Double Proton Transfer in Hydrated Formic Acid Dimer: Interplay of Spatial Symmetry and Solvent-Generated Force on Reactivity

TL;DR

This study uses machine learning-enhanced molecular dynamics to explore how solvent effects and molecular symmetry influence double proton transfer in hydrated formic acid dimer, revealing temperature-dependent reaction dynamics and solvent roles.

Contribution

Developed a reactive ML-based potential energy surface enabling extensive MD simulations to analyze solvent and structural influences on double proton transfer in hydrated FAD.

Findings

Water promotes the first proton transfer via Coulomb forces.

DPT rate peaks at around 350 K and decreases at higher temperatures.

The barrier height for DPT varies by about 10% between 300 K and 600 K.

Abstract

The double proton transfer (DPT) reaction in hydrated formic acid dimer (FAD) is investigated at molecular-level detail. For this, a global and reactive machine learned (ML) potential energy surface (PES) is developed to run extensive (more than 100 ns) mixed ML/MM molecular dynamics (MD) simulations in explicit molecular mechanics (MM) solvent at MP2-quality for the solute. Simulations with fixed - as in a conventional empirical force field - and conformationally fluctuating - as available from the ML-based PES - charge models for FAD shows significant impact on the competition between DPT and dissociation of FAD into two formic acid monomers. With increasing temperature the barrier height for DPT in solution changes by about 10% ($\sim 1$ kcal/mol) between 300 K and 600 K. The rate for DPT is largest, $\sim 1$ ns$^{-1}$, at 350 K and decreases for higher temperatures due to destabilisation and increased probability for dissociation of FAD. The water solvent is found to promote the first proton transfer by exerting a favourable solvent-induced Coulomb force along the O-H$\cdots$O hydrogen bond whereas the second proton transfer is significantly controlled by the O-O separation and other conformational degrees of freedom. Double proton transfer in hydrated FAD is found to involve a subtle interplay and balance between structural and electrostatic factors.