The electron spin resonance spectra of the penta-alkylcyclopentadienyl radicals MeR4C5˙(R = Et, Pr, and Bu) and 1,3,5-Me3-2,4-Et2C5˙. Relative substituent effects of the alkyl groups

Abstract

The penta-alkylcyclopentadienyl radicals Me₄EtC₅˙, Me₄PrC₅˙, Me₄BuC₅˙, and 1,3,5-Me₃-2,4-Et₂C₅· have been prepared by photolysis of the corresponding penta-alkylcyclopentadienes. The e.s.r. spectra which are observed are strongly temperature-dependent. It is assumed that the unsymmetrical penta-alkylation breaks the degeneracy of the ψ_A and ψ_S molecular orbitals of the cyclopentadienyl radical, and that there is a Boltzmann distribution of the unpaired electron between these two levels. The e.s.r. spectra which are observed then represent a population-weighted average of the spectra associated with the radical in the ψ_A¹ψ² and ψ_S²ψ_A¹ configurations. The McLachlan equation is used to give the spin-density distributions in these two configurations, and the induced separation between the ψ_A and ψ_S energy levels can then be derived. It is concluded that in the π-radical R–Ċ, the relative electron-releasing interaction between the alkyl group R and a;unit p-electron on the attached carbon atom falls in the sequence (kJ mol^–1) Me (0) > Et (1.79)∼ Pr (1.84)∼ Bu (1.99).