vibrational energy encourages ISC (by virtue of the LZ rule) through a small barrier from CCO bending mode (crossing coordinate from MECP calculations, see Fig. in SI) or through a small barrier by internal rotation. This mechanism is different from the one originally proposed by Cvetanović, who assumed ISC of the hot intermediate, followed by collisional stabilization of the singlet intermediate. Considering the overall shape of the PES, the triplet intermediate acts as a trap: once the system is there, the relaxation due to collisions closes pathways on the triplet surface due to high barriers, while the ISC channel remains competitive owing to low MECP heights.