A Temperature-Dependent Hydrating Water Structure Around Chlorine Anion

The hydrating water structure in NaCl aqueous solution has long been a controversial topic. We employ the Multivariate Curve Resolution method to get the solute-correlated (SC) OD/OH stretch bands from the Raman spectra of D2O/H2O-NaCl solutions at temperatures from 293 to 573 K. The largely narrowed SC bands remain a nearly constant wavenumber of the main peak with increasing temperature but get sunken around 2611 cm(-1)/3591 cm(-1) to let the similar to 2660 cm(-1)/similar to 3635 cm(-1) modes be more prominent above a pivoting temperature (T-P, approximate to 453 K/433 K for OD/OH bands). These spectral features incline to support a more flexible water structure in the Cl- hydration shell (CHS) balanced by the water-water hydrogen bond and water-Cl- bond. Most hydrating water molecules are engaged in hydrogen bonding configurations of single donor (SD) and single hydrogen-bond water (SHW). As temperature rises, the SD/SHW ratio in CHS first decreases below T-P because of the SD - SHW transition but then increases above T-P owning to the rapid increase of the contact ion pairs. Moreover, we ascribe the differences between the SC OD and OH bands primarily to the stronger D2O-D2O hydrogen bond and D2O-Cl- bond. (C) 2020 Elsevier B.V. All rights reserved.