The acidity of analogous ammonium cations: A description of the solvent effect through the attainment of hydration clusters using the AGOA methodology

The optimized geometries of ethylaminium (1), ethyldiaminium (2), n-butylaminium (3) and tri-ethylaminium (4) cations were calculated using the B3LYP/6-311++G(d,p) theoretical level. Subsequently, the hydration clusters of these structures were explored using the AGOA methodology. Analysing the molecular electrostatic potential of the solute and using the TIP4P model to orient the water molecules around the ammonium cations, systematic increases were observed in hydration energy, from 1 to 4. This result does not correlate with the experimental acidity of 1-4, suggesting that the AGOA methodology is not efficient in modelling cationic structures in an aqueous medium. The AGOA hydration clusters were, therefore, re-optimized (AGOA-OPT) using the B3LYP/6-311++G (d,p) theoretical level. This new AGOA-OPT procedure revealed the hydration energy of the 1, 2, 3 and 4 cations to be in accordance with the experimental acidity.