To rationalize the trend in the tunneling contribution, a comparison of barrier height and width is instructive. An apparent observation in the tunneling transmission coefficient for 1-5 shows that kT decreases with decreasing barrier height. This might suggest that lower barriers in these systems are associated with broader barrier widths, thereby lowering the kT values. Since the imaginary vibrational frequency (u) of a TS can dictate the topology in the vicinity of the TS and is a measure of the curvature of the reaction coordinate at the TS, it might be expected that a larger barrier height would correspond to a larger imaginary frequency for the reaction coordinate, and in turn to a thinner barrier. However, for our studied reactions, there is no clear correlation between barrier height and TS imaginary frequency. For instance, in 1H,H the TS has a u of 1386i cm -1 and a ∆G ‡ of 26.0 kcal mol -1 , whereas 5Me,Me the one with a large u of 1664i cm -1 , has a ∆G ‡ of 22.1 kcal