A Computational Study of the Interactions among the Nitro Groups in Octanitrocubane

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• 作者：David A. Hrovat ; Weston Thatcher Borden ; Philip E. Eaton ; and Bart Kahr
• 刊名：Journal of the American Chemical Society
• 出版年：2001
• 出版时间：February 21, 2001
• 年：2001
• 卷：123
• 期：7
• 页码：1289 - 1293
• 全文大小：231K
• 年卷期：v.123,no.7(February 21, 2001)
• ISSN：1520-5126

文摘

Ab initio molecular orbital and density functional calculations on isodesmic disproportionationreactions of nitrated cubanes indicate that the repulsion between nitro groups on adjacent carbons inoctanitrocubane (ONC) is surprisingly small and that the energy required for successive substitutions of nitrogroups on the cubane frame diminishes progressively with increasing substitution. Mulliken population analysesshow that, as a consequence of the inductive effects of the nitro groups on one another, the negative chargeson the oxygens also decrease with increasing substitution reducing the magnitudes of the Coulomb repulsionbetween nearest-neighbor nitro groups. The computed D₄ symmetric equilibrium conformation of ONC, whichis essentially the same as the conformation found in the crystalline state, effectively interdigitates the nitrosubstituents, thereby avoiding close O···O contacts. The O···O interactions remain very small throughout180 rotations about the C-NO₂ bonds, a process that is computed to be nearly barrierless. The mechanisminvolves dynamic gearing of the eight nitro groups in ONC by correlated disrotation of two tetrahedral subsetsof four nitro groups. Enantiomerization of the chiral, D₄ equilibrium conformation of ONC by this thresholdmechanism passes through a D_2d transition structure. Dynamic gearing in ONC effects pseudorotation of theunique 4-fold axes to an orthogonal position with every 15 rotation of the nitro groups, thereby generating inONC the time-averaged O_h symmetry of the parent hydrocarbon.