Hydrogen bond lifetime definitions and the relaxation mechanism in water solutions

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, 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. © Editorial office of Acta Physico·Chimica Sinica.