NMR study of environment modulated proton tautomerism in crystalline and amorphous phthalocyanine

Abstract

Using high resolution 15N and 13C CPMAS NMR spectroscopy (CP - cross polarization, MAS - magic angle spinning) we have detected fast thermally activated proton transfer processes in solid 15N enriched phthalocyanine (Pc). The synthesis of the latter is described. NMR experiments were performed on the crystalline α- and β-modifications, as well as on a novel amorphous modification (am-Pc). In order to extract thermodynamic and kinetic data from the NMR spectra an appropriate lineshape theory of bistable molecules in ordered and disordered matrices is developed. Bistable molecules are subject to exchange between at least two molecular states. The lineshape theory includes the possibility of site dependent perturbations of the rate and equilibrium constants of state exchange, as well as of exchange between different sites. For disordered environments bi-Gaussian distributions of the reaction enthalpies of the state exchange and of the enthalpies of activation are proposed. Different possibilities, including Marcus theory, of reducing this two-dimensional site distribution function to a one-dimensional distribution are discussed. The analysis of the NMR spectra gave the following results. Whereas the proton potential in α-Pc is quasisymmetric, the degeneracy of the tautomers in the β modification is lifted because of a subtle interplay between inter- and intramolecular interactions. The amorphous modification is characterized by a broad distribution of differently perturbed asymmetric double-minimum potentials, as expected for a disordered environment. Rotation of Pc in either of these modifications can be excluded. Proton transfer in the β phase is faster than in the α phase due to smaller energy of activation. This finding is interpreted with a different geometric arrangement of the inner nitrogen atoms in both phases. In addition, the proton transfer in the β phase is characterized by a smaller pre-exponential factor than in the α phase. This effect indicates substantial but different tunnel contributions to the reaction rates in both phases. The implications of the environment modulated proton transfer processes in Pc for the mechanism of hydrogen transfer reactions in liquids is discussed.