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.

1D 1H - 31P HOESY

2D Heteronuclear Overhauser Effect SpectroscopY (HOESY) is an effective way to determine whether or not a pair of heteronuclear spins are close to one another in space.  It is particularly effective for ¹H and ³¹P where both nuclides are 100% naturally abundant.  2D experiments, however, can be quite time consuming.  Alternatively, one can obtain 1D ¹H detected ¹H - ³¹P HOESY data to save data collection time.  When only one ³¹P resonance is present, the data can be obtained using nonselective ³¹P pulses.  An example of this, using the, using the pulse sequence from the reference¹ below, is shown in the figure.  The HOESY spectrum is on top while the simple ¹H spectrum is on the bottom.  One can see that heteronuclear ¹H - ³¹P NOE's are apparent on the bridging methylene protons and the ortho-aromatic protons.  Neither the meta- nor para-aromatic protons show significant heteronuclear NOE's.

1.  L.E. Combettes, P. Clausen-Thue, M.A. King, B. Odell, A.L. Thompson, V. Gouverneur and T.D.W. Claridge. Chem. Eur. J. 18, 13133 (2012).   

1D Selective 1H - 19F HOESY

2D Heteronuclear Overhauser Effect SpectroscopY (HOESY) is an effective way to determine whether or not a pair of heteronuclear spins are close to one another in space.  It is particularly effective for ¹H and ¹⁹F where both nuclides are 100% naturally abundant.  2D ¹⁹F detected ¹⁹F - ¹H HOESY data are typically obtained which provide all NOE correlations.  2D experiments, however,  can be quite time consuming, especially when only a few NOE correlations are sought after.  In such cases, 1D ¹H detected ¹H - ¹⁹F HOESY experiments¹ are very desirable and can save a great deal of time.  When only one ¹⁹F resonance is present, they can be obtained by using hard ¹⁹F pulses.  This was recently illustrated well by Dr. Michael Lumsden of Dalhousie University.  When more than one ¹⁹F resonance is present, one can use a selective ¹⁹F pulse and repeat the experiment selecting each type of fluorine.  An example of this is shown in the figure below.  Selective 1D ¹H detected ¹H - ¹⁹F HOESY spectra were collected for 2,3-difluoropyridine using a selective ¹⁹F pulse.  The simple ¹⁹F spectra are shown on the left with the selected ¹⁹F resonance color coded.  The upper two spectra on the right are the HOESY spectra while the spectrum on the bottom right is a simple ¹H spectrum.  One can see that when the fluorine in the 3-position is selected, there is a strong NOE to the nearest proton, C.  Alternatively, when the fluorine in the 2-position is selected, there are no strong NOE's as there are no adjacent protons.

1.  L.E. Combettes, P. Clausen-Thue, M.A. King, B. Odell, A.L. Thompson, V. Gouverneur and T.D.W. Claridge. Chem. Eur. J. 18, 13133 (2012).