The prototype Diels–Alder (DA) reaction between butadiene and ethene (system 1) and the DA reaction involving 1-methoxy-butadiene and cyano-ethylene (system 2) are investigated with an explicit-time-dependent Density Functional Theory approach. Bond orders and atomic net charges obtained in the dynamics at the transition state geometry and along the reaction coordinate toward reactants are used to provide a picture of the process in terms of VB/Lewis resonance structures that contribute to a resonance hybrid. The entire dynamics can be divided into different domains (reactant-like, product-like, and transition state domains) where different Lewis resonance structures contribute with different weights. The relative importance of these three domains varies along the reaction coordinate. In addition to the usual reactant-like and product-like covalent Lewis structures, ionic Lewis structures have non-negligible weights. In system 2, the electron-donor OCH₃ on the diene and the electron-acceptor CN on the dienophile make more important the contributions of ionic Lewis structures that stabilize the transition state and determine the decrease of the reaction barrier with respect to system 1.

中文翻译：

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具有[4 + 2] Diels–Alder反应的显式时间密度泛函理论的电子动力学

丁二烯和乙烯之间的原型Diels–Alder（DA）反应（系统1）和涉及1-甲氧基丁二烯和氰基乙烯的DA反应（系统2））的研究采用了显式时间相关的密度泛函理论方法。在过渡态几何构型以及沿着反应坐标朝向反应物的动力学中获得的键序和原子净电荷用于提供有助于共振混合的VB / Lewis共振结构的过程图。整个动力学可分为不同的域（类似反应物，类似产物和过渡态域），其中不同的Lewis共振结构以不同的权重起作用。这三个域的相对重要性沿反应坐标变化。除了通常的类似反应物和类似产物的共价路易斯结构外，离子路易斯结构的重量也不可忽略。在系统2中，电子给体OCH二烯上的₃和亲二烯上的电子受体CN使得离子路易斯结构的贡献更加重要，该离子路易斯结构稳定了过渡态并决定了相对于系统1的反应势垒的降低。