Comparison of the relative stabilities of polycyclic aryl nitrenium ions and arylmethyl cations: ab initio and semiempirical molecular orbital calculations

Ab initio HF/3-21G calculations predict the stabilities of a series of nitrenium ions to increase in the order, phenyl < 2-naphthyl < 1-naphthyl < 2-fluorenyl. Quantitatively similar results are obtained at the MP/6-31G(d) level for the phenyl and naphthyl systems. The relative stabilities of the nitrenium ions qualitatively parallel those of the corresponding isoconjugate arylmethyl cations but are significantly more sensitive to the nature of the aryl group. Semiempirical AM1 calculations of the same quantities are in reasonable agreement with the ab initio values and are used to extend the comparison to a further 20 polycyclic aromatic derivatives. The relative stabilities of the larger set of nitrenium ions are found to be very precisely linearly related to those of the corresponding arylmethyl cations if allowance is made for the electron donating ability of the aryl group as measured by the charge on the exocyclic carbon atom. A similar but approximate relationship can also be demonstrated in terms of simple perturbational molecular orbital theory.