Linear free energy relationships on rate constants for the gas-phase reactions of hydroxyl radicals with PAHs and PCDD/Fs

Abstract

Polyparameter linear free energy relationships (LFERs) on rate constants ( k OH) for gas-phase reactions of hydroxyl radicals with polycyclic aromatic hydrocarbons (PAHs) and polychlorinated dibenzo- p-dioxins and dibenzofurans (PCDD/Fs) were developed. Quantum chemical descriptors and partial least squares (PLS) regression were used for model development. Acenaphthylene was found to be an outlier and was excluded in the final model development. The cumulative variance of the dependent variable explained by the PLS components and determined by cross-validation ( Q cum 2 ), for the optimal model, is 0.97, indicating good robustness and predictive power of the model. The main molecular structural factor governing k OH values of PAHs and PCDD/Fs is molecular ability to donate electrons, as described by the energy of the highest occupied molecular orbital ( E HOMO), the average of net atomic charges on hydrogen atoms ( q H + ), and the average of net atomic charges on carbon atoms ( q C). PAH and PCDD/F molecules with high E HOMO and low q H + and q C values tend to have high log k OH values. The LFER model indicates the temperature dependence of log k OH is weak.