Kinetic and equilibrium solvent isotope effects on the ionisation of a hydrogen-bonded proton and studies of the intramolecular hydrogen bond in phenylazoresorcinols

Abstract

The kinetic solvent isotope effect on the removal of the hydrogen-bonded proton from 2-methyl-4-(4-nitrophenylazo)resorcinol monoanion by hydroxide ion has been measured at different hydroxide-ion concentrations in 60%(v/v) Me₂SO–L₂O (L = H or D) and in 80%(v/v) Me₂SO–L₂O. The isotope effect in 80%(v/v) Me₂SO–L₂O has been analysed to give isotope effects on the rate coefficient (k₁) and equilibrium constant (k₁) for formation of an open non-hydrogen-bonded species from the intramolecularly hydrogen-bonded form and the results (k₁)^H/(k₁)^D 1.26 and (k₁)^H/(k₁)^D 0.65 were obtained. One contribution to the equilibrium isotope effect is made by the difference in the values of the isotopic fractionation factors (φ) for the protons in the hydrogen-bonded and open forms. To provide estimates of these values, fractionation factors have been measured in 80%(v/v) Me₂SO–L₂O for the hydrogen-bonded proton in 2-isopropyl-4-(4-nitrophenylazo)resorcinol monoanion (φ 0.97) and for the hydroxy proton in phenol (φ 1.08). The isotope effect on the chemical shift of the hydrogen-bonded proton in 2-isopropyl-4-(4-nitrophenylazo)resorcinol monoanion has been determined and the result Δ[δ(¹H)–δ(²H)] 0.61 shows that the hydrogen bond is of the double minimum type.