Temperature dependence of proton chemical shifts in aqueous silver salts. The association shift of water

Abstract

¹ H N.m.r. chemical shifts are reported for aqueous AgNO₃ and AgBF₄ between 15 and 100°. The salts have a very small influence on the chemical shift of water and the cationic hydration number is apparently less than unity. The small shift is attributed to almost exact cancellation of upfield anionic and downfield cationic contributions. The calculated cationic hydration water shift is unusually large and cannot be accounted for by the ionic electric field alone. Since the hydration complex cannot have spherical symmetry it is proposed that the dipole moment induced in the hydration water by the ionic field cannot be ignored and acts as an anti-screening mechanism which increases the hydration water chemical shift. The introduction of this anti-screening effect into calculations of the association shift of water results in close agreement between calculation and experiment. The results appear to support bond bending and stretching rather than bond breaking as affecting the temperature dependence of the association shift.