Ab initio study of the formation and degradation reactions of chlorinated phenols

The formation, stability, and reactivity of chlorinated phenoxyl radicials was studied using ab initio methods. All 19 congeners from mono- to penta-chlorinated species were considered. The radical species are formed in combustion reactions via unimolecular scission of the phenoxyl-hydrogen bond or hydrogen atom abstraction by hydrogen atom or hydroxyl radical. The resulting radicals are stable with respect to unimolecular decomposition and reaction with molecular oxygen is relatively slow. Activation energies are similar to those of the phenoxyl radical for both the decomposition pathway and the reaction with molecular oxygen at the more reactive para-position. Calculations were performed with the model chemistries B3LYP/6-31G(d,p), BHandHLYP/6-31G(d,p), BHandHLYP/aug-cc-pVDZ and QCISD(T)/6-31G(d.p)//BHandHLYP/6-31G(d,p) (for selected reactions.) The results suggest the radicals are sufficiently stable and unreactive to be moderately persistent in the atmosphere, especially when associated with some types of particulate matter. An additivity analysis is made to decompose the relative energetics of the congeners into contributions from hydrogen bonding, resonance stabilization. and repulsive interactions. The results of this analysis correlate well with the results of the calculations. (C) 2009 Elsevier B.V. All rights reserved.