AB-INITIO STUDIES OF THE THERMAL-DECOMPOSITION OF AZAAROMATICS - FREE-RADICAL VERSUS INTRAMOLECULAR MECHANISM

The energetics of the key reactions in the thermal decomposition of th e azaaromatic molecules pyridine, pyrimidine, pyrazine and pyridazine have been investigated using ab initio quantum-chemical techniques. Tw o distinct mechanisms are considered: (a) intramolecular rearrangement where a bicyclic, Dewar isomer is formed as intermediate, followed by its fission, producing acetylene and hydrogen cyanide, and (b) the fo rmation of azyl radicals in a CH bond fission initiation reaction foll owed by decomposition as well as hydrogen abstraction reactions. The c alculated energetics of the free radical mechanism are found to be con sistent with the results of earlier experimental studies. The estimate s of the critical energies associated with the intramolecular Dewar me chanism are, however, such as to rule it out as a viable alternative. A detailed ab initio study of the benzene --> Dewar benzene rearrangem ent is also reported.