NMR and NIR Studies of the Tautomerism of 5,10,15,20‐Tetraphenylporphyrin including Kinetic HH/HD/DD Isotope and Solid State Effects

Abstract

AbstractThe liquid and solid state tautomerism of 5,10,15,20‐tetraphenylporphyrin‐15N4 (meso‐tetraphenylporphyrin, TPP) has been studied by dynamic NMR spectroscopy and by NIR spectroscopy. The kinetic HH/HD/DD isotope effects on the liquid state tautomerism were reinvestigated in the temperature region between 200 K and 300 K. It was confirmed that the kinetic HH/HD isotope effects are large and the HD/DD isotope effects small. At 298 K the values 9.7 and 1.8 were obtained. The Arrhenius curve of the HH reaction is non‐linear indicating that the proton transfer takes place by thermally activated tunneling at low temperatures. The kinetic isotope effects are consistent with a stepwise proton transfer pathway involving intermediate tautomeric states in which protons are bound to adjacent nitrogen atoms. Solid state NMR experiments were performed on triclinic TPP, where the reaction barrier is higher compared to TPP in the liquid state. In addition, the degeneracy of the tautomerism in the solid state is lost due to intermolecular interactions. The latter conclusion is supported by the finding that for tetragonal mixed crystals of [(TPP—14N4—Ni)0.1(TPP—15N4—H2)0.9] the degeneracy of the tautomerism and its barrier is equal to the values found for the liquid solution. The NIR measurements show the presence of NH‐stretching overtones in the region of about 6450 cm−1. The anharmonicity of this vibration is of the usual order, which indicates that the reaction coordinate of proton transfer is not identical with the normal modes of the NH‐stretching vibrations, but rather a combination of various vibrational modes including the stretching, bending and skeletal modes. As a consequence, there will be a large number of vibrational states from which tunneling might occur.