Thermodynamic parameters of complexation of sterically hindered phenols with hydrogen bond acceptor solvents: determination by 1H NMR spectroscopy

The temperature and concentration dependences of the chemical shift of the phenolic hydroxyl proton in 2,6-diisobornyl-4-methylphenol ( 1 ) and 2,6-di- tert -butyl-4-methyl-phenol (ionol, 2 ) were measured by 1 H NMR spectroscopy in the hydrogen bond acceptor (HBA) solvents (diethyl ether, acetone, ethyl acetate). The equilibrium constants, K , of the formation of hydrogen-bonded phenol—solvent complexes were determined and the thermodynamic parameters Δ H ° and Δ S ° of the complexes were calculated. It was found that the K values of 1 are higher than those of 2 by a factor of about 1.3 in diethyl ether and by a factor of 1.8 in other solvents. The enthalpies of complexation of all systems studied lie in a narrow range of −(12.9−15.5) kJ mol −1 . The results of density functional theory calculations of the complex structures suggest that low K values are due to effective shielding of the OH group in 1 by the isobornyl groups. Steric hindrances force the H atom to deviate from the aromatic ring plane to form a hydrogen bond with the O atom of the HBA solvent. The torsion angle α between the C-O-H plane and the ring plane is in the range of 57–70°. Steric hindrances created by the ortho -substituents in ionol are even more pronounced (α ≈ 90°).