Minor processes in the photolysis of azomethane at low pressure: An upper limit for the rate of the unimolecular elimination of H 2 from vibrationally excited ethane

The photolysis of azomethane in the near UV has been studied at room temperature and pressures from 10 mtorr to 10 torr. The main products, C 2 H 6 and N 2 , accounted for more than 99% of the reaction. Minor hydrocarbon products observed were (with quantum yields) C 3 H 8 (3.5 × 10 −3 ), C 2 H 4 (3.2 × 10 −4 ), CH 4 (3 × 10 −3 ), and n ‐C 4 H 10 (trace). Quantum yields of H 2 of 4 × 10 −5 and 2 × 10 −5 were measured at azomethane pressures of 0.1 and 1.0 torr, respectively. The minor hydrocarbon products can be accounted for by reactions of CH 3 and C 2 H 5 radicals following hydrogen abstraction from azomethane by CH 3 . The H 2 product observed represents an upper limit for the H 2 elimination from vibrationally excited C 2 H 6 formed by CH 3 combination in the system, corresponding to a rate of elimination ca. 5 × 10 −5 times the competing rate of dissociation to 2CH 3 . Assuming a frequency factor of 10 13 s −1 for the H 2 elimination, a lower limit of about 90 kcal mol −1 was estimated for the energy barrier.