[Rutherford]

I attached the problem. It is asked to Assign ²⁷Al-NMR signals in the spectrum to the appropriate Al cation(s) in the right figure. How to do that? Some are tetrahedral some octahedral, but which ones?

[Borek]

Is this whole question?

[Rutherford]

No. Attached what I can't solve. Here is the whole problem: http://icho2014.hus.edu.vn/document/01-24-2014%20IChO46-Preparatory.pdf . It is the 15th problem.

[Rutherford]

The axis of symmetry for the right figure cations would pass through 3, 7 and 13, so maybe these cations are tetrahedral and all the others are octahedral, but I could really use a second opinion.

[Radu]

From the relative intensity of the peaks I found that Td:Oh = 1:11, which can be interpretated as 1:12, too, given the peaks are not integrated. I think that Al7 is Td, given that it is the most symmetrical( it is almost perfectly symetrical).
     

[Borek]

From the relative intensity of the peaks I found that Td:Oh = 1:11, which can be interpretated as 1:12, too, given the peaks are not integrated.

Yep, better than nothing, and it yields conclusion that there is only one tetrahedral cation.

I think that Al7 is Td, given that it is the most symmetrical( it is almost perfectly symetrical).

That was my first idea as well, however, as it is not clear what the right part of the image is intended to show - it is most likely some kind of axonometric projection, but it can be interpreted in many ways - there is no way to say for sure.

Edit: http://en.wikipedia.org/wiki/Keggin_structure



Three layers containing 4 Al atoms each, plus a single Al atom inside the middle layer.

Seems like what they used is the same structure cut at an angle, so that there are six atoms in the plane. But it wasn't clear to me without seeing the the wikipedia image.

[Rutherford]

And from that spectrum I should be able to draw the structure you uploaded. Perfect.

[CrazyAssasin]

Is the Keggin ion shown in the picture just part of the structure or is it only one molecule?
I don't get it because oxygen atoms, that are linked with center aluminum atom, are four coordinated

[Radu]

 Yes, they are μ₄ bridges,  because they are O^2-.

[Rutherford]

What's the reaction between ammonium carbonate and aluminum hexahydrate chloride to yield Al13 Keggin ions? I tried:
(NH₄)₂CO₃ + Al(H₂O)₆Cl₃ [Al₁₃O₁₈H₂₄(H₂O)₁₂]Cl₇ + CO₂ + NH₄Cl + H₂O but it can't be balanced. What are the correct products?

[CrazyAssasin]

Well, I balanced it with the same products. You wrote Keggin ion with 18 O atoms instead of 28, maybe that is why you can't balance it

[Rutherford]

Well, that shouldn't happen . Thanks for the correction.

[Big-Daddy]

How do you do the last part? (Determine the number of Al₁₃-Keggin ions produced by the reaction.)

Also, is my interpretation of the reaction correct - we are mixing two solids, which will produce water, as well as CO2 gas which escapes, but the water should then dissolve the Al₁₃-Keggin ions (produced originally as part of a solid crystal with chloride anions) along with dissolved ammonium and chloride.

[Rutherford]

Carbon dioxide is the gas which produces the pressure, ideal gas law and simple stochiometry give the answer.

[Big-Daddy]

Carbon dioxide is the gas which produces the pressure, ideal gas law and simple stochiometry give the answer.

Sure. But if water is produced we also have to assume the volume of the water is negligible, in order to say that the volume available for the gaseous phase remains constant.