[superg]

hi all,

i've just done the evans measurment on one of my compounds, but i'm struggling with the calculations. do anyone of us got an "easy" formula for this? especially with unit...

i've he difference between the two peaks of the reference (around 140Hz), but for the moment i'm stuck at that point...

also how do you calculate the number of unpaired electron from the magnetic susceptibility?

thanks

[Yakimikku]

Hello! I just found your post a couple months ago when I was having problems getting my head over understanding Evans method.
Since then I've figured a few things out!
Although you posted this a year ago, hopefully my answer will get to you or others in need of help.

First of all, be sure that you use the correct "shape factor" in the equation for Evans method. Note that the original Evans paper is generally not the equation you use nowadays, because it applies for old types of NMR spectrometers. A reference for this is [Schubert E. M. J Chem Ed, 1992, pg. 62] <--not sure about the volume

The second important point is the solvent correction in the formula. There are two corrections in the formula--one correction for the solvent's diamagnetic contribution and another for a difference in densities. It was previously thought that ignoring the density correction was safe, but it turns out (and maybe still with debate) that ignoring both the solvent and density corrections is a better approximation. Here are some references for that: [Grant, D. H. J Chem Ed, 1995, 72, no. 1, 39-40] and [Piquet, C. J Chem Ed, 1997, 74, no. 7, 815-816]

Thus, from the frequency difference that you have, you can calculate the mass susceptibility by:

Xg = -3(:delta:f) / (4πFm)

where :delta:f is the frequency difference, F is the frequency of the NMR spectrometer, and m is the g of your compound per mL of solution. As is, the units for your answer would be (emu g^-1).

You then generally would want to convert to molar susceptibility by using the molar mass of your compound. Then the units should be in (emu mol^-1). This value needs to be corrected because there is a underlying diamagnetic contribution contributing to your observed susceptibility. To do this, you need to estimate the diamagnetic contribution of your molecule using Pascal's constants. A good comprehensive resource for those constants and how to use them can be found [Bain, G. A.; Berry, J. F. J Chem Ed, 2008, 85, no. 4, 532-536]. I believe this paper also explains your last question on unpaired electrons.

So, to get the paramagnetic contribution (Xp), you subtract the diamagnetic contribution calculated by Pascal's constants (Xd) from the observed value you calculated before (Xobs). i.e. Xp = Xobs - Xd

Generally, you want to calculate the effective magnetic moment, μ_eff.

μ_eff = sqrt(8 Xp T)

where 8 is the value of a bunch of constants calculated together, Xp discussed above, and T is the temperature you took the measurement at (in Kelvin). μ_eff is in units of Bohr magnetons.

Well, I hope this answers your questions and many others! I personally had a few headaches figuring it out, but I think I got it now. I'll poke in now and then to see if there are anymore questions.