Abstract
The effect of Kékulé representation and hybrid function of O‐atoms in the PCILO‐CNDO framework of conformation and internal rotation in mono‐sub stituted benzenes Ph‐X (XNH~2~, OH, OCH~3~, CH~3~, CHO, NO~2~) is studied. Three variational criteria for the choice of the appropriate third‐order energy, proposed to symmetrize the PCILO results, are critically examined in relation with the height of rotational barrier in these molecules. The study shows that, in all cases, the most stable conformation is qualitatively correct predicted by the PCILO method. Since the barrier to internal rotation in the studied aromatic systems arises predominantly from delocalization effect, it is proposed to employ the arithmetic mean of the third‐order energy of the two Kékulé structures. In molecules, in which the third‐order energy between the two Kékulé structures is larger than 2 kcal/mol, however, the lower third‐order energy representation alone seems to be appropriate. In phenol and anisole the spa‐hybridization type of the O‐atoms offers better values of rotational barrier, whereas in the sp^3^‐type the delocalization is overestimated in the planar conformation.