Lower the electron affinity by halogenation: an unusual strategy to design superalkali cations.

A new kind of cationic superatom compounds (M-F)(+) (M = OLi4, NLi5, CLi6, BLi7, and Al14) with low vertical electron affinities (VEA) has been designed based on the distinctive electronic structure of superalkaline-earth atom. The stability of the studied superatom architectures is guaranteed by strong M-fluorine interactions, considerable HOMO-LUMO gaps, as well as large dissociation energies. What is extraordinary is that fluorination plays an important role in lowering the VEA value of M(+) and enables the resulting (M-F)(+) fluorides to join the superalkali family. However, the same strategy does not work as well for the alkaline-earth atoms whose valence electrons are more tightly bound. The comparative study on (OLi4-X)(+) (X = F, Cl, Br) reveals that fluorination is more effective than chlorination and bromination to reduce the VEA value of the OLi4(+) cation. As for the (Al14-X)(+) species, there is no obvious dependence of VEA values on halogen atomic number.