正丙醇水溶液准弹性中子散射光谱的分子动力学模拟

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丙醇水溶液中羟基氢原子的自相关中间散射函数FS(Q,t)和去耦合近似函数FP(Q,t),以及相关性质来评价它们的合理性.结果表明,在低动量转移范围内平-转去耦合近似相对合理,水分子的平-转耦合贡献较小,混合溶液中水分子的平-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.对于混合溶液中的正丙醇羟基氢原子,由于FS(Q,t)和质心自相关中间散射函数FCM(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-转耦合项贡献较大,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.

Quasi-Elastic Neutron Scattering Spectroscopy of the 1-Propanol/Water Solution by Molecular Dynamics Simulations

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,the self-intermediate scattering function F S(Q,t)and the decoupling approximation function F P(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 is 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 F S(Q,t)and the center-of-mass translational function F CM(Q,t).The translational diffusion constants by the three translation models used in our current work are consistent with experimental results and a little higher than those predicted by the Einstein method.The jump rotation,as opposed to continuous rotation,is observed for the water molecule in both bulk water and mixture.For the 1-propanol molecule,rotations are anisotropic,being continuous along the axis from the hydroxyl hydrogen to the center-of-mass,and jumping along the hydroxyl bond vector.Simulations indicate that neither the rotational diffusion constant nor the relaxation time at high momentum transfer Q are adequately determined by the decoupling models,since the coupling effects become significant.Within the low momentum transfer range,the translation properties can be reasonably derived,due to the negligible contributions from the rotation and the coupling terms,as well as the canceling effect between them.