The four possible mechanisms of ring closure in dioxin formation from chlorophenols 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 transition rates. We show, however, that the static approximation overestimates the actual rates by 33%-345%, depending on the intermediate/temperature. Comparing our results with the available experimental data, we conclude that, contrary to what is widely assumed, the pathways starting from two chlorophenols 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 catalysts and/or other precursors.