C₂H₂⁺ + CH₄:

Different vibrational modes are often found to have very different effect on reactivity. In the example here, we are looking at the reaction of acetylene cations with methane. There are three main product channels: C₂H₃⁺ + CH₃, C₃H₄⁺ + H₂, and C₃H₅⁺ + H. From isotope labeling we can tell that the C₃H₄⁺ and C₃H₅⁺ products form in a mechanism mediated by a long-lived (nsec) complex. You can see that this mechanism is inhibited by collision energy and acetylene CC stretch excitation, and modestly enhanced by acetylene cis-bend excitation.

From product ion recoil angular and energy distributions (see example below) we can tell that the C₂H₃⁺ + CH₃ channel goes by two mechanisms. About 10% of the C₂H₃⁺ is formed by CH₃-elimination from the same complex that mediates the other two channels. 90% of the C₂H₃⁺ is formed by a direct mechanism with collision times varying from about 1 psec at low collision energy to <100 fsec at high energies. The angular distributions (see final figure) are strongly forward-peaked at high energies, indicating a stripping-type mechanism. This direct reaction mechanism is enhanced by collision energy , slightly enhanced by the acetylene CC stretch, and greatly enhanced by the cis-bending vibration. The effect is related to the fact that the transition state for the reaction is cis-bent.

Center of Mass recoil velocity distribution for C₂H₃⁺ produced at a collision energy of 2.7 eV. Note that the products are strongly forward scattered (positive axial velocity). The peak of the distribution is very close to the limit expected from spectator stripping.