The observed frequency of a spin
I = 1/2 nucleus in a molecule depends on the orientation of the molecule with respect to the magnetic field. For powdered solid samples, all possible orientations of the molecules are present in the magnetic field and one obtains an envelope of peaks called a powder pattern. If one can neglect all other interactions (such as dipolar coupling), the powder line shapes are dominated by the chemical shielding interaction. These line shapes provide valuable electronic, geometrical and dynamic information about the molecule. The chemical shift parameters can be read directly or calculated trivially from the spectrum. These parameters are indicated in the figure below.
The parameters shown in the figure are those recommended by IUPAC. There are other conventions used in the older literature and at times it can be confusing. The other conventions are described
here by
Klaus Eichele. The site also contains a very useful tool to convert the parameters from one convention to another. Thank you Klaus!
2 comments:
What about powder patterns in metals (I>1/2)? How can one tell if the chemical shift is important versus quadrupolar effects or anisotropic knight shifts?
Dear Magdalena,
Thank you for the questions.
Nuclei with spins greater than 1/2 can give very complicated powder specra affected by both the quadrupolar interaction and chemical shielding anisotropy. These spectra are described with many more parameters than those of the simple spectra of spin 1/2 nuclei shown in this post, inluding the relative orientations of the quadrupolat and chemical shielding tensors.
The only way to know if the chemical shift effects will be significant in the powder spectrum of a quadrupolar nucleus is to know something about the symmetry of the site, the span of the chemical shift and an estimate of the quadrupolar coupling constant. In general, quadrupolar effect are larger than chemical shift effects, so one expects that chemical shift effects will be important for nuclei with small quadrupolar coupling constants.
I hope this helps.
Glenn
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