INFRARED SPECTRAL STUDY OF CROWN ETHER MOLECULAR COMPLEXES WITH 4-NITROANILINE

Abstract The IR spectra of the crystalline complexes of 4-nitroaniline with crown ethers were studied, viz., 18-crown-6, benzo-18-crown-6, dibenzo-18-crown-6, dicyclohexano-18-crown-6 and dibenzo-24-crown-8. The hydrogen bonding interaction was found to be predominant in these complexes. The strongest among them was the one formed with the most flexible crown ether dicyclohexano-18-crown-6, in which both the N-H bonds of 4-nitroaniline were shown to be equivalently hydrogen bonded to the crown ether oxygens. The shift of the NO2 absorption bands indicated that the resonance form (−C2N-C6H4=N+H2) is favored upon complexation with the larger ring size dibenzo-24-crown-8, while for the other complexes, the resonance form (C2N-C6H4-NH2) is favored; a fact which was attributed to geometrical factors.