BALANCING STERIC AND ELECTRONIC FACTORS IN PUSH-PULL BENZENES - AN AB-INITIO STUDY ON THE MOLECULAR-STRUCTURE OF 1,3,5-TRIAMINO-2,4,6-TRINITROBENZENE AND RELATED-COMPOUNDS

The structures of 1,3,5-triamino-2,4,6-trinitrobenzene (2) and related compounds (3-5) are studied using ab initio Hartree-Fock (6-31G(D)) and local density functional (LDF) computational methods. In contradiction to published structures for 2 predicted from semiempirical data, both ab initio methods predict a planar structure in accord with the experimental X-ray crystallographic study. It is concluded that appreciable ''push-pull'' conjugation is present in 2 and that the interaction between flanking groups is mainly attractive through cyclic hydrogen bonding. Computations on 3-5 were used to gauge the affinity for some rings to distort into boat instead of chair forms. The relative electron density distributions for 2 and 5 are discussed. The planar forms of 2 and 5 are shown to support the concept that static ''gear clashing'' in combination with push-pull effects are responsible for the warped structure of alkyl substituted cognates of 2.