Decrypting the Unusual Structure and σ-Hole Interactions of the XC(NO2)3 (X=F, Cl, Br, and I) Compounds Using Quasi-Atomic Orbitals

This work reports the quasi-atomic orbital analysis of the XC(NO₂)₃ (X=F, Cl, Br, and I) compounds and shows that the interactions between the C-N σ bonds and the lone electron pairs on the halogen atom and oxygen atoms of the nitro groups may contribute to the unusually short C-X distances observed. While the presence of a σ-hole on the halogen atom of the XC(NO₂)₃ compound may not be obvious from the electron density distribution, an analysis of the intermolecular forces of the NH₃--XC(NO₂)₃ complexes suggests a σ -hole interaction between the nitrogen lone pair and halogen atom X (X=Cl, Br, and I) in the linear N--X-C configuration, where electrostatics and exchange forces dominate. The linear N--X-C bond in these systems is shown to have a noticeable covalent character, which is captured in the polarization energy term. Complexation with the ammonia nucleophile is shown to affect the electronic structure of the entire compounds, notably the oxygen/halogen lone electron pairs interactions with the C-N σ bonds.