Michel Cameron S, Lampkin Philip P, Shezaf Jonathan Z, Moll Joseph F, Castro Claire, Karney William L
Department of Chemistry , University of San Francisco , 2130 Fulton Street , San Francisco , California 94117 , United States.
J Am Chem Soc. 2019 Apr 3;141(13):5286-5293. doi: 10.1021/jacs.8b13131. Epub 2019 Mar 18.
Midsized annulenes are known to undergo rapid π-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 planar π-bond shifting in the major (CTCTCTCT, 5a) and minor (CTCTTCTT, 6a) known isomers of [16]annulene. UM06-2X/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 5a, the computed SCT rate constant for bond shifting at 80 K is 0.16 s, corresponding to a half-life of 4.3 s, and indicating that bond shifting is rapid at cryogenic temperatures despite a 10 kcal/mol barrier. This contrasts with the CVT rate constant of 8.0 × 10 s at 80 K. The minor isomer 6a 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 5a and 6a. 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.
已知中型轮烯会发生快速的π键迁移。鉴于重原子隧穿在环丁二烯的平面键迁移中起作用，我们通过计算探索了重原子隧穿对[16]轮烯的主要（CTCTCTCT，5a）和次要（CTCTTCTT，6a）已知异构体中平面π键迁移的贡献。UM06 - 2X/cc - pVDZ计算得出的键迁移势垒约为10 kcal/mol。结果还显示势垒宽度极窄，表明这些键迁移反应发生隧穿的概率很高。速率常数通过正则变分过渡态理论（CVT）以及包含小曲率隧穿（SCT）贡献的直接动力学方法进行计算。对于主要异构体5a，计算得出在80 K时键迁移的SCT速率常数为0.16 s⁻¹，半衰期为4.3 s，这表明尽管存在10 kcal/mol的势垒，但在低温下键迁移仍很快。这与80 K时CVT速率常数8.0×10⁻⁶ s⁻¹形成对比。预测次要异构体6a即使在10 K时也会通过隧穿快速发生键迁移。对于这两种异构体，尽管后者过程的势垒较低，但预测键迁移比竞争性构象变化要快得多。对键迁移的偏好代表了隧穿控制的情况，其中优先反应由重原子运动主导。在低于 - 50°C的所有温度下，预测隧穿将主导5a和6a的键迁移过程。因此，[16]轮烯被预测为16个碳原子进行隧穿的一个例子。预计这两种异构体在溶液相核磁共振光谱可达到的温度下键迁移都很快，并提出了一个实验来验证这些预测。
