I have optimized several of my diastereomers on spartan (M06-2X, 311+G**, CPCM ACETONITRILE) I managed to have all positive frequencies. However I'm getting the syn isomer as the more favorable, however, my experimental data clearly indicates that the anti is the more favorable product. Can anyone suggest to me what to do? I have tried other methods and the results are the same.
Maybe try SMD as a solvent model, I often get more accurate results with M06-2X. Also adding explicit solvent molecule sometimes can improve the situation. If you would like to change the basis set for a different with comparable level of theory I could propose you AUG-cc-PVTZ. What type of molecules are you working with and what is the energy difference?
Thank you Michal. I will try right now your suggestions. I have invested a lot of time in this. I am working with an organic molecule furan type medium size 350 gr/mol. I just need the optimized geometry of the 2 isomers and compare the energy difference. The energy difference usually is between 0.5 to 1 Kcal/mol always favoring the syn isomer. in another case, I got a 6 kcal difference. My experimental data is clear and it is indeed the anti isomer the major product.
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