Unimolecular Decomposition of Chemically Activated Propyl Radicals. Normal Intermolecular Secondary Kinetic Isotope Effect

Abstract

The vibrationally excited radical pairs n-propyl-d0, n-propyl-d6 and isopropyl-d0, isopropyl-d6 were produced at 25°C by the addition of H atoms to propylene-d0 and propylene-d6. Decomposition of n-propyl is by C–C rupture, whereas C–H rupture appears to be the predominant path for isopropyl decomposition, possibly to the exclusion of C–C rupture. The average rates of decomposition ka were determined relative to collisional stabilization by complete product analysis: for n-propyl-d0 C–C rupture, kan≃108 sec—1; for isopropyl-d0 C–H rupture, kai(—1)≃10—2 kan. For vibrationally excited propane, formed by the addition of H atoms to isopropyl radicals, kapr≃6.6×105 sec—1. The secondary kinetic isotope effect for n-propyl unimolecular decomposition in this nonequilibrium system was found to be ∼5.0 at the low-pressure limit, and ∼4.5 at the high-pressure limit; that found for isopropyl decomposition is ∼6. The Marcus—Rice specific rate expression was used with a quantum statistical harmonic oscillator model to calculate theoretical rate constants for the decomposition of n-propyl radicals which are in good agreement with the experimental values. A loosened activated complex was assumed, in which all vibrational modes and internal rotations were taken as active. The effect of different treatments of the internal rotations on the distribution function f(E)dE was demonstrated to be small.