Stability and Spectroscopic Properties of Singly and Doubly Charged Anions

Using density functional theory and hybrid B3LYP exchange-correlation energy functional we have studied the structure, stability, and spectroscopic properties of singly and doubly charged anions composed of simple metal atoms (Na, Mg, Al) decorated with halogens such as Cl and pseudohalogens such as CN. Since pseudohalogens mimic the chemistry of halogen atoms, our objective is to see if pseudohalogens can also form superhalogens much as halogens do and if the critical size for a doubly charged anion depends upon the ligand. The electron affinities of MCl_<i>n</i> (<i>M</i> = Na, Mg, Al) exceed the value of Cl for <i>n</i> ≥ (<i>k </i>+ 1), where <i>k</i> is the normal valence of the metal atom. However, for M­(CN)_<i>n</i> complexes this is only true when <i>n</i> = <i>k</i> + 1. In addition, while the electron affinities and vertical detachment energies of MCl_<i>n</i> complexes are close to each other, they are markedly different when Cl is replaced by pseudohalogen, CN. The origin of these anomalous results is found to be due to the large binding energy of cyanogen, (NCCN) molecule. Because of the tendency of CN molecules to dimerize, the ground state geometries of the neutral and anionic M­(CN)_<i>n</i> complexes are very different when their number exceed the normal valence of the metal atom. While our calculations support the conclusion of Skurski and co-workers that pseudohalogens can form the building blocks of superhalogens, we show that there is a limitation on the number of CN moieties where this is true. Equally important, we find large differences between the ground state geometries of the neutral and anionic M­(CN)_<i>n</i> complexes for <i>n</i> ≥ (<i>k</i> + 2) which could play an important role in interpreting future experimental data on M­(CN)_<i>n</i> complexes. This is because the electron affinity defined as the energy difference between the ground states of the anion and neutral can be very different from the adiabatic detachment energy defined as the energy difference between the ground state of the anion and its structurally similar neutral isomer