Surface vibrations enhance intramolecular hydrogen tunneling in (some) molecular switches

Hydrogen atoms in supported molecular-switch architectures can tunnel close to room temperature, which calls for the inclusion of nuclear quantum effects (NQE) in the calculation of reaction rates even at high temperatures. However, standard computations of NQE that rely on standard parametrized dimensionality-reduced models, quickly become inadequate in these environments. Here, a paradigmatic molecular switch based on porphycene molecules adsorbed on metallic surfaces is addressed by full-dimensional calculations that combine density-functional theory for the electrons with the semi-classical ring-polymer instanton approximation for the nuclei. The double intramolecular hydrogen transfer (DHT) rate can be enhanced by orders of magnitude due to surface fluctuations in the deep tunneling regime [1]. In addition, the origin of an Arrhenius temperature-dependence of the rate below the tunneling crossover temperature, as well as the transition to different regimes, is explained. With these considerations, a simple model is proposed to rationalize the temperature dependence of porphycene DHT rates spanning diverse fcc [110] surfaces. [1] Y. Litman, M. Rossi, Phys. Rev. Lett., accepted (2020) arXiv:2005.13314.