Ammonium-Based Ionic Liquids: Cross-Validation of Energetics Using Solution Calorimetry, Quartz Crystal Microbalance, Quantum Chemistry, and Structure–Property Relationships

The solution enthalpies of tetra-methyl- and tetra-butyl-ammonium tetrafluoroborates were measured using solution calorimetry. The sublimation enthalpies and vaporization enthalpies of ammonium-based ionic liquids with the anions [BF₄] and [NO₃] were derived from temperature dependencies of the vapour pressures, measured with a quartz crystal microbalance and adjusted to the reference temperature 298.15 K. The solution calorimetry results were used to derive the solid-phase enthalpies of formation of the compounds studied. The latter results were combined with the sublimation enthalpies to obtain the experimental gas-phase formation enthalpies of the ionic liquid containing [BF₄] and [NO₃] anions. The theoretical gas-phase formation enthalpies were calculated using the quantum chemical method G3MP2 and agree well with the experimental results. Different types of structure–property relationships were used to establish the consistency of the alkyl-ammonium-based ionic liquids studied in this work.