Relaxed 3D plots in hyperspherical coordinates (Varandas 1987) of the nuclear-mass-independent PESs of ground-state (a) and (b) . The zero of energy is set relative to the infinitely separated C+C₂ fragments. Stationary points and minimum energy paths (solid yellow lines) as in Fig. 1. Solid white lines show the time evolution (in coordinate space) of sample reactive trajectories for the processes (a) and (b) . An isotopically distinct carbon atom is schematically represented in yellow.