The electron spin resonances spectra of the [4]annulene (cyclobutadiene) radical cations, R4C4+˙

Abstract

A series of simple (R = R′) and mixed (R ≠ R′)cyclobutadiene radical cations, R₂R₂′C₄⁺˙, have been prepared by photolysis of the alkynes, RCCR′ or of mixtures of the alkynes RCCR and R′CCR′, in dichloromethane in the presence of aluminium chloride, and the e.s.r. spectra have been recorded.

The magnitude of the ¹³C hyperfine coupling in Et₄C₄⁺˙ confirms that it is a π-radical, with no evidence for out-of-plane or in-plane (Jahn–Teller) distortion.

The value of a(H_β) for the radicals (RCH₂)₄ C₄⁺˙ and (RCH₂)₂R′₂C₄⁺˙ indicate that, as the bulk of the alkyl substituents increases, the group R is pushed out of the plane of the ring. Some unusual temperature effects on a(H_β) are ascribed to interaction of the radical cation with the solvent or with the counterion. As the bulk of the alkyl groups increases, the g-value decreases from 2.0030 to about 2.0022.

cis- and trans-Isomers of the radicals Me₂Bu^t₂C₄⁺˙, Et₂Bu^t₂C₄⁺˙, Buⁱ₂Bu^t₂C₄⁺˙, and probably Me₂Et₂C₄⁺˙ have been identified, and the spectra of cis- and trans-Me₂Bu^t₂C₄⁺˙ are analysed in terms of breaking of the degeneracy of the molecular orbital of the cycrobutadiene system by differential electron release by the alkyl groups (Bu^t > Me).