Multidimensional Hydrogen Tunneling in Supported Molecular Switches: The Role of Surface Interactions

TL;DR

This study demonstrates the significant impact of surface interactions on hydrogen tunneling rates in supported molecular switches, emphasizing the importance of full-dimensional quantum calculations for accurate rate predictions near room temperature.

Contribution

It introduces a full-dimensional computational approach combining DFT and ring-polymer instanton methods to study hydrogen tunneling in supported molecular switches, revealing surface fluctuation effects.

Findings

Surface fluctuations can enhance hydrogen transfer rates by orders of magnitude.

The temperature dependence of rates varies with different metallic surfaces.

A simple model explains the diverse temperature behaviors across surfaces.

Abstract

The nuclear tunneling crossover temperature ($T_c$) of hydrogen transfer reactions in supported molecular-switch architectures can lie close to room temperature. This calls for the inclusion of nuclear quantum effects (NQE) in the calculation of reaction rates even at high temperatures. However, standard computations of NQE relying on parametrized dimensionality-reduced models, quickly become inadequate in these environments. In this letter, we study the paradigmatic molecular switch based on porphycene molecules adsorbed on metallic surfaces with full-dimensional calculations that combine density-functional theory for the electrons with the semi-classical ring-polymer instanton approximation for the nuclei. We show that the double intramolecular hydrogen transfer (DHT) rate can be enhanced by orders of magnitude due to surface fluctuations in the deep tunneling regime. We also explain the origin of an Arrhenius temperature-dependence of the rate below $T_c$ and why this dependence differs at different surfaces. We propose a simple model to rationalize the temperature dependence of DHT rates spanning diverse fcc [110] surfaces.