Friday, December 19, 2014
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.
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4 comments:
David,
Thank you for your comment. I produced the stacked plot using the "stack" option in the plotting facility in TOPSPIN 3.0.
Glenn
Another very nice application of the Co-59 sample is to measure the temperature gradients in the sample. Simply said, the narrower the peak the more uniform the temperature is across the sample. One can experiment with increasing the temperature, altering the VT gas flow rate or trying different probes. Running the sample in a cryogenically cooled probe gives some pretty spectacular insight on the quality of the probe.
Anonymous,
Thank you for the comment. Yes, this would be an excellent sample to show temperature gradients across the sample. This can also be shown with the proton signal of HDO (albeit on a smaller scale). See this link:
http://u-of-o-nmr-facility.blogspot.ca/2009/12/variable-temperature-nmr-thermal.html
Glenn
...One way I sometimes run variable temperature spectra is set the VT unit to do a temperature ramp, say 5C per minute, then do a 'psuedo 2d' experiment where I acquire a 1 scan spectrum every 5 seconds or so. With the Bruker software you can scroll through the spectra quite easily in the multiple display mode, or get a 3d plot using the oblique mode.
Its probably not precise enough to get any physical constants out, but can be a quite nice way of confirming the system has correctly equilibrated.
I've got I vague feeling I did this with a cobalt sample, the lines being broad when the temperature was stable, but sharp when the sample was transiting between temperatures. I had the idea that the broadness was caused by temperature gradient across the sample,but got busy again and wasn't able to investigate further.
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