The non-bonding contact between C2 atom and the moving proton H4 must somehow play a role in the process (Fig. ), as it seems to introduce an element of stiffness to the system, considerably decreasing the vibration energy around TS. Otherwise, the skeleton atoms play little role in the process, with O3-C2 bond barely sensing the events (Fig. ), despite evident decrease of its formal bond order; the impact of the process on the C2-H6 bond is larger.