Insignificance of second coordination sphere interactions in cobalt-59 nuclear magnetic resonance relaxation

Abstract

Nuclear magnetic resonance relaxation time measurements are reported which permit evaluation of the electrostatic contribution to field gradients from ions external to the first coordination sphere of the observed nucleus. It is concluded that for the tris(ethylenediamine)cobalt(III) ion the presence of phosphate ion in the second coordination sphere makes little if any contribution to the electric field gradient at the cobalt nucleus.Considerable attention has been focused on the origins of NMR relaxation in aqueous electrolyte solutions.2 For many nuclei the nuclear electric quadrupole interaction dominates; but definitive evidence that any of the theoretical approaches to this problem is correct is lacking. A theory developed by Hertz focuses on electrostatic contributions to the electric field gradient at the observed nucleus and is able to obtain approximate agreement with experimental observations.3,4 In spite of this result some discussions of NMR data for quadrupole relaxed nuclei have ignored electrostatic contributions to relaxation originating external to the first sphere of interaction and focused only on that part which results from molecules or ions in the first coordination sphere of the observed ion. The present experiment is directed at assessing the validity of this approach.

Influence of solvent and counterion on sodium-23(1+) spin relaxation in aqueous solution

“…This is supported by measurements of the 59C03+ and I3C relaxation times in the C0(en)3~+ (tris-(ethylenediamine)cobalt(III))-phosphate ion pair. [35][36][37] Correspondingly, the interaction mode between counteranion and sodium ion is of interest. Therefore, exploring the dependence of the structure of the polyanion on the 23Na+ quadrupolar relaxation rate is of importance.…”

Appreciable Effects of Ion Pairings on the 59Co Electric Field Gradient in the NMR Relaxation of [Co(chxn)3]3+

“…Reasonable explanations have been developed for the concentration dependences of the relaxation rates and the chemical shifts by assuming the ion-association model. [3][4][5][6][7]10 Although the presence of 1:1 ion pairs is usually assumed for the analysis of the experimentally obtained NMR parameters, significant amounts of multiple ion pairs are also present for the multivalent cations at high anion concentrations. If a system containing a probe nuclei is in more than two states and the exchange between these states is much faster than each relaxation rate, the relaxation rate is generally expressed as13…”

Asymmetry-enhanced 59Co NMR thermometry in Co(iii) complexes

“…An alternative explanation of the concentration dependence is that aggregation is modifying how the individual 59 Co nuclei interact with the solvent cage. Changes in this interaction may induce changes in the electric field gradient at the cobalt nucleus, [40][41][42][43] which might also contribute to the concentration dependence of the chemical shift. Dynamic light scattering experiments as a function of concentration and solvent suggest that some degree of aggregation may be happening (see ESI †), though these changes do not trend with Δδ/ΔT, or any other magnetic resonance characteristic, across all studied concentrations and solvent choice.…”