MULTINUCLEAR NMR-STUDY OF COUPLED BOND MOTIONS INVOLVING AMIDE GROUPSATTACHED TO IODINATED AROMATIC RINGS

The amide groups of 2,4,6-triiodo-3,5-diacetamidobenzoates ate displac ed above or below the plane of the aromatic ring by the adjacent, bulk y iodine atoms. The NMR spectra of representative compounds at 303 K i n dimethyl sulfoxide solution were decomposed into signals from six di fferent molecular conformations. The amide groups in the most stable c onformations are trans and are either on the same face of the ring (sy n) or on opposite faces (anti). In dimethyl sulfoxide there is essenti ally no interaction across the ring between the amide groups. The dist ance 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 o f conformational interchange is a jump of a trans amide from one face of the aromatic ring to the other. Detailed analysis showed that the j ump, 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 e xtensive 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.