As discussed above, we have seen two main mechanisms by which chemical reactions can proceed in our model system : (a) the initial vibrational excitation as the limiting reaction step (as described by Eq. 7) and (b) the direct tunneling from reactants to products as the main reactive pathway. Whether a reaction proceeds by the first or second mechanism is dictated by the position of the reaction barrier and the conditions are E ‡ I > ω 0 (for case a) or E ‡ II < ω 0 (for case b), respectively. Thus, we propose that for two chemically similar reactions only the one obeying mechanism (a) will be modified by the VSC effects. Experimental measurements of the barrier height E ‡ based on infrared absorption spectroscopy (in conjunction with DFT simulations) suggest that (I) alcoholysis reaction between phenylisocyanate and cyclohexanol has a E ‡ I = 6.7 kcal/mol (2343 cm -1 ), and for a similar alcoholysis reaction (II) between 2,4toluene-diisocyanate and chloraldhydrate, the activation energy is E ‡ II = 2.8 kcal/mol (973 cm -1 ). Interestingly,