MO Theory of Ease of Formation of Carbocations Derived from Nonalternant Polycyclic Aromatic Hydrocarbons

Abstract

The energy required for carbocation formation from diol epoxide derivatives of polycyclic aromatic hydrocarbons (PAHs) is of interest in understanding molecular carcinogenicity. Here we examine the effects upon such (computed) energies of changing from alternant to nonalternant PAHs. Quantum chemical calculations indicate that the presence of an external unsaturated five-membered ring on an otherwise alternant PAH makes “bay-region” carbocation formation more difficult. However, the presence of a saturated external five-membered ring has the opposite effect. A “peninsular” ring can undergo ionization in either of two ways, the resulting ions often having significantly different stabilities. Simple interactive-fragment frontier-orbital models are found to rationalize all of these results and to offer predictive shortcuts. © 1984, American Chemical Society. All rights reserved.