AbstractFrom the photoelectron spectra of benzonorbornadiene (1) and its 9‐isopropylidene derivative (2) the energy and an approximate wave function for the highest occupied orbitals is obtained. With these results the differential reactivity of the systems inDiels‐Alderadditions with ‘inverse electron demand’ is discussed on the basis of frontier orbitals. For the ground state of2similar non‐bonded interactions between the isopropylidene unit and the double bond or the benzene ring are indicated. Conformational equilibria in 9‐aza derivatives of1are discussed in view of the