Major
Chemistry and Philosophy
Research Abstract
Mid-sized annulenes are known to undergo rapid π-bond shifting. Given that heavyatom 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. (U)M06-2X/cc-pVDZ calculations yield bond-shifting barriers of ca. 10 kcal/mol. The results also reveal extremely narrow barrier widths, suggesting a high probability of tunneling for these bond-shifting reactions. Rate constants were calculated using canonical variational transition state theory (CVT) as well as with small curvature tunneling (SCT) contributions, via direct dynamics. For the major isomer, the computed SCT rate constant for bond shifting at 80 K is 0.16 s–1, indicating that bond shifting is rapid at cryogenic temperatures despite a 10 kcal/mol barrier. The minor isomer is predicted to undergo rapid bond shifting via tunneling even at 10 K. For both isomers, bond shifting is predicted to be much faster than competing conformation change despite lower barriers for the latter process. The preference for bond shifting represents cases of tunneling control in which the preferred reaction is dominated by heavy-atom motions. At all temperatures below –50 °C, tunneling is predicted to dominate the bond shifting process for both isomers. Thus, [16]annulene is predicted to be an example of tunneling by 16 carbons. Bond shifting in both isomers is predicted to be rapid at temperatures accessible by solution-phase NMR spectroscopy, and an experiment is proposed to verify these predictions.
Faculty Mentor/Advisor
William Karney
Included in
Heavy-Atom Tunneling in the Planar Bond Shifting of [16]Annulene
Mid-sized annulenes are known to undergo rapid π-bond shifting. Given that heavyatom 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. (U)M06-2X/cc-pVDZ calculations yield bond-shifting barriers of ca. 10 kcal/mol. The results also reveal extremely narrow barrier widths, suggesting a high probability of tunneling for these bond-shifting reactions. Rate constants were calculated using canonical variational transition state theory (CVT) as well as with small curvature tunneling (SCT) contributions, via direct dynamics. For the major isomer, the computed SCT rate constant for bond shifting at 80 K is 0.16 s–1, indicating that bond shifting is rapid at cryogenic temperatures despite a 10 kcal/mol barrier. The minor isomer is predicted to undergo rapid bond shifting via tunneling even at 10 K. For both isomers, bond shifting is predicted to be much faster than competing conformation change despite lower barriers for the latter process. The preference for bond shifting represents cases of tunneling control in which the preferred reaction is dominated by heavy-atom motions. At all temperatures below –50 °C, tunneling is predicted to dominate the bond shifting process for both isomers. Thus, [16]annulene is predicted to be an example of tunneling by 16 carbons. Bond shifting in both isomers is predicted to be rapid at temperatures accessible by solution-phase NMR spectroscopy, and an experiment is proposed to verify these predictions.
