In consequence, similar bending excitations would be expected in both reactions, and this is the behaviour found in our theoretical studies with methane and ethane, with only 10 % and 22%, respectively, of bendless states, i.e., the HCN product appears mainly with bending excitation. The second factor is coupling of vibrational modes and, therefore, transfer of energy between them, along the reaction path. Figure shows the changes of the vibrational frequencies along the reaction path, where for a clearer presentation only the most important changes are presented and the vibrational degeneracy is ignored. In this Figure we distinguish the largest stretch modes which practically do not change, named spectator modes (black line, about 3000 cm -1 ); the lowest modes corresponding to free rotations and translations in reactants that evolve to vibrations in products, called transitional modes; the C-H stretching mode which is broken and transformed in the H-CN stretching mode of the new bond formed, named reactive mode (red line) and finally, of special interest in this mechanism of bending excitation, is the change from the bending mode in the ethane reactant (blue line) which evolves to bending mode in the HCN product. We observe that many vibrational modes (with the exception of the spectator and transitional modes) present strong coupling, especially visible in the zone [0-1 bohr] of the reaction coordinate, already in the product zone. Note that s=0 corresponds to the saddle point. However, this does not seem to be the complete picture. The third factor is related with the known QCT limitations due to its classical nature (ZPE violation), and therefore an artificial bending/stretching flux of energy is possible, which would explain the theory/experiment discrepancies in the bending vibrational population. Figure shows this artificial transfer of energy between modes, where the percentage of reactive trajectories with bending energy, CN stretching energy or CH stretching energy, below its respective ZPE are highlighted. So we observe that 17% of the reactive trajectories appear with bending energy below its ZPE, 32% in the case of the CN stretching energy (which shows that this mode does not conserve the adiabaticity along the reaction) and only 7% in the case of the newly formed CH stretch mode. This artificial transfer of energy could explain, at least partially, the theory/experimental discrepancy about the bending excitation in the HCN product.