NMR study of kinetic HH/HD/DD isotope, solvent and solid-state effects on the double proton transfer in azophenine

Abstract

Listen

Translate article

Azophenine (AP, N,N'-diphenyl-3,6-bis(phenylimino)- 1,4-cyclohexadiene-l,4-diamine) is subject in liquid solution to a fast intramolecular double proton transfer involving two degenerate tautomers. Rate constants of this reaction have been measured as a function of temperature by applying different methods of dynamic NMR spectroscopy to various isotopically labeled AP species dissolved in different organic solvents. The rate constants do not depend on the dielectric constant of the solvent, which was varied between 2 (toluene) and 25 (benzonitrile). For C2D2CI4 as solvent, the full kinetic HH/HD/DD isotope effects were obtained at different temperatures. The observed kinetic isotope effects of kHH/kHD = 4.1 and kHD/kDD = 1.4 at 298 K indicate a breakdown of the rule of the geometric mean. I5N CPMAS NMR experiments on crystalline azophenine showed that the reaction also takes place in the solid state. However, the degeneracy of the tautomerism is lifted in this phase because of intermolecular interactions. The mechanism of this reaction is discussed in detail, especially with respect to the questions of whether tunneling is involved and whether one or two protons are transferred in the rate-limiting step. The kinetic isotope effects can best be explained in terms of a stepwise consecutive single proton transfer mechanism involving either a highly polar zwitterion or an apolar singlet-biradical as intermediate. The observation that solvent effects on the reaction rates are absent and that the activation entropy of the reaction almost vanishes excludes the formation of a strongly solvated zwitterionic intermediate. Static medium effects on the double minimum potential of the proton transfer are discussed, taking into account previous results of IR experiments on AP and of solid-state NMR experiments on double proton transfers in organic glasses.