STERIC AND ELECTRONIC EFFECTS ON CONFORMATIONS OF N-ACYLINDIGO AND N,N'-DIACYLINDIGO DYE MOLECULES. Kenneth J. Haller, School of Chemistry, Suranaree University of Technology, Nakhon Ratchasima 30000 Thailand, Bradley D. Smith, Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame IN 46556 USA

Solid and solution state structures of N-mono- and N,N'-di-chloroacetyl substituted indigo were compared to further understanding of spectral changes in substituted indigos (J. Chem. Soc. Perkin 2 1993 165). Steric repulsions between the acyl groups and the chromophore carbonyl groups induce large conformational twisting (8 and 20deg. respectively) about the central C=C bond of the chromophore; the twisting is offset in the monosubstituted compound by the remaining N--H...O intramolecular hydrogen bond. The near equivalence of the central C=C bond lengths is explained by resonance effects. The inductive effect of the N-acyl groups completes the description of the spectral shifts.

More subtle conformation changes were explored in comparing the diacyl series, N,N'-diacetylindigo, N,N'-bis(chloroacetyl)indigo, and N-acetyl-N'-chloroacetyl-indigo. Steric crowding again determines the gross conformation. The N-acyl groups form double intramolecular C--H...O hydrogen bond relays with the N-acyl C--H's as bifurcated hydrogen bond donors to the chromophore carbonyls and the N-acyl carbonyls as acceptors to chromophore aryl C--H's. The relative order of observed C--H...O hydrogen bond distances correlates with C--H acidity and O basicity (J. Org. Chem. 1993 58 6493).

Additional structures, including N-methoxyacetylindigo, N,N'-dimethoxyacetylindigo, and N,N'-diethoxymalonylindigo will be presented along with further analysis of the effects of steric interactions, C--H...O hydrogen bonding, and electrostatic interactions on the conformations of indigo dye molecules.