FT-IR spectra of hydrogen bond complexes involving ortho-halogenophenols and tri-n-butylamine

The complexes between 2,4,6-trichloro-, 2,4,6-tribromo- and 2,4,6-triiodophenol and tri-n-butylamine (TBA) are studied by FT-IR spectrometry at 298K. The equilibrium constants characterizing the formation of complexes of 1-1 (K-1) and 2-1(K-2) stoichiometry are determined in tetrachloroethylene. The K-2/K-1 ratio is higher for the trichlorophenol-TBA complex than for the two other complexes. For the trichlorophenol-TBA system, the proton transfer constants for the 1-1 and 2-1 complexes are respectively equal to 0.33 and 9. This feature is explained by the enhancement of the proton donor ability of the phenol dimer as compared with the monomer. Comparison with other systems suggests that the pK(a), of the 2,4,6-trichlorophenol dimer is lower than that of the monomer by at least 0.7 pK(a) unit. The IR band shapes are discussed and more specifically the broad absorption observed in the 700-400 cm(-1) region, where a strong coupling between the proton donor and proton acceptor vibrations is operating. This coupling is only possible in hydrogen bonds where the O...N distance is relatively short and the OH or NH+ bonds strongly stretched.