A study of the conformational profile and the vibrational spectra of the plant hormone indole-3-acetic acid

The structural stability of indole-3-acetic acid was investigated by DFT-B3LYP calculations with the 6-311G** basis set. From the calculations the gauche-gauche (gg) structure was predicted to be the second lowest energy minimum for the acid. It is energetically only 0.57 kcal/mol above the lowest conformer which is the trans-cis (tc) structure. A further stable conformer, however, highest in energy, is the trans-trans (tt) one, which is by 2.68 kcal/mol higher in energy than tc. The tc conformer upon full optimization turned a little bit away from real tc to a near tc (ntc) structure (defining torsional angles only changed by a few decigrades). However, the X-ray data indicate a structure in the solid, which is most similar to gg, stabilized by intermolecular eight ring hydrogen bonds. In the present DFT calculations such stabilizations cannot be accounted for, because the calculations treat only isolated molecules. To take such interactions into account at least dimers would have to be optimized. Therefore the vibrational frequencies of the gg conformer were computed at the B3LYP level of theory and tentative vibrational assignments were provided on the basis of normal coordinate analysis, normal mode calculations and experimental infrared and Raman data. However, some of the observed lines are obviously due to a small amount of the tc conformer present.