Skara Gabriella, De Vleeschouwer Freija, Geerlings Paul, De Proft Frank, Pinter Balazs
Quantum Chemistry Group, Member of the QCMM VUB-UGent Alliance Research Group, Vrije Universiteit Brussel (VUB), Pleinlaan 2, B-1050, Brussels, Belgium.
Sci Rep. 2017 Nov 22;7(1):16024. doi: 10.1038/s41598-017-16244-1.
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 H splitting reaction of various Lewis pairs are in good agreement with the experimentally observed feasibility of H 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 H splitting characterizes activation processes corresponding to high-energy transition states and, accordingly, is not operative for H-activating systems. One of the criteria of H-activation, actually, is the availability of a low-energy transition state that represents a different H splitting mechanism, in which the electrostatic field generated in the cavity of Lewis pair plays a critical role: to induce a strong polarization of H that facilities an efficient end-on acid-H interaction and to stabilize the charge separated "H-H" moiety in the transition state.
我们运用一系列最先进的量子化学技术，仔细研究了受阻路易斯酸碱对和经典路易斯酸碱对与分子氢反应时显著不同的反应活性。为各种路易斯酸碱对的氢裂解反应计算出的反应机理和反应剖面图，与实验观察到的氢活化可行性高度吻合。更重要的是，对活化参数的分析明确揭示了存在两条通过低能量和高能量过渡态的反应途径。对这些过渡态进行详尽的研究，包括它们的稳定性、几何结构和电子结构，结果表明低能量过渡态中的电子重排与高能量过渡态的电子重排存在根本差异。我们的研究结果表明，普遍认可的氢裂解机理表征了对应于高能量过渡态的活化过程，因此，对于氢活化体系并不适用。实际上，氢活化的标准之一是存在一个低能量过渡态，它代表了一种不同的氢裂解机理，其中路易斯酸碱对空腔中产生的静电场起着关键作用：诱导氢的强烈极化，促进有效的端基酸-氢相互作用，并在过渡态中稳定电荷分离的“H-H”部分。
