Tomut Alex-Cristian, Moraru Ionut-Tudor, Nemes Gabriela
Faculty of Chemistry and Chemical Engineering, Department of Chemistry, Babeș-Bolyai University, 1 M. Kogalniceanu Street, 400084 Cluj-Napoca, Romania.
Molecules. 2024 Oct 16;29(20):4902. doi: 10.3390/molecules29204902.
The current study investigates the influence of several R substituents (e.g., Me, SiH, F, Cl, Br, OH, NH, etc.) on the aromaticity of borazine, also known as the "inorganic benzene". By performing hybrid DFT methods, blended with several computational techniques, e.g., Natural Bond Orbital (NBO), Quantum Theory of Atoms in Molecules (QTAIM), Gauge-Including Magnetically Induced Current (GIMIC), Nucleus-Independent Chemical Shift (NICS), and following a simultaneous evaluation of four different aromaticity indices (para-delocalization index (PDI), multi-centre bond order (MCBO), ring current strength (RCS), and NICS parameters), it is emphasized that the aromatic character of B-substituted (BRNH) and N-substituted (BHNR) borazine derivatives can be tailored by modulating the electronic effects of R groups. It is also highlighted that the position of R substituents on the ring structure is crucial in tuning the aromaticity. Systematic comparisons of calculated aromaticity index values (i.e., via regression analyses and correlation matrices) ensure that the reported trends in aromaticity variation are accurately described, while the influence of different R groups on electron delocalization and related aromaticity phenomena is quantitatively assessed based on NBO analyses. The most relevant interactions impacting the aromatic character of investigated systems are (i) the electron conjugations occurring between the p lone pair electrons (LP) on the F, Cl, Br, O or N atoms, of R groups, and the π*(B=N) orbitals on the borazine ring (i.e., LP(R)→π*(B=N) donations), and (ii) the steric-exchange (Pauli) interactions between the same LP and the π(B=N) bonds (i.e., LP(R)↔π(B=N) repulsions), while inductive/field effects influence the aromaticity of the investigated trisubstituted borazine systems to a much lesser extent. This work highlights that although the aromatic character of borazine can be enhanced by grafting electron-donor substituents (F, OH, NH, O, NH) on the N atoms, the stabilization due to aromaticity has only a moderate impact on these systems. By replacing the H substituents on the B atoms with similar R groups, the aromatic character of borazine is decreased due to strong exocyclic LP(R)→π*(B=N) donations affecting the delocalization of π-electrons on the borazine ring.
本研究考察了几种R取代基（如甲基、硅氢基、氟、氯、溴、羟基、氨基等）对硼氮六环（又称“无机苯”）芳香性的影响。通过采用混合密度泛函理论方法，并结合多种计算技术，如自然键轨道（NBO）、分子中原子的量子理论（QTAIM）、含规范的磁诱导电流（GIMIC）、核独立化学位移（NICS），并同时评估四种不同的芳香性指标（对位离域指数（PDI）、多中心键级（MCBO）、环电流强度（RCS）和NICS参数），强调了通过调节R基团的电子效应可以定制B取代（BRNH）和N取代（BHNR）硼氮六环衍生物的芳香特性。还强调了R取代基在环结构上的位置对于调节芳香性至关重要。通过对计算得到的芳香性指标值进行系统比较（即通过回归分析和相关矩阵），确保准确描述了所报道的芳香性变化趋势，同时基于NBO分析定量评估了不同R基团对电子离域和相关芳香性现象的影响。影响所研究体系芳香特性的最相关相互作用包括：（i）R基团中F、Cl、Br、O或N原子上的p孤对电子（LP）与硼氮六环环上的π*(B=N)轨道之间发生的电子共轭作用（即LP(R)→π*(B=N)给予作用），以及（ii）同一LP与π(B=N)键之间的空间交换（泡利）相互作用（即LP(R)↔π(B=N)排斥作用），而诱导/场效应对所研究的三取代硼氮六环体系芳香性的影响程度要小得多。这项工作强调，虽然通过在N原子上接枝给电子取代基（F、OH、NH、O、NH）可以增强硼氮六环的芳香特性，但芳香性导致的稳定性对这些体系的影响仅为中等程度。通过用类似的R基团取代B原子上的H取代基，硼氮六环的芳香特性会降低，这是因为强烈的环外LP(R)→π*(B=N)给予作用影响了硼氮六环环上π电子的离域。
