Direct NMR studies of ionic solvation

Abstract

The nuclear magnetic resonance of quadrupolar ions provide a wealth of information about solvation thermodynamics and dynamics. The talk will deal with main topics: 1. Use of cobalt(III) complexes. Cobalt-59 ( I = 7 2 ) has an enormous range of chemical shifts, due to low-lying excited states. The types of ion-solvent interactions to be monitored can be tailored at will simply by changing the nature of the ligands coordinated to the central Co 3+ cation and/or changing the counter ions. For instance, the Co(CN) 3− 6 ion measures the polarity of dipolar solvents, and will rank protic solvents according to their hydrogen-bond donor ability. Conversely, an ion such as Co(NH 3) 3+ 6 will interact with solvents depending upon their strength as hydrogen-bond acceptors. Changes in net ionic charge can also be expoited to reveal the details of the interactions between these cobalt complexes and other solute and solvent molecules. 2. Investigation of sodium cation solvats. Sodium-23 ( I = 3 2 ) nmr has good enough sensitivity that it can be applied to rather dilute (c M/m M) solutions of sodium salts. The observed changes in both the 23Na chemical shifts and linewidths, in binary solvent mixtures, are successfully interpreted on the basis of competition for tetracoordination of the sodium cation by organic solvents such as THF, glymes, pyridine and other unidentate or polydentate amins. We have applied, to account jointly for the chemical shift and the relaxation rate data, the Hill formalism: mathematically, preferential solvation of a tetracoordinated ion, and binding of a ligand to one of four equivalent sites in a biomelecule, constitute one and the same problem. The powerful Hill formalism allows the determination of the magnitude of the chelate effect present for certain bidentate and polydentate ligands. It also leads to fairly accurate measurement of microscopic equilibrium constants for the successive steps in the replacement, in the sodium coordination sphere, of a solvent A by another solvent B.