Structure Determination of Auxin Phytohormones.

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T1	T2	E [kJ/mol]
0.000°	0.000°	0.000
112.508°	103.570°	0.499
99.061°	-96.409°	1.998
111.850°	1.601°	4.361

The potential energy surface of 3-indole acetic acid (IAA) contains four symmetry unique local minima with the COOH group in cis-orientation. The geometries of these conformers are shown below this paragraph in the order of their energy. The global minimum is mirror symmetrical and stabilized by a weak C=O···H-C2 hydrogen bond; the other three conformers all exhibit a tilted side chain, with three different orientations of the COOH group. The 6-31G*-optimized positions in T1/T2-space and the corresponding relative energies are collected in the table next to this paragraph.

A number of basis sets, ranging from STO-3G to 6-311G** have been employed to optimize these conformers at RHF-level. The comparison of the calculated rotational constants with the experimental values shows that the 4-31G values are already within the experimental error of 1%; in contrast, the electron densities indicate that one set of d-polarization functions is necessary to obtain an adequate description of the aromatic ring.

The ab initio results compare in an interesting manner with those of molecular mechanics (MM) calculations, which were performed by Tomić et al. with a number of different force fields: none of the force fields used was able to reproduce the ab initio results; the best agreement was obtained with CVFF and MM3. All of the MM calculations gave only two symmetry unique minima, which correspond to the ab initio conformers of second and third energy, but failed to reproduce the two other conformations as energy minima. The planar conformation, which is the global minimum in the ab initio case, is described as a second order saddle point by MM methods.

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