Mid-sized annulenes are known to undergo rapid p-bond shifting. Given that heavy-atom tunneling plays a role in planar bond shifting of cyclobutadiene, we computationally explored the contribution of heavy-atom tunneling to the planar bond shifting in the major (CTCTCTCT) and minor (CTCTTCTT) isomers of [16]annulene. Intrinsic reaction coordinate calculations using the UM06-2X/cc-pVDZ level of theory revealed extremely narrow barrier widths with Ea = 10 kcal/mol and suggest a high probability of tunneling for these bond-shifting reactions. Rate constants were calculated using transition state theory (TST) as well as with small curvature tunneling (SCT) contributions. For the major isomer, the computed SCT rate constant for bond shifting at 10 K is 1.3 s–1, indicating that bond shifting is rapid at cryogenic temperatures despite a 10 kcal/mol barrier. This contrasts with the TST rate constant of 1.1 x 10–199 s–1 at 10 K. Similar behavior is predicted for the minor isomer. At all temperatures below –50 °C, tunneling is predicted to dominate the bond shifting process. Thus, [16]annulene is predicted to be an example of tunneling by 16 carbons.

Heavy-atom tunneling in planar bond shifting of [16]annulene

2019 Spring National Meeting

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