Aromatic Stabilization Energy Calculations for the Antiaromatic Fluorenyl Cation. Issues in the Choice of Reference Systems for Positively Charged Species

Aromatic stabilization energy (ASE) calculations for the fluorenyl cation show substantial destabilization in comparison to suitable reference systems (16.3 +/- 1.6 kcal/mol), supporting its categorization as an antiaromatic species. The choice of appropriate reference systems is exacting for cationic systems because of the need to match strain energies, convolved with allylic-type resonance terms and other potential structural effects that stabilize charge. Several homodesmotic ASE reaction systems are examined to demonstrate the role played by these factors in the calculation of an ASE value for the fluorenyl cation. The magnitudes of the derived ASE are quite similar for four very different determinative, homodesmotic reaction systems, giving strong support to the inherent accuracy of the final derived ASE value. The results of nucleus independent chemical shift calculations for the components of each one of the ASE reactions add additional weight to this conclusion.