Neon and argon bonding in first-row cations NeX⁺ and ArX⁺ (X = Li-Ne)

Theoretically determined equilibrium distances, vibrational frequencies, and dissociation energies for the first-row diatomic cations NeX⁺ and ArX⁺ (X = Li-Ne) in their ground and selected excited states are reported at the MP4(SDTQ)/6-311G(2df,2pd)//MP2/6-31G(d,p) level and compared with the results for HeX⁺. The dissociation energies D_e for the electronic ground states increase for a given X in the order HeX⁺ < NeX⁺ < ArX⁺, with the exception of X = Ne. The differences in D_e values between NeX⁺ and ArX⁺ are significantly larger than between HeX⁺ and NeX⁺. The binding energies for NeX⁺ and ArX⁺ show a distinct maximum for X = N. The trends in the calculated dissociation energies are rationalized by invoking donor-acceptor interactions between the weak electron donors Ne and Ar, respectively, and the electron acceptor X⁺. For this purpose, both frontier orbitals, electron density, energy density, and Laplace concentration are investigated. The analysis of the electronic structure shows that, in the case of the relatively weak acceptors Li⁺, Be⁺, and B⁺, the stability of the corresponding NeX⁺ and ArX⁺ ions in the ground state is solely due to charge-induced dipole interactions. Covalent bonding, however, is predicted for the ground state of NeN⁺, ArC⁺, ArN⁺, ArF⁺, and possibly NeNe⁺, as well as for most excited states of NeX⁺ and ArX⁺.