Structure—activity correlations for phenoxyacetic acids and indoleacetic acids used for plant growth regulation

Abstract Experiments on substituted indole‐3‐acetic acid and halogenated indoles have led to the suggestion that fractional positive charge development on the—NH group of the indole ring at a fortuitous distance (5.5 Å) from the carboxylic acid function is requisite for plant auxin activity exhibited by indoleacetic acids [W. L. Porter and K. V. Thimann, Phytochem. 4 , 229 (1965)]. Calculations using self‐consistent field (scf) molecular orbital (mo) theory with complete neglect of differential overlap (cndo) were carried out on a series of halo‐substituted phenoxyacetic and indoleacetic acids. No net positive charge development calculated for the N—H group in the substituted indoleacetic acids gave the observed order of auxin activity. No development of positive charge in the ∏‐ p z system calculated for N was in any way comparable to the observed auxin activity increase. However, the energies of the lowest unoccupied molecular orbitals (lumo) decreased nearly linearly with increasing auxin activity. Twelve substituted phenoxyacetic acids, also treated, showed a correlation between the lumo energies and the square of the lumo coefficients at the ortho position, respectively, and plant auxin activity when the lipophilicity of the molecules in the series was taken into account. The correlation coefficient of the activity as function of lumo energy was virtually identical to that calculated from the data of Hansch and coworkers [see J. Amer. Chem. Soc. 85 , 2817 (1963)], using ring substituent Hammett sigma values for the ortho position instead of the lumo energies.