Electron proton-coupled transfers in [NH4][H3N]n (n = 1, 2) Rydberg clusters: A machine learning-path integral study

Using a combination of path-integral molecular dynamics and machine-learning techniques, we investigate the structure, energetics, and proton-transfer isomerizations in small Rydberg complexes of the form [NH4][NH3]n (n = 1, 2), at temperatures near T = 50 K. In both clusters, the unpaired negative charge resides predominantly outside the molecular framework and is localized near the unit exhibiting the strongest NH4 character. Relative to classical estimates, the combined effects of thermal and nuclear quantum fluctuations shift the computed vertical detachment energies upward, yielding improved agreement with available experimental measurements. Qualitative and quantitative distinctions emerge between the molecular-orbital Rydberg description and solvated electron scenarios in analogous isoelectronic species of the form (NH3)n-. We further characterize proton-transfer-mediated interconversions between isomeric structures. For the dimer, the reactant and product states are equivalent, whereas in the trimer, the rearrangement involves migration of the NH4 radical from a distal to a central position along a hydrogen-bonded chain. Nuclear quantum effects also substantially modify the free energy landscapes associated with proton-transfer isomerizations, most notably through pronounced tunneling induced reductions of the magnitudes of the activation barriers. At the transition states, a strong correlation is registered between the degree of quantum delocalization of the transferring proton and the spatial distribution of the unpaired electron.