Cooperativity in a New Role: Stabilization of the Ammonium Salts in the Solid State over Their H‑Bonded Complexes in the Gas Phase

Crystal structure of ammonium halides, carbonates, and sulfates like NH₄X (X = F^–, Cl^–, Br^–, NO₃^–) and (NH₄)₂X (X = CO₃^2– and SO₄^2–) exhibit a mode of aggregation in which the cation (NH₄⁺) and counterion are well separated, typical of ionic salts. However, in the stoichiometric limit of the gas phase, they exist only as H-bonded molecular complexes of the type, H₃N···HX. Following a bottom up approach, calculations were performed on these molecular complexes by increasing the number of molecules to investigate the limit in which these molecular complexes transformed to their respective salts. Molecular complex → salt transition is shown to occur for the 2:2 complexes in NH₄Cl, NH₄Br, NH₄HCO₃, and NH₄NO₃, 3:3 complexes for NH₄F, and 4:2 complex for (NH₄)₂SO₄. The relative stability of the salt form in comparison to the H-bonded molecular complex is shown to exhibit interesting cooperative enhancement as the number of molecules increases. Dispersion corrected solid state density functional theory calculations for the crystalline salts reveal that the structures of the higher order aggregates of these complexes resemble the bulk salt-like structures. The computed terahertz (THz) spectra for both the H-bonded complexes and the solid state ionic structures are well resolved to distinguish between the two forms. Calculations for three solid phases of NH₄Cl are in agreement with experimental temperature-dependent relative order of their stability, and the low frequency THz spectra decipher the orientational disorder of the phases due to tumbling/rotational motion of the NH₄⁺ ion within the crystals