EFFECTS OF THE SIMULTANEOUS PRESENCE OF NITRO AND AMINE SUBSTITUENTS IN CUBANE AND SOME AZACUBANES
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We have carried out an ab initio self-consistent-field molecular orbital study of the structures and bond orders (as measures of relative bond strengths) of nitro and aminonitro derivatives of cubane and some azacubanes. The NO2 group is found to have, in general, a conformation-dependent effect on relative bond strengths. When the plane of the nitro group is perpendicular to that of an endocyclic bond, that bond is weakened. An aza nitrogen adjacent to C-NO2, however, eliminates the bond-weakening due to a "perpendicular" nitro group, except when a C-NH2 is also adjacent to the C-NO2. The NH2 group has a direction-specific bond-weakening effect on one [and only one] endocyclic bond; the particular bond undergoing this weakening is determined by the orientation of the amine nitrogen lone pair. The influences of the NO2 and NH2 are essentially independent; when these substituents are on adjacent carbons the effects are reinforcing. Thus, the combination of a "perpendicular" NO2 and an NH2 on adjacent carbons in cubane and azacubanes produces a marked bond weakening. Our findings suggest that strained cage systems that have alternating aza nitrogens and C-NO2 groups should be further investigated as potential high-energy systems. 1991 VCH Publishers, Inc.
