Multiple Bonding in AeN⁻ (Ae=Ca, Sr, Ba)

Abstract

Quantum chemical calculations using ab initio methods at the MRCI+Q(8,9)/def2‐QZVPPD and CCSD(T)/def2‐QZVPPD levels as well as density functional theory are reported for the diatomic molecules AeN‐ (Ae = Ca, Sr, Ba). The nature of the bonds is analyzed with a variety of methods. The anions CaN‐ and SrN‐ have electronic triplet (3Π) ground states with nearly identical bond dissociation energies De ~57 kcal/mol calculated at the MRCI+Q(8,9)/def2‐QZVPPD level of theory. In contrast, the heavier homologue BaN‐ has a singlet (1Σ+) ground state, which is only 1.1 kcal/mol below the triplet (3Σ‐) state. The computed bond dissociation energy of (1Σ+) BaN‐ is 68.4 kcal/mol. The calculations at the CCSD(T)‐full/def2‐QZVPPD and BP86‐D3(BJ)/def2‐QZVPPD levels of theory are in reasonable agreement with the MRCI+Q(8,9)/def2‐QZVPPD data except for the singlet (1Σ+) state, which has a large multireference character. The calculated atomic partial charges given by the CM5, Voronoi and Hirshfeld methods suggest small to medium‐sized charge donation toward Ae atom Ae←N‐ for most electronic states. In contrast, the NBO method predicts for all species medium to large electronic charge donation toward nitrogen Ae→N‐, which is due to the neglect of the (n)p AOs of Ae atoms as genuine valence orbitals.