The direction of epoxide ring opening may be predicted using the techniques of theoretical chemistry by comparing the computed total energy of the two possible carbocations formed. To predict the direction of epoxide ring opening and the potential binding of aceanthrylene 1,2-epoxide to biopolymers, quantum mechanical calculations were performed on the two potential hydroxy carbocations. The 2-hydroxy carbocation (II) was favored over the 1-hydroxy carbocation by 11.8 kcal/mol. Molecule II had more positive charge at the meso carbon group than at the nominally electrostatic potential confirm this result, and indicate the possibility of unusual adducts to biopolymers. Similar calculations on the equivalent epoxides of acenaphthylene and acephenanthrylene do not show the same results. Modeling the addition products of II with small nucleophiles indicates that these unusual addition products do not form, and that the interaction is controlled by electronic effects and not electrostatic effects. The calculations on acephenanthrylene demonstrate the importance of including the hydroxyl group when making predictions relative to epoxide ring opening. Molecular descriptors are surrogates for the interactions of that molecule with an often unknown biological target. In cases where molecular descriptors are used without information about the target, small quantitative differences may not be appropriate discriminators. (Copyright (c) 1991 Taylor and Francis Ltd.)