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

Ab initio molecular orbital and density functional calculations on isodesmi c disproportionation reactions of nitrated cubanes indicate that the repuls ion between nitro groups on adjacent carbons in octanitrocubane (ONC) is su rprisingly small and that the energy required for successive substitutions of nitro groups on the cubane frame diminishes progressively with increasin g substitution. Mulliken population analyses show that, as a consequence of the inductive effects of the nitro groups on one another, the negative cha rges on the oxygens also decrease with increasing substitution reducing the magnitudes of the Coulomb repulsion between nearest-neighbor nitro; groups . The computed D-4 symmetric equilibrium conformation of ONC, which is esse ntially the same as the conformation found in the crystalline state, effect ively interdigitates the nitro substituents, thereby avoiding close O . . . O contacts. The O . . .O interactions remain very small throughout 180 degr ees rotations about the C-NO2 bonds, a process that is computed to be nearl y barrierless. The mechanism involves dynamic gearing of the eight nitro gr oups in ONC by correlated disrotation of two tetrahedral subsets of four ni tro groups. Enantiomerization of the chiral, D-4 equilibrium conformation o f ONC by this threshold mechanism passes through a D-2d transition structur e. Dynamic gearing in ONC effects pseudorotation of the unique 4-fold axes to an orthogonal position with every 15 degrees rotation of the nitro group s, thereby generating in ONC the time-averaged O-h symmetry of the parent h ydrocarbon.