Photolysis of ketene in the presence of propylene: Multistep collisional deactivation of methylcyclopropane and the excess energy of singlet methylene

Abstract

The photolysis of ketene in the presence of propylene-5% oxygen mixtures was investigated at average wavelengths of 3550, 3340, 3130, and 2680 Å, and ketene photolysis in the presence of propylene-n-pentane-5% O2 mixtures was investigated at average wavelengths of 3130 and 2680 Å. Reaction pressures ranged approximately between 10 and 760 torr. Chemically activated methylcyclopropane, arising from singlet methylene addition to propylene, isomerized to the four butene isomers at each wavelength. The rate of isomerization increased monotonically with decreasing wavelength, demonstrating that the methylcyclopropane average energy increased with increasing photon energy. Of the chemically activated butenes formed either by methylcyclopropane isomerization or by insertion of singlet methylene into the carbon-hydrogen bonds of propylene, butene-1 decomposed significantly at the lowest pressures. A multistep process was required to describe the collisional deactivation of chemically activated methylcyclopropane. Theoretical analysis of data at 3550 Å, where methylcyclopropane was considered to have an average energy of 93 kcal mole−1, gave a deactivation step size of 1.5 kcal mole−1 per collision for a stepladder model. Extension of the theoretical treatment to shorter wavelengths predicts that singlet methylene retains only 3 kcal mole−1 of the approximately 25 kcal mole−1 of excess photon energy available at 2680 Å when it adds to the propylene double bond.