The effect of including vs. excluding diffuse functions while calculating numerous parameters of PAH anions by various calculation methods is discussed. The omission of diffuse functions appears to have a negligible effect while calculating geometry parameters or total energy; thus, acceptable results may be obtained without them. The conclusions for charge density appear to be the same; however, limited results make an unambiguous claim unachievable. Calculating (1)H- and (13)C-NMR shifts undoubtedly requires the use of these functions.

A detailed investigation has been made into the antiaromaticity and chemical reactivity of a planar cyclooctatetraene (COT) molecule fully annelated with bicyclo[2.1.1]hexane units 2. In spite of its planar 8pi-electronic structure, theoretical calculations have indicated that the antiaromaticity of COT 2 is considerably decreased in comparison with a planar COT 16 with D 4h symmetry. This behavior appears to be related to the wider HOMO-LUMO gap of 2 relative to 16, which is caused by the raised LUMO level as a result of the effective sigma-pi* orbital interaction between the strained bicyclic framework and the COT pi system. The two-electron reduction of 2 required the use of potassium mirror or a combination of lithium/corannulene in highly dried [D8]THF at -78 degrees C under vacuum. In contrast, the [4+2] cycloaddition of 2 with tetracyanoethylene (TCNE) proceeded quite smoothly owing to the high-lying HOMO. Reaction of 2 with meta-chloroperbenzoic acid gave all-trans tetraepoxide 23 in the same way as the corresponding benzene derivative 3. While the Simons-Smith-type cyclopropanation of benzene 3 gave tricyclopropanated derivative 21, the reaction of 2 only afforded isomers of dicyclopropanated derivatives 25 and 26. Yet, the reactivity of 2 is higher than the parent COT, which does not show any reactivity under the same conditions. On the basis of homodesmic reactions, it was concluded that release of strain is also an important factor for such relatively high reactivity in the epoxidation and cyclopropanation of bicycloannelated COT 2 as well as benzene 3.

Various cross-conjugated enediynes undergo "Bergman-type" cycloaromatizations upon reduction with potassium metal, generating anions of fulvenes and fulvalene derivatives. This new anionic cyclization is considerably more facile than the classic Bergman cyclization with linear enediynes, creating highly reactive diradicals at -78 degrees C. Not all cross-conjugated enediynes yield cyclized dianions upon reduction; some give uncyclized, Y-shaped, cross-conjugated dianions, while others apparently yield radical-anions that either dimerize or persist as monomers. One system yields both a cyclized and an uncyclized dianion. The substituents are thus shown to be a critical factor in determining the outcome of the reduction. Cyclization occurs within a specific "window of opportunity" that is governed by the substituents.

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