The photoelectron spectra of 5-methylenenorborn-2-ene (I), 2-methylenenorborane (III), and norbornene show that the π-orbitals in the diene (I) interact strongly but the interacation leads to little or no stabilization of the π-system; simple HMO arguments and consideration of the photoelectron spectrum of norbornadiene (II) suggest that the interaction is mostly π-bonding through-space.
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Cowling et al. (1973) studied this question.
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