Conversion from a π to a σ structure on replacing oxygen by sulphur: an ESR study

The radical cations of 1,3-dioxacylohexane and 1,3-dioxacyclopentane have π-structures involving delocalisation within the [-OCH₂O-]⁺ unit with high spin-density on the CH₂ group, whilst, in complete contrast, the corresponding sulphur derivatives have a weak σ bond between the two sulphur atoms, the unpaired electron being in the σ^* orbital with negligible spin-density on the CH₂ group.