Epoxide ring opening and related reactivities of cyclopenta polycyclic aromatic hydrocarbons: quantum mechanical studies.

A series of 13 cyclopenta polycyclic aromatic hydrocarbons have been studied using quantum mechanical methods. The three-dimensional molecular structure of each carbocation that might result from the opening of a protonated epoxide ring formed between the carbon atoms completing the cyclopenta ring was computed with AM1. AM1 and ab initio calculations, using a split valence basis set, were then used to predict the direction of ring opening and obtain information about the reactivity of the carbocation. These calculations have shown that for all carbocations studied the cationic charge is well distributed throughout the molecule. The largest CH group charges are approximately 0.3 electron. If the protonated epoxide ring can open so that the nominal charge is on a CH group that is attached to the central ring of an anthracenic core, that carbocation will be greatly favored. For carbocations of this type, the unoccupied alpha' position (the CH group opposite the position of attachment to the anthracenic core) has as much or more of the cation charge as the nominally charged CH position. The group charges, and other properties related to electrostatic reactivity, clearly favor addition of nucleophiles at the unoccupied alpha' position over addition at the nominally charged position. However, when the addition of small nucleophiles at both of these positions is modeled for two such examples, the results favor addition at the nominally charged position in one case and are equivocal in the other case. The group charges and other reactivities considered characterize the electrostatic part of the interaction.(ABSTRACT TRUNCATED AT 250 WORDS)