Predicting thermodynamic stability of diatomic dications: a case study of BeF²⁺

Abstract

General criteria are discussed for the prediction of thermodynamically stable diatomic dications (i.e., cations of + 2 charge). For a diatomic AB 2+ , the most useful quantity for this purpose is Δ, the difference in energy between the attractive channel with asymptote A + B 2+ and the repulsive channel with asymptote A + and B + . Excluding noble gas compounds and using this measure, diatomic dications that may be thermodynamically stable include BeF 2+ , MgO 2+ , MgF 2+ , MgCl 2+ , and SiF 2+ , as well as several other less likely combinations. The dication BeF 2+ was selected for further analysis. Ab initio calculations show that the X 2 Π state of this ion is thermodynamically stable by 1.08 eV and undergoes an avoided crossing that causes the effective dissociation energy to be 2.07 eV. The excited state A 2 ∑ + is also very low-lying and predicted to be long-lived. The X 2 Π–A 2 ∑ + band system is electric dipole allowed but very weak in intensity. Possible mechanisms of formation of BeF 2+ are discussed.