School of Chemistry, Institute of Science, Suranaree University of Technology, Nakhon Ratchasima 30000, Thailand.
J Comput Chem. 2012 Jan 15;33(2):175-88. doi: 10.1002/jcc.21957. Epub 2011 Oct 18.
Dynamics and mechanism of proton transfer in a protonated hydrogen bond (H-bond) chain were studied, using the CH(3)OH(2)(+)(CH(3)OH)(n) complexes, n = 1-4, as model systems. The present investigations used B3LYP/TZVP calculations and Born-Oppenheimer MD (BOMD) simulations at 350 K to obtain characteristic H-bond structures, energetic and IR spectra of the transferring protons in the gas phase and continuum liquid. The static and dynamic results were compared with the H(3)O(+)(H(2)O)(n) and CH(3)OH(2)(+)(H(2)O)(n) complexes, n = 1-4. It was found that the H-bond chains with n = 1 and 3 represent the most active intermediate states and the CH(3)OH(2)(+)(CH(3)OH)(n) complexes possess the lowest threshold frequency of proton transfer. The IR spectra obtained from BOMD simulations revealed that the thermal energy fluctuation and dynamics help promote proton transfer in the shared-proton structure with n = 3 by lowering the vibrational energy for the interconversion between the oscillatory shuttling and structural diffusion motions, leading to a higher population of the structural diffusion motion than in the shared-proton structure with n = 1. Additional explanation on the previously proposed mechanisms was introduced, with the emphases on the energetic of the transferring proton, the fluctuation of the number of the CH(3)OH molecules in the H-bond chain, and the quasi-dynamic equilibriums between the shared-proton structure (n = 3) and the close-contact structures (n ≥ 4). The latter prohibits proton transfer reaction in the H-bond chain from being concerted, since the rate of the structural diffusion depends upon the lifetime of the shared-proton intermediate state.
质子在质子化氢键（H 键）链中的转移动力学和机制被研究，使用 CH(3)OH(2)(+)（CH(3)OH)(n) 复合物，n = 1-4，作为模型系统。本研究使用 B3LYP/TZVP 计算和 350 K 下的 Born-Oppenheimer MD（BOMD）模拟，以获得气相和连续液体中转移质子的特征 H 键结构、能量和红外光谱。静态和动态结果与 H(3)O(+)（H(2)O)(n)和 CH(3)OH(2)(+)（H(2)O)(n)复合物进行了比较，n = 1-4。结果发现，n = 1 和 3 的 H 键链代表最活跃的中间状态，并且 CH(3)OH(2)(+)（CH(3)OH)(n)复合物具有最低的质子转移阈频率。BOMD 模拟得到的红外光谱表明，热能量波动和动力学有助于通过降低在振荡穿梭和结构扩散运动之间的互变的振动能量，促进具有 n = 3 的共享质子结构中的质子转移，从而导致结构扩散运动的占据数高于具有 n = 1 的共享质子结构。引入了对以前提出的机制的附加解释，重点是转移质子的能量、H 键链中 CH(3)OH 分子数量的波动以及共享质子结构（n = 3）和紧密接触结构（n ≥ 4）之间的准动态平衡。后者阻止 H 键链中的质子转移反应是协同的，因为结构扩散的速率取决于共享质子中间状态的寿命。
