Energetic and dynamic aspects of intramolecular proton transfer in 4-methyl-2,6-diformylphenol: a detailed analysis with AM1 potential energy surfaces

Abstract

The energetics of ground and excited state proton transfer in an isolated 4-methyl-2,6-diformylphenol molecule has been systematically studied by the configuration interaction method at the AM1 level of approximation. The ground singlet and the excited triplet are predicted to have rather large activation barriers on the respective proton transfer paths while the barrier height is much lower on the corresponding singlet surface. The process is predicted to be endothermic in the ground state and exothermic in the excited singlet and more so in the triplet state. From an analysis of the under-barrier vibrational levels supported by the asymmetric double-well potential characterizing two forms, it appears that proton transfer may occur by a vibrationally assisted over-barrier process as well as by a tunneling mechanism following the S₀ → S₁ excitation. Complex coordinate rotation calculation in the Fourier grid Hamiltonian (FGH) framework shows that the tunneling rate constant from the v= 0 vibrational level in the S₁ state is of the same order of magnitude as the experimentally obtained rate constant.