Ab initio SCF study of the barrier to internal rotation in simple amides. Part 3. Thioamides

The free energies of activation for rotation about the thiocarbonyl C-N bond in X-C(S)N(CH3)(2) (X = H, F, Cl, CH3, CF3) were calculated at the MP2(fc)/6-31 + G*//6-31G* and MP2(fc)/6-311++G**//6-311++G** levels and compared with literature NMR gas-phase data. The results of calculations indicate that the nonbonded interactions in ground state (GS) are mainly responsible for the differences in the rotational barriers. For X = H, CH3 and CF3, the anti transition state (TS) is more stable; for the case X = Cl, the syn TS is more stable, while for the X = F, the two TS are energetically almost equivalent. (C) 2003 Elsevier Science B.V. All rights reserved.