Combustion Pathways of the Alkylated Heteroaromatics: Bond Dissociation Enthalpies and Alkyl Group Fragmentations

The bond dissociation enthalpies (BDEs) of the alkyl groups of the alkyl-substituted heterocycles have been studied and compiled using DFT methodology, with the intent of modeling the larger heterocyclic functionalities found in coal. DFT results were calibrated against CBS-QB3 calculations, and qualitative trends were reproduced between these methods. Loss of hydrogen at the benzylic position provided the most favorable route to radical formation, for both the azabenzenes and five-membered heterocycles. The ethyl derivatives had lower BDE values than the methyl derivatives due to increased stabilization of the corresponding radicals. Calculated spin densities correlated well with bond dissociation enthalpies for these compounds, while geometric effects were minimal with respect to the heterocycles themselves. Temperature effects oil the bond dissociation enthalpies were minor, ranging by about 5 kcal/mol from 298 to 2000 K; the free energies of reaction dropped significantly over the same range due to entropic effects. Monocyclic heteroaromatic rings were seen to replicate the chemistry of multicyclic heteroaromatic systems.