Ji Xiaoyan, Zhao Chongyang, Xie Jing
Key Laboratory of Cluster Science of Ministry of Education, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, China.
Phys Chem Chem Phys. 2021 Mar 21;23(11):6349-6360. doi: 10.1039/d0cp06299e. Epub 2021 Feb 15.
The dynamics of bimolecular nucleophilic substitution (S2) reactions in the gas phase are of great interest and several new mechanisms have been observed recently by theoretical studies. Here we investigate a recent-discovered S2 reaction mechanism, called front-side complex (FSC) or halogen-bonded complex (XC) mechanism that couples the traditional front-side attack (FSA) and back-side attack (BSA) Walden-inversion mechanism. This XC-pathway begins with a front-side attack on the leaving group, then goes through a bending transition state (XTS) that is followed by Walden-inversion, and results in a configuration inverted product. We characterized the potential energy surface of the microsolvated Y(HO) + CHI S2 reaction using the B97-1/ECP/d method, where Y = HO, F, Cl, Br, and I, and n is the number of water molecules. It is found that the XCs have a deeper well depth than the back-side attack (BSA) pre-reaction complexes for HO/F nucleophiles, indicating that the system can easily become trapped in the halogen-bonded complex well. The barriers of both FSA- and BSA-paths increase with incremental solvation, whereas the change of XTS depends on the type of nucleophile. When Y = HO/F, the order of the barriers is BSA < XC < FSA for n = 0-2, and the order inverts to XC < BSA < FSA for n = 1 (Y = Br/I) and n = 2 (Y = Cl/Br/I), where the latter suggests an increasing participation of the halogen-bonded complex in the S2 reactions. Comprehensive analyses on the structure, charge distribution, and energetics of XC and XTS are provided. This work indicates that the halogen-bonded complex mechanism may be common for alkyl iodides and the information on the potential energy surface is useful in understanding the dynamics behavior of the title and analogous reactions.
气相中双分子亲核取代（S2）反应的动力学备受关注，近期的理论研究发现了几种新机制。在此，我们研究一种最近发现的S2反应机制，称为前侧复合物（FSC）或卤键复合物（XC）机制，它将传统的前侧进攻（FSA）和后侧进攻（BSA）瓦尔登反转机制结合起来。这种XC途径始于对离去基团的前侧进攻，然后经过一个弯曲过渡态（XTS），接着是瓦尔登反转，最终产生构型反转的产物。我们使用B97-1/ECP/d方法表征了微溶剂化的Y(HO) + CHI S2反应的势能面，其中Y = HO、F、Cl、Br和I，n为水分子的数量。研究发现，对于HO/F亲核试剂，XC的阱深比后侧进攻（BSA）预反应复合物更深，这表明体系很容易被困在卤键复合物阱中。FSA和BSA路径的势垒都随着溶剂化程度的增加而升高，而XTS的变化则取决于亲核试剂的类型。当Y = HO/F时，对于n = 0 - 2，势垒顺序为BSA < XC < FSA；对于n = 1（Y = Br/I）和n = 2（Y = Cl/Br/I），顺序变为XC < BSA < FSA，后者表明卤键复合物在S2反应中的参与度增加。文中还对XC和XTS的结构、电荷分布和能量学进行了综合分析。这项工作表明卤键复合物机制可能在烷基碘化物中普遍存在，势能面的信息有助于理解该反应及类似反应的动力学行为。
