Hyperfine coupling constants of CH2 protons in paramagnetic aromatic systems

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Abstract
It is shown that the splitting constants observed for methylenic protons in π-electron radicals can be accounted for by hyperconjugation only. The theory predicts correctly both magnitude and the ratio of the splittings in the positive and negative ion of pyracene. Application of the method to the negative ion of acenaphthene and the cyclohexadienyl radical also yields satisfactory results. Spin polarization makes only a minor contribution to the splitting. It is shown that one can distinguish two different types of spin polarization: a direct and a consecutive polarization. The contributions of these two types to the splitting of the aliphatic protons are opposite in sign and tend to cancel each other. Objections are raised against the hyperconjugation method in a pseudohetero atom version. The neglect of the anti-bonding orbital in the methyl or methylene substituents leads to erroneous results for the spin densities on the aliphatic protons.

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