In the present work, a number of R–X⋯NH3 (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⋯NH3 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 NH3 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]n system (n = 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 n = 2 to n = 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.

中文翻译：

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R–X⋯NH3 卤素键配合物的理论描述：R 基团对配合物稳定性和 Sigma-Hole 电子耗尽的影响

在目前的工作中，通过在 ωB97XD/6-31+G(d,p) 水平上进行的 DFT 计算，对许多 R–X⋯NH3 (X = Cl、Br 和 I) 卤素键合系统进行了理论研究为了深入了解不同 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）的给电子或吸电子特性对卤素键的稳定性。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 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) 水平上进行的后一个结果的细化表明 DFT 和 MP2 结合能非常相关，表明 Grimme 型色散校正泛函产生了卤素键系统的合理结构和能量特征。还观察到 DFT 结果与在 CCSD(T) 级别执行的更精确的计算一致。在进一步的阶段，通过新的电子定位/离域工具对 R-Br⋯NH3 复合物进行了更彻底的分析，该工具根据信息理论 IT 定义，基于从条件对密度获得的数量。对于后者，采用了我们内部开发的 C++/CUDA 程序，称为 KLD（Kullback-Leibler 分歧的首字母缩写词）。KLD 结果映射到以 0.04 au 等值绘制的单电子密度上，表明 (i) 正如预期的那样，Br sigma-hole 的局部电子耗尽在很大程度上受 R 取代基的吸电子特性的影响，并且 (ii) 由于配合物中 NH3 分子的存在，R-X 键显着极化。上述内容清楚地表明了静电对卤素键稳定的主要特征，这与主要文献中报道的许多研究一致。最后，在 MP2/6-31+G(d,p) 水平上评估了对 [Br-CN]n 系统（n = 1-8）的协同效应，观察到增加了约 14.2当从 n = 2 到 n = 8 时，获得了复杂稳定性的百分比。 后者的结果通过对卤素键区费米空穴局域化模式变化的分析得到证实，