IR spectra and structures of 3-hydroxyquinoline, 4-hydroxyisoquinoline, and their derivatives

Abstract

From an examination of the frequencies, integral intensities, and half-widths of the absorption bands of the stretching vibrations of the hydroxyl groups in the IR spectra, it was concluded that 3-hydroxyquinoline, 4-hydroxyisoquinoline, and their derivatives exist in the phenol form in dilute CCl₄ solutions. Strong intermolecular hydrogen bonds, which are destroyed on dilution, exist in 3-hydroxyquinoline, 4-methyl-3-hydroxyquinoline, 4-hydroxyisoquinoline, and 1-chloro-3-methyl-4-hydroxyquinoline at concentrations above 10² M in CCl₄. From an examination of the values of halo derivatives of 3-hydroxyquinoline and 4-hydroxyisoquinoline, it was concluded that an intramolecular hydrogen bond of the OH...Hal type is present in these compounds. The shift in the OH band increases in the order Cl< Br< I. Weak bands of a free OH group that pertain to the s-trans form are observed in the spectrum. Strong intramolecular hydrogen bonds exist in 3-nitro-4-hydroxyisoquinoline and 3-piperidinomethyl-4-hydroxyisoquinoline.