Resonance and solvent effects on absorption spectra of some 2- and 4-(–M)-substituted aniline derivatives

Abstract

The N-H protons of N-methyl-2-X-aniline derivatives remain intramolecularly hydrogen bonded in HBA (hydrogen bond acceptor) solvents where X = COOMe and NO₂, but form intermolecular hydrogen bonds to the HBA solvents where X = CN. The order of response of v_max to solvent HBA basicity for the Cl → C2 bands of N-unsubstituted-2-(-M)-substituted aniline derivatives and the Cl → C4 bands of 4-(-M)-substituted aniline derivatives follows the orders of the hydrogen bond donor acidities of the amine protons, which are determined primarily by the mesomeric effects of the 2- and 4-substituents. The spectral data indicate that intramolecularly hydrogen bonded 2-(-M)-substituted aniline derivatives are near sp² hybridized in non-HBA solvents and, unlike several 4-substitute anilines, do not undergo significant rehybridization in HBA solvents. Bath ochromic shifts resulting from hydrogen bonds by corresponding 2- and 4-substituted aniline derivatives to HBA solvents are of similar magnitudes.