Although increase of Zn 2+ number or decrease of Zn 2+ …F -distance or hydrolyzed H + increase the shield of 19 F nucleus, these cations should be still in the outer shell of F -. As previously reported, the pH value is 4.9 at 2.7 mol•L -1 ZnSO4 solution due to the hydrolysis of Zn 2+ and this value increase with the Zn 2+ concentration decreasing . This suggest that the hydrolyzed H + concentration is very low and consequently predominantly form H + -F -SSIPs (with single solvent layer existing in the space between the ion partners of the pair) if F -exists in the solution. When the NaF is gradually added to the ZnF2 solutions and the ZnF2 concentration is kept at 0.03 and 0.05 mol•kg -1 , respectively, the chemical shifts (A) gradually shift downfield (Fig. ). This is because the addition of NaF introduces excess solvated F -and decreases the average number of Zn 2+ around F -, resulting in the deshielding effect. In contrast, the second signal at ~-130 ppm (with weak satellite peaks, reading SI for the detailed analysis) displays a slight shift upfield and development of peak intensity upon the increase of F:Zn. Therefore, it is supposed that the formation of Zn 2+ -F -2SSIPs and SSIPs is assigned to the first signal, whereas the second signal results from the formation of Zn 2+ -F -contact ion pairs (CIP). As expected, the single 19 F NMR signal at ~-120 ppm for initial pure 0.005 mol•kg -1 ZnF2 solution gradually shifts downfield with F:Zn increasing by addition of the NaF. After F:Zn > 8, the second signal at ~-130 ppm appears and its strength gradually develops which is indicative of Zn 2+ -F -CIPs formation (Fig. ).