Investigation of B ⋯ HA ⇌ B+H ⋯ A− equilibrium in complexes of trifluoroacetic acid with pyridines in dichloromethane by second derivative infrared spectroscopy

Abstract

The second derivative of FTIR spectra in the carbonyl—carboxyl region has been used in order to study the interaction of trifluoroacetic acid with eight R-pyridines in dichloromethane. In the case of the equimolar base—acid mixture, the following species (a) B · HA, (b) B · HA · HA, (c) B+H · A− and (d) B+H · A− · HA are recognized. All of the species are present in complexes of the medium strong pyridines (R = H or D, 3-Me and 4-Me). Two species (a) and (b) are found for complexes of the weakest, when R = 4-CN and 3-Br, and another two (c) and (d) for complexes of the strongest, when R = 3-NMe2 and 2,4,6-Me3, pyridines. Complete formation of the 1:1 complex requires an excess of pyridines. Fermi resonance of overtones due to pyridines and the acid with the continuous absorption is observed.