MULTINUCLEAR NMR STUDY OF COUPLED BOND MOTIONS INVOLVING AMIDE GROUPS ATTACHED TO IODINATED AROMATIC RINGS

摘要

The amide groups of 2,4,6-triiodo-3,5-diacetamidobenzoates ate displaced above or below the plane of the aromatic ring by the adjacent, bulky iodine atoms. The NMR spectra of representative compounds at 303 K in dimethyl sulfoxide solution were decomposed into signals from six different molecular conformations. The amide groups in the most stable conformations are trans and are either on the same face of the ring (syn) or on opposite faces (anti). In dimethyl sulfoxide there is essentially no interaction across the ring between the amide groups. The distance is too great for steric interactions, and the solvent screens the carbonyl oxygens from electrostatic interactions with each other. Two-dimensional exchange NMR at 313 K showed that the fastest component of conformational interchange is a jump of a trans amide from one face of the aromatic ring to the other. Detailed analysis showed that the jump, which at the end amounts to rotation about the aniline bond alone, must proceed with internal rotation about both the aniline and amide bonds and possibly distortions of bond angles. For a cis amide such extensive rotation about the amide bond takes place in conjunction with rotation about the aniline bond that the cis amide almost invariably is converted into a trans amide as it transfers from one ring face to the other. [References: 28]