Spectroscopic studies of 1-piperidineacetic acid complexes with phenols

Eleven 1:1 crystalline complexes of 1-piperidineacetic acid with phenols (pKa from 9.55 to 0.33) have been synthesized and analyzed by UV–vis, FTIR, 1H and 13C NMR spectroscopies. The UV–vis spectra of the complexes with moderately strong phenols show separate bands assigned to the molecular complex and the hydrogen bonded ion pair. The sigmoidal character of the plots of the proton and carbon chemical shifts from the N+CH2 group versus the pKa values of phenols, and the change in the FTIR spectra in the ranges 3100–1800 and 1800–400 cm−1 have confirmed the occurrence of prototropic equilibrium in the complexes investigated.