Electron paramagnetic resonance spectra of the radical cations of some benzocyclobutenes, benzocyclopentenes and benzocyclohexenes

1,2,4,5-Tetrahydrobenzo[1,2:4,5]dicyclobutene, 1,2,3,5,6,7-hexahydrobenzo[1,2:4,5]dicyclopentene and 1,2,3,4,6,7,8,9-octahydrobenzo[1,2:4,5]dicyclohexene, and their dimethyl derivatives, and tetramethylbenzocyclo-butene, -pentene and -hexene have been prepared. All except the first compound have been oxidised to their corresponding radical cations, and the EPR spectra have been analysed by assigning McConnell-type Q-values to the substituents in the benzene ring. It is suggested that the ordering of the orbital energy levels (psi(A) above psi(S)) in 3,6-dimethylbenzo[1,2:4,5]biscyclobutene and -pentene is a manifestation of the Mills-Nixon effect, and results from rehybridisation of the strained molecular framework. In the radical cations of 5,10-dimethyl-1,2,3,4,6,7,8,9-octahydrobenzo[1,2:4,5]dicyclohexene and 5,6,7,8-tetramethyl-1,2,3,4-tetrahydrobenzocyclohexene the total unpaired electron density, as implied by the McConnell type of relationship, appears to be less than unity. Various possible causes of this are examined.