正丙醇水溶液准弹性中子散射光谱的分子动力学模拟.pdf

物理化学学报( Wuli Huaxue Xuebao)
1037
ay
Acta Phys.- Chin. Sin.2012,28(5),1037-1044
Articlel
doi:10.3866/PKU.WHXB201203072
www.whxb.pku.edu.cn
正丙醇水溶液准弹性中子散射光谱的分子动力学模拟
张霞1张强赵东霞
(渤海大学化学化工与食品安全学院,辽宁锦州121000:2辽宁师范大学化学化工学院,辽宁大连11029)
摘要:准弹性中子散射(QENS)光谱是获取溶液分子动力学性质的重要方法,但光谱解析模型的有效性和去
耦合近似的合理性仍存在争议.本文利用分子动力学模拟方法获取纯水和正内醇水溶液中羟基氢原子的自相
关中间散射函数Fs(Q,)和去耦合近似函数F(Q,0,以及相关性质来评价它们的合理性.结果表明,在低动量传
移范围内平-转去耦令近似相对合理,水分子的平-转耦合贡献较小,混合溶液中水分子的γ-转耦合项和转动项
随动量转移Q值增大而增大,二者显现相互抵消趋势.对于泥合溶液中的正内醇羟基氢原子,由于Fs(Q,)和质
心自相关中间散射函数Fo(Q.t偏差较大,利用实验光谱直接拟合分子平动扩散系数是不合适的.三种平动模
型获取的纯水和正丙醇水溶液分子平动扩散系数与实验结果一致,略高于 Einstein均方位移方法所得结果.水
分子在纯水和混合溶液中表现为跳跃转动,而不是连续转动.正丙醇分子存在转动各向异性,羟基氢原子沿羟
基向量为跳跃转动,沿相对质心向量可近似为连续转动.模拟结果显示,高动量转移范围平-转耦合顼贡献较
大,直接拟合实验光谱获取分子转动扩散系数或弛豫时间是不合适的.鉴于低动量转移范围内转动和平转耦合
贡献较小,以及二者的抵消作用,在此范围内获取水分子平动信息是现实可行的.
关键词:准弹性中子散射光谱;分子动力学模拟;非相干结构因子;自相关中间散射函数;跳跃扩散
中图分类号:O643
Quasi-elastic Neutron Scattering Spectroscopy of the
1-propanol/Water Solution by Molecular Dynamics Simulations
ZHANG Xia ZHANG Qiang
ZHAO Dong-xia
('institute of Chemistry, Chemical Engineering and Food Safety, Bohai University, Jinzhou 121000, Liaoning Province, P. R. China
Institute of Chemistry and Chemical Engineering, Liaoning Normal University, Dalian 116029, Liaoning Province, P R. China)
Abstract: Quasi-elastic neutron scattering(QENS) spectroscopy as an important tool can be used to
extract the molecular dynamic properties. However, the validity of the dynamical models and the
decoupling approximation used in QENS spectral analysis is a topic of ongoing debate. In this paper, th
self-intermediate scattering function F(, and the decoupling appromation function F(Q, t)of the
hydroxyl hydrogen in pure water and in 1-propanol/water mixture, and certain dynamic properties predicted
by three translation models, are derived from molecular dynamics simulations to assess their reasonability
The results suggest that the decoupling approximations for the water hydrogen in pure water and in mixture
are reasonable at low momentum transfer Q. The contribution from the translation-rotation coupling term
small for the pure water. The coupling effect is strengthened for the water hydrogen when 1-propanol is
added to the water. Under these conditions, the coupling and rotation terms both increase with the
momentum transfer Q and largely cancel each other. For the hydroxyl hydrogen of 1-propanol in the
mixture, the translational diffusion constant cannot be directly derived from the experimental spectrum, due
to large deviation between Fs(Q, and the center-of-mass translational function FM(Q, t). The translational
diffusion constants by the three translation models used in our current work are consistent with
Received: December 20. 20] 1: Revised March 4. 2012: Published on Web: March 7. 2012
Correspondingauthor.Email:zhangqiang@bhu.edu.cn;Tel:+86-416-3400698
The project was supported by the National Natural Science Foundation of China(20873055, 21176029)
国家自然科学基金(20873055,2116029)资功项目
C, Editorial office of Acta Physico-chimica Sinica