Delocalization quantitatively mapped for prototypic organic nitroanions as well as azidoform anions

Abstract

Delocalization of occupied orbitals impacts the chemical bonding in the simplest known pernitroanions [(NO₂)₃C]⁻ (1) and [(NO₂)₂N]⁻ (2) as well as other functionalized organic anions. By quantitatively mapping it onto molecular backbones of 1, 2, [CH₂NO₂]⁻ (3), [CH₃NNO₂]⁻ (4) and [C(N₃)]⁻ (6) anions (all modeled by QM calculations), the Weinhold's NBO analysis refines their chemical structure, enabling to explain and even predict their essential chemical behaviour. In detail, the HOMO of 1 and 2 is associated with the central atom to the degree of 70.7% and 80.4%, respectively, while the HOMO localization on O atoms for 3 and 4 is 85.3% and 81.1%, respectively. Predomination of C-alkylation for 1 and that of O-alkylation for 3 in non-coordinating solvents thus becomes clear. The important news is that the easiness of homolytically disrupting the N–N bond in 2, a constituent of inexpensive powerful explosives, is because of the occupancy of the related σ*orbital increases with stretching this bond. The same is true for electrocyclic extrusion of NO₃⁻ from this molecule. This antibonding effect may be assumed to be the common cause of the proneness of aliphatic nitro compounds to decompose. Pyramidal anion 6 is a highly localized carbanion. Its isomer of molecular symmetry C_S has a unique chemical structure of its azido substituents: each of them is represented by one high-weight resonance structure, e.g., N–NN. The prediction is that the dinitrogen-eliminating decomposition of this isomer is more facile than of the isomer of C₃ symmetry. In summary, this study affords three novel particular insights into the chemical structure and reactivity of these anions: chemically telling delocalization-augmented molecular structures, a reasonable hypothesis of the common cause of thermally triggered instability of aliphatic nitro compounds, and discovered one-resonance structure azido groups.