Quest of Quadruple Bonding Between Two Main‐Group Atoms in AeB⁻ and AeC (Ae=Ca, Sr, Ba) and the Role of d Orbitals of Heavier Alkaline‐Earth Atoms in Covalent Interactions

Abstract

Quantum chemical calculations using ab initio methods at the MRCI+Q(6,8)/def2‐QZVPP and CCSD(T)/def2‐QZVPP levels as well as density functional theory are reported for the diatomic molecules AeB− and isoelectronic AeC (Ae=Ca, Sr, Ba). The boride anions AeB− have an electronic triplet (3Σ−) ground state. The quintet (5Σ−) state is 5.8–12.3 kcal/mol higher in energy and the singlet (1Δ) state is 13.1–15.3 kcal/mol above the triplet. The isoelectronic AeC molecules are also predicted to have a low‐lying triplet (3Σ−) state but the quintet (5Σ−) state is only 2.2 kcal/mol (SrC) and 2.9 kcal/mol (CaC) above the triplet state. The triplet (3Σ−) and quintet (5Σ−) states of BaC are nearly isoenergetic. All systems have rather strong bonds. The calculated bond dissociation energies of the triplet (3Σ−) state are between De=38.3–41.7 kcal/mol for AeB− and De=49.4–57.5 kcal/mol for AeC. The barium species have always the strongest bonds whereas the calcium and strontium compounds have similar BDEs. The bonding analysis indicates that there is little charge migration in AeB− in the direction Ae→B− where the alkaline earth atoms carry positive charges between 0.09 e–0.22 e. The positive charges at the Ae atoms are much larger in AeC where the charge migration Ae→C is between 0.90 e–0.91 e. A detailed analysis of the interatomic interactions with the EDA‐NOCV method shows that all diatomic species AeB− and AeC are built from dative interactions between Ae (1S, ns2) and B− or C (3P, 2 s22pπ12pπ′1). The eventually formed bonds in AeC are better described in terms of interactions between the ions Ae+ (2S, ns1)+C− (4S, 2 s22pπ12pπ′12pσ1). Inspection of the orbital interactions suggests that the alkaline earth atoms Ca, Sr, Ba use mainly their (n‐1)d AOs besides the (n)s AOs for the covalent bonds. This creates a second energetically low‐lying σ‐bonding MO in the molecules, which feature valence orbitals with the order ϕ1 (σ‐bonding)<ϕ2 (σ‐bonding)<ϕ3 (degenerate π‐bonding). All four occupied valence MOs of AeB− and AeC are bonding orbitals. Since the degenerate π orbitals ϕ3 are only singly occupied, the formal bond order is three.

Genuine quadruple bonds between two main-group atoms. Chemical bonding in AeF− (Ae = Be–Ba) and isoelectronic EF (E = B–Tl) and the particular role of d orbitals in covalent interactions of heavier alkaline-earth atoms

“…30 The finding lead to a theoretical study of the diatomic anions AeB − and the isoelectronic neutral species AeC (Ae = Ca, Sr, Ba), which have an electron triplet ( 3 Σ − ) ground state. 31 The analysis of the bonding situation has shown that the six valence electrons occupy two bonding σ-orbitals with two electrons each and a degenerate π-orbital, in which each component has one electron. Accordingly, there are four occupied bond orbitals, but since there are only six valence electrons, the formal bond order is only three.…”

“…Accordingly, there are four occupied bond orbitals, but since there are only six valence electrons, the formal bond order is only three. 31 …”

“…Accordingly, there are four occupied bond orbitals, but since there are only six valence electrons, the formal bond order is only three. 31 In order to nd a main-group molecule with a genuine quadruple bond where four bonding orbitals are doubly occupied, we searched for analogous systems with a larger number of valence electrons. A previous study of AeO showed that heavier molecules with Ae = Ca, Sr, Ba have only three doubly occupied MOs, because the oxygen atom is too electronegative for Ae)O s-backdonation into a vacant d s AO of the Ae atom.…”

Computational modelling of graphene/aluminum nitride (GP/AlN) hybrid materials for the detection of 2,4 dichlorophenoxyacetic acid (DCP) pollutant

“…On the other hand, weaker bond interactions are understood by longer bonds. 49,50 Notably, many properties are determined by the bond length, such as the biological activity, polarity, phase of matter, and reactivity. 51 Graphene has a monolayer (single layer) of carbon atoms tightly bonded in a hexagonal honeycomb, and AlN wurzite has a thermodynamically stable crystal structure.…”