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I have calculated optimized geometries and energies in water of two diastereomers of one compound. One turned out to be 1.9 kcal/mol more stable than the other (level of theory PBEo-G3bj/6-311g++(d,p)/SMD). There is no sterical hinderance in less stable isomer, and I see no significant difference in its NBO.

The only big difference that I found are the values of dipole moments: more stable isomer - 3.9, less stable - 5.9. The question is how can the difference in dipole moments be used to explain the difference in energy in aqueous solution?

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  • $\begingroup$ There is not enough to go on here. Does this level of theory actually describe your system well? What NBO version have you used, and what is 'no significant difference'? Sterical hinderence is a figment of imagination, how did you qualify and quantify your statement? $\endgroup$ – Martin - マーチン Aug 14 at 16:51
  • $\begingroup$ What's the size of your molecule (roughly)? $\endgroup$ – Buck Thorn Aug 14 at 19:25
  • $\begingroup$ Thanks for your replies. I believe the level of theory describes my system well, this difference in stability was observed in experiment too. I used NBO verison 3.1 from Gaussian09. I compared the values of E(2) from Second order perturbation theory analysis of Fock matrices of two diastereomers and all values were equal ±5%, so I decided that there is no orbital interaction that lowers energy of one isomer and is blocked in the other. $\endgroup$ – Bobik Aug 15 at 6:54
  • $\begingroup$ My statement about sterical hindrance is only qualitative, I made it because distances between neighboring groups in less stable isomer are greater or equal than in more stable. I think I can say that isomer with lower dipole moment is more sterically hindered because bigger substituent is in axial position of 6-membered ring (higher dipole moment - equatorial). The size of a molecule is roughly 35 atoms of I and II periods, 120 electrons. $\endgroup$ – Bobik Aug 15 at 6:55