INFRARED SPECTROSCOPIC STUDY OF THE ROLE OF WATER IN CROWN-ETHERS ANDTHEIR MOLECULAR-COMPLEXES WITH 3-NITROPHENOL AND 4-NITROPHENOL

The IR spectra of the crystalline complexes of 3- and 4-nitrophenol wi th crown ethers were studied, viz., 18-crown-6 (18C6), benzo-18-crown- 6 (B18C6), dibenzo-18-crown-6 (DB 18C6), dicyclohexano-18-crown-6 (DC1 8C6) and dibenzo-24-crown-8 (DB24C8). The spectra of uncomplexed crown ethers showed water absorption bands which indicate the presence of t wo types of bound water molecules, viz., cavitant water enclosed by th e strong ether-cavity field and outer-layer hydrogen-bonded water mole cules. Upon complexation with 3- and 4-nitrophenol, the bands attribut ed to cavitant water disappeared, leaving the outer layer water to act as a bridge between the host crown ether and the guest phenols. The r esults further showed that of the crown ethers and of the phenols, B18 C6 and DC18C6 and 3-nitrophenol, have the strongest interaction. The b ehaviour of the phenols was explained by the increased contribution of the inductive sigma-moment over the resonance pi-moment in the comple xes.