NMR Relaxation Times: Fe3+ & Mn2+ Ratios Explained

[Baggio]

Hello

We conducted an experiment recently for our physics lab to determine the relaxation times for a solution of Fe3+ and Mn2+ ions we then had to compare the ratios of T1/T2 for the two solutions and we found that for Fe3+ ions the ratio came to ~1 and for Mn2+ ions it came to 4 (i.e >1)

My question is why is there a difference in the ratios? It seems that it is all dependent on T2 (relaxation time eliminating field inhomogeneity). I've been hearing things about scalar coupling but I don't understand that idea fully. :-/

 

[Mike H]

Think about the nuclear properties of each transition metal. Fe(III) and Mn (II) may have the same number of valence electrons, but the nuclear properties differ in a not insignificant way.

Scalar (or J) coupling is just spin-spin coupling through bonds, usually within three or so bonds. It is one of those delightfully wacky quantum mechanical results that you eventually learn to just deal with and accept in NMR.

 

[Amos]

does anyone know exactly why the T1/T2 ratio is smaller for Fe and larger for Mn? Their electronic configs are virtually the same?

 

[Mike H]

T2 measures spin-spin relaxation, right? So if you have two quadrupolar nuclei interacting (e.g. Mn), you're going to have a really fast relaxing nucleus, a really short T2, and therefore a much higher T1/T2 ratio.

The reason I said in my older post to look at the nuclear properties is because iron iron-57) has a nuclear spin of 1/2 (ergo no electric quadrupole moment) and manganese has a nuclear spin of 5/2 (ergo a quadrupole moment).

But I didn't want to give it away. It is important to understand that quadrupolar nuclei are like this, and therefore NMR studies on them are very tricky.