Photolysis of diazo-n-butane. Study of the unimolecular decomposition of activated intermediates and energy partitioning

Abstract

The gas-phase photolysis of diazo-n-butane has been performed at various pressures and with various added gases. A mechanism is proposed that satisfactorily explains the experimental observations. The mechanism has been tested by photolysis of diazo-n-butane labelled with ¹⁴C. The photolysis affords a clean and convenient way of activating but-1-ene and methylcyclopropane, and therefore, provides a suitable means of studying the unimolecular decomposition of those molecules at well defined energy levels. In addition, the comparison of experimental and calculated (RRKM) rates allows the estimation of the energy partition between products. 60 ± 9 % of the energy available in the diazo-n-butane photolysis is retained by the n-propylcarbene as internal energy. The activated complex models used permitted the calculation by absolute rate theory (ART) of the rate of the reaction CH₃·+·CH₂—CHCH₂, the reverse of the but-1-ene decomposition. The calculated values are, at best, ten times smaller than the experimental data, indicating a failure of ART when applied to reactions with very small energy barriers.