Hydrogen bonds and hydrogen-bonded systems in Mannich bases of 2,2′-biphenol: an FTIR study of the proton polarizability and Fermi resonance effects as a function of temperature

Abstract

5,5′-Dibromo-3-diethylaminomethyl-2,2′-biphenol ( 1) and 5,5′-dibromo-3,3′-bis(diethylaminomethyl)-2,2′-biphenol ( 2) were synthesized and studied by FTIR and 1H NMR spectroscopy. In dichloromethane solution, 1 forms a hydrogen-bonded system with large proton polarizability due to collective tunnelling of protons in the two intramolecular hydrogen bonds. In acetonitrile the system is polarized to a greater extent. If this system is protonated, the HO⋯HO⋯bond is broken and hence the collective system destroyed. The FTIR spectrum of protonated 2 shows an intense continuum indicating a collective hydrogen-bonded system with large proton polarizability due to collective proton motion. The butyronitrile solution of 1 shows an increase of the Fermi resonance bands with decreasing temperature (above the glass transition). The hydrogen bonds become stronger. Furthermore, the proton polarizability increases. With the transition in the glass state the continuum remains unchanged since the local fields are frozen. Only the Fermi resonance effect becomes slightly stronger. With the butyronitrile solution of 2 the IR continuum caused by the polarizable OH⋯N ⇌ O −⋯H +N bonds increases above the glass transition with decreasing temperature. The equilibrium is shifted in favour of the polar structure. In the glass state no change of the continuum and thus of the proton polarizability is observed, since the local fields are frozen.