Department of Chemistry and Biochemistry, The Ohio State University, Columbus, Ohio 43210, USA.
J Chem Phys. 2014 Mar 21;140(11):114305. doi: 10.1063/1.4868098.
Protonated hydrogen dimer, H₅⁺, is the intermediate in the astrochemically important proton transfer reaction between H₃⁺ and H2. To understand the mechanism for this process, we focus on how large amplitude motions in H₅⁺ result in scrambling of the five hydrogen atoms in the collision complex. To this end, the one-dimensional zero-point corrected potential surfaces were mapped out as functions of reaction coordinates for the H₃⁺ + H2 collision using minimized energy path diffusion Monte Carlo [C. E. Hinkle and A. B. McCoy, J. Phys. Chem. Lett. 1, 562 (2010)]. In this study, the previously developed approach was extended to allow for the investigation of selected excited states that are expected to be involved in the proton scrambling dynamics. Specifically, excited states in the shared proton motion between the two H2 groups, and in the outer H2 bending motions were investigated. Of particular interest is the minimum distance between H₃⁺ and H2 at which all five hydrogen atoms become free to exchange. In addition, this diffusion Monte Carlo-based approach was used to determine the zero-point energy E0, the dissociation energy D0, and excitation energies associated with the vibrational motions that were investigated. The evolution of the wave functions was also studied, with a focus on how the intramolecular vibrations in H₅⁺ evolve into motions of H₃⁺ or H2. In the case of the proton scrambling, we find that the relevant transition states become fully accessible at separations between H₃⁺ and H2 of approximately 2.15 Å, a distance that is accessed by the excited states of H₅⁺ with two or more quanta in the shared proton stretch. The implications of this finding on the vibrational spectroscopy of H₅⁺ are also discussed.
质子化氢二聚体 H₅⁺ 是 H₃⁺ 和 H2 之间在星际化学中重要的质子转移反应的中间体。为了理解这个过程的机制，我们专注于 H₅⁺ 中的大振幅运动如何导致碰撞复合物中五个氢原子的混合。为此，使用最小能量路径扩散蒙特卡罗方法[C. E. Hinkle 和 A. B. McCoy，J. Phys. Chem. Lett. 1，562（2010）]，将 H₃⁺ + H2 碰撞的反应坐标的一维零修正势能表面映射出来。在这项研究中，之前开发的方法得到了扩展，以允许研究预计涉及质子混合动力学的选定激发态。具体来说，研究了两个 H2 基团之间共享质子运动和外部 H2 弯曲运动中的激发态。特别感兴趣的是 H₃⁺ 和 H2 之间的最小距离，在这个距离下，所有五个氢原子都可以自由交换。此外，这种基于扩散蒙特卡罗的方法还用于确定零能 E0、离解能 D0 以及与所研究的振动运动相关的激发能。还研究了波函数的演化，重点是 H₅⁺ 中的分子内振动如何演变成 H₃⁺ 或 H2 的运动。在质子混合的情况下，我们发现，在 H₃⁺ 和 H2 之间的分离约为 2.15 Å 时，相关的过渡态变得完全可访问，H₅⁺ 的激发态具有两个或更多量子的共享质子伸展可以达到这个距离。还讨论了这一发现对 H₅⁺ 振动光谱的影响。
