Combined ab initio SCF and molecular mechanics studies of propionic and isobutyric acids and their indole derivatives related to the phytohormone auxin (indole-3-acetic acid)

Detailed conformational analyses of propionic and isobutyric acids were performed to contribute to a better understanding of the stereochemical characteristics of biologically active indole-3-aliphatic acids. The studies are based on ab initio SCF (RHF/6-31G*) and molecular mechanics (force fields used: MM2, MM3, CFFS1, AMBER, CVFF, ESFF) methods. The results obtained with the CFFS1 and MM3 force fields revealed the best agreement with the experimental values and those from ab initio calculations. Normal mode frequencies in the harmonic oscillator approximation were calculated for the geometry optimized conformers with C-s symmetry of these compounds as well as of indole-3-acetic acid (IAA) and some of its biologically important derivatives (4-Cl-IAA, 6-Cl-IAA, 7-Cl-IAA, 4-Me-IAA) and indole-3-isobutyric acid (IIBA). The influence of the indole ring on the C=O and O-H stretching frequencies was analyzed. A small decrease of the C=O frequency was determined in the indole-3-acetic acid derivatives and a larger one in indole-3-isobutyric acid.