Hydrogen bonding in the binary water/ammonia complex

A detailed study of the interactions leading to stabilization of the NH₃(H₂O)_n=1,2 clusters is presented in this work. The Potential Energy Surface for the trimers was explored using an adapted version of the simulated annealing optimization procedure that produced cluster candidate structures that were further optimized, refined, and characterized at the MP2/6–311++G(d, p) level. Our results indicate that hydrogen bonding of the N⋯H type is stronger and more covalent than of the O⋯H type. We provide evidence that suggests that the topological complexity of the electron distributions is directly correlated with cluster stability and that most of the stabilization energy originates in electrostatic and exchange contributions. Our calculated trimerization enthalpy, ΔH°₂₉₈ −41.75 kJ/mol, is in excellent agreement with the experimental enthalpy of adsorption of NH₃ into surface water, reported to be −(41 ± 5) kJ/mol.