Monday, June 9, 2008
Shifts Due to Paramagnetism
The paramagnetic susceptibility of a material can be determined using NMR spectroscopy. Because of its sensitivity, proton NMR is most often used to measure the paramagnetic shift for a reference compound in the presence and absence of a paramagnetic material. Since the paramagnetic shift (in ppm) is independent of the observation frequency, it is also independent of the isotope observed. This is illustrated in the figure below for the 1H, 13C and 29Si NMR spectra of TMS in a CDCl3 solution of Cr(acac)3 and in a similar solution without the Cr(acac)3. In these spectra the high frequency peak is due to TMS in the presence of Cr(acac)3 and the low frequency peak is due to TMS in the absence of Cr(acac)3. In all three cases the chemical shift difference (in ppm) between the peaks is identical.