Ab initio study of the barrier to internal rotation in simple amides. Part 2. N, N-dimethylacetamide and N, N-dimethyl-2-(mono-, di- and tri-) halogenacetamides

Abstract

The free energies of activation for rotation about the amide C–N bond in R–C(O)N(CH 3) 2 (R=CH 3, CH 2F, CHF 2, CF 3 and CCl 3) were calculated at the MP2(fc)/6-31+G ∗//6-31G ∗ level and compared with the NMR gas-phase data. The results of calculations indicate that the repulsion between the substituent R and methyl group in ground state and the conformation of R are largely responsible for the difference in the free energies of the studied amides. The anti transition state is the preferred one in all the studied compounds.