths of 2.71–2.81 Å. The 3,5-dichloropyridine forms a 2/1 adduct without proton transfer, in accordance with its pk a (0.6), and strong OH⋯N hydrogen bonds occur (vOH about 2000 cm −1 and R OH⋯N = 2.65 A ̊ ). Finally, the 2,6-dihalogenopyridine derivatives do not form complex with oxalic acid, presumably because of steric hindrance.
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