1] Department of Chemical Physics, Weizmann Institute of Science, Rehovot 76100, Israel [2].
Department of Chemical Physics, Weizmann Institute of Science, Rehovot 76100, Israel.
Nat Chem. 2014 Apr;6(4):332-5. doi: 10.1038/nchem.1857. Epub 2014 Feb 2.
Quantum phenomena in the translational motion of reactants, which are usually negligible at room temperature, can dominate reaction dynamics at low temperatures. In such cold conditions, even the weak centrifugal force is enough to create a potential barrier that keeps reactants separated. However, reactions may still proceed through tunnelling because, at low temperatures, wave-like properties become important. At certain de Broglie wavelengths, the colliding particles can become trapped in long-lived metastable scattering states, leading to sharp increases in the total reaction rate. Here, we show that these metastable states are responsible for a dramatic, order-of-magnitude-strong, quantum kinetic isotope effect by measuring the absolute Penning ionization reaction rates between hydrogen isotopologues and metastable helium down to 0.01 K. We demonstrate that measurements of a single isotope are insufficient to constrain ab initio calculations, making the kinetic isotope effect in the cold regime necessary to remove ambiguity among possible potential energy surfaces.
在室温下通常可以忽略不计的反应物平移运动中的量子现象，在低温下可以主导反应动力学。在如此寒冷的条件下，即使是微弱的离心力也足以产生一个势能障碍，使反应物保持分离。然而，反应仍然可以通过隧道效应进行，因为在低温下，波动性质变得很重要。在某些德布罗意波长下，碰撞粒子可能会被困在长寿命的亚稳态散射态中，从而导致总反应速率急剧增加。在这里，我们通过测量氢同位素与亚稳态氦之间的绝对彭宁电离反应速率，证明了这些亚稳态对剧烈的、数量级强的量子动力学同位素效应负责，反应速率可以低至 0.01 K。我们证明了仅测量单个同位素不足以约束从头计算，因此在冷区动力学同位素效应是消除可能势能面之间歧义的必要条件。
