Article

Abstract

Recent theoretical work on amide systems has brought into question the application of the concept of resonance. In particular, the role of the oxygen atom was questioned, since the calculations showed little change in its properties when the amide bond was rotated. This paper investigates, both experimentally and computationally, the effect of a substituent on the carbonyl carbon on the amide barrier, in order to test this view. The barriers to the amide rotation in seven spiro-fused oxazolidines were measured by NMR, to within 1 kJ mol-1. A subset of three of them was modelled to the 6-31G** level. For all three substituents the computed and measured barriers corresponded to within 7 kJ mol-1. The electron densities were analyzed using the Atoms in Molecules (AIM) theory. The AIM analysis revealed that the oxazolidines behaved similarly to formamide. The substituent effect was described in terms of the atomic populations and energies of the amide C, O, and N. A substituent on the carbonyl carbon caused electron redistributions between N and C, changing their basin attractive energies. Neither the population nor the energy of oxygen changed significantly. When interactions outside the basin of interest were considered, the energy of C was seen to be more sensitive to changing the substituent than the energy of N. However, the atomic parameters from the AIM analysis did not fully reflect the substituent effects observed. For these molecules, the barrier includes contributions from several sources - there is no single, dominant contribution.Key words: amides, barrier, substituent, nuclear magnetic resonance spectroscopy, chemical exchange, atoms in molecules, slow exchange, NMR lineshape analysis.