Photoelectron‐spectroscopic Evidence Concerning “Homo‐aromaticity”

The first of the two π‐bands in the photoelectron spectrum of cis‐cis‐cis‐1, 4, 7‐cyclononatriene (I, symmetry C3v) shows a Jahn‐Teller split. This is consistent with the prediction of molecular orbital theory that the top occupied orbitals of I are e(π) and a1(π) respectively. From the difference ϵ(e(π)) ‐ ϵ(a1(π)) = 0.90 to 0.97 eV a value of β1,3 = −0.68 eV = 0.27 β (β = ‐2.5 eV) is obtained for the homoconjugative interaction of two π‐orbitals in I. This value represents almost exclusively through‐space interaction between the π‐orbitals. Through‐bond interaction (hyperconjugation) is a minor effect in I. A comparison of the photoelectron data of bicyclo [4.2.1] nonatriene with those of norbornene and cycloheptadiene shows that homoconjugation (homo‐aromaticity) can only be detected by photoelectron spectroscopy if the interacting π‐bonds (basis orbitals) are symmetry equivalent or have accidentally (almost) degenerate energies.