The Ionization Potentials of Butadiene, Hexatriene, and their Methyl Derivatives: Evidence for through space interaction between double bond π‐orbitals and non‐bonded pseudo‐π orbitals of methyl groups?

It is shown that the correlation of the π‐ionization potentials of ethylene ( 1 ), butadiene ( 2 ) and trans ‐1,3,5‐hexatriene ( 4 ) favours the orbital sequence π, π, σ in butadiene and π, π, σ, π in the hexatriene in excellent agreement with the results of SPINDO calculations. Within the experimental error the π‐ionization potentials of cis ‐1,3,5‐hexatriene ( 3 ) and trans ‐1,3,5‐hexatriene ( 4 ) are the same. Methyl‐substitution of 2 lowers the π‐ionization potentials I 1 (π) and I 2 (π). For 1 and/or 4 substitution the difference I 2 (π)− I 1 (π) remains constant (≈ 2.5 eV). On the other hand 2 and/or 3 substitution leads to a smaller gap of I 2 (π)– I 1 (π) ≈ 1.6 to 2.0 eV without changing the mean π‐ionization potential I (π) relative to the corresponding 1,4 substituted derivatives. This result is rationalized in terms of a through space interaction between double bond π‐orbitals and non‐bonded pseudo‐π‐orbitals of the substituting methyl groups. The reduced split I 2 (π)– I 1 (π) in cyclopentadiene is attributed to hyperconjugation across the methylene group.