Out of several pericyclic reactions, Diels-Alder (DA) reaction is one of the most widely used synthetic processes. In the present work, several models and methodologies have been used to determine and to analyze the plausible mechanism of some representative DA cycloaddition reactions. A comparison between the dual descriptor and the bond reactivity indices corresponding to the natural bond orbital of the reagents is included, which provides a complete description of the plausible reaction mechanism. In the next step, two very recent models are used to determine the local electronic density transfer and redistribution between the reactants involved. The description of the local electronic density transfer has been made in two stages; first, the variation in the net charge on the atoms is obtained, and then, the electronic density transfer between the natural bond orbitals is calculated. The values obtained using the two models are correlated with the experimental rate constants of the reactions. Finally, the natural bond orbitals are obtained at several steps along the reaction path and the variation in their partial occupation is compared with the corresponding electron density transfer among these orbitals. Furthermore, frontier molecular orbital (FMO) approach has been employed to understand the more feasible way of interaction between the DA pair. Relative electrophilicity descriptors like net electrophilicity ( ∆ω ± ), net reactivity index (NRI, Δ ω R ± $$ {\omega}_R^{\pm } $$ ), and electrophilicity difference ( ∆ω ) between DA pairs have also been employed to describe the studied reaction mechanisms especially whether they follow non-polar-concerted/polar-stepwise pathway along with their classification in terms of normal or inverse electron demand. Furthermore, adaptive natural density partitioning method (AdNDP) and energy decomposition analyses (EDA) in conjunction with natural orbital for chemical valence (NOCV) have been made use of in order to analyze the actual bonding situation in the transition state (TS).

中文翻译：

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一些周环反应的合理机制的概念 DFT 分析

在几个周环反应中，Diels-Alder (DA) 反应是应用最广泛的合成过程之一。在目前的工作中，已经使用了几种模型和方法来确定和分析一些有代表性的 DA 环加成反应的合理机制。包括双描述符和对应于试剂自然键轨道的键反应性指数之间的比较，它提供了对合理反应机制的完整描述。在下一步中，使用两个非常新的模型来确定所涉及的反应物之间的局部电子密度转移和重新分布。局部电子密度转移的描述分两个阶段进行；首先，获得原子上净电荷的变化，然后，计算自然键轨道之间的电子密度转移。使用两种模型获得的值与反应的实验速率常数相关。最后，沿着反应路径的几个步骤获得自然键轨道，并将它们部分占据的变化与这些轨道之间相应的电子密度转移进行比较。此外，前沿分子轨道 (FMO) 方法已被用于了解 DA 对之间更可行的相互作用方式。相对亲电性描述符，如净亲电性 ( Δω ± )，净反应性指数 (NRI, Δ ω R ± $$ {\omega}_R^{\pm } $$ )，DA 对之间的亲电性差异 ( Δω ) 也被用来描述所研究的反应机制，特别是它们是否遵循非极性协调/极性逐步路径以及它们在正常或反向电子需求方面的分类。此外，自适应自然密度分配方法（AdNDP）和能量分解分析（EDA）结合化学价的自然轨道（NOCV）已被用于分析过渡态（TS）中的实际键合情况。