Importance of nuclear quantum effects on the hydration of chloride ion

Path-integral ab initio molecular dynamics (PI-AIMD) calculations have been employed to probe the nature of chloride ion solvation in aqueous solution. Nuclear quantum effects (NQEs) are shown to weaken hydrogen bonding between the chloride anion and the solvation shell of water molecules. As a consequence, the disruptive effect of the anion on the solvent water structure is significantly reduced compared to what is found in the absence of NQEs. The chloride hydration structure obtained from PI-AIMD agrees well with information extracted from neutron scattering data. In particular, the observed satellite peak in the hydrogen-chloride-hydrogen triple angular distribution serves as a clear signature of NQEs. The present results suggest that NQEs are likely to play a crucial role in determining the structure of saline solutions.

Figure 1

In PI-AIMD, the distribution of distances from the intramolecular oxygen to the MLWF centers for bonding pair electrons; vertical lines denote average positions of two types of protons bonded to solvent water or to ${\mathrm{Cl}}^{-}$, as shown in the inset together with density isosurfaces of MLWF for bonding pair electrons.

Figure 2

Spatial distributions of proton transfer coordinate $\nu$ for water molecules in ${\mathrm{Cl}}^{-}$(aq) within (a) the solvent (b) anion's first hydration shell from AIMD and PI-AIMD.

Figure 3

Cl-O RDFs in ${\mathrm{Cl}}^{-}$(aq) from (a) AIMD and (b) PI-AIMD simulations, with neutron diffraction result [12]. (c) Isosurface of probability of finding a ${\mathrm{Cl}}^{-}$ ion first neighbor of a water molecule in AIMD and PI-AIMD simulations. The contrast level of the isosurface is set to 0.70. (d) Probability distribution of H-Cl-H angles in the first hydration shell of ${\mathrm{Cl}}^{-}$ from AIMD, PI-AIMD simulation, and neutron diffraction experiment [12]. The inset shows polarized ${\mathrm{Cl}}^{-}$ hydration structures, where water molecules in the first hydration shell are classified by bonded (orange) and nonbonded (yellow) to ${\mathrm{Cl}}^{-}$.

Figure 4

(a) Normalized tetrahedral structure order parameter $\overline{Q}$ decay as a function of the distance to ${\mathrm{Cl}}^{-}$ from AIMD and PI-AIMD. For water molecule $i$, $Q_i=\frac{3}{32}{\sum }_{j=1}^3{\sum }_{k=j+1}^4{(\cos {\psi }_{j,k}+\frac{1}{3})}^2$, where $j$ and $k$ are the $j\mathrm{th}$ and $k\mathrm{th}$ nearest neighbor of water molecule $i$, and ${\psi }_{j,k}$ is the angle between molecule $i,j$ and $i,k$. $\overline{Q}$ defined by ${\langle Q_i\rangle }_{\text{solution}}/{\langle Q_i\rangle }_{\text{water}}$ is the averaged and normalized $Q$. (b) AIMD and PI-AIMD O-O RDFs within the first ${\mathrm{Cl}}^{-}$ hydration shell computed by water bonded to ${\mathrm{Cl}}^{-}$, along with regular O-O RDFs of bulk water. O-O RDFs of (c) AIMD and (d) PI-AIMD ${\mathrm{Cl}}^{-}$(aq) simulations, with AIMD and PI-AIMD pure water simulations, and diffraction experiment [51].