The electronic effect of polar substituents on the barrier of internal rotation around the amide carbon-nitrogen bond in a series of 10 p-substituted acetanilides is studied by applying density functional theory at the B3LYP/6-31G(d,p) level. The theoretical results are supplemented by experimental data on the amide C=O and N-H stretching mode frequency shifts. It is shown that computations at the theoretical level employed provide a valuable approach in studying the factors determining the conformational stability of the studied series of compounds. It is found that an excellent linear dependence between the barriers of rotation and frequency shifts exists. It is concluded that the variations of the amide C=O stretching mode frequency can be used for quantitative characterization of the amide group conformational flexibility in the studied series of acetanilides.