Hydrocarbon bond dissociation enthalpies: From substituted aromatics to polyaromatics

Hydrocarbon-bond dissociation enthalpies (BDE) at 298 K are calculated for a set of hydrocarbons. An efficient method for calculating the BDE values is derived on the basis of a comparative study with experimental data. The methods considered are based on density functional theory (DFT) including the B3LYP MPW1PW91, B3P86, B3PW91, MPW1P86, KMLYP, MPW1K and BMK functionals. The commonly known sequence for radical stability is quantified on the basis of BDE values. The recommended procedure is extrapolated to substituted aromatics and large polyaromatic hydrocarbons (PAHs) to obtain insight into the factors that govern the stability of the radicals. Furthermore it is shown that BDEs are also good reactivity descriptors for subsequent additions involving the formed radicals. Linear correlations, similar to classical Evans-Polanyi-Semenov plots, between the BDE and the reaction barriers for addition reactions with ethene, ethyne, propene, propyne and butadiene are found, as the exothermicity is primarily determined by the stability of the originating reactant radical.