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
