NMR study of the tautomerism of porphyrin including the kinetic HH/HD/DD isotope effects in the liquid and the solid state

Dynamic liquid-state 1H NMR and solid-state 15N cross polarization magic angle spinning NMR spectroscopies help determine rate constants and kinetic HH/HD/DD isotope effects, associated with the intramolecular double-proton transfer of 15N-labeled porphyrin in liquid and polycrystalline states. These parameters are same for both states and liquid state degeneracy of tautomerism is conserved in the solid-state. Arrhenius diagram shows that the stepwise tautomerism involves a metastable intermediate having a thermally activated tunneling of low temperature.