Variable-pressure dynamic NMR studies: effects of paramagnetic metal ions on NMR parameters in nonexchanging systems

The possible effects of paramagnetic relaxation on the apparent volumes of activation for exchange reactions in solution, as measured by NMR at high pressures, are considered. Two model paramagnetic systems that do not undergo ligand exchange on the NMR time scale were examined: tri(acetylacetonato)chromium(III) in various perdeuterated solvents, and tris(ethylenediamine)nickel(II) ion in ethylenediamine solvent. No pressure dependence was discernible up to 200 MPa for the chemical shifts of 1H (exemplifying nuclei of spin 1/2) in the Cr(III) complex, or of solvent 14N (representing quadrupolar nuclei) in the Ni(II)–ethylenediamine case. The line widths Δv1/2, however, were significantly dependent on pressure. For 1H in the Cr complex, the increase of Δv1/2 with pressure was less than expected from the theory of scalar interactions, and was small enough to imply that any contribution from this source to the observed volume of activation in exchanging systems may be neglected. For 14N in liquid ethylenediamine, the increase of Δv1/2 with pressure was significantly greater when a paramagnetic solute was present. Thus, before the observed Δv1/2 at a pressure P (in MPa) measured by the NMR of a quadrupolar nucleus can be used to obtain a chemical exchange rate for a paramagnetic solute, it should be reduced by an amount Δv1/20 exp{θ(P − 0.1)(ΔVV‡/RT + κ)}, where ΔVV‡ is the activation volume for viscous flow and κ the compressibility of the solvent, Δv1/20 is the linewidth at 0.1 MPa in the absence of chemical exchange, and θ is a scaling factor between 0 and 1. The factors Δv1/20 and θ(ΔVV‡/RT + κ) are obtainable from measurements with a chemically equivalent, nonexchanging, paramagnetic solute.