Statistical effects in the thermal deazetization reaction of 2,3-diazabicyclo(2.2.1)hept-2-ene

Abstract

The deazetization reaction of 2,3‐diazabicyclo(2.2.1)hept‐2‐ene‐exo, exo‐5,6‐d₂ is investigated using Monte Carlo classical variational transition‐state theory, implemented by the efficient microcanonical sampling procedure. Comparison is made with the results of trajectory calculations performed on the same global potential‐energy surface. The microcanonical reaction rates have been determined for both reaction channels, i.e., the stepwise and concerted cleavage of the two C–N bonds of the reactant. The results demonstrate that the thermal decomposition of 2,3‐diazabicyclo (2.2.1)hept‐2‐ene‐exo,exo‐5,6‐d₂ is well described by statistical theories that assume equal weighting for all energetically accessible phase‐space points. It is found that the rate coefficients of the statistical calculations are close upper bounds of the rates determined in trajectory calculations. Previously reported classical trajectory simulations have shown that the distribution of internal energy in the reactant configuration, at the transition state and beyond is very nearly microcanonical for the range of excitation energies analyzed (60–175 kcal/mol). Under such conditions, the agreement obtained between the present statistically computed rate coefficients and those extracted from the trajectory results is not surprising. It is suggested that nonstatistical post‐transition‐state events account for the nonunity ratio of the exo/endo reaction products observed experimentally. These events are not considered in the present statistical theories of the reaction rates.