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 AB2+, the most useful quantity for this purpose is Δ, the difference in energy between the attractive channel with asymptote A + B2+ 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 BeF2+, MgO2+, MgF2+, MgCl2+, and SiF2+, as well as several other less likely combinations. The dication BeF2+ was selected for further analysis. Ab initio calculations show that the X2Π 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 A2∑+ is also very low-lying and predicted to be long-lived. The X2Π–A2∑+ band system is electric dipole allowed but very weak in intensity. Possible mechanisms of formation of BeF2+ are discussed.