Ab initio studies of the thermal decomposition of azaaromatics: free radical versus intramolecular mechanism

Abstract

The energetics of the key reactions in the thermal decomposition of the azaaromatic molecules pyridine, pyrimidine, pyrazine and pyridazine have been investigated using ab initio quantum-chemical techniques. Two distinct mechanisms are considered: (a) intramolecular rearrangement where a bicyclic, Dewar isomer is formed as an intermediate, followed by its fission, producing acetylene and hydrogen cyanide, and (b) the formation of azyl radicals in a CH bond fission initiation reaction followed by decomposition as well as hydrogen abstraction reactions. The calculated energetics of the free radical mechanism are found to be consistent with the results of earlier experimental studies. The estimates of the critical energies associated with the intramolecular Dewar mechanism are, however, such as to rule it out as a viable alternative. A detailed ab initio study of the benzene → Dewar benzene rearrangement is also reported.