Structure Determination of Auxin Phytohormones.

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The potential energy surface of 4-chloro-indole-3-acetic acid contains four symmetry unique local minima with a cis-orientation of the COOH group, which are shown on top of this paragraph in the order of their energies. The 6-31G* optimized positions in T1/T2-space and the relative energies are collected in the table following this paragraph. As in the case of unsubstituted indole-3-acetic acid (IAA), the global minimum (A) is mirror symmetrical and stabilized by a weak C=O···H-C2 hydrogen bond. Unlike IAA, the conformers with tilted side chain offer only two different orientations for the COOH group, the torsion angles of which differ by approximately 180°; these conformers are second and third in energy. The fourth symmetry unique local minimum (D) is not only remarkable because of its high relative energy. It is located at a T1/T2-position, which is close to that of a saddle point in unsubstituted IAA. Furthermore, D is no stable conformer: the lowest potential barrier (in the reaction to A) is only 0.452 kJ/mol, which is lower than the zero point energy of the corresponding vibration mode (that has an unscaled, harmonic frequency of 103.53 cm).

Conformer	T1	T2	E [kJ/mol]
A	0.000°	0.000°	0.000
B	105.623°	-14.059°	2.683
C	110.991°	162.977°	6.316
D	5.789°	115.002°	8.988

These four energy minima and their enantiomers are connected by a network of reaction paths, which contains the following connections (lower case symbolizing mirror images): A-B, A-b, A-C, A-c, A-D, A-d, B-c, B-D, C-D, and D-d. It should be noted that there is no direct reaction path between B and C, i.e., the COOH group cannot rotate freely in either of these conformers. Starting from B or C, an internal rotation of the COOH group is always coupled to a torsion of the acetic acid side chain.  

4-Cl-IAA has the possibility of forming a O-H···Cl hydrogen bond that closes a eight-membered ring. The geometry of this H-bonded conformer is shown next to this paragraph, as seen from two different directions. Due to the rigid nature of the indole nucleus, the resulting hydrogen bond is considerably weaker than the hydrogen bonds formed by -aminopentanoic acid or -hydroxypentanoic acid, which also lead to eight-membered rings: the 6-31G* relative energy of this conformer is 24.914 kJ/mol and the hydrogen bond order is only 0.027.

IAA.
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