水溶液中氢键寿命的定义和弛豫机理

氢键弛豫过程决定水溶液中分子的动力学行为,氢键寿命作为理论和实验结果中一个重要的参数通常用以描述溶液中的氢键动力学性质.本文采用SPC/E-P2和SPC/E-OPLS两种力场方法对二甲基亚砜(DMSO)水溶液体系进行分子动力学模拟,计算了四种不同定义下的氢键寿命,并进行了对比阐述,连续性氢键寿命Tc和基于动力学平衡方法下的氢键寿命TR较短,它们忽略了不成功的氢键交换过程,稳定态模型下的氢键寿命TPR能够真实体现氧键转换过程,可作为标度衡量其它量值.间歇性氢键寿命T相对最长,重复统计了氢键成功交换后的恢复概率.随着二甲基亚砜浓度增大,TC、TI、TR和TPR不断增大,分子平动扩散系数在中等浓度达到极小,说明氢键寿命与分子活动性无决定性关联,同类型氢键在不同浓度下的寿命差异表明氢键寿命具有分子环境依赖性;水分子和二甲基亚砜分子氢键个数降低,氢键受激角度扭曲和拉伸概率下降,TC和TR不断接近,氢键交换受体密度降低使氢键交换速率下降,TPR不断增大,氢键寿命与其周围氢键密度密切相关.TI与TPR的比值体现了氢键交换的局域性,其变化趋势与分子平动活动性趋势相同.两种力场下氢键寿命的差异也说明氢键的寿命具有明显的理论模型依赖性.

Hydrogen Bond Lifetime Definitions and the Relaxation Mechanism in Water Solutions

The molecular dynamics behaviors in water solutions are determined by the hydrogen bond (H-bond) relaxations.The H-bond lifetime,as an important experimental and theoretical parameter,is often used to explore the general kinetics of H-bond dynamics.In this work,four different H-bond lifetimes were defined and calculated in dimethyl sulfoxide (DMSO)-water mixtures with two widely-used combined force fields,SPC/E-P2 and SPC/E-OPLS.The continuous and kinetic based H-bond lifetimes,Tc and TR,are always shorter than the TPR of stable states due to neglecting of unsuccessful H-bond exchanges.The intermittent H-bond lifetime TI was found to be the longest because of a recount of the reforming events after the successful switching event.The H-bond lifetimes,TC,TI,TR,and TPR increase with the mole fraction of DMSO (XD).This trend is not consistent with that of the molecular diffuse constants.This shows that the molecular mobility is not a decisive factor to the H-bond lifetime.The environment-dependent H-bond lifetimes suggest that the stronger H-bonds should not always remain longer time.The H-bond coordination numbers of water and DMSO decrease with XD.The distortion and elongation probability of the H-bond that was induced by surrounding molecules decreases and,therefore,so the TC and TR approach each other at the limiting concentrations in this work.The facts above show that the labeled H-bond lifetime is closely related to the H-bond density around it.One H-bond switching event only takes place on one new available acceptor there.The localized character of H-bond relaxation is consistent with the trend of the molecular mobility trend.The H-bond lifetimes also rely on the theoretical model used in the simulations.