Department of Chemistry, Faculty of Science, Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic.
Molecules. 2021 Apr 25;26(9):2501. doi: 10.3390/molecules26092501.
The experimentally motivated question of the acetylene bromoboration mechanism was addressed in order to suggest possible radical isomerization pathways for the -adduct. Addition-elimination mechanisms starting with a bromine radical attack at the "bromine end" or the "boron end" of the C=C bond were considered. Dispersion-corrected DFT and MP2 methods with the SMD solvation model were employed using three all-electron bases as well as the ECP28MWB ansatz. The rate-determining, elimination step had a higher activation energy (12 kcal mol) in case of the "bromine end" attack due to intermediate stabilization at both the MP2 and DFT levels. In case of the "boron end" attack, two modes of C-C bond rotation were followed and striking differences in MP2 vs. DFT potential energy surfaces were observed. Employing MP2, addition was followed by either a 180° rotation through an eclipsed conformation of vicinal bromine atoms or by an opposite rotation avoiding that conformation, with 5 kcal mol of elimination activation energy. Within B3LYP, the addition and rotation proceeded simultaneously, with a 9 (7) kcal mol barrier for rotation involving (avoiding) eclipsed conformation of vicinal bromines. For weakly bound complexes, ZPE corrections with MP2 revealed significant artifacts when diffuse bases were included, which must be considered in the Gibbs free energy profile interpretation.
为了提出 - 加合物可能的自由基异构化途径，研究了乙炔溴化硼的实验驱动的反应机制问题。考虑了从 C=C 键的“溴端”或“硼端”的溴自由基攻击开始的加成-消除机制。使用三种全电子基和 ECP28MWB 逼近，采用了色散校正的 DFT 和 MP2 方法以及 SMD 溶剂化模型。由于在 MP2 和 DFT 水平上都有中间体稳定化，因此在“溴端”攻击的情况下，决定速率的消除步骤具有更高的活化能（12 kcal mol）。在“硼端”攻击的情况下，遵循了两种 C-C 键旋转模式，并观察到 MP2 与 DFT 势能面之间的明显差异。在 MP2 中，加成后要么通过相邻溴原子的重叠构象进行 180°旋转，要么通过避免该构象进行相反的旋转，消除活化能为 5 kcal mol。在 B3LYP 中，加成和旋转同时进行，涉及（避免）相邻溴重叠构象的旋转的势垒为 9（7）kcal mol。对于弱结合复合物，当包括弥散基时，MP2 的 ZPE 校正会产生显著的伪影，在解释吉布斯自由能曲线时必须考虑到这一点。
