Trends in MH2n+ ion–quadrupole complexes (M = Li, Be, Na, Mg, K, Ca; n = 1, 2) using ab initio methods

Abstract

The ground state potential energy surfaces (PESs) of MH₂ⁿ⁺ (M = Li, Be, Na, Mg, K, Ca; n = 1, 2) have been investigated using relativistically corrected, coupled-cluster (CC) and multi-reference configuration interaction (MRCI) methods. The PESs for MH₂⁺ (M = Li, Na, K) and MH₂ⁿ⁺ (M = Be, Mg, Ca; n = 1, 2) exhibit global minima corresponding to C_2v symmetry equilibrium structures, with local minima for D_∞h and C_∞v symmetry states. Conversely, the ground state PESs of LiH₂²⁺, NaH₂²⁺ and KH₂²⁺ are repulsive. In all cases, the D_∞h states resulting from the insertion of Mⁿ⁺ into the H₂ moiety were significantly higher in energy than the co-linear C_2v states. It is generally assumed a priori that these species are the result of the interaction between the metal ion charge state and the quadrupole moment of the H₂ moiety. However, analysis of the functional Δ_αα(R(Mⁿ⁺–H₂)) = _αα(MH₂ⁿ⁺) − _αα(Mⁿ⁺) (which is effectively the difference between traceless quadrupole moments (_αα) of MH₂ⁿ⁺ and the isolated Mⁿ⁺ ion computed using MRCI) as a function of Mⁿ⁺–H₂ distance demonstrates that a local maximum in Δ_αα along the molecular C₂ axis is necessary for the formation of a thermodynamically stable complex. It is concluded that the topology of Δ_αα provides a convenient indicator of the stability of such molecular ion–quadrupole complexes.