Photolysis of 3-methylcyclobutanone in the gas phase

Abstract

The photolysis of 3-methylcyclobutanone has been carried out in the gas phase mainly at 100 °C using light of 353, 337, 325, 313, ca. 250, 249 and 193 nm wavelengths. In addition, some sensitized decompositions using benzene and mercury were performed. The major products were propene (and ketene) and methylcyclopropane (and carbon monoxide). In some experiments significant quantities of butenes were also produced. The relative product yields depended on the wavelengths of the photolytic radiation and the pressure and were different when sensitizers were present. The results obtained are rationalised on the basis of the initial formation of an excited singlet state which may undergo an energy-dependent decomposition to products or inter-system cross to a triplet state, which on decomposition yields predominantly methylcyclopropane. It can also undergo internal conversion to yield a vibrationally excited ground-state molecule, which at low pressure can decompose to yield predominantly propene and ketene. The orders of magnitude of rate constants of some of the processes involved are consistent with RRKM calculations when reasonable assumptions about energy distributions are made. In these experiments where significant quantities of the butene isomers were detected they are thought to arise from the decomposition of vibrationally excited methylcyclopropane.