The decomposition of chemically activated n–butane, isopentane, neohexane, and n–pentane and the correlation of their decomposition rates with radical recombination rates.

Abstract

AbstractThe total decomposition rates of the chemically activated alkanes n‐butane, n‐pentane, isopentane, and neohexane were measured using an internal comparison technique. Chemical activation was by the CH insertion reaction of excited singlet‐state methylene radicals. A total of ten rate constants ranging from 4.6 × 105 to 2.3 × 107 sec−1 were measured for these alkanes at different excitation energies. These rates correlate via RRKM theory calculations with thermal A‐factors in the range of 1016.1 to 1017.1 sec−1 for free rotoractivated complex models and in the range of 1016.4 to 1017.8 sec−1 for vibrator‐activated complex models. It was found that high critical energies for decomposition, “tight” radical models, and activated complex models with free internal rotations were required to correlate the decomposition rates of these alkanes with estimated alkyl radical recombination rates. The correlation is just barely possible even for these favorable extremes, indicating that there may be a basic discrepancy between the recombination rate and decomposition rate data for alkanes.