From cyclobutane (four-centre/three-electron) radical cations to the Mills-Nixon effect

One-electron oxidation of pagodane-type molecules (2-9) leads to rather unusual radical cations. The corresponding EPR spectra are interpreted in terms of two different forms of four-centre/three-electron radical cations being embedded inside rigid polycyclic skeletons. With the help of quantum mechanical calculations these two forms have been assigned to an 'extended' and to a 'tight' structure representing two stages on the (D-2h) reaction coordinate of a [2+1] cycloaddition. A cyclobutane moiety can change the ordering of the two highest occupied orbitals in the radical cation of an alternant hydrocarbon, This unexpected observation on the radical cation of binaphthylene (11) is traced back to the Mills-Nixon effect introduced in 1930.