Chemical Forums
Chemistry Forums for Students => Inorganic Chemistry Forum => Topic started by: DonkeyHody1 on March 02, 2017, 01:39:31 PM
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The problem statement, all variables and given/known data I'm am trying to find out the magnetic susceptibility,Xm in order to calculate the number of unpaired electrons in Mn(acac)3 via the Evans Method
Relevant equations
Xm = (477) Δv/(Qv1C) Xm= X'm+ Xm(metal)+ Xm (ligands) Where X'm is the magnetic susceptibilty of just the unpaired electrons. Δv is the frequency difference between the two chloroform peaks, 648.2 MHz (7.2722 ppm-5.6517 ppm)*400 MHz Q=2 because a superconducting NMR was used v1 is the frequency of the NMR, 400MHz and c is the molarity of the solution used, 0.0948M X'mT=(1/8)n(n+2) where n is number of unpaired electrons
The Xm I calculated is 4179.1878. I have nothing to base the accuracy of this off of. the Xm (metal) = 511. THis was for elemental Mn. I couldn't find anything for Mn(III) so this might be the problem. And from the lab manual the Xm(ligands) is 3*52 Solving for X'm I got 3512.1878 This gives an unpaired electron count of of nearly 3,000 so this cannot be correct. Unless of course this is number of unpaired electrons in the sample or in a mole?
Any help would be appreciated
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I actually have used Evan's method before to determine magnetic susceptibility (actually, spin state) of iron centers in heme complexes. It's a little hard to follow your post because of the equation formatting and lack of units anywhere. I had to look up my own publication but this is the equation we used:
[tex]\chi_i=\frac{-3}{4m\pi}\frac{\nu_o-\nu_i}{\nu_s}+\chi_0[/tex]
Here, νi and νo are the respective chemical shift values, expressed in Hertz, of the standard in the absence (outer tube) and presence (inner tube) of the paramagnetic species; νs is the resonance frequency of the spectrometer (e.g. 500 MHz); m is the concentration of paramagnetic species in the inner tube expressed in grams per cubic centimeter; and χ0 is the magnetic susceptibility per gram of the standard. The magnetic susceptibility per mole of the paramagnetic species, χm, is determined by multiplying χi by the molar mass.
You may also look up Evans's original paper:
Evans, D. F.; James, T. A. J. Chem. Soc., Dalton Trans. 1979, 723-726.
If you can provide numbers and units for some of the input values, maybe we can find where you're going wrong.
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I actually figured it out. I had my units mixed up... A rookie mistake.