Liu Peng, Zhang Jingxue, Wang Dunyou
College of Physics and Electronics, Shandong Normal University, Jinan, 250014, China.
Phys Chem Chem Phys. 2017 Jun 7;19(22):14358-14365. doi: 10.1039/c7cp02313h.
A double-inversion mechanism of the F + CHI reaction was discovered in aqueous solution using combined multi-level quantum mechanics theories and molecular mechanics. The stationary points along the reaction path show very different structures to the ones in the gas phase due to the interactions between the solvent and solute, especially strong hydrogen bonds. An intermediate complex, a minimum on the potential of mean force, was found to serve as a connecting-link between the abstraction-induced inversion transition state and the Walden-inversion transition state. The potentials of mean force were calculated with both the DFT/MM and CCSD(T)/MM levels of theory. Our calculated free energy barrier of the abstraction-induced inversion is 69.5 kcal mol at the CCSD(T)/MM level of theory, which agrees with the one at 72.9 kcal mol calculated using the Born solvation model and gas-phase data; and our calculated free energy barrier of the Walden inversion is 24.2 kcal mol, which agrees very well with the experimental value at 25.2 kcal mol in aqueous solution. The calculations show that the aqueous solution makes significant contributions to the potentials of mean force and exerts a big impact on the molecular-level evolution along the reaction pathway.
运用多级量子力学理论与分子力学相结合的方法，在水溶液中发现了F + CHI反应的双反转机制。由于溶剂与溶质之间的相互作用，特别是强氢键的作用，反应路径上的驻点与气相中的驻点结构差异很大。发现一个中间复合物，即平均力势面上的一个极小值，作为抽象诱导反转过渡态与瓦尔登反转过渡态之间的连接环节。采用DFT/MM和CCSD(T)/MM理论水平计算了平均力势。在CCSD(T)/MM理论水平下，我们计算得到的抽象诱导反转的自由能垒为69.5 kcal/mol，这与使用玻恩溶剂化模型和气相数据计算得到的72.9 kcal/mol相符；我们计算得到的瓦尔登反转的自由能垒为24.2 kcal/mol，这与水溶液中25.2 kcal/mol的实验值非常吻合。计算结果表明，水溶液对平均力势有显著贡献，并对沿反应路径的分子水平演化产生重大影响。
