ENDOR study of pentaphenylcyclopentadienyl radicals in solution. Lifting of orbital degeneracy by methyl substitution

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Abstract
An electron-nuclear-double-resonance (ENDOR) study has been made of pentaphenylcyclopentadienyl (PPCPD) and a series of para-methylated PPCPD radicals in liquid solution. The hyperfine splitting (hfs) constants in PPCPD and pentamethyl PPCPD (MPPCPD) do not show a significant π-σ spin delocalization effect. This indicates that the phenyl substituents are twisted less than 60° out of the plane of the five-membered ring. A MO calculation based on σ-π separability using the McLachlan approximation gives good agreement between experimental and theoretical hfs constants for a twist angle of 40°. The ring proton hfs constants in PPCPD and MPPCPD are practically identical, indicating a negligible effect of the methyl groups on the spin density distribution. Moreover, the hfs constants in both radicals are not affected noticeably by temperature variations. The ring proton splittings in the partially methylated PPCPD radicals, on the other hand, vary widely and show a marked temperature dependence. The same is true for the methyl proton splittings in these compounds. This remarkable difference between the PPCPD and the MPPCPD radicals on the one hand and the partially para-substituted radicals on the other can be explained satisfactorily by considering the effect of partial methylation on the orbital ground states of the radicals. It is assumed that partial methyl substitution lifts the orbital degeneracy in the PPCPD radical giving rise to a symmetric (or antisymmetric) orbital ground state with an antisymmetric (or symmetric) thermally accessible excited state. The values of the methyl hfs constants are used to determine the symmetry of the orbital ground states in the partially methylated compounds. The temperature dependence of these couplings is used to derive the values of the energy gaps between the symmetric and antisymmetric states. The experimental data are in good agreement with the results of simple HMO calculations in which the effect of the methyl substituents is treated as purely inductive.