DFT calculations on the reaction coordinate were then carried out (Figure and Table ) using the B3LYP hybrid functional and the 6-311G (d,p) basis set. As shown in Figure , the radical process has an activation barrier for the initial step (57 kJ mol -1 ) comparable to the experimentally determined value of 67 kJ mol -1 for cysteine (1) (Figure ). S-C bond formation is followed by rotation of the C-C bond via TS2. In the thiyl radical process, the elimination step via TS3 will give the product, formally as the retro-Michael addition, with exclusive S-C scission leading to the stabilization by π-conjugation. In fact, the homolytic dissociation of another S-C bond at the cysteine side via TS3' was kinetically and thermodynamically demanding to form the episulfide (Figure ), which explains the selective elimination. Although the computationally determined activation energy for TS3 (37 kJ mol -1 ) is much smaller than the experimentally determined value of 106 kJ mol -1 , the DFT calculations supported the existence of kinetically and thermodynamically feasible pathways for thiyl radical catalyzed reactions.