Kinetics of the iodine atom catalyzed gas phase geometrical isomerization of diiodoethylene. Rotation about a single bond

The spectrophotometric determination of the rate of iodine atom catalyzed geometrical isomerization of diiodoethylene in the gas phase from 502.8 to 609.1°K leads to a rate constant for the bimolecular reaction between I and trans‐diiodoethylene of log kt−c(M−1 sec−1) = 8.85 ± 0.12 − (11.01 ± 0.30)/θ. Estimates of the entropy and enthalpy change for the addition of I atoms to trans‐diiodoethylene (process a.b) lead to log Ka.b(M−1) = −2.99 − 4.0/θ, and thus to log kc (sec−1) = log kt−c – log Kab = 11.8 −7.0/θ for the rate constant for rotation about the single bond in the adduct radical. The theory for calculation of the rotation rate constant is presented and it is shown that while the exact value depends on the barrier height, a value of 6.8 kcal/mole for this quantity leads to log k (sec−1) = 11.8 −6.7/θ. The activation energy points to a better value of the group contribution to heat of formation of the group C‐(I)2(H)(C) than one based on bond additivity.