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

Quantum chemical calculations using ab initio methods at the MRCI+Q(8,9)/def2-QZVPPD and CCSD(T)/def2-QZVPPD levels as well as using density functional theory are reported for the diatomic molecules AeN^- (Ae=Ca, Sr, Ba). The anions CaN^- and SrN^- have electronic triplet (³Π) ground states with nearly identical bond dissociation energies D_e ~57 kcal/mol calculated at the MRCI+Q(8,9)/def2-QZVPPD level. 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 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 Ae←N^- charge donation for most electronic states. In contrast, the NBO method predicts for all species medium to large Ae→N^- electronic charge donation, which is due to the neglect of the (n)p AOs of Ae atoms as genuine valence orbitals. Neither the bond orders nor the bond lengths correlate with the bond dissociation energies. The EDA-NOCV calculations show that the heavier alkaline earth atoms Ca, Sr, Ba use their (n)s and (n-1)d orbitals for covalent bonding.