Intending to gain further insights in the reaction mechanism, we carried out extensive quantum chemical calculations (DLPNO-CCSD(T)/def2-QZVP// PBE0/def2-TZVP/D4 in toluene (see SI for details), through which we were able to determine a complex branched radical pathway, resulting in the formation of the benzyl aldehyde with and without loss in 18 O-labelling, shown in Scheme 3. The reaction mechanism can be separated into an initiation reaction (Path A) and several branched pathways (paths B to E) that depend on intermediates generated in preceding pathways. Overall, the reaction mechanism can be subdivided into two subsequent hydrogen atom abstraction (HAT) steps.