Formation of the oxyl species. For the formation of the oxyl species from [CuOOH] + two possible pathways were considered. The O-O-cleavage can occur prior (4b→6a) or after the second reduction (4c→6b). In ref. 37 protonation of 4b was shown to be kinetically feasible (48 kJ/mol) and thermoneutral with the TPSS functional. However, the reaction energy turned out to be highly functional dependent and employing B3LYP the reaction was reported uphill (54 kJ/mol).