Ab initio study of the barrier to internal rotation in simple amides - 1. N,N-dimethylformamide and N,N-dimethylcarbamic halogenides

Free energies of activation for rotation about the amide C-N bond in X-C(O) N(CH3)(2) (X = H, F, Cl and Br) were calculated at the MP2(fc)/6-31+G*//6-3 1G* and MP2(fc)/6-311++G*//6-311++G** levels and compared with NMR gas-phas e data. The results of calculations indicate that the repulsion between X a nd methyl group in ground state and the repulsion between X or oxygen and n itrogen lone pair in transition states (TS) are largely responsible for the difference in the free energies of the studied amides. For X = H (DMF), th e anti TS is more stable; for the cases X = Cl, Br, the syn TS is more stab le, while for the case X = F the two transition states are energetically al most equivalent. (C) 1999 Elsevier Science B.V. All rights reserved.