Reaction of singlet methylene with methylenecyclopropane. Part 1.—Evidence for multistep collisional deactivation of chemically activated spiropentane

Abstract

The reaction of singlet methylene, formed by the 325 nm photolysis of ketene, with methylene-cyclopropane has been investigated over a wide range of pressures. The products are ethylene, allene, spiropentane, methylenecyclobutane, methylmethylenecyclopropane, ethylidenecyclopropane and isoprene. Their formation was interpreted in terms of decomposition, rearrangement and partial stabilisation of the initially formed products of addition and insertion. Studies of the pressure dependence of the product distribution, despite its complexity lead to the unequivocal conclusion that chemically activated spiropentane molecules can not be characterised by a unique lifetime. RRKM calculations used in conjunction with a step-ladder model for collisional deactivation give a fit to the data for a deactivation step size of 7.20 kcal mol^–1 at an energy of 105.5 kcal mol^–1 for the initially formed spiropentane. This result is consistent with a figure of 101 kcal mol^–1 for the heat of formation of methylene, although uncertainties, particularly in the entropy of activation for spiropentane isomerisation, mean that a lower value cannot be ruled out.