Variable‐temperature high‐pressure investigation of the cobalt‐59 NMR spectroscopy of aqueous K3[Co(CN)6]

Cobalt‐59 NMR measurements were made on an aqueous solution of K3[Co(CN)6] with pressure as the main variable. We determined the effect of temperature and pressure (up to 5 kbar) on (i) the cobalt‐59 chemical shift, (ii) the carbon–cobalt‐59 isotope shift, (iii) the cobalt‐59–carbon‐13 coupling constant and (iv) the cobalt‐59 spin–lattice relaxation time. The marked effects of temperature and pressure on the chemical shift can be attributed to changes in the cobalt–carbon bond distance and are consistent with a radius of 2.5 Å for the [Co(CN)6]3− ion. Analysis of the slightly non‐linear dependence of the chemical shift on pressure has, for the first time, allowed the dependence of (∂δ/∂p)T and (∂δ/∂T)p on temperature and pressure to be evaluated. For a given temperature change, the fractional change in (∂δ/∂p)T is independent of pressure and, for a given pressure change, independent of temperature. The ion is more compressible at high temperatures. The same explanation can be applied to the small effect of temperature on the carbon‐13 induced isotope shift and this has confirmed previous theoretical work. The lack of a pressure dependence for the cobalt‐59 spin–lattice relaxation time is attributed to a dominant quadrupolar mechanism arising from the motion of one or more water molecules in the solvent sheath surrounding the ion. This motion induces an electric field gradient at the cobalt nucleus. This conclusion is supported by nitrogen‐14 and carbon‐13 using {K3[Co(13CN)6]} NMR relaxation studies made at ambient pressure over a range of temperatures and at different radiofrequencies: these showed that, unlike the cobalt‐59 nucleus, these nuclei are relaxed by a molecular re‐orientational process. The nitrogen‐14 and carbon‐13 data independently provide values of 18 ps for τc at 298 K and 16 kJ mol−1 for the associated activation enthalpy for the re‐orientation of the [Co(CN)6]3− ion in water. Copyright © 2001 John Wiley & Sons, Ltd.

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