The thermal decomposition of diimide in the gas phase: Kinetics and stoichiometry

The kinetics and stoiehiometry of the decomposition of N2H2 and N2D2 have been studied as a function of sample size, pressure, and temperature. The reaction follows a single first order kinetic expression over most of its time course. It is suggested that the rate‐determining step in the mechanism is a first‐order homogeneous gas‐phase isomerization of trans‐diimide with rate constants:k N 2H 2 = 1.8 exp (‐4.2 kcal/mol/RT) sec−1 and k N 2D 2 = 1 exp (‐4.4 kcal/mol/RT) sec−1. The detailed mechanism of this isomerization, however, is not evident. At temperatures above room temperature, self‐heating has been observed which leads to an initial fast decay. At room temperature the reaction exhibits autocatalysis with the rate increasing as the reaction proceeds. This has been attributed to enhancement by a surface decay process involving adsorbed hydrazine. The only significant products from the decomposition of N2H2 are N2, H2, and N2H4, and the results are interpreted in terms of two parallel reactions: The decomposition of N2D2 occurs almost completely by the single reaction giving N2 + N2D4. No azide formation has been detected from either N2D2, or N2D2, and limits have been put on the yield of ammonia. Extinction coefficients at 365 nm of 3.9 ± 0.2 for N2H2 and 3.3 ± 0.1 for N2D2 have been measured. Both the rate of decay and the stoichiometry of products show pressure dependence below 150 torr, and this is suggested to be due to direct decomposition of cis‐N2H2 on the surface.