FROM CYCLOBUTANE (4-CENTRE 3-ELECTRON) RADICAL CATIONS TO THE MILLS-NIXON EFFECT/

One-electron oxidation of pagodane-type molecules (2-9) leads to rathe r unusual radical cations. The corresponding EPR spectra are interpret ed in terms of two different forms of four-centre/three-electron radic al cations being embedded inside rigid polycyclic skeletons. With the help of quantum mechanical calculations these two forms have been assi gned to an 'extended' and to a 'tight' structure representing two stag es on the (D-2h) reaction coordinate of a [2+1] cycloaddition. A cyclo butane moiety can change the ordering of the two highest occupied orbi tals in the radical cation of an alternant hydrocarbon, This unexpecte d observation on the radical cation of binaphthylene (11) is traced ba ck to the Mills-Nixon effect introduced in 1930.