Furthermore, the consequences of tunneling in 1-5 can also be seen from the Arrhenius graph (see Fig. for six selected systems), for which the kT rates (solid curves) show a distinct concave curve diverging from the semiclassical kSC rates (dotted straight line) and not reaching a flat plateau as the temperature decreases, which is an indication of a reaction assisted by thermally-activated tunneling. The Arrhenius graph conspicuously reveals that as the temperature is lowered, the contribution of the tunneling grows, reaching a kT on the order of ~ 103-104 for all the above systems at 225 K, making the reaction possible even when the classical over-the-barrier process is forbidden (see Supporting Information for rates table). As the temperature increases beyond 298.15 K, the contribution of tunneling, albeit lowered as thermal reaction begins to dominate at We now turned our attention to investigating the kinetic reversibility of the experimentally reported tautomerization of fluorinated systems (5). Previous gas-phase experimental study on 5CF3,CF3 and 5CF3,C2F5 revealed only the exclusive presence of the P(III) form by IR and NMR spectroscopy. To understand this observation, we analyzed the kinetic reversibility of their reaction by comparing the tunneling rate constants and their half-life time for the reverse reaction, i.e., P(III)2w → P(V)2w to those of the corresponding forward rates P(V)2w → P(III)2w. Noteworthy, the ratio between P(III) and P(V) concentration will depend on the energy difference between the tautomers. Still, we can explain this ratio from the relationship of the kinetic rate constants and their corresponding half-lives. Since the reverse reaction is endergonic, we obtained their rates through scaling the rate constants calculated in the exergonic direction by their microscopic reversibility. For 5CF3,C2F5, the reverse rate constants yield a kT of 1 × 10 -4