Département de Chimie, Université de Montréal, CP 6128, succursale Centre-Ville, Montréal, H3C3J7, Canada.
J Chem Phys. 2011 Mar 7;134(9):094505. doi: 10.1063/1.3554654.
We investigate the role played by the coordination state of pre-existing water wires during the dissociation of moderately strong acids by means of first-principles molecular dynamics calculations. By preparing 2,4,6-tricyanophenol (calc. pKa∼0.5) in two different initial states, we are able to observe sequential as well as concerted trajectories of dissociation: On one hand, equilibrium dissociation takes place on a ∼50 ps timescale; proton conduction occurs through three-coordinated water wires in this case, by means of sequential Grotthus hopping. On the other hand, by preparing 2,4,6-tricyanophenol in a hydration state inherited from that of equilibrated phenol (calc. pKa=7.6), the moderately strong acid finds itself in a presolvated state from which dissociation can take place on a ∼1 ps timescale. In this case, concerted dissociation trajectories are observed, which consist of proton translocation through two intervening, four-coordinated, water molecules in 0.1-1.0 ps. The present results suggest that, in general, the mechanism of proton translocation depends on how the excess proton is injected into a hydrogen bond network. In particular, if the initial conditions favour proton release to a fourfold H-bonded water molecule, proton translocation by as much as 6-8 Å can take place on a sub-picosecond timescale.
我们通过第一性原理分子动力学计算研究了在中等强度酸解离过程中预先存在的水键配位状态所起的作用。通过将 2,4,6-三氰基苯酚（计算的 pKa∼0.5）制备在两种不同的初始状态下，我们能够观察到顺序和协同解离轨迹：一方面，平衡解离在 ∼50 ps 的时间尺度上发生；在这种情况下，质子通过三配位水键通过连续的 Grotthus 跳跃进行传导。另一方面，通过将 2,4,6-三氰基苯酚制备在从平衡苯酚（计算的 pKa=7.6）继承的水合状态下，中等强度酸处于预溶剂化状态，从该状态可以在 ∼1 ps 的时间尺度上发生解离。在这种情况下，观察到协同解离轨迹，其中质子通过 0.1-1.0 ps 内的两个中间的四配位水分子进行迁移。本研究结果表明，一般来说，质子迁移的机制取决于多余质子如何注入氢键网络。特别是，如果初始条件有利于质子释放到四配位的水分子上，则质子可以在亚皮秒时间尺度上迁移高达 6-8 Å。
