On the Origin of Higher Rotational Barriers in Thioamides than in Amides. Remote Substituent Effects on the Conformational Stability of the Thioamide Group in Thioacetanilides

Abstract

The variations of barrier heights for rotation around the amide C−N bond in para-substituted thioacetanilides was studied in a comparative context with data for a series of acetanilides by applying density functional theory at the B3LYP/6-31G(d,p) level. The shifts of the energy barriers induced by remote substituents in the para position of the aromatic ring are interpreted in terms of changes of the bond lengths and electric-charge parameters for the equilibrium configurations, as well as in terms of charge fluctuations that accompany the internal rotation. A satisfactory explanation for the distinctly greater shifts of rotational barriers in the thioacetanilide series upon substitution in the aromatic ring, as compared with data for the respective acetanilides, is obtained by analyzing the dependences of barriers heights on the charge shifts upon substitution in the aromatic ring and upon rotation. Several linear relationships are established linking the barrier heights with structural and electronic parameters that characterize the amide and thioamide groupings. A common linear equation quantifying the dependence of barrier heights on the shifts of natural bond orbital partial charges at the amide S and O atoms upon substitution in the aromatic ring in the two series is found. The relationship is confirmed by data from a Mulliken population analysis. The results obtained are consistent with the views for a classical amide resonance as being the origin of higher rotational barriers in thioamides than in amides.