Fig 3 . Energetic of the reaction between CI and HCl in water cluster.
The first reaction route is normal proton abstraction by CI from HCl which proceed via TS1 with a barrier of 13.56 kcal/mol and generates a chloride radical, which can further initiate chain reactions. The second reaction channel involves 1,3-insertion of HCl to CI which forms a 5-memebered TS2 with a barrier of 17.75 kcal/mol. This TS2 gives a relevant INTClCH2OOH (chlorinated methylhydroxyperoxide) or thermally fragmented into chlorinated methoxy and hydroxyl radicals (PROD-1 ). INT may lead to the most stable products chlorinated methanal and water (PROD-3 ) via a cyclicTS3 . The energy barrier for this process is 19.1 kcal/mol. The formation of PROD-3 is highly exergonic and is the most stable product.
3.4 Reactions between formic acids and CI: Four reaction pathways have been shown for the reaction between CI and formic in this study as shown in Fig 4.