Ab initio study of geometrically metastable multiprotonated species: MHk+n

Abstract

The geometries and stabilities relative to fragmentation of H₄O²⁺, H₄S²⁺, H₃F²⁺, H₃Cl²⁺, H₂Ne²⁺, H₂Ar²⁺; H₄F³⁺, H₄Cl³⁺, H₃Ne³⁺, H₃Ar³⁺; H₄Ne⁴⁺, and H₄Ar⁴⁺ have been studied using the quadratic configuration interaction (with single and double excitations plus approximate triple excitations included) QCISD(T)/6–311G(2df,2p) method at second‐order Mo/ller–Plesset optimized geometries MP2(full)/TZP+ZPE/6–31G**. All of the triply charged H₄F³⁺, H₄Cl³⁺, H₃Ne³⁺, and H₃Ar³⁺ and more highly charged H₄Ne⁴⁺ and H₄Ar⁴⁺ ions, as well as doubly charged H₂Ne²⁺ were not found to possess local minima on their zero‐point corrected ground state surfaces, although H₄Cl³⁺ is only slightly unstable. Tetrahedral (T_d) structures for H₄O²⁺ and H₄S²⁺, planar triangular (D_3h) H₃F²⁺, triangular pyramidal (C_3v) H₃Cl²⁺ and bent (C_2v) H₂Ar²⁺ were found to be local minima on the respective ground state surfaces. The latter five species lie above their respective ion‐plus‐ion dissociation products by 61, 91, 111, 67, and 116 kcal/mol, and have barriers to dissociation of 38, 20, 12, 34, and 5 kcal/mol (all energies being zero‐point corrected). Such multiply charged cations store a great deal of energy, which may be released by the addition of a single extra electron to form the corresponding cation of one less charge.