The Kinetics of the gas-phase isomerization of 1,trans-3,trans-5-heptatriene into the cis-5-isomer catalyzed by Nitric Oxide and the stabilization energy in the pentadienyl radical

The kinetics of the nitric oxide catalyzed, homogeneous, gas-phase isomerization of 1,trans-3,trans-5-heptatriene have been studied for temperatures ranging between 130°C and 241°C. The very clean reaction involves exclusive geometrical isomerization about the 5,6-π-bond. The observed rate constants for \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm NO} + trans - {\rm 3,}trans{\rm - 5}\stackrel{1}{\rightarrow}trans - 3,cis - 5 + {\rm NO} $\end{document} can be represented (with standard errors) by log k₁ = (7.18 ± 0.06) – (16.75 ± 0.12)/θ, where θ = 2.303 R T in kcal/mole. The consecutive-step reaction mechanism involves addition of NO to the double bond (K_{a, b} = k_a/k_b), followed by rotation of the 5,6-C? C bond in the adduct radical (k_c.) Analysis of the observed activation parameters shows, that k_c is rate-controlling and consequently k₁ = k_cK_{a, b}. Estimates of k_c and K_{a, b} lead to a value of k₁ in good agreement with experiment. Comparing our data with those previously obtained for the similar 1,3-pentadiene system results in a value for the extra stabilization energy generated in the 1,3-heptadienyl radical of 18.5 ± 1.7 kcal/mole. This value is discussed in view of comparable data in the literature.