Departing from thio-(hydro)peroxy acid two possible mechanisms for the formation of NOS are shown in Figure , namely the ionic and the radical pathways. The ionic pathway in (a) proceeds by transferring the hydroxyl group to the nitrogen, followed by a proton transfer to the oxygen bonded to the sulfur. This pathway was obtained by restricting the wave function to a closed-shell formalism. Then, a dehydration reaction takes place, removing the hydroxyl group from the newly formed sulfenic acid and one proton from the amino oxide, with the assistance of an external water, leading to the NOS bridge formation. The fatal flaw of such pathway, however, is uncovered by comparing the reaction barrier of transferring the hydroxyl group to the amine (17.3 kcal/mol) and to the sulfur (9.0 kcal/mol). The sulfinic acid is thereby the preferred product through the ionic pathway and virtually impossible to reverse due to its high stability.