[ironnica]

Hi all,

I'm curious more than anything else about this if anyone has experience (should have done it before). If I have a mix of a complex with paramagnetic metal centre and for instance free ligand, assuming there is no covalent bonding between the two species, will the presence of the paramagnetic complex affect the clarity/region of free ligand as compared to a pure free ligand spectrum?

(A side question if anyone can be so kind as to field, I am completely new to EPR Spectroscopy. If I have a complex with paramagnetic metal centre, does EPR tell me anything much about the overall structure or just that of the metal itself or of metal with atoms bound directly to it etc. Eg. Is it anywhere near as informative as NMR? Sorry, realise it is a broad question, if you can give any answer at all it's appreciated)

Thank you

[Babcock_Hall]

To the second question, EPR signals may show broadening due to NMR-active nuclei that are directly bound to them.  An example is Mn(II) and O-17.  ENDOR and ESEEM give additional information about the bound ligands and in favorable cases about nearby nuclei that are not bound.

To the first question, I believe that one factor that has to be taken into consideration is the exchange rate between bound and unbound ligands.

[ATMyller]

I have noticed that paramagnetic center metal atoms do broaden the NMR signals (¹H, ¹³C and HSQC) of the complex, but only the complex. Any free ligand impurities are completely visible with similar FWHM values as in a sample with only free ligand.

Then again the complex I'm working with is very stable and large and does not exchange ligands after it has been synthesized.

[Irlanur]

This questions is a bit hard to answer generally, because it all depends on the relative magnitudes of the relaxation times and the hyperfine-coupling (coupling between unpaired electrons and nuclei, analogous to spin-spin couplings in NMR). Also Ligand/solvent exchange plays a big role.


Roughly one can say that the lines of close nuclei will be broadened, sometimes so much that you won't see anything. If the relaxation of the electron spin is very fast, you will observe huge coupling constants, easily in the range of MHz, which means that you won't see the lines in  normal NMR.

The influence on unbound ligands is not easily predicted. The main contribution probably comes from ligand exchange, but if your complex also binds solvent molecules, the peaks of the whole sample can be broadened. There are "paramagnetic relaxation enhancers" which are made exactly for that purpose in NMR, but mainly in MRI for contrast improvement.


for very distant spins, e.g. in proteins, the electron can lead to pseudo-contact shifts, which can be used as distant restraints.

EPR includes many techniques which give quite different information content. The Metalcenter and its surrounding can be characterized very well by CW-EPR, also the symmetry is quite easily extracted from the spectra (e.g. tetrahedral or quadratic-planar). for nuclei further away from the paramagnetic center see Babcock_Hall's post. There are pulsed EPR techniques which give a huge amount of structural information. In general these are not easily extracted and need simulations of the spectra.

All this can of course be understood very well in quantum mechanical framework.

[ironnica]

Much appreciated in the info guys!

I don't believe that solvent exchange is happening, or to any appreciable level anyway.

[Babcock_Hall]

Nuclear Magnetic Resonance of Paramagnetic Molecules
by Gerd N. La Mar might be a good resource.