Substituent Effects by Deuterium and Alkyl Groups and 13C Hyperfine Coupling Constants of Cyclopentadienyl Radicals as Studied by Electron Spin Resonance

Article abstract of DOI:10.1021/ja00536a014

ESR spectra of the parent and six substituted cyclopentadienyl radicals (RC5H4; R = H, D, Me, Et, i-Pr, t-Bu, Me3SiCH2), generated from the corresponding RC5H4SnMe3 by an S_H2 attack on Sn with the photochemically generated tert-butoxy radical, have been recorded over the temperature range -95 to +24 deg C.Judging from the proton coupling constants, the electron-releasing perturbation of alkyl groups is in the order (CH3)3SiCH2 > CH3 > C2H5 > (CH3)2CH > (CH3)3C.Preferred conformations of these alkyl groups are discussed from the temperature effects on the spectra.Notably, the methyl group rotates freely, while the (CH3)3SiCH2 group has a fixed conformation in which the (CH3)3Si group eclipses the p orbital on the C1 atom of the cyclopentadienyl.The resonance-integral perturbation effect is more important than the Coulomb-integral perturbation in the deuterium substitution.Finally the coupling constants of 13C for CnHn (n = 5-8) radical species are discussed.