Concerted Swing-Coupled Double Proton Tunneling Induced by Dispersion Interaction: Quantum Dynamics for the Formic Acid Dimer-Fluorobenzene System.

The proton (or hydrogen atom) transfer via tunneling plays a key role in chemical and biological processes. However, our understanding of multiple motion or proton concerted tunneling is very limited. Herein, we find that the weak dispersion interaction in the formic acid dimer (FAD)-fluorobenzene (PhF) system does not change the double proton transfer (DPT) barrier in FAD, but induces the FAD swing coupled with DPT. We reveal that it is a triple concerted phenomenon involving the DPT and FAD swing, which reduces the tunneling rate. This is achieved by reduced-dimensionality quantum dynamics calculations using an efficient scheme proposed by us and on a new highly accurate 30-dimensional ab initio neural network potential energy surface constructed in this work, yielding tunneling splittings in excellent agreement with experiment. Our findings are important for acquiring a deeper understanding of nuclear quantum effects, and FAD-PhF undergoing the concerted swing-coupled DPT may be useful in such quantum science applications as construction of molecular clocks or qubits.