An energy decomposition analysis approach to the rotational barriers of amides and thioamides

Abstract The most popular interpretation for the origin of rotational barrier in amides is the electron delocalization of the nitrogen lone pair over the NCO π system. Many attempts have been made to quantify the extent of delocalization in amide and amide like systems. An alternative approach to understand rotational barriers is the energy decomposition analysis (EDA) class of methods. In this work, the rotational barrier of a series of amides is investigated using the Canonical Molecular Orbital Energy Decomposition Analysis (CMOEDA) method. Based on CMOEDA, it was possible to explain i) the origin of the rotational barriers, ii) the energy difference between the two transition states (TS1/TS2) and iii) the variation in the rotational barrier as structural modifications are introduced. Regarding the origin of the rotational barrier, for instance, it was verified that electrostatic and polarization interactions are responsible for building the barrier, while exchange-repulsion contributes to decrease its value.