THE WEAKENING OF THE INTRAMOLECULAR HYDROGEN-BOND NH...O IN COMPLEXES OF ORTHO-SUBSTITUTED ANILINES WITH PROTON ACCEPTORS

The IR spectra of ortho-nitroaniline and methyl anthranilate and their complexes with protonacceptors B have been studied in the region of the stretching vibrations of the NH2, ND2 and NHD groups. Weakening of the intramolecular hydrogen bond NH...O under the influence of the intermolecular bond NH...B has been found to occur. The cooperativity effect increases with the strength of the inter- and intramolecular hydrogen bonds. A method of estimating the intramolecular hydrogen-bond enthalpy in ortho-substituted anilines has been proposed, based on the assumption of the equality of two hydrogen-bond enthalpies in the complex with the symmetrically perturbed amino group. The equivalence of inter- and intramolecular perturbations were determined by the coincidence of cis- and trans-NH (and/or ND) frequencies of the NHD group.