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

摘要

The vibrationally excited radical pairsnhyphen;propylhyphen;d0,nhyphen;propylhyphen;d6and isopropylhyphen;d0, isopropylhyphen;d6were produced at 25deg;C by the addition of H atoms to propylenehyphen;d0and propylenehyphen;d6. Decomposition ofnhyphen;propyl is by Csngbnd;C rupture, whereas Csngbnd;H rupture appears to be the predominant path for isopropyl decomposition, possibly to the exclusion of Csngbnd;C rupture. The average rates of decompositionkawere determined relative to collisional stabilization by complete product analysis: fornhyphen;propylhyphen;d0Csngbnd;C rupture,kansime;108secmdash;1; for isopropylhyphen;d0Csngbnd;H rupture,kai(mdash;1)sime;10mdash;2kan. For vibrationally excited propane, formed by the addition of H atoms to isopropyl radicals,kaprsime;6.6times;105secmdash;1. The secondary kinetic isotope effect fornhyphen;propyl unimolecular decomposition in this nonequilibrium system was found to be sim;5.0 at the lowhyphen;pressure limit, and sim;4.5 at the highhyphen;pressure limit; that found for isopropyl decomposition is sim;6.The Marcusmdash;Rice specific rate expression was used with a quantum statistical harmonic oscillator model to calculate theoretical rate constants for the decomposition ofnhyphen;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 functionf(E)dEwas demonstrated to be small.