Thursday, April 24, 2008
17O is a spin I = 5/2 quadrupolar nucleus with a natural abundance of only 0.037%. Its receptivity with respect to 13C is 0.061. Unlike the complications of the NMR spectra of quadrupolar nuclei in the solid state, the spectra in solution are quite simple as all of the energy levels are equally spaced by the Zeeman interaction, the first and second order quadrupolar effects averaged by the rapid isotropic motion in solution. 17O has a chemical shift range of more than 1600 ppm, making it quite sensitive to the local chemical environment of the oxygen and therefore potentially useful to the chemist. The figure below is a 17O NMR spectrum of a mixture of water, acetone, ethanol, isopropyl alcohol and ethyl acetate acquired in less than 15 minutes on a 300 MHz NMR spectrometer using a standard broadband probe with a 5 mm NMR tube. One can see that every single oxygen site is resolved in the spectrum. In order to overcome the problem of low sensitivity, one should use as much sample as possible either as a neat liquid (if the sample is a liquid) or in a very concentrated solution with a solvent free of oxygen (if the sample is a solid) - the more sample the better. There may also be problems if the quadrupolar dominated relaxation rate is too fast resulting in resonances that are very broad.