Three-dimensional electron delocalization: a theoretical study based on the pentacyclo[3.3.1.13,7.01,3.05,7]decane system

Abstract

The concept of electron delocalization, and the possibility of aromaticity in three dimensions has been introduced, but remains relatively unexplored. This work examines the structure and energy of a series of ions generated from pentacyclo[3.3.1.13,7.01,3.05,7]decane (1). The ions considered are the dication (12+), the radical cation (1+•), the radical anion (1−•), and the dianion (12−). The dication belongs to a small group of organic compounds that shows three-dimensional electron delocalization and it has been classified as "three-dimensionally homoaromatic." The relative stability of these ions is estimated by the enthalpy change of isodesmic and disproportionation reactions using energies obtained from HF, MP2, and DFT methods. The structural changes and the reorganization of the frontier orbitals when electrons are successively added to or removed from the neutral molecule (1) are also explored. The results confirm the three-dimensional electron delocalization in 12+ and indicate that it is stable relative to a localized structure with similar features; however, it is highly unstable compared to the radical cation (1+•). The dianion (12−), on the other hand, did not show any evidence for electron delocalization and it is unstable relative to the radical anion (1−•). Keywords: three-dimensional aromaticity, electron derealization, resonance, radical cations, radical anions.