A computational study of the interactions among the nitro groups in octanitrocubane

Ab initio molecular orbital and density functional calculations on isodesmic disproportionation reactions of nitrated cubanes indicate that the repulsion between nitro groups on adjacent carbons in octanitrocubane (ONC) is surprisingly small and that the energy required for successive substitutions of nitro groups on the cubane frame diminishes progressively with increasing substitution. Mulliken population analyses show that, as a consequence of the inductive effects of the nitro groups on one another, the negative charges on the oxygens also decrease with increasing substitution reducing the magnitudes of the Coulomb repulsion between nearest-neighbor nitro groups. The computed D₄ symmetric equilibrium conformation of ONC, which is essentially the same as the conformation found in the crystalline state, effectively interdigitates the nitro substituents, thereby avoiding close O···O contacts. The O···O interactions remain very small throughout 180° rotations about the C-NO₂ bonds, a process that is computed to be nearly barrierless. The mechanism involves dynamic gearing of the eight nitro groups in ONC by correlated disrotation of two tetrahedral subsets of four nitro groups. Enantiomerization of the chiral, D₄ equilibrium conformation of ONC by this threshold mechanism passes through a D_2d transition structure. Dynamic gearing in ONC effects pseudorotation of the unique 4-fold axes to an orthogonal position with every 15° rotation of the nitro groups, thereby generating in ONC the time-averaged O_h symmetry of the parent hydrocarbon.