Using a set of state-of-the-art quantum chemical techniques we scrutinized the characteristically different reactivity of frustrated and classical Lewis pairs towards molecular hydrogen. The mechanisms and reaction profiles computed for the H2 splitting reaction of various Lewis pairs are in good agreement with the experimentally observed feasibility of H2 activation. More importantly, the analysis of activation parameters unambiguously revealed the existence of two reaction pathways through a low-energy and a high-energy transition state. An exhaustive scrutiny of these transition states, including their stability, geometry and electronic structure, reflects that the electronic rearrangement in low-energy transition states is fundamentally different from that of high-energy transition states. Our findings reveal that the widespread consensus mechanism of H2 splitting characterizes activation processes corresponding to high-energy transition states and, accordingly, is not operative for H2-activating systems. One of the criteria of H2-activation, actually, is the availability of a low-energy transition state that represents a different H2 splitting mechanism, in which the electrostatic field generated in the cavity of Lewis pair plays a critical role: to induce a strong polarization of H2 that facilities an efficient end-on acid-H2 interaction and to stabilize the charge separated “H+–H−” moiety in the transition state.
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机译:我们使用一套最先进的量子化学技术,研究了沮丧的和经典的路易斯对对分子氢的特征不同的反应性。计算各种路易斯对的H2裂解反应的机理和反应谱与实验观察到的H2活化的可行性非常吻合。更重要的是,对活化参数的分析清楚地揭示了通过低能和高能过渡态存在两种反应途径。对这些过渡态的详尽研究,包括其稳定性,几何形状和电子结构,反映出低能过渡态的电子重排与高能过渡态的电子重排根本不同。我们的发现表明,H2分裂的广泛共识机制表征了与高能跃迁状态相对应的活化过程,因此不适用于H2活化系统。实际上,H2活化的标准之一是低能量跃迁状态的可用性,该状态代表了不同的H2分裂机理,其中路易斯对腔中产生的静电场起着至关重要的作用: H2的极化可促进酸与H2的有效末端相互作用,并在过渡态中稳定电荷分离的“ H + sup> –H - sup>”部分。
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