How do hydrogen bonds break in supercooled water?: Detecting pathways not going through saddle point of two-dimensional potential of mean force

Abstract

Supercooled water exhibits remarkably slow dynamics similar to the behavior observed for various glass-forming liquids. The local order of tetrahedral structures due to hydrogen-bonds (H-bonds) increases with decreasing temperature. Thus, it is important to clarify the temperature dependence of the H-bond breakage process. This was investigated here using molecular dynamics simulations of TIP4P supercooled water. The two-dimensional (2D) potential of mean force (PMF) is presented using combinations of intermolecular distance and angle between two water molecules. The saddle point of the 2D PMF suggests the presence of the transition state that distinguishes between H-bond and non H-bond states. However, we observed pathways not going through this saddle point particularly at supercooled states, which are due to translational rather than rotational motions of the molecules. We quantified the characteristic time scales of rotational and translational H-bond breakages. The time scale of the translational H-bond breakage shows a non-Arrhenius temperature dependence comparable to that of the H-bond lifetime. This time scale is relevant for the temperature dependence of the transmission coefficient based on the transition state theory. The translational H-bond breakage is also related to cage-jumps observed in glass-forming liquids, which mostly involve spatially correlated molecules. Our findings warrant further exploration of an appropriate free-energy surface or reaction coordinates beyond the geometrical variables of the water dimer to describe a possible saddle point related to collective jump motions.

The physics of empty liquids: from patchy particles to water

“…These anomalies are linked to the structural properties of the underlying network, but differently from hard-sphere type liquids, the relaxation processes are not dominated by excluded volume effects (i.e. caging) but by bonding [27,[213][214][215]. Patchy particles are an ideal model to study network glasses because, with appropriately chosen parameters, the liquid phase can be cooled down to very low temperature without interference from phase separation (empty liquids) or crystallization (ultra-stable liquids).…”

Diffusion dynamics of supercooled water modeled with the cage-jump motion and hydrogen-bond rearrangement

“…Moreover, we have also examined the pathways of hydrogen-bond breakages on the profile of the two-dimensional potential of mean force. 41 It has been clarified that H-bonds break due to translational, rather than rotational motions of the molecules, particularly at supercooled states. Although these studies suggest the strong relationship between the H-bond dynamics and molecular diffusivity in liquid water, the connection between the microscopic change of the Hbond network and molecular displacement remains elusive.…”

Concentrated suspensions of Brownian beads in water: dynamic heterogeneities through a simple experimental technique

“…Indeed, the scenario just illustrated for χ 4 is qualitatively found in a wide variety of liquids, even including supercoleed water [20]. In addition, we notice that experimental and numerical studies have very recently highlighted the relevance of cage-jump dynamics, at the atomic scale, for supercooled water [63][64][65].…”