Möbius Aromaticity : Which Side Are You on ?

INTRODUCTION The idea of Möbius aromaticity was first proposed by Heilbronner in 1964. He postulated that a molecule could gain Möbius aromatic stabilization if the nuclear framework were twisted so the porbitals composing the π-system would contain an odd number of nodes. To introduce a node in the orbitals, Heilbronner theorized that a series of p-orbitals could itself be twisted by contorting the nuclear framework. The twisted p-orbitals would then form a C2-symmetric π-system that lies on the surface of a Möbius strip (Figure 1). A Möbius strip is a onesided, three-dimensional surface that is constructed from a rectangle by holding the first end fixed, rotating the second end 180°, and joining the two ends together (Figure 1). In normal planar annulenes like benzene, the 4n + 2 electrons in the delocalized π-system give the molecule an overall stability associated with a closed-shell Hückel occupancy. In contrast, if 4n electrons are present, the Hückel molecular orbital analysis results in an open-shell configuration, leading to a destabilization of the molecule relative to the same molecule without delocalization of electrons; this destabilization is also known as “anti-aromatic” character. However, if a 180° phase shift is introduced into the array, Hückel analysis predicts a closed-shell electron configuration for the 4n electron case with one phase change and an open-shell configuration for the 4n + 2 case, leading to a reversal in the pattern of stabilization. Heilbronner used Hückel molecular orbital analysis (Figure 2) to calculate the π–electron energy for the twisted p-orbitals in the Möbius case and found that the energy lost from smaller orbital overlap is exactly equal to the energy gained from pairing the two electrons in non-bonding orbitals into an orbital degenerate with the lowest energy molecular orbital (α + 2β). Zimmerman provided a mnemonic for remembering the new Möbius molecular orbital system (Figure 2). Figure 1. A theoretical Möbius strip bearing p-orbitals.