“…[26] TheA rrhenius plots of the rate constants for 7a!7a' ' and 8a!8a' ' reveal pronounced curvature that also indicates al arge tunneling contribution ( Figure 6). Thel evel portion of the solid blue curve in Figure 6b indicates "deep tunneling", that is,tunneling from the lowest vibrational state of 8a,w hereas the curved portions of the Arrhenius plots in Figure 6r epresent temperatures where thermally activated tunneling [17] is occurring,t hat is,f rom excited vibrational states of 7aand 8a.Inboth systems,bond-shifting is predicted to occur much faster than the corresponding (lower-barrier) conformation change below acertain temperature,providing aunique example of tunneling control in which the preferred process is dominated by motions of heavy atoms.A dditionally,t unneling of 16 carbons suggests that synchronous tunneling by multiple heavy atoms might be more prevalent than expected. Coupled with the work by Kozuch, it is not unreasonable to expect tunneling to dominate most planar p-BS reactions at temperatures up to 200 K(or higher in some cases);that is,automerization without tunneling would be the surprise.O ther candidates for degenerate p-BS reactions in annulenes in which tunneling might be present are [4n+ +2]annulenes,such as [14]-and [18]annulene.However, since these are aromatic systems,t he p-BS reactions would be expected to have extremely low barriers,m aking them rapid even without tunneling.…”