Deuterium-substituted water-ammonia mixed trimer clusters, (H2O)n−3(NH3)n (n = 0,1,2,3): Interaction energy, hydrogen bond structures, and Mulliken population

The interaction energies and hydrogen bond structures of water-ammonia mixed trimer clusters, (H2O)n−3(NH3)n (n = 0, 1, 2, 3), were analyzed by means of a multi-component molecular orbital method, which directly accounts the quantum effects of protons. We found that the protonic quantum effects directly influenced the flexible description of geometrical parameters and electronic populations. Deuterated water and ammonia were used to estimate the intermolecular interaction energies and hydrogen bond structures. The interaction energies of deuterated complexes were smaller than those of protonated ones, because the hydrogen bond distances in the deuterated complexes were longer than those in their protonated analogs. We also analyzed the stability afforded by formation of cyclic trimer structures and induced by the hydrogen/deuterium isotope effect. Through the replacement of protons with deuterons, the difference in quantum effects between protons and deuterons was reflected in the interaction energies and hydrogen bond structures.