THE NATURE OF INTRAMOLECULAR HYDROGEN-BONDED AND NON-HYDROGEN-BONDED CONFORMATIONS OF SIMPLE DIAMIDES AND TRIAMIDES

The nature and the magnitude of the N-H...O = C hydrogen-bonding interaction in amides were examined in terms of ab initio and AM1 calculations on formamide dimer. On the basis of AM1 calculations, we examined the relative stabilities of various intramolecular hydrogen-bonded and non-hydrogen-bonded conformations for several di- and triamides. AM1 calculations were also performed for certain amide structures solvated with CH2Cl2 molecules. The energy surface of the N-H...O = C hydrogen-bonding interaction is soft with respect to the angle N-H...O and the angle H...O = C angle variation. For each of the amides studied, the most stable conformation is a hydrogen-bonded structure. For an amide which forms a hydrogen-bonded 6- or 7-membered ring containing one CH2 group, the corresponding non-hydrogen-bonded form is not a stationary (i.e., minimum energy) structure but is made stationary when solvated with CH2Cl2 Molecules. The stability of triamide 1 increases with an increase in the number of intramolecular hydrogen-bonding contacts. The conformation 1c of this triamide, which makes 7- and 9-membered hydrogen-bonded rings involving the H(b) atom but leaves the H(a) atom non-hydrogen-bonded, is not a stationary structure even when solvated with CH2Cl2 molecules.