High-level electronic structure calculations have been used to study the factors contributing to the barriers to degenerate hydrogen-atom transfer (HAT) reactions. Understanding of these reactions is a prerequisite to the development of any more general theory of HAT reactions, and yet, the existing models for such reactions perform quite poorly when applied to even simple self-exchanges. The reasons behind these failures are elucidated in the present work. They include a near cancellation of bond-strength effects between reactant and transition state, as well as a strong dependence of the geometry of the transition state on the nature of the heavy atoms.

Aromatic Möbius [4n]annulenes with 4n pi electrons, originally conceived by Heilbronner, are characterized computationally. These (CH)(12), (CH)(16), and (CH)(20) minima have nearly equal C-C bond lengths, small twist angles around the rings, and magnetic properties (NICS, nucleus-independent chemical shifts–see above at various positions in [16]annulene–and magnetic susceptibility exaltations) indicating significantly diatropic ring currents. The Möbius forms are not the most stable isomers but may contribute significantly to the chemistry of these annulenes. [structure: see text]