Spin density distribution and stereochemistry of triphenylmethyl radical in solution

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Abstract
13C-E.S.R. coupling constants of triphenylmethyl radical are readily obtained from the model compound tris-(3,5-di-t-butylphenyl)-methyl. Twisting of the phenyl substituents in solution is proven for the first time by the observation of a considerable contribution from hyperconjugation to acortho C. This twisting causes the unusually large a meta H by π-σ interaction as proposed by Pople for twisted benzyl radicals. Two different sets of spin densities are obtained for C1 and C2 by employing ac2 C, coupling constants of the ring protons and , depending on whether the σ-π parameters of Karplus and Fraenkel or those recently proposed by Strom, Underwood and Jurkowitz are used. The comparison of ac1 C calculated from these spin densities with the experimental carbon-13 splitting at C1 indicates some deviation from trigonal planarity at C1 in solution when the σ-π-parameters of Karplus and Fraenkel are used. Use of the Strom, Underwood and Jurkowitz parameters, on the other hand, lead to pure sp2 hybridization at C1. The latter result is in contrast to the deviations from trigonal planarity observed previously for the central carbon atoms of triphenylmethyl radical in the gas phase and of tris-(p-nitrophenyl)-methyl in the solid state.