Abstract
The `tight` (T), cyclobutanoid, and the `extended` (E), {pi}-complex-like, four-centred-three-electron radical cations (4c/3e) derived from (iso)pagodanes and from the valence isomeric (iso)dienes can be viewed as well-separated stationary points on the reaction coordinate for the ethylene/ethylene radical cation [2+1]cycloaddition. So far, experimentally verified examples (ESR, CV, PE) are the extended [1.1.1.1](2{sup .+}) and iso-[2.2.1.1](4{sup .+})ions, persistent in fluid solution at room temperature, and the tight iso-[1.1.1.1]{sup .+} and [2.2.1.1]{sup .+} ions, observable only in Freon matrix (CFCl{sub 3}, {gamma}-rays, 77K). According to calculations, these are the thermodynamically more stable ones of the respective pairs of valence isomers. In this communication the direct observation of tight 1{sup .+} and the activation barrier for its isomerization into extended 2{sup .+} are reported. 16 refs., 3 figs., 1 tab.
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