Electric field gradient modulation and nuclear magnetic relaxation in hexacyanocobaltate ion

The ⁵⁹Co, ¹⁵N, and ¹³C NMR relaxation rates are reported for potassium hexacyanocobaltate in aqueous solutions as a function of temperature and counter ion. The ⁵⁹Co relaxation rate is dominated by the nuclear electric quadrupole relaxation mechanism, but measurements of the ¹³C and ¹⁵N relaxation rates show that modulation of the electric field gradient at the cobalt nucleus is not coupled to rotation of the symmetrical complex ion. The field gradient change at the cobalt nucleus resulting from ion-pair formation with magnesium ion is on the order of or less than 30%. This observation supports the hypothesis that larger changes in the relaxation rate of the central ion are due to perturbations in the first-coordination sphere of the observed ion.