Roberts Sean T, Petersen Poul B, Ramasesha Krupa, Tokmakoff Andrei, Ufimtsev Ivan S, Martinez Todd J
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
Proc Natl Acad Sci U S A. 2009 Sep 8;106(36):15154-9. doi: 10.1073/pnas.0901571106. Epub 2009 Jul 31.
It is generally accepted that the anomalous diffusion of the aqueous hydroxide ion results from its ability to accept a proton from a neighboring water molecule; yet, many questions exist concerning the mechanism for this process. What is the solvation structure of the hydroxide ion? In what way do water hydrogen bond dynamics influence the transfer of a proton to the ion? We present the results of femtosecond pump-probe and 2D infrared experiments that probe the O-H stretching vibration of a solution of dilute HOD dissolved in NaOD/D(2)O. Upon the addition of NaOD, measured pump-probe transients and 2D IR spectra show a new feature that decays with a 110-fs time scale. The calculation of 2D IR spectra from an empirical valence bond molecular dynamics simulation of a single NaOH molecule in a bath of H(2)O indicates that this fast feature is due to an overtone transition of Zundel-like H(3)O(2)(-) states, wherein a proton is significantly shared between a water molecule and the hydroxide ion. Given the frequency of vibration of shared protons, the observations indicate the shared proton state persists for 2-3 vibrational periods before the proton localizes on a hydroxide. Calculations based on the EVB-MD model argue that the collective electric field in the proton transfer direction is the appropriate coordinate to describe the creation and relaxation of these Zundel-like transition states.
一般认为，氢氧根离子的反常扩散源于其从相邻水分子接受质子的能力；然而，关于这一过程的机制仍存在许多问题。氢氧根离子的溶剂化结构是怎样的？水的氢键动力学以何种方式影响质子向离子的转移？我们展示了飞秒泵浦 - 探测和二维红外实验的结果，这些实验探测了溶解在NaOD/D₂O中的稀HOD溶液的O - H伸缩振动。加入NaOD后，测量的泵浦 - 探测瞬态和二维红外光谱显示出一个以110飞秒时间尺度衰减的新特征。从H₂O浴中单个NaOH分子的经验价键分子动力学模拟计算二维红外光谱表明，这种快速特征归因于类似尊德耳（Zundel）的H₃O₂⁻态的泛频跃迁，其中质子在水分子和氢氧根离子之间显著共享。考虑到共享质子的振动频率，这些观察结果表明共享质子态在质子定位于氢氧根之前持续2 - 3个振动周期。基于经验价键 - 分子动力学（EVB - MD）模型的计算表明，质子转移方向上的集体电场是描述这些类似尊德耳跃迁态的产生和弛豫的合适坐标。
