关键词: 氢键寿命, 分子动力学模拟, 氢键交换, 相关函数, 分子力场

Abstract: 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, τ_C and τ_R, are always shorter than the τ_PR of stable states due to neglecting of unsuccessful H-bond exchanges. The intermittent H-bond lifetime τ_I was found to be the longest because of a recount of the reforming events after the successful switching event. The H-bond lifetimes, τ_C, τ_I, τ_R, and τ_PR increase with the mole fraction of DMSO (x_D). 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 x_D. The distortion and elongation probability of the H-bond that was induced by surrounding molecules decreases and, therefore, so the τ_C and τ_R 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.

Key words: Hydrogen bond lifetime, Molecular dynamics simulation, Hydrogen bond exchange, Correlation function, Force field