59Co : Temperature Dependent Chemical Shifts

⁵⁹Co is a very receptive, 100% naturally abundant, spin I = 7/2 quadrupolar nuclide with a chemical shift range spanning some 18,000 ppm.  The ⁵⁹Co NMR spectra of symmetric diamagnetic cobalt III complexes are characterized by relatively sharp resonances of a few Hz to tens of Hz.  The chemical shifts are extremely sensitive to temperature, pressure and solvent effects.  The temperature sensitivity of the chemical shift is largely due to the shortening or elongation of the chemical bonds between the cobalt and the surrounding ligands as a function of temperature.  The figure below shows ⁵⁹Co NMR spectra of K₃[Co(CN)₆] in D₂O on a 300 MHz NMR spectrometer collected as a function of temperature and time.  The spectrum in the bottom trace of the stacked plot was for a sample equilibrated at 21°C. The temperature was them set at 60°C and 80 single scan spectra were collected over a 9 minute period of time.  One can see that as the sample begins to warm up, the resonance moves to higher chemical shifts and broadens severely owing to a temperature gradient over the length of the sample.  As time passes and the temperature (read at the thermocouple in the probe) becomes stable, the chemical shift approaches a constant value while the line width narrows as the temperature gradient over the length of the sample becomes smaller.  The chemical shift change was measured to be 1.56 ppm/°C.  The data emphasize that temperature regulation is extremely important when collecting or reporting ⁵⁹Co NMR data.