10]Annulene: The Wealth of Energetically Low-Lying Structural Isomers of the Same (CH)10 Connectivity

Abstract

Ab initio quantum mechanical methods, as well as molecular mechanics (MM2 and MM3) and semiempirical theoretical methods (AM1), have been used to predict the molecular structures and energetics of the plausible isomers of [10]annulene. Standard double-[zeta] (DZ) and double-[zeta] plus polarization (DZP) basis sets have been used for the ab initio self-consistent-field (SCF) studies. Full geometry optimizations have been carried out by these different methods. The harmonic vibrational frequencies and their infrared intensities are also predicted for all these isomers. The most likely observable isomers are the two structures with C[sub 2] symmetry, one of which could be called naphthalene-like and the other twisted. Isomers with C[sub s] (including a low-lying boat-like structure) and C[sub 1] (including a low-lying azulene-like structure) symmetries are also discussed. The aromatic character for the planar isomers, which are not local minima, has been studied using second-order perturbation theory. A random conformational searching procedure based on the MM3 method was also carried out to avoid losing any possible energetically low-lying conformations. The latter procedure uncovered one low-lying structure that had never been suggested previously. A comparison with the experimental conclusions of Masamune and co-workers is made. 21 refs., 4 tabs.