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? |
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Authors: | Michael Beez Gerhard Bieri Hans Bock Edgar Heilbronner |
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Abstract: | 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 I1(π) and I2(π). For 1 and/or 4 substitution the difference I2(π)?I1(π) remains constant (≈ 2.5 eV). On the other hand 2 and/or 3 substitution leads to a smaller gap of I2(π)–I1(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 I2(π)–I1(π) in cyclopentadiene is attributed to hyperconjugation across the methylene group. |
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