Generally, phosphates have chemical shifts in the range -20 to 10 ppm. For vanadyl phosphates containing $\ce{V^{4+}}$, the isotropic chemical shift is about 1500 ppm. Why?


1 Answer 1


$\ce{V^{4+}}$ (and $\ce{V^{3+}}$) is paramagnetic, and therefore you would expect a very large change in chemical shift. Paramagnetic shifts arise from the interaction of the observed nucleus with unpaired electron spin of the paramagnetic metal ion. The observed isotropic shift comes from two interactions: the contact shift and pseudo-contact shift.

The contact shift comes from the direct interaction between the observed nucleus and the unpaired electron - a through bond effect. This gives very large shifts - many hundreds of ppm.

The indirect pseudocontact shifts essentially come from the interaction of the unpaired electron density with the electron density of the orbitals of the observed nucleus, and are effectively compensating for the nonzero magnetic field of the paramagnetic source - a through space effect. These are much smaller shifts, often within the usual observed chemical shift range.

Whilst the measurement of contact shift information is infrequently used for structural insight, pseudocontacts using paramagnetic lanthanides are used extensively for 3D solution structures of proteins.


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