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 ψA and ψS 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 ψA1ψ2 and ψS2ψA1 configurations. The McLachlan equation is used to give the spin-density distributions in these two configurations, and the induced separation between the ψA and ψS 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).