complexes (Ng = He–Xe): An ab initio and DFT theoretical investigation

The HNgOH2+ cations (Ng = He–Xe), formally arising from the insertion of a Ng atom into the O–H bond of H3O+, were characterized by MP2, CCSD(T), B3LYP, and BP86 calculations as ion–dipole complexes, best described by the resonance form (HNg+)(OH2). While the MP2, CCSD(T), and B3LYP methods predict planar structures of C2v symmetry, the BP86 predicts non-planar structures of Cs symmetry. The structural differences are however only minor, and do not affect the bonding situation, as described by the atomic charges, and the AIM bond topologies. The energy decomposition analysis performed by the ETS method at the BP86 level of theory revealed that the interaction between NgH+ and H2O is prevailingly electrostatic for Ng = Ne, Ar, Kr, and Xe, while the electrostatic and the orbital contributions become comparable for Ng = He. The NOCV analysis unraveled also that, for any Ng, the dominant orbital contribution is the donor–acceptor interaction between the σ(O–H) orbital of H2O (3 A1) and the empty σ orbital of NgH+. All the (HNg+)(OH2) are however largely unstable with respect to dissociation into H3O+ and Ng, and only the heaviest (HAr+)(OH2), (HKr+)(OH2), and (HXe+)(OH2) are predicted to be metastable, and conceivably observable at low temperature. The structure and stability of the Ng-H3O+ intermediates involved in the decomposition of the (HNg+)(OH2) were also briefly examined.