Importance of Solvents' Translational-Rotational Coupling for Translational Jump of a Small Hydrophobic Solute in Supercooled Water.

Despite clear evidence of sudden translational jump occurrence of a solute in supercooled water, a detailed mechanism of this jump is still lacking. A previous work [ Indra , S. ; Daschakraborty , S. Chem. Phys. Lett. 2017 , 685 , 322 - 327 ] put forward a mechanism of this jump from an initial solvent cage to a final one. The proposed mechanism is astoundingly similar to that of the electron/proton transfer reaction in aqueous solution. The above study identified the spatial prearrangement (rearrangement before the jump occurrence) of cage forming water solvent molecules as the actual reaction coordinate. However, the study completely ignored the contribution of the orientational prearrangement of solvent water molecules. In this study, we have monitored both the spatial and the orientational prearrangements of water solvent molecules at subzero temperatures during the jump occurrence of the solute. We have found overwhelming contributions of both the spatial and orientational prearrangements of water, which symmetrize the hydration structure at the initial and final cage positions to facilitate the jump event. Through a systematic temperature dependence study (from T = 240 to 270 K), we have found clear evidence that a strong synchronization between translational and rotational prearrangements of the solvent water molecules is crucial for the solute's jump from one solvent cage to another in supercooled water (below T = 252 K). The above translation-rotation synchronization is probably due to the cooperative movement of solvent water molecules forming clusters in the supercooled region. Since these cooperative dynamics are the consequence of the spatiotemporal heterogeneity in the medium, we infer that the large-amplitude translational jump of the nonpolar solute probably stems from the spatiotemporal heterogeneity of supercooled water. At temperatures above the melting point, this cooperativity is partly lost since the translational and orientational prearrangements become somewhat independent of each other.