Author Topic: Problem of conformer population (butane)  (Read 2196 times)

Bricklayer

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Problem of conformer population (butane)
« on: March 10, 2015, 11:01:00 am »
Hello,

We would like to study conformer population of butane at 25C. We have anti and gauche conformers. We produced conformers from frog2. With the .sdf file we performed calculation with Turboloe and get two .cosmo files. We then used cosmotherm, went to global option and checked wconf (print conformer information), selected both conformers butc0 and butc1, added water, and performed activity calculations.

Tthe calculus gives us that the normalized weight factor of molecule in compound of c0 (anti butane) is 1.00000. No gauche butane is present...

The problem is that, at 25C, 0.75 is anti, 0.25 is gauche.

What's wrong with our calculation ?

Could you help me please ?

Thank you very much.

Sylvain

Arnim

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Re: Problem of conformer population (butane)
« Reply #1 on: March 10, 2015, 03:12:22 pm »
Hi,

the conformer distribution in cosmotherm is computed by the Boltzmann partition and depends on three contributions: the QM energy of the conformer COSMO-files, the symmetry of the conformer geometries (which determines the degeneracy prefactor of the Boltzmann term), and the chemical potential of the conformers in the given solvent (the COSMOtherm mixture/solution contribution).

For nonpolar compounds like alkanes the COSMOtherm mixture contribution will be almost identical for all possible conformers, because the electrostatics of the alkane COSMO-surfaces is the same. The only remaining contribution to the chemical potential is the combinatorial one, resulting from shape difference of the conformers. However for a compound like butane the differences between the gauche and trans conformers typically are <<0.1 kcal. Hence from the view of COSMOtherm, the butane conformers are actually identical.

The only significant contributions to the conformational distribution hence are the conformer's QM energy (and to a much smaller extent also their symmetry). Obviously the QM energy of the computed gauche conformer is way too high above the anti conformer to contribute to the conformer equilibrium. Now assuming that all conformers were optimized properly and on the same QM level, the one remaining cause for the discrepancy between experiment and prediction is the QM level that is used. The QM level used, obviously is not good enough to predict the energy difference between the conformers correctly.

With some quick handmade conformers, I get a trans:gauche ratio of 0.66 : 0.35. So, there might be something wrong in your calculation.

Since this is not really a Turbomole question, please write to cosmotherm@cosmologic.de for further discussion.

Cheers,

Arnim

Bricklayer

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Re: Problem of conformer population (butane)
« Reply #2 on: March 12, 2015, 03:57:58 pm »
Tank you very much sir.

Best regards,