Conformational preferences of N-ethyl,N-methylformamide and N-ethyl,N-methylacetamide and their correlations with the NMR chemical shifts: a theoretical study

The stabilities of N-ethyl,N-methylformamide and N-ethyl,N-methylacetamide in their syn and anti forms have been calculated using HF/6-311++G**, MP2(FC)/6-311++G**//HF/6-311++G** and B3LYP/6-311++G**//HF/6-311++G** methods. The calculated NMR chemical shifts were evaluated using the last method. The relative position of the methyl part of the N-ethyl group has been found to be almost perpendicular to the plane of the heavy atoms in disagreement with the previous report. The conformational preference is discussed in terms of both steric and hyperconjugative/anomeric effects. The consequences of these effects on the geometry and NMR spectra have been analyzed.