In this study, we demonstrate that quantum-chemical calculations can be successfully employed in a priori estimation of the thermal properties of alkylammonium hydrogen sulfate protic ionic liquids (PILs) and show how these characteristics can be controlled by altering the cation structure. The geometries, charge distributions, energies of the highest occupied and lowest unoccupied molecular orbitals, proton affinities, and dipole moments of the ammonium cation and some of its alkyl derivatives are investigated by the dispersion-corrected density functional theory (DFT-D) method in the gas phase and in the dielectric medium applying the conductor-like polarizable continuum model. The correlations between the calculated characteristics of the single cations and the thermal properties of the alkylammonium hydrogen sulfates are analyzed. The findings indicate that the melting temperature of the specified PILs has a more pronounced correlation with the dipole moment of the cation while the decomposition temperature correlates well with the lowest unoccupied molecular orbital energy. These cation parameters are used to construct one-parameter models to evaluate the thermal properties of the PILs. The resultant data form the basis for designing new alkylammonium hydrogen sulfates. The experimental values of the melting and decomposition temperatures of the newly synthesized PIL, methylpropylammonium hydrogen sulfate, are found to be in exact agreement with the values calculated using the models.
