A theory for stabilization of carbonium ions or other cations, by delocalization of neighboring bonds, is described. Such delocalization is available without changing the reactant geometry and is termed "vertical stabilization." The stabilizing influence is contrasted to the bridged-ion theory of neighboring group participation, in which the neighboring group moves toward the reaction center as the transition state is approached, and to frangomeric acceleration, in which the neighboring group moves away from the reaction center as the transition state is approached. The effects of structural changes on the magnitude of (vertical) -conjugation are discussed. In addition, further evidence is offered against significant effects of C-H hyperconjugation on bond lengths. Delocalization of bonds was first discussed in detail by Mulliken6 in his treatment of carbon-hydrogen hyperconjugation as an explanation of certain properties (1) Supported by the Air Force Office of Scientific Research, Grants AFOSR-69-1639 and AFOSR-69-1639A.(2) The effects ofconjugation on singlet neutral molecules, on free radicals, and on carbanions will be considered in subsequent papers.(3) The major part of these ideas was presented at the 155th National Meeting of the American Chemical Society, San Francisco, Calif., April 1968, p 39. Previous discussions ofconjugation in cations are found in ref 4a-g.(4) (a) W.

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