Ab initio SCF study of the barrier to internal rotation in N,N-dimethylcinnamamides

The barrier to internal rotation around the amide C-N bond in R-C6H4-CH=CH-C(O)N(CH3)(2) (R =p-NO2, m-NO2, m-Cl,p-Cl, m-OCH3, H, p-CH3, p-OCH3) was studied at the MP2(fc)/6-31+G*//6-31G* level of theory. The solvent effect was estimated using polarizable continuum model (PCM) and the free energies of activation were compared with the NMR data in CHCl3. For all studied compounds the anti transition state (anti TS) is more stable and determines the rotational barrier. A linear relationship is established linking the barrier heights and the shift of C-N bond order difference between anti TS and ground state (GS) upon substitution in the aromatic ring.