35Cl nuclear quadrupole resonance and infrared spectroscopic studies of hydrogen bonding in complexes of trichloroacetic acid with various nitrogen and oxygen bases

Abstract

The ³⁵Cl n.q.r. spectra of 29 trichloroacetic acid complexes with various bases, mainly pyridines and their N-oxides, have been studied. The dependence of the average ³⁵Cl n.q.r. frequencies, ν_Q, on ΔpK_a have been analysed in terms of the proton-transfer equilibrium, and the data can be explained either on the basis of a double-minimum potential function with rapid flipping of the proton between two equilibrium sites or one broad minimum. Infrared spectra of such complexes show very broad absorptions over the whole region, with numerous so-called ‘Evans holes’. Variations of the absorption in complexes from the inversion region are similar to those observed in solution. In solids the range of the inversion is shifted to the lower ΔpK_a values. Nitrogen and oxygen bases show similar spectral behaviour. In the vicinity of 50% proton transfer the spectral features are similar for A—H…B and A^–…HB⁺ complexes.