A comprehensive qualitative and quantitative molecular orbital analysis of the factors governing the dichotomy in the dinorcaradiene reversible arrow 1,6-methanol[10] annulene system

The interconversion between the valence tautomers 1,6-methano[10]-annulene (1) and dinorcaradiene (2) was computationally investigated by qualitative extended Huckel molecular orbital methods (with CACAO visualization) and quantitative semiempirical and ab initio methods. The fundamental bonding interactions are described in terms of perturbation theory arguments for both tautomers, the influence of pi-acceptor or pi-donor substituents at the C11 position is rationalized, and bonding changes during the interconversion are monitored. The electronic molecular structural preferences can be modified by through-space interactions, and residual C1-C6 a bonding remains even at the >2.1 Angstrom separations of the annulenic structures. Hitherto unprecedented calculations of the ge ometries of C11 homo-disubstituted derivatives of 1 and 2 have been carried out at semiempirical and ab initio levels to identify the mon stable tautomer and find whether two stable minima may exist for each derivative. Searches at the AM 1 level to determine transition-state structures for the interconversion of 1 to 2 and of their C11 homo-disubstituted derivatives ale also reported.