Proton Solvation Free Energy in Liquid Ammonia at Room Temperature

The investigation of experimental pK(a) values in liquid ammonia at room temperature for a series of 30 organic compounds containing eight distinct chemical groups enabled an accurate assessment of the proton ammoniation free energy. The deprotonation free energies, as well as the solvation free energies of acids and conjugate bases, were computed using a combination of quantum chemical and electrostatic approaches. Two thermodynamic schemes were employed to reproduce experimental pK(a) values while scanning a large interval of probable values of proton ammoniation free energy. The desired quantity corresponded to the lowest mean absolute deviation of the overall reproduced pK(a) values of the series. As a result, the proton ammoniation free energy was evaluated to be -275.0 Kcal/mol. Test results show that this proposed value is well suited for pK(a) calculation in liquid ammonia at room temperature.