Benchmark binding energies of ammonium and alkyl-ammonium ions interacting with water. Are ammonium–water hydrogen bonds strong?

Alkyl-ammonium ion/water interactions are investigated using high level quantum computations, yielding thermodynamics data in good agreement with gas-phase experiments. Alkylation and hydration lead to weaken the NHO hydrogen bonds. Upon complete hydration by four water molecules, their main features are close to those of the OHO bond in the isolated water dimer. Energy decomposition analyses indicate that hydration of alkyl-ammonium ions are mainly due to electrostatic/polarization effects, as for hard monoatomic cations, but with a larger effect of dispersion.