Nonuniform Proton Transfer and Strong Hydrogen Bonding within Cation, Anion, and Neutral Clusters of Ammonia and Hydrogen Fluoride.

Structural studies of ammonium halide nanoparticles can help to reveal fundamental information about the detailed nature of intermolecular forces. This study focuses on small ammonium fluoride clusters, which exhibit complex behavior in comparison to other ammonium halide clusters due to the weak acidity and strong hydrogen-bonding ability of HF. Calculations of optimized structures and binding energies are presented for cation, anion, and neutral clusters using MP2, CCSD(T), FNO-CCSD(T), ωB97M-V, and MN15 methods. The extent to which proton transfer occurs between two given cluster components was quantified using a dimensionless proton-transfer parameter (ξ), leading to a classification of different types of hydrogen bonds within the clusters. Whereas the neutral clusters exhibit a complex transition from ordinary hydrogen bonding to a combination of shared-proton hydrogen bonds and complete proton transfers, the anion and cation systems exhibit a rapid transition toward complete proton transfer from HF to NH, with incomplete proton transfer observed only in the smallest anion and cation clusters. Ionic interaction energies of these clusters were also computed and found to exhibit trends which can be interpreted by the size-dependent behavior of ξ. This work extends our understanding of the size-dependent trends in intermolecular forces which govern the formation of anhydrous ammonium halide clusters as well as the relationship between strong hydrogen bonding and proton transfer.