A σ and not a π complex has been identified for the prototypical arenium intermediate, protonated benzene. The activation of the infrared multiphoton dissociation of C6H7+ ions (obtained by the gas-phase protonation of benzene) by the absorption of free electron laser radiation shows a wavelength-dependent dissociation of H2 (see spectrum) that matches the transitions calculated for the σ complex benzenium ion.
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Jones et al. (2003) studied this question.
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