Instituto de Simulación Computacional (ISC-USFQ), Universidad San Francisco de Quito, Diego de Robles y Vía Interoceánica, Quito 17-1200-841, Ecuador.
Departamento de Ingeniería Química, Grupo de Química Computacional y Teórica (QCT-USFQ), Universidad San Francisco de Quito, Diego de Robles y Vía Interoceánica, Quito 17-1200-841, Ecuador.
Molecules. 2020 Jan 25;25(3):530. doi: 10.3390/molecules25030530.
In the present work, a number of R-X⋯NH (X = Cl, Br, and I) halogen bonded systems were theoretical studied by means of DFT calculations performed at the ωB97XD/6-31+G(d,p) level of theory in order to get insights on the effect of the electron-donating or electron-withdrawing character of the different R substituent groups (R = halogen, methyl, partially fluorinated methyl, perfluoro-methyl, ethyl, vinyl, and acetyl) on the stability of the halogen bond. The results indicate that the relative stability of the halogen bond follows the Cl < Br < I trend considering the same R substituent whereas the more electron-withdrawing character of the R substituent the more stable the halogen bond. Refinement of the latter results, performed at the MP2/6-31+G(d,p) level showed that the DFT and the MP2 binding energies correlate remarkably well, suggesting that the Grimme's type dispersion-corrected functional produces reasonable structural and energetic features of halogen bond systems. DFT results were also observed to agree with more refined calculations performed at the CCSD(T) level. In a further stage, a more thorough analysis of the R-Br⋯NH complexes was performed by means of a novel electron localization/delocalization tool, defined in terms of an Information Theory, IT, based quantity obtained from the conditional pair density. For the latter, our in-house developed C++/CUDA program, called KLD (acronym of Kullback-Leibler divergence), was employed. KLD results mapped onto the one-electron density plotted at a 0.04 a.u. isovalue, showed that (i) as expected, the localized electron depletion of the Br sigma-hole is largely affected by the electron-withdrawing character of the R substituent group and (ii) the R-X bond is significantly polarized due to the presence of the NH molecule in the complexes. The afore-mentioned constitutes a clear indication of the dominant character of electrostatics on the stabilization of halogen bonds in agreement with a number of studies reported in the main literature. Finally, the cooperative effects on the [Br-CN] system ( = 1-8) was evaluated at the MP2/6-31+G(d,p) level, where it was observed that an increase of about ~14.2% on the complex stability is obtained when going from = 2 to = 8. The latter results were corroborated by the analysis of the changes on the Fermi-hole localization pattern on the halogen bond zones, which suggests an also important contribution of the electron correlation in the stabilization of these systems.
在本工作中，通过在 ωB97XD/6-31+G(d,p)理论水平上进行的 DFT 计算，对一系列 R-X⋯NH（X = Cl、Br 和 I）卤键体系进行了理论研究，以便深入了解不同 R 取代基（R = 卤素、甲基、部分氟化甲基、全氟甲基、乙基、乙烯基和乙酰基）的供电子或吸电子特性对卤键稳定性的影响。结果表明，考虑到相同的 R 取代基，卤键的相对稳定性遵循 Cl < Br < I 的趋势，而 R 取代基的吸电子特性越强，卤键越稳定。在 MP2/6-31+G(d,p)水平上对后者结果进行的细化表明，DFT 和 MP2 结合能非常好地相关，表明 Grimme 型色散校正函数产生了卤键体系合理的结构和能量特征。DFT 结果也与在 CCSD(T)水平上进行的更精细计算一致。在进一步阶段，通过一种新的基于信息论（IT）的电子定域/离域工具对 R-Br⋯NH 配合物进行了更深入的分析，该工具是根据从条件对密度获得的基于 IT 的定量来定义的。对于后者，我们使用了内部开发的 C++/CUDA 程序，称为 KLD（Kullback-Leibler 发散的缩写）。将 KLD 结果映射到在 0.04 a.u.等密度面上绘制的单电子密度上，结果表明：（i）正如预期的那样，R 取代基组的吸电子特性对 Br sigma-空穴的电子耗尽有很大影响，（ii）由于配合物中存在 NH 分子，R-X 键明显极化。上述结果清楚地表明，静电对卤键稳定的主导作用与主要文献中报道的许多研究一致。最后，在 MP2/6-31+G(d,p)水平上评估了 [Br-CN] 体系（ = 1-8）的协同效应，结果表明，当从 = 2 增加到 = 8 时，配合物稳定性增加约 14.2%。后者结果得到了卤键区域的 Fermi-hole 定域模式变化分析的证实，这表明电子相关对这些体系稳定化的重要贡献。
