Institute of Quantum Beam Science, Graduate School of Science and Engineering, Ibaraki University, Hitachi 316-8511 Ibaraki, Japan.
Frontier Research Center for Applied Atomic Sciences, Ibaraki University, 162-1 Shirakata, Tokai, 319-1106 Ibaraki, Japan.
J Phys Chem B. 2022 Jun 23;126(24):4565-4571. doi: 10.1021/acs.jpcb.2c01971. Epub 2022 Jun 11.
Autodissociation in liquid water is one of the most important processes in various topics of physical chemistry, such as acid-base chemistry. Molecular simulations have elucidated most of the molecular mechanisms at the atomic level, yet quantitative analysis to compare with experiments using the potential of mean force (PMF) remains a hurdle, including the definition of reaction coordinates and the accuracy of liquid structures by molecular dynamics (AIMD) simulations with density functional theory (DFT) methods. Here, we perform AIMD simulations with the revPBE-D3 exchange-correlation functional to compute the PMF profiles of autoionization, or proton transfer (PT), in liquid water. For the quantitative analysis with physically meaningful reaction coordinates, we employ a PT coordinate, donor-acceptor (OH-HO) distance, and hydrogen (H)-bond number. The one-dimensional (1D) PMF profile along the PT coordinate shows no local minimum in the product state of PT (OH and HO), which is necessary to accurately compute the acid dissociation constant (or p). On the other hand, the 2D PMF profiles along the PT coordinate and donor-acceptor distance show local minima in the product state and reaction barriers, and the computed p is comparable to the experiment. In addition, the 2D PMF profiles along the PT coordinate and the H-bond number reveal the molecular mechanism of the H-bond rearrangement concomitant with PT, in which the H-bond breaking before PT is slightly preferable. These findings indicate that an accurate evaluation of p by MD simulations requires the donor-acceptor distance in addition to the conventional PT coordinate.
在各种物理化学主题中，如水的质子自解离是最重要的过程之一，如酸碱化学。分子模拟已经阐明了原子水平上的大多数分子机制，但使用平均力势（PMF）进行与实验的定量分析仍然存在障碍，包括反应坐标的定义和分子动力学（AIMD）模拟与密度泛函理论（DFT）方法的液体结构的准确性。在这里，我们使用 revPBE-D3 交换相关泛函进行 AIMD 模拟，以计算质子转移（PT）在液态水中的自离解或质子转移（PT）的 PMF 分布。为了进行具有物理意义的反应坐标的定量分析，我们采用了 PT 坐标、供体-受体（OH-HO）距离和氢键数。沿着 PT 坐标的一维（1D）PMF 分布在 PT（OH 和 HO）产物状态下没有局部最小值，这是准确计算酸离解常数（或 p）所必需的。另一方面，沿着 PT 坐标和供体-受体距离的二维 PMF 分布在产物状态和反应势垒中显示出局部最小值，计算出的 p 值与实验值相当。此外，沿着 PT 坐标和氢键数的二维 PMF 分布揭示了与 PT 伴随的氢键重排的分子机制，其中在 PT 之前的氢键断裂稍微优先。这些发现表明，MD 模拟准确评估 p 需要除传统的 PT 坐标外还需要供体-受体距离。
