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 {article}{empty}{document}NO + trans - 3,trans - 51→trans - 3,cis - 5 + NO{document} can be represented (with standard errors) by log k 1 = (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 k 1 = k c K a, b . Estimates of k c and K a, b lead to a value of k 1 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.