Helium bonding in singly and doubly charged first-row diatomic cations HeXⁿ⁺ (X = Li-Ne; n= 1, 2)

With the use of ab initio calculations at the MP4(SDTQ)/6-311G(2df,2pd)//MP2/6-31G(d,p) level of theory, equilibrium geometries, dissociation energies, and vibrational frequencies are reported for singly and doubly charged HeXⁿ⁺ cations (X = Li-Ne; n = 1, 2). The calculations were performed for the electronic ground states and selected excited states of HeXⁿ⁺. The trends in the interatomic distances and bond strengths of the helium bonds are discussed in terms of donor-acceptor interactions between neutral He as the donor and the cationic Xⁿ⁺ fragment as the acceptor. In addition, the mechanism of bonding is analyzed by utilizing the properties of the calculated electron density ρ(r) and its associated Laplace concentration -∇²ρ(r). HeX⁺ ions in their ground states represent van der Waals complexes stabilized by charge-induced dipole interactions. In those excited states in which X⁺ becomes a stronger acceptor, covalent bonds are predicted. In contrast to the singly charged HeX⁺ ground-state species, all HeX²⁺ dications investigated in this work can be considered as covalently bonded molecules. Calculated properties of HeXⁿ⁺ such as interatomic distances r_e and dissociation energies D_e are nicely explained within the donor-acceptor model. The results show that the electronic structure of Xⁿ⁺ is more important for the stability of the corresponding HeXⁿ⁺ system than the positive charge of Xⁿ⁺.