Ring closure in dioxin formation process: An ab initio molecular dynamics study

The four possible mechanisms of ring closure in dioxin formation from chlor ophenols are studied using ab initio molecular dynamics, within generalized gradient approximation. Free energy barriers, derived as the potential of mean constraint force, directly lead to a static approximation for the tran sition rates. We show, however, that the static approximation overestimates the actual rates by 33%-345%, depending on the intermediate/temperature. C omparing our results with the available experimental data, we conclude that , contrary to what is widely assumed, the pathways starting from two chloro phenols are not among the most dominant pathways in the formation of highly toxic dioxins, at least in the gas phase. This signifies the role of catal ysts and/or other precursors. (C) 2001 American Institute of Physics.