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In this molecule, which is 3,5-dimethyl-4-carboxypyrazolate (unless I got it wrong):

enter image description here

I do not think there is free rotation around the bond indicated in red, because one can write a resonance structure with significant reasonable weight where a double bond would be located between the two groups.

However, how can I go further than that and qualitatively describe the energy barrier for the rotation. I have found (here and there) that in the case of benzoates, the barrier is in the range 3–6 kcal/mol. Should I expect it to be higher or lower in the above molecule?

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  • $\begingroup$ According to ChemDraw, the name is 4-carboxylato-3,5-dimethylpyrazol-1-ide $\endgroup$ – CHM May 14 '12 at 21:45
  • $\begingroup$ @F'x - can't you transit the carboxylate dihedral angle? $\endgroup$ – Richard Terrett May 15 '12 at 4:11
  • $\begingroup$ @RichardTerrett I could, but I really would like to have some qualitative argument about which way it is expected to be $\endgroup$ – F'x May 18 '12 at 9:30
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One of way to answer this question is to put it in some QM software. The system is small enough and has higher symmetry so computations shouldn't take to long, even on an older computer.

For co-planar geometry the energy calculated at B3LYP/6-31G(d,p) is –492.132206748 a.u. The geometry with dihedral angle of 90° between $\ce{CO2}$ and $\ce{C3N2}$ is –492.116719625 a.u. That makes roughly 41 kJ/mol (10 kcal/mol).

Now if you really want to be sure if that's the right energy you should make frequency calculations on both structures to find out if they are minima or transition states. This can take a bit longer (I haven't done it) and I would expect that the better structure (with lower energy) should have no imaginary frequencies and the higher one should have one (TS). If by some accident both of them are minima than TS has probably a dihedral angel somewhere around 45°.

Of course this is only computation. It gives as clues but not the exact answer. You can go for higher level of theory, add solvent modelling and... or you can take your compound and try to make temperature depending NMR spectra. With a bit of luck you can freeze your TS and find out how high is your barrier.

p.s.

It would be good if you mention in your post what tools (in this case computational) are available to you. I really would prefer to explain how to get the number instead of giving just dry answer.

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  • $\begingroup$ I know how I can calculate the numbers, but I was wondering if one could intuitively argue the energy barrier (carboxypyrazolate vs. benzoate) one way or another. Thanks anyway for the calculations… $\endgroup$ – F'x Jun 10 '12 at 19:51

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